Regional Aquifer-System Analysis

U.S. Department of the Interior

U.S. Geological Survey

Puerto Rico

C A R I B B E A N S E A

A T L A N T I C O C E A N

Isla de Vieques

St. Thomas

St. John

St. Croix

Isla Mona

Isla Desecheo

Isla de Culebra

Professional Paper 1419

Geology and Hydrogeology of the Caribbean Islands

Aquifer System of the Commonwealth of Puerto Rico

and the U.S. Virgin Islands
Geology and Hydrogeology of the

Caribbean Islands Aquifer System

of the Commonwealth of Puerto Rico

and the U.S. Virgin Islands

By ROBERT A. RENKEN1, W.C. WARD2, I.P. GILL3, FERNANDO GÓMEZ-GÓMEZ1,

JESÚS RODRÍGUEZ-MARTÍNEZ1

, and others

REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1419

1

U.S. Geological Survey, 2Department of Geology and Geophysics, University of New Orleans, 2000 Lakeshore Drive, New Orleans, LA 70148 (retired), 3

Department of Geology, University of Puerto Rico, Mayagüez, PR 00681
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Any use of trade, product, or firm names in this publication is for

descriptive purposes only and does not imply endorsement by the

U.S. Government.

Library of Congress Cataloging-in-Publication Data

For sale by U.S. Geological Survey, Branch of Information Services, Box 25286,

Federal Center, Denver, CO 80225

Reston, Virginia 2002

Geology and hydrogeology of the Caribbean islands aquifer system of the 

Commonwealth of Puerto Rico and the U.S. Virgin Islands / by Robert A. Renken ... [et al.].

 p. cm. — (Regional aquifer-system analysis—Caribbean islands) (U.S. Geological 

 Survey professional paper; 1419)

 Includes bibliographical references.

 ISBN 0-607-99361-8

 1. Hydrogeology—Puerto Rico. 2. Hydrogeology—Virgin Islands of the United States.

 3. Aquifers—Puerto Rico. 4. Aquifers—Virgin Islands of the United States. I. Renken,

 Robert A. II. Series: U.S. Geological Survey professional paper ; 1419

GB1055 .G46 2002

551.49'097295—dc21

 2002029991
FORWARD

THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM

The Regional Aquifer-System Analysis (ASA) Program represents a 

systematic effort to study a number of the Nation's most important aquifer 

systems, which, in aggregate, underlie much of the country and which represent an important component of the Nation's total water supply. In general, 

the boundaries of these studies are identified by the hydrologic extent of 

each system and, accordingly, transcend the political subdivisions to which 

investigations have often arbitrarily been limited in the past. The broad 

objective for each study is to assemble geologic, hydrologic, and geochemical 

information; to analyze and develop an understanding of the system; and to 

develop predictive capabilities that will contribute to the effective management of the system. The use of computer simulation is an important element 

of the RASA studies to develop an understanding of the natural, undisturbed 

hydrologic system and the changes brought about in it by human activities, 

and to provide a means of predicting the regional effects of future pumping 

or other stresses.

The final interpretive results of the RASA Program are presented in a 

series of U.S. Geological Survey Professional Papers that describe the 

geology, hydrology, and geochemistry of each regional aquifer system. Each 

study within the RASA Program is assigned a single Professional Paper 

number beginning with Professional Paper 1400.

Charles G. Groat

Director
CONTENTS

Forward

Abstract

Introduction by Robert A. Renken

 Physiographic setting

 Puerto Rico

 Puerto Rico's offshore islands

 Isla de Vieques

 Isla de Culebra

 Isla Desecheo

 Isla Mona

 U.S. Virgin Islands

 Previous investigations

 Puerto Rico

 Puerto Rico's offshore islands

 U.S. Virgin Islands

 Method of investigation

Geology of the South Coast ground-water province of Puerto

 Rico by Robert A. Renken

 Stratigraphy

 Juana Díaz Formation

 Unnamed pelagic carbonate rocks

 Ponce Limestone

 Subsurface stratigraphic relations

 Pleistocene to Holocene fan-delta and alluvial 

 deposits

 Configuration of basal Quaternary contact and

 thickness of deposits

 Lithofacies

 Vertical profile

 Sand and gravel percentage

 The South Coast Tertiary Basin: Controls on clastic

 deposition

 Structural features and evidence of tectonic movement 

 in the South Coast ground-water province

 Changes in base level: Its record in the stratigraphic

 sequence

 Climate

Geology of the West Coast ground-water province of Puerto

 Rico

Geology of the East Coast ground-water province of Puerto

 Rico

Geology of the Interior ground-water province of Puerto Rico

Geology of the North Coast ground-water province of Puerto

 Rico by W.C. Ward, R.A. Scharlach, and J.R. Hartley

 Introduction

 Geologic setting of the North Coast Tertiary Basin

 Stratigraphy

 Methods of correlation

 Lithostratigraphic nomenclature

 San Sebastián Formation ("middle" to upper 

 Oligocene)

 Outcrop

 Subsurface

 Lares Limestone (upper Oligocene to lower Miocene)

 Outcrop

 Subsurface

 Mucarabones Sand (upper Oligocene to lower 

 Miocene)

 Outcrop

Geology of the North Coast ground-water province of Puerto

 Rico—continued

 Stratigraphy—continued

 Mucarabones Sand (upper Oligocene to lower

 Miocene—continued

 Subsurface

 Cibao Formation (lower Miocene)

 Outcrop

 Subsurface

 Montebello Limestone Member

 Mudstone unit

 Río Indio/Quebradas Arenas Limestone Members

 Undifferentiated Cibao Formation

 Aguada (Los Puertos) Limestone (lower to middle

 Miocene)

 Outcrop

 Subsurface

 Aymamón Limestone (middle Miocene)

 Outcrop

 Subsurface

 Quebradillas Limestone (uppermost Miocene to

 Pliocene)

 Outcrop

 Subsurface

 Depositional history

 Introduction

 Middle to late Oligocene

 Late Oligocene to early Miocene

 Early Miocene

 Late early Miocene to early middle Miocene

 Middle Miocene

 Latest Miocene-Pliocene

 Effects of structural movements on deposition of upper

 Oligocene to lower Miocene rocks

 Sequence stratigraphy of Oligocene to middle Miocene

 rocks

 Introduction

 Sequence boundaries in northern Puerto Rico

Geology of central St. Croix, U.S. Virgin Islands by I.P. Gill,

 D.K. Hubbard, P.P. McLaughlin, and C.H. Moore

 Introduction

 Stratigraphy and sedimentology of the central limestone

 plain region

 Jealousy Formation

 Lithology, facies, and depositional environment

 Structure and distribution

 Age

 Kingshill Limestone

 La Reine (lower) Member of the Kingshill

 Limestone

 Lithology, facies, and depositional

 environment

 Structure and distribution

 Age

 Mannings Bay (upper) Member of the Kingshill

 Limestone

 Lithology, facies, and depositional

 environment

 Structure and distribution

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VI CONTENTS

Geology of central St. Croix, U.S. Virgin Islands—continued

 Stratigraphy and sedimentology of the central limestone

 plain region—continued

 Mannings Bay (upper) Member of the Kingshill

 Limestone—continued

 Age

 Blessing Formation

 Lithology, facies, and depositional 

 environment

 Structure and distribution

 Age

 Dolomitization and diagenesis

 Alluvial deposits

 Sedimentary and structural setting of the Kingshill 

 Basin

Hydrogeologic framework of the U.S. Caribbean Islands by

 Robert A. Renken, Fernando Gómez-Gómez, and Jesús

 Rodríguez-Martínez

 The South Coast ground-water province

 The South Coast aquifer

 The vertical sequence—major water-bearing 

 units

 Hydraulic conductivity and the lateral continuity

 of water-bearing units

 Alluvial valley aquifers of the South Coast

 The Ponce-Juana Díaz aquifer

 The West Coast ground-water province

 Alluvial valley aquifers of the East Coast ground-water

 province

 Alluvial valley aquifers of the Interior ground-water

 province

 North Coast ground-water province: The North Coast

 limestone aquifer system

 Occurrence and movement of ground water

 Alluvial valley aquifers and local confining units

Hydrogeologic framework of the U.S. Caribbean Islands

 —continued

 North Coast ground-water province: The North Coast

 limestone aquifer system—continued

 Upper aquifer

 Middle confining unit

 Lower aquifer

 Basal confining unit

 Distribution of transmissivity and hydraulic 

 conductivity

 Controls on porosity and permeability in carbonate

 aquifers of northern Puerto Rico by W.C. Ward

 Stratigraphic control

 Lithologic and diagenetic control

 Upper aquifer

 Middle confining unit

 Lower aquifer

 Basal confining unit

 Fracture control

 U.S. Virgin Islands by Robert A. Renken

 Distribution and types of porosity within the

 Kingshill aquifer by I.P. Gill, D.K. Hubbard, P.P. 

 McLaughlin, and C.H. Moore

 Kingshill Limestone in the Central and Northern

 Basin

 Mannings Bay Member and the Blessing 

 Formation along the southern coastline

 Dissolution and karsting

 Dolomitization

 Alluvial aquifers by Robert A. Renken

 Weathered mantle-bedrock aquifer

 Vieques Island

Summary

References

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Page Page

ILLUSTRATIONS

PLATE 1.

2.

3.

4.

Geology, hydrogeology, and hydrology of the South Coast ground-water province between Ponce and Patillas, 

 Puerto Rico

Maps showing:

 Geology of the South Coast

 Configuration of bedrock surface underlying fan-delta deposits of Quaternary age

 Thickness of Holocene to Pleistocene fan-delta and alluvial deposits

 Percentage of sand and gravel in the fan-delta deposits

 Interpretive structure and major lineaments and/or faults underlying the fan-delta plain

 Estimated potentiometric surface of the South Coast aquifer, February, 1968

 Estimated potentiometric surface of the South Coast aquifer, March, 1986

 Estimated thickness of freshwater lens contained within the South Coast aquifer, 1986–87

 Distribution of hydraulic conductivity within the South Coast aquifer

Geology and hydrogeology of the South Coast ground-water province between Cabo Rojo and Ponce, southwestern

 Puerto Rico

Maps showing:

 Geology of the South Coast ground-water province

 Configuration of bedrock surface underlying alluvial-valley and fan-delta sediments of Quaternary age

 Percentage of sand and gravel contained within alluvial-valley and fan-delta areas

Cyclic depositional sequences within the southern fan-delta plain

Geologic map and sections showing principal stratigraphic units and lithofacies of rocks of Oligocene to Pliocene

 age in the North Coastal limestone aquifer system

In pocket

In pocket

In pocket

In pocket

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CONTENTS

Page

PLATE

FIGURE

5.

1.

2.

3–6.

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13–15.

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18–21.

22.

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25–32.

33.

Hydrogeology and hydrology of the North Coast limestone aquifer system

 Section A–A' showing principal aquifers and confining units of the North Coast limestone aquifer system

Maps showing:

 Estimated potentiometric surface of the upper aquifer showing average conditions between 1980 and 1990

 Estimated predevelopment potentiometric surface of the lower aquifer

 Potentiometric surface of the lower aquifer in area of greatest development (Río Grande de Arecibo to Río

 Cibuco) showing average conditions during 1987

 Estimated thickness of the freshwater lens within the upper aquifer

 Top of the middle confining unit

 Distribution of transmissivity in the upper aquifer

 Distribution of hydraulic conductivity in the upper aquifer

 Distribution of transmissivity in the lower aquifer

Map showing location of Puerto Rico and the U.S. Virgin Islands

Map showing ground-water provinces of Puerto Rico

Maps showing geology of:

 Puerto Rico

 Isla de Vieques

 St. Croix, U.S. Virgin Islands

 St. Thomas and St. John, U.S. Virgin Islands

Map showing location of Tertiary sedimentary basins of Puerto Rico

Correlation chart showing stratigraphic terminology in Puerto Rico's South Coast ground-water province

Map showing major facies and thickness of the Juana Díaz Formation, south-central Puerto Rico

Map showing major facies and thickness of the Ponce Limestone and equivalent beds, south-central Puerto Rico

Section A–A' across southern Puerto Rico's fan-delta plain

Section B–B' down the axis of the Coamo fan delta, south-central Puerto Rico

Maps showing:

 Location and extent of fan deltas of southern Puerto Rico

 Location of principal alluviated valleys and small, restricted fan-delta plains in the western half of Puerto 

 Rico's south coast

 Areas flooded in the central part of the southern plain during the October 6–7, 1985 flood

Photograph of a southward-facing seacliff exposure at Central Machete-Puerto Arroyo in the South Coast ground-

 water province, Puerto Rico, showing an exhumed channel within Quaternary fan-delta plain

Chart of major biochronozones and lithostratigraphic units of late Tertiary and Quaternary age in the South Coast

 ground-water province and estimates of relative sea level

Maps showing:

 Fan-delta deposition in south-central Puerto Rico during late Pleistocene-early Holocene sea level rise

 Physical processes within the fan-delta plain of south-central Puerto Rico following the last Holocene

 transgression

 Thickness of the Caguas-Juncos alluvial valley aquifer

 Location of cored test holes and other wells that penetrate Tertiary rocks beneath the North Coast ground-

 water province of Puerto Rico

Correlation chart showing stratigraphic nomenclature and ages for Oligocene, Miocene, and Pliocene sedimentary

 rocks in the North Coast Tertiary Basin

Map showing thickness of Lares Limestone, northern Puerto Rico

Generalized sections showing stratigraphic relations of Cibao Formation and its members at the outcrop (A) and in

 the subsurface (B)

Maps showing:

 Thickness of Montebello Limestone, northern Puerto Rico

 Thickness of the mudstone unit of the Cibao Formation, northern Puerto Rico

 Thickness of the undifferentiated Cibao Formation, northern Puerto Rico

 Combined thickness of Aguada (Los Puertos) and Aymamón Limestones, northern Puerto Rico

 Major depositional environments and facies patterns during deposition of the upper Lares Limestone, 

 northern Puerto Rico

 Major depositional environments and facies patterns during the deposition of the lower Cibao Formation,

 northern Puerto Rico

 Major depositional environments and facies patterns during the deposition of the upper Cibao Formation,

 northern Puerto Rico

 Major depositional environments and facies patterns during the deposition of the Aguada (Los Puertos)

 Limestone, northern Puerto Rico

Conceptual model of the southern part of the North Coast Tertiary Basin in Puerto Rico showing influence of 

 structural movement on late Oligocene and early Miocene deposition

In pocket

VII

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CONTENTS Page

FIGURE 34.

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General model for a sequence-stratigraphic framework

Depositional-sequence model for an inclined carbonate platform or "ramp" located in a humid environment

Sequence-stratigraphic framework of the Oligocene to middle Miocene sedimentary rocks of the North Coast Tertiary

 Basin, Puerto Rico

Generalized section showing sequence boundaries related to the updip Cibao Formation, northern Puerto Rico

Map showing generalized geology of St. Croix, U.S. Virgin Islands, and location of control points

Generalized stratigraphic column of St. Croix, U.S. Virgin Islands

Expanded stratigraphic column showing rocks of late Tertiary age in St. Croix, U.S. Virgin Islands

Map showing structural configuration of the top of the Jealousy Formation

Section A–A' from Krause to Judiths Fancy

Section B–B' from Estate Hesselberg to Pearl

Section C–C' from Williams Delight to Sion Hill showing relation between formational units and biostratigraphy

Section D–D' from Fairplain to St. Johns showing relation between formational units and biostratigraphy

A south-facing view of the disconformity separating the Mannings Bay Member from the underlying beds of the La

 Reine Member lies approximately 7 m above the Highway 66 road bed, north of Alexander Hamilton Airport. Arrows

 mark the position of the unconformity

Map showing thickness of the Kingshill Limestone

An east-facing view of the unconformity that separates the Mannings Bay Member and the Blessing Formation at the

 Hamilton Airport Quarry exposure. Arrows mark the position of the unconformity

Map showing major facies within the Blessing Formation in the industrial area north of Krause Lagoon

Hardground and exposure surfaces in the Blessing Formation at the Hess Oil exposure. Upper photo shows an oblique

 view to the north-northwest. Apparent "bedding" dipping upper left to lower right is caused by bulldozer scarring.

 Lower photo shows hardground (H) and caliche/exposure surfaces (C) in the Blessing Formation reef tract. Rock

 hammer and field book (FH) for scale

Block models of St. Croix during the early Miocene to Pliocene

Chart showing relation of major stratigraphic and hydrogeologic units of the U.S. Caribbean Islands

Maps showing distribution of transmissivity (A) and percentage of sand and gravel (B) in the Yauco alluvial valley

 aquifer, western south-central Puerto Rico

Map showing distribution of transmissivity in the Caguas-Juncos alluvial valley aquifer

Diagrammatic section A–A' showing principal aquifers and confining units of the North Coast limestone aquifer

 system

Block diagram showing area of probable discharge from lower aquifer to the alluvial aquifer in the Río Grande de

 Arecibo valley, northern Puerto Rico

Block diagram showing exploded view of aquifers in the Río Grande de Arecibo valley and patterns of ground-water

 flow, northern Puerto Rico

Maps showing:

 Basal configuration of Arecibo alluvial valley aquifer, northern Puerto Rico

 Drawdown in the lower aquifer between 1968 and 1987, north-central Puerto Rico

 Major fracture patterns inferred from strike of surface lineaments in karst topography between the Río 

 Grande de Arecibo and Río Grande de Manatí, northern Puerto Rico. Rose diagrams, constructed using

 10-degree sectors, show the orientation of 99 lineaments from the Aymamón and Aguada Limestones (A)

 and orientation of 100 lineaments from the Montebello Limestone Member and the Lares Limestone (B)

 Thickness of the alluvial aquifer in St. Croix, U.S. Virgin Islands

Section A-A' showing hydrogeology of the Esperanza alluvial valley aquifer, Isla de Vieques

VIII

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128

TABLE

TABLE 1. Principal springs within the North Coast limestone aquifer system ........................................................................................ 112

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58.

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61.
CONTENTS IX

Sea Level: In this report "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)— a

geodetic datum derived from a general adjustment of the first-order level nets of both the United States and

Canada, formerly called "Sea Level Datum of 1929".

CONVERSION FACTORS

Multiply SI unit By To obtain inch-pound unit

VERTICAL DATUM

ABBREVIATIONS

Length

Area

Flow

Hydraulic conductivity

Specific capacity

Transmissivity

millimeter (mm)

meter (m)

kilometer (km)

centimeter (cm)

inch (in)

foot (ft)

mile (mi)

inch (in)

square mile (mi2

)

foot per day (ft/d)

square foot per day (ft2

/d)

gallon per minute per foot [(gal/min)/ft]

square kilometer (km2

)

liter per second (L/s)

cubic meter per day (m3

/d)

cubic meter per second (m3/s)

gallons per minute (gal/min)

gallons per day (gal/d)

cubic foot per second (ft3

/s)

meter per day (m/d)

square meter per day (m2

/d)

liter per second per meter [(L/s)/m]

0.03937

3.281

0.6214

2.590

0.3861

15.85

264.2

35.31

3.281

4.831

10.76

yr BP

m.y.

years before present

millions of years
1

REGIONAL AQUIFER-SYSTEM ANALYSIS

GEOLOGY AND HYDROGEOLOGY OF THE CARIBBEAN ISLANDS 

AQUIFER SYSTEM OF THE COMMONWEALTH OF PUERTO RICO 

AND THE U.S. VIRGIN ISLANDS

BY ROBERT A. RENKEN1, W.C. WARD2, I.P. GILL3, FERNANDO GÓMEZ-GÓMEZ1, JESÚS RODRÍGUEZMARTÍNEZ1

, AND OTHERS

ABSTRACT

Poorly lithified to unconsolidated carbonate and clastic sedimentary rocks

of Tertiary (Oligocene to Pliocene) and Quaternary (Pleistocene to Holocene)

age compose the South Coast aquifer and the North Coast limestone aquifer

system of Puerto Rico; poorly lithified to unlithified carbonate rocks of late

Tertiary (early Miocene to Pliocene) age make up the Kingshill aquifer of St.

Croix, U.S. Virgin Islands. The South Coast aquifer, North Coast limestone

aquifer system, and Kingshill aquifer are the most areally extensive and function as the major sources of ground water in the U.S. Caribbean Islands

Regional Aquifer-System Analysis (CI-RASA) study area.

In Puerto Rico’s South Coast ground-water province, more than 1,000

meters of clastic and carbonate rocks of Oligocene to Pliocene age infill the

South Coast Tertiary Basin. The pattern of lithofacies within this basin

appears to have been controlled by changes in base level that were, at times,

dominated by tectonic movement (uplift and subsidence), but were also influenced by eustasy. Deposition of the 70-kilometer long and 3- to 8-kilometer

wide fan-delta plain that covers much of the South Coast ground-water province occurred largely in response to glacially-induced changes in sea level and

climate during the Quaternary period. Tectonic movement played a much less

important role during the Quaternary.

The North Coast ground-water province of Puerto Rico is underlain by a

homoclinal coastal plain wedge of carbonate and siliciclastic rocks that infill

the North Coast Tertiary Basin and thicken to more than 1,700 meters. A thin

basal siliciclastic sequence of late Oligocene age is overlain by a thick section

of mostly carbonate rocks of Oligocene to middle Miocene age. Globigerinid

limestone of late Miocene to Pliocene age crops out and lies in the shallow

subsurface areas of northwestern Puerto Rico. Oligocene to middle Miocene

age rocks tentatively can be divided into five depositional sequences and associated systems tracts; these rocks record carbonate and minor siliciclastic deposition that occurred in response to changes in relative sea level. The Cibao

Formation represents the most complex of these sequences and contains a varied facies of carbonate, mixed carbonate-siliciclastic, and siliciclastic rocks

that reflect differential uplift, subsidence, and transgression of the sea.

Uplift, graben formation, and gradual shallowing of the sea are reflected

within the bathyal-dominated sedimentary facies of the Kingshill Limestone in

St. Croix, U.S. Virgin Islands. Reef-tract limestone beds of Pliocene age were

subject to exposure, resubmergence, and meteoric leaching of aragonitic skeletal debris; these beds contain patchy lenses of dolomite that are restricted to a

small, structurally-controlled embayment.

The South Coast aquifer, the principal water-bearing unit of Puerto Rico’s

South Coast ground-water province, consists of boulder- to silt-size detritus

formed by large and small coalescing fan deltas of Pleistocene to Holocene

age. Deep well data indicates that it is possible to vertically separate and

group a highly complex and irregular-bedded detrital sequence that underlies

distal parts of the fan-delta plain into discrete water-bearing units if correlated

with 30- to 40-meter thick, eustatically-controlled depositional cycles. Lithofacies maps show that greatest hydraulic conductivity within the fan-delta

plain is generally associated with proximal fan and midfan areas. Distal and

interfan areas are least permeable. Alluvial valley aquifers located in the western part of the South Coast ground-water province are important local sources

of water supply and appear to contain some of the same physical and hydraulic

characteristics as the South Coast aquifer. Older sedimentary rocks within the

basin are poor aquifers; conglomeratic beds are well-cemented, and carbonate

beds do not contain well-developed solution features, except locally where the

beds are overlain by alluvium. Ground-water occurs under unconfined conditions in proximal and midfan areas. Confined conditions within deeper parts

of the system and in interfan and some midfan areas are created largely by the

intercalated nature of discontinuous fine-grained beds that retard vertical

ground-water movement.

The development of water resources in southern Puerto Rico has modified

the hydrologic system of the South Coast aquifer considerably. Under predevelopment conditions, the South Coast aquifer was recharged in the unconfined, proximal fan and some midfan areas by infrequent rainfall and seepage

from streams near the fan apex. Discharge occurred as seabed seepage, baseflow discharge along the lower coastal reach of streams, seepage to coastal

wetlands, or evapotranspiration in areas underlain by a shallow water table.

Under development conditions, seepage from irrigation canals and areal

recharge from furrow irrigation represented a principal mechanism for

recharge to the aquifer. Increased ground-water withdrawals in the 1960’s and

1970’s resulted in declines in the water table to below sea level in some places

and intrusion of salt water into the aquifer. By the middle 1980’s, a reduction

in ground-water withdrawals and a shift from furrow irrigation to drip-irrigation techniques resulted in the recovery of water levels. Under present-day

(1986) conditions, regional ground-water flow is coastward but with local

movement to some well fields. In addition to the discharge mechanisms

described above, ground-water discharges also to coastal canals.

The North Coast limestone aquifer system consists of limestone, lesser

amounts of dolomite, and minor clastic detritus of Oligocene to Pliocene age 

1

 U.S. Geological Survey; 

2

 Department of Geology and Geophysics, University of 

New Orleans, 2000 Lakeshore Drive, New Orleans, LA 

70148 (retired); 

3 Department of Geology, University of Puerto Rico, 

Mayagüez, PR 00681
2 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

that form an unconfined upper aquifer and a confined lower aquifer; these

aquifers are separated by a clay, mudstone, and marl confining unit. Topographic relief and incision of carbonate coastal plain rocks by streams are the

principal factors controlling the direction of ground-water flow. The North

Coast limestone aquifer system is recharged principally by precipitation that

enters the upper and lower aquifers where they crop out. Regional groundwater movement from the upper aquifer is to the major rivers, wells, coastal

wetlands, coastal, nearshore, and offshore springs, or as seabed seepage.

Regional discharge from the lower aquifer is to the major rivers along its

unconfined parts or where the confining unit has been breached by streams.

Discharge from the lower aquifer also occurs in the San Juan area where the

Mucarabones Sand provides an avenue for diffuse upward ground-water flow.

Transmissivity within the upper limestone aquifer appears to be largely regulated by the thickness of the freshwater lens. The lens is thickest and transmissivity is greatest in interstream areas that lie in a zone that closely corresponds

to the landwardmost extent of the underlying saltwater wedge. Hydraulic conductivity of the upper aquifer generally increases in a coastward direction and

reflects lithologic control, karstification in the upper 30 to 100 meters of the

section, and enhanced permeability in a zone of freshwater and saltwater mixing. Transmissivity of the lower aquifer is an order of magnitude smaller than

that of the upper aquifer; highest transmissivities in the lower aquifer largely

correspond to a coarse grainstone-packstone and coral-patch-reef depositional

facies contained within the outcropping parts of the Montebello Limestone

Member and its subsurface equivalents. Porosity within the North Coast limestone aquifer system is high in grainstone-packstones and low in wackestone

and marl. Dolomitized zones and moldic grainstone-packstone strata are the

most porous carbonate rocks, but occur in thin beds that usually are only a few

meters thick. Processes of karstification that include the development of

caverous zones and large vugs, and dissolution along possible regional fracture sets has enhanced permeability within the upper part of the aquifer system. Stratigraphic and lithologic control play an important role controlling

permeability within the lower part of the system.

The Kingshill aquifer of St. Croix, in large part, is composed of deepwater limestone that contains only microscopic pores and is poorly permeable; however, the upper part of the aquifer, a shallow-water skeletal and

reef limestone, is fairly permeable, but restricted in areal extent. Permeability

within these uppermost beds of the aquifer has been enhanced by meteoric

leaching, dissolution within a mixing zone of saltwater and fresh water, and

dolomitization. However, most large-yield wells completed in the Kingshill

aquifer are also screened in alluvium that overlies or infills incised channels.

The alluvial deposits serve as a temporary storage zone for rainfall, runoff, and

ground water slowly entering the Kingshill aquifer.

INTRODUCTION

BY ROBERT A. RENKEN1

Three major aquifers underlie the North Coast and South

Coast ground-water provinces of Puerto Rico and the central

plain of St. Croix, U.S. Virgin Islands. Together, they combine to form the principal aquifers of the U.S. Caribbean

Island aquifer system (Gómez-Gómez, 1987). The Caribbean

Island aquifer system is one of twenty-eight major aquifer

systems that are located in the United States, its territories,

and the Commonwealth of Puerto Rico. Collectively, these

28 aquifer systems contain most of the Nation’s ground-water

supplies. The U.S. Caribbean Island Regional Aquifer-System (CI-RASA) investigation was studied as part of the U.S.

Geological Survey’s (USGS) Regional Aquifer-System Analysis (RASA) program (Sun, 1986; Weeks and Sun, 1987), a

series of investigations that present an overview and assessment of hydrogeologic, hydrologic, and hydrochemical conditions within regionally extensive aquifer systems. The

major objectives of the Caribbean Island aquifer system study

are to (1) identify and map the permeability distribution

within major aquifers and aquifer systems of Puerto Rico and

St. Croix, (2) describe the hydrochemistry of major aquifers,

(3) examine the pattern of ground-water flow, and

(4) simulate flow patterns by use of digital computer.

Deposits of boulders, gravel, sand, and silt that underlie

most of the eastern two-thirds of Puerto Rico’s South Coast

ground-water province, form one of two of the island’s principal aquifers, and supply nearly one-half of all ground-water

withdrawn on the island. In the North Coast ground-water

province, highly karstified reef and platform carbonate rocks

and minor clastic beds form an aquifer system that contains

two aquifers separated by a confining unit. The upper limestone aquifer contains water mostly under unconfined conditions. A lower aquifer is more restricted in extent and

contains water in middip and downdip areas under artesian

conditions; unconfined conditions prevail at outcrop. Alluvial deposits, mostly those associated with stream deposition,

are locally important aquifers in the East Coast, West Coast,

and Interior ground-water provinces and in Lajas Valley

(Valle de Lajas). Shelf and shelf-slope hemipelagic limestone, interbedded turbidites, and shallow water limestone

make up the Kingshill aquifer, the principal aquifer in the

island of St. Croix. Alluvial-fan, debris flow, and alluvial

deposits that overlie the aquifer in some areas function as a

temporary storage zone slowly releasing water to the aquifer. Alluvial and alluvial fan deposits are also locally important in Isla de Vieques, St. Thomas, and St. John. Yields to

wells completed in volcaniclastic, fractured bedrock and

weathered-rock mantle aquifers in Puerto Rico and the U.S.

Virgin Islands are small (less than 1 liter per second). The

area underlain by these rocks in each of the islands is large

and in those areas represent the only source of ground water.

Collectively, these rocks constitute about 6 percent of total

ground-water withdrawals in Puerto Rico and 75 percent of

the total ground water withdrawn in the Virgin Islands (U.S.

Geological Survey, 1985).

Understanding the physical framework and facies of an

aquifer or aquifer system is useful in predicting hydraulic

conductivity. Hydraulic conductivity is a measure of how

rapidly water will pass through an aquifer, and is a good indication of the probable yield of wells completed in the aquifer—the higher the hydraulic conductivity, the greater the

expected yield. Permeability is another way of measuring the

ability of an aquifer to transmit water under a hydraulic gradient. Permeability is equal to the hydraulic conductivity of an

aquifer multiplied by the gravitational constant and divided

by the dynamic viscosity of the water. Transmissivity is a

1

 U.S. Geological Survey
INTRODUCTION 3

third way of measuring the capacity of an aquifer to transmit

water of the prevailing viscosity. The transmissivity of an

aquifer is equal to the horizontal hydraulic conductivity of an

aquifer multiplied by the saturated thickness of the aquifer.

The hydraulic conductivity, transmissivity, and permeability

of an aquifer are directly influenced by the particle size, particle shape, degree of packing, sorting, amount of material that

fills pore space, and cementation of the mineral and rock

material comprising the aquifer. These factors are, in large

part, a reflection of the of the depositional history of the rock.

Therefore, the distribution of the hydraulic conductivity of an

aquifer might be estimated from a map of the geologic facies,

assuming a direct correlation exits between rock type and

aquifer permeability. Hydraulic and lithofacies data can also

be compared to help understand lateral variations within the

geologic sequence. To a degree, lithofacies maps can be used

as a predictive tool in the search for overlooked or undeveloped sites that might prove suitable for ground-water

resource development.

This report provides a comprehensive description of the

hydrogeology and hydrology of the Caribbean Island aquifer

system. This report (1) defines the regional and local geology; (2) identifies, delineates, and maps the permeability distribution within clastic and carbonate aquifers; and (3)

develops a suitable framework that helps explain major tectonic, depositional, and diagenetic controls on transmissivity

and permeability.

PHYSIOGRAPHIC SETTING

Puerto Rico, its satellite islands, and the Virgin Islands are

located in the northeastern Caribbean Sea region between latitude 17 ̊30' and 19 ̊00' and between longitude 64 ̊15' and

68 ̊00'. Puerto Rico is the easternmost island of the chain of

large islands that forms the Greater Antilles (fig. 1). Puerto

Rico is shaped roughly like a rectangle (170 kilometer [km]

long and 60 km wide) and forms the western half of the

Puerto Rico-Virgin Island platform. The Virgin Islands are

the northwesternmost chain of small islands that form the

Lesser Antilles. Much of the Lesser Antilles extends eastward and then southward. Together the islands of the Greater

and Lesser Antilles geographically separate the Atlantic

Ocean and the Caribbean Sea. The Virgin Islands include 80

islands and cays, 48 of which are U.S. territories with the

remainder governed by the United Kingdom (U.K.). St. Thomas, the westernmost of the Virgin Islands, lies 70 km east of

Puerto Rico’s eastern shore. St. Croix, largest of the Virgin

Islands (207 square kilometer [km2]), is located 88 km southeast of Puerto Rico. St. Thomas (83 km2) and St. John (49

km2) both lie north of St. Croix and to the east of Puerto Rico.

Puerto Rico, the smaller islands of Vieques, Culebra and

Mona, and the Virgin Islands (except St. Croix) are surrounded by an insular shelf in which water depth is less than

200 meters (m). In Puerto Rico, the insular shelf is narrow,

varying in width from less than 2 km northwest of the island

to greater than 25 km southwest of the island. A precipitous

shelf break borders the island on three sides at depths less

than 200 m. Vieques, Culebra, and the Virgin Islands lie in

the southern half of the broad (40 to 50 km) insular shelf that

extends eastward from Puerto Rico for about 140 km to the

easternmost island of Anegada. St. Croix, located on the St.

Croix ridge, is separated from the Puerto Rico-Virgin Island

platform by the Anegada Passage. A narrow shelf surrounds

the island of St. Croix.

PUERTO RICO

Puerto Rico’s varied topographic relief, a function of the

island’s complex and varied geology, can be separated into

several ground-water provinces. These ground-water provinces include upland (Interior ground-water province), northern (North Coast ground-water province), and southern

(South Coast ground-water province) and several minor

ground-water provinces (McGuinness, 1948) that conform

well with major geologic areas (figs. 2, 3). 

The mountains of the Cordillera Central and other highlands that form its eastern extensions, the Sierra de Cayey and

Sierra de Luquillo, are underlain mostly by volcaniclastic,

volcanic, and some plutonic rocks. These rocks have been

uplifted and erosionally dissected to form an asymmetric

mountain range in which the southern slopes dip more steeply

than the northern slopes. The mountains of the Cordillera

Central in the Interior ground-water province rise to more

than 1,300 m above sea level and constitute the principal

drainage divide. The drainage divide runs east-west, and lies

an average of 35 km south of the northern coast and 15 to 25

km north of the southern coast of Puerto Rico. Geologic

structure influences both topographic relief and drainage patterns in the central mountains. Drainage patterns in the central mountains and foothills lie parallel to subparallel to

northeast and northwest fault and fracture patterns. Fractures

or faults underlie many valleys or form notches; some faultline scarps form prominent mountain and ridge fronts. 

Orographic relief of the Cordillera Central greatly influences Puerto Rico’s mildly tropical Caribbean climate. A

rain shadow in the leeward side of the island intercepts most

of the moisture carried by northeast trade winds. Windward

northern slopes receive 2,000 to 5,000 millimeter per year

[mm/yr] of precipitation as compared to the less than 1,000

mm/yr of precipitation on the leeward southern slopes

(Calvesbert, 1970). Precipitation on the southern side of the

island is, in places, less than 500 mm/yr.

Coastal plain deposits flank the northern (North Coast

ground-water province) and the southern (South Coast

ground-water province) sides of the island. The western one-
4 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

Tropic of Cancer

Florida

Cuba

Hispaniola

Jamaica

Puerto Rico

B a h a m a s

G R E A T E R

A N T I L L E S

L E S S E R

A

N T I L L E S

A T L A N T I C O C E A N

C A R I B B E A N S E A

S O U T H A M E R I C A

C

E

N

T R A L A M E R I C A

MAP AREA

Puerto Rico

C A R I B B E A N S E A

A T L A N T I C O C E A N

San Juan

Ponce

Mayagüez Isla de Vieques

Isla de Culebra

Isla Desecheo

Isla Mona

St. Thomas

St. John

Tortola

Virgin Gorda

St. Croix

Anegada

V I R G I N I S L A N D S

UNITED STATES UNITED KINGDOM

Anegada Passage

Muertos Shelf 200

200

10°

15°

20°

25°

85° 80° 75° 70° 65° 60°

19 ̊00'

18 ̊30'

18 ̊00'

17 ̊30'

67 ̊30' 67 ̊00' 66 ̊30' 66 ̊00' 65 ̊30' 65 ̊00' 64 ̊30'

0 400 800 KILOMETERS

0 200 400 MILES

0 20 40 KILOMETERS

0 10 20 MILES

FIGURE 1.—Location of Puerto Rico and the U.S. Virgin Islands.

EXPLANATION

Puerto Rico–Virgin Island Platform—Delimited

 by 200 meter bathymetric contour
INTRODUCTION 5

third of the South Coast ground-water province contains an

upland area of low-ridge foothills underlain by carbonate and

clastic rocks of late Tertiary age. The eastern two-thirds of

the South Coast ground-water province is underlain by fandelta deposits of Quaternary age that form a low-lying plain.

The North Coast ground-water province, characterized by

scenic karst topography, is underlain by limestone and minor

clastic deposits of Oligocene to Holocene age. A series of

discontinuous low-lying alluviated valleys occur along the

eastern (East Coast ground-water province) and western

(West Coast ground-water province) sides of the island and in

the eastern part of the interior (Interior ground-water province). The physiography of Puerto Rico’s semiarid South

Coast ground-water province is highlighted by a steep-faced

mountain front and thin soil cover on the lower mountains

and foothill areas. Coastal plain foothills in the western half

of the South Coast ground-water province are incised by alluvial-filled stream valleys along the Ríos Tallaboa, Macaná,

Guayanilla, Yauco, and Loco. In the Bahía de Guayanilla

area, the alluvial valleys of Macaná-Guayanilla-Yauco coalesce to form a low-lying plain that is about 8-km-wide (pl.

2A). A fan-delta plain, formed by several large and small

fans, extends 70 km eastward from Ponce to Patillas and varies from 3 to 8 km wide (pl. 1A). The southern plain slopes

gently coastward (1 to 6 meters per kilometers [m/km]) near

the coast but at a steeper gradient (4 to 8 m/km) near the apex

of the fans. A narrow coastal zone separates the plain from

the Caribbean Sea. Bedrock hills, or cerros, protrude the

plain in several localities. Mountain streams that enter the

southern plain are comparatively straight and have incised

deep valleys in the lower mountains and foothills. Recent

erosion by the trunk streams where they enter the plain has

resulted in different degrees of fanhead incisement.

Trunk streams that traverse the fan-delta plain display a

straight to slightly sinuous channel morphology. For example, the stream that drains the Salinas fan, the Río Nigua at

Salinas, is relatively straight and extends southwest from the

fan apex. A similar channel morphology is present along the

Río Coamo that drains the Coamo fan delta. Several of the

fans that lie to the west are drained by two streams, which are

Río Culebrinas

Río Grande de Añasco

Río Yagüez

Río Guanajibo

Lajas Valley

Tertiary uplands

Cayey

Caguas-Juncos Fajardo

NaguaboYabucoa Humacao

Maunabo

SOUTHERN ALLUVIAL VALLEYS SOUTHERN FAN-DELTA PLAIN

S O U T H C O A S T P R O V I N C E

N O R T H C O A S T P R O V I N C E

I N T E R I O R P R O V I N C E

WEST COAST PROVINCE

EAST COAST PROVINCE

67°00' 66°30' 66°00"

18°30'

18°00'

Base modified from U.S. Geological Survey digital data

ATLANTIC OCEAN

CARIBBEAN SEA

Isla

Mona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

0 20 40 KILOMETERS

0 10 20 MILES

EXPLANATION

Principal rock types

 Limestone

 Gravel, sand, silt, and clay

 Volcaniclastic, volcanic, and

 igneous intrusive rocks

FIGURE 2.—Ground-water provinces of Puerto Rico (modified from McGuinness, 1948).
6 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

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Qa Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qa

































y

Qb

Qb

Qs

Qu

Qu Qu

Qu

Qu

Qu

Tq

Tq

Ta Ta Ta Ta Ta Tal

Tal Tal Tal

Tc Tc

Tc Tc

Tm Tm

Tcg

Tlf Tlf Ts

Ts

Ts

TKv

TKv

Tms

Tg

Tp

Tp

Tp

Tp

Tp Tp

TKm

Tj Tj

Tj

Tj

TKp

TKp

TKp

TKp

Qb Qs

Qs

Qb

Qb

Qs

Qs

Qs

Qs

Qs

Qaf

Qu Ta

Tr Ts

Tlf

Ts

TKv

TKv

Tqa

Tms

TKp

TKp

Qs

Qb

Qb

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0 10 20 KILOMETERS

0 5 10 MILES

Base modified from U.S. Geological survey digital data

67°00'

18°30'

66°30' 66°00'

18°00'

FIGURE 3.—Geology of Puerto Rico (modified from Briggs and Ackers, 1965; Monroe, 1980; and R.D. Krushensky, written commun., 1993).

EXPLANATION

Quaternary deposits Alluvium Landslide deposits Beach deposits Swamp, marsh deposits Artificial fill Undifferentiated surficial depositsPliocene and Miocene rocks Quebradillas Limestone Ponce Limestone

















yy

Miocene rocks Aymamón Limestone Aguada (Los Puertos) Limestone Guanajibo Formation Cibao Formation Montebello Limestone Member Quebrada Arenas Limestone Member— Includes Miranda Sand Member Río Indio Limestone Member—Includes Almirante Sur Lentil Guajataca Member

Miocene and Oligocene rocks Mucarabones Sand Juana Díaz Formation Lares Formation San Sebastián FormationEocene, Paleocene, and Cretaceous rocks Volcanic and sedementary rocks Plutonic rocks—Mostly quartz diorite and granodiorite Metamorphic (serpentine), sedementary, and igneous rocksFault

Qa

Ql

Qb

Qs

Qaf

Qu

TqTp

Ta

Tal

Tg

Tc

Tm

Tqa

Tr

Tcg

Tms

Tj

Tlf

Ts

TKv

TKp

TKm
INTRODUCTION 7

fairly straight where they enter the fan-delta plain, but

develop a sinuous to meandering pattern as they extend coastward. Prior to being channelized for flood control, the confining channels of these western streams became

distributaries and were lost in the distal coastal lowland areas

allowing water to spread out as sheetflow.

Carbonate rocks of the North Coast ground-water province form the caprock of intensely karstified, northward-dipping cuesta slopes. Northern cueseta slopes conform to the

gentle northward dip of beds and have been greatly modified

by processes of karstification. Differential erosion of these

carbonates has created a belted series of steep, south-facing

scarps that range in height from 10 to 400 m and are scalloped or indented by blind valleys. The Lares cuesta scarp

(Monroe, 1976, p. 19), the southernmost of the three eastwest trending cuesta scarps, is nearly continuous and mappable for 100 km. Karst features are widely- and well-distributed within the North Coast ground-water province and

documented in reports by Lehmann (1954), Gurnee (1967,

1972), Thrailkill (1967), Birot and others (1968), Blume

(1968, 1970), Moussa (1969), Miotke (1973), Monroe (1976,

1980), Guisti (1978), and Troester and White (1984). Dissolution along a poorly defined fracture system and mass transport have contributed to the wide variety of karst relief

features. Positive relief structures of the North Coast groundwater province include cone and tower karst. Negative relief

structures include fracture-controlled zanjones, closed sinkhole depressions or dolines, and dry valleys. Cave collapse

locally reveals an extensive, subterranean river system. With

the exception of the Río Tanamá caves, Río Camuy cave, the

Quebrada Infierno cave system, and the Río Encantado cave

system (near Florida), however, most Puerto Rican caves

have not been surveyed. 

East Coast and West Coast ground-water provinces of

Puerto Rico (figs. 2, 3) are indented by several alluvial valleys. The low-lying Humacao and Naguabo Valleys of the

East Coast ground-water province are drained by several

streams whose headwaters lie in the foothills and mountains

of the Sierra de Cayey or Sierra de Luquillo. Both valleys are

surrounded by steep-faced slopes and separated by a ridge

that rises more than 90 m above the valley floor. A third alluvium-filled valley, the Yabucoa Valley, lies south of the Naguabo-Humacao area. Alluvium-filled valleys of the West Coast

ground-water province include those drained by the Río Guanajibo, Río Grande de Añasco, Río Culebrinas, Río Yagüez,

and Lajas Valley. Surrounded by dissected hills and ridges,

the valleys are low-lying and narrow with the alluvial surface

of the valley floor rising from the coast to no more than 60 m

above sea level. The Lajas Valley (Valle de Lajas) is relatively narrow (1.5 to 9.5 km) and low-lying and extends

29 km in a nearly east-west direction. Foothills surround the

valley on the northern and southern sides, rising nearly 300 m

above sea level. Alluvium represents the principal aquifer in

the Interior ground-water province and is located in the eastcentral part of the island. Alluvial deposits underlie a 91 km2

area in this valley.

PUERTO RICO’S OFFSHORE ISLANDS

The principal offshore islands of Puerto Rico include Isla

de Vieques and Isla de Culebra that lies to the east, Isla Mona

and Isla Desecheo that are to the west. A generalized geologic description follows.

Isla de Vieques

Vieques, largest of Puerto Rico’s adjacent islands, is

located 14 km east-southeast of Puerto Rico’s eastern coast.

Vieques is approximately 23 km long, but no more than 5 km

wide. The island’s central ridge trends east-northeast with

the altitude of its summit nearly 300 m above sea level at

Monte Pirata (fig. 4). The central ridge is asymmetric, with

northern slopes steeper than the south. The island’s land surface is hilly, but includes two alluvial valleys, the Esperanza

and Resolución alluvial valleys (fig. 4). The island of Vieques

is underlain by highly weathered, plutonic rocks (dominated

by granodiorite and quartz similar to the San Lorenzo

Batholith in eastern Puerto Rico) and marine volcaniclastic

rocks of Cretaceous age (Learned and others, 1973; U.S.

Department of Defense, 1980). These older rocks are overlain by limestone of late Tertiary age in several localities,

most notably along the northern coastline near Desembarcadero Mosquito, Punta Salinas on the eastern peninsula, and in

isolated areas between Esperanza and Ensenada Honda on the

southern coast. Alluvial deposits, consisting of sand, silt, and

clay lie within the narrow (0.5 to 1 km wide by 5 to 6 km

long) Esperanza alluvial valley on the south-central coast of

Vieques. Alluvial fan deposits fringe the north side of Monte

Pirata and coalesce to form Resolución alluvial valley located

on the northwestern coast (McGuinness, 1945). Reported

thickness of the alluvial section in Esperanza alluvial valley

near Ensenada Sombe is 27 m. Geophysical studies in the

Resolución alluvial valley area indicate alluvial deposits are

less than 30 m (Torres-Gonzaléz, 1989).

Isla de Culebra

The island of Culebra is approximately 27 km east of

Puerto Rico and 9 km north of Vieques (fig. 1); the island

encompasses an area of 28 km2. The island rises to a maximum altitude of only 195 m with ridge crests that average

from 75 to 175 m in altitude. Culebra is underlain by volcanic, volcaniclastic, and dioritic rocks of Late Cretaceous age

(Donnelly, 1959). Some minor alluvial deposits of silt, clay

and some sand and gravel are located in major stream valleys

near the coast (Jordan and Gilbert, 1976). Bedrock is

exposed at land surface in 50 to 70 percent of the island and a
8 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS Laguna PlayaGrande Laguna Kiana Laguna Algodones Laguna Monte Largo Laguna Puerto Diablo Laguna Anones CARIBBEAN SEA Bahía Salina del Sur Ensenada Sombe Ensenada Honda Laguna Yanuel Punta Arenas Punta Vaca Punta Mulas Punta Conejo Punta Salinas Punta Este Isla Chive Cayo de Afuera Cayo deTierra Punta Boca Quebrada Desembarcádero Mosquito Esperanza al ul v ai l val el y Resolución alluvial valley Mount Pirata Playa Grande ? ? ? ? Qa Qa Qa Qb Qb Qb Qb Qs Qs Qs Qs Qb Qb Tld Tld Tld Qb Mosquito Isabel Segunda Esperanza ATLANTIC OCEAN

CARIBBEAN SEA

IslaMona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

Base modified from U.S. Geological Survey,Humacao, 1:100,000, unpublished 0 10 KILOMETERS 5

0 5 MILES

65 ̊30' 65 ̊22'30" 65 ̊15'

18 ̊25'18 ̊15'18 ̊05'

EXPLANATION

Quaternary deposits Alluvium Beach deposits Swamp depositsPliocene rocks Limestone and dolomite

Miocene and Oligocene rocks Intrusive rocks—Quartz diorite, diorite, granodiorite, apilite, and porphyryEocene, Paleocene, and Cretaceous rocks Andesite, tuff, and breccia, and minor limestoneFault—Queried where uncertain ?

Qa

Qb

Qs

Tld

TKp

TKs

TKp

TKp

TKp

TKp TKp

TKs

TKs

TKs

TKs

TKs

TKs

FIGURE 4.—Geology of Isla de Vieques (modified from Briggs and Ackers, 1965; Learned and others, 1973).
INTRODUCTION 9

thin granulated soil (0.3 to 0.6 m-thick) soil occurs in pockets

between rocky areas (Jordan and Gilbert, 1976).

Isla Desecheo

Isla Desecheo is a small island encompassing 1.2 km2

and located approximately 21 km west of northwestern

Puerto Rico and within the northeastern part of the Mona Passage (fig. 1). Ridge crests are only 100 to 200 (m) in height.

Isla Desecheo is largely underlain by folded and faulted volcaniclastic sandstone, mudstone and breccia of Eocene age.

Marine terrace deposits of calcite-cemented sand and gravel

(1 m-thick) are discontinuously exposed along the coast of

the island.

Isla Mona 

Isla Mona encompasses an area of approximately 55

km2 and is located in the Mona Passage and lies approximately 72 km due west of Puerto Rico’s western coastline,

about midway between Puerto Rico and the Dominican

Republic. Isla Mona is underlain by carbonate rocks of

Miocene age (fig. 1) and characterized by a relatively flat,

gently sloping upland surface or tableland bordered by 45 to

85 m-tall, vertical sea cliffs. A narrow coastal lowland of

beach and raised, relict reef deposits of Quaternary age are

located along the western, southwestern and southeastern

margins. The stratigraphic sequence of Isla Mona can be separated into two formational units that include in ascending

order, the Isla Mona Dolomite and the Lirio Limestone of

Miocene age (Briggs and Seiders, 1972). The thick-bedded,

dense, finely crystalline dolomite and two 5 m-thick limestone zones compose the Isla Mona Dolomite. The maximum

exposed thickness of the formation is approximately 80 m.

The Lirio Limestone is approximately 10 m thick and forms

the cap of the island. The Lirio Limestone is composed of

thick to very thick bedded, fine-grained limestone; it is well

indurated possibly due to surficial solution and recrystallization. Solution development of caves is mostly limited to the

Lirio Limestone, with most located along the periphery of the

island. Many caves contain flowstone, stalactites, and stalagmites, collapse rock rubble and some phosphorite deposits

produced by alteration of bat guano by percolating groundwater. Lirio Limestone caves roofed near the island’s upland

surface often have openings that lead to the bottom of tableland sinkholes.

U.S. VIRGIN ISLANDS

The 212 km2 island of St. Croix is the largest of the U.S.

Virgin Islands and lies 88 km southeast of Puerto Rico. St.

Croix is approximately 34 km long with an average width less

than 9.6 km. St. Croix is separated into two physiographic

areas: (1) incised and rugged hills and low mountain areas on

the northern (Northside Range) and eastern sides (East End

Range) of the island and (2) low-lying to gently rolling terrain that forms a central plain (fig. 5). The Northside Range,

whose northern slopes steeply dip to the sea, forms the northern spine of the island, rising to more than 300 m above sea

level. The East End Range is lower and somewhat less rugged. Summit altitudes do not exceed 244 m and two broad

low areas separate the East End Range into the eastern and

western parts. A nearly flat to gently rolling land surface typifies the southwestern part of St. Croix’s central plain, and is

a stark contrast to the hilly and dissected carbonate highland

area that characterizes the northeastern part of the central

plain. St. Croix’s two upland areas, the Northside and East

End Ranges, are underlain by consolidated volcanogenic sedimentary strata that include turbidites, mudstones, reworked

sandstones, debris flow deposits, minor chert, and Maestrichtian plutons and dikes (fig. 5). These rocks are folded and

faulted within an imbricated nappe structure that formed

within the south- to southwest-facing Late Cretaceous islandarc accretion complex (Speed and Joyce, 1989). The central

plain overlies a southward-facing demi-graben structure that

has been infilled with late Tertiary rocks that include foraminiferal-rich clay or mud of Miocene to Oligocene(?) age; shelf

and shelf-slope hemipelagic limestone and interbedded turbidite deposits of Miocene age; and shallow-water limestone

of Pliocene age. Clayey to gravelly alluvium, alluvial fan,

slope wash, and debris flow deposits of Quaternary age blanket these strata along the central plain’s northern margin.

Incisement by River Gut, Bethelem Gut, and the Salt River

during the late Pleistocene lowstand resulted in the overdeepening of stream valleys that traverse the central plain. Stream

valleys infilled during the Holocene contain clayey to gravelly alluvium ranging from 10 to 25 m thick.

St. Thomas (83 km2) is the second largest island of the

U.S. Virgin Islands and lies 72 km east of Puerto Rico. St.

Thomas is 22.5 km long and 3 to 5 km wide. The spine of the

island is formed by a 245- to 366-m-high central ridge that

slopes steeply on its northern and southern slopes, and is dissected by streams that flow intermittently. St. John lies

immediately east of St. Thomas, and is the smallest of the

three principal U.S. Virgin Islands (47 km2). St. John also is

traversed by a east-trending central ridge 200 to 350 m high

with steep northern slopes and prominent spur ridges on the

southern side of the island and dissected by intermittent

streams. 

Rock units that underlie the highland ridges of St. Thomas and St. John consist of kerophyric lava, flow breccias and

tuffs, spilitic lava, andesitic breccia and tuff, and minor limestone beds, all of Cretaceous age and intruded by Cretaceous

and early Tertiary dikes and plugs (fig. 6). These rocks have

been uplifted and faulted forming a northward-dipping

homocline (Donnelly, 1966). Unconsolidated alluvial and
10 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS Christiansted Frederiksted CARIBBEAN SEA Krause Lagoon Salt River Bethlehem GutRiver Gut NORTHSIDE RANGE CENTRAL LIMESTONE PLAIN EAST END RANGE Qab Qab Qab Qab Qr Qr Qr Qr Qr Qr Tbf Tbf Tmb Tlr Tlr Tlr Kg Kd Km Km Km Km KmATLANTIC OCEAN CARIBBEAN SEA Isla Mona Isla de Culebra Isla de Vieques St. ThomasSt. John St. Croix Puerto Rico Base modified from U.S. Geological Survey, Fredriksted, Chistiansted, East Point, 1:24,000, 1958 0 10 KILOMETERS 5 0 5 MILES 64 ̊48' 64 ̊55'30" 64 ̊40'30" 64 ̊30' 17°48' 17 ̊40'30" EXPLANATION Quaternary deposits Alluvium, minor beach deposits Reef Pliocene rocks

 Blessing FormationMiocene rocks Kingshill Limestone Mannings Bay Member La Reine Member

Cretaceous rocks Igneous intrusive rocks Gabbro Diorite Mt. Eagle GroupContact—Dashed where inferred

Qab

Qr

Tbf

Tmb

Tlr

Kg

Kd

Km

FIGURE 5.—Geology of St. Croix, U.S. Virgin Islands (modified from Whetten, 1966; Gill, 1989).
INTRODUCTION 11

UNITED KINGDOM UNITED STATES

Tortola

CARIBBEAN SEA

ATLANTIC OCEAN

D

U U

D

D

U

D

U

D

U

St. Thomas

St. John Qab

Qab

Qab

Qab

Qab

Qab

Tdp

Kd

Kd

Kd

Kh

Kt

Kt Kt

Kt

Ko

Ko Kl

Kl

Kl

Kl

Kl

Kdp

Kdp

Kd

Kw

Kw Kw

Kw Kw

Qab

Kt

Ko Kl

Qab

Kdp

Tdp

DU

Charlotte Amalie

ATLANTIC OCEAN

CARIBBEAN SEA

IslaMona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

Base modified from U.S. Geological Survey,Western St. Thomas, Eastern St. Thomas,Western St. John, Eastern St. John, 1:24,000 0 10 KILOMETERS 5

0 5 MILES

65 ̊00' 64 ̊52'30" 65 ̊45'

18 ̊22'30"

18 ̊15'

EXPLANATION

Quaternary deposits Alluvium, minor beach depositsEarly Tertiary to Cretaceous rocks Dikes and plugsCretaceous rocks Dioritic rocks Hans Lollik Formation

 Tutu Formation Outer Brass Limestone Lousenhoj Formation Dikes and plugs Water Island FormationFault—Arrows show relative horizontal movement, dashed where inferred. U, upthrown side; D downthrown side

Qab

Tdp

KdKh

Kt

Ko

Kl

Kdp

Kw

UD

FIGURE 6.—Geology of St. Thomas and St. John, U.S. Virgin Islands (modified from Donnelly, 1966).
12 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

coastal deposits occur within several small stream valleys and

within shoreline reentrants.

The St. Thomas-St. John homocline is cut by two strikeslip fault sets that strike in northwest and northeast directions.

The northeast fault set parallel the orientation of the Anegada

trough that lies to the south of these two islands. A third fault

set strikes in a north-south direction. The striking parallelism

of major stream valleys and their drainage systems with welldefined joint sets and major faults is strongly suggestive of

structurally-controlled selective erosion and weathering of

the bedrock (Jordan and Cosner, 1973).

PREVIOUS INVESTIGATIONS

PUERTO RICO 

Some of the earliest geologic investigations in Puerto

Rico were conducted by the New York Academy of Sciences;

the findings of the Academy were published in a series of

reports between 1919 and 1933 (Berkey, 1915, 1919;

Semmes, 1919; Hodge, 1920; Mitchell, 1922; Hubbard, 1923;

Fettke, 1924; and Meyerhoff, 1931, 1933). In the 1950’s, the

USGS initiated a mapping program of Puerto Rico and has

subsequently published fifty-one 1:20,000 scale geologic

quadrangle maps (1960-84) showing outcropping rock units

and geologic structure. A geologic map of Puerto Rico, its

offshore islands, and the Virgin Islands was published by

Briggs and Ackers (1965). An updated geologic map of

Puerto Rico is in preparation (R.D. Krushensky, oral commun., 1993).

Reports that describe the geology of Puerto Rico’s South

Coast ground-water province include Pessagno (1960, 1963),

McClymonds and Ward (1966), Moussa and Seiglie (1970,

1975), Glover (1971), Monroe (1973, 1980), Krushensky and

Monroe (1975, 1978, 1979), Frost and others (1983), and

Erikson and others (1990). Geologic investigations of Puerto

Rico’s North Coast ground-water province are published in

reports by Berkey (1915, 1919), Hubbard (1923), Zapp and

others (1948), Bermúdez and Seiglie (1969), Monroe (1973,

1980), Seiglie and Moussa (1974, 1975, 1984), Moussa and

Seiglie (1975), Meyerhoff and others (1983), and Moussa and

others (1987). The geology of the East Coast, West Coast,

and Interior ground-water provinces has been documented in

U.S. Geological Survey geologic quadrangle maps (1960-84)

and in reports by Mattson (1960, 1973). The thickness and

character of alluvial deposits found in the East Coast groundwater province is described by Robison and Anders (1973),

Adolphson and others (1977), and Graves (1989). The physical character of alluvial deposits in the West Coast groundwater province and Lajas Valley include work by Anderson

(1977), Colón-Dieppa and Quiñones-Marquéz (1985), Díaz

and Jordan (1987), and Graves (1991). Puig and Rodríguez

(1993) provide data regarding the thickness of alluvial deposits in the Interior ground-water province’s Caguas-Juncos

Valley.

Many of the early reports (1946-76) as well as subsequent

reports (1976-94) that describe the hydrogeology and hydrology of Puerto Rico’s North and South Coast ground-water

provinces focus principally on hydrologic conditions within

local areas and did not examine conditions within the entire

ground-water province. Bennett’s (1976) analog model study

of the South Coast ground-water province is a notable exception and the first to study an entire hydrogeologic province.

Guisti and Bennett (1976), Guisti (1978), and Heisel and others (1983) followed with regional hydrogeologic and hydrologic studies of the North Coast ground-water province.

Ward and others (1990), Rodríguez-Martinez (1990), Renken

and Goméz-Goméz (1994), and Torres-González and others

(1996) have updated the knowledge of the geology, hydrogeology, and hydrology of the north coast and information presented earlier is expanded herein. Goméz-Goméz and Heisel

(1980) provide a summary appraisal of the entire U.S. Caribbean region.

Reports that describe the local hydrology and hydrogeology of the South Coast ground-water province include:

McClymonds and Ward (1966), McClymonds (1967, 1972),

Crooks and others (1968), Guisti (1971), Grossman and others (1972), McClymonds and Díaz (1972a, b), Heisel and

Gonzáles (1979), Quiñones-Aponte (1986b, 1990), Dacosta

and Gómez-Gómez (1987), Quiñones-Aponte and GómezGómez (1987), Román-Mas and Ramos-Ginés (1987),

Torres-Gonzáles and Gómez-Gómez (1987), Gómez-Gómez

(1990), Renken, Díaz, and others (1990), Renken, GómezGómez, and others (1990), Rodríguez-del-Río and QuiñonesAponte (1990), and Quiñones-Aponte and others (1996).

Reports that address local hydrology and hydro-geology

within the North Coast ground-water province include Jordan

(1970, 1977), Bennett and Guisti (1972), Anderson (1976),

Gómez-Gómez (1984), Torres-González and Díaz (1984),

Zack and Class-Cacho (1984), Torres-González (1985),

Quiñones-Aponte (1986a), Gómez-Gómez and Torres-Sierra

(1988), Quiñones-Aponte and others (1989) and TorresGonzález and others (1996).

PUERTO RICO’S OFFSHORE ISLANDS

Few geologic reports of the island of Vieques have been

published since Meyerhoff’s (1927) work. A map showing

the geology of Isla de Vieques was first published as part of

the Briggs and Ackers (1965) map of Puerto Rico and adjacent islands. An updated map completed by Grove was published in a geochemical reconnaissance study of the island by

Learned and others (1973). Hydrologic reports include those

by McGuinnes (1945) and Torres-González (1989).

The geology of Isla de Culebra is described in a report by

Donnelly (1959) and as part of the Briggs and Ackers (1965)
INTRODUCTION 13

map of Puerto Rico and adjacent islands. Some modifications

to Donnelly’s work are presented in a report by the members

of the Culebra Study Group (Puerto Rico Environmental

Quality Board, 1971). The hydrology of the island is

described by Jordan and Gilbert (1976).

The geology of the island of Mona is described in reports

by Kaye (1959), Briggs and Seiders (1972), and Aaron

(1973). Isla Mona hydrology is presented in a report by Jordan (1973).

A geologic description of Isla Desecheo is presented by

Seiders, Briggs, and Glover (1972). There are no hydrologic

reports available for Isla Desecheo.

U.S. VIRGIN ISLANDS

Early work on the geology of St. Croix dates to the early

1800’s; examples include Maclure (1817) and Hovey (1839),

both of which are summarized in Cederstrom (1950). Geologic work continued into the early 20th century with contributions by Vaughan (1916,1923) and Kemp (1926) who

suggested names and ages for the sedimentary units, as well

as a bio-geographical relation between Puerto Rico and St.

Croix. Cederstrom’s (1950) report contains substantial information on the subsurface geology and ground-water conditions resulting from a federally sponsored drilling program.

Test wells drilled for this program in 1938 and 1939 are the

deepest on record in St. Croix, and much of our knowledge of

the subsurface of St. Croix comes from this information

(Cederstrom, 1950).

The first modern interpretation of the structure and sedimentology of Cretaceous age rocks on St. Croix is described

by Whetten (1961, 1966). More recent interpretations of the

Kingshill-Jealousy Basin were published by Frost and Bakos

(1977), Multer and others (1977), Gerhard and others (1978),

Lidz (1982), Gill and Hubbard (1986, 1987), Gill (1989), and

Gill and others (1989).

The biostratigraphy of the Tertiary age carbonate rocks on

St. Croix is presented in reports by Cushman (1946), Bold

(1970), Lidz (1982, 1984), Andreieff and others (1986), and

Gill (1989). Reports that deal specifically with ground-water

conditions and the hydrogeology of St. Croix include Cederstrom (1950), Hendrickson (1963), Jordan (1975), Robison

(1972), Black and others (1976), Geraghty and Miller (1983),

Gill and Hubbard (1986,1987), Torres-González and Rodriguéz del Río (1990) and Torres-González (1990).

The geology of St. Thomas and St. John is described in a

paper by Donnelly (1966). Hydrologic investigations include

those by Cosner (1972) and Jordan and Cosner (1973).

Graves and González (1988) constructed a map showing the

September 1987 potentiometric surface of the bedrock and

local alluvial deposits that make up the aquifer in eastern St.

Thomas.

METHOD OF INVESTIGATION

A regional hydrogeologic system, or aquifer system, can

be categorized as one of two types. The first type of regional

aquifer system is one that contains a body of water-bearing

strata having a wide areal distribution, an extensive set of

aquifers and confining units, and acts hydrologically as a single regional system. The second type of aquifer system is one

comprising independent aquifers that can be hydrologically

characterized such that common principles can be established

and serve as a prototype to similar hydrologic terrains (Bennett, 1979). The Caribbean Islands aquifer RASA investigation is considered to be this second type of regional system.

Hydrologic features and hydrogeologic conditions presented

herein could parallel other Caribbean islands or tropical

island regimes elsewhere.

The tectonic and lithostratigraphic nature of sedimentary

basins are studied by exploration and development geologists

in an effort to assess the mineral-resource potential of a particular area. Maps and cross sections showing plate reconstructions, regional geology, facies, stratigraphy, and

paleogeography all help to identify areas underlain by sedimentary strata in which geologic conditions and past depositional processes could prove favorable to the occurrence of a

specific mineral resource. Sedimentary basin analysis is a

useful investigative technique in that it helps identify the

important geologic processes, outlines factors that control the

occurrence of a mineral resource, and provides information

and knowledge that can be transferrable and applicable to the

study of other sedimentary basins (and aquifers or aquifer

systems). This report applies basin analysis techniques to a

regional hydrologic evaluation of U.S. Caribbean Island aquifers and aquifer systems. Depositional, tectonic, paleoclimatic, eustatic, and diagenetic factors are important geologic

processes critical to understanding the distribution, character,

and evolution of permeability and porosity within aquifers

and aquifer systems of the U.S. Caribbean Islands. A hydrogeologic framework for U.S. Caribbean Island aquifers and

aquifer systems is defined herein by use of cross sections,

structure contour, isopach, and lithofacies maps that illustrate

the spatial distribution, and physical attributes of the individual aquifers and confining units that serve as conduits or that

impede the movement of ground water within the regional

flow system. Geologic maps in this report are compared with

transmissivity and hydraulic conductivity maps to examine

and evaluate the geologic processes that influence permeability and transmissivity. The hydrologic and hydrogeologic

factors are considered within the context of a regional geologic framework of the northeastern Caribbean. 

The hydrogeologic framework described in this report

was defined using lithologic, paleontologic, geophysical, and

hydrologic data obtained from the files of the U.S. Geological

Survey, published reports, and from unpublished data of non-
14 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

Survey workers. Although borehole geophysical data were

collected during the course of the investigation, most records

require further analysis, particularly in terms of identifying

mappable marker horizons. Driller’s and lithologic sample

logs, in combination with hydrologic data, were used to map

the attitude of beds, their thickness, and to assess their physical and hydraulic character. The majority of these records are

contained within U.S. Geological Survey files.

The geologic and hydrogeologic framework of Puerto

Rico’s North Coast ground-water province is based largely on

core data collected from 17 deep test wells drilled to depths

that exceed 750 m. These deep test wells were drilled as part

of a cooperative Commonwealth of Puerto Rico-U.S. Geological Survey investigation to evaluate the resource potential

and to map the extent of an artesian limestone aquifer of

northern Puerto Rico. Cores were collected using a reverseair dual-tube drilling method that continuously collects core

of 7.5-cm diameter. Limestone core samples considered representative of a certain bed, or sequence of beds, were

selected, slabbed and thin sectioned for study at the University of New Orleans. Analysis included study of carbonate

rock texture, porosity, fossil content, bedding, and identification of other possible depositional and diagenetic features.

Detailed geologic and hydrologic descriptions of the cored

test holes are presented in Hartley (1989), Scharlach (1990),

Rodríguez-Martínez, Hartley and others (1991), RodríguezMartínez, Scharlach and others (1991,1992), Rodríguez-Martínez and Hartley (1994), and Rodríguez-Martínez and

Scharlach (1994).

As part of the CI-RASA investigation, five deep exploration test wells were drilled and continuously cored in separate

localities of the southern fan-delta plain. For the most part,

detailed study was limited to the clastic part of the sequence.

Core studies of the clastic sequence included study of grain

size, bed thickness, and other depositional features. Lithologic core data were supplemented with auger cuttings and

split-spoon samples collected from several shallow wells

drilled during the tenure of the investigation. Driller’s log

data and other well data from more than 500 wells were used

to map the thickness of alluvial deposits, map the configuration of the underlying bedrock surface and determine the percent of coarse-grained clastic detritus contained within the

stratigraphic section.

In St. Croix, fourteen test holes were drilled as part of a

subsurface exploration study and information derived from

these wells was first outlined in Gill and Hubbard (1986,

1987), Gill (1989), and Gill and others (1989). Cross sections, structure, and isopach maps presented herein were constructed using information derived from the field studies as

well as data presented by earlier workers. Split-spoon samples and cores from these test wells were supplemented with

other cutting samples and lithologic descriptions from other

test holes and water wells. Split-spoon and core samples

were analyzed by thin section analysis, X-ray diffraction, and

micropaleontological identification. Mud-rich, split-spoon

samples collected below the water table were usually unlithified, and contained nearly pristine tests of benthic and planktonic foraminifera. During the later phase of the RASA

project, more than 20 wells were drilled in St. Croix using the

dual-tube method described previously, and the data collected

provided some additional insight regarding the thickness or

attitude of various geologic units, particularly the thickness

of alluvium filling some of the incised paleochannels.

GEOLOGY OF THE SOUTH COAST 

GROUND-WATER PROVINCE OF PUERTO 

RICO

BY ROBERT A. RENKEN1

Puerto Rico’s South Coast ground-water province encompasses 596 km2 between Guánica and Patillas and flanks

much of the southern side of the island. The South Coast

ground-water province can be physiographically separated

into two distinct areas: a western area that extends eastward

from Guánica to Ponce characterized by low coastal plain

ridges and foothills and underlain by carbonate and clastic

strata of the Juana Díaz Formation and Ponce Limestone; and

an eastern area that is characterized by a low-lying, narrow

fan-delta plain underlain throughout most of its 70 km extent

from Ponce to Patillas by gravel, sand, and silt of Quaternary

age.

Most of the South Coast ground-water province is underlain by the South Coast Tertiary Basin, a structural and depositional basin that extends from Bahía Montalva (west of

Guánica) to Bahía de Rincon (Meyerhoff and others, 1983)

(fig. 7). The South Coast Tertiary Basin contains a coastal

plain wedge that thickens seaward and is infilled with onlapping shelf and reef tract limestone and minor clastic rocks

that range from Oligocene to Miocene age (pls. 1A, 2A). Outliers of these late Tertiary rocks occur as far west as Cabo

Rojo (Volckmann, 1984a) and to the east at Cerro Central

Aguirre (Glover, 1971). Oligocene to Miocene age strata are

exposed nearly continuously from Bahía de Montalva to

Juana Díaz. Equivalent rocks extend southward beneath the

Isla Caja de Muertos shelf (Trumbull and Garrison, 1973) and

southeastward where they underlie unconsolidated Quaternary sediments that make up the fan-delta plain (Glover,

1971, p. 75). The structural dip of Oligocene and Miocene

strata is southward at less than 30 ̊, except where normal

fault movement has locally reversed structural dip to the

north (Krushensky and Monroe, 1978).

1

 U.S. Geological Survey
GEOLOGY OF THE SOUTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 15

Early workers have variously characterized Quaternary

age deposits of southern Puerto Rico between Ponce and

Patillas as low alluvial flats (Lobeck, 1922), an alluvial

coastal plain (Hodge, 1920), alluvial fans (Lobeck, 1922;

Berryhill, 1960; McClymonds and Ward, 1966), alluvial lowlands or alluvial plain (Zapp and others, 1948), and Piedmont

alluvial plain formed by coalescing alluvial fans (Glover,

1971, p. 77). Friedman and Saunders (1978, p. 302) characterize clastic deposits of southern Puerto Rico as “modern

fans of the sea shore.” In a global analysis of the tectonic and

geologic settings of Holocene fan deltas, Wescott and

Ethridge (1980, p. 379) were the first workers to classify

Puerto Rico’s fans as fan deltas; they catalogued Puerto

Rico’s fans as gravel-fan types that occur within island-arc

collision zones. Although Wescott and Ethridge (1980)

describe them as examples, they did not provide detail regarding their physical character or geologic history, nor did they

conduct any field investigations (F.G. Ethridge, oral commun., 1989).

In the western part of the South Coast ground-water province, foothill and ridges are dissected by five large stream valleys of the Ríos Tallaboa, Macaná, Guayanilla, Yauco, and

Loco (Gúanica valley). Each valley is underlain by moderately thick deposits of alluvium. The Ríos Macaná, Guayanilla, and Yauco flow across an 8-km-wide alluvial plain

before entering the Caribbean Sea at Bahía de Guayanilla.

The surface of the alluvial plain rises 10 m above sea level at

its landward coastal alluvial plain edge and to an altitude of

40 m where it extends inland within the different alluvial valleys. In places, low-lying swamps and beaches separate the

coastal alluvial plain from the Caribbean Sea. The fanlike

morphology and the narrow coastal zone of beaches and

swamps associated with the Ríos Macaná and Tallaboa valleys are geomorphically similar but less extensive than the

fan-delta plain east of Ponce.

STRATIGRAPHY

Unconsolidated to lithified, clastic, and carbonate deposits of Tertiary and Quaternary age make up the principal

stratigraphic units of hydrologic interest within the South

Coast ground-water province. In ascending order, they are

the Juana Díaz Formation (Oligocene age), Ponce Limestone

(middle Miocene to Pliocene(?) age), and unnamed gravel,

sand, and silt deposits of Pleistocene to Holocene age.

Stratigraphic boundaries within the Oligocene to

Pliocene(?) sequence have been revised a number of times

since Berkey (1915) (fig. 8) separated the Tertiary sequence

into a lower clastic unit and an upper limestone unit. Later

revisions to the stratigraphic sequence were made in light of

subsequent studies of macrofaunal assemblages, recognition

of mappable lithofacies, identification of unconformities, correlation of planktonic foraminiferal assemblages, and revisions in the age range of these foraminifera assemblages.

Citing paleontologic evidence, Seiglie and Bermúdez (1969)

proposed that the top of the Juana Díaz Formation be shifted

upward to include limestone strata that were previously

defined by Zapp and others (1948) as the lower member of

the Ponce Limestone. Moussa and Seiglie (1970) later recognized five planktonic foraminiferal zones within the prePonce Limestone sequence and correlated them with worldwide assemblage zones. In addition, Seiglie and Bermúdez

(1969) and Moussa and Seiglie (1970, 1975) identified a

minor sequence of pelagic limestone beds of early Miocene

age that were exposed in several quarries between the Río

Tallaboa and Ponce; noting the occurrence of overlying and























Isla Desecheo

Isla Mona

Isla de Culebra

Isla de Vieques

Puerto Rico

SOUTH COAST TERTIARY BASIN

NORTH COAST TERTIARY BASIN

18 ̊30'

18 ̊00'

67 ̊30' 67 ̊00' 66 ̊00' 66 ̊30' 65 ̊30'

0 20 40 KILOMETERS

0 10 20 MILES

FIGURE 7.—Location of Tertiary sedimentary basins of Puerto Rico (modified from Meyerhoff and others, 1983).
16 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

underlying unconformities, they referenced these rocks separately as the Angola limestone beds, Río Tallaboa marlstone

beds, and Quarry No. 2 sandstone beds. Some of their work

served as the basis for Monroe’s (1980) stratigraphic revision

of the Tertiary section that is currently applied by the U.S.

Geological Survey. Monroe, however, included the Angola

limestone, Río Tallaboa marlstone, and Quarry No. 2 sandstone beds as part of the Juana Díaz Formation, in part, due to

the limited occurrence and exposure of these beds and the difficulty in reliably tracing them from outcrop (Monroe, 1980,

p. 66). Presumably, the early Miocene pelagic rocks were

part of a sequence that was largely removed by erosion prior

to deposition of the Ponce Limestone.

As a result of a detailed investigation of coral faunal communities contained within the Oligocene and early Miocene

section of southern Puerto Rico, Frost and others (1983)

refined correlation of shallow- and deep-water lithofacies

within the Juana Díaz Formation beyond that proposed by

Moussa and Seiglie (1970); they suggested that the name,

Juana Díaz Formation, be further restricted and that its usage

be limited to the dominately clastic part of the section.

Although no names were suggested, Frost and others (1983)

proposed that it would be more appropriate to apply separate

and “new” formation names to (a) reef/shelf tract and some

minor slope carbonate facies now considered part of the

Juana Díaz Formation and (b) the pelagic slope Angola limestone-Río Tallaboa chalk facies.

1 Berkey (1915) suggested the Arecibo Formation may be, in part, Eocene in age

2 Considered by Galloway and Hermingway (1914) and Zapp and others (1948) to be Middle Oligocene in age

3 Todd and Low (1976) suggested possibility of Miocene age Juana Díaz Formation rocks in deep test holes 2 CPR and 3 CPR

4 Hodges (1920) also described Santa Isabel Formation as a series

Alluvium Alluvium

TERTIARY QUATERNARY

PALEOGENE NEOGENE

OLIGOCENE MIOCENE

EARLY LATE EARLY MIDDLE LATE

Alluvial deposits

Berkey (1915) Hodge (1920) Hubbard (1920) Mitchell (1922)

Galloway and

Hemingway

(1941)

Zapp and others

(1948) Gordon (1961a)

Juana Díaz Marls

and Shales

Ponce Limestone

and Marls Guánica Coral Reefs

Arecibo

Formation

Ponce Formation (text)

Juana Díaz Formation

(chart) Ponce Formation (chart)

Ponce Limestone

Ponce Limestone

Ponce Limestone

Not described

Not described

San Juan

Formation

San Juan Formation

Undifferentiated

HOLOCENE

PLEISTOCENE

PLIOCENE

Upper

Member Lower Member

Lower Upper

Guánica beds

Upper Ponce

Limestone

Lower Ponce

Limestone

Juana Díaz Shale

Arecibo Formation1 (Younger Series)

Santa Isabel

Formation4

Juana Díaz2

Formation

Juana Díaz2

Formation

Juana Díaz

Formation

FIGURE 8.—Correlation chart showing stratigraphic terminology in Puerto Rico's South Coast ground-water province.
GEOLOGY OF THE SOUTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 17

Glover (1971, p. 75), Todd and Low (1976, p. 2), and

Meyerhoff and others (1983) tentatively extended formational

contacts of the Juana Díaz Formation and Ponce Limestone to

buried strata that underlie the fan-delta plain. Sparse well

control and limited paleontologic data have impeded further

division of the subsurface stratigraphic section. 

Unconsolidated to poorly lithified boulder- to silt-size

detritus of Pleistocene to Holocene age that forms the fandelta plain east of Ponce, or infills incised valleys that lie to

their north, or west, is not formally named (pls. 1A, 2A).

Hodge (1920) referred these and other alluvial deposits,

including alluvium of the upland Cayey valley, to his Santa

Isabel Formation. The Santa Isabel terminology was never

widely applied, however, even among Hodges’ colleagues

(Mitchell, 1922).

Current USGS usage separates the late Tertiary sequence

into a lower clastic-carbonate unit of Oligocene to middle

Miocene age (Juana Díaz Formation) and overlying limestone

unit of Miocene age (Ponce Limestone). For purposes of this

report, this author separates the Oligocene-Holocene

sequence into (1) the lower clastic-carbonate Juana Díaz Formation of Oligocene age; (2) an unnamed pelagic carbonate

rock sequence of early Miocene age equivalent to the Angola

limestone, Río Tallaboa marlstone, and Quarry No. 2 sandstone beds of Seiglie and Bermúdez (1969) and Moussa and

Seiglie (1970, 1975), and the upper “new” formation of Frost

and others, (1983); (3) the Ponce limestone (carbonate and

clastic facies) of middle Miocene to early Pliocene age; and

(4) fan delta and alluvial deposits of Quaternary age (fig. 8).

The geologic ages assigned to the Tertiary sequence are based

strictly on foraminiferal assemblage zones reported by

Moussa and Seiglie (1970, 1975); however, the age of these

zones has been refined by Haq and others (1988) and is the

zonation used in this report.

TERTIARY QUATERNARY

PALEOGENE NEOGENE

OLIGOCENE MIOCENE

EARLY LATE EARLY MIDDLE LATE

HOLOCENE

PLEISTOCENE

PLIOCENE

Bold (1969)

Seiglie and

Moussa (1974)

Moussa and

Seiglie

(1970, 1975)

Todd and

Low (1976)

(subsurface) Monroe (1980)

Frost and others

(1983) This report

Alluvial

deposits

Alluvial

deposits

Limestone

Not described

Not described Not described Not described

"Angola"

sand

Angola

Beds

Angola

Beds

Marlstone

Rio Tallaboa

Marlstone

Marlstone and

Sandstone

Quarry

No.2 Sandstone

Coral and

Lepidocyclina

reefs

Las Delicias

Marlstones

Sabana Llana

Clastic facies (subsurface)

Fan delta and

alluvial deposits

Ponce

Limestone

Ponce

Limestone

Ponce

Limestone

Ponce

Limestone

Ponce

Limestone

Ponce

Limestone

? ?

Unnamed

Pelagic

Deposits

"New

Formation"

"New Formation"

? ?

Juana Díaz

Formation

Juana Díaz

Formation

Juana Díaz

Formation

Juana Díaz

Formation

Juana Díaz

Formation

Juana Díaz

Formation

Juana Díaz

Formation

Coral

and

Lepidocyclina

reefs

FIGURE 8.—Continued. Correlation chart showing stratigraphic terminology in Puerto Rico's South Coast ground-water province.
18 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

JUANA DÍAZ FORMATION

Except where covered by alluvium in major river valleys,

the Juana Díaz Formation is exposed almost continuously

from Bahía de Guánica to the city of Ponce. Large outliers of

the Juana Díaz Formation crop out northeast and east of

Ponce. Small outliers occur at Cerro de los Muertos near

Santa Isabel and have been reported at Cerro Central Aguirre

near Salinas (Berryhill, 1960) (pl. 1A).

The basal part of the Juana Díaz Formation consists of a

basal clastic mudstone or facies equivalents that consist of

clastic conglomerate of boulders, cobble, sand, and mudstone

of possible alluvial fan or fan-delta origin (fig. 9). Deposition

of the Juana Díaz Formation’s conglomeratic beds probably

occurred in response to uplift and increased erosion in highland areas during a comparatively low base level. Northeast

and east of Ponce, basal Juana Díaz beds exhibit horizontal to

parallel stratification and some beds are reported to locally

contain silicified wood and fossil leaves. Some mudstone

beds are burrowed and contain interbeds of sandstone and

claystone that have marine fauna. Reef faunal assemblages

occur throughout the finer clastic facies of the Juana Díaz

Formation (Frost and others, 1983, p. 13). An angular unconformity separates the lower part of the Juana Díaz Formation

from older, underlying Cretaceous and Eocene bedrock units

and records pre-late Oligocene emergence and erosion in

southern Puerto Rico. North of Ponce, basal conglomeratic

beds of the Juana Díaz Formation are horizontally juxtaposed

with Cretaceous and Eocene strata by strike-slip and normal

faults (Krushensky and Monroe, 1975). In the western part of

the basin, nonmarine and clastic shelf rocks of the Juana Díaz

Formation grade upward (coastward) to a limestone sequence

deposited within carbonate shelf (reef, forereef, deep forereef) and island-slope environments (the lower “new formation” of Frost and others, 1983). The transition in facies from

terrigenous to carbonate slope deposition in the north-central

part of the basin is rapid and lacks the reef facies seen to the

west (Frost and others, 1983 p. 13-15) (fig. 9).

Limestone of the Juana Díaz Formation contains a rich

and diverse fauna of coral, coralline algae, large benthic foraminifera, and mollusks that record alternating periods of reef

growth and destruction. These phases of growth and destruction have been attributed to a combination of factors that

include tectonic movement, upwelling of colder oceanic

waters, and eustatic changes in sea level (Frost and others,

1983). The reef facies extends eastward from Guánica to the

Río Tallaboa; farther eastward, this sequence grades to forereef and island slope deposits. Carbonate and clastic rocks of

Oligocene age equivalent to the Juana Díaz Formation lie buried in the deep subsurface between Ponce and Santa Isabel

(Glover, 1971). Here, shallow-water benthic foraminiferal

assemblages contrast with underlying, deep-water benthic

and planktonic faunal assemblages, highlighting the contact

that separates sedimentary strata of the Ponce Limestone

from the Juana Díaz Formation (Todd and Low, 1976). The

Juana Díaz Formation here is reported to consist of silty shale

with limestone, sandstone, shale and conglomerate (Glover,

1971, p. 76), and marly quartz conglomerate (Meyerhoff and

others, 1983, fig. 18). The reported occurrence of quartz conglomerate beds within this buried section suggests a nearby

source of terrigenous material.

At outcrop, unconformable relations mark the upper and

lower contacts of the Juana Díaz Formation. A

lowermost angular unconformity that separates the clastic

Juana Díaz Formation from the older early Tertiary and Cretaceous rocks has been described earlier. Paleontologic evidence (Moussa and Seiglie, 1970, 1975) and an apparent

erosion surface mark the unconformity separating the carbonates of the Juana Díaz Formation from overlying “pelagic carbonate deposits” of early Miocene age. This unconformity

suggests a second major period of emergence immediately

followed deposition of the Juana Díaz Formation.

Sparse well data, structural tilting, and complex fault relations make it difficult to assess formation thickness. The

Juana Díaz Formation may range from 200 to more than 700

m thick where it crops out or lies buried in the subsurface

between Guayanilla and Santa Isabel (Todd and Low, 1976;

Monroe, 1980) (fig. 9). The Juana Díaz Formation is probably thickest in the Río Tallaboa-Playa de Ponce area (400 to

more than 700 m thick) and thins to 200 m thick or less on the

eastern and western margin of the basin. Clastic beds that

make up the lower part of the formation range from 100 to

420 m thick, whereas the estimated thickness of the limestone

facies varies from 150 m to more than 600 m. 

Three planktonic foraminiferal zones, the Globigerina

ampliapertura, Globorotalia opima opima and Globigerina

ciperoensis Zones, have been identified within the Juana Díaz

Formation (Moussa and Seiglie, 1970). Frost and others

(1983, p. 18) correlate clastic mudstones that make up the

basal part of the formation in the Guayanilla area with the

Globigerina ampliapertura zone contained in beds northwest

of the city of Juana Díaz, placing this part of the section in

the middle part of the early Oligocene. Moussa and Seiglie

(1970) and Frost and others (1983) correlate the shallowshelf carbonate rocks that make up the upper part of the formation in the west with deep-water carbonate rocks of the

north-central part of the basin which contain planktonic foraminifera assigned to the Globorotalia opima opima Zone.

Accordingly, this part of the section is late-early to early-late

Oligocene (Haq and others, 1988). There appears to be some

difference of opinion between Moussa and Seiglie (1970) and

Frost and others (1983) as to the correlation of shallow-water

limestone beds near Guanica in the western edge of the basin

with slope chalk beds near Guayanilla containing faunal

assemblages assigned to the Globigerina ciperoensis Zone of

late Oligocene age. Moussa and Seiglie (1970) show the
GEOLOGY OF THE SOUTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 19 >180 230 230 to620 290 to580 695 710 530 382 387 234 CARIBBEAN SEA 200

400

400

600

SOUTHERN MARGIN - GREAT SOUTHERN

PUERTO RICO FAULT ZONE

NORTHERN MARGIN - GREAT SOUTHERN

PUERTO

RICO

FAULT ZONE

U

D

U

D

U

D

HINGE LINE

? ?

?

?

?

Guánica

Guayanilla

Tallaboa

Ponce

Santa

Isabel Salinas Cerro Central

Aguirre

Juana Díaz

Playa de

Ponce

ATLANTIC OCEAN

CARIBBEAN SEA

IslaMona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

66 ̊52'30" 67 ̊00' 66 ̊45' 66 ̊30' 66 ̊37'30" 66 ̊22'30" 66 ̊15' 66 ̊07'30"

18 ̊15'

18 ̊07'30"

18 ̊00'

17 ̊52'30"

Base modified from U.S. Geological Surveydigital data

0 10 KILOMETERS 50 5 MILES

EXPLANATION

Fan delta and alluvial fan depositsReef depositsForereef, deep shelf, or slope depositsDeep shelf depositsIsland slope and deep shelf deposits with clasticsContact—Queried where uncertain

Line of equal thickness of Juana Díaz Formation—Interval 200 meters. Dashed where approximately locatedInferred fault—U, upthrown; D, downthrownInferred fault—Arrows show relative movementControl point Well—Thickness in meters Outcrop—Thickness in meters

382

530 ?

U

D

200

FIGURE 9.—Major facies and thickness of the Juana Díaz Formation, south-central Puerto Rico (distribution of western facies from Frost and others, 1983).
20 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

limestones as being correlative, whereas Frost and others

(1983) did not identify coeval shallow-water equivalents.

UNNAMED PELAGIC CARBONATE ROCKS

An unnamed carbonate-rock sequence of early Miocene

age unconformably overlies the Juana Díaz Formation and

crops out in restricted localities of the north-central part of

the basin. These strata include the Angola limestone or

island-slope chalk beds of Seiglie and Bermúdez (1969), the

pelagic Río Tallaboa marlstone of Moussa and Seiglie (1970),

and the upper “new formation” of Frost and others (1983)

(fig. 8). Chalk and marlstone beds deposited within an

island-slope environment crop out in restricted localities

between Guayanilla and Ponce (Moussa and Seiglie, 1970, p.

1894) and contain planktonic foraminifera assigned to the

Globorotalia kugleri Zone. Moussa and Seiglie (1970) suggest that shallow-water reef and Lepidocyclina undosa (large

benthic foraminifera) forereef strata in the north-central part

of the basin are, at least in part, correlative with the chalky

“Angola beds” of early Miocene age. However, Frost and

others (1983) do not show these shallow-water limestone

beds as correlative. The Río Tallaboa marlstone beds, bedded

Globergina ooze (sandstone at Quarry No. 2), and other

equivalent interbedded marlstone and claystone beds crop out

only in a few exposures between Ponce and Río Macaná.

These beds contain planktonic foraminifera of the Globigerinatella insueta Zone, suggesting these rocks are late-early

Miocene in age (Haq and others, 1988). Moussa and Seiglie

(1970) imply an unconformity separates the Angola and Río

Tallaboa beds and their equivalents, whereas Frost and others

(1983, p. 62) suggest continuous deposition within the early

Miocene sequence. Taken together, these early Miocene

strata are the lenticular remnant of a bathyal sequence that

was deposited almost until the end of the early Miocene. If

rocks equivalent to these bathyal deposits lie buried beneath

the fan-delta plain, they were not described by Todd and Low

(1976).

PONCE LIMESTONE 

The Ponce Limestone unconformably overlies the

unnamed carbonate-rock sequence previously described and,

where this sequence is missing, the Ponce unconformably

overlies the Juana Díaz Formation. The Ponce Limestone

consists largely of yellowish-orange, soft to moderately hard,

fossiliferous limestone and crops out nearly continuously as a

narrow band that extends from Bahía Montalva to the Río

Pastillo (pls. 1A, 2A). Outliers of the Ponce Limestone are

also exposed north and northeast of the city of Ponce and in

the southwestern part of the island between Punta Aguila to

Isla Cueva, and unconformably overlie early Tertiary bedrock

units on Isla Caja de Muertos. 

Outcropping strata of the Ponce Limestone contain an

abundant marine fauna; molds of gastropods, pelecypods,

coral heads, and large foraminifera are indicative of deposition in shallow-water lagoon and back-reef environments.

The large foraminifera, Lepidocyclina undosa and the ahermatypic “deep sea” coral Flabellum are reported within the

Ponce Limestone (Monroe, 1980, p. 78). Such fauna reflect

deeper shelf environments also contained within the Ponce

Limestone sequence. Minor clastic beds equivalent to the

Ponce Limestone crop out near Ponce and are considered

indicative of deposition in nearshore environments. 

Clastic beds equivalent to the Ponce Limestone are buried

beneath the fan-delta plain between Playa de Ponce and Santa

Isabel (see discussion below). Cutting samples collected

from the Kewannee oil wells show that shallow-water beds of

the Ponce Limestone grades west to east from a shallowwater facies to terrigenous sand, gravel, and mudstone interbedded with some limestone (Glover, 1971; Todd and Low,

1976, p. 2; Meyerhoff and others, 1983).

On the basis of stratigraphic position and planktonic foraminiferal data collected from the underlying unnamed islandslope sequence described previously, Moussa and Seiglie

(1975) suggest that the lower part of Ponce Limestone is

probably middle Miocene age. Frost and others (1983, p. 10-

11) correlate coral faunal assemblages within the lower Ponce

Limestone with the Globorotalia foshi Zone of middle

Miocene age. Bold (1969) studied the ostracod fauna within

the upper part of the Ponce Limestone and correlated faunal

assemblages with the Globorotalia margaritae planktonic

foraminiferal zone. The age of this zone was revised (Haq

and others, 1988) and is currently considered to extend from

latest Miocene to early Pliocene time. Therefore, the Ponce

Limestone may range in age from middle Miocene with the

uppermost unconformity recording post-early Pliocene emergence and erosion. 

A brackish and nearshore marine, benthonic foraminiferal

fauna constitute the Amphistegina angulata-Elphidium lens

marker horizon identified from Kewannee oil well cutting

samples. This fauna is also reported to occur within outcropping rocks of the Ponce Limestone (Seiglie and Moussa,

1974, p. 257) and is considered to be middle Miocene. Todd

and Low (1976, p. 2) showed that the clastic mudstone strata

that lie deeply buried in the Playa de Ponce to Santa Isabel

area are stratigraphically equivalent to the Ponce Limestone. 

Structural complexity and sparse well control limits accurate thickness estimates of the Ponce Limestone. The limestone facies and equivalent clastic strata appear to be thickest

(300 to more than 400 m) (fig. 10) in the Ponce and Santa Isabel area. The Ponce Limestone has been estimated to be as

much as 850 m thick in the Río Tallaboa area (Monroe, 1980,

p. 81), a thickness estimate that seems to be inconsistent with

regional trends in the area between Ponce and Guayanilla

(fig. 10). Perhaps, complex fault relations and vegetative
GEOLOGY OF THE SOUTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 21 200 400 250 to 300 44 365 790+ CARIBBEAN SEA ? ? 400 200 600 800 400 ? ? ? ? ? U U D D UD Guánica Guayanilla Tallaboa Ponce Santa Isabel Salinas Cerro Central Aguirre

Juana Díaz

Playa de

Ponce

ATLANTIC OCEAN

CARIBBEAN SEA

IslaMona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

66 ̊52'30" 67 ̊00' 66 ̊45' 66 ̊30' 66 ̊37'30" 66 ̊22'30" 66 ̊15' 66 ̊07'30"

18 ̊07'30"

18 ̊00'

17 ̊52'30"

Base modified from U.S. Geological Surveydigital data

0 10 KILOMETERS 50 5 MILES

EXPLANATION

Nonmarine (fan delta, alluvial fan) and transitional marine (nearshore, shallow marine) depositsReef tract and shallow carbonate shelf depositsContact—Dashed where approximately locatedLine of equal thickness of Ponce Limestone— Interval 200 meters. Dashed where approximately located, queried where uncertain

Inferred fault—U, upthrown; D, downthrownControl point Well—Thickness in meters Seismic data—Estimated thickness in meters Outcrop—Thickness in meters

44

250 to 300

200 200

U

D

?

FIGURE 10.—Major facies and thickness of the Ponce Limestone and equivalent beds, south-central Puerto Rico.
22 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

cover obscured geologic relations and contributed, as suggested by Frost and others (1983, p. 24), to what they considered to be an overestimate by Monroe of the thickness of the

Ponce. Middle to upper Miocene deposits of the Ponce Limestone, however, are thickest between Ponce and Santa Isabel

and are associated with clastic deposition (alluvial fan or fan

delta) in a subsiding basin.

A shallow-water and possibly some deeper shelf carbonate paleoenvironments prevailed in much of the western basin

during middle to late Miocene time. During the early

Pliocene, relative sea level continued to decline and resulted

in subaerial exposure of the late Miocene and early Pliocene

carbonate-clastic shelf. It is not known if the Pliocene hiatus

that marks the top of the Ponce Limestone is restricted to

rocks that crop out or if the unconformity extends eastward to

the clastic facies that underlies the fan-delta plain. Deposition of alluvial-fan or fan-delta deposits may have continued

during the Pliocene in response to a low base level in the eastern part of the basin.

SUBSURFACE STRATIGRAPHIC RELATIONS

Stratigraphic and structural relations of Oligocene to

Quaternary lithologic units are complex where they lie buried

beneath the southern fan-delta plain. The relations are illustrated in sections A–A ́ and B–B ́ (figs. 11, 12). A paleontologic marker horizon (the Amphistegina angulata-Elphidium

lens benthic zone) found within the Ponce Limestone

sequence shows important age-related clastic-carbonate relations. This marker horizon occurs in limestone and clastic

facies and verifies equivalent age of deposits and a facies

change in deposition in the eastern part of the basin. The subsurface contact separating the Ponce Limestone and underlying Juana Díaz Formation is recognized by an abrupt faunal

change from varied and abundant assemblage of planktonic

foraminifera of Oligocene age to overlying shallow-water

benthic foraminifera of Miocene age (Todd and Low, 1976).

Unnamed pelagic strata of early Miocene age (Angola limestone and Río Tallaboa marlstone) that crop out between

Guayanilla and Tallaboa were not reported within the subsurface sequence located between Ponce and Santa Isabel (Todd

and Low, 1976). The subsurface contact between the carbonate facies of the Ponce Limestone and overlying clastic fandelta deposits of Quaternary age is readily identified in wells

located in the western part of the fan-delta plain; however,

this contact is difficult to recognize in the central part of the

fan-delta plain where buried carbonate rocks of the Ponce

Limestone grade eastward to a nearshore, brackish marine,

and nonmarine sequence of clastic and minor interbedded

limestone beds. Here, clastic deposits of Miocene to Pliocene

(?) age underlie clastic deposits of Quaternary age. Similar

difficulties were encountered during the present investigation

when studying driller’s log descriptions of wells located in

areas where fan-delta and valley-fill deposits directly overlie

conglomeratic beds of the Juana Díaz Formation. For the

most part, these wells are located on the northern margin of

the fan-delta plain and in the Tallaboa-Guánica area. In general, the contact between Cretaceous-Tertiary bedrock units

and overlying strata can be easily identified in well cuttings,

cores, and geophysical log data. 

Sections A–A ́ and B–B ́ illustrate an important structural

feature of Puerto Rico’s south coast. An inferred high-angle

fault vertically juxtaposes the Ponce Limestone and coeval

clastic beds with bedrock of Cretaceous and Tertiary age.

The carbonate-clastic facies change within the Ponce Limestone and its clastic equivalents are shown to be the same age,

but basinal subsidence helps explain a large increase in thickness of the Miocene section.

PLEISTOCENE TO HOLOCENE FAN-DELTA AND ALLUVIAL 

DEPOSITS

Four large and seven smaller fan deltas coalesce to

form the low-lying fan-delta plain between Ponce and Patillas

(fig. 13). Bordered to the north by the Cordillera Central, the

insular drainage divide lies less than 24 km north of the Caribbean coastline. High-gradient streams that flow southward

and transport bedload deposits of boulders, cobbles, sand,

and silt during upper flow regimes have incised deep mountain and foothill valleys. The altitude of most of the fan deltas located between Ponce and Patillas do not exceed 50 m

above sea level at their apex, except for the Capitanejo fan

that rises to 80 m above sea level (pl. 1A). The surface of the

different fans slopes coastward at a gradient that varies from

25 m/km within the apex or proximal upland channel to 2 m/

km near the coast. From fan apex to shoreline, fan radii average nearly 10 km; all of the fan deltas east of Ponce contain a

well-developed, semi-radial, subaerial morphology. However, bedrock hills protrude the fan-delta plain in several

localities, more commonly where deposits thin against the

foothills. One of the larger bedrock hills is at Central Aguirre, located adjacent to the Caribbean Sea. Cerro Central

Aguirre is the only locality within the fan-delta plain where

pre-Miocene strata have been reported to crop out along the

coast (Berryhill, 1960; Glover, 1971).

West of Ponce in the Tallaboa to Guánica area, five major

valleys contain moderately thick gravel, sand, and silt deposits and separate an otherwise continuous outcropping belt of

Oligocene to Pliocene (?) rocks that make up the Juana Díaz

Formation and Ponce Limestone (pl. 2). These alluvial-filled

valleys extend coastward where three river valleys, the Ríos

Guayanilla, Macaná, and Yauco, combine to form a more

extensive low-lying plain (fig. 14). Although the extent of

this plain is considerably smaller than the fan-delta plain
GEOLOGY OF THE SOUTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 23

described above, fan-like topography of the coalesced lower

valleys and plain parallels similar features seen to the east.

Similar features are present in the Guánica and Tallaboa valleys. 

By definition, a fan delta is an alluvial fan that progrades

into a standing body of water from an adjacent highland

(Holmes, 1965). Fan deltas are characterized by their semiradial or fan-like shape in plain view, concave-upward

radial and convex-upward cross-fan profiles, and the relatively narrow space that they occupy between a highland

mountain front and the nearby water body (Wescott and

Ethridge, 1980; McPherson and others, 1987). Fan deltas

contain subaerial and subaqueous depositional components

that are separated by a narrow transitional marine coastal

zone of beach, mangrove swamps, and tidal flats. Sedimentary strata deposited within the subaerial component are dominated by streamflow, sheetflow, and gravity depositional

processes. The subaqueous part of the fan-delta system lies

offshore and is subject to marine shelf and slope depositional

processes. 

The fan-delta plain between Ponce and Patillas and the

alluviated valley-fan-delta plain between Tallaboa and

Guánica is bordered to the north by steep, southward-facing

foothill and mountain slopes of the Cordillera Central, and to

the south by the Caribbean Sea. The subaqueous part of the

fan-delta system of Puerto Rico lies offshore where it forms

the upper sediments of the Muertos insular shelf. Quaternary

deposits that make up the bulk of the fan-delta plain consist

almost entirely of subaerial deposits, but include some transitional, marine deposits. 

The arcuate-cuspate coastline morphology of much of the

south coast (Kaye, 1959a) is the result of the interaction of

wave energy, longshore current, and detritus carried to the

coast by streams (Hayes and Michel, 1982). Present-day

streams and floodflow conditions apparently are not of significant magnitude or sufficient frequency to maintain the lobate

form that typifies active fan deltas such as the Yallahs fan

delta of Jamaica (Wescott and Ethridge, 1980). The lobate

Río Guayanilla fan delta, is located within the protected confines of Bahía Guayanilla, is a notable exception. The apparent reduction in wave energy and lessened longshore currents

within this embayed area is the principal factor controlling

the shape of the coastline in the vicinity of Río Guayanilla.

The supply of sediment from the river system that feeds an

adjoining fan delta, that of the Río Yauco, has sufficient magnitude, in combination with low wave energy and minimal

longshore currents, to allow the development of a small progradational delta at the mouth of the river.

Subaerial parts of southern Puerto Rico’s fan deltas are

separated from the Caribbean Sea by a narrow marginal

marine zone of supratidal or salty scrub flats, marsh and mangrove swamps, and beach deposits. Fan-delta coastal zones

are largely characterized by broad sand and gravel beaches.

Mangroves, marshes, and tidal flats are mostly restricted to

those areas protected by offshore, fringing reefs. The most

extensive area of mangroves, marshes, and tidal flats is along

the Salinas fan-delta margin. Spits partially enclose Bahía de

Jobos and Bahía de Guayanilla. The compound spit at Bahía

de Jobos shelters expansive mangrove swamps that rim the

bay adjacent to the Salinas fan delta. The beach here and

along the southern edge of the Playa de Salinas consists, at

least in part, of skeletal carbonate sand. The occurrence of

carbonate sand is largely restricted to the lee of fringing reefs

that extend from Punta Ola Grande to Playa de Salinas.

Fringing reefs are also the principal source of carbonate sand

supplying an elongate compound spit at Punta Pozuelo. Carbonate sand beaches bordering the fan-delta plain are unusual

in southern Puerto Rico, however. The Playa de Salinas to

Punta Ola Grande area represents the only place where the

transitional beach zone is dominated by, or contains a large

component of, carbonate detritus. 

Major streams that traverse the 70-km wide fan-delta plain

and alluviated valley-fan delta areas to the west generally display a straight channel morphology, drain one fan, and mostly

extend in a southwestward direction from the fan apex. The

floor of stream channels traverse the southern alluviated valleys and fan-delta plain slope at a gradient of 2 to 8 m/km near

the coast and higher inland gradients of 8 to 18 m/km. The

coastward slope of streams draining the large coastal fan-delta

plain near Ponce is 1 to 6 m/km. The Portugúes-Bucana and

Capitanejo fans are drained by two, relatively straight streams.

These streams develop a sinuous to meandering pattern as they

extend coastward, except where they have been channelized.

Flood control channelization efforts, such as that developed for

the Bucana-Portugúes-Cerillos flood control and reservoir

project have altered depositional conditions within the fandelta plain. Prior to these changes, the confining channel of

many streams altered to a distributary system in the distal fan

area allowing surface water to spread out as sheetflow onto the

coastal lowlands (Mitchell, 1922, p. 236).

Streamflow within the fan-delta plain is minimal except

during flood events. For example, streamflow within the

landward parts of the fan-delta plain usually is less than 1.0

cubic meters per second (m3/s) 90 percent of the time

(Quiñones and others, 1984) and the midstream reach is often

dry. The distal stream channel can contain standing water,

the result of upward ground-water discharge or equilibrium

with the ground-water table. Intense storm precipitation that

can last several days occasionally results in floodflow that is

widespread and catastrophic. In 1985, Puerto Rico was

affected by a stationary tropical depression that produced

more than 580 millimeters (mm) of rainfall in 24 hours and

caused extensive flooding over much of the southern part of

the island (Quiñones and Johnson, 1987) (fig. 15). Surfacewater records show that flooding has been more extensive and

widespread in the western half of the South Coast ground-
24 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

PC-1

CPR-3

SC-5 CPR-2

CPR-1

SC-4

SC-2 SC-3

CA-55

AY-16

PA-2

U

D

U

D

U

D

CARIBBEAN SEA

Santa Isabel 

Salinas

Patillas

Juana

Díaz

Ponce

Central

Aquirre

Guayama

Arroyo

Playa de

Ponce

Playa de

Santa Isabel

A A'

66°37'30" 66°30' 66°22'30" 66°15' 66°07'30" 66°00"

18°00'

17°52'30" Base modified from U.S. Geological Survey digital data 0 5 10 KILOMETERS

0 5 MILES ATLANTIC OCEAN

CARIBBEAN SEA

Isla

Mona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

Rocks of Miocene to Quaternary age

 Unconsolidated clastic sediments—Boulder- to silt- or clay-size clastic detritus 

 deposited under subaerial conditions, mostly during Pleistocene to Holocene time

 Unconsolidated to poorly consolidated sediments—Boulder- to silt- or clay-size detritus, minor limestone and marl deposited under

 subaerial, nearshore, and shallow marine conditions. Deeply buried sediments may be as old as Miocene age and are equivalent

 to the Ponce Limestone. Uppermost deposits are Pleistocene and/or Holocene in age

 Limestone—Consists of limestone with brackish- to shallow-marine fauna. Considered equivalent to Ponce Limestone

Rocks of Oligocene age

 Limestone-shale sequence—Consists of deep-water limestone, silty shale, marly conglomerate dominated by planktonic and deep-

 water benthic foraminfera. Considered equivalent to the Juana Díaz Formation

Rocks of Cretaceous to early Tertiary age

 Bedrock—Consists of bedded, lithified sequence of conglomeratic sandstone, siltstone, volcanic and igneous rocks, and minor

 limestone

Line showing approximate position of freshwater-saline water interface

Line showing top of Amphistegina Angulata-Elphidum lens fauna—Considered to be a Miocene stratigraphic datum (Todd and

 Low, 1976)

Fault—U, upthrown side; D, downthrown side. Dashed where inferred

Line of section

Control point

Well name abbreviations

 SC South Coast

 CPR Kewanee Interamerican Oil Company—Commonwealth of Puerto Rico

 PC Punta Cuchara

 CA Central Aquirre

 AY Arroyo

 PA Patilla

 SI Santa Isabel

EXPLANATION

SC-5

U

D

A A'

FIGURE 11.—Section A–A' across southern Puerto Rico's fan-delta plain.
GEOLOGY OF THE SOUTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 25

D

U

D

U

PC-1 CPR-3 SC-5 CPR-2 CPR-1 SC-4 SC-2 SC-3 CA-55 AY-16 PA-2 Section B–B' 0 5 10 KILOMETERS

0 5 MILES

200

METERS

WEST

A

FEET

EAST

A'

Sea level

200

400

600

800

1,000

1,200

1,400

1,500

VERTICAL SCALE GREATLY EXAGGERATED

4,500

4,000

3,500

3,000

2,500

2,000

1,500

1,000

500

Sea level

500

FIGURE 11—Continued. Section A–A' across southern Puerto Rico's fan-delta plain.
26 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

SC-4

CPR-1

SI-66

SI-45

SI-24

SI-19

SI-12

SI-10

U

D

U

D

B'

B

CARIBBEAN SEA

Santa Isabel Bahía de Rincón

Playa de

Santa Isabel 17°57'30"

18°00'

18°02'30"

Base modified from U.S. Geological Survey digital data

66°25' 66°22'30" 66°20'

0 4 1 KILOMETERS

0 1 2 MILES

2 3

ATLANTIC OCEAN

CARIBBEAN SEA

Isla

Mona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

Rocks of Miocene to Quaternary age

 Unconsolidated clastic sediments—Boulder- to silt- or clay-

 size clastic detritus deposited under subaerial conditions, 

 mostly during Pleistocene to Holocene time

 Unconsolidated to poorly consolidated sediments—Boulder- 

 to silt- or clay-size detritus, minor limestone and marl 

 deposited under subaerial, nearshore, and shallow 

 marine conditions. Deeply buried sediments may be as 

 old as Miocene age and are equivalent to the Ponce 

 Limestone. Uppermost deposits are Pleistocene and/or 

 Holocene in age

 Limestone—Consists of limestone with brackish- to shallow-

 marine fauna. Considered equivalent to Ponce Limestone

Rocks of Cretaceous to early Tertiary age

 Bedrock—Consists of bedded, lithified sequence of 

 conglomeratic sandstone, siltstone, volcanic and 

 igneous rocks, and minor limestone

EXPLANATION

SC-4

U

D

B B'

Well lithology

 Sand-size or larger

 Silt-size or smaller

 Limestone, limey clay, or marl

 Bedrock

Line showing approximate position of freshwater-

 saline water interface

Line showing top of Amphistegina Angulata-

 Elphidum lens fauna—Considered to be a Miocene 

 stratigraphic datum (Todd and Low, 1976)

Fault—U, upthrown side; D, downthrown side. Dashed

 where infered

Line of section

Control point—Well name abbreviations on figure 11

FIGURE 12.—Section B–B' down the axis of the Coamo fan delta, south-central Puerto Rico.
GEOLOGY OF THE SOUTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 27

D

U

SC-4 CPR-1

SI-66

SI-19 SI-21

SI-10

SI-45

Section

SI-24

B–B'

0 4 1 KILOMETERS

0 1 2 MILES

2 3

100

METERS

SOUTH

B

FEET

NORTH

B'

Sea level

100

200

300

400

900

1,000 VERTICAL SCALE GREATLY EXAGGERATED

3,250

3,000

1,250

1,000

750

500

250

Sea level

250

FIGURE 12—Continued. Section B–B' down the axis of the Coamo fan delta, south-central Puerto Rico.
28 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

 

           yy

yy

y

yy

yyy

 yyy  yyy yyyyyy y yyyyyy yyyyyy  yyy  yyy  yyy yyy

yyy yyyy yyyy

CARIBBEAN SEA

Santa Isabel 

Salinas

Patillas

JuanaDíaz

Ponce

Playa de

Ponce

Playa de

Santa Isabel

PastilloCañas Portugúes- Bucaná

Capitanejo Cañas

Descalabrado

Coamo Jueyes

Salinas

Seco Guayama

Arroyo

FIGURE 13.—Location and extent of fan deltas of southern Puerto Rico.

ATLANTIC OCEAN

CARIBBEAN SEA

IslaMona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

Alluviated valley

EXPLANATION

Fan deltaInterfan or alluvial plain area

yy 

0 10 KILOMETERS 50 5 MILES

66°37'30" 66°30' 66°22'30" 66°15' 66°07'30" 66°00'

18°00'

17°52'30"

Base modified from U.S. Geological Survey digital data
GEOLOGY OF THE SOUTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 29

water province than in the eastern half. Widespread, catastrophic flooding of the western fan-delta plain is reported to

have occurred six times between 1899 and 1985; extensive

flooding of the Río Tallaboa Valley has occurred at least four

times between 1928 and 1975 (Johnson, 1981).

The subaqueous component of Puerto Rico’s

modern-day fan-delta plain lies offshore and within the Isla

Caja de Muertos shelf area. Bathymetric data indicates a

semi-radial fan-like morphology of the fan-delta plain that

extends to depths of 10 m (pl. 1A). Uppermost deposits

within the subaqueous part of the fan-delta system are made

up of inner shelf deposits. Inner shelf deposits grade seaward

to outer shelf and shelf slope deposits; all of these shelf

deposits are considered to be an active part of the fan-delta

depositional system. The inner shelf extends from the shoreline to water depths of 20 to 30 m and can be separated into

nearshore, shelf platform, and shelf basin areas (Beach and

Trumbull, 1981). Nearshore deposits lie within the turbid

coastline and are made-up of terrigenous fine sand. The shelf

platform zone consists of a layer of gravel- to silt-size bioclastic detritus that is underlain by a cemented hardground

surface. Live, recurved fringing reefs of intergrown coral and

coral-algae lie offshore of the Coamo and Salinas fan deltas.

These reefs function as a source of carbonate sand that form

carbonate banks or cays on their leeward side. A bathymetric

and structural shelf basin lies south of Playa de Ponce. In this

area, longshore currents have carried fine terrigenous detritus

from the nearshore zone and moved them farther downslope

than elsewhere along the coastline. An outer shelf area lies

seaward of the fringing reefs and is covered with biogenic

sand and gravel; brown-stained sediment (Beach and Trumbull, 1981) suggests that this outer shelf was subject to subaerial exposure. Drowned reefs that border the outer shelf

slope rim and may record a lower, relict sea stand. 

Although terrigenous fine sand and mud cover inner-bay

areas of Bahía de Tallaboa, Guayanilla, and Guánica, the fanlike nearshore bathymetry seen east of Ponce is not present in

these shallow marine areas. Biogenic silt covers much of the

outlying shallow shelf platform and hard-bottom areas are

generally covered by coral and coralline algae. Patch reefs

MONA PASSAGE

CARIBBEAN SEA

Yauco

Tallaboa

Guayanilla

Guánica

Drainage divide Lajas Valley

FIGURE 14.—Location of principal alluviated valleys and small, restricted fan-delta plains in the western half of Puerto Rico's south coast.

ATLANTIC OCEAN

CARIBBEAN SEA

Isla

Mona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

Area underlain by alluvial

and fan-delta deposits

EXPLANATION

0 5 10 KILOMETERS

0 5 MILES

Base from U.S. Geological Survey,

Arecibo, 1:100,000, 1980, unpublished

18 ̊07'30"

17 ̊52'30"

18 ̊00'

67 ̊15' 07'30" 67 ̊00' 52'30" 66 ̊45'
30 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS yyyy  yyyyyyyyyyyyy  yyyyyyyyyyyyy  yyyyyyyyyyyyy  yyyyyyyyyyyyy  yyyyyyyyyyyyy  yyyyyyyyyyyyy  yyyyyyyyyyyyy  yyyyyyyyyyyyy

Canal de

CARIBBEAN SEA

Río Jueyes

ío C R añas

Río Cañas

Río Bucaná

Río Cuyón

R oí Jacaq s au

L Coama

Lago

Bronco

Lago

Ponceña Lago No. 1 Lago No. 2

Lago No. 5

Río 

Río 

Río 

Río 

Río 

Río 

Río 

San Patricio

Cerrillos

Río Inabón

Portugués

Guayo

Toa Vaca

e D sca al brado

Co ma o

R oí aS l ni as

Lago Guayabal

Juana Díaz

Santa

Isabel 

Playa de Ponce

Ponce

Villalba

Salinas 

Coamo

JuanaDíaz

Playa de

Santa Isabel

LANDSLIDES AREABARRIO MAMEYES

DESTROYED BRIDGES

 ON THE RÍO COAMO

FIGURE 15.—Areas flooded in the central part of the southern plain during the October 6–7, 1985 flood (modified from Quiñones and Johnson, 1981).

ATLANTIC OCEAN

CARIBBEAN SEA

IslaMona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

Flooded area

EXPLANATION

Alluvium

Drainage boundary yyyy Boundary of fan-delta plain

0 10 KILOMETERS 5

0 5 MILES

Base modified from U.S. Geological Survey digital data

18 ̊07'30"

18 ̊00'

66 ̊37'30" 66 ̊30' 66 ̊22'30"
GEOLOGY OF THE SOUTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 31

and leeward sandy cays are present within the inner bay areas

(indicating limited influx of terrigenous sediment) and in the

outlying shallow shelf platform areas.

Configuration of Basal Quaternary Contact and Thickness 

of Deposits

Driller’s logs and other well data for more than 500 wells

were used to estimate the thickness of fan-delta and alluvial

valley deposits and map the configuration of the underlying

bedrock surfaces (pls. 1B, 1C, 2B). Of these wells, many do

not reach underlying bedrock that is generally described by

drillers as “hard, blue, or gray rock.” These shallow well data

are considered useful data control points because they indicate a minimum depth a well must be drilled in order to penetrate the underlying bedrock surface. Some wells drilled in

the eastern half of the fan-delta plain penetrate a sequence of

weathered clay, silty clay, or silty limestone reported on

driller’s logs as “tosca.” Tosca has been previously interpreted as a weathered bedrock zone of highly variable thickness that overlies unweathered bedrock (McClymonds and

Ward, 1966). Ramos-Ginés (1994) reports that alluvial

deposits in the Río Majada and Río Lapa area are underlain

by a 3 to 24 m-thick zone of highly weathered volcanic rocks

which are, in turn, underlain by partly weathered and

unweathered bedrock. Test borings drilled in the Central

Aguirre area reportedly penetrated a weathered bedrock zone

of 3 to 15 m thick (Weston Geophysical Research, 1967).

Graves (1994) describes a 1 to 33 m-thick regolith or saprolite zone in the Aguirre area that preserves original rock structure. Driller’s log data suggest that, in some places, the

weathered bedrock zone is as much as 18 to 24 m thick. Core

samples collected from the SC 3 well drilled during this study

show gravel, sand, or silt deposits lying immediately above

partly weathered bedrock; a saprolitic zone of weathered clay,

silty clay, or silty limestone was not observed. In the western

part of the fan-delta plain near Ponce and in valley-fill areas

between the Río Tallaboa and Guánica, many wells are

reported to bottom in “white, gray, or blue clay or limestone.” On the basis of lithofacies maps shown for Oligocene

and Miocene strata, it seems likely that wells drilled in the

western part of the fan-delta plain and in the valley-fill areas

to the west have penetrated a carbonate facies within either

the Juana Díaz Formation or the Ponce Limestone. However,

some “limestone” beds could consist of caliche rather than

Oligocene to Miocene bedrock. Poorly developed soil caliche has been observed elsewhere within the fan-delta plain at

isolated exposures and the occurrence of caliche in the subsurface is consistent with semiarid environmental conditions

during the Pleistocene. In the Central Aguirre area, drillers’

log descriptions of cutting samples report the occurrence of

“white clay” contained within the interbedded sequence of

sand, gravel, silt, and clay deposits.

Fan-delta deposits usually do not exceed 10 to 30 m in

thickness where they are found within the proximal part of

the fan or within inland alluvial channels that extend landward from the fan apex (pl. 1B). The thickness of the

Holocene to Pleistocene fan-delta sequence east of Ponce

increases from a featheredge at its landward margin to 80 to

100 m in most coastal areas. Where the sequence overlies the

structural basin between Playa de Ponce and Bahía de Rincon, it can locally thicken to more than 400 m; the actual

thickness of Quaternary deposits within this basin is poorly

known because of sparse control and the lack of age-diagnostic faunal assemblages. Between Bahía de Jobos and Patillas,

Fan-delta deposits usually are less than 40 m-thick. The

thickness of unconsolidated alluvium within all fan-delta/valley-fill areas west of Ponce is greatest near the coast, and

reportedly in excess of 45 m in some coastal areas.

In offshore areas west of Ponce, deposits of Quaternary

age that underlie the Isla Caja de Muertos shelf are estimated

to average 150 m thick, but these deposits progressively thin

along the flanks of submarine canyons that border and incise

the insular shelf (Trumbull and Garrison, 1973). The thickness of Quaternary deposits that lie in the eastern offshore

areas of the South Coast ground-water province is not known. 

The valleys of the Ríos Tallaboa, Guayanilla, Macaná,

Yauco, and Loco-Guánica are incised into bedrock units of

the Juana Díaz Formation and Ponce Limestone to depths as

much as 45 m below sea level (pl. 2B). A period of subaerial

exposure and fluvial downcutting (late Pliocene(?) to early

Pleistocene) of the Muertos shelf is also reflected by canyon

bathymetry within the Bahía de Guayanilla and Bahía de

Guánica. The confluence of the Ríos Macaná, Guayanilla,

and possibly Yauco was located beneath Bahía de Guayanilla

during lower sea stands. Bathymetric data in the Punta Verraco area may indicate a different drainage pattern for the Río

Yauco; this river discharges across the dune deposits west of

Punta Verraco during extreme floods. Canyon development

south of the dunes suggests a southward drainage direction.

A delta formed at the mouth of the Río Yauco has covered

much of the canyon that incises the Bahía de Guayanilla, visible to the east. In any case, these three rivers combine to

form a larger river system that may have extended farther

southward on to the Isla Caja de Muertos shelf (Trumbull and

Garrison, 1973).

Lithofacies

A seacliff exposure located in the Central Machete-Puerto

Arroyo area extends 2.5 km along the Caribbean Sea and represents the only extensive cross-profile fan in the South Coast

ground-water province. An extensive cutbank exposure

occurs on both sides of the Río Coamo channel south of Paso

Seco and shows a longitudinal section of a fan for approximately 5 km downstream. Other cutbank exposures within
32 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

the fan-delta plain are poorly preserved because the unlithified nature of deposits promotes sloughing. Subaeriallydeposited silt represents the best preserved lithofacies, in

large part because it is better lithified and can support a nearly

vertical exposure face. Exposures located within the fandelta plain are limited to sediments deposited under subaerial

conditions.

Subaerial fan-delta deposits include thick to very thick,

crudely-stratified, clast-supported, and rare open-framework

conglomerates; horizontal and planar cross-stratified boulders, cobbles, pebbles; and sand, and thickly bedded to massive silt. All of these different subaerial facies usually occur

within thick, horizontal to subhorizontal beds; however, conglomeratic beds also occur as channel-fill deposits.

Conglomeratic sediments range in size from fine sand to

boulders and usually fine-upward, but sometimes gradationally coarsen upward, or show no discernible vertical change

in grain size. Some sequences interpreted to be coarseningupward could be the result of poor definition between thinner,

discrete conglomerate beds. Up-current imbrication of cobbles and boulders within conglomeratic beds is poorly to

well-exhibited. Conglomerates of boulder- to pebble-size

material are usually thickly bedded, clast-supported, and contain a sandy matrix. The lower contact of coarse-grained

beds usually is abrupt and suggestive of a predepositional

phase of channel scouring. These conglomerate beds are

interpreted to have been deposited in a braided stream as part

of migrating channel bars. 

An example of an exhumed conglomeratic channel-fill

sequence is visible at the coastal sea cliff exposure near Central Machete-Puerto Arroyo (fig. 16). This channel sequence

ranges from 10 to 30 m wide and is 1 to 4 m thick. The

channel-fill consists of a clast-supported, cobble- and boulder-conglomerate with a sandy to pebbly matrix; the

sequence exhibits large-scale, low-angle crossbeds that were

possibly associated with channel bar migration and gradual

channel aggradation. The channel conglomerate overlies and

downcuts a massive silt bed. Rare open-channel fill conglomerates have been seen along a cutbank exposure of the Río

Nigua at Salinas. The conglomeratic stream deposits are

probably associated with deposition during high flow conditions in which most fine sediment is carried away in suspension. Smaller pebbles and sand were deposited during lower

flow regimes and filled only the uppermost pore space.

Bedded coarse-grained deposits of fine to coarse sand and

pebbles, with minor cobbles, are dominated by horizontal and

planar crossbedding. Sandy and pebbly, planar cross-bedded

deposits are interpreted as being associated with migrating

alluvial bedforms. Horizontal or parallel crossbeds within

the sand and pebble lithofacies and are attributed to deposition within an upper flow regime. Rust (1972) concluded that

migrating longitudinal bars form poorly defined horizontal

beds possibly due to transport in planar sheets under highflow. A few, poorly preserved, large-scale trough beds of fine

to coarse sand and pebbly sand, probably associated with

braided-stream deposition, have been observed at the Central

Machete-Puerto Arroyo sea cliff exposure and in a cutbank

exposure along the upland channel fan-head reach of the Río

Coamo near the city of Coamo. 

Silt beds were largely deposited under sheet flow conditions in which flood waters spread outside the confines of an

incised alluvial channel. Bedded silt deposits are often massive and tend to be blocky, especially where large pieces have

sloughed off the incised channel walls, as in the Río Coamo

south of Paso Seco. Horizontal bedding features present

within the silt facies are not everywhere evident, partly due to

the quality of preservation within the section at most exposures. At the Río Coamo exposure south of Paso Seco, very

thin, discontinuous lenses of sand and pebbles (less than a

few cm) separate discrete horizontal beds. The uppermost

contact that separates massive silt beds from overlying

coarse-grained sand, pebbles, cobble, and boulder-bedded

deposits is abrupt, erosional, and, in many places, shows evidence of fluvial scouring.

Poorly-preserved paleosols were seen in a few locations

and represent periods of nondeposition and subaerial exposure. Paleosols are represented by the occurrence of calcium

carbonate deposits contained within the bedded fan-delta

sequence. Calcification generally occurs in subhumid to arid

regions where there is insufficient precipitation to drive soil

water to the water table. Calcium carbonate, carried into

solution from the A soil horizon, is reprecipitated in the B soil

horizon. Calcium carbonate deposits have been observed at a

road cut into the coastal terrace north of Bahía de Rincón,

west of Las Ochenta, and within the streambed of the Río

Coamo near Paso Seco. The reported occurrence of white

“tosca” in driller’s well log reports, often underlain by sand

and gravel deposits, can be interpreted as paleosol-related

calcium carbonate.

Vertical Profile

Well-to-well correlation of discrete subsurface cobble,

sand, and silt beds that underlie the fan-delta plain is impossible because of the lack of continuity of coarse- and finegrained strata, lack of correlable marker beds, and poor geophysical-log control. Core samples were continuously collected from test wells drilled using the reverse-air dual-tube

drilling method previously described. Two wells (SC 4 and

SC 5) are located within the Ponce Basin; a third well (SC 2)

was drilled northeast of the basin but within the distal part of

the Salinas fan delta. Due to the unconsolidated nature of

coarse-grained fan-delta deposits, crossbed current structures

were destroyed and many bed-to-bed contacts were “blurred”

by coring operations; however, progressive changes in grain
GEOLOGY OF THE SOUTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 33

size and bed thickness could, for the most part, be observed

(pl. 3).

The analysis of sedimentary cyclicity, as defined by progressive changes in grain size, bed thickness, and their interpreted depositional process, has been shown to be a viable

approach to evaluate alluvial fan depositional systems and

their short-term and long-term depositional behavior

(Heward, 1978; Steel and Aasheim, 1978; Gloppen and Steel,

1981; Steel, 1988). One purpose of this analysis is to identify

cyclic patterns of deposition within the vertical sequence that,

at first glance, appear to be part of a chaotic distribution of

coarse-grained and fine-grained sediments. Recognition of

such cyclicity was used to aid well-to-well correlation

between the different cored test holes in this study (pl. 3) and

to suggest how the fan-delta sequence might be separated into

regionally-extensive water-bearing units.

Three scales of cyclic deposition were identified. Smallest-scale cyclicity (less than 1 m to 20 m) is associated with

discrete coarse- and fine-grained beds and largely characterized by a progressive-upward change in grain size within a

discrete bed. An intermediate-scale of cyclic deposition (15

to 70 m) was recognized by progressive changes in bed thickness. Some cycles were separated by the occurrence of limestone or limy marl; limestone and limy marl strata were

considered to represent the initial phase in a new cycle of

deposition. Coarse- (gravel- and sand-size detritus) and finegrained (silt-size detritus) beds were grouped as an intermediate-scale cycle if bed thickness appeared to progressively

increase or progressively decrease. The largest scale (400 to

500 m) of cyclicity referred to overall changes in bed thickness that occurred within the entire gravel, sand, and silt section that was penetrated by drilling.

In general, most individual beds of coarse-grained sediment either fine-upward or coarsen-upward; in some

instances, they do not exhibit any obvious change in grain

size. Fining-upward bed cycles are the most common vertical

change in lithology, and they usually consist of a basal conglomerate with a sandy matrix, fining upward to coarse to fine

sand. Whereas the lower silt-to-sand contact is distinct and

sharp, the upper sand-to-silt contact ranged from sharp to gradational. Bedded units of cobble- to sand-size detritus range

from 1 to 20 m thick with thickest coarse-grained beds more

commonly seen within the upper part of the section. 

FIGURE 16.—Southward-facing seacliff exposure at Central Machete-Puerto Arroyo in the South Coast

ground-water province, Puerto Rico, showing an exhumed channel within Quaternary fan-delta plain.
34 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

Thin lenses of sand, used to separate discrete silt beds at

outcrop, were not easily recognized in cored samples. Smallscale fining-upward cycles probably formed in response to

flood deposition, such as aggradation or lateral accretion

within channels. Small-scale coarsening-upward cycles are

either associated with channel bar migration or are the result

of poor definition between thinner, discrete coarse-grained

beds. Silt bed thickness varies from less than 1 to 20 m;

thickest silt beds are generally present within the deep subsurface. Silt beds are associated with low-energy sheetflow

deposition in areas away from major trunk streams, in distal

parts of the fan, or in interfan areas. Some or many of these

beds do not represent a single flood, but are probably the

result of the stacking of deposits from several floods. 

Four separate, intermediate-scale depositional cycles that

range from 30 to 50 m thick can be delineated within the vertical section penetrated by test wells SC 4 (near Santa Isabel)

and SC 5 (near Boca Chica, Playa de Ponce). Two intermediate-scale depositional cycles were identified within the fandelta section penetrated by SC 2 (Salinas fan delta) and range

from 15 to 70 m thick. Coarsening-upward cycles, characterized by upward-thickening of coarse-grained beds and

upward-thinning of fine-grained beds, dominate the cored

section of SC 4 and SC 5. Thin marl, limy clay, and limestone beds separate several of the depositional cycles and can

indicate a minor period of marine onlap prior to continued fan

outbuilding. Fining-upward cycles, recognized by the

upward-thinning of coarse-grained beds, and the thickeningupward of fine-grained beds, dominate the SC 2 core. Only

one intermediate-scale cycle (SC 4 well) displays a gradual

coarsening upward followed by a fining-upward change contained within a single cycle.

Unlike grain size variations (small-scale cyclically) that

occur within discrete beds and reflect short-term depositional

(stream and floodflow) events, intermediate- and large-scale

cyclicity probably reflect overall progradational (or aggradational) and retrogradational fan activity. This fan activity was

controlled by a complex interaction of eustasy, climate, and

tectonics. The specific role of these contributory factors is

considered later.

Large-scale cyclicity, or the progressive change in bed

thickness throughout the entire penetrated section, differs

through the fan-delta plain. The SC 5 core shows an overall

coarsening-upward. The SC 4 core exhibits a similar coarsening-upward pattern in the lower two-thirds of the section,

but appears to fine-upward near the top of the section. The

SC 2 core exhibits a third pattern, in which the fan-delta section appears to fine-upward.

Sand and Gravel Percentage

Sand and gravel percentage maps show the spatial distribution of coarse- and fine-grained lithofacies within the fandelta plain. Sand and gravel percentage data are mostly based

on lithologic descriptions provided in drillers’ reports. The

majority of wells located in the South Coast ground-water

province and used as part of this analysis, were drilled prior

to 1980 mostly using a cable-tool percussion method. Drillers’ lithologic descriptions provided information concerning

the type of lithologic material, their sample depth and the

thickness of discrete beds of gravel, sand, silt and clay. The

amount of gravel plus sand at a given locality is expressed as

a percent of the total sedimentary section. The writer

assumed that the sand and gravel calculated for a partial

sequence could be assigned as being representative of the

entire undrilled section. However, most coastal area wells are

shallow and do not penetrate deeper parts of section; the character of these deeply-buried sediments is poorly known.

Accordingly, the percent of coarse-grained detritus is likely

over- or underestimated in these areas. Therefore, the areal

distribution is intended to illustrate the areal prevalence of

coarse-grained deposits in context of the conceptual depositional model previously described. 

A lithofacies map for the fan-delta plain shows that sand

and gravel deposits were concentrated as lobes and separated

from one another by interlobe areas of fine-grained detritus

(pl. 1D). The relative size of the different fan-delta lobes can

reflect differences in drainage basin size, topographic relief,

elevation, stream gradients, and possibly the type of rock that

underlies feeder basins. The Capitanejo, Coamo, Salinas, and

Guamani fan deltas contain large concentrations of coarse

detritus that extend considerably downfan. Small lobes are

associated with the Ríos Pastillo-Cañas, Cañas at Juana Díaz,

Descalabrado, and Jueyes; the Portugués-Bucaná, Seco, and

Arroyo sand and gravel lobe are moderate in size. 

The percentage of gravel plus sand in the different fan

deltas decreases in a downfan and downflow direction. The

percent lithofacies map indicates that highest concentrations

of coarse-grained (sand and gravel) sediment occur in proximal fan areas or within fluvial channels that reach inland

where flows are restricted and velocities are the greatest.

Within the proximal parts of the Salinas, Coamo, and Capitanejo fan deltas, sand and gravel deposits exceed 40 percent

of the entire section. Within the Río Cerrillos, one of two

streams that drains proximal parts of the Portugués-Bucaná

fan at Ponce, the percent of sand and gravel exceeds 60 percent. High sand and gravel percentages are also present

within the Río Jacaguas alluvial valley, one of two feeder

streams to the Capitanejo fan delta. High concentrations of

sand and gravel are locally found within the upland channel

of the Coamo fan delta.

Where channeled, surface water spreads out and covers

the midfan floodplain, floodwaters diminish in depth and

velocity, and coarse detritus is deposited as a lobe. In midfan

areas, the amount of coarse material diminishes to 20 to 40

percent of the sedimentary section. As floodwaters extend
GEOLOGY OF THE SOUTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 35

downfan, their bedload consists primarily of silt and clay. In

these downfan areas, the coarse fraction can represent less

than 20 percent of the fan-delta distal sequence.

The size, extent, and amount of sand and gravel within the

Salinas and Guamani fan-delta lobes suggest that distal subaerial fan-delta deposits lie buried in the offshore subsurface.

Well cuttings collected from a well in the Playa de Salinas area

indicate that subaerial deposits are buried beneath mud and

sand deposited within a transitional marine environment. Conversely, an erosional coastal marine scarp marks the point

where Holocene sea level rise has truncated the seaward part of

the Guamani fan delta.

Finger-like projections of coarse-grained detritus extend

southeast and southwest from the main channel and Salinas

fan-delta lobe; similar features extend from the Arroyo and

Jueyes fan delta. They are interpreted as paleostream channels

that fed fan segments that lie farther offshore and were active

during a lower sea level. Perhaps, these paleochannels initially

formed during a phase of fan incision and sea level decline,

and later aggraded during a eustatic rise.

The distribution of sand and gravel in the Capitanejo fandelta area is more complex than elsewhere within the fan-delta

plain. The lithofacies pattern here suggests that the Río Inabon

and Río Jacaguas fed separate fan deltas that coalesced and

now make up a single fan. To the south of the PortuguésBucaná fan, a buried sequence of fine, dark gray sand and

minor shell debris lies at depths less than 10 to 15 m below

land surface. The lateral extent of this sequence suggests that

it forms a narrow strip trending in an east-west direction that

parallels the modern-day coastline in the Playa de Ponce area.

These sandy beds are, in turn, underlain by dark gray to blue

organic clay and shell debris. Similar clay beds buried in the

subsurface also occur in a landward direction. Given their

proximity to a prograding beach that can be mapped at land

surface, it is likely that these beds were part of a former beach

and marsh area which bordered the coast.

It is difficult to characterize the distribution of coarsegrained detritus within fan-delta/valley-fill areas between Tallaboa and Guánica because of sparse well control (pl. 2C). It

appears that sand plus gravel-size detritus is concentrated

within the narrower confines of inland incised-valley channels.

Sand and gravel within the Guánica (Río Loco) and Río Yauco

Valleys ranges from less than 25 to more than 75 percent of the

vertical section (pl. 2C). In the Río Tallaboa area, sand and

gravel is present in more than 60 percent of the section in the

valley-fill area immediately north of the coastward low-lying

fan-delta plain; the amount of coarse-grained material diminishes to less than 20 percent within the fan-delta plain south of

the channelized area.

THE SOUTH COAST TERTIARY BASIN: CONTROLS 

ON CLASTIC DEPOSITION 

As discussed above, a significant part of the Oligocene to

Pliocene (?) sedimentary sequence and nearly the entire Quaternary sequence that lies onshore consists of clastic coarsegrained detrital sediments that were deposited as part of a

series of fan deltas or alluvial fans. A principal physiographic

requisite to fan-delta or alluvial-fan deposition is the cooccurrence of a highland area that immediately adjoins a lowland area (Denny, 1967, p. 83). Puerto Rico’s Cordillera Central highland area has served as the source for abundant coarsegrained detritus throughout Oligocene to Quaternary time. As

rivers exit the highland and streamflow moves outside the confines of the channel and onto the fan surface there is an abrupt

reduction in stream competence, which results in deposition of

bedload material.

Structural setting and associated tectonic movement is a

second factor considered important as a control to fan-delta

and alluvial-fan deposition. Accordingly, tectonics is often

emphasized in the fan-delta and alluvial-fan literature. Wescott

and Ethridge (1980) catalogued modern and ancient fan deltas within a tectonic-physiographic classification scheme.

They observed that fan deltas are, in part, recognized by their

close association and proximity to relatively young, high-relief

mountains that are commonly fault-bound on the proximal

margins of the fan deltas. Tectonic movement can increase the

gradient of streams, rejuvenate nearby highland sources of

clastic detritus (Blissenbach, 1954), shift sites of deposition

(Crowell, 1974), and control patterns of fan growth (Heward,

1978). Tectonic movement provides the initial relief required

for the formation of fans; continued movement creates basinal

conditions that are well-suited to the continued accumulation

and preservation of detritus.

Although the role of tectonically-induced base level change

should not be underestimated, other factors must also be considered. Fan formation can occur in response to rapid rates of

eustatic sea level rise. Sea level rise is often accompanied by a

eustatically-related change in climate (McGowen, 1970; Soegaard, 1990). A world-wide acceleration of fan growth

occurred in response to sea level rise and climate change during the Quaternary (Nilsen, 1982).

STRUCTURAL FEATURES AND EVIDENCE OF TECTONIC 

MOVEMENT IN THE SOUTH COAST GROUND-WATER PROVINCE

The Great Southern Puerto Rico fault zone is a principal

structural feature on the southern side of the island and contains a wide variety of fault structures. The fault zone is

formed by parallel to subparallel sinistral faults that strike

northwest and extend diagonally across the south-central part

of Puerto Rico. Glover (1971) postulated a fault zone extending westward to the central-west coastline, separating the central and southwestern blocks of the island. However, Glover’s
36 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

thoughts on the subject were made prior to much of the needed

mapping of western Puerto Rico, and the proposal of such a

continuous fault zone has been questioned; detailed delineation

of a southwestern block is not possible because of the widespread cover of younger Cretaceous and Tertiary rocks and

pervasive normal and strike-slip faults (Krushensky, written

commun., 1994). Oceanic rocks that lie southwest of the fault

zone were emplaced by an abducting plate during the middle

Eocene or even later (Krushensky and Schellekens, written

commun., 1994).

Gravity (Glover, 1971, p. 84) and seismic data (Garrison,

1969) indicate that the Great Southern Puerto Rico fault zone

extends beneath the fan-delta plain and the Isla Caja de Muertos shelf, possibly forming the south wall of the Whiting Basin

(Western Geophysical Company of America, 1974). As

many as 12 northwest striking sinistral faults have been

mapped in the insular highland areas west of Coamo (pls. 1A,

1E). The Santo Domingo, San Patricio, Paraiso, and Río

Jueyes faults, and the splay fault, Río Cañas Abajo, form the

northern margin of the fault zone. The Bartolomei, Lago Garzas, Portugués, and Machuelo sinistral faults form the southwestern margin of the fault zone in an area west of the fandelta plain. Northwest of Ponce, the width of the fault zone

that lies between the northernmost (San Patricio) and southernmost (Bartolomei) sinistral faults is more than 9-km wide. An

interpretative structure map of the bedrock surface that underlies the fan-delta plain (discussion follows) suggests the Great

Southern Puerto Rico fault zone narrows to a width of 3 km.

However, coastward parts of the fan-delta plain possibly conceal a much wider zone, but control given by wells is lacking.

The strike of the fault zone varies from N 54 ̊ W to N 55 ̊ W

where it crosses the Cordillera Central to the northwest and

west of Ponce (Glover, 1971; Krushensky and Monroe, 1975,

1978, 1979) to N 75 ̊ W where it underlies the fan-delta plain.

Low-angle thrust faults mapped within the fault zone occur

only within a narrow belt of Eocene to Paleocene age rocks

located in the highland areas immediately north of the fandelta plain and between the Ríos Coamo and Jacaguas (Glover

and Mattson, 1973). These faults have been interpreted as

(1) northward-moving gravity slide plates (Glover, 1971), (2)

southward-moving gravity glide plates (Krushensky and Monroe, 1975), (3) part of a transgressive flower structure (Erickson and others, 1990), or (4) formed by compressional thrust

movement that preceded sinistral movement (Erickson and

others, 1990). High-angle cross faults that strike northwest

have been mapped throughout the highland areas north and

northwest of Ponce, the majority of which do not crosscut sinistral faults. West and north of Ponce, normal faults strike

east-west and vertically displace the Juana Díaz Formation and

the Ponce Limestone.

Glover (1971) thought first sinistral movement occurred

during the Early Cretaceous, with later episodes of movement

occurring during Maestrichtian and Eocene time. McIntyre

(1971, 1975) suggests that inception of the Great Southern

Puerto Rico fault zone was a post-middle Eocene event.

Krushensky and Monroe (1975, 1978) constrained sinistral

movement to Eocene to Oligocene time with horizontal movement continuing along the Lago Garzas and Bartolomei faults

as late as the early Miocene. Sinistral movement along the

Lago Garzas and Bartolomei faults could possibly correspond

with the development of the large structural and depositional

basin that underlies the fan-delta plain between Ponce and

Santa Isabel (figs. 9, 10, and discussion below). Other faults

strike east-west, have a normal slip component, and displace

rocks of the Ponce Limestone and Juana Díaz Formation that

are exposed west and north of Ponce. These latter faults probably postdate sinistral movement. 

Whereas rapid rates of denudation by fluvial erosion and

mass wastage processes help mask structural relations in many

parts of Puerto Rico, other lines of geomorphic evidence support ancient and more recent structural movement. For example, closely aligned karstic features in the North Coast

limestone ground-water province nearly parallel joint and fracture patterns of older, underlying volcaniclastic rocks (Tobisch

and Turner, 1971). Fault control of mountain areas has also

been inferred by the apparent alignment of faults with linear

valleys and scarps (Kaye, 1959a, p. 51; Pease, 1968; Rodgers,

1977; M’Gonigle, 1978). North of the Salinas fan-delta,

streams parallel fractures, joints, and faults (Berryhill and

Glover, 1960) and in highland areas of the Cordillera Central,

streams often exhibit a drainage pattern that seems to parallel

the orientation of faults within the Great Southern Puerto Rico

fault zone. Snow (1993) has noted fault control along some

segments of several drainage divides in the interior mountains.

Recent tectonic movement within the Ponce Basin is suggested

by linear bathymetric scarps that conform with the position of

Bajo Tasmanian fault zone and Caja de Muertos Fault (Garrison, 1969). Seaward tilting of the Isla Caja de Muertos shelf

during Quaternary time has caused the drowning of some reefs

(Beach and Trumbull, 1981).

The configuration of the bedrock surface that underlies

Puerto Rico’s southern fan-delta plain (pl. 1B) appears to parallel structural and geomorphic features in the highland areas.

The distinctive pattern of bedrock low and high areas that

underlie the eastern half of the fan-delta plain (“buried valleys

and ridges” of McClymonds and Ward, 1966) suggests that

fault movement or fracturing, or both, controlled pre-Quaternary paleotopography in southern Puerto Rico. Bedrock low

areas probably correspond to sites that have been subject to

greater rates of mechanical and chemical subaerial erosion.

However, differential rates of pre-fan erosion could have

resulted in topographic inversion; that is, structural and paleotopographic “highs and lows” do not necessarily correspond. 

It is suggested here that the pattern of buried bedrock

highs and lows are relict features of a braided sinistral fault

system. Horst and graben fault blocks, formed by movement
GEOLOGY OF THE SOUTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 37

along the Great Southern Puerto Rico fault zone, were subjected to subaerial erosion, then buried and preserved as a

series of paleotopographic ridges and valleys. The position

and extent of these buried faults were largely inferred by the

sharp breaks in bedrock slope (compare pls. 1B and 1E).

Most inferred fracture or faults strike northwest, northeast, or

east-west. 

A distinctive change in bedrock slope caused by faulting

occurs in the western half of the fan-delta plain (pls. 1B, 1E).

Two faults border the margins of a large graben and delimit

the landward border of the Ponce Basin. The west-northwest

oriented fault (N 70 ̊ W to N 80 ̊ W) is remarkably linear and

extends 21 km as a single trace from Aguilita to Bahía de

Rincón exhibiting dip-slip movement to the south. Rocks of

Miocene to Oligocene age have been vertically displaced by

as much as 50 m near Aguilita but more than 200 m where the

fault crosses the Bahía de Rincon coastline. This northwest

fault trace is well-aligned with the sinistral Portugués fault,

but concealed by unfaulted strata of the Juana Díaz Formation

and Ponce Limestone that crop out near Ponce. Sinistral

movement changing to down-to-the-south and southwest normal movement also occurs along the San Patricio fault, horizontally juxtaposing rocks of Oligocene and early Tertiary age

(Krushensky and Monroe, 1975). If outcrop relations are correctly illustrated (Krushensky and Monroe, 1975), latest horizontal movement along this buried hinge line occurred prior

to, with latest movement during, Oligocene time. The

throughgoing linearity and steeply vertical nature of this

northwest-oriented buried hinge line, and its parallel alignment with other northwest-oriented, sinistral faults suggests it

too was a former sinistral fault. Perhaps, this fault contained a

normal component or was reactivated by normal movement.

Sinistral movement along the Bartolomei fault, located southwest of the Portugués fault, could have occurred during early

Miocene time (Krushensky and Monroe, 1978), but on the

basis of available well data its extension beneath the fan-delta

plain can not be demonstrated.

A second set of lineaments that underlie the fan-delta plain

strike N 40 ̊ E to N 60 ̊ E and parallel some faults mapped in

the highland areas north of the fan-delta plain (Glover, 1971;

Krushensky and Monroe, 1975, 1978, 1979) as well as several

other faults that displace rocks beneath the Isla Caja de Muertos shelf (Garrison, 1969; Beach and Trumbull, 1981). It is

impossible to accurately determine crosscutting relations

between various faults that underlie the fan-delta plain; some

northeast-oriented faults probably formed in response to sinistral movement, whereas others could reflect a later phase of

normal movement. For example, a buried northeast-striking

fault trace (pl. 1E) is shown near Aguilita to truncate a northwest-oriented hinge line that together delimit the landward

margin of the Ponce Basin. This suggests that movement of

the northeast-striking fault trace postdates movement along

the northwest hinge line. 

The third buried fault or lineament orientation located

south of Ponce also exhibits down-to-the-south normal movement. These east-west buried faults parallel other normal

faults that displace the Juana Díaz Formation and Ponce

Limestone west of the fan-delta plain (Krushensky and Monroe, 1975, 1978, 1979). These east-west buried faults also

parallel normal faults that have been mapped offshore along

the Isla Caja de Muertos shelf (Garrison, 1969; Western Geophysical Company of America, 1974; Meyerhoff and others,

1983).

A strike-slip structural model is postulated here to explain

Oligocene depositional patterns (fig. 9) and is partly based on

the fault pattern inferred for the buried part of the Great

Southern Puerto Rico fault zone that lies east and southeast of

Ponce. Two southeast-striking sinistral faults, assumed to be

coincident with northeastern and southwestern margins of the

fault zone, diverge and form a large wedge-shaped basin.

Uplift and convergence of the two sinistral faults at their

wedge-tip coincides with areas containing conglomeratic

detritus of the basal Juana Díaz Formation. Carbonate rocks,

thickest between Ponce and Bahía de Guánica, form a northeast-southwest oriented band. The thickest reef deposits accumulated along a shelf hinge line, formed along a fault striking

northeast-southwest with down-to-the-southeast normal

movement. Deep-water deposition occurred to the southeast

within a subsiding basin. Fault and outcrop relations indicate

last sinistral movement within the Great Southern Puerto Rico

fault zone occurred along the Lago Garzas-Bartolomei faults

during early Miocene time (Krushensky and Monroe, 1978).

Strike-slip movement was succeeded by normal fault

movement during middle to late Miocene time, possibly along

a former sinistral fault. Subsidence or uplift, or both, in the

Ponce-Santa Isabel area was substantial. The clastic and

minor limestone sequence that infills the large graben structure from Santa Isabel eastward exceeds 800 m in thickness

(fig. 11). Denning (1955) estimated that as much as 1,100 m

of vertical displacement occurred along one of two faults that

form the Ponce Basin, a graben structure identified in Isla

Caja de Muertos shelf bathymetry (Beach and Trumbull,

1981). Post-early Pliocene movement can be inferred by the

occurrence of normal faults which displace the Ponce Limestone and locally reverse the southward-dip of beds to the

north.

Elevated marine terrace data collected in the northwestern

part of Puerto Rico show that the island was subject to slow

insular uplift during the Quaternary. As much as 50 to 90 m of

insular uplift has occurred since the onset of the Pleistocene

(Taggart and Joyce, 1990, Taggart, 1992). If Moore’s (1982)

eustatic curves correctly depict present seas at their highest

level in the last 120,000 years, Taggart and Joyce’s (1990)

assumption of an average rate of uplift (.03 to .05 mm/yr)

might not be correct, nor can this assumed average rate be

applied to the south coast. A relict marine shoreline near
38 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

Bahía de Rincón lies 2 to 3 m above sea level. A late

Holocene uplift rate of 1.8 mm/yr (Geomatrix Consultants,

1988) or episodic movement on the south coast is suggested if

one can correctly correlate the Bahía de Rincon relict shoreline with the 1,500 to 3,000 year-old marine terrace in northwestern Puerto Rico.

CHANGES IN BASE LEVEL: ITS RECORD IN THE 

STRATIGRAPHIC SEQUENCE

Clastic, basal conglomeratic deposits of the Juana Díaz

Formation, considered coeval with the planktonic foraminifera zone, Globergerina ampliapertura (Moussa and Seiglie,

1970; Frost and others, 1983), were deposited during early

Oligocene time. Conglomeratic alluvial-fan or fan-delta

deposits were probably deposited in direct response to a low

base level. If eustatically high seas prevailed during early Oligocene time (Haq and others, 1988), deposition of conglomeratic fan-delta deposits probably reflect tectonic movement

(lowering base level), possibly associated with sinistral movement along the Great Southern Puerto Rico fault zone

(fig. 17). Limestone strata of the Guánica and Peñuelas area

are considered coeval with the Globergina opima opima Zone

and exhibit periods of reef growth interrupted by episodes of

reef destruction; destructive events have been attributed to tectonic uplift or subsidence of the shelf margin, or both (Frost

and others, 1983, p. 39). The influx of clastics and rapid subsidence in the Ponce-Juana Díaz area during late-early Oligocene time is indicated by a deep shelf facies of sandy shale

and minor limestone beds that directly overlie conglomeratic

beds. 

Moussa and Seiglie (1970) correlated shallow-water limestone strata in areas west of Río Tallaboa with the Globergerina ciperoensis ciperoensis Zone, whereas Frost and others

(1983) believed that reef deposition in the western part of the

basin terminated by this time. Pelagic mudstone and turbidite

beds overlie reef-shelf deposits in the Guayanilla area indicating rapid shift from shallow marine, reef-shelf deposition to

deep water, pelagic deposition. Subsidence within the basin

probably continued to control pelagic conditions as global sea

level was relatively low during late Oligocene (Haq and others, 1988). An erosion surface and unconformity spans most,

if not all, of the Globergerina ciperoensis ciperoensis Zone

(Frost and others, 1983, p. 58, 61, 76) and separates the slope

facies of late Oligocene age from overlying slope chalk beds

(“Angola limestone” and “Río Tallaboa marlstone” of Moussa

and Seiglie, 1970, 1975) of early Miocene age. Frost and others (1983, p. 61) believed this unconformity reflected uplift

and subaerial erosion during the latest Oligocene that was followed by subsidence and flooding in the early Miocene. Submarine erosion or a submarine gravity slide of the late

Oligocene slope might be an alternative explanation to the

occurrence of “slope-on-slope” deposits. If the basin experienced a fluctuation in relative sea level greater than 100 m,

one might expect deposition of a reef or shallow shelf

sequence that would separate the two different slope

sequences. An erosive submarine event would eliminate the

need for a 100 m or greater eustatic or tectonic change in base

level to explain the late Oligocene-early Miocene hiatus. 

Bathyal submergence and deposition of the island slope

carbonate rocks is recorded during earliest Miocene time

(Globergerina kugleri Zone), a period of high global sea levels. Moussa and Seiglie (1970, p. 1893) implied an unconformity separated the chalky unnamed pelagic deposits into two

separate units (“Angola Limestone” and “Tallaboa marlstone”), whereas Frost and others (1983, p. 62) indicate that

submergence probably continued without interruption

throughout early Miocene time. Uplift and a period of subaerial erosion are suggested by an unconformity that spans the

early-middle Miocene. This hiatus separates uppermost

island-slope pelagic chalk beds from overlying strata of the

Ponce Limestone (Moussa and Seiglie, 1970; Frost and others, 1983).

The Ponce Limestone consists of a shallow-water carbonate sequence that was largely deposited during middle early

Miocene to early Pliocene time. The Ponce Limestone grades

eastward to a conglomeratic facies that underlies the western

part of the fan-delta plain. This clastic facies could be, in part,

equivalent to paralic crossbedded sand and gravelly channelfill deposits (Monroe, 1980, p. 73; Moussa and Seiglie, 1975,

p. 166) that crop out in a restricted area west of Ponce. Shallow-water and some deeper shelf environments contained

within the Ponce Limestone sequence indicate that basin subsidence could have, at times, exceeded eustatic decline. The

more than 500-m thick clastic sequence of Miocene age near

Santa Isabel indicates this area was subject to a rapid rate of

subsidence or uplift, or both, in the adjoining highland areas.

Moussa and Seiglie (1975, p. 168) and Frost and others

(1983, p. 10) consider lower beds of the Ponce Limestone to

be early middle Miocene age. Frost and others (1983) correlate coral assemblages contained within these strata with

Globorotalia fohsi fohsi Zone of early middle Miocene age.

Bold (1969) correlated uppermost strata of the Ponce “Formation” with the Globorotalia margaritae Zone on the basis

of ostacode and bethonic foraminiferal assemblages. Under

current USGS usage (Monroe (1980, p. 82), the Ponce Limestone is considered to be late Miocene in age. However, the

Globorotalia margaritae foraminiferal zone is currently considered to range from latest Miocene to early Pliocene age

(Haq and others, 1988). Therefore, uppermost beds of the

Ponce could be younger than the late Miocene age suggested

by Monroe (1980). The unconformity that marks the upper

surface of the Ponce Limestone probably spans late Pliocene

time, a period of subaerial exposure, declining sea levels, and

possibly tectonic uplift. 

In summary, changes in base level during Oligocene to

early Pliocene time and clastic-carbonate deposition appear
GEOLOGY OF THE SOUTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 39

Planktonic

foram

biochronozones

Lithostratigraphic

units

Relative sea level

in meters

100 50 0

EPOCH

HOLOCENE

PLEISTOCENE G.

truncatulinoides

Alluvium

fan-delta deposits

Ponce to

Patillas

area

LATE

PLIOCENE

EARLY G.

margaritae

P. obliquiloculata

LATE

G.

acostaensis

MIOCENE

MIDDLE EARLY LATE EARLY

OLIGOCENE

G. menardii

G. siakensis

G. fohsi robusta

G. fohsi lobata

G. fohsi fohsi

G. peripheroronda

P. glomerosa

G. insueta

G. stainforthi

C. dissimius

G. kugleri

G.

ciperoensis

ciperoensis

G.

opimaopima

G. ampliapetura

G. chipolensis

-P. micra

Ponce to Cabo Rojo area

Guayanilla to Ponce

area

Santa Isabel area

Ponce

Limestone

Clastic

deposits

Unnamed

carbonate

rock

Juana Diaz

formation

area

? ? ?

`

Ponce – Juana Díaz area Guánica

Gu

a

a

re

yanilla

a

(Haq and others, 1988)

FIGURE 17.—Chart of major biochronozones and lithostratigraphic units of late Tertiary and Quaternary age 

in the South Coast ground-water province and estimates of relative sea level.
40 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

to have been controlled by tectonic movement and eustacy.

Of the two variables, however, tectonic movement seems to

represent a more important factor in terms of late Tertiary

clastic and carbonate deposition. 

Change in eustatic base level was the primary control on

coarse-grained clastic deposition in southern Puerto Rico during the Quaternary with tectonic movement playing a secondary role. Sea level rose as much as 130 m during the last

glacial maxima (18,000 years before present [yr BP]) (Chappell and Shackleton, 1986; Fairbanks, 1989) with eustatic

variations of similar magnitude occurring during the early

Pleistocene and Pliocene time (Haq and others, 1988). 

Study of the Pleistocene (?) to Holocene vertical sequence

as seen at outcrop and on the basis of deep well control (pl. 3)

provides considerable insight regarding short- and long-term

depositional behavior of the South Coast fan deltas. Smallscale fining- and coarsening-upward cycles that occur within

individual beds, or a series of beds (1 to 20 m thick), are

probably related to specific short-term hydrologic events.

Longer-term depositional behavior is probably related to fluctuations in base level; these changes are exhibited within the

intermediate (15 to 70 m) and large-scale (400 to 500 m)

cycles. To understand the relation between intermediatescale and large-scale depositional cycles within the South

Coast fan-delta plain, one must assume that eustatic change

in base level during Quaternary occurred at a greater frequency and was larger in magnitude than base level change

associated with tectonic movement.

The stacked series of intermediate-scale, coarseningupward (aggradation) and fining-upward (retrogradation)

cycles defined within three continuously-cored, deep test

wells (pl. 3) reflect fan-delta deposition in response to the relative change in base level. This change in relative base level

includes two principal components, change in base level due

to eustacy and change in base level due to tectonics. Tectonic

base level change is further complicated in that it includes

uplift and subsidence that, in some cases, could have occurred

simultaneously within different parts of the fan-delta plain.

This eustatic-tectonic balance could explain markedly different depositional responses within the western and eastern

parts of the fan-delta plain, namely, aggradation that occurs

within landward parts of the slowly subsiding Ponce Basin

area, and retrogradation that occurs on the adjoining, structural block to the east that is slowly being uplifted. Two deep

wells (SC 4 and SC 5) that penetrate the western half of the

fan-delta plain lie well within the Ponce basin; the easternmost well (SC 2) is located in the upthrown side of a fault

block that defines the northeastern side of the basin. If intermediate-scale depositional cycles of the Capitanejo, Portugués-Bucaná, Coamo, and Salinas fans are correctly

correlated, it is apparent that fans located in the western part

of the south coast actively aggraded or even prograded during

sea level rise. This eustatically-induced sedimentation process, characterized by the intermediate-scale, thickening

upward of coarse-grained beds, is analogous to deposition

that occurs in a reservoir following closure of a dam (Leopold

and others, 1964, p. 259). The locus of coarse-grained deposition during lowstand periods probably remained in close

proximity to the mountain front or fault line, or both, during

each successive lowstand period. Conversely, the eastern part

of the fan-delta plain was slowly being uplifted, and the Salinas fan (and other eastern fans) were gradually being abandoned as the locus of coarse-grained deposition shifted

seaward. This is supported by a progressive, upward-decrease

in the thickness of coarse-grained beds (pl. 3) and a lithofacies map (pl. 1D) that indicates buried channels fed fan segments that were probably located in offshore areas. Although

large-scale depositional cycles within the Ponce Basin parallel intermediate-scale behavior, this 400 to 500 m scale probably reflects the long-term structural movement; namely,

subsidence within the Ponce Basin. Alternately, long-term

cyclicity (fining upward) on the eastern edge of the basin suggests gradual fan abandonment as the highland structural

block was slowly uplifted.

Two geomorphic features of the fan-delta plain that further illustrate the relation between Quaternary eustacy and

uplift include multilevel fan-head terraces and the raised

relict shoreline at Bahía de Rincón. The raised shoreline may

reflect a lower base level that existed during the late Holocene

eustatic rise in sea level. If uplift along the south coast was at

a slower rate than eustatic rise, the presence of 1,500- to

3,000-year-old relict shoreline seems, at first glance, contradictory. A possible explanation is that base level along the

south coast of Puerto Rico has lowered due to an increase in

the rate of uplift approximately 1,500 to 3,000 years ago. An

alternative explanation is that the rate of uplift did not significantly increase, but could have exceeded the rate of sea level

rise. The rate of eustatic change is sinusoidal and as seas

approach their interglacial maxima, the rate of change diminishes. Another possible explanation is that Holocene eustatic

sea level has been higher than the present level (Stapor and

Matthews, 1983; Shackleton, 1987; Stapor and others, 1991;

Taggart, 1992), a view not considered to be resolved by all

workers (Kidson, 1982; Lajoie, 1986, p. 102; Bloom and

Yonekura, 1990, p. 111).

Fan-head terraces (pl. 1A) are interpreted to record shortterm fluctuations in sea level. Fan-head trenches probably

were incised during interglacial highstands; diminished

source area relief was accompanied by an interglacial wet climate, increased vegetation cover that stabilized slopes, and a

reduction in fluvial bedload. Multiple terrace levels within

apex of different fan-deltas reflect different episodes of

interglacial fan-head incision.

Bathymetric data provide additional insight to the change

in Quaternary eustatic base level. Fringing reefs separate a

relatively shallow, inner shelf and a deeper, outer shelf. A
GEOLOGY OF THE WEST COAST GROUND-WATER PROVINCE OF PUERTO RICO 41

submerged reef borders the Isla Caja de Muertos shelf and

slope edge and records a relict lowstand (Beach and Trumbull, 1991). Submerged early Holocene or late Pleistocene

reefs located on the shelf edge are common to other eastern

Caribbean islands; during later stages of Holocene sea-level

rise, reefs were adjacent to deep ocean water but were unable

to keep pace with the rising sea level (Macintyre, 1972)

(fig. 18). Fan-like nearshore bathymetry and sand percentage

data also indicate that these fans extended further seaward

and that the Caribbean Sea has subsequently transgressed

coastward parts of Puerto Rico’s fan-delta plain. Terrigenous

deposition has not kept pace with the eustatic rise in sea level,

despite pulses of sediment resulting from episodic, catastrophic floods. Thus, it appears that fan deltas of southern

Puerto Rico have entered a period of relative quiescence

(fig. 19). Wave energy, longshore currents, and carbonate

sedimentation are the principal processes operating during

sea-level highstands; these processes have reshaped the coastline morphology and promoted the longshore transport of

coarse-grained detritus. Beach progradation in the Playa de

Ponce area is the direct result of longshore transport of clastic

detritus deposited within a slowly subsiding basin. The

buildup of carbonate-dominated detritus on the inner Isla

Caja de Muertos shelf occurred largely in response to the

Holocene rise in sea level combined with lessened fan activity, clastic sediment input, and sediment stress.

CLIMATE

Climate represents the third factor influencing coarsegrained deposition on the south coast. Temporal and spatial

variability of moisture and temperature in the Caribbean and

Central America region during Pleistocene and Holocene

time has been the subject of considerable study and speculation (Douglas and Spencer, 1985). It is theorized that the

southern Caribbean-South American region experienced a

significant reduction in rainfall (Damuth and Fairbridge,

1970) and an expansion of coastal land areas during glacial

lowstands. These dry periods alternated with humid, interglacial periods accompanied by a reduction in land area (Tricart,

1985). A similar reduction in rainfall has also been reported

along the western and southern (Tricart, 1985) Caribbean

region during the most recent glacial lowstand (18,000 yrs

BP) (Street-Perrott and others, 1985). Eustatic lowstand periods within northernmost South America were accompanied

by fewer tropical storms and easterly rains, and cooler, drier

winds (Damuth and Fairbanks, 1970). It seems plausible that

the rain shadow effect caused by Puerto Rico’s central highlands and leeward aridity could have been further accentuated. 

In Puerto Rico, evidence for Pleistocene climate change is

supported by the occurrence of eolianite deposits that are preserved on the northern coastal margin. Coastal sand dunes

were active during glacial lowstands on an expanded inner

shelf covered by sand and exposed to cool, dry wind. Discrete eolianite beds are separated by paleosols that developed

during the wet, interglacial periods subject to milder winds,

warmer temperatures, and humid conditions that enhanced

dune stabilization by vegetation (Kaye, 1959a). Quaternary

climate change in Puerto Rico’s westernmost satellite island,

Isla Mona, has been inferred on the basis of cemented red

residuum and the solution development of caverns (Kaye,

1959b, p. 173). Cemented red residuum indicates the island

was subjected to a glacial period of greater aridity or smaller

average rate of precipitation. Remnants of the residuum

occur on cavern walls and suggest that the caves, once filled

with guano, formed earlier in response to greater precipitation and meteroric dissolution, presumably during an interglacial Pleistocene highstand. 

The type and distribution of vegetation on the leeward

side of Puerto Rico during sea level lowstands is unknown.

Assuming Puerto Rico experienced a period of tropical aridity during glacial lowstands, vegetative cover on the fan-delta

plain and lower mountain slopes could have been comparatively sparse. It also seems likely that a dry forest, common

to lower foothill slopes in Puerto Rico before colonial deforestation, could have extended even further upslope and was

accompanied by a reduction in the size of mountain rain forests. 

Fan deposition is likely during periods of high runoff and

sediment yield. In southern Puerto Rico, such conditions correspond to the period of time immediately following a glacial

maximum. If vegetation was sparse, it could not effectively

protect the steep mountain slopes from erosion of chemicallyweakened saprolitic soil. These soils formed during more

humid, interglacial periods were subject to mechanical erosion as precipitation increased in response to a change from a

dry, glacial to wetter, interglacial climate. Following geomorphic models proposed by Schumm (1968) and Street-Perrott and others (1985), the transition period phase to greater

humidity and more frequent rainfall during eustatic recovery

probably represented the most effective period of runoff and

sediment yield in southern Puerto Rico. As the recovery

period continued, the increased amount and frequency of

rainfall was accompanied by an expansion of vegetative cover

that helped reduce the sediment yield to streams and eventually resulted in fan-head dissection.

GEOLOGY OF THE WEST COAST GROUNDWATER PROVINCE OF PUERTO RICO

Located in the southeastern part of the island (pl. 2), Lajas

valley forms a broad (6 km), elongate (35 m), lowland valley

whose floor rises no more than 50 m above sea level. The

Sierra Bermeja Mountains border the southern side of the val-
42 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS                



            





Santa Isabel 

Salinas

Patillas

JuanaDíaz

Ponce

APPROXIMATE

SEAWA

R

D

L

MI TI OF

TERR GI ENO

U

S

DEPO

SIT

OI N

Reef

Shallow water carbonate deposition

Reef Reef

EDGE OF ALSI

CAJA

D

E

MUER OT S SHELF

FIGURE 18.—Fan-delta deposition in south-central Puerto Rico during late Pleistocene-early Holocene sea level rise.

ATLANTIC OCEAN

CARIBBEAN SEA

IslaMona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

Fan delta

EXPLANATION

Interfan or alluvial plain areaShallow marine depositsExtent of modern-day coastal plainLine showing principal facies— Dashed where inferred



0 10 KILOMETERS 50 5 MILES

66°37'30" 66°30' 66°22'30" 66°15' 66°07'30" 66°00'

18°00'

17°52'30"

Base modified from U.S. Geological Survey digital data
GEOLOGY OF THE WEST COAST GROUND-WATER PROVINCE OF PUERTO RICO 43 Santa Isabel Salinas Patillas

JuanaDíaz

Ponce

Active carbonate deposition

EDGE OF

ALSI CAJA D E MUER

OT S SHELF

Infrequent catastrophic flooding

Patch and

fringing reefs

Coastal progradation Longshore deposition

Basin subsidence (Late Pleistocene to Recent)

Relict shelf

Coastal erosion Coastal erosion Longshore deposition

M i n o r f a n a c t i v i t y a n d f l u v i a l d e g r a d a t i o n

D r o w n e d r e e f s

Drowned reefs

Fring ni g reefs

FIGURE 19.—Physical processes within the fan-delta plain of south-central Puerto Rico following the last Holocene transgression.

ATLANTIC OCEAN

CARIBBEAN SEA

IslaMona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

0 10 KILOMETERS 50 5 MILES

66°37'30" 66°30' 66°22'30" 66°15' 66°07'30" 66°00'

18°00'

17°52'30"

Base modified from U.S. Geological Survey digital data
44 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

ley and form a low foothill ridge with a maximum altitude of

246 m; the northern side of the valley is flanked by dissected

hills and ridges that rise to altitudes of 200 m. Lajas Valley,

open at its eastern and western ends, drains to Guánica valley

and Bahía de Boqueron, respectively. Ephermeral streams

drain from the north and south and once fed three large

lagoons. The two easternmost lagoons have been drained for

agricultural purposes, while Laguna Cartegena in the western

part of the valley is maintained as a wildlife refuge.

Upland areas adjacent to Lajas Valley are underlain by

Cretaceous and Tertiary volcanic and volcaniclastic bedrock

and minor limestone beds, Cretaceous to Jurassic serpentinite, amphibolite, and siliceous chert. However, the distribution and character of bedrock units that underlie the clayey

alluvium of the central valley is poorly known, largely due to

scattered well control. Limestone strata that represent potential aquifers underlie the central part of the valley and are

possibly equivalent to irregularly-distributed sequences of

Cretaceous limestone strata exposed along nearby foothills.

West of Laguna Cartegena, clayey alluvium overlies a carbonate-clastic sequence that could be equivalent to nearby

outliers of the Juana Díaz Formation and Ponce Limestone.

Alluvial deposits of Lajas valley range are 60 to 90 m

thick and consist mostly of clayey silt, interspersed with

stringers of sand. Deposits of sand and gravel are restricted

largely to several small alluvial fans located on the perimeter

of the valley. Clayey silt underlying the central part of the

valley is distal fan or lacustrine in origin. 

The Añasco Valley (39 km2) (figs. 2, 3) and the Guanajibo

Valley (34 km2) are among the largest alluvial valleys of the

West Coast ground-water province. The Añasco Valley,

drained by two streams, is formed by a low-lying alluvial

plain having a maximum altitude of 15 m above sea level and

is surrounded on three sides by hills and mountains. The

east-southeast trending Guanajibo alluvial valley lies south of

the Añasco Valley and north of Lajas Valley. The 22-km-long

valley is 1 to 4.5 km wide and is drained principally by Río

Guanajibo. Two small alluvial fans border the north side of

the valley, formed and fed by tributary streams.

The vertical sequence of stratigraphic units within the

Guanajibo and Añasco Valleys suggest that both contain a

similar geology. In both valley areas unconsolidated alluvial

to marginal marine deposits are underlain by clay and limestone of Pleistocene(?) age that, in turn, unconformably overlie volcaniclastic bedrock. Alluvial deposits in the Añasco

Valley consist of clay, minor sand, and occasional channel

gravel; clastic nonmarine deposits grade coastward to beach,

dune and marsh deposits of clay, silt, sand and skeletal carbonate debris. Alluvial deposits are locally 45 m thick with

coarse-grained detritus forming 15 to 30 m-thick beds. A 75

m-thick zone of hard, dense clay interlayered with 15 m-thick,

soft limestone beds of Pleistocene(?) age underlies alluvial

deposits of the Añasco Valley; the underlying bedrock floor

occurs at depths of 100 m below sea level (Díaz and Jordan,

1987). In the Guanajibo Valley, alluvial deposits consist of

sand, gravel, and clay reported to be, in some places, 30 m

thick. An approximately 50 m-thick limestone unit of Pleistocene(?) age separates alluvial deposits, and unconformably

overlies volcaniclastic and volcanic bedrock.

GEOLOGY OF THE EAST COAST GROUNDWATER PROVINCE OF PUERTO RICO

Ridges and mountains of Puerto Rico’s East Coast

ground-water province (figs. 2, 3) are flanked by a discontinuous plain of alluvial fan fanglomerates and floodplain alluvium that infill several incised stream valleys and grade

coastward to lagoonal, swamp, and beach deposits. These

valleys are, in turn, underlain by Cretaceous volcaniclastics

and Tertiary intrusive and extrusive volcanic rocks akin to the

adjoining highlands. The alignment of the alluvial lowland

areas and intervening highland ridges indicates that bedrock

topography is probably structurally-controlled. Upland

stream linearity, trellis or rectilinear drainage patterns, shape

of the alluviated valleys, and the configuration of buried bedrock surfaces all strongly reflect joint and fault control.

Alluvial floodplain deposits of unconsolidated, poorly

sorted to moderately well-sorted, thick bedded sand, gravel,

and clay form a 35-m-thick Piedmont plain in the Río Fajardo

valley (Briggs and Aguilar-Cortes, 1980). Sandy lime mud

and organic clay deposited within a mangrove swamp and

pebbly, coarse to fine-grained carbonate and terrigenous sand

separate alluvial plain deposits from Sonda de Vieques.

Twenty-five m-thick fanglomerate deposits of poorly- to wellsorted, clay to boulder-size detritus dominates much of the

coastal area between the Río Fajardo floodplain and Naguabo

(M’Gonigle, 1978). 

The Humacao-Naguabo alluvial area encompasses

approximately 39 km2 (figs. 2, 3). Infilled with clay, silt, and

sand deposits, the Humacao alluvial valley is formed as a

broad triangular-shaped valley and plain that widens coastward. Fanglomerate deposits fringe the edge of the valley

with coastal marsh and beach deposits lining the coastline.

The thickness of alluvial deposits can exceed 50 m near the

coast (Graves, 1989).

The Yabucoa Valley (31 km2) is a broad, 23-km-wide by

33-km-long valley incised into the granodiorite of the San

Lorenzo Batholith (figs. 2, 3). Boulder- to clay-size alluvial

fan fanglomerates dominate the landward margin of the valley, but in the central part of the valley these sediments grade

to alluvial plain deposits of stratified, poorly consolidated

sand, silt, clay, with scattered pebbles and boulders. The

thickness of the alluvial deposits is reported to be as much as

100 m, but usually averages less than 50 m near the coast

(Robison and Anders, 1973; Rodgers, 1977). The Río Maun-
GEOLOGY OF THE INTERIOR GROUND-WATER PROVINCE OF PUERTO RICO 45

abo alluvial valley (8 km2) (figs. 2, 3) is also incised into the

granodioritic rocks of the San Lorenzo Batholith and most

valley fill consists of fanglomerate and alluvial plain deposits

of clay- to boulder-size detritus. The thickness of these

deposits can locally exceed 60 m (Adolphson and others,

1977).

GEOLOGY OF THE INTERIOR GROUNDWATER PROVINCE OF PUERTO RICO

The Interior ground-water province of Puerto Rico forms

the bedrock core of the island and is underlain by a deformed

and faulted sequence of volcanic, volcaniclastic, plutonic, and

sedimentary rocks of Jurassic to early Tertiary age (fig. 3).

Two large batholiths of granodiorite of Late Cretaceous-early

Tertiary age occur in the eastern and western parts of the Interior ground-water province. Volcanic, volcaniclastic, and

other sedimentary rocks that underlie the eastern half of the

Interior ground-water province are mostly Early and Late Cretaceous in age. Except for the Bermeja Complex, volcanic

and sedimentary rocks in the western half of the Interior

ground-water province mostly range in age from Late Cretaceous to Eocene age. The oldest rocks identified in Puerto

Rico are part of the ophiolitic Bermeja Complex in southwestern Puerto Rico and range from Jurassic to Cretaceous age.

Primary porosity of the rocks of the Interior ground-water

province was destroyed by processes of compaction, diagenesis, and tectonic movement. Consolidated rocks of the Interior ground-water province do not represent a regionally

important source of water. However, they are capable of

yielding small amounts of water to wells and function as a

local source of water through fractures, joints, faults and the

weathered-bedrock mantle that lies within the upper 15 to

90 m of the surface. 

Alluvium is the principal aquifer in Puerto Rico’s Interior

ground-water province, as it is along the coast. The most

extensive deposits are present within the 91 km2 Caguas-Juncos Valley in the east-central part of the island. Located

within the upper part of the Río Grande de Loiza Basin, the

alluvial valley is divided into two subareas (Puig and

Rodríguez, 1993); the circular Caguas Valley with a 6.4 km

diameter lies immediately southwest of the narrow, 0.8- to

2.4-km-wide, elongate Juncos Valley that extends east-west 19

km (fig. 20).

Alluvial deposits within the Caguas-Juncos Valley consist

of boulder- to clay-size detritus and range from 10 to 50 m in

thickness (Puig and Rodríguez, 1993). The thickness of the

clay and silt-dominated deposits of the Caguas Valley increase

in the center of the basin, locally exceeding 50 m; alluvial

deposits within the Juncos Valley are dominated by sand-size

or coarser detritus with a reported thickness that can locally

exceed 50 m.

GEOLOGY OF THE NORTH COAST 

GROUND-WATER PROVINCE OF PUERTO 

RICO

BY W.C. WARD1, R.A. SCHARLACH2, AND

J.R. HARTLEY3

INTRODUCTION

The North Coast ground-water province of Puerto Rico is

underlain by a homoclinal wedge of rocks that infill the North

Coast Tertiary Basin. The geology of the middle and upper

Tertiary rocks of northern Puerto Rico is known mostly from

several studies of the outcrop belt (Hubbard, 1923; Meyerhoff,

1933; Zapp and others, 1948; Monroe, 1973, 1980; Seiglie

and Moussa, 1984). Until recently, however, there was little

information on the regional stratigraphic framework of subsurface Tertiary rocks underlying the northern coastal plain.

Before 1986, only a few deep wells provided data on subsurface Tertiary stratigraphy of northern Puerto Rico (Briggs,

1961; Seiglie and Moussa 1984). In 1986, an extensive coredrilling program was initiated, providing new data on the

stratigraphy and depositional history of Oligocene and

Miocene sedimentary rocks of Puerto Rico’s North Coast Tertiary Basin (Hartley, 1989; Scharlach, 1990) (fig. 7). Much of

the geologic and hydrologic core data collected between 1986

and 1989 are described in a series of reports (Hartley, 1989;

Scharlach, 1990; Rodríguez-Martínez, Hartley, and TorresGonzález, 1991; Rodríguez-Martínez, Scharlach, and TorresGonzález, 1991, 1992; Rodríguez-Martínez and Hartley, 1994;

Rodríguez-Martínez and Scharlach, 1994; Todd, 1996). Core

recovery from all these wells (fig. 21) was about 85 percent of

the total footage drilled, resulting in nearly 9,000 m of core of

Tertiary rocks. These new subsurface data are the primary

focus of this section.

GEOLOGIC SETTING OF NORTH COAST TERTIARY 

BASIN

The northern coastal plain of Puerto Rico is underlain by

as much as 2,000 m of carbonate and siliciclastic sedimentary

rock, which accumulated in the North Coast Tertiary Basin

(fig. 7) during Oligocene to Pliocene time (Briggs, 1961;

Monroe, 1973, 1980; Meyerhoff, 1975; Meyerhoff and others,

1983; Seiglie and Moussa, 1984; Hartley, 1989; Larue, 1990;

1

 Department of Geology and Geophysics, University of 

 New Orleans, 2000 Lakeshore Drive, New Orleans, LA

 70148 (retired); 

2 Texas Commission on Environmental Quality, 12100 Park 

Circle, Bldg. D, Austin, TX 78753

3

U.S. Corps of Engineers, Omaha District, 215 North 17 Street,

Omaha, NE 68102
46 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

and Scharlach, 1990). These sedimentary rocks rest on a basement of folded and faulted Cretaceous and lower Tertiary sedimentary and igneous rocks (Monroe, 1980; Larue, 1991;

Montgomery, 1998).

Maximum drilled thickness of the Oligocene-Pliocene section is 1,701 m in the CPR-4 well 8 km east of Arecibo

(Briggs, 1961). Meyerhoff and others (1983) estimate maximum onshore thickness of 1,905 m and offshore thickness of

2,500 to 3,500 m. Maximum thickness of the onshore Oligocene-Pliocene section occurs under the northwestern coastline between Aguadilla and the Rio Grande de Arecibo (fig. 3;

pl. 4). Under the eastern part of the northern coastal plain, the

Oligocene-Pliocene section is less than 750 m and thins to less

than 100 m over paleohighs in the basement complex.

Oligocene-Pliocene sedimentary rocks of the northern

coastal plain strike generally east-west and dip gently toward

the north (fig. 3; pl. 4). Projections from outcrop altitudes

(Monroe, 1980) to correlative points in the core holes show

that the regional dip is 2 to 4 degrees northward. Dips of 4 to

5 degrees (locally up to 10 degrees) were measured across the

southern part of the Oligocene-Pliocene outcrop belt and 1 to

3 degrees across the northern part (Monroe, 1963a, b, 1967,

1969a, b, 1971; Berryhill, 1965; Briggs, 1965, 1968; Nelson

and Monroe, 1966; Nelson, 1967a, b; Nelson and Tobisch,

1968; Tobisch and Turner, 1971). Geologic maps of these

areas show only a few minor faults which displace OligocenePliocene rocks (Berryhill, 1965; Monroe, 1969a). Regional

faults which disrupt the regional homoclinal dip are unknown,

although Briggs (1961) postulated a down-to-the-north fault

with as much as 150 m of displacement, trending generally

east-west from the mouth of the Río Grande de Arecibo to the

mouth of the Río Cibuco. Briggs (1961) based the presence of

40

30

20

20

10

10

30

50 40

20

10

30 20

10

30

10

30

20

 Lago

Loíza

de

Gurabo

Turabo

Río

Río

Loíza

Grande

Río

Juncos

Caguas

CAGUAS

RÍO GRANDE

GURABO

AGUAS

BUENAS

JUNCOS

GUAYNABO

CAROLINA

LAS PIEDRAS

HUMACAO

SAN LORENZO

SAN JUAN TRUJILLO

 ALTO

CANÓVANAS

FIGURE 20.—Thickness of the Caguas-Juncos alluvial valley aquifer (modified from Puig and Rodríguez, 1993).

ATLANTIC OCEAN

CARIBBEAN SEA

Isla

Mona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

Area underlain by alluvial aquifer

EXPLANATION

Line of equal thickness of alluvial deposits of

 Quaternary age—Dashed where uncertain.

 Interval 10 meters

Well control point

10

0 4 1 3 2 KILOMETERS

0 1 2 MILES

18 ̊18'45"

18 ̊11'15"

18 ̊15'

66 ̊00' 65 ̊52'30"

Base modified from U.S. Geological Survey digital data
GEOLOGY OF THE NORTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 47 Lago Caonillas Lago de la Plata Lago Loíza Camuy de Culebrinas de Arecibo Grande de de de Cibuco la Plata Grande Loíza de Lago Guajataca Lago Dos Bocas Añasco

Bayamón

ATLANTIC OCEAN Bahía de

San Juan

Río

Río

Río

Río

Río

Río

Río

Río

ío R

Río

Río

Río

Guajataca

Grande

aT n

ma á

Grande

Manatí

nI

i d o

Ciales

Florida

Morovis

Aguadilla

Moca

Quebradillas

Lares

Arecibo Barceloneta

VegaAlta

San Juan

Bayamón

San

Sebastián

NC11

NC6

AI0

4CPR

IAS3

IAS4

NC10

UC

NC5

A3

A1 NC4 IAS2

IAS1

D5

NC14

NC9A

NC9 NC2

NC8

TB NC13

NC1

NC15

NC3

NC12

Sou t h e rn

e x tent

of roc k s o f Middle Oli g o c ene to Plio c e ne

age

Punta

Higüero

FIGURE 21.—Location of cored test holes and other wells that penetrate Tertiary rocks beneath the North Coast ground-water province of Puerto Rico.

ATLANTIC OCEAN

CARIBBEAN SEA

IslaMona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

Extent of North Coast ground-water province

EXPLANATION

Well control pointWell name abbreviations

NCIASADUCTBAIOCPR

North CoastInteraquifer StudyAbbottDupontUnion CarbideToa BajaArecibo Ionosphere ObservatoryKewanee Interamerican Oil Company— Commonwealth of Puerto Rico

NC6

0 20 KILOMETERS 5 10 150 5 10 MILES

67°15' 67°00' 66°45' 66°30' 66°15' 65°45' 66°00'

18°30'

18°15'

Base modified from U.S. Geological Survey 1:100,000 quadrangles:Arecibo, 1980, unpublished, and Humacao, 1979, unpublished
48 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

this fault, in part, on “anomalous” geomorphic (low-lying land

and swamps) and hydrologic features (presence of coastal

springs) and, in part, on the dip of strata that crop out (more

steeply dipping beds lie to the south). Seismic data collected

in the area failed to support the “Briggs Fault” (Western Geophysical Company of America and Fugro, Inc., 1973) and the

springs near the coast can be explained in terms of regional

flow, ground-water discharge patterns, and the distribution of

hydraulic conductivity within the upper aquifer (Renken and

Gómez-Gómez, 1994; Renken, Gómez-Gómez, and

Rodríguez-Martínez, this volume).

STRATIGRAPHY

Lower and upper Tertiary sedimentary rocks of northern

Puerto Rico comprise two major lithologic units: (1) a relatively thin basal section of siliciclastic sedimentary rocks of

“middle” to late Oligocene age and (2) a thick upper section of

predominantly carbonate rocks of late Oligocene to middle

Miocene age (pl. 4). The boundary between these major units

is conformable and diachronous. In the central part of the

North Coast Tertiary Basin between the Río Guajataca and

Río Grande de Manatí, the upper section is divided into a

lower unit of carbonate rocks, a middle unit of mixed carbonate and siliciclastic rocks, and an upper unit of carbonate

rocks. Toward the western and eastern margins of the basin,

the lower carbonate unit passes laterally into mixed carbonatesiliciclastic rocks. Uppermost Miocene-Pliocene limestone

unconformably overlies middle Miocene limestone on the

northwestern coastal plain (pl. 4).

METHODS OF CORRELATION

For the most part, the ages attributed to the various rock

units are those already established by earlier workers, and no

attempt will be made to further document or justify the ages

used. General ages of the lithologic units described here (pl.

4; fig. 22) are known from paleontological studies (Gordon,

1961b; Bold, 1965; Bermúdez and Seiglie, 1969; Seiglie,

1978; Frost and others, 1983; Seiglie and Moussa, 1984;

Montgomery and others, 1991), but biostratigraphic data are

few for precise correlation of time-equivalent units. For this

study, preliminary biostratigraphic correlation for some lithologic units is done with planktonic foraminifers identified by

P. McLaughlin (Exxon, written commun., 1991) and

A. Melillo (Chevron, written commun., 1992), but most of

the cored Tertiary rocks contain predominantly benthonic foraminifers (C. Young, University of New Orleans, written

commun., 1989).

In the absence of sufficient biostratigraphic control, an

attempt is made to establish relative time equivalency by correlating sedimentary cycles that resulted from third-order sealevel fluctuations (Seiglie and Moussa, 1984; Scharlach and

others, 1989; and Scharlach, 1990). This will be attempted

by establishing the sequence-stratigraphic framework of the

Oligocene-Miocene sedimentary rocks of the northern coastal

plain.

LITHOSTRATIGRAPHIC NOMENCLATURE

For the most part, stratigraphic names used here for units

of the middle and upper Tertiary section follow the nomenclature currently used by the U.S. Geological Survey

(MacLachlan and others, 1992). These rocks are divided into

six widespread lithostratigraphic units and four less-extensive

units (pl. 4; fig. 22). The major stratigraphic divisions are, in

ascending order: the San Sebastián Formation, the Lares

Limestone, the Cibao Formation, the Aguada (Los Puertos)

Limestone, the Aymamón Limestone, and the Quebradillas

Limestone. In the west-central part of the basin, the Montebello Limestone Member is the lower part of the Cibao Formation, and in the east-central basin, the mudstone unit and

Río Indio and Quebrada Arenas Limestones occupy this

stratigraphic position (pl. 4). On the eastern side of the basin,

the Lares Limestone and Cibao Formation interfinger with the

Mucarabones Sand. Most of these stratigraphic subdivisions

were established from outcrop studies, particularly Hubbard

(1923), Zapp and others (1948), and Monroe (1973) as summarized in Monroe (1980). The mudstone unit was recognized from subsurface studies by Seiglie and Moussa (1984).

Some recent workers use other names for units above the

Cibao Formation. “Aguada Limestone” (Zapp and others,

1948) is essentially the same section previously named “Los

Puertos Limestone” by Hubbard (1920), although the type

locality of the Aguada (Zapp and others, 1948) is within the

Cibao outcrop. To deal with this problem, Monroe (1968)

established a reference section for the Aguada limestone,

which Moussa and Seiglie (1975) considered to be contrary

to the Code of Stratigraphic Nomenclature (American Commission on Stratigraphic Nomenclature, 1961, 1970).

Although the name “Los Puertos Limestone” has priority

over “Aguada Limestone” (Moussa and Seiglie, 1975; Meyerhoff, 1975), “Aguada (Los Puertos) Limestone” is used here

to be more consistent with USGS usage. Nomenclatural difficulties associated with the geologic name “Los Puertos” are

further complicated because the same name has been applied

(Glover and Mattson, 1967) and is currently used

(MacLachlan and others, 1992; Krushensky and Schellekens,

written commun., 1994) to describe rocks of Paleocene age

located in Puerto Rico’s central interior on the leeward side of

the island. The “Aymamón Limestone” (Zapp and others,

1948) is essentially equivalent to the “Moca Limestone” of

Meyerhoff and others (1983). In addition, the uppermost unit

called “Camuy Formation” by Monroe (1963b) was redefined

as the “Quebradillas Limestone” by Moussa and Seiglie
GEOLOGY OF THE NORTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 49

AGE

PLIOCENE

LATE

MIDDLE

EARLY

LATE

"MIDDLE"

Hubbard, 1923 Zapp and others, 1948 Meyerhoff, 1975 Monroe, 1980 Seiglie and Moussa, 1984 This report (subsurface)

MIOCENE OLIGOCENE Quebradillas Limestone Los Puertos Limestone Cibao Limestone

Lares Formation

San Sebastián

Shale

Cibao marl

Lares Limestone

San Sebastián

Formation

Aymamón Limestone

Quebradillas Limestone

Los Puertos

Limestone

San Sebastián

Formation

San Sebastián

Formation

Camuy Limestone

Aguada Limestone

Cibao Formation

Cibao Formation

Mucarabones Sand

Quebradillas Limestone

Aymamon Limestone

Los Puertos Limestone

Cibao Formation

Mudstone

unit

Montebello

Limestone

Quebradillas Limestone

Aymamón Limestone

Aguada (Los Puertos) Limestone

Undifferentiated Cibao Formation

Lares Limestone

?

?

Aguada Limestone

Aymamón Limestone

`

San Sebastian

Formation

`

San Sebastián

Formation

Lares Limestone Lares Limestone

Lares Limestone

Mucarabones Sand Montebello Limestone Member Mudstone unit Quebrada Arenas and Río Indio Limestone Members

FIGURE 22.—Stratigraphic nomenclature and ages for Oligocene, Miocene, and Pliocene sedimentary rocks of the North Coast Tertiary Basin.
50 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

(1975), who used the name originally applied to the upper

Tertiary section by Hubbard (1920).

SAN SEBASTIÁN FORMATION (“MIDDLE” TO UPPER 

OLIGOCENE)

Outcrop

As mapped by Meyerhoff (1933), Zapp and others (1948),

and Monroe (1980), the predominantly siliciclastic San

Sebastián Formation rests unconformably on basement rocks

across most of the North Coast Tertiary Basin, but is locally

absent over paleohighs, where basement rock is overlain by

younger units. The thickness of this formation is highly variable, reflecting both the irregular paleotopography and the

paralic depositional system. The average outcrop thickness

between San Juan and the town of San Sebastián is about 70

m, but it is as thick as 150 m west of San Sebastián (Monroe,

1980).

The San Sebastián Formation is divided into three members (Meyerhoff and others, 1983). The basal part consists of

yellow-brown to red-brown sandstone, conglomerate, and

mudstone (Monroe, 1980; Meyerhoff and others, 1983; Frost

and others, 1983). A middle unit comprises varicolored sandstone and mudstone, locally interbedded with lignite, and the

upper San Sebastián Formation is gray and greenish-gray fossiliferous glauconitic mudstone, sandstone, and marl with

fossiliferous limestone. The upper 30 m of the San Sebastián

Formation just north of the town of Lares consists of at least

five depositional cycles of carbonaceous mudstone and fine

sandstone, fossiliferous siltstone and sandstone, and thin layers of skeletal packstone and wackestone (Frost and others,

1983).

Subsurface

Five test wells (NC 2, 5, 8, 11, and 13; fig 21) were drilled

into the San Sebastián Formation, but none penetrated the

entire thickness. There is, therefore, little control on the subsurface thickness of this formation except in two downdip

wells. The Kewanee Interamerican oil-test well, 4 CPR,

drilled near Arecibo in 1960 encountered 494 m of predominantly siliciclastic rocks unconformably overlying “basement” rock (Briggs, 1961). In addition, the Toa Baja test well

(TB) drilled about 16 km west of San Juan in 1990 penetrated

about 75 m of siliciclastic sand and mudstone above Paleogene volcaniclastic sedimentary rocks (Larue, 1990).

Under the northwestern corner of the coastal plain (NC

11), the upper 82 m of the San Sebastián Formation is predominantly olive-green mudstone and greenish-gray marl

with red-brown mudstone, thin black carbonaceous mudstone, and skeletal wackestone-packstone containing miliolids, mollusks, ostracodes, and some red algae and echinoids

(Hartley, 1989). Sandy mudstone and thinly bedded fine

sandstone are common in the upper half of this section. The

San Sebastián in this area is conformably overlain by limestone and marl of Cibao-like lithology.

In the west-central basin, the upper 65 m of San Sebastián

Formation penetrated by NC 5 (fig. 21; pl. 4) is greenish-gray

partly fossiliferous mudstone with red-brown mudstone and

thin black carbonaceous mudstone in the lower part and yellow-gray glauconitic skeletal wackestone, packstone, and

grainstone in the upper part (Hartley, 1989). Mollusks and

benthic foraminifers are the predominant fossils in the lower

part of this section, with Lepidocyclina, Hetrostegina, mollusks, red algae, bryozoans, echinoids, and a few corals in the

upper glauconitic layers (Hartley, 1989). Downdip from NC

5 in the 4 CPR well (fig. 21), the upper 175 m of the San

Sebastián Formation (as defined in this report; pl. 4) is pinkish- to orangish-gray sandy marl, calcareous claystone, and

“punky” limestone (Briggs, 1961). The lower 320 m, which

is the “San Sebastián Formation” of Briggs (1961), consists

of olive-gray pyritic calcareous mudstone and thin sandstone

with volcanic-rock fragments. This unit also includes thin

highly carbonaceous claystone layers and, in the lower 30 m,

sandstones and pebble conglomerates rich in volcanic-rock

fragments. The middle two-thirds of this “San Sebastián”

section is rich in planktonic foraminifers (Globigerina),

which decline in abundance toward the top and toward the

base of the unit (Gordon, 1961b). Pale-yellow-brown clay at

the base of the section likely represents a paleosol developed

on the volcaniclastic sandstone and siltstone of Eocene(?) age

(Briggs, 1961). This downdip “San Sebastián” probably is

older than the San Sebastián at the outcrop and that penetrated by the NC core tests. More likely, the updip San

Sebastián is age-equivalent to the 175 m of sandstone, calcareous mudstone, and clayey limestone with inner-shelf foraminifers that Briggs (1961) included in the basal Lares

Limestone.

About 120 m of the San Sebastián Formation was penetrated by NC 8 in the east-central part of the basin (pl. 4).

The lower half of this section is variably colored nonfossiliferous claystone and siltstone grading up-ward into olivegreen calcareous claystone with oyster fragments, high-spired

gastropods, bivalves, and carbonaceous matter (Scharlach,

1990). The upper half of the San Sebastián in NC 8 is made

up of three depositional sequences, which grade upward from

calcareous claystone to alternating layers of clay and silty

marl and finally to glauconitic muddy wackestone and packstone. Species diversity increases upward in this sequence

from mostly mollusks and miliolids at the base to red algae,

echinoids, oysters, pecten, green algae, and various benthic

foraminifers including Lepidocyclina sp., Heterostegina sp.,

and Miogysina sp. in upper parts of the unit. Downdip from

NC 8, the 45 m of upper San Sebastián cored in NC 2 is similar to that in NC 8. Farther eastward, the NC 13 well (fig. 21;

pl. 4) recovered 26 m of San Sebastián composed of a lower
GEOLOGY OF THE NORTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 51

unit that is like the lower claystone of NC 8. This unit is

overlain by a coarsening-upward unit of fossiliferous fine to

coarse quartz and lithic sandstone, which is overlain by

clayey marl with bryozoans and Lepidocyclina sp.

(Scharlach, 1990).

LARES LIMESTONE (UPPER OLIGOCENE TO LOWER MIOCENE)

Outcrop

Across the central and west-central outcrop belt (fig. 3),

the siliciclastic beds of San Sebastián Formation grade

upward into carbonate rocks of the Lares Limestone (pl. 4).

According to Monroe (1980, p. 23), the Lares Limestone at

outcrop has a fairly constant thickness of approximately 270

m, but thickens to more than 310 m east of the town of Lares.

Projections from the outcrop map (Monroe, 1980) to the

cored test wells, however, indicate that the thickness of the

Lares generally is 75 to 150 m where it crops out (fig. 3;

pl. 3). Frost and others (1983) measured 95 m of Lares Limestone in a new highway cut just north of the town of Lares.

The Lares Limestone thins and pinches out in the western

and eastern margins of the outcrop belt. West of the town of

San Sebastián, the Lares Limestone interfingers with siliciclastic rocks of San Sebastián lithology, and the limestone

pinches out a few miles east of Aguadilla (Monroe, 1980). In

the westernmost area of Lares outcrop, Monroe (1980)

divided the Lares Limestone into three parts: a lower unit of

about 25 m of oyster-bearing marl passing upward to fossiliferous limestone with abundant coral heads, a middle member

of 40 to 80 m of “chalk” and calcareous clay containing abundant oysters and a few corals, and an upper unit of 30 m of

fossiliferous limestone with common corals interbedded with

“chalk” and calcareous clay. Toward the eastern part of the

basin, Lares Limestone interfingers with and onlaps the

Mucarabones Sand. Lares lithology is unknown east of the

Río de la Plata (Monroe, 1980).

In the Lares Limestone exposed along Highway 129 north

of the town of Lares (figs. 3, 21), Frost and others (1983)

described wackestone and packstone with abundant large foraminifers and finger corals (Porites sp.) as well as reef limestone containing several species of corals and red algae. This

section records three cycles of reef growth during Lares deposition. Frost and others (1983) noted two angular discontinuities within the lower 15 m of the Lares at about the same

stratigraphic position where Monroe (1980, fig. 13, p. 22)

pictures an apparent angular discontinuity northeast of Morovis in the eastern part of the basin (Frost and others, 1983).

Subsurface

Entire sections of Lares Limestone were cored in North

Coast test wells located in the eastern and east-central part of

the basin (wells NC 2, 5, 8, and 13; fig. 21). In addition, the

entire thickness was penetrated in wells IAS 1, 4 CPR,

PREPA Toa Baja (TB), and probably in the well drilled at the

Arebico Ionosphere Observatory (AIO) (fig. 21). Wells NC

4, 6, and 9A, IAS 4, Abbott 3 (A3), Dupont 5 (D5), and

Union Carbide 1 (UC) bottomed in the Lares Limestone.

Wells NC 1, 3, 12, and 15 were drilled east of the eastern

margin of Lares lithology, and NC 11 probably was drilled

beyond the western limit of this formation.

Lares Limestone probably is thickest in the central part of

the basin between the Río Cibuco and Río Grande de Arecibo

(fig. 23). Estimations of subsurface thickness between the

Río Grande de Arecibo and Río Grande de Manatí (fig. 23)

are tenuous because of the difficulty in distinguishing the

Lares Limestone from the overlying Montebello Limestone

and in the northwestern area (NC 6 and NC 11), from the

overlying Cibao Formation.

Between the Río Grande de Arecibo and Río de la Plata,

the Lares Limestone can be subdivided into three major units.

The lower Lares is characterized by several cycles of olivegray and greenish-gray skeletal wackestone passing upward

into skeletal packstone and lesser grainstone. Upper parts of

these cyclic sequences commonly are rich in rhodolites, the

large benthic foraminifer Lepidocyclina sp., or the branching

coral Porites sp. Other common skeletal constituents of the

lower Lares are bivalves including oysters, codiacean and

dasyclad green algae, and benthic foraminifers Heterostegina

sp. and Amphistegina sp. Some lowermost wackestones are

clayey and glauconitic. The upper part of the lower Lares is

dolomitic.

The middle part of the Lares comprises a few shoalingupward sequences of greenish-gray and yellowish-gray burrowed skeletal wackestone-packstone grading upward to redalgae/large-foraminifer/mollusk packstone or grainstone and

thin patch-reef limestones containing Porites sp., Montastrea

sp., and Goniastrea sp. (identified by S.H. Frost, UNOCAL,

written commun., 1991). Test well NC 9A penetrated thicker

coral-bearing units made up of Alveopora chiapeneca, Porites

panamensis, Montastrea sp. cf. M. altissima, Acropora sp. cf.

A. saludensis, and Astreopora(?) sp. (corals identified by S.

H. Frost, UNOCAL, written commun., 1991). The middle

unit also contains oysters and the bivalve Kuphus sp. Many

coral- and rhodolite-rich layers are dolomitized.

The lithology of the upper Lares Limestone varies from

place to place. In the eastern part of the basin, between the

Río Cibuco and Río de la Plata near wells NC 2 and NC 8

(fig. 21), the upper Lares passes upward from lithology similar to the middle unit to yellowish-gray partly dolomitic

clayey wackestone and packstone with soritid and miliolid

foraminifers. Between the Río Grande de Manatí and Río

Cibuco, the upper Lares is composed of skeletal wackestone

and packstone with corals, red algae, and planktonic foraminifers interbedded with clayey wackestone, silt, and fine

sandstone. In the area between the Río Grande de Arecibo
52 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS Lago Caonillas Lago de la Plata Lago Loíza Camuy de Culebrinas de Arecibo Grande de de de Cibucola Plata Grande Loíza de Lago Guajataca Añasco

Bayamón

ATLANTIC OCEAN Bahía de

San Juan

Río

Río

Río

Río

Río

Río

Río

Río

Río

Río

Río

Río

Guajataca

Grande

aT n

ma á

Grande

Manatí

nI

i d o

Lago Dos  Bocas

Ciales

Florida

Morovis

Aguadilla

Moca

Quebradillas

Lares

Arecibo

Barceloneta

VegaAlta

San Juan

Bayamón

San

Sebastián

100150 200 200 250 250

150100

0

10050 150

050

75±

110

20

75±

145±

NC11

NC2

156 NC8

164 NC13 79

4CPR

290±

NC5

217

AI0

245±

IAS1

210

NC1

0

NC3

0

Sou t h e rn

e x tent

of roc k s o f Middle Oli g o c ene to Plio c e ne

age

Punta

Higüero

FIGURE 23.—Thickness of Lares Limestone, northern Puerto Rico (outcrop extent modified from Monroe, 1980).

ATLANTIC OCEAN

CARIBBEAN SEA

IslaMona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

Area of outcrop of Lares Limestone

EXPLANATION

Line of equal thickness of Lares Limestone— Interval 50 meters. Dashed where approximately locatedWell control point—Thickness in meters  (well name abbreviations on figure 21)Outcrop—Thickness in meters

100

75±

4CPR

290±

0 20 KILOMETERS 5 10 150 5 10 MILES

67°15' 67°00' 66°45' 66°30' 66°15' 65°45' 66°00'

18°30'

18°15'

Base modified from U.S. Geological Survey 1:100,000 quadrangles:Arecibo, 1980, unpublished, and Humacao, 1979, unpublished
GEOLOGY OF THE NORTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 53

and Río Grande de Manatí, upper Lares is dolomitic wackestone and clayey wackestone with coral, red algae, rhodolites,

and benthic foraminifers Heterostegina sp., Amphistegina sp.,

and Miogypsina sp. Soritid and miliolid foraminifers are

common near the top of the formation.

An eastern tongue of the Lares Limestone was cored in

well NC 13 (pl. 3; fig. 21). The dominant rock type is clayey

soritid wackestone (Scharlach, 1990), similar to carbonate

rocks of the overlying Cibao Formation. In this eastern area,

the lower Lares is marly wackestone and lesser packstone

containing bryozoans, Lepidocyclina sp., Heterostegina sp.,

mollusks, and red algae with clay and marly sandstone layers.

Middle Lares is predominantly clayey wackestone with soritids, Heterostegina sp., oysters, pecten, branching and crustose red algae, and lesser miliolids, ostracodes, and

bryozoans (Scharlach, 1990). The upper part of the Lares in

well NC 13 is composed of mixed carbonate and terrigenous

lithology. Much of the upper interval is glauconitic sandy

skeletal grainstone with marly fossiliferous sandstone. The

common fossils in these beds are Heterostegina sp., soritid

foraminifers, red algae, echinoids, and mollusks.

In the western part of the basin, the upper Lares equivalent penetrated by test well NC 6 is similar to the carbonate

rocks of the Cibao Formation. Here, the stratigraphic equivalent of the upper Lares consists of clayey wackestone and

dolomitized wackestone with bivalves, echinoids, miliolids,

some red algae, and Miogypsina sp. This section also contains thin carbonaceous claystone. No Lares Limestone was

identified in NC 11. In this area, the lithology of rocks that

apparently are time-equivalent to the Lares is more typical of

the upper San Sebastián Formation.

Seiglie and Moussa (1984) describe the Lares Limestone

as a depositional cycle in their study of the drill cuttings from

two wells located in the north-central part of the basin

(Dupont 5 (D5) and Union Carbide 1 (UC); fig. 21). In the

Dupont 5 well, the lower, regressive part of the Lares exhibits

a progressive upward increase in the percentage of planktonic

foraminifers through the middle of the formation. The upper,

transgressive section exhibits an upward decrease in planktonic foraminifers toward the top of the formation. Lepidocyclina sp. and Heterostegina sp. also are common throughout

the Lares section in this well, but are slightly more abundant

in the lower part of the formation. In the Union Carbide 1,

the transgressive phase of the depositional cycle is represented by an upward increase in the percentage of Amphistegina sp. From the middle Lares upward, the decrease in

abundance of this foraminifer records the regressive phase,

the close of which is marked by oyster beds at the base of the

Montebello Limestone.

In the most downdip well, the 4 CPR (fig. 21), Briggs

(1961) describes 504 m of Lares “calcarenite,” the lower 175

m of which are interbedded with calcareous claystone and

sandy marl. Gordon (1961b) reported this unit to contain an

outer-neritic foraminiferal assemblage, except near the top

and bottom where middle-neritic foraminifers are more abundant. These foraminiferal assemblages indicate that the

downdip Lares also records a cycle of deposition.

MUCARABONES SAND (UPPER OLIGOCENE TO LOWER 

MIOCENE)

Outcrop

In the eastern part of the North Coast Tertiary Basin, the

Lares Limestone and the lower part of the overlying Cibao

Formation interfinger with sandstone, conglomerate, and

mudstone, which were shed off the eastern highlands during

the late Oligocene and early Miocene. These terrigenous

deposits were assigned to the Mucarabones Sand (Nelson,

1966).

Monroe (1980) mapped nearly continuous exposures of

Mucarabones Sand between the Río Grande de Manatí and

the outskirts of San Juan (fig. 3; pl. 4). In the vicinity of the

Río Grande de Manatí, the Mucarabones is a 10-m-thick unit

overlying the San Sebastián Formation and underlying and

interfingering with the lower Lares Limestone. East of the

Río de la Plata, the Mucarabones consists of as much as 120

m of sandstone and conglomerate that grades laterally into

the Lares and lower two-thirds of the Cibao Formation. East

of the Río de Bayamón, this formation is "indistinguishable"

from the underlying San Sebastián Formation and the overlying alluvial deposits (Monroe, 1980).

The basal Mucarabones Sand is composed of coarse sandstone with lenses of conglomerate and greenish-gray clay.

The middle and upper parts of this unit are characteristically

yellow to grayish-orange crossbedded partly glauconitic fine

to medium sandstone with scattered lenses of fossiliferous

sandy limestone and fossiliferous sandy claystone (Monroe,

1980). Fossils include echinoids, mollusks, and large benthic

foraminifers. Lepidocyclina undosa is common in the lower

and middle parts of the Mucarabones Sand (Monroe, 1980).

Subsurface

The Mucarabones Sand is the basal Tertiary sedimentary

unit penetrated by two wells drilled in the San Juan metropolitan area (NC 1 and NC 3) (Scharlach, 1990). In the NC 3 well

(pl. 4), 7.5 m of Mucarabones rests on weathered basaltic basement rock. The NC 1 well was completed in Mucarabones

Sand after penetrating 103 m of the unit (pl. 4). In both wells,

this formation underlies the Cibao Formation. The upper 24 m

of terrigenous rocks that underlie the Lares Limestone in well

NC 13 (pl. 4) consists of calcareous claystone and fine- to

medium-sandstone that could be equivalent to westernmost

outcrops of the Mucarabones Sand. This interval in NC 13, in

turn, is underlain by San Sebastián lithology.
54 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

The Mucarabones Sand in the subsurface consists of intermixed olive-green and greenish-gray silty claystone, siltstone,

sandstone, and conglomerate. Sandstones are immature to

submature fine to medium lithic arkose to subarkose

(Scharlach, 1990). Monocrystalline and polycrystalline

quartz, fresh and weathered plagioclase, and volcanic-rock

fragments are the predominant constituents of the sandstones

(Scharlach, 1990). Conglomerates consist of rounded volcanic-rock fragments up to 4 cm.

A 14-m-thick fossiliferous interval was encountered in the

upper part of the Mucarabones in well NC 1 (pl. 4). This rock

is cross-laminated quartz-sandy grainstone that consists of

encrusting foraminifers, small benthic foraminifers, red algae,

and echinoids with lesser amounts of bryozoans, ostracodes,

and planktonic foraminifers (Scharlach, 1990). This grainstone is underlain and overlain by non-fossiliferous siltstone

and grainstone.

CIBAO FORMATION (LOWER MIOCENE)

Outcrop

The Cibao Formation is a complex unit of carbonate,

mixed carbonate-siliciclastic, and siliciclastic sedimentary

rocks, which Monroe (1980) divided into six members on the

basis of surface exposures (fig. 24). Typically, the formation

consists of clayey skeletal wackestone and fossiliferous calcareous claystone (marl) with thin units of mudstone and sandstone. In the central part of the North Coast Tertiary Basin,

however, much of the Cibao section is limestone similar to the

underlying Lares.

In the western part of the basin, clayey wackestone and

marl (“typical Cibao lithology”) interfinger with volcaniclastic

sandstone and conglomerate and shale of the Guajataca Member. This apparent fan-delta deposit is as much as 160 m thick

between the towns of San Sebastián and Moca (figs. 3, 21).

Between the Río Camuy and Río Grande de Manatí, the

lower member of the Cibao Formation is the Montebello

Limestone, formerly included in the Lares Limestone (Zapp

and others, 1948). Nelson and Monroe (1966) separated the

Montebello from the Lares on the basis of an upward change

from “fine-grained calcarenite” of the Lares to “weakly indurated medium to coarse calcarenite” of the Montebello (Monroe, 1980, p. 33). Across most of the Montebello outcrop belt,

beds of large oysters mark the base of this limestone member.

The Montebello is about 200 m thick in the vicinity of the Río

Grande de Arecíbo, where, in some places, this limestone rests

directly on a paleohigh of basement rock. In this area, the

“typical” Cibao limestone/marl is only a few meters thick. The

Montebello packstone and grainstone commonly contain abundant large foraminifers Lepidocyclina (L. undosa, L. gigas, L.

cancellei, and L. yurnagunensis), Heterostegina sp., and Nummulites dia (Monroe, 1980; Frost and others, 1983). The Montebello Limestone is overlain by marl and clayey limestone of

the upper Cibao Formation, except in the area between the Río

Grande de Arecibo and northeast of the town of Florida, where

the Montebello is overlain by the Aguada (Los Puertos) Limestone (fig. 3).

About 3 km west of the Río Grande de Manatí, the Montebello Limestone thins abruptly and is replaced eastward by

marl and clayey limestone of the Cibao Formation (fig. 3).

Eastward of the Río Indio, these “typical deposits of Cibao . . .

grade laterally eastward and intertongue with a predominantly

limestone sequence” (Monroe, 1980, p. 34). Monroe (1962)

divided this limestone sequence into two members: the lower

Río Indio Limestone Member and the upper Quebrada Arenas

Limestone Member. These units crop out from just west of the

Río Indio to 4 to 5 km west of Bayamón (figs. 3). A thin limestone in the upper part of the predominantly clay and marl

Cibao Formation between Río Grande de Manatí and Río Indio

is mapped as a westward extension of the Quebrada Arenas

Limestone (Berryhill, 1965; Monroe, 1980).

The Río Indio Limestone Member is about 66 m thick near

the Río Indio, where it consists of three units: at the base (1)

40 m of partly glauconitic intraclastic fossiliferous limestone

with layers of large oysters, overlain by (2) 6 m of cross-bedded fossiliferous glauconitic sandstone and pebble conglomerate (Almirante Sur Sand Lentil), overlain, in turn, by

(3) limestone containing quartz sand (Monroe, 1980). The Río

Indio Limestone Member contains Lepidocyclina sp., and

other benthic foraminifers, mollusks, and red algae (Monroe,

1980). This member is increasingly sandy and clayey eastward and grades into the Mucarabones Sand about 4 km west

of Bayamón.

Monroe (1980) considers the Quebrada Arenas Limestone

Member to be an eastward extension of the Montebello Limestone. This fossiliferous limestone member is about 60 m

thick south of Vega Alta, where it consists of “alternating beds

of very pale orange to grayish-orange indurated limestone in

strata about 50 cm thick and grayish-orange chalky limestone

in strata about 50 cm thick” (Monroe, 1980, p. 36). The base

of this member in this area is coral-rich limestone. The Quebrada Arenas Limestone also grades eastward into the Mucarabones Sand about 5 km west of Bayamón.

In at least three areas (fig. 24A), the upper part of the Quebrada Arenas Limestone is incised by channels filled with

mottled grayish red and yellow-gray noncalcareous, nonfossiliferous clayey coarse sand and gravel. The gravel consists

of pebbles and cobbles of quartz and volcanic-rock fragments. These discontinuous terrigenous deposits are the

Miranda Sand Member of Monroe (1980).

Across most of the outcrop belt, the upper Cibao is characterized by clayey limestone and calcareous mudstone,

informally termed “upper member” (Monroe, 1962). This

widespread unit crops out from the western tip of Puerto Rico

near Punta Higüero to the area south of Bahía de San Juan
GEOLOGY OF THE NORTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 55

  

 

  

 

  





 

10 20 KILOMETERS

50

100 METERS

VERTICAL EXAGGERATION X 100

67 ̊00'66 ̊00'

NC 11NC 6NC 5IAS 1NC 14NC 9NC 2NC 13NC 1NC 3

Mucarabones Sand

Mucarabones Sand

Lares Limestone

Lares Limestone

San Sebastián

Formation

San Sebastián

Formation

WEST EAST

WEST EAST

A

B

Line of section same as A-A', Plate 4

Lares Limestone

EXPLANATION

Outcrop—Cibao Formation

 Cibao undifferentiated

 Miranda Sand Member

 Quebrada Arenas Limestone Member

 Río Indio Limestone Member

 Montebello Limestone Member

 Guajataca Member







 





Subsurface—Cibao Formation

 Cibao undifferentiated

 Quebrada Arenas Limestone and Río Indio

 Limestone Members undifferentiated

 Montebello Limestone Member

 Mudstone unit Questionable normal fault

 









FIGURE 24.—Stratigraphic relations of Cibao Formation and its members at the outcrop (A) and in the subsurface (B)

(section A modified from Monroe, 1980, figure 16).
56 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

(pl. 4). Thickness of the upper member is about 80 m in the

western part of the basin, about 10 m where it overlies the

Montebello Limestone in the west-central part of the basin,

and about 50 m in much of the eastern basin (Monroe, 1980).

Fossils include oysters and other marine mollusks as well as

large benthic foraminifers Lepidocyclina sp., Nummulites sp.,

Planorbulinella sp., and Marginopora sp. (Monroe, 1980).

From the descriptions of Monroe (1980), the Cibao Formation is sharply overlain by the Aguada (Los Puertos) Limestone.

Subsurface

For this report, the Cibao Formation of subsurface

northern Puerto Rico is differentiated into four members: (1)

Montebello Limestone, (2) mudstone unit, (3) Río IndioQuebrada Arenas Limestone, and (4) undifferentiated Cibao

(fig. 24B).

Montebello Limestone Member

Wells which document the entire Montebello section are

NC 5, IAS 4, and Union Carbide 1 (fig. 21). Well NC 10 bottomed in the Montebello about 140 m below the top of the

unit (Hartley, 1989), and the Arecibo Ionospheric Observatory (AIO) well was drilled through the basal 8 m of the Montebello (Monroe, 1980). It is questionable that Montebello

lithology extends downdip into the area of the 4 CPR well,

and only a few thin limestones in the NC 6 core test (Hartley,

1989) could be equivalent to the Montebello.

In the subsurface, the Montebello Limestone Member is

separated from the underlying Lares Limestone on the basis

that the Montebello represents a separate transgressiveregressive depositional cycle (Seiglie and Moussa, 1984;

Hartley, 1989). Seiglie and Moussa (1984) use the abundance

of Miogypsina tani to define a 60-m-thick Montebello section

overlying a basal oyster-rich layer.

Thickness of the Montebello in NC 5 is 197 m (fig. 25).

The lower 45 m of the Montebello is characterized by yellowgray and pale-orange Heterostegina sp. and Lepidocyclina sp.

packstone and wackestone with mollusks, red algae, and corals. Overlying this are 84 m of yellow-gray and pale-orange

large-foraminifer packstone with thin packstone-grainstone

and coral-rich layers. Heterostegina sp., Miogypsina sp., and

Amphistegina sp. are abundant. The upper 87 m of Montebello is light-gray partly dolomitic packstone, wackestone,

and grainstone with rhodolites, corals, and large foraminifers

Amphistegina sp., and Miogypsina sp. In wells NC 5 and NC

10 (fig. 25), the Montebello is composed of generally coarsening-upward depositional units a few meters thick. Typically these cycles consist of skeletal wackestone passing

upward to large-foraminifer-bearing packstone or packstone-grainstone. Some cycles are capped by coral and red

algal boundstone.

Mudstone unit 

The term mudstone unit is adapted from an informal designation by Seiglie and Moussa (1984) for a calcareous mudstone overlying the Lares Limestone in the Dupont 5 (D5)

well (figs. 21, 22). According to Seiglie and Moussa (1984),

this planktonic-foraminifer-bearing unit is age equivalent to

the lower part of the subsurface Montebello Limestone to the

west and the subsurface Río Indio Limestone to the east.

The mudstone unit is restricted to a narrow area east of

the Río Grande de Manatí (Scharlach, 1990). The nature of

the apparently abrupt stratigraphic or structural boundaries

with lateral equivalents is unknown. This unit was encountered in wells NC 4, NC 14, IAS 1, IAS 2, and Dupont 5 (D5)

(fig. 21). Thickness ranges from about 137 m in NC 4 to

about 60 m in Abbott 1 (A1) (fig. 26). The mudstone unit

overlies the Lares Limestone and underlies either the Río

Indio/Quebrada Arenas Limestone or undifferentiated Cibao

Forma-tion. In the subsurface, neither the lower nor

upper contact is sharp. The lower boundary is placed where

terrigenous strata become more abundant and thicker than

carbonate strata. The upper boundary is placed where calcareous mudstone and wackestone-packstone bearing planktonic

foraminifers pass upward to interbedded mudstone, marl, and

clayey-sandy limestone with benthic foraminifers and other

shallow-marine fossils.

Scharlach (1990) divided the mudstone unit penetrated by

NC 4 into three parts. The lower 30 m of the mudstone unit is

composed of grayish-green and light-olive-green marly sandstone, siltstone, wackestone, and rare packstone. The carbonate rocks are locally dolomitic. The diverse fauna includes

planktonic and benthic foraminifers, thin-shelled mollusks,

echinoids, and, in places, large thin Lepidocyclina sp., Halimeda sp., red algae, and bryozoans.

The middle part of the mudstone unit in NC 4 is predominantly grayish-green clay, silt, carbonate mudstonewackestone, and dolomudstone-dolowackestone. This interval is less fossiliferous than the lower part, but bears planktonic foraminifers, thin-shelled bivalves, scaphopods, and

echinoid fragments. In the lower half of the middle mudstone

unit are thin beds of marly quartz and lithic sandstones and

glauconitic sandy-clayey wackestone-packstone. These

coarser rocks contain scattered Lepidocyclina sp. and other

coarse skeletal grains.

The uppermost mudstone unit is composed of pale-olive

and grayish-olive marl and claystone with upwardly increasing skeletal wackestone and packstone. Planktonic foraminifers and large benthic foraminifers (mostly Miogypsina sp.)

are common. Other fossils include echinoids, ostracodes,

pychnodontid oysters, other mollusks, and bryozoans.
GEOLOGY OF THE NORTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 57   Lago Caonillas Lago de la Plata Lago Loíza Camuy de Culebrinas de Arecibo Grande de de de Cibuco la Plata Grande Loíza de Lago Guajataca Lago Dos Bocas Añasco

Bayamón

ATLANTIC OCEAN Bahía de

San Juan

Río

Río

Río

Río

Río

Río

Río

Río

Río

Río

Río

Río

Guajataca

Grande

aT n

ma á

Grande

Manatí

nI

i d o

Ciales

Florida

Morovis

Aguadilla

Moca

Quebradillas

Lares

Arecibo

Barceloneta

VegaAlta

San Juan

Bayamón

San

Sebastián

050 100150 200

45± 150± 200± 200±

NC5

197

UC

64

NC10

139

IAS2

0

4CPR

0

NC6

0

Sou t h e rn

e x tent

of roc k s o f Middle Oli g o c ene to Plio c e ne

age

Punta

Higüero

FIGURE 25.—Thickness of Montebello Limestone, northern Puerto Rico (outcrop extent modified from Monroe, 1980

A

).

TLANTIC OCEAN

CARIBBEAN SEA

IslaMona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

Area of outcrop of Montebello Limestone

EXPLANATION Line of equal thickness of Montebello Limestone Member—Interval 50 meters

Well control point—Thickness in meters  (well name abbreviations on figure 21)Outcrop—Thickness in meters

100

200±

NC5

197

0 20 KILOMETERS 5 10 150 5 10 MILES

67°15' 67°00' 66°45' 66°30' 66°15' 65°45' 66°00'

18°30'

18°15'

Base modified from U.S. Geological Survey 1:100,000 quadrangles:Arecibo, 1980, unpublished, and Humacao, 1979, unpublished
58 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

















 

















Ciales

Florida Montebello

Barceloneta

Manatí

Vega

Baja

Vega Alta

Corozal

Caño

Tíburones Río

Grande

de

Manatí

Río

Toro Negro

Río

Bauta

Río

Yunes

Río

Sana Muerto

Río Grande de Manatí

Río

de

la Plata

Río Lajas

Río

Cibuco

Río

Indio

Laguna

Tortuguero

ATLANTIC OCEAN

MANATÍ VEGA BAJA

FLORIDA

MOROVIS

CIALES COROZAL UTUADO

JAYUYA

VEGA

ALTA

NARANJITO

TOA

ALTA

DORADO

BAJA

ARECIBO

BARCELONETA

OROCOVIS

A1

60±

D5

55±

NC4

137

IAS-1

112

IAS-2

NC14

>105

0

50

100

100

50

0

0 24 8 6 10 KILOMETERS

0 4 2 6 MILES

18°30'

18°22'20"

18°15' Base modified from U.S. Geological Survey

digital data

66°30' 66°22'30"

ATLANTIC OCEAN

CARIBBEAN SEA

Isla

Mona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

Area of outcrop of Cibao Formation

 Cibao Formation, undifferentiated

 Quebrada Arenas and Río Indio Limestone Members

 Montebello Limestone Member

Line of equal thickness of the mudstone unit—Interval 50 meters.

 Dashed where approximately located

Control point

 Well—Thickness in meters (well name abbreviations on figure 22)

EXPLANATION

 









50

IAS-1

112

FIGURE 26.—Thickness of the mudstone unit of the Cibao Formation, northern Puerto Rico.
GEOLOGY OF THE NORTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 59

Río Indio/Quebrada Arenas Limestone Members

Between the Lares Limestone and the Cibao Formation in

the eastern part of the basin is a transition zone of variable

lithology, here termed Río Indio/Quebrada Arenas Limestone

Members. No attempt is made to distinguish between these

two members in the subsurface.

The Río Indio/Quebrada Arenas Limestone section was

encountered in test wells NC 2, NC 4, NC 8, NC 9A, NC 13,

and NC 14 (fig. 21; pl. 4). In NC 8, the section of variable

lithology is 63 m thick, but it thins downdip to about 27 m in

NC 2. The Río Indio/Quebrada Arenas section encountered in

NC 8 is light-gray and yellowish-gray sandy skeletal grainstone, packstone, and wackestone interbedded with fossiliferous lithic sandstone with thin layers of claystone and siltstone.

The large benthic foraminifers Miogypsina sp., Heterostegina

sp., and Lepidocyclina sp. are common. The Río Indio/Quebrada Arenas Limestone section also contains a thin cobblepebble conglomerate composed of volcanic-rock fragments,

corals, and mollusks. In NC 2, the Río Indio/Quebrada Arenas is composed of glauconitic sandy and clayey skeletal

packstone and wackestone and glauconitic fossiliferous marl.

Skeletal grains include bryozoans, large benthic foraminifers

Heterostegina sp., Miogysina sp., Amphistegina sp., and Lepidocyclina sp., echinoids, red algae, and mollusks, including

oysters and pectens.

The Río Indio/Quebrada Limestone lithology is not recognized in the subsurface west of NC 4 between Río Grande de

Manatí and Río Cibuco (fig. 21; pl. 4). In NC 4 in the central

part of the basin, there is a 30 m-thick transition zone above

the mudstone unit. East of the Río de la Plata, where the

Cibao Formation apparently becomes more terrigenous and

the Lares Limestone interfingers with the Mucarabones Sand,

the Río Indio/Quebrada Arenas Limestone section is difficult

to distinguish from the sandy Lares Limestone. For this study,

26 m of glauconitic sandy wackestone-packstone and fossiliferous sandstone cored in the NC 13 well is tentatively

assigned to the Río Indio/Quebrada Arenas Limestone section

(pl. 4). Fossils in these beds include red algae, small benthic

foraminifers, Heterostegina sp., mollusks, and echinoids.

Undifferentiated Cibao Formation

For this report, all of the subsurface sequence that is composed predominantly of the “typical” Cibao lithology of marl,

calcareous mudstone, and clayey wackestone is referred to as

“undifferentiated Cibao Formation.” The thickness of the

undifferentiated Cibao ranges from more than 300 m in the

western downdip area (between Río Camuy and Río Grande

de Manatí) (fig. 27) to tens of meters in the eastern part of the

basin (east of Río de la Plata). Over most of the area, this unit

is the upper part of the Cibao Formation, but in the western

end of the basin the entire Cibao section is composed of this

lithology (pl. 4). The lower part of the Cibao Formation in the

western part of the basin in the vicinity of the Río Camuy

most likely is time equivalent to the uppermost Lares Limestone and the Montebello Limestone Member. Cibao lithology penetrated by NC 11 rests directly on San Sebastián

lithology (pl. 4). Well NC 6, downdip from the Lares and

Montebello outcrop (pl. 4), encountered only a few thin layers

of carbonate rocks which could be attributed to the Lares or

Montebello Limestones. For this report, therefore, the entire

lower 285 m of section in the NC 6 well is considered to be

Cibao lithology, although a thin limestone at 747 m could be

equivalent to the top of the updip Lares Limestone. The other

downdip well in the west-central part of the basin, the 4 CPR

(pl. 4), drilled through more than 300 m of the Cibao Formation above the Lares Limestone (Briggs, 1961). Over a paleohigh in the eastern end of the basin, undifferentiated Cibao

overlies the Mucarabones Sand (NC 1 and NC 3; pl. 4).

The predominant lithology of the undifferentiated Cibao

Formation in the subsurface of northern Puerto Rico is

medium-gray, yellowish-gray, and olive-gray soritid-miliolid

wackestone and packstone interbedded with marl and claystone. Much of the limestone is clayey and sandy. Dolomitic

zones occur in the upper and lower parts of the formation in

several places. Several gray green yellow-gray and redbrown paleosols are preserved in the Cibao of the eastern and

central parts of the basin (Scharlach, 1990), and caliche crusts

occur in a few zones of the Cibao of the central and western

part of the basin (Hartley, 1989; Scharlach, 1990).

In addition to abundant soritid and miliolid foraminifers,

typical limestones of the undifferentiated Cibao Formation

also contain small rotalid foraminifers, encrusting porcellaneous foraminifers, agglutinated foraminifers, a variety of

mollusks, ostracodes, red algae, echinoids, and less commonly, bryozoans and small corals. The large benthic foraminifer Miogypsina sp. is restricted to the lower part of the

undifferentiated Cibao (Scharlach, 1990), except in the west,

where this foraminifer occurs in a few limestones within the

lower three-quarters of the undifferentiated Cibao section

(Hartley, 1989). Most fine-grained terrigenous layers in the

Cibao contain benthic foraminifers and mollusks, but some

mudstones, sandstones, and conglomerates are non-fossiliferous. Dark-gray to dark-green carbonaceous claystones occur

locally.

AGUADA (LOS PUERTOS) LIMESTONE (LOWER TO MIDDLE 

MIOCENE)

Outcrop

At outcrop as well as in the subsurface, a 100- to more

than 500-m thick section of carbonate rock overlies the Cibao

Formation across the whole North Coast Tertiary Basin

(fig. 28). The lower part of this lower to middle Miocene

sequence is the Aguada (Los Puertos) Limestone.
60 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS





Lago Caonillas Lago de la Plata Lago Loíza

Camuy

de

Culebrinas

de

Arecibo

Grande de

de

de

Cibuco

la Plata

Grande

Loíza

de Lago

Guajataca

Lago Dos  Bocas

Añasco

Bayamón

ATLANTIC OCEAN Bahía de

San Juan

Río

Río

Río

Río

Río

Río

Río

Río

Río

Río

Río

Río

Guajataca

Grande

aT n ma á

Grande

Manatí

nI i d o

Ciales

Florida

Morovis

Aguadilla

Moca

Quebradillas

Lares

Arecibo Barceloneta

VegaAlta

San Juan

Bayamón

San

Sebastián

50 50

100

100

150

150

150

200

200

200

250

300

0

180± 150±

NC11

196 NC6

285

4CPR

305

IAS1

154

NC14

145

NC4

136

NC9

186 NC8 91

NC13

91

NC1

52

NC3

48

NC5

98

NC10

88 D5

155

NC2

149

Sou t h e rn e x tent of roc k s o f Middle Oli g o c ene to Plio c e ne age

Punta

Higüero

FIGURE 27.—Thickness of undifferentiated Cibao Formation, northern Puerto Rico (outcrop extent modified from Monroe, 1980).

ATLANTIC OCEAN

CARIBBEAN SEA

IslaMona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

Area of outcrop of Cibao Formation

EXPLANATION

 Undifferentiated Cibao—Only the upper unit of the undifferentiated Cibao between Río Grande de Manatí and Río Indio is considered for this map Montebello Limestone Member

 Río Indio Limestone Member and Quebrada Arenas Limestone Member

 Guajataca MemberLine of equal thickness of Cibao Formation— Interval 50 metersWell control point—Thickness in meters 

 (well name abbreviations on figure 21)Outcrop—Thickness in meters

100

150±

NC5

98

0 20 KILOMETERS 5 10 150 5 10 MILES

67°15' 67°00' 66°45' 66°30' 66°15' 65°45' 66°00'

18°30'

18°15'

Base modified from U.S. Geological Survey 1:100,000 quadrangles:Arecibo, 1980, unpublished, and Humacao, 1979, unpublished
GEOLOGY OF THE NORTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 61                                        Lago Caonillas Lago de la Plata Lago Loíza Camuy de Culebrinas de Arecibo Grande de de de Cibuco la Plata Grande Loíza de Lago Guajataca Lago Dos Bocas Añasco

Bayamón

ATLANTIC OCEAN Bahía de

San Juan

Río

Río

Río

Río

Río

Río

Río

Río

ío R

Río

Río

Río

Guajataca

Grande

aT n

ma á

Grande

Manatí

nI

i d o

Ciales

Florida

Morovis

Aguadilla

Moca

Quebradillas

Lares

Arecibo Barceloneta

VegaAlta

San Juan

Bayamón

San

Sebastián

500

400

300

300

200

200

100

100

NC3

11E

0

NC1

10E

NC13

205E

NC2

271E

NC8

92E

NC9

272E

NC14

300E

D5

280E

IAS1

300E

NC4

189E

NC10

232E

NC6

436

NC11

340E 4CPR 400

NC12

244E

NC5

204E Sou t h e rn

e x tent

of roc k s o f Middle Oli g o c ene to Plio c e ne

age

Punta

Higüero

FIGURE 28.—Combined thickness of Aguada (Los Puertos) and Aymamón Limestone, northern Puerto Rico (outcrop extent modified from Monroe, 1980).

ATLANTIC OCEAN

CARIBBEAN SEA

IslaMona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

Area of outcrop

EXPLANATION  Aymamón Limestone Aguada (Los Puertos) Limestone  

Line of equal thickness of Aguada (Los Puertos)  and Aymamón Limestone—Interval 100 metersWell control point—Thickness in meters. E, existing thickness  after erosion (well name abbreviations on figure 21)

100

NC5

205E

0 20 KILOMETERS 5 10 150 5 10 MILES

67°15' 67°00' 66°45' 66°30' 66°15' 65°45' 66°00'

18°30'

18°15'

Base modified from U.S. Geological Survey 1:100,000 quadrangles:Arecibo, 1980, unpublished, and Humacao, 1979, unpublished
62 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

The Aguada (Los Puertos) Limestone crops out across the

entire northern coastal plain, except east of Río Bayamón

where the outcrop is narrow and discontinuous (fig.3; pl. 4).

Although the Aguada (Los Puertos) is predominantly carbonate rocks, it has been described as a transitional unit between

the underlying Cibao Formation and the overlying Aymamón

Limestone (Zapp and others, 1948; Monroe, 1980). At the

surface, the lower part of this unit consists of pink to grayishorange “chalky” fine crystalline limestones and fine- to

medium-grained calcarenite, commonly containing scattered

quartz sand (Monroe, 1980). Upper parts of the Aguada (Los

Puertos) commonly are grayish-pink and pale-orange thinly

bedded, partly cross-bedded, fossiliferous to “relatively

unfossiliferous” calcarenite (Monroe, 1980). Fossils include

Marginopora sp., Archaias angulatus, Gypsina globula and

other benthic foraminifers, mollusks, red algae, and green

algae. Meyerhoff and others (1983) describe the Los Puertos

Limestone as white, yellow, pink, and light-orange massive

limestone, light-brownish-gray to white recrystallized massive limestone, and white dolomitic limestone. East of the

Río de la Plata, the lower Aguada (Los Puertos) passes into

chalky limestone and calcareous clay indistinguishable from

the upper Cibao (Monroe, 1980).

In the western part of the outcrop, the Aguada (Los Puertos) Limestone is about 90 m thick (Monroe, 1980). East of

the Río Grande de Manatí thickness varies from about 60 m

updip to 110 m downdip, and east of the Río de la Plata the

formation thins to approximately 35 m (Monroe, 1980).

The Aguada (Los Puertos) Limestone rests conformably

on the Cibao Formation and in many places “it is difficult to

distinguish the two formations” (Monroe, 1980, p. 45). In

most places, Monroe (1980) marks the base of this unit at the

lowest occurrence of “calcarenite.” The top of this formation

is in sharp contact with the overlying Aymamón Limestone

(Monroe, 1980), and Meyerhoff and others (1983) believe

this contact to be an island-wide unconformity. In the easternmost outcrops, the Aguada (Los Puertos) intertongues

with the uppermost Cibao (Monroe, 1980; Meyerhoff and

others, 1983).

Subsurface

All of the NC core holes (figs. 21, 28) penetrated the

Aguada (Los Puertos) Limestone, with the possible exceptions of test holes NC 1 and NC 3, which cored sandy carbonate rock of uncertain equivalency (Scharlach, 1990). The

lower contact of the Aguada (Los Puertos) Limestone is gradational with the Cibao Formation, and for this report the

lower boundary of the Aguada (Los Puertos) is placed where

carbonate rock becomes the predominant lithology. The

upper boundary is marked by paleokarst in updip wells (NC

2, NC 9, and NC 10), but the subaerial unconformity apparently passes into a conformable contact downdip (Hartley,

1989; Scharlach, 1990), as previously suggested by Meyerhoff and others (1983). The upper contact of the Aguada

(Los Puertos), therefore, is poorly defined in downdip wells

(NC 14 and IAS 1). In the eastern part of the basin, the

Aguada (Los Puertos) Limestone is about 60 to 90 m. In the

western half, it is about 90 to 120 m thick and thickens in the

downdip well 4 CPR downdip to between 115 and 175 m,

according to where the top of the formation is placed (Briggs,

1961).

The Aguada (Los Puertos) Limestone typically consists of

shoaling-upward cycles of grayish-orange and yellowish-gray

skeletal wackestone to packstone several meters thick. In the

lower part of the formation, the predominant fauna are soritid

and miliolid benthic foraminifers and mollusks, and in many

places the limestone is sandy. Amphistegina sp., red algae,

corals, Halimeda sp., Kuphus sp., and echinoids become more

abundant up section and downdip (Hartley, 1989; Scharlach,

1990). Amphistegina sp. is the most abundant foraminifer in

all samples from the Aguada penetrated by 4 CPR (Gordon,

1961b). Calichified zones cap some of the depositional cycles

in the lower part of the Aguada (Los Puertos) in the west-central area (NC 5, NC 6, and NC 10). Elsewhere in the basin,

cycles consist of skeletal wackestone passing upward to skeletal packstone, skeletal packstone to packstone/grainstone, and

packstone to coral/red algae patch-reef limestone. The small

branching coral Stylophora affinis is common in the coralbearing layers.

In the updip wells NC 1 and NC 3 (pl. 4), the section overlying the Mucarabones Sand is sandy packstone and grainstone with thinner interbeds of clay and sand (Scharlach,

1990). Most limestones in this sequence have fauna similar to

that of the Aguada (Los Puertos) Limestone cored in the other

wells. The upper part of this updip sandy limestone sequence,

therefore, probably is time equivalent of the Aguada (Los

Puertos) Limestone.

In the downdip part of the east-central basin (NC 9 and

IAS 1), much of the Aguada (Los Puertos) Limestone is dolomitized. Elsewhere in the basin, the Aguada (Los Puertos)

contains scattered yellowish-brown dolomitic zones. Briggs

(1961) described the Aguada in the 4 CPR well as “largely calcarenite, possibly largely magnesian,” suggesting the mostbasinward Aguada (Los Puertos) also is dolomitic.

AYMAMÓN LIMESTONE (MIDDLE MIOCENE)

Outcrop

The Aymamón Limestone is exposed in topographic highs

across a wide belt that extends from the northwestern coast to

the Río de la Plata, but is covered in broad areas by surface

deposits of Quaternary and Tertiary(?) age. East of the Río de

la Plata, where much of the Aymamón Limestone is buried

under Quaternary alluvial and coastal deposits, the outcrops
GEOLOGY OF THE NORTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 63

are narrow and discontinuous (fig. 3). Monroe (1980) considers this limestone to be the most uniform in lithology and

thickness of all the Tertiary formations cropping out in northern Puerto Rico. Estimated thickness across the outcrop belt is

190 to 200 m (Monroe, 1980; Meyerhoff and others, 1983).

The Aymamón is “thick-bedded to massive commonly

quartz-free, very pure limestone” (Monroe, 1980, p. 53). The

lower half of the formation is very pale-orange to white, thickbedded quartz-free limestone. In some places, there are many

layers of limestone breccia, some of which may be organicreef debris, but much of which is solution breccia (Monroe,

1980). On northwestern coastal plain, the upper part of the

Aymamón is pale-grayish-orange unconsolidated chalk that

alternates with thick beds of recrystallized porcelain-like pure

limestone (Monroe, 1980, p. 53). Near the coast, the formation is dolomitic (Monroe, 1980). Some beds contain abundant fossils, including “Ostraea” haitensis in the upper part of

the formation, corals Montastrea limbata, Psammocora gasparillensis, and Porites sp., the echinoid Clypeaster cubensis,

and benthic foraminifers Marginopora sp., Archais sp., and

Gypsina sp. (Monroe, 1980).

In a study of the northwestern part of the outcrop, Seiglie

and Moussa (1984) observed the following stratigraphic

sequence, in ascending order: (1) branching-red-algae/miliolid/soritid back-reef lagoonal deposits, (2) open-shelf coralreef debris and gastropod-bivalve deposits, and (3) a forereef

containing encrusting coralline algae, an Amphistegina angulata assemblage and rare planktonic foraminifers. Meyerhoff

and others (1983) described a similar sequence of rock types

and included a locally absent basal unit of cross-bedded calcarenite. Osbourne and others (1979) also described an

Amphistegina angulata microfacies in the upper Aymamón.

The Aymamón Limestone rests sharply on the Aguada (Los

Puertos) Limestone (Monroe, 1980; Meyerhoff and others,

1983). Monroe (1980) describes a typical abrupt change from

reddish thin-bedded Aguada (Los Puertos) to very-pale-orange

and white thicker-bedded Aymamón. Meyerhoff and others

(1983) believe the Aymamón overlies a widespread erosional

unconformity. Judging from the descriptions of Seiglie and

Moussa (1975), Monroe (1980), and Meyerhoff and others

(1983), the upper boundary of the Aymamón Limestone is a

karsted surface. In the northwestern half of the Tertiary outcrop, the Aymamón is overlain by the Pliocene Quebradillas

Limestone (fig. 3; pl. 4).

Subsurface

Most of the deeper water-supply wells and test wells in

northern Puerto Rico are located within the outcrop belt of

the Aymamón Limestone; therefore, the total thickness of this

formation was penetrated only by NC 6 and 4 CPR, which

started in the Quebradillas Limestone. The Aymamón Limestone is 330 m in NC 6 and is estimated to be approximately

300 m thick in the 4 CPR (pl. 4; fig. 28).

The Aymamón Limestone is lithologically similar across

most of the North Tertiary Basin. Typically, this formation is

yellowish-gray and pale-orange skeletal wackestone and

packstone with lesser grainstone and common thin coral rudstone and coral-red algae boundstone. For the most part, the

Aymamón is composed of shoaling-upward depositional

cycles that range from a meter to several meters thick. Cycles

commonly consist of skeletal wackestone or wackestonepackstone passing upward to packstone-grainstone. Some

cycles are capped by coral rudstone or coral-red algae boundstone.

Red algae and Amphistegina sp. are the most abundant

grain types. Other common skeletal constituents include miliolids, soritids, homotremids, Gypsina sp., echinoids,

bivalves including Kuphus sp., and Halimeda sp. The small

branching coral Stylophora affinis is scattered throughout the

formation. Other corals identified in cores from NC-2 (S. H.

Frost, UNOCAL, written commun., 1991) include Porites

waylandi, Porites sp. cf P. baracoaensis, Porites sp. cf P. portoricoensis, Siderastrea siderea, Montastrea sp. cf M. annularis, Montastrea sp. cf M. costata, Montastrea sp. cf M.

bainbridgensis, and Goscinaraca coleti.

Dolomitized zones are common but unevenly distributed

in the subsurface Aymamón Formation. Many coral-rich layers in the depositional cycles are selectively dolomitized.

Most of the Aymamón in NC 9 is dolomitized. Dolomite and

dolomitic limestone also are common in the lower half of the

Aymamón in NC 2, NC 4, and NC 11 (Hartley, 1989;

Scharlach, 1990). The lower and upper parts of the Aymamón

in NC 6 have dolomitic zones, and the upper Aymamón in 4

CPR is dolomite (Briggs, 1961).

QUEBRADILLAS LIMESTONE (UPPERMOST MIOCENE TO 

PLIOCENE)

Outcrop

The uppermost Tertiary unit exposed in northern Puerto

Rico is the Quebradillas Limestone (pl. 4). Most outcrops of

this uppermost Miocene-Pliocene limestone are in the northwestern part of the coastal plain west of the Río Grande de

Arecibo (fig. 3). From Arecibo to north of Manatí there are

only a few small isolated outliers of Quebradillas, and this

unit is absent in northeastern Puerto Rico. The Quebradillas

Limestone overlies a paleokarst developed on the Aymamón

Limestone and underlies, in some places, alluvial and coastal

deposits of Quaternary age.

Monroe (1980) divides the Camuy (Quebradillas) Limestone into three parts. The lower unit is about 30 to 40 m of

pale-yellowish-orange thin- to medium-bedded “chalky limestone” and pale-orange to pale-reddish-brown thin-bedded
64 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

and cross-bedded calcarenite. Some layers contain scattered

quartz sand, and some basal beds are highly ferruginous.

Oyster shells are common near the base of the formation in

some localities. Much of this lower unit is rich in planktonic

foraminifers. A middle unit consists of about 25 to 30 m of

“hard very-pale-orange to light-brown ferruginous calcarenite

commonly containing blebs of limonitic clay and scattered

grains of quartz and magnetite” (Monroe, 1980, p. 61). These

fossiliferous beds contain abundant planktonic foraminifers,

benthic foraminifers including Nummulites sp., bryozoans,

echinoids, and mollusks. An upper unit is recognized only in

the central part of the outcrop belt between Río Camuy and

Río Grande de Arecibo (fig. 3, 21). It is composed of more

than 40 m of fossiliferous “chalk, sandy chalk, sandstone,

sandy limestone, and limestone” containing abundant planktonic foraminifers (Monroe, 1980).

Based on a study of outcrops along Highway 2 northwest

of the town of Quebradillas (fig. 3), Seiglie and Moussa

(1975) also divided the “Quebradillas Limestone” into three

units, for which they give no thicknesses. The lower unit,

which overlies and infills a dissolution surface on the

Aymamón Limestone, is planktonic-foraminifer limestone

containing abundant “deep-water” foraminifers such as

Sphaeroidinella sp. and Spheroidinellopsis sp. In some

places, the lower unit contains layers of broken oyster shells

interbedded with the globigerinid limestone. The middle unit

is nummulitid-globigerinid limestone with abundant Nummulites cojimarensis as well as other benthic foraminifers

Amphistegina sp. Planorbulina sp., Carpenteria sp., and Sporadotrema sp. Planktonic foraminifers decrease up section,

and the upper part of the Quebradillas Limestone is skeletal

limestone rich in the benthic foraminifer Amphistegina.

Other fossils are red algae, some large miliolids, and bryozoans. Seiglie and Moussa (1975) mention terrigenous material

only in their upper unit and at the base of the formation in the

fill of solution cavities.

Subsurface

The Quebradillas Limestone was encountered only in test

wells NC 6 and 4 CPR (pl. 4). The basal 26 m of this formation in NC 6 is yellow-orange globigerinid packstone, which

sharply overlies uppermost Aymamón packstone containing

abundant Amphistegina sp. and red algae (Hartley, 1989).

Briggs (1961) did not recognize the Quebradillas in the 4

CPR well, but the upper 120 m of his “Aymamón” contain

abundant planktonic foraminifers (Gordon, 1961b). Monroe

(1980, p. 63-64) estimates that 170 m of Camuy (Quebradillas) Limestone were penetrated in this well. The upper 45 m

in the 4 CPR are interbedded dense limestone and limestone

rubble and marl with quartz and clay in the uppermost zone

(Briggs, 1961). Below this, the “upper Aymamón” is grayishorange, pale-orange-pink, and yellowish-orange “calcarenite”

with scattered quartz grains. Gordon (1961b, p. 33) characterizes the fauna in this interval as an “extremely diverse assemblage of fossils in which globigerinids and amphisteginids are

both abundant.”

DEPOSITIONAL HISTORY

INTRODUCTION

Rock types and faunal assemblages were the principal data

used to interpret the depositional environments of the major

stratigraphic units of the North Coast Tertiary Basin. After

deposition of the terrigenous sediments of the San Sebastián

Formation, the general depositional setting during the rest of

the Tertiary was a gently northward-inclined carbonate platform, i.e., ramp (Ahr, 1973) or distally steepened ramp (Read,

1985). The landward margin of the carbonate platform was

fringed by paralic siliciclastic systems. “Inner platform” and

“inner neritic” refer to the landward margin of the platform,

which generally is characterized by a less diverse fauna than

the middle platform. This zone contains subtidal and intertidal

settings. The terms “middle platform” and “middle neritic”

refer to open, generally subtidal environments from many tens

of meters deep to shallow shoals several centimeters deep.

“Outer platform” and “outer neritic” refer to parts of the platform from many tens of meters to a few hundreds of meters

deep. Although several authors describe northern Puerto Rico

depositional environments during the middle and late Tertiary

in terms of “backreef” and “forereef,” this usage is avoided

here inasmuch as there is no evidence for any shelf-break reef

tracts which would have created these environments.

MIDDLE TO LATE OLIGOCENE

At outcrop (Monroe, 1980; Frost and others, 1983) and in

the shallow subsurface (Hartley, 1989; Scharlach, 1990), the

San Sebastián Formation records a general change from nonmarine to shallow-marine deposition. The basal red- and

green-mottled nonfossiliferous mudstone and sandstone represent alluvial-plain or delta-plain sedimentation. The overlying

mudstone and marl with restricted-marine fauna and highly

carbonaceous layers were deposited in coastal bays, marshes,

and swamps. Glauconitic siliciclastic and carbonate rocks

with diverse, normal-marine fauna near the top of the San

Sebastián Formation were deposited on the shallow open platform as sea level rose and terrigenous influx waned. Repeated

depositional cycles within the upper San Sebastián Formation

suggest high-order fluctuations of relative sea level during the

larger-scale transgression.

The thick lowermost “San Sebastián Formation” that is

penetrated only by the 4 CPR well (pl. 4) apparently records

one major depositional cycle and is not exposed in outcrop.

The ratios of planktonic to benthonic foraminifers reported by

Gordon (1961b) indicate that the lowermost San Sebastián
GEOLOGY OF THE NORTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 65

was deposited in nonmarine to inner-platform environments,

but most of the typical part of the formation was deposited on

the middle and outer platform at depths perhaps greater than

100 m (Gordon, 1961b). The fauna of the upper part of the

unit, however, suggests a return to a shallower-platform environment.

LATE OLIGOCENE TO EARLY MIOCENE

A major rise of sea level began during the late Oligocene.

This is recorded by the gradation of shallow-marine deposits

of the upper San Sebastián Formation into the inner- to midplatform carbonate and terrigenous deposits of the lower Lares

Limestone. Small coral patches as well as rhodolite and largeforaminifer packstone and grainstone of the lower Lares represent tidal shoals on a shallow, normal-salinity inner platform.

Burrowed large-foraminifer/red algae wackestone accumulated in subtidal parts of the inner to middle platform as sea

level continued a general rise.

Over most of the northern Puerto Rico platform, the middle

part of the Lares Limestone records the maximum transgression of the late Oligocene sea (fig. 29). The middle unit was

deposited in middle to outer-neritic environments, except in

the western and eastern extremities of the basin, where predominantly terrigenous inner platform to nonmarine sediment

accumulated (fig. 29). Coral reefs developed on the western

middle platform (Frost and others, 1983) and in the east-central platform in the vicinity of the NC 9A test well (fig. 21).

Rhodolite and large-foraminifer deposits accumulated in

somewhat deeper parts of the open platform in the central

basin. An increase in planktonic foraminifers in the middle

Lares in the west-central part of the basin (NC 5, 4 CPR, and

Dupont 5) further attest to the deepening of the northern Puerto

Rico platform.

During deposition of the upper Lares Limestone, sea level

fell. Soritid- and miliolid-rich limestone of the uppermost

Lares indicate that inner- to middle-platform environments

spread seaward over much of the early Miocene platform.

Packstone and wackestone with corals, red algae, and planktonic foraminifers interbedded with fine terrigenous rocks in

the upper Lares of the east-central part of the basin suggest

that this area was on a middle to outer platform that was

receiving terrigenous influx from the east. At this same time in

the west-central basin, the carbonate platform shoaled to shallow water, as recorded by the soritid-miliolid carbonate rocks

at the top of the Lares. On the western flank of the basin,

paralic and inner-platform terrigenous sediments continued to

accumulate. Strata equivalent to upper Lares in the NC 11

well are predominantly claystone and marl with only thin

limestone, mostly with shallow-marine fossils. On the flanks

of paleohighs in the eastern part of the basin (the vicinity of

wells NC 1 and NC 3; pl. 4), the mudstone, sandstone, and

conglomerate of Mucarabones Sand were deposited in nonmarine to shallow-marine environments.

EARLY MIOCENE

Following the regression of sea level which marked the end

of Lares deposition, sea level began to rise again across the

gently inclined insular shelf of northern Puerto Rico. The patterns of deposition during this transgression were the most

complicated of any time during the Neogene (fig. 30). Apparently, the complex facies patterns of the Cibao Formation

reflect differential uplift or subsidence, or both, in the highlands and under the marine platform. The western part of the

basin received an influx of terrigenous material associated with

the Guajataca delta system. The inner-platform clayey wackestone and marl of the lower Cibao Formation in the subsurface,

as well as the terrigenous Guajataca Member at the outcrop

(fig. 24A), attest to a large input of siliciclastic sediment in

northwestern Puerto Rico. Deposition of the Montebello carbonate sediments at this same time indicates that the west-central part of the basin remained an inner- to middle-platform

environment with little terrigenous influx from the land. Contemporaneous with Montebello deposition, a narrow area in

the east-central part of the basin was receiving the abundant

influx of fine siliciclastic sediment that makes up the mudstone

unit. This predominantly terrigenous member has the deepestwater fauna of any Cibao unit indicating that the east-central

basin was undergoing preferential subsidence during the early

Miocene. The abrupt eastern and western boundaries of both

the mudstone unit and its outcrop equivalent may reflect faultcontrolled downdropping of a narrow trough that trended

north-northwest. Farther east in the east-central basin, the carbonate and terrigenous sedimentary rocks of the Río Indio/

Quebrada Arenas Limestone accumulated in inner- to middleplatform environments. At the eastern edge of the basin, fluvial deltaic deposition of the Mucarabones terrigenous sediments continued.

The lower Cibao Formation, except perhaps in the westernmost basin, contains a faunal succession which records a general deepening of the sea, followed by a general fall in sea

level. In the east-central part of the basin, inner-platform sediments of undifferentiated Cibao were deposited over mid-platform sediments of the uppermost mudstone unit and Río Indio/

Quebrada Arenas Limestone. In the western part of the basin,

the area of accumulation of carbonate sediments (Montebello

Limestone Member) became progressively restricted as clayey

carbonate sediments (undifferentiated Cibao) became increasingly predominant. 

The maximum fall of sea level in this major fluctuation of

sea level is recorded by several lithologic changes across the

basin. In the east, channels subsequently filled with Miranda

Sand (fig. 24A) were incised into the upper Quebrada Arenas

Limestone (Monroe, 1980), and Miogypsina-bearing lime-
66 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS        Lago Caonillas Lago de la Plata Lago Loíza Camuy de Culebrinas de Arecibo Grande de de de Cibuco la Plata Grande Loíza de Lago Guajataca Añasco

Bayamón

ATLANTIC OCEAN Bahía de

San Juan

Río

Río

Río

Río

Río

Río

Río

Río

Río

Río

Río

Río

Guajataca

Grande

aT n

ma á

Grande

Manatí

nI

i d o

Lago Dos  Bocas

Ciales

Florida

Morovis

Aguadilla

Moca

Quebradillas

Lares

Arecibo Barceloneta

VegaAlta

San Juan

Bayamón

San

Sebastián

NC11

NC6

AI0

4CPR

IAS3

IAS4

NC10

UC

NC5

A3

A1 NC4 IAS2

IAS1

D5

NC14

NC9A

NC9 NC2

NC8

TB NC13

NC1

NC15

NC3

NC12

Sou t h e rn

e x tent

of roc k s o f Middle Oli g o c ene to Plio c e ne

age

Punta

Fluvial-deltaic

Higüero

Middle platform

Middle platform

Deeper platform

Inner platform Deltaic– fluvial Inner platform

FIGURE 29.—Major depositional environments and facies patterns during deposition of the upper Lares Limestone, northern Puerto Rico.

ATLANTIC OCEAN

CARIBBEAN SEA

IslaMona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

Siliciclastic sediment, marl, and carbonate sediments

EXPLANATION 

Predominantly carbonate sediments Predominantly siliciclastic sediments Well control point (well name abbreviations on figure 21) NC5

0 20 KILOMETERS 5 10 150 5 10 MILES

67°15' 67°00' 66°45' 66°30' 66°15' 65°45' 66°00'

18°30'

18°15'

Base modified from U.S. Geological Survey 1:100,000 quadrangles:Arecibo, 1980, unpublished, and Humacao, 1979, unpublished
GEOLOGY OF THE NORTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 67             Lago Caonillas Lago de la Plata Lago Loíza Camuy de Culebrinas de Arecibo Grande de de de Cibuco la Plata Grande Loíza de Lago Guajataca Añasco

Bayamón

ATLANTIC OCEAN Bahía de

San Juan

Río

Río

Río

Río

Río

Río

Río

Río

Río

Río

Río

Río

Guajataca

Grande

aT n

ma á

Grande

Manatí

nI

i d o

Lago Dos  Bocas

Ciales

Florida

Morovis

Aguadilla

Moca

Quebradillas

Lares

Arecibo

Barceloneta

VegaAlta

San Juan

Bayamón

San

Sebastián

NC11

NC6

AI0

4CPR

IAS3

IAS4

NC10

UC NC5

A3

A1

NC4

IAS2

IAS1

D5

NC14

NC9A

NC9 NC2

NC8

TB NC13

NC1

NC15

NC3

NC12

Sou t h e rn

e x tent

of roc k s o f Middle Oli g o c ene to Plio c e ne

age

Punta

Higüero Middle platformMiddle platform Inner platform

Inner platform

Deltaic–

fluvial Deeper platform

Deltaic?

FIGURE 30.—Major depositional environments and facies patterns during deposition of the lower Cibao Formation, northern Puerto Rico.

ATLANTIC OCEAN

CARIBBEAN SEA

IslaMona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

Predominantly siliciclastic sediments

EXPLANATION Mixed siliciclastic and carbonate sediments Predominantly carbonate sediments Predominantly siliciclastic mud Well control point (well name abbreviations on figure 21) NC5

0 20 KILOMETERS 5 10 150 5 10 MILES

67°15' 67°00' 66°45' 66°30' 66°15' 65°45' 66°00'

18°30'

18°15'

Base modified from U.S. Geological Survey 1:100,000 quadrangles:Arecibo, 1980, unpublished, and Humacao, 1979, unpublished
68 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

stone and marl of the lower Cibao are overlain by poorly to

non-fossiliferous clay and silt (Scharlach, 1990). In the updip

part of the west-central part of the basin, the maximum fall

may be recorded by the apparently sharp (partly karsted) top of

the thickest Montebello Limestone and, farther west, by the

top of the Guajataca fanglomerate. 

Another rise in sea level brought deposition of the widespread blanket of inner- to mid-platform clayey limestone,

marl, and claystone of the upper undifferentiated Cibao

(fig. 31). Soritid- and miliolid-rich carbonate and terrigenous

sediments were deposited across most of the basin. Less common were inner-platform foraminifer-sand shoals and Stylophora affinis patches. Prograding fluvial systems deposited

sand and conglomerate onto the inner parts of the platform

and introduced finer terrigenous sediment onto more basinward areas. Several zones of caliche and paleosol horizons

within the upper Cibao record repeated subaerial exposure

that result from high-frequency sea-level fluctuations and perhaps shifting delta lobes. At the eastern edge of the basin, fluvial-deltaic sedimentation (Mucarabones Sand) continued.

LATE EARLY MIOCENE TO EARLY MIDDLE MIOCENE

As the major sea-level rise continued, terrigenous sedimentation waned, and the predominantly carbonate sediments of

the Aguada (Los Puertos) Limestone were deposited basinwide

(fig. 32). Aguada limestones are predominantly middle platform packstone and grainstone rich in foraminifers and red

algae with patches of coral. An abundance of soritid and miliolid foraminifers in the lower part of the formation passing

upward to more abundant Amphistegina sp. records a general

rise in sea level during deposition of the Aguada (Los Puertos)

Limestone. Caliche zones within this formation in the westcentral part of the basin indicated that parts of the carbonate

platform were periodically exposed because of high-frequency

fluctuations of sea level. The karst zone at the top of updip

parts of the Aguada (Los Puertos) also is evidence of a regional

fall in relative sea level.

MIDDLE MIOCENE

After the end of Aguada (Los Puertos) deposition, the North

Tertiary Basin was flooded by another rise in sea level. The

shoreline moved farther inland than during previous OligoceneMiocene sea-level rises, and middle-platform carbonate sediments of the Aymamón Limestone were deposited across the

entire area of the platform that is preserved in outcrop and the

subsurface. The Aymamón Limestone, rich in Amphistegina

sp., red algae, and coral is typical of middle-platform carbonates.

Sometime after deposition of the Aymamón Limestone,

uplift or a major fall in sea level, or both, exposed much of the

northern Puerto Rico carbonate platform to subaerial weathering. The extensive karst zone at the top of the Aymamón began

to develop at this time.

LATEST MIOCENE-PLIOCENE

A rise in sea level deeply flooded northern Puerto Rico during the late Miocene (Seiglie and Moussa, 1984). The globigerinid limestone of the lower Quebradillas Limestone records

the outer-platform environments that existed during the latest

Miocene and early Pliocene. An upward decrease in planktonic

foraminifer and increase in Nummulites sp. in the Quebradillas

at the outcrop (Monroe, 1980) suggests that sea level began to

fall during deposition of the upper part of this formation.

EFFECTS OF STRUCTURAL MOVEMENTS ON 

DEPOSITION OF UPPER OLIGOCENE TO LOWER 

MIOCENE ROCKS

The thickness of the San Sebastián Formation is variable,

apparently reflecting paleotopography. On the outcrop, this

formation is thin or absent from just west of the Río Grande de

Arecibo to just east of the Río Grande de Manatí (pl. 4; fig. 33),

which suggests basement rocks in this area were elevated during deposition of the San Sebastián. Indeed, the Lares Limestone also thins and locally is absent over the western part of

this same area, indicating that paleohighs persisted into the

early Miocene (fig. 33). Another area where the San Sebastián

Formation is relatively thin over an apparent paleohigh is from

the Río Grande de Arecibo westward to about 2 km east of the

town of San Sebastián. West of that point, the San Sebastián

Formation thickens, apparently reflecting subsidence in the

western end of the basin and influx of terrigenous sediment

from an uplifted region to the south.

Lares Limestone accumulated across the central basin on

the flanks of basement highs. Carbonate sediments may have

been deposited in this area not only because of the terrigenous

influx in the eastern and western ends of the basin, but also

because basement highs provided platforms for carbonate

buildup (fig. 33). Angular discontinuities within the Lares

Limestone (Frost and others, 1983) probably reflect slight vertical movements of basement blocks during deposition of the

Lares.

The distribution of the Montebello Limestone and the mudstone unit may represent the most striking control of differential structural movement during the early Miocene

(figs. 25, 26). The Montebello carbonate buildup is directly

above the paleohighs over which the San Sebastián is thin.

This implies that this area remained a slowly subsiding and

probably relatively high platform during Montebello deposition. The area of the deeper-water mudstone unit, on the contrary, apparently was a downdropped trough bounded on the

west and east by shallow carbonate platforms. These facies

relations might be explained by reactivated vertical movement
GEOLOGY OF THE NORTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 69       Lago Caonillas Lago de la Plata Lago Loíza Camuy de de Arecibo Grande de dede Cibuco la Plata Grande Loíza de Lago Guajataca Añasco

Bayamón

ATLANTIC OCEAN Bahía de

San Juan

Río

Río

Río

Río

Río

Río RíoRío

ío R Río

Río

Río

Guajataca

Grande

aT n

ma á

Grande

Manatí

nI

i d o

Lago Dos  Bocas

Culebrinas

Ciales

Florida

Morovis

Aguadilla

Moca

Quebradillas

Lares

Arecibo Barceloneta

VegaAlta

San Juan

Bayamón

San

Sebastián

NC11

NC6

AI0

4CPR

IAS3

IAS4

NC10

UC

NC5

A3

A1 NC4 IAS2

IAS1

D5

NC14

NC9A

NC9 NC2

NC8

TB NC13

NC1

NC15

NC3

NC12

Sou t h e rn

e x tent

of roc k s o f Middle Oli g o c ene to Plio c e ne

age

Punta

Higüero Deltaic–fluvial Deltaic?

Inner platform

Inner platform Inner platform

Middle platform

FIGURE 31.—Major depositional environments and facies patterns during deposition of the upper Cibao Formation, northern Puerto Rico.

ATLANTIC OCEAN

CARIBBEAN SEA

IslaMona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

Predominantly siliciclastic sediments

EXPLANATION Mixed siliciclastic and carbonate sediments Well control point (well name abbreviations on figure 21) NC5

0 20 KILOMETERS 5 10 150 5 10 MILES

67°15' 67°00' 66°45' 66°30' 66°15' 65°45' 66°00'

18°30'

18°15'

Base modified from U.S. Geological Survey 1:100,000 quadrangles:Arecibo, 1980, unpublished, and Humacao, 1979, unpublished
70 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS      Lago Caonillas Lago de la Plata Lago Loíza Camuy de Culebrinas de Arecibo Grande de de de Cibuco la Plata Grande Loíza de Lago Guajataca Añasco

Bayamón

ATLANTIC OCEAN Bahía de

San Juan

Río

Río

Río

Río

Río

Río

Río

Río

Río

Río

Río

Río

Guajataca

Grande

aT n

ma á

Grande

Manatí

nI

i d o

Lago Dos  Bocas

Ciales

Florida

Morovis

Aguadilla

Moca

Quebradillas

Lares

Arecibo Barceloneta

VegaAlta

San Juan

Bayamón

San

Sebastián

NC11

NC6

AI0

4CPR

IAS3

IAS4

NC10

UC

NC5

A3

A1 NC4 IAS2

IAS1

D5

NC14

NC9A

NC9 NC2

NC8

TB NC13

NC1

NC15

NC3

NC12

Sou t h e rn

e x tent

of roc k s o f Middle Oli g o c ene to Plio c e ne

age

Punta

Higüero Inner platform

Middle platform

FIGURE 32.—Major depositional environments and facies patterns during deposition of the Aguada (Los Puertos) Limestone, northern Puerto Rico.

ATLANTIC OCEAN

CARIBBEAN SEA

IslaMona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

Predominantly carbonate sediment

EXPLANATION Mixed siliciclastic and carbonate sediment Well control point (well name abbreviations on figure 21) NC5

0 20 KILOMETERS 5 10 150 5 10 MILES

67°15' 67°00' 66°45' 66°30' 66°15' 65°45' 66°00'

18°30'

18°15'

Base modified from U.S. Geological Survey 1:100,000 quadrangles:Arecibo, 1980, unpublished, and Humacao, 1979, unpublished
GEOLOGY OF THE NORTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 71       

  

 



Sea level near the end of deposition of Lares Limestone

Readjustment of basement blocks during deposition of Lares resulted in

angular unconformities within Lares Limestone

Sea level during deposition of Montebello Limestone

Montebello carbonates built up on high block

Mud accumulated in

down-dropped block

0 5 10 15 20 KILOMETERS0 5 10 MILES

0

100

FEET

1,000

500

0

500400300200

METERS

0

100

FEET

1,000

500

0

500400300200

METERS

WEST EAST

LATE OLIGOCENE

WEST EAST

EARLY MIOCENE

VERTICAL EXAGGERATION X 25VERTICAL EXAGGERATION X 25

EXPLANATION 



Fan deltaCibao siliciclastic and carbonate sedimentsMontebello carbonatesMudstone unitQuebrada Arenas and Río Indio LimestoneMucarabones and San Sebastián siliciclastic sedimentsLares carbonatesBasement volcanic, sedimentary, and intrusive igneous rocks

FIGURE 33.—Conceptual model of the southern part of the North Coast Tertiary Basin in Puerto Rico showing influence of structural movement on late Oligocene and early

Miocene deposition.
72 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

of basement segments during deposition of the lower Cibao

Formation (fig. 33).

Beginning with the deposition of the uppermost Cibao, the

whole northern Puerto Rico basement underwent more uniform

subsidence. This is indicated by the relatively minor variation

in thickness of the Aguada (Los Puertos) and Aymamón Limestones along depositional strike (fig. 28).

SEQUENCE STRATIGRAPHY OF OLIGOCENE TO 

MIDDLE MIOCENE ROCKS

INTRODUCTION

High-resolution biostratigraphy of the northern Puerto Rico

limestone province is impractical, because fossils are mostly

shallow-water forms with fairly wide stratigraphic ranges. This

sedimentary section, however, can be subdivided into major

packages of chronostratigraphically related strata using the

concepts of sequence stratigraphy, which was developed from

seismic stratigraphy (Mitchum and others, 1977; Van Wagoner and others, 1988). “Depositional sequences” are stratigraphic units “composed of a relatively conformable

succession of genetically related strata and bounded by

unconformities or their correlative conformities” (Mitchum

and others, 1977, p. 53). The sequence boundary is meant to

be a chronostratigraphic marker.

For the most part, depositional sequences and their

boundaries are the products of an interplay between eustatic

sea-level fluctuations, regional tectonic movements, and sedimentological processes, all of which affect relative sea level.

Ideally, the sequence can be divided into “systems tracts,”

units deposited during specific intervals of a third-order (0.5

to 5 m.y.) fluctuation of relative sea level. A type 1 sequence

is deposited during a third-order rise in relative sea level (or

base level) that followed a fall in base level great enough to

expose the shelf to subaerial erosion. This type of sequence

is made up of lowstand, transgressive, and highstand systems

tracts (fig. 34). According to the siliciclastic-based model

(Van Wagoner and others, 1988), the lowstand systems tract

(LST) commonly can be divided into a lowstand fan (LSF),

deposited during the base-level fall, and a lowstand wedge

(LSW), deposited at the lowstand and during the early stages

of base-level rise. The LST is best developed in basins with a

break in slope (shelf break). Where the LST is deposited in a

basin without a striking break in slope, such as the North

Coast Tertiary Basin of Puerto Rico, it consists of a relatively

thin lowstand wedge (Van Wagoner and others, 1988). In a

type 2 sequence, which is deposited after a base-level fall not

great enough to expose the shelf, the lowermost unit is the

shelf-margin wedge systems tract (SMW) (fig. 34). The middle systems tract of type 1 and 2 sequences is the transgressive systems tract (TST) deposited during the subsequent rise

of relative sea level. The base of this systems tract is the top

of the LST or SMW. Its upper boundary is the maximum

flooding surface, which marks the point at which the thirdorder depositional framework changes from the retrogradational TST to the aggradational/progradational highstand systems tract (HST). This upper unit (HST) is deposited during

the late part of a sea-level rise, a stillstand, and the early part

of a fall.

Although sequence stratigraphy was derived from seismic

stratigraphy of siliciclastic basin fills, the general concepts

also can be applied, with some modifications, to analyses of

carbonate sections (Sarg, 1988; Handford and Loucks, 1993)

(figs. 34 and 35). The response of siliciclastic depositional

systems to changes in relative sea level, however, is somewhat different than that of carbonate depositional systems

because of the fundamental differences in the source of these

sediments. Siliciclastic deposits are derived from outside the

basin; carbonate sediments are generated within the basin.

Marine carbonate deposits accumulate most rapidly in parts

of basins where water depth, temperature, and geochemistry

are optimal for carbonate-producing organisms and where

siliciclastic influx is slight. The rate of carbonate production,

therefore, generally is greatest during times of high sea level

when large areas of continental margins are flooded with relatively shallow water and siliciclastic deposition is confined to

coastal zones. Lithofacies patterns indicate that the Tertiary

limestones of northern Puerto Rico were deposited on a ramp.

The coral-algal constituents of these limestones imply they

accumulated under a tropical climate. In addition, the associated deltaic deposits and the lack of evaporites suggest the

climate was humid (fig. 35). Falls in relative sea level may

terminate or retard carbonate production. Commonly, even

slight drops in sea level may expose broad areas of a carbonate platform, halting carbonate sedimentation and bringing on

karstification or calichification of the inner platform.

SEQUENCE BOUNDARIES IN NORTHERN PUERTO RICO

The distinctive responses of carbonate production and

siliciclastic sedimentation to changes in relative sea level

were a major consideration in interpreting the sequence

stratigraphy of the North Coast ground-water province. This

stratigraphic analysis, however, suffers from the sparsity of

published detailed descriptions of outcrop sections as well as

from the cursory descriptions of cores taken during later

stages of the core drilling project and lack of downdip control. Nevertheless, the Oligocene through middle Miocene

sedimentary rocks (San Sebastián through Aymamón) can be

divided tentatively into five major depositional sequences

(fig. 36). Well and outcrop control is insufficient to resolve

minor cycles on a basinwide scale.

The oldest sequence was encountered only in the “San

Sebastián Formation” penetrated by the 4 CPR well between

about 1,430 to 1,690 m (pl. 4; fig. 36). This interval, accord-
GEOLOGY OF THE NORTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 73

  























 

























   























KEY TO FACIES

Tidalsupratidal

flat

Shelf Shelf

margin

Slope Basin

fine grained

EUSTASY

TECTONIC

SUBSIDENCE

SEDIMENT

ACCOMMODATION

POTENTIAL

RELATIVE

CHANGE OF

SEA LEVEL

+

-

CANYON

SB1

SB2

HST

TST

mfs

HST

TS

LSW

SMW

LOW HIGH

SB2

SMW

SB1

LSF

LSW

TST

HST

high

sea-level

low

SMW

A SMW

B

C

D

LSF

LSW

TST

TST

TST

TST

HST HST HST

HST

HST

A

B

C

D

shelf/platform

emergence

LSF

LSW

LST

TST

HST

HST

DLS

time

time

sea-level sea-level

sea-level

sea-level

RAPID SEA LEVEL FALL GIVES

TYPE 1 SEQUENCE:

LOWSTAND SLOPE DEBRIS

HIGHSTAND DEPOSITION

TYPE 1 SEQUENCE:

LOWSTAND WEDGE AND

TRANSGRESSIVE DEPOSITS

TYPE 2 SEQUENCE:

SHELF-MARGIN WEDGE AND

TRANSGRESSIVE DEPOSITS

basin

slope

LSF



SUPRATIDAL/TIDAL

PLATFORM/SHELF

PLATFORM-MARGIN

GRAINSTONES/REEF

MEGABRECCIAS/SANDS

FORESLOPE

TOE-OF-SLOPE/BASIN

HST

TST

LST

 

SMW

=

=

=

=

HIGHSTAND SYSTEMS TRACT

TRANSGRESSIVE SYSTEMS TRACT

LOWSTAND SYSTEMS TRACT

LST = LOWSTAND FAN

LSW = LOWSTAND WEDGE

SHELF MARGIN WEDGE

SYSTEMS TRACT

SYSTEMS TRACTS

SB

DLS

TS

=

=

=

SEQUENCE BOUNDARIES

SB 1 = TYPE-1

SB 2 = TYPE-2

DOWNLAP SURFACE

mfs = maximum flooding surface

TRANSGRESSIVE SURFACE

SURFACES

LITHOFACIES

FIGURE 34.—General model for a sequence-stratigraphic framework (reprinted from Tucker and Wright, 1990,

and published with permission).
74 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS               4TH ORDER 3RD ORDERRELATIVE SEA LEVEL SUBSIDENCE HST LST

LSTTST

LST

HST

LST

TST

TIME

Caves and karst

Type 1 SB

Type 1 SB

TIME

EUSTASY

THICKNESS

Sea level

NOT TO SCALE

Cave Cave

EXPLANATION





System tracts Highstand Transgressive LowstandSequence boundaryMaximum flooding surface

Carbonate lithofacies Pertidal-lagoonal mustone and dolomite Grainstones and packstones Outer shelf wackestones Basinal organic-rich mudstones

FIGURE 35.—Depositional-sequence model for an inclined carbonate platform or "ramp" located in a humid environment

(reprinted from Hanford and Loucks, 1993, and published with permission).
GEOLOGY OF THE NORTH COAST GROUND-WATER PROVINCE OF PUERTO RICO 75

ing to sample descriptions and micropaleontology (Gordon,

1961b; Briggs, 1961), represents an onlap of the basement

complex during the Oligocene. The maximum transgression is

recorded at about 1,525 m, where the foraminiferal assemblage indicates outer neritic to bathyal depositional environments (Briggs, 1961; A. Melillo, Chevron, oral commun.,

1993). Briggs (1961, p. 8) speculated that the lower part of the

San Sebastián in 4 CPR core is older than any San Sebastián

at outcrop. In support of this, it is dubious that most of this

San Sebastián section can be correlated with the San

Sebastián of updip core tests (pl. 3). Rather, any record of

this rise of relative sea level probably was removed by erosion during a subsequent major fall in relative sea level. It is

possible that the interval about 1,370 and 1,430 m, which is

composed of claystone with common sand layers and some

coal layers (Briggs, 1961), represents a lowstand wedge

deposited during this fall in base level. Without core data,

however, the character of this lithologic unit is speculative.

A second depositional sequence comprises the San

Sebastián Formation and the Lares Limestone. The upper San

Sebastián-lower Lares interval records progressive onlap onto

the subaerially exposed basement rock during a rise in relative

sea level. The top of this TST is within the middle Lares,

which records the maximum transgression during deposition

of this carbonate unit (Seiglie and Moussa, 1984; Hartley,

1989; Scharlach, 1990). Upper parts of the Lares are the HST.

The upper boundary of this depositional sequence is the top of

the Lares Limestone as mapped by Monroe (1980). In the subsurface, the boundary is less distinct, but it is placed where the

generally progradational upper Lares gives way to the various

generally retrogressive units of the lower Cibao Formation. A

drop in base level after deposition of the Lares Limestone is

indicated by the influx of terrigenous sediment of much of the

lower Cibao. In the central part of the basin, this drop in relative sea level is suggested by the widespread oyster beds at the

base of the Montebello and Río Indio Members of the Cibao

Formation (Monroe, 1980).

UPPER

MIDDLE

UPPER

LOWER

"MIDDLE"

SEQUENCE

BOUNDARY SERIES STRATIGRAPHIC UNITS

MIOCENE OLIGOCENE

5

4

3

2

1

EXACT RELATION OF SEQUENCE BOUNDARIES AND TIME-STRATIGRAPHIC

BOUNDARIES IS UNCERTAIN DUE TO POOR BIOSTRATIGRAPHIC CONTROL

Aymamón Limestone

Aguada (Los Puertos)

Limestone

Undifferentiated Cibao

Lares Limestone

San Sebastián

Formation

"San Sebastián Formation"

of No. 4CPR

Montebello

Limestone

Member

Mudstone

unit

Quebrada Arenas

and Río Indio

Limestone Members

FIGURE 36.—Sequence-stratigraphic framework of the Oligocene to middle Miocene sedimentary rocks of the North Coast Tertiary Basin,

Puerto Rico.
76 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

A third depositional sequence is represented by the lower

three-quarters to two-thirds of the Cibao Formation. Interpretation of this unit is complicated by the several facies changes

parallel to depositional strike. These probably reflect the influence of tectonic movements on relative sea level and sediment

dispersal. Throughout much of the basin, however, there is

evidence of general transgression after the lowstand that terminated Lares deposition. The Montebello and the mudstone unit

record a cycle of deposition from generally shallower to deeper

(TST) and then to shallower environments (HST). The upper

boundary of this sequence is marked by the tops of the Montebello and Quebrada Arenas Members (fig. 37). As diagrammed by Monroe (1980, p. 30-31), the distribution of the

upper part of the Guajataca suggests basinward progradation of

a fan-delta complex, probably during a lowering of sea level.

The upper Guajataca and incised-channel deposits of the nonmarine Miranda Sand Member (fig. 37, Section A modified

from Monroe, 1980, figure 16) likely were deposited during a

low stand of sea level (LST).

The upper part of the Cibao Formation and the Aguada

(Los Puertos) Limestone make up another depositional

sequence. The TST is represented by the upper Cibao and

lower Aguada. Resolution of a HST with present outcrop and

subsurface data is tenuous. The upper boundary of this

sequence is the widespread karst surface in the outcrop and

shallow subsurface. The basinward equivalent of this surface

apparently is a conformity, which is difficult to recognize in the

deeper cores.

Finally, the widespread Aymamón Limestone is a fifth depositional sequence. In the updip area, this lithologic unit is

bounded both at the bottom and top by karst zones. Whether

the remaining part of the Aymamón represents more than the

TST is undetermined.

GEOLOGY OF CENTRAL ST. CROIX, U.S. 

VIRGIN ISLANDS

BY I.P. GILL1, D.K. HUBBARD2, P.P. MCLAUGHLIN3, 

AND C.H. MOORE4

INTRODUCTION

The use of water on St. Croix traditionally has depended

on the catchment and storage of rainwater and scattered

wells. In pre-Columbian times, Native American settlements

were apparently located close to the few areas of reliable surface-water supply. Since the 1960’s, water from several

desalination plants has replaced some dependence on rainwater.

Today, the Tertiary carbonate rocks of St. Croix are still

being utilized as a source of ground water, augmenting rainwater catchment and desalinization. In addition, recent

development and population growth is placing increased

demand on water supplies; therefore, knowledge of the subsurface geology of central St. Croix is of greater importance

now than it has been in the last several decades. The purpose

of this discussion is to provide up-to-date information regarding the geology of St. Croix’s Tertiary limestone sequence.

The following section of the report describes the stratigraphy

and setting of the major Tertiary geologic units. A later section examines the pattern of porosity and permeability within

this sequence and relates these patterns to the island’s structural and sedimentological setting.

Of major concern here are the laterally extensive waterbearing rocks of the Kingshill Basin region (fig. 38). These

relatively flat-lying rocks of Tertiary age are bounded to the

east by the East End Range and to the northwest by the

Northside Range, areas of hills and low mountains with a

maximum altitude approaching 335 m. The East End Range

and the Northside Range consist of well-lithified Cretaceous

rocks of the Mount Eagle Group, a diverse assemblage of

deep-water volcaniclastics, tuffaceous sandstones, pelagic

sediments, and gabbroic and dioritic intrusives (Whetten,

1966; Speed, 1989; Speed and Joyce, 1989). A number of

alluvial-filled valleys abut the coastline of St. Croix. They

are limited in extent and are discussed in a later section. 

The Cretaceous rocks of the Northside and East End

ranges are interpreted to be horst blocks, bounding the graben

represented by the Central Limestone Plain (fig. 38; Whetten,

1966). Deposition of Jealousy Formation, Kingshill Limestone, and Blessing Formation limestones in a structural

basin, herein named the Kingshill Basin, occurred during the

early Miocene to Pliocene (Gill, 1989; McLaughlin and others, 1995). The thickness of sediments in the basin, perhaps

as great as 1,800 m as inferred from gravity surveys (Shurbet

and others, 1956), indicates that basinal sedimentation may

have begun in the Oligocene or earlier. Early Tertiary sedimentation in the Kingshill-Jealousy Basin is supported by the

presence of resedimented Eocene foraminifera within the

Kingshill limestone (Lidz, 1984). However, the earliest documented age of the Jealousy Formation is early Miocene

(Todd and Low, 1976; Gill and others, 1989; McLaughlin

and others, 1995), and the deepest drilling of the Kingshill

Basin is 460 m below land surface (Cederstrom, 1950) and

did not penetrate the basal Jealousy Formation. In ascending

order, the principal rock units of St. Croix include the Mount

Eagle Group, the Jealousy Formation, Kingshill Limestone,

its newly named lower and upper members, the La Reine and

1

 Department of Geology, University of Puerto Rico,

Mayagüez, PR 00681;

2

Department of Geology, Oberlin College, Oberlin, OH 44074

3

Delaware Geological Survey, Newark, DE 19716-7501.

4

Department of Geology and Geophysics, Lousiana State

University, Baton Rouge, LA 70803, (retired)
GEOLOGY OF CENTRAL ST. CROIX, U.S. VIRGIN ISLANDS 77

Mannings Bay Members of Gill (1989) and McLaughlin and

others (1995), and newly named Blessing Formation of Gill

(1989) (figs. 39, 40).

The Mount Eagle Group of Cretaceous age underlies the

Northside and East End Ranges and presumably forms the

floors of the graben underlying the Kingshill Basin. Additional information on the sedimentologic character and structure of these rocks is provided by Whetten (1966), Speed

(1989), and Stanley (1989). The Jealousy Formation of

Miocene age consists largely of grey-blue, planktonic foraminifera-rich muds. Locally, the units contains several conglomeratic layers encountered during deep drilling

(Cederstrom, 1950). The Kingshill Limestone of Miocene

age consists of two members. The basal La Reine Member,

named herein, is a lithologically diverse unit dominated by

planktonic foraminiferal carbonate mud and marl, and an

upper member of shelf and slope facies is named herein the

Mannings Bay Member. The Blessing Formation of Pliocene

age consists of reef and shelf limestone that unconformably

overlies the Kingshill Limestone along St. Croix’s southern

coast (fig. 5). 

STRATIGRAPHY AND SEDIMENTOLOGY OF THE 

CENTRAL LIMESTONE PLAIN REGION

JEALOUSY FORMATION

Lithology, facies, and depositional environment

The type section of the Jealousy Formation is defined

within a 426 m section penetrated by the deep test well no. 41

drilled in 1939 by the Civilian Conservation Corps near the

estate of Bethlehem (Cederstrom, 1950) (fig. 38). This well

was drilled to a depth of 459 m below land surface, of

which the lowest 426 m were identified as Jealousy Formation sediments (Cederstrom, 1950). The Jealousy Formation

type section was described by Cederstrom (1950, p. 19) as

dark grey, clay-rich strata interrupted by calcareous conglomeratic deposits: “ . . . 1,398 feet of dark clayey strata lying

below the Kingshill marl were penetrated, the lowest stratum

consists of 305 feet of gray clay in which a few streaks of

limestone, not more than a few inches in thickness, are

present." Two intervals of calcareous conglomerate were

noted within the section, containing rounded and subangular

boulders identified as Mount Eagle volcanics in a “limy

matrix” (Cederstrom, 1950.

  

 

   

  

Aguada (Los Puertos) Limestone

Mucarabones Sand

Lares Limestone

WEST EAST

San Sebastián

Formation

10 20 KILOMETERS

50

100 METERS

DATUM IS THE TOP OF THE

CIBOA FORMATION

EXPLANATION

Cibao Formation

 Cibao undifferentiated

 Miranda Sand Member

 Quebrada Arenas Limestone Member

 Río Indio Limestone Member

 Montebello Limestone Member

 Guajataca Member



  

 







Sequence boundary

 Type I

 Type II

FIGURE 37.—Sequence boundaries related to the updip Cibao Formation, northern Puerto Rico (modified from Monroe, 1980, figure 16).

.
78 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

U

D

FD

M11

WD272

WD951

U

D

PA2

PA7

PA8

PA10

PW17

PW19

PW18

34a

PL149

GP41

39

U

D

U

D

45a

M1

M4

M8

M3

M9

M6

M7 T2

M5

M12

M14 M13

PW6

41

C15

5b

7b

WI-LLO

M2 SH46

C20

M10

U

D

LGP50b

C26

CH-ALK

4

Mannings Hill

(Airport/Penitentiary)

(Airport Quarry) Subsidiary graben

or demi-graben

Evans Highway

Hesselburg

Wheel of

Fortune

Williams

Delight

Carlton

Fairplain

Anguilla

Kingshill

Jealousy Bethlehem

Villa

La Reine Barren

Spot Castle

Coakley

Sion Hill

Work and Rest

Hess

Oil

Pearl

Blessing

Beeston

Hill

St. John

Rattan

Morningstar

Montpellier

Judiths

Fancy

B7

D

U

CARIBBEAN SEA

Krause Lagoon

Christiansted

Frederiksted

NORTHSIDE RANGE

CENTRAL LIMESTONE PLAIN

EAST

END

RANGE

Salt

River

Bethlehem

Gut

River Gut

ATLANTIC OCEAN

CARIBBEAN SEA

Isla

Mona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

Base modified from U.S. Geological Survey,

Fredriksted, Chistiansted, East Point,

1:24,000, 1958

64 ̊55'30" 64 ̊48' 17°48'

17 ̊40'30"

0 5 10 KILOMETERS

0 5 MILES

EXPLANATION

Blessing Formation

Kingshill Limestone

 Mannings Bay Member

 La Reine Member

Mt. Eagle Group

Inferred fault—U, upthrown; 

 D, downthrown

Contact—Dashed where

 approximately located

Control points

 Test hole—Core and cutting samples

 Well—Cutting samples

 Well— Driller's log information

 Outcrop

Fredericksted

Williams Delight

Sion Hill

Windsor

Paradise

La Grande Princesse

Plessen

Constitution Hill

Grove Place

Civilian Conservation Corps

Gill-Hubbard drilling project

Public Works

Tibbits, Abbott, McCarthy, and

 Stratton, (TAMS) Inc.

Martin Marietta Alumna—

 Caribbean Drilling Services

U

D

FD

W D

S H

WI-LLO

P A

LGP

P L

CH-ALK

G P

C

M

P W

T

B

FIGURE 38.—Generalized geology of St. Croix, U.S. Virgin Islands, and location of control points (modified from Gill, 1989).
GEOLOGY OF CENTRAL ST. CROIX, U.S. VIRGIN ISLANDS 79

To the west, near Estate Jealousy (test well 39; fig. 38),

dark clay of the Jealousy Formation contains more sandy

material than do rocks of the type-section. The basal 18.6 m

section penetrated in the Estate Jealousy well consists of

hard, calcareous conglomerate that directly overlies the basement rock of the Mount Eagle Group. Cederstrom (1950)

correlated these rocks with lithologically similar calcareous

conglomerate exposed in stream beds west and northeast of

well no. 39.

A sharp, distinctive color change marks the boundary

between the light buff Kingshill Limestone and the underlying dark, blue-grey to grey Jealousy Formation. Well drillers

commonly describe poorly lithified sediments of the Jealousy

Formation as “blue clay.” However, the Jealousy Formation

consists largely of calcareous muds containing a sand fraction

composed almost entirely of planktonic foraminifera. The

percentage of sand in Jealousy Formation samples ranges

from less than 10 percent to almost 25 percent; of the sand

fraction, planktonic foraminifera averaged 89 percent ± 5

percent.

X-ray diffraction analyses of samples from the Jealousy

Formation indicate a dominant calcite mineralogy, having an

insoluble fraction of quartz, feldspar, and clay minerals ranging from 30 to 51 percent (Gill, 1989). On the basis of information available from the five wells drilled into the Jealousy

Formation, the mineralogy of the Jealousy Formation does





 

?

?

SYSTEM SERIES GEOLOGIC UNIT

QUATERNARY

TERTIARY

CRETACEOUS

LOWER

PLIOCENE

MIDDLE

OLIGOCENE

MIOCENE

EOCENE

PALEOCENE

UPPER

CRETACEOUS

UPPER

Alluvium, beach rock,

 and terraces

Blessing Formation

Mannings Bay Member,

 Kingshill Limestone

La Reine Member,

 Kingshill Limestone

Jealousy Formation

(unexposed)

Unamed, unexposed

Mount Eagle Group

Tuffaceous sandstone

Siliceous sandstone

Limestone

Volcanic pebble

 conglomerate

Volcanic siltstone and

 sandstone

Mudstone

Dioritic and gabbroic

 intrusives

(Whetten, 1966)

FIGURE 39.—Generalized stratigraphic column of St. Croix, U.S. Virgin Islands (modified from Gill, 1989).
80 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS       STANDARD CHRONOSTRATIGRAPHY BIOCHRONO- STRATIGRAPHY KINGSHILL BASIN STRATIGRAPHIC SECTIONS LITHOSTRATIGRAPHY HOLOCENE PLEISTOCENE

PLIOCENE MIOCENE

UPPER LOWER UPPER MIDDLE

SERIES

05

1015

TIME (Ma)

Stages

Piacenzian

Zanclean

Messinian

Tortonian

Serravallian

Langhian P. glomerosa

G. peripheroronda

G. fohsi robata

G. fohsi fohsi

G. fohsi robusta

G. menardii

G. mayerii

G. numerosa

G. acostaensis

G. M. margaritae

G. F. fistulosus

G. M. evoluta

G. exilis

G. miocenica G. margaritae

Milazzian

Sicilian

Emilian

Calabrian

Planktonic

Foram

Biochronozones

G. truncatulinoides

Evan's

Highway

Estate work

and rest

Villa

La Reine

Centerline

Road

Morningstar

East

Well M4

Well M2

Well 45

Well 41

Well 39

Well M1 Well M10

Well PL149 Well GP41

Five

Corners

RattanBelvedere Morningstar

LA REINE MEMBER

BLESSING FORMATION

JEALOUSY FORMATION

KINGSHILL BASIN

interbedded limestone

and marl

MANNINGS BAY MEMBER

predominantly limestone

predominantly

tan marl

blue-gray marl

KINGSHILL LIMESTONE

EXPLANATION

Mannings Bay Member limestoneLa Reine Member limestone and marlJealousy Formation marl

Chronostratigraphic rangeEstimated range

FIGURE 40.—Expanded stratigraphic column showing rocks of late Tertiary age in St. Croix, U.S. Virgin Islands (modified and reprinted from McLaughlin, P.P., Gill, I.P., and

Bold, W.A. van den, 1995, and published with permission).
GEOLOGY OF CENTRAL ST. CROIX, U.S. VIRGIN ISLANDS 81

not vary systematically within the Kingshill Basin or within

the stratigraphic range sampled. The dominant calcite mineralogy of the Jealousy Formation is not detectably different

from Kingshill Limestone samples taken immediately above

the contact. However, well cuttings and core samples collected from older wells penetrating the Jealousy Formation

are not currently available and thus no direct comparison to

lower sections of the Jealousy Formation is possible. However, it is clear that the uppermost Jealousy Formation is best

described as a marl or foraminiferal mud, rather than an estuarine clay (Multer and others, 1977), and does not differ significantly from the Kingshill Limestone except for its marked

color contrast.

The cause of the color contrast is uncertain, because the

calcitic rocks that lie above and below the Kingshill-Jealousy

contact are identical in bulk X-ray diffraction analysis.

Although the blue-grey color of the Jealousy Formation is

striking in fresh, wet split-spoon samples, it often fades upon

drying and exposure to air. In many samples, the color does

not entirely disappear, but remains as a very faint blue or

greyish tinge; in other samples, color differences between

Kingshill and Jealousy samples become almost indistinguishable.

The color difference associated with the Kingshill Limestone and upper Jealousy Formation could be caused by a

number of factors, including minor mineralogic or elemental

differences induced either by depositional or diagenetic

effects such as redox state differences. In any case, the Kingshill-Jealousy boundary is not marked by a detectable change

in lithology, paleoenvironment or depositional patterns, and

the significance and cause of the color change remains

unclear. Until the significance of this contact can be clarified,

the use of the formation boundary alone for structural interpretation is of questionable value.

Whetten (1966) interpreted the Jealousy Formation to be

a marine deposit of altered pyroclastic rock, with its volcanic

source located upwind toward the Lesser Antilles. Multer

and others (1977) suggested that the Jealousy Formation was

an estuarine deposit of Oligocene age with the exposed and

eroding horst blocks of the Northside and East End Ranges

serving as insular sediment sources (Multer and others,

1977). However, Gerhard and others (1978) noted the possibility that an external sediment source, as opposed to the

exposed horst blocks of the structural basin, was required to

account for the great thickness of the Jealousy Formation.

It can be shown, however, that Jealousy Formation samples taken from the Kingshill Basin are marine in origin (Gill,

1989), not estuarine, as suggested by Multer and others

(1977). The water depth at the time of the deposition of the

Jealousy Formation was probably between 600 and 800 m, on

the basis of the abundance and type of planktonic and benthic

foraminifera (Gill, 1989; Gill and others, 1989; McLaughlin

and others, 1995). In addition, the depth of the basin did not

vary appreciably from basin center to its outer margins. This

could indicate that early Jealousy Formation deposition preceded movement along the Kingshill Basin boundary faults,

and thus deposition preceded formation of the basin itself

(Gill, 1989).

Structure and distribution

The surface of the Jealousy Formation, as determined

from well-log data, is characterized by (1) marked upbowing

of its surface beneath the limestone highlands in the northeastern section of the central plain; (2) a gentle dip toward the

northern and southern coasts of St. Croix; and (3) a pronounced rise of its surface close to the boundary imposed by

the Northside Range (fig. 41; Robison, 1972; Jordan, 1975;

Gill and Hubbard, 1987). The altitude of the top of the Jealousy Formation in the southeastern Central Limestone Plain

area is poorly known and probably lies at depth greater than

80 m relative to sea level, the maximum depth of well penetration in that area (figs. 41, 42, 43). 

The Jealousy Formation underlies the Kingshill Basin

throughout the study area. In addition to the subsurface samples, Cederstrom (1950) and Whetten (1966) considered

exposures on the northern margin to be part of the Jealousy

Formation. These strata should be included as part of the

Kingshill Limestone because the exposed strata bear no

resemblance to the subsurface Jealousy Formation. They contain a lithologic facies that is similar to other exposures of the

Kingshill Limestone, and associated fauna are within the biostratigraphic range of the Kingshill Limestone (Bold, 1970;

Gill and Hubbard, 1986; McLaughlin and others, 1995).

Including these strata as Kingshill Limestone follows the suggestion of Gerhard and others (1978).

Deposits of the Jealousy Formation are apparently not

restricted to the main structural basin. “Blue clays” of the

Jealousy Formation are reported in a well that is located

northeast of the structural basin (well C 26, Cederstrom,

1950). The occurrence of blue clay outside the limits of the

basin suggests that deposition of the Jealousy Formation preceded fault movement. However, samples collected from the

C 26 well are no longer available, and their identification is

considered to be tentative (Cederstrom, 1950).

Age

The age of the Jealousy Formation ranges from the late

early Miocene (N8) to the early middle Miocene (N12)

(fig. 40; Gill, 1989; McLaughlin and others, 1995). Biostratigraphic assignments of the Jealousy Formation presented here

take into account the revision in the position of the Oligocene/

Miocene boundary (Todd and Low, 1976), and utilize planktonic foraminifera abundances (McLaughlin and others,

1995).
82 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

30

20

10

0

–10

–20

20

10

0

–10

–10 0

10

20

30

40

50

0

–10

U

D

–11

>–14

–15

–6

U

D

–13

–14

–10

–5

24

12

11.9

U

D

U

D

–57

–29

>–80

>21

>–58

>–63

>–69

>–36

>–23

>–30

–11

–6

17

15 33

6

36 6.1

4

–0.3

U

D

–15

–26

52

Depth of Jealousy Formation

probably greater than –80 meters

>–42

>–58

D

U

CARIBBEAN SEA

Krause Lagoon

Christiansted

Frederiksted

ATLANTIC OCEAN

CARIBBEAN SEA

Isla

Mona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

Base modified from U.S. Geological Survey,

Fredriksted, Chistiansted, East Point,

1:24,000, 1958

64 ̊55'30" 64 ̊48' 17°48'

17 ̊40'30"

0 5 10 KILOMETERS

0 5 MILES

EXPLANATION

Area underlain by Jealousy Formation

Structure contour—Line shows altitude of top of the Jealousy Formation. Contour interval 10 meters. 

 Datum is sea level

Inferred fault—U, upthrown; D, downthrown

Inferred northern limit of Blessing Formation

Inferred northern limit of Mannings Bay Member

Well—Control point. Shows altitude of Jealousy Formation in meters

U

D

30

FIGURE 41.—Structural configuration of the top of the Jealousy Formation (modified from Gill, 1989).
GEOLOGY OF CENTRAL ST. CROIX, U.S. VIRGIN ISLANDS 83









        



  Section B–B'

150

100

50

Sea level

50

100

500

400

300

200

100

Sea level

100

200

300

METERS

SOUTH

A

FEET

NORTH

A'

0 2 3 4 5 KILOMETERS

0 2 1 3 MILES

1

0 5 10 KILOMETERS

0 5 MILES

Northside Range

East

End

Range

A

A'

M10

M2

M8

M3

M6

M7

B7

Line of section A–A'

Krause

Lagoon

Peak height

146 meters

M7 M6 B7

M3

M8

M2

Judiths

Fancy

? ?

?

M10

D

U

EXPLANATION

Alluvium

Dolomite

Blessing Formation and

 Mannings Bay Member

Kingshill Limestone

 (La Reine Member)

Jealousy Formation

Mount Eagle Group

Fault—U, upthrown; 

 D, downthrown

Contact—Dashed where

 approximately located,

 queried where uncertain

Test hole and number—Sample 

 depths shown as dark lines. 

 Dark intervals represent sample 

 collected by diamond-bit coring

M3

U

D

  

?

FIGURE 42.—Section A–A' from Krause Lagoon to Judiths Fancy (reprinted with permission from Gill and others, 1989).
84 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

The Jealousy Formation and Kingshill Limestone contact

is diachronous and ranges in age from the late early Miocene

to the middle Miocene (Gill, 1989; McLaughlin and others,

1995). These ages are based on planktonic foraminifera in

core samples located immediately above and below the contact (figs. 44, 45). Based on gravity data (Shurbet and others,

1956), it is likely that the thickness of the Jealousy Formation

and underlying strata is substantial and the Jealousy Formation could, therefore, be Oligocene or older in age. However,

it is important to note that no samples confirmed as being of

Oligocene age or older have been recovered from the Jealousy

Formation, and assignment to the Oligocene or Eocene is

speculative.

KINGSHILL LIMESTONE

The nomenclature of the carbonate stratigraphic units on

St. Croix has been revised considerably over the years. The

name “Kingshill Series” was first used by Kemp (1926) to

describe the outcropping Tertiary section of St. Croix. Cederstrom (1950) introduced the name Kingshill Marl differentiating the upper carbonate rocks from the underlying “clay” of

the Jealousy Formation. Whetten (1961, 1966) followed the

usage of Cederstrom (1950) as did Multer and others (1977),

whereas Bold (1970) referred to the unit as the Kingshill Formation. Gerhard and others (1978) formalized the name

Kingshill Limestone and designated the section at Villa La

Reine to be the type locality. In this report, the Kingshill



     











    



Section

A–A'

50

Sea level

50

100

400

100

Sea level

100

200

300

METERS

WEST

B

FEET

EAST

B'

0 5 10 KILOMETERS

0 5 MILES

0 2 3 4 5 KILOMETERS

0 2 1 3 MILES

1

Northside Range

East

End

Range

Line of section B–B'

B

B' M4 B7 M9 M5

M1

PA10

M11 WD272

PA7

M11

WD272

PA7

PA10

M1

M4

B7 M9

M5 Pearl

Estate

Hesselberg

U

D

?

?

? ?

EXPLANATION

Alluvium

Dolomite

Blessing Formation and

 Mannings Bay Member

Kingshill Limestone

 (La Reine Member)

Jealousy Formation

Fault—U, upthrown; 

 D, downthrown

Contact—Dashed where

 approximately located,

 queried where uncertain

Test hole and number—Sample 

 depths shown as dark lines. 

 Dark intervals represent sample 

 collected by diamond-bit coring

M1

U

D







?

FIGURE 43.—Section B–B' from Estate Hesselberg to Pearl (reprinted with permision from Gill and others, 1989).
GEOLOGY OF CENTRAL ST. CROIX, U.S. VIRGIN ISLANDS 85

Limestone is divided into the La Reine (lower) Member and

the Mannings Bay (upper) Member.

La Reine (lower) Member of the Kingshill Limestone

The La Reine Member of the Kingshill Limestone is

herein defined as the lower member of the Kingshill Limestone described in the type section for the formation by Gerhard and others (1978). The La Reine Member contains the

lower three of the five depositional facies of the Kingshill

Limestone (Gerhard and others, 1978); the molluscan packstone facies, the clastic grainstone facies and the polymictic

packstone facies. The La Reine Member is unconformably

overlain by the Mannings Bay (upper) Member. Both members and the unconformity are exposed in the north-facing

side of Mannings Hill along the Melvin Evans Highway

(Lidz, 1982, 1984; Gill and others, 1989).

Lithology, facies, and depositional environment

Outcropping rocks of the La Reine Member of Kingshill

Limestone consist of a variety of sedimentary lithofacies that

include packstones, wackestones, and clastic grainstones. In

the subsurface, the lower part of the formation is dominated

by packstone, predominantly planktonic foraminifera-rich

carbonate muds with lithic grains or pebbles occurring at

some levels. Less common are lithic-pebble or foraminiferarich wackestones.

 Section A–A'

100

50

Sea level

50

400

300

200

100

Sea level

100

200

METERS

SOUTHWEST

C

FEET

NORTHEAST

C'

0 5 10 KILOMETERS

0 5 MILES

0 2 3 4 5 KILOMETERS

0 2 1 3 MILES

1

Northside Range

East

End

Range

Line of section C–C'

C

C'

M2 SH46

WDP51

PL149

GP41

C41

Sion Hill

WD951 PL149 GP41

M2

SH

46

Total depth = 450 meters

N8 N8

N12

N12

N12

N11

N11

N10

N10 to N13

C41

Williams Delight

EXPLANATION

Kingshill Limestone

 (La Reine Member)

Jealousy Formation

Contact

Test hole and number—Sample 

 depths shown as dark lines. 

 Planktonic foraminiferal 

 biochronozones

 M2

N12

FIGURE 44.—Section C–C' from Williams Delight to Sion Hill showing relation between formational units and biostratigraphy.
86 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

The lowermost Kingshill Limestone strata (La Reine

Member) in the Estate St. John and Judith’s Fancy areas have

been interpreted by Gerhard and others (1978), Lidz (1982),

and Andreieff and others (1986) as shelf and lagoon deposits

based on the presence of rounded terrigenous gravel, as well

as a shallow-water fauna that includes echinoids and benthic

forams. However, these deposits lie stratigraphically between

the nearby basinal Kingshill Limestone and Jealousy Formation deposits recovered in test well M10 and outcropping in

the hills to the south. Interpreting these deposits as in-place

would require a major shoaling from bathyal depths, to littoral conditions, and back again. Alternatively, it is possible

that the reefs are in-place and positioned on a small step

block. If so, they could be the only example of in-place



















Section

A–A'

150

100

50

Sea level

50

100

500

400

300

200

100

Sea level

100

200

300

METERS

SOUTHWEST

D

FEET

NORTHEAST

D'

0 5 10 KILOMETERS

0 5 MILES

0 2 3 4 5 KILOMETERS

0 2 1 3 MILES

1

Northside Range

East

End

Range

Line of section D–D'

D

D'

M10

M2

M1

M7

C45a

D 

Fairplain

St. John

M1

C45a

M2

M10

N9

N9

N8

N12

N12

N12

N12

N11

N10

N10?

N8

N8

N8

N9

N9

U

EXPLANATION

Alluvium

Blessing Formation and

 Mannings Bay Member

Kingshill Limestone

 (La Reine Member)

Jealousy Formation

Mount Eagle Group

Fault—U, upthrown; 

 D, downthrown

Contact—Dashed where

 approximately located

Test hole and number—Sample 

 depths shown as dark lines. 

 Planktonic foraminiferal

 biochronozones

U

D





 

M2

N10

FIGURE 45.—Section D-D' from Fairplain to St. Johns showing relation between formational units and biostratigraphy.
GEOLOGY OF CENTRAL ST. CROIX, U.S. VIRGIN ISLANDS 87

Miocene reef material in the basin (Gerhard, oral commun.,

1994), and could have served as the source of the rounded terrigenous pebbles in Jealousy and Kingshill material in test

well M10. A third hypothesis would be that the reef material

is allochthonous, deposited in bathyal depths from a source

now distant or eroded from the basin. There is certainly

abundant allochthonous reef material throughout the rest of

the Kingshill Limestone.

Lithofacies exposed at the La Reine Member type section

at Villa La Reine are characterized as polymictic packstones

(Gerhard and others, 1978), and include foraminiferal chalk,

soft marly interbeds and debris flows of coral and terrigenous

gravel. The debris flow beds often have erosional bases and

include cobble- to boulder-size clasts, primarily coral heads. In

most cases, the corals are well-cemented and have been

replaced by calcite, but may still be recognized by morphologic type. In some cases, the corals can be identified to the

genus and species level.

An exposure located at Estate Work and Rest approximately 5 km to the east, displays foraminifera-rich lithologies

similar to those seen at Villa La Reine. Interbedded with the

foraminifera-rich beds, however, are beds composed predominantly of terrigenous breccia presumably derived from Cretaceous Mount Eagle Group. Gerhard and others (1978)

interpreted this outcrop to be an example of a syntectonic breccia.

The youngest rocks of the Kingshill Limestone (Mannings

Bay Member) are exposed along the Melvin Evans Highway

(fig. 46) near the southern coastline. The exposure, referred to

here as the Airport/Penitentiary outcrop, can be divided into

upper and lower parts, separated by a disconformity. Below

the disconformity, the exposure is characterized by regularly

bedded intercalations of softer, planktonic foraminifera-rich

beds and more indurated shelf-derived debris beds similar to

that of the other La Reine Member exposures discussed.

Above the disconformity, the lithologic character of beds

indicates shoaling of the basin. These strata are included

within the Mannings Bay Member of the Kingshill Limestone. The Mannings Bay Member is characterized by channelled beds of shelf-derived carbonate debris, including

sediment flows of larger foraminifera interbedded with softer

beds containing poorly preserved planktonic foraminifera (foraminiferal wackestone facies of Gerhard and others, 1978).

The strata referred herein to as the Mannings Bay Member

were formerly included as part of the “post-Kingshill” Limestone by Lidz (1982) and Andreieff and others (1986), whereas

Gerhard and others (1978) included them as part of the Kingshill Limestone. The lower contact of the Mannings Bay

Member is distinctive where exposed, but is not easily identified in core samples (Gill and Hubbard, 1986, 1987).

The lithologic character and the benthic foraminiferal

fauna associated with the La Reine Member of the Kingshill

Limestone are indicative of middle bathyal conditions between

600 and 800 m below sea level (Gill, 1989; McLaughlin and

others, 1995). This estimate is similar to the depth estimates

of Multer and others (1977) or Lidz (1982), and the inferred

environmental conditions for the Kingshill Limestone are

identical to those inferred for the underlying Jealousy Formation.

The depth of the basin does not appear to have varied

throughout the time of lower Kingshill Limestone deposition

(exclusive of the Mannings Bay Member), or spatially

throughout the basin until late in the Miocene or early in the

Pliocene. Basinal shoaling is marked by the deposition of the

thinly bedded limestone of the Mannings Bay Member (discussed below), which contains higher concentrations of shelfderived carbonate clasts and faunal remains.

The large quantity of shelf-derived material in the Kingshill

Limestone appears to be allochthonous, and transported to

bathyal depths by sediment-gravity flows. The concentrations

of sand- to boulder-sized debris indicate the importance of offshelf transport of coarse material as well as the close proximity

of coral reefs and sources of rounded terrigenous clasts.

Because there are no known Miocene reefs exposed on modern

St. Croix, reefs must either have existed in an area currently

separated from the island, or they have been erosionally

removed from the uplands during island emergence.

Structure and distribution

Faulting has placed the La Reine Member of the Kingshill

Limestone into contact with Cretaceous rocks of the Mount

Eagle Group in the Northside and East End Ranges (Whetten,

1966; Multer and others, 1977). Biostratigraphic evidence

(McLaughlin and others, 1995) collected from strata thought

to be syntectonic breccias (Gerhard and others, 1978) that

crop out on the western margin of the East End Range indicates that basin faulting occurred prior to late middle

Miocene time (outcrop 4, fig. 38). However, the fault contact

along the Northside Range is obscured by alluvium, and

structural relationships are difficult to observe directly

(figs. 5, 38). Gerhard and others (1978) suggest that there

was less displacement along this northern fault boundary than

along the eastern basin margin. A north-south cross section

through the island (fig. 42) also shows the marked upbowing

of the Jealousy Formation and Kingshill Limestone where

these strata underlie the carbonate highlands close to the

northern coast.

Fault-juxtaposed rocks of the La Reine Member of the

Kingshill Limestone and the Cretaceous Mount Eagle Group

strata along the eastern boundary fault indicate that the Kingshill Limestone was at least in part deposited prior to basinal

faulting (Gill and Hubbard, 1986, 1987). Initiation of the St.

Croix normal fault system occurred after the late early

Miocene. However, it is not certain whether the Kingshill

Basin fault boundaries had formed penecontemporaneously
88 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

with the deposition of the Kingshill Limestone, or whether the

bounding horst blocks were exposed during Kingshill Limestone deposition. Gerhard and others (1978) support an

exposed horst model largely on the basis of syntectonic breccias along the eastern fault boundary of the Kingshill Basin. If

the terrigenous clasts within the Kingshill are indeed derived

from Mount Eagle Group strata, then the graben boundaries

must have formed no later than middle Miocene time, because

strata in the eastern fault zone can be assigned to zones

between N14 and N16 (McLaughlin, written commun., 1978;

Gill, 1989).

Strata thickness patterns of the Kingshill Limestone reveal

three major trends (fig. 47): (1) the Kingshill Limestone

pinches out toward the north and northwest margins of the

basin; (2) there is a pronounced thickening of the Kingshill

Limestone in the carbonate highlands near the northern coast

of St. Croix; and (3) gentle thickening of the Kingshill Limestone toward the south of the basin is interrupted by post-depositional faults located along St. Croix’s southern coast (fig. 47).

The maximum estimated thickness of the Kingshill Limestone

is 180 m in the carbonate highlands areas near Rattan Hill

(Cederstrom, 1950), whereas the maximum documented

thickness of the Kingshill Limestone, encountered in the

same area, is nearly 140 m (fig. 47). Well control for the

Kingshill Limestone section is poor in the vicinity of the

post-Kingshill carbonate section on the south coast, and to

the west of Estate Williams Delight (fig. 38). If faulting and

folding are assumed to be minor, Kingshill Limestone thickness patterns imply a basin opening to the south but deepest

in the section presently occupied by the carbonate highlands.

The Jealousy Formation underlies the Kingshill Limestone across most of the south coast of St. Croix. The position

of the Kingshill Limestone-Jealousy Formation contact is

unknown west of Estate Williams Delight due to poor core

control, and east of Estate Anguilla due to faulting within the

Tertiary section between test holes M1 and M4 (figs. 38, 43).

In the latter areas, extensive thicknesses of Mannings Bay

Member and the Blessing Formation accumulation put the

Kingshill/Jealousy boundary beyond the reach of shallow

drilling.

The Kingshill Limestone has a general southerly dip that

averages less than 12 degrees, and exposures of the formation

therefore are progressively younger to the south (Lidz, 1982).

However, Cederstrom (1950) mapped a number of localized

structures in the exposed parts of the Kingshill not mentioned

in subsequent reports. In particular, Cederstrom (1950)

mapped a series of low-angle synclinal structures with axial

traces oriented in a crudely radial pattern that ranged from

WSW-ENE in the southwestern part of the basin to SW-NE in

the north, and back to WSW-ENE in the southeastern part of

FIGURE 46.—A south-facing view of the disconformity separating the Mannings Bay Member from underlying beds of the 

La Reine Member lies approximately 7 meters above the Highway 66 road bed, north of Alexander Hamilton Airport. 

Arrows mark the position of the unconformity (reprinted and published with permission from Gill and others, 1989).
GEOLOGY OF CENTRAL ST. CROIX, U.S. VIRGIN ISLANDS 89

the basin (Cederstrom, 1950). The synclines mapped by Cederstrom (1950) correspond to topographic highs, with the synclinal axes mapped along ridgelines. A possible, but

speculative, interpretation of this pattern is post-Kingshill

deformation of the Kingshill Basin, with the long axis of St.

Croix being bent northward around a pivot point in the northern part of the Kingshill Basin. Such a pattern might explain

the extensional faulting along the south shore, and the gentle

folding mapped by Cederstrom (1950) in the northern part of

the Kingshill Basin.

A wide variation in dip is evident near the fault contact of

the Kingshill Limestone and the Cretaceous Mount Eagle

Group in the Northside and East End Ranges. Two steep

scarps outline the edge of the limestone highlands, and are

30

20

10

30

10

20

30

40

50

60

70

80

90 100

110

120

100 110

40

30

U

D

26

35

U

D

37

30

33

14

29

28

U

D

U

D

10

>36

29

>~20

>~40

27 27

18

51

82

19

U

D

17

0

49

Thickness of Kingshill Limestone is

unknown. Overlain by younger

carbonate rocks

>~20

D

U

>10 >10

CARIBBEAN SEA

Krause Lagoon

Christiansted

Frederiksted

Williams

Delight

Anguilla

Rattan Hill

ATLANTIC OCEAN

CARIBBEAN SEA

Isla

Mona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

Base modified from U.S. Geological Survey,

Fredriksted, Chistiansted, East Point,

1:24,000, 1958

64 ̊55'30" 64 ̊48' 17°48'

17 ̊40'30"

0 5 10 KILOMETERS

0 5 MILES

EXPLANATION

Area underlain by Kingshill Limestone

Structure contour—Line shows altitude of top of the Kingshill Limestone. Dashed where uncertain.

 Contour interval 10 meters. Datum is sea level

Inferred fault—U, upthrown; D, downthrown

Inferred northern limit of Blessing Formation

Inferred northern limit of Mannings Bay Member

Well—Control point. Thickness in meters

U

D

30

FIGURE 47.—Thickness of the Kingshill Limestone (modified from Gill, 1989).
90 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

suggestive of structural control. These scarps trend NW-SE

from Beeston Hill to Morningstar and then NNE to SSW to

Fair Plain. However, despite geomorphic evidence and the

marked upbowing of the Jealousy Formation beneath the

limestone highland area, available biostratigraphic data do not

support faulting in this area (Gill, 1989).

Age

The Kingshill Limestone ranges in age from the early

Miocene (N8) to close to the Miocene and Pliocene boundary

(N17). The La Reine Member can range in age from early

Miocene (N8) to as young as late Miocene in age (lower

N17). In the subsurface, the Kingshill Limestone spans a narrower range, from early Miocene (N8) to middle Miocene

(N12). The age of the Kingshill Limestone (at the Mannings

Bay Member-La Reine Member boundary) is placed within

the upper N17 zone (Globorotalia numerosa Zone). The

determination of age (Gill, 1989) was based on the presence

of planktonic foraminifera Globorotalia numerosa, Candeina

nitida (d’Orbigny) and the absence of Pliocene marker species, Globorotalia margaritae and Sphaeroidinella dehiscens

(McLaughlin and others, 1995).

Mannings Bay (upper) Member of the Kingshill Limestone

The Mannings Bay Member is herein named for uppermost well-bedded limestone strata of late Miocene to early

Pliocene age (McLaughlin and others, 1995) that are characterized in part by larger benthic foraminifera and shelfderived debris. Previous workers have included these strata

either as part of the Kingshill Limestone (Gerhard and others,

1978) or considered them to be part of the post-Kingshill

Limestone sequence (Lidz, 1982; Andreieff and others, 1986;

Gill and Hubbard, 1987). They are defined herein as the

uppermost member of the Kingshill Limestone and include

the foraminiferal wackestone facies and foraminiferal grainstone facies of Gerhard and others (1978). The Mannings

Bay Member is exposed along the Evans Highway near the

Alexander Hamilton Airport (fig. 46). A quarry located on the

southeastern side of Mannings Hill adjacent to the airport is

proposed as the type section (fig. 48). More than 20 m of

Kingshill Limestone, including the top of the unit, are

exposed in this outcrop (Gerhard and others, 1978; Gill and

others, 1989). Limestone strata of the Mannings Bay Member were also penetrated in several test wells described in this

report, but the strata are difficult to differentiate from the

lower Kingshill Limestone without the detail of an exposed

section.

Lithology, facies, and depositional environment

The Mannings Bay Member is bracketed by unconformable surfaces that separate it from the lower part of the

Kingshill Limestone and from the overlying Blessing Formation, a lagoonal and reefal limestone. The lower unconformable surface is erosional, and characterized by coarse benthic

foraminiferal and other skeletal debris of the Mannings Bay

Member cutting into the underlying alternating hemipelagic

and pelagic beds of the La Reine Member of the Kingshill

Limestone. There are numerous pinchouts of the coarsergrained beds of the Mannings Bay Member, probably indicating channelization (Lidz, 1984), but no caliche, karstification,

or paleosols have been observed (Gill, 1989).

The unconformity separating the Mannings Bay Member

of the Kingshill Limestone from the overlying Blessing Formation sharply separates the two units, but there is no clear

evidence of subaerial exposure at the Airport Quarry exposure. To the east, the unconformity separating the Mannings

Bay Member from the Blessing Formation was observed to

have the red oxidized coloration and the clay content of terra

rosa (Behrens, 1976; Gerhard and others, 1978; S.H. Frost,

UNOCAL, oral commun., 1986; L.C. Gerhard, Kansas Geological Survey, oral commun., 1994), but this exposure has

since been lost to industrial development.

The Mannings Bay Member of the Kingshill Limestone

has at its base, channelized beds containing shelf-derived skeletal material including larger benthic foraminifera. The Mannings Bay Member tends to be more thinly bedded and contain

a significantly larger component of shelf-derived skeletal

clasts than the La Reine Member, and is dominated by bioclastic wackestones and packstones. Within the Blessing fault

block to the east of the Fairplain fault, the Mannings Bay

Member has been patchily dolomitized and heavily leached by

meteoric weathering.

The occurrence of larger foraminifera within the Mannings Bay Member signals the establishment of a shallowing

Kingshill Limestone environment that does not have a modern

St. Croix analog. The fossil assemblage includes and is dominated in places by the operculinoid foraminifera Operculinoides cojimarensis and Paraspiroclypeus chawneri (Behrens,

1976; Gerhard and others, 1978; S.H. Frost, UNOCAL, oral

commun., 1986). These foraminifera occasionally show evidence of transport or reworking, such as fracturing, abrasion,

and imbrication. The foraminiferal wackestones also contain

ubiquitous planktonic foraminifera, including Orbulina universa and Globigerina spp.

Foraminifera such as Operculinoides and Paraspiroclypeus were more than likely photic zone forms (S.H. Frost,

UNOCAL, oral commun., 1986) and temporarily dominated

St. Croix shallow carbonate environments. Other bioclasts

that contribute significantly to the facies are coralline algal

crusts and rhodoliths, benthic foraminifera such as Archaeis

and Amphistigina, and echinoid spines and plate fragments.

Minor coral fragments and molluscan debris are also part of

the assemblage, but are primarily represented by external

molds and pore space in the cores.
GEOLOGY OF CENTRAL ST. CROIX, U.S. VIRGIN ISLANDS 91

The bioclast assemblage in the Mannings Bay Member

was analyzed primarily from thin sections. Benthic foraminiferal assemblages were not statistically analyzed due to

extensive diagenetic alteration. However, the presence of

shallow and poorly developed deep-water planktonic foraminifera within the Mannings Bay Member suggests deposition in a marine setting approximately 100 m deep

(McLaughlin and others, 1995).

Structure and distribution

On the basis of core material, the lower boundary of the

Mannings Bay Member is 44 m below sea level in well M4

(fig. 43). The transition is marked by a change in dominance

from the deep-water planktic fauna of the Kingshill Limestone to the mixture of planktonic and shelf-derived benthic

fauna of the Mannings Bay Formation. The core material

from well M4 indicates that the Mannings Bay Member is at

least 56 m thick.

The Mannings Bay Member of the Kingshill Limestone

occurs only close to the southern central coastline of St.

Croix, and is exposed along Evans Highway from Estates

Barren Spot and Castle Coakley in the East to the western end

of Manning Hill in the West. Poor exposures make identification outside of this region difficult. The Mannings Bay Member (and the overlying Blessing Formation) is interrupted by a

fault near Fairplain (fig. 43). Well M1 at Fairplain was completed in the Jealousy Formation at 29 m below sea level,

whereas well M4, located less than 180 m to the east, did not

encounter the Jealousy Formation despite penetration to 80 m

below sea level. Similarly, a well (45a) drilled 60 m to the

east of M1 (fig. 38) penetrated the Jealousy Formation 24 m

deeper than at M1 indicating that the intervening fault zone is

steeply dipping. 

The existence of a fault at Fairplain is also supported by

the vertical displacement of the lower contact of the Mannings Bay Member. West of well M1, Kingshill Limestone

strata in Mannings Hill dip toward the fault, and the lower

Mannings Bay contact is elevated 24 m above sea level. In

contrast, the lower Mannings Bay contact is reached at

approximately 55 m below sea level in well M4, suggesting a

minimum displacement of 79 m. In addition, River and BethFIGURE 48.—An east-facing view of the unconformity that separates the Mannings Bay Member from the Blessing Formation at the 

Hamilton Airport Quarry exposure. Arrows mark the position of the unconformity (reprinted and published with permission from 

Gill and others, 1989).
92 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

lehem Guts, two ephemeral streams, run along the presumed

line of the fault as they extend to the coast (fig. 38).

Vertical displacement of the Mannings Bay Member indicates that tectonic activity occurred after deposition of the

Kingshill Limestone. However, the specific orientation and

extent of the Fairplain fault is uncertain, although disrupted

streamflow occurs between wells M8 and M3, extending from

River Gut eastward to hole M5. Streamflow here is oriented

parallel to the coastline rather than into the sea and corresponds to the lateral extent of the exposed reef facies.

Holocene topography in this area could be partly the result of

Tertiary reef development patterns, but may also mark the

location of the fault trace of a half-graben or graben block

that contains most of the Mannings Bay Member and Blessing Formation strata (fig. 49). A similar location for a fault

boundary was published by Cederstrom (1950).

Age

The age of the Mannings Bay Member can range from the

late Miocene (upper N17) to as young as early Pliocene in

age (upper N19). It is not possible to further refine the biostratigraphic placement due to extensive diagenetic alteration

(McLaughlin and others, 1995).

Biostratigraphic assignment of the Mannings Bay Member has been done only on the basis of species presence in

core and outcrop samples, because species absences are unreliable due to extensive diagenetic alteration, and the likelihood of unfavorable ecological factors (McLaughlin and

others, 1995). Samples from well M4 (21.8 m subsurface) are

placed near the Miocene/Pliocene boundary based on the

presence of Globoquadrina altispira, Globigerinoides trilobus subspp., and Globigerinoides obliquus extremus. Tentative identification of a small, menardiform Globorotalia

tumida would indicate placement in the Pliocene portion of

this interval. In outcrop, the Mannings Bay Member appears

to lie in the lower Pliocene, due to the tentative identification

of Globorotalia crassaformis and Globorotalia crassaformis

ronda in a sample taken from immediately above the basal

unconformity of the member. A more detailed discussion of

the biostratigraphy is given by McLaughlin and others

(1995).

BLESSING FORMATION

The name Blessing Formation is herein used to describe

reef and lagoonal carbonate rocks of Pliocene age that extend

across the central southern and western coasts of St. Croix

(fig. 38). The greatest exposure of the Blessing Formation is

in the Hess Refinery outcrop which is suggested here as the

type-section (figs. 38, 50). The name Blessing Formation, as

used here, combines the strata separated into the Annaberg

and Blessing Formations by Behrens (1976) because the two

units would be difficult to map individually, and the outcrops

used to differentiate the two have deteriorated to the point

that much of the original lithologic detail has been lost. Like

the Mannings Bay Member, the greatest thickness of Blessing

Formation strata is contained within a subsidiary graben

located in the southeastern coastal section of the central plain

region.

Lithology, facies, and depositional environment

The Blessing Formation unconformably overlies the Mannings Bay Member of the Kingshill Limestone in the quarry

face on the eastern side of Mannings Hill adjacent to Alexander Hamilton airport. In this exposure, there is an erosional

truncation of the soft, poorly indurated, well-bedded Kingshill

Limestone strata, and an abrupt transition to a massively bedded, well-indurated molluscan bioclastic wackestone. There is

no strong evidence of subaerial exposure at this unconformity,

although Behrens (1976) and Frost (oral commun., 1984)

observed a reddish, oxidized zone interpreted as a paleosol

between the upper Kingshill Limestone and the Blessing Formation in an outcrop within the Hess Refinery. This latter

exposure has since been obliterated by continued construction

in the Hess Refinery.

Throughout the Blessing Formation, mollusks and corals

are preserved as moldic porosity, and the unit is generally classified as coral- and molluscan-dominated bioclastic wackestones and occasionally packstones. Diagenetic alteration of

the Blessing Formation is extensive, with evidence of subaerial

exposure and meteoric leaching commonplace. Locally, the

unit has been patchily dolomitized within the Blessing graben

block. The unit is generally quite porous and friable.

Exposures and core samples from several wells show the

Blessing Formation to contain a macrofaunal assemblage represented by external molds of scleractinian corals, gastropods

and pelecypods, as well as skeletal debris from foraminifera,

coralline algae and a wide variety of shallow-water invertebrates. Scleractinian corals include several extant genera

(Agaricia, Diploria, Montastrea, Siderastrea, among others)

as well as extinct corals such as Stylophora sp., Teliophyllia

sp., and Thysanus sp. In general, different faunal assemblages

within the Blessing Formation represent coexisting reef, forereef, and lagoon environments that extended along the southern and western coastlines of St. Croix during Pliocene time.

Structure and distribution

The Blessing Formation occurs only along the southern

and western coasts of St. Croix, and its distribution suggests

that reef and lagoon systems in the Pliocene followed a coastline similar to that of modern St. Croix. The thickest remnant

of Blessing Formation strata lies in the subsurface east of the

Fairplain fault described above. The thickness of the Blessing

Formation could reach 30 m and its extent suggests that sub-
GEOLOGY OF CENTRAL ST. CROIX, U.S. VIRGIN ISLANDS 93







   

Inferred limit of Pliocene carbonates

17°43'

64°45'30"

Near surface and

subsurface dolomite

Subsurface

dolomite

Subsurface

dolomite

Subsurface

dolomite

Surficial and

near-surface

dolomite

Pliocene reef and near-reef facies

Krause Lagoon

? U

D

U

D

?

?

Subsurface

indurated

calcite Fairplain Fault

River Gut

Bethlehem

Gut

C A R I B B E A N S E A

MAP AREA ST. CROIX

0 1 2 KILOMETERS

0 1 MILE

EXPLANATION

Dolomite

Fault—Dashed and queried where infered.

 U, upthrown; D, downthrown

Inferred facies boundary—Queried where uncertain

Well control point

U

D ?

? 

FIGURE 49.—Major facies within the Blessing Formation in the industrial area north of Krause Lagoon

(reprinted from Gill and others, 1989, and published with permission).
94 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

FH

H

C

FIGURE 50.—Hardground and exposure surfaces in the Blessing Formation at the Hess Oil exposure. Upper photo shows 

an oblique view to the north-northwest. Apparent "bedding" dipping upper left to lower right is caused by bulldozer 

scarring. Lower photo shows hardground (H) and caliche/exposure surfaces (C) in the Blessing Formation reef tract. 

Rock hammer and field book (FH) for scale (reprinted and published with permission from Gill and others, 1989).
GEOLOGY OF CENTRAL ST. CROIX, U.S. VIRGIN ISLANDS 95

sidence controlled both the accumulation and preservation of

the Blessing reef facies. In western St. Croix, well control is

poor and exposures sparse; west of the Airport/Evans Highway outcrop, exposures are limited to scattered outcrops along

the highway, around Fredericksted, and an exposure of reefal

limestone in Estate Carlton described by Gerhard and others

(1978). For this reason, the age, nature, and extent of the

Blessing Formation reef facies in this area are speculative.

The maximum thickness of the Blessing Formation west of the

fault at Fairplain is estimated to be between 10 and 20 m.

The structure of the Mannings Hill Member and Blessing

Formation is discussed in detail in the previous section. The

Fairplain fault displaces Blessing Formation strata as well as

that of the Kingshill Limestone. Vertical displacement of the

Blessing Formation by faulting indicates tensional fault activity occurred as recently as the early Pliocene, and perhaps

later.

Age

The Blessing Formation was deposited during the early

Pliocene. This age assignment is based on the tentative identification of Globorotalia margaritae in subjacent beds (Lidz,

1982), the co-occurrence of Operculinoides cojimarenis and

Paraspiroclypeus chawneri (Andreieff and others, 1986) in

underlying Mannings Bay strata, and the common presence of

scleractinians Stylophora, Teliophyllia, and Thysanus within

the Blessing Formation (Behrens, 1976; Frost, oral commun.,

1984; Gill, 1989; Gill and others, 1989).

Dolomitization and diagenesis

The Blessing Formation displays patchy areas of dolomitization where exposed as well as in the subsurface

(figs. 42, 43). Dolomite has not been detected elsewhere on

St. Croix. On the basis of stratigraphic position, the dolomitization occurred during or after Pliocene time, and is discussed

in more detail in subsequent sections. Subsurface dissolution

has resulted in voids in the Blessing Formation that were

encountered during drilling. In contrast, karst has not been

observed in rocks of the Kingshill Limestone.

Well-cemented, undulating layers within the Hess outcrop

(fig. 50) correspond to karst surfaces with marked light stableisotopic excursions, and indicate subaerial exposure of the

Blessing Formation several times during the Pliocene. The

exact timing of the exposure episodes remains uncertain.

ALLUVIAL DEPOSITS

Dark, terrigenous material makes up most of the alluvium

in the stream valleys, and is derived from the weathering of the

Cretaceous siliciclastics of the highland areas. However, alluvium in areas immediately overlying the Kingshill Limestone tends to be light-colored, and contains carbonate

material difficult to distinguish from the unweathered parts of

the Kingshill Limestone. For this reason, identifying the

uppermost contact of the Kingshill Limestone in well cuttings

or cores can be difficult. Red, clay-rich alluvium fills karst

cavities in Blessing Formation and Mannings Bay Member

strata at the Barren Spot well field, and terrigenous alluvium is

penetrated in many of the active well fields on St. Croix.

SEDIMENTARY AND STRUCTURAL SETTING OF 

THE KINGSHILL BASIN

The Jealousy Formation was deposited in 600 to 800 m of

water, and represents deep-marine depositional conditions. On

the basis of the evidence discussed above, the graben block

which now forms the Kingshill Basin might not have been

active before the early Miocene (fig. 51A) and the horst blocks

were probably not exposed. If the Kingshill Basin did not

form before the middle Miocene, then shelf-derived sediments

in the Jealousy Formation must be derived from outside of the

present island of St. Croix.

The lowermost strata of the Kingshill Limestone were

deposited in the same bathyal conditions as the immediately

underlying strata of the Jealousy Formation (fig. 51B). There

are no changes in sediment character, and no changes in basin

depth. Tectonic or eustatic changes, if they occurred during

this period, either were not substantial enough to be detectable, or cancelled each other out. The origin of the sharp color

change between the two formations remains undetermined, but

could reflect differences in clay mineralogy or diagenetic

effects.

Basinal shallowing only becomes apparent in upsection

parts of the Kingshill Limestone along the southern edge of

the central plain. Here, the Kingshill Limestone contains

greater quantities of shelf-derived sand, is burrowed, and contains an intraformational disconformity separating the La

Reine Member of the Kingshill Limestone from the overlying

Mannings Bay Member. The presence of a planktonic foraminiferal assemblage of both shallow and deep water forms

implies that the deposition of the upper La Reine Member

occurred in approximately 200 m of water (Gill, 1989;

McLaughlin and others, 1995) (fig. 41).

The structural relation between the Kingshill Limestone

and adjacent Cretaceous strata along the eastern margin of the

Kingshill Basin indicates that the Kingshill Limestone was, at

least in part, deposited prior to basin faulting (Gill and Hubbard, 1986; Gill and others, 1989). Initiation of the St. Croix

normal fault system occurred after early Miocene time. It is

not entirely clear, however, whether the basin fault boundaries

formed during the deposition of the Kingshill Limestone, or if

the bounding horst blocks of the Northside and East End

Ranges were exposed during this time. Gerhard and others

(1978) supported a model with subaerially exposed horst
96 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS



 



 



  

EXPLANATION

Blessing Formation

Kingshill Limestone (Mannings Bay Member)

Kingshill Limestone (La Reine Member)

Jealousy Formation

Coral reef deposits

Benthic formaminiferal deposits

Areas covered by prehistoric seas 







A

C

E

B

D

F

FIGURE 51.—Block models of St. Croix during the early Miocene to Pliocene 

(reprinted and published with permission from Gill and others, 1989).
HYDROLOGIC FRAMEWORK OF THE U.S. CARIBBEAN ISLANDS 97

blocks (fig. 51C), interpreting exposures of the Kingshill

Limestone along the eastern fault boundary as syntectonic

breccias. If the terrigenous clasts in the Kingshill are indeed

derived from Mount Eagle Group strata, then the graben

boundaries must have formed no later than the middle

Miocene (N14 and N16; Gill, 1989; McLaughlin and others,

1995).

The unconformable contact between the Kingshill Limestone and its upper Mannings Bay Member signals basin shallowing and the development of a source of shallow-water

benthic foraminifera, in particular Operculinoides cojimarensis and Paraspiroclypeus chawneri. An alternative interpretation is that the disconformity represents submarine erosion

caused by the transport of coarse, shelf-derived gravity deposits.

The foraminiferal-algal facies of this member marks a

period of deposition when shallow-water carbonate production

was dominated by benthic foraminifera and coralline algae

perhaps at the expense of scleractinian communities. These

deposits indicate basin shallowing from bathyal depths to

outer platform or upper slope environments of about 100 m

water depth (fig. 51D).

The amount of shallowing suggested by the character of

the Mannings Bay Member is too great to be explained by

either basinal fill or eustatic change alone, and must have been

caused by tectonic uplift. Neglecting eustatic variation, the

Kingshill Basin shallowed from approximately 750 m of water

depth in the middle Miocene to approximately 100 m water

depth in the lower Pliocene (N17). The average rate of uplift

suggested by these estimates of 650 m of vertical movement

over a period of roughly 9 million years translates to a minimum average uplift of 72 m.y., or slightly less than 0.1 mm/yr.

These calculations assume continuous uplift from bathyal

depths between the middle Miocene and the early Pliocene,

and that the foraminiferal biozones on St. Croix are equivalent

to the biozones established elsewhere in the Caribbean. This

tectonic uplift culminated in the shoaling of the Kingshill

Basin and deposition of the Blessing Formation reef tract that

extended around St. Croix’s southern and western coastlines

(fig. 51E). The reef tract consisted of interspersed reefs and

shelf lagoon systems similar to the arrangement of reefs

around the southern coastlines of St. Croix today. The reefs

apparently formed planar deposits with little topographic

relief. This planar geometry was common in Caribbean Tertiary reef deposits (Frost, oral commun., 1984).

The greatest thickness of Pliocene reef growth is found

within the Blessing graben, a small, subsidiary graben in

what is now the industrial area on the south-central coastline.

The geographical distribution of reefs suggests that faulting

in the subsidiary graben affected sedimentation of the Blessing Formation and the Mannings Bay Member. The arcuate

distribution of reef and lagoonal lithologies indicates that the

Blessing graben area was an embayment during the Pliocene

(fig. 49), with the embayment’s size and shape controlled by

faulting in the Krause Lagoon area.

From a tectonic standpoint, the Fairplain fault that cuts

through the Mannings Bay Member and Blessing Formation

units demonstrates that normal faulting, and therefore a tensional tectonic regime, extends at least into the Pliocene if not

later. The orientation of this fault is poorly controlled, but

suggests that the mechanism for the faulting was the same for

the basin boundary faults and the subsidiary south coast graben.

Deposition of Mannings Bay Member and Blessing Formation sediments was concentrated in the basin formed by

this subsidiary graben, and the strata were preserved by additional subsidence during island uplift. The incorporation of

reworked, cemented planktonic forams from the Kingshill

Limestone into the Blessing Formation rocks demonstrates

that erosion of the uplands area has removed a significant section of the Kingshill Limestone and, by inference, the postKingshill rocks as well.

Normal faulting of Blessing Formation strata indicates

that tectonic activity continued on St. Croix through the latest

periods of Tertiary deposition, and therefore extended into

Pliocene time or later. Uplift continued during the Pliocene,

and eustatic variation along with tectonic uplift account for

the repeated exposure of Blessing Formation strata (fig. 51F).

Preferential uplift of the northern part of the island accounts

for the more extensive erosion in the northern central plain,

and the general southerly dip of Tertiary strata in the Kingshill Basin.

HYDROGEOLOGIC FRAMEWORK OF THE 

U.S. CARIBBEAN ISLANDS

BY ROBERT A. RENKEN1, FERNANDO GÓMEZ-GÓMEZ1, 

AND JESÚS RODRÍGUEZ-MARTÍNEZ1

The two most important aquifer systems of the U.S. Caribbean Islands, in terms of areal extent and productivity are

the North Coast limestone aquifer system and the South

Coast aquifer; they are located on the windward-Atlantic

(North Coast ground-water province) and leeward-Caribbean

(South Coast ground-water province) sides of the Puerto

Rico, respectively. In 1985, withdrawals from Puerto Rico’s

North Coast limestone aquifer system and South Coast aquifer accounted for approximately 80 per cent of the total

amount of ground water withdrawn in the U.S. Caribbean

Island study area (U.S. Geological Survey, 1990).

Poorly lithified carbonate rocks combine with a thin overlying alluvial veneer in St. Croix’s central plain and compose

the only areally extensive sedimentary-rock aquifer in the

1

 U.S. Geological Survey
98 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

U.S. Virgin Islands. The importance of the Kingshill aquifer

is related more to its physical extent and lack of alternative

ground-water supplies on that island than its productivity,

which is relatively poor. In 1985, the Kingshill aquifer supplied less than 1 percent of the total water withdrawn from

wells in the U.S. Caribbean region (U.S. Geological Survey,

1990).

Puerto Rico contains a number of alluvial valley aquifers

that represent important local sources of ground water. Collectively, alluvial valley aquifers supply nearly 14 percent of

the total amount of ground water withdrawn in the U.S. Caribbean region, with all but a very small fraction from Puerto

Rico (U.S. Geological Survey, 1990). Alluvial aquifers of

Puerto Rico are relatively permeable and comparatively

restricted in extent. Alluvial aquifers are reported to yield to

wells 3 to 10 L/s, and as much as 125 L/s. Alluvial valley

aquifers of the U.S. Virgin Islands are much more limited in

extent and importance and function only as a local source of

water. 

Volcanic and volcaniclastic rocks on the different U.S.

Caribbean Islands collectively yield about 482,000 m3/s or

about 6 percent of the total ground water withdrawn from the

islands (U.S. Geological Survey, 1990). The bulk of that

water is derived from Puerto Rico; volcanic and volcaniclastic rocks on St. Thomas, St. John, and St. Croix collectively

supply only about 1,360 m3/d. 

A previous section of this report defined the stratigraphic

section in the major ground-water provinces of Puerto Rico

and in the U.S. Virgin Islands. The major purpose of this section is to establish equivalency between stratigraphic and

hydrogeologic units (fig. 52), illustrate how the different geologic processes previously described affect permeability and

porosity within water-bearing strata, and compare the pattern

of deposition within the various aquifers with the areal distributions of transmissivity and hydraulic conductivity.

THE SOUTH COAST GROUND-WATER PROVINCE

The South Coast ground-water province of Puerto Rico

can be separated into three distinct hydrogeologic entities.

The South Coast aquifer is comprised of fan-delta and alluvial deposits of Quaternary age that underlie and form a continuous 70 km-long low-lying plain located between Ponce

and Patillas. West of Ponce, fan-delta and alluvial deposits

do not form a continuous alluvial plain. These deposits are

restricted to narrow valleys incised into the underlying strata

of the Ponce Limestone and the Juana Díaz Formation or lithified bedrock units of Cretaceous and early Tertiary age that

form the mountainous core of the island. These restricted

alluvial deposits are considered to be the second major hydrogeologic entity of the South Coast ground-water province. 

Rocks of Oligocene and Miocene age (Juana Díaz Formation and Ponce Limestone) represent the third major

hydrogeologic unit of the South Coast. For reasons described

below, bedded carbonate and clastic strata are poorly permeable or contain ground water of poor quality, or both.

THE SOUTH COAST AQUIFER

Previously referred as the Southern aquifer (Renken,

Díaz, and others, 1990; Renken, Gómez-Gómez, and others,

1990), the South Coast aquifer is the most important and

widespread clastic aquifer of the U.S. Caribbean Islands and

one of Puerto Rico’s two largest sources of ground water.

Consumptive ground-water withdrawals from this aquifer in

1987 were estimated to be 274,000 cubic meters per day (m3/

d), 67 percent of which was used for irrigation with remaining amounts mostly used for municipal supply (GómezGómez, 1990).

The South Coast aquifer consists of unlithified deposits of

boulder- to silt- and some clay-size detritus, mostly of volcaniclastic origin. Boulder- to sand-size detritus composes the

major water-bearing units. Beds of coarse-grained sediments

are intercalated with thick beds of silt and some clayey material that impede vertical ground-water movement in the

downfan areas. The South Coast aquifer contains ground

water under both confined and unconfined conditions.

Unconfined conditions dominate the coarser-grained proximal and midfan areas, whereas confined conditions appear to

be more prevalent within the deep subsurface, or in interfan,

and distal fan areas. Interfan and distal fan deposits are

important hydrologically because they tend to restrict seaward discharge of fresh ground water and retard the landward, as well as upward, encroachment of saltwater.

Quiñones-Aponte (1990) reported a coastal 3-m thick confining unit that lies approximately 10 m below land surface and

extends 3 km landward in the Salinas-Jobos area. Given the

intercalated, laterally-discontinuous nature of fine- and

coarse-grained beds that make up the fan-delta plain, it is

unlikely that there are any confining units that can be mapped

over a wide area, let alone the entire plain. In places, coastal

confinement of the aquifer can be attributed to fine-grained

sediment deposited within coastal marsh and mangrove areas.

Elsewhere, confinement is due, in part, to the numerous but

discontinuous fine-grained beds that collectively retard the

vertical movement of water. In any case, the aquifer contains

water under confined conditions at depths below 10 m near

the coast. 

Under predevelopment conditions, the South Coast aquifer was principally recharged by infrequent rainfall and by

streambed seepage near the fan apex. Aquifer discharge

occurred primarily as (1) seabed seepage, (2) base flow discharge along the lower coastal reach of stream channels, (3)

seepage into coastal wetlands, or (4) evapotranspiration in

areas underlain by a shallow, coastal water table. In addition
HYDROLOGIC FRAMEWORK OF THE U.S. CARIBBEAN ISLANDS 99

to those sources of recharge cited above, recharge to the aquifer also comes from irrigation canal seepage and area irrigation practices. On a regional basis, 30 percent of the surface

water used for irrigation recharges the aquifer (Bennett, 1976)

and enters the flow system largely within proximal and midfan areas of the fan-delta plain. 

The development of surface water resources within the

fan-delta plain has modified the hydrologic system considerably. Diverted streamflow by a channel-pond irrigation system served as the principal source of water for irrigation

between 1900 and 1930. Irrigation deliveries between 1914 to

the 1930’s increased recharge to the aquifer which, in turn,

increased discharge to the coastal reach of streams and coastal

wetlands (Quiñones-Aponte, 1990). Coastal drainage canals

were constructed as a consequence of irrigation practices;

canals were used to control the water-logging of coastal soils,

to recover land for sugar cane cultivation, and to control formation of coastal swamps and spread of water-borne diseases

(malaria) (Quiñones-Aponte and others, 1996). The development of surface water for irrigation between 1910 and 1960

increased ground water flow within the aquifer by a factor of

three when compared to the predevelopment water budget

(Quiñones-Aponte and others, 1996). 

During the 1960’s and 1970’s, the net increase in groundwater withdrawals for public supply contributed to a decrease

in discharge to coastal drains. By February 1968, there were

large cones of depression south of Ponce, near Santa Isabel,

and in the Salinas area (pl. 1F). Water levels were as much as

6 m below sea level in places. The intrusion of salt water into

the aquifer was also of concern. The chloride concentration

of water near the Coamo fan-delta coastline was reported to

have increased from 50 mg/L in 1960 to 250 mg/L in 1967

(Guisti, 1971). Concern about water-level declines led to a

gradual shift from furrow irrigation to drip irrigation and

sugar cane was replaced by vegetable crops (Ramos-Ginés,

1994). 

Under present day conditions (1986), the reduction in

ground-water withdrawals has resulted in a recovery of water

levels and has mitigated the problem of coastal salt-water

intrusion. Regional ground-water movement within the South

Coast aquifer is coastward away from the fan apex or adjoining foothills with local movement of ground water toward

streams and coastal drainage canals (pl. 1G). In general,

ground-water flow closely parallels topography. Although

most movement is to the coast or streams, development has

locally altered the direction of flow toward several well fields.

For example, a small cone of depression was formed at a well

field north of Central Aguirre in 1986; withdrawals were

reported to be nearly 19,000m3/d (Torres-González and

Gómez-Gómez, 1987). Although there has been a reduction

in surface water irrigation and well withdrawals since 1968,

present-day (1986) ground-water flow within the aquifer

exceeded predevelopment conditions by a factor of two

(Quiñones-Aponte and others, 1996). 

The thickness of the South Coast aquifer’s fresh-water

lens (pl. 1H) was estimated by applying Ghyben-Herzberg

relations to 1986-1987 potentiometric surface maps (Dacosta

and Gómez-Gómez, 1987; Quiñones-Aponte and GómezGómez, 1987; Román-Mas and Ramos-Ginés, 1987; TorresGonzález and Gómez-Gómez, 1987; Rodríguez-del-Río and

Quiñones-Aponte, 1990; and Rodríguez-del-Río and GómezGómez, 1990). The authors assumed that the interface

between fresh and saline water was relatively sharp and the

depth to the interface, zs, in meters below sea level, could be

estimated by the relation: 

zs = 40 zf (1)

where zf = height of freshwater, in meters above sea level. 

Specific conductivity data collected during the dual-tube

drilling operations also served as a source of additional information to estimate the depth of the interface.

The freshwater lens within the South Coast aquifer varies

considerably in thickness across the fan-delta plain and, for

the most part, is limited to the Quaternary units of the system.

In easternmost areas, where the aquifer is relatively thin, the

lens is less than 20 m thick; in westernmost areas, the lens

ranges from 20 to 40 m thick. The thickest part of the freshwater lens underlies the Coamo fan delta in the central part of

the fan-delta plain. Here, the freshwater lens locally exceeds

150 to 200 m in thickness with some freshwater contained

within Miocene and Pliocene (?) alluvial sediments (fig. 9). 

The Vertical Sequence—Major Water-Bearing Units

Given the inherent non-continuous nature of bedded layers within alluvial fan and fan-delta deposits, it is difficult to

separate coarse- and fine-grained layers into widespread mappable zones of permeability with any degree of accuracy. In

southern Puerto Rico, these correlation problems are compounded by sparse borehole geophysical data and limited

lithologic log control, reported mostly from well driller’s

completion reports. Therefore, an understanding of the pattern of deposition within an alluvial-fan and fan-delta system

is extremely important. Recognition of such spatial depositional patterns, however, can provide a degree of hydrostratigraphic congruity within what otherwise might appear, at first

glance, to be a random distribution of permeable and poorly

permeable water-bearing units. 

Detailed study of the continuously-cored wells that were

described earlier in this report (pl. 3) reveals the occurrence

of layering of coarser- and finer-grained beds, but also provides an understanding of the cyclic nature of deposition

within the vertical sequence as well as eustatic, climatic, and

tectonic controls. In a broad sense, distal Portugúez-Bucana,
100 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

SERIES

HOLOCENE

PLEISTOCENE

PLIOCENE

EOCENE

PALEOCENE

UPPER

LOWER MIOCENE OLIGOCENE

JURASSIC CRETACEOUS TERTIARY QUATER- NARY SYSTEM

UPPER MIDDLE LOWER

PUERTO RICO

North Coast Ground-water Province South Coast Ground-water Province

Geologic Unit Geologic Unit Hydrogeologic Unit Hydrogeologic Unit

Alluvial, terrace, beach and swamp deposits

Alluvial-valley aquifers

(may locally function as

confining unit)

Quebradillas Limestone

Aguada (Los Puertos) Limestone

Undifferentiated Cibao Formation

Lares Limestone

Volcanic, sedimentary, and

intrusive igneous rocks Confining unit

Upper aquifer

North Coast limestone

aquifer system

Confining unit

Confining unit

Lower aquifer

Alluvial and fandelta deposits

South Coast aquifer;

Yauco, Tallaboa,

Guayanilla, Macana,

and Guanica alluvialvalley aquifers

Ponce

Limestone

Not a fresh-water aquifer

Mucarabones

Sand

Aymamón Limestone

Unnamed clastic deposits

Unnamed

pelagic rocks

???

Río Guatemala Group

Juana Díaz

Formation

1 Mostly or entirely unsaturated.

2 Quebrada Arenas and Río Indio Limestone Members of the Cibao Formation may function as confining unit in downdip areas.

3 San Sebastián Formation contains some conglomeratic and sandy deposits in updip areas that may function as part of the lower aquifer.

4 Juana Díaz Formation clastic beds are poorly permeable. Juana Díaz Formation and Ponce Limestone yield fresh water locally, mostly where they are adjacent to or 

 underlie stream-valley alluvial aquifers. Clastic beds equivalent to the Ponce Limestone are deeply buried and contain saline water.

5 There are no data available to evaluate water-bearing characteristics.

Ponce-Juana

 Díaz aquifer4

Not an aquifer 1

San Sebastián Formation3

Montebello

Limestone

Member

Mudstone

unit

Quebrada Arenas

and Río Indio

Limestone Members2

Volcanic,

sedimentary,

and igneous rocks

Poorly

permeable;

wells may

yield limited

quantities for

domestic or

livestock

purposes

FIGURE 52.—Relation of major stratigraphic and hydrogeologic units of the U.S. Caribbean Islands.
HYDROLOGIC FRAMEWORK OF THE U.S. CARIBBEAN ISLANDS 101

Geologic

Unit

Hydrogeologic

Unit

Geologic

Unit

Hydrogeologic

Unit

Geologic

Unit

Hydrogeologic

Unit

Igneous

rocks

Volcanic and minor

limestone rocks

Mannings

Bay

Member

Igneous

intrusive rocks

Volcanic,

sedimentary, 

and igneous

rocks

Volcanic,

sedimentary,

and igneous

rocks

La Reine

Member

Kingshill

aquifer

Blessing

Formation

Jealousy

Formation

Not an

aquifer

Not an

aquifer

Limestone

and

 dolomite5

? ?

?

? ?

?

Kingshill Limestone

PUERTO RICO ISLA DE VIEQUES U.S. VIRGIN ISLANDS

St. Croix St. Thomas

and St. John

Weathered

mantlebedrock

aquifer

Geologic

Unit

Hydrogeologic

Unit

Volcanic,

sedimentary,

and igneous

rocks

Metamorphic,

volcanic, and

sedimentary

rocks

Not an

aquifer

Aquifer is poorly

permeable; wells

may yield limited

quantities for

domestic and 

livestock purposes

East and West Coasts and

Interior Ground-water Provinces SERIES

HOLOCENE

PLEISTOCENE

PLIOCENE

EOCENE

PALEOCENE

UPPER

LOWER MIOCENE OLIGOCENE

JURASSIC CRETACEOUS TERTIARY QUATER- NARY SYSTEM

UPPER MIDDLE LOWER

Alluvial, alluvial

fan, terrace,

beach, and

swamp deposits

Lajas, Guanajibo, Añasco,

Humacao-Naguabo,

Maunabo, Yabucoa,

and Caguas-Juncos

alluvial-valley aquifers

Alluvium,

beach, and

swamp

deposits

Resolución

and Esperanza

alluvial-valley

aquifers

Alluvial fan,

alluvium,

debris flow,

and beach

deposits

Alluvial aquifer

and coastal

embayment

aquifers

Alluvial aquifer

and coastal

embayment

aquifers

Alluvium

and beach

deposits

Aquifer is

poorly

permeable;

wells may

yield limited

quantities for

domestic and 

livestock

purposes

Weathered

mantlebedrock

aquifer

FIGURE 52.—CONTINUED. Relation of major stratigraphic and hydrogeologic units of the U.S. Caribbean Islands.
102 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

Capitanejo, and Coamo fan-delta areas (fig. 13) can be separated into regionally-extensive water-bearing zones, distinguished by their intermediate-scale cyclic, coarseningupward, sand- to boulder-sized detrital nature (mostly 30 to

40 m thick). In the Salinas fan, intermediate-scale layers fineupward, but can also be viewed as cyclic, 30 to 40 m-thick

hydrogeologic layers. The recognition of such stratigraphic

layering clarifies the physical characteristics of the hydrogeologic system in general and improves our understanding of

the depth and frequency of occurrence of small-scale (less

than 1 m to 20 m thick) coarse- and fine-grained bedded layers that could be encountered during drilling operations. This

type of stratigraphic evaluation proved to be useful to a limited degree in Puerto Rico’s south coast. However, additional

deep well information is needed before detailed correlation of

regional water-bearing units can be considered.

Hydraulic conductivity and the lateral continuity of waterbearing units

Sand and gravel percentage facies described earlier

(pl. 1D) show the proximal-to-distal fan spatial variability in

grain size. Coarse-grained deposits are concentrated in lobes

with the highest concentrations of sand and gravel within

proximal and midfan areas or within the paleochannels that

extend inland. Coarse-grained sediment is also concentrated

within paleochannels that were infilled with conglomeratic

deposits during channel aggradation. The highest percentage

of fine-grained material occurs within the interfan areas that

separate fan lobes and within the distal fan areas.

Maps showing lithofacies variations of the fan-delta plain

were compared with maps showing the areas distribution of

hydraulic conductivity (pl. 1I). Hydraulic conductivity and

transmissivity maps are important to an understanding of the

hydrogeologic system, in constructing ground-water flow

models, and in simulating dissolved-mineral transport. Few

aquifer tests have been conducted within the fan-delta plain

because of the high cost and lack of opportunity. Within a

474 km2 area, aquifer test data are available from only three

separate sites; these are considered representative of local

conditions only (V. Quiñones-Aponte, USGS, written commun., 1989). Accordingly, it would be imprudent to characterize regional hydraulic conductivity of the South Coast

aquifer given the inherent limitations of extending aquifer test

data to even nearby areas, and the aquifer’s depositional heterogeneity.

Hydraulic conductivity estimates based on specific capacity data are available for most wells in the study area. Specific-capacity is defined as the yield per unit of water level

drawdown in a pumped well for a specific pumping rate.

Estimates of apparent transmissivity (T  ́ ) for the South

Coast aquifer were made from specific-capacity data using

empirical equations developed by Theis (1963) and Brown

(1963) for water-table or artesian conditions, respectively.

The distribution of apparent hydraulic conductivity (K  ́ ) was

computed by dividing estimated transmissivity by the thickness of the aquifer penetrated by a particular well. The areal

distribution of hydraulic conductivity shown in plate 1I, is

subject to qualification. It is based on specific-capacity methods which assume 100 percent well efficiency, full penetration of the aquifer, a 15 to 45 cm well diameter, a storage

coefficient of 1 x 10-4 for wells subject to artesian conditions

or 0.15 for wells subject to water-table conditions, and a 24-

hour pumping time. An additional qualification is that the

distribution of hydraulic conductivity shown in plate 1I is

based on wells that penetrate, at the minimum, the upper 50

m of the saturated zone.

Lithofacies maps (pl. 1D) were used to indirectly corroborate the distribution of hydraulic conductivity shown for the

South Coast aquifer. The estimated hydraulic conductivity

for fan-delta deposits of the South Coast aquifer ranges from

less than 1 to greater than 100 meters per day (m/d). A comparison of sand and gravel percentage maps with hydraulic

conductivity maps indicates that the highest conductivities

are generally associated with the proximal and midfan parts

of each fan delta. Within the Coamo and Salinas fan-delta

areas, the areas of high hydraulic conductivity lie near the

present position of the Río Coamo and the Río Nigua at Salinas. The high hydraulic conductivity areas are underlain by

deposits that have a large percentage of sand and gravel. An

area of moderate hydraulic conductivity (7.5 to 15 m/d) parallels a paleostream channel and the buried fault trace of the

Esmeralda Fault that lies north of the bedrock hill at Central

Aguirre. The Arroyo, Capitanejo, and Portugúes-Bucana fan

deltas, underlain by large concentrations of sand and gravel,

also exhibit relatively high hydraulic conductivities exceeding 7.5 to 30 m/d in proximal and midfan areas. Conductivity

values of greater than 30 m/d are associated with the fluvial

“string” of the Ríos Patillas and Chico.

Maps of sand-gravel percentage and hydraulic conductivity exhibit a poorer correspondence in the Guayama fan area.

The highest conductivity values seem to correlate better with

the present position of the Río Guamani rather than a depositional lobe. The more restricted occurrence of sand and

gravel is indicative of a more channelized sequence, such

as along a high-gradient stream.

Specific capacity-derived estimates of conductivity suggest that the Guayama fan-delta deposits are poorly permeable in the Central Machete area. However, favorable

conditions for the occurrence of sand and gravel is indicated

on the basis of a vertical exposure face visible along the

Puerto Arroyo coastline. The exhumed channel seen here

(fig. 16) is corroborated further by the percent lithofacies map

(pl. 1D), and can indicate that permeable water-bearing units

are still available for development, but have not yet been

used.
HYDROLOGIC FRAMEWORK OF THE U.S. CARIBBEAN ISLANDS 103

ALLUVIAL VALLEY AQUIFERS OF THE SOUTH COAST

Boulder- to sand-size sediments form the principal waterbearing units within alluvial and fan-delta deposits of the

Ríos Tallaboa, Guayanilla, Yauco, Macaná and in the Guánica

alluvial valley area. Recharge to the aquifer is principally by

stream and irrigation ditch seepage (McClymonds, 1967;

Crooks and others, 1968) with minor recharge occurring from

infiltration of infrequent precipitation that is usually short in

duration (Quiñones-Aponte, 1986b). Ground water within

the different alluvial valleys occurs mostly under water-table

conditions; local artesian conditions also are reported and

probably due to stratification of silt and sand-to-boulder sized

material. Core data are not currently available within the

coastward portions of these alluviated/fan-delta valleys. It is

likely, however, that vertical stratification of coarse- and finegrained bedded units also occurs here, possibly similar to that

present within the South Coast aquifer. Ground water is

available locally from volcanic and volcaniclastic, fractured

or weathered bedrock, or both, or from caverous limestone

sequences that underlie the alluvial sequences.

The estimated hydraulic conductivity for alluvial deposits

within the Yauco alluvial valley aquifer ranges from 4 m/d

near the coast to 60 m/d in upland channel areas (Bennett,

1976). Estimates based on simulation (Quiñones-Aponte,

1986b) indicate that transmissivity within the Yauco alluvial

valley ranges between 1,400 to 2,300 square meters per day

(m2/d) in the upland valley and diminishes down valley.

Transmissivity in the midfan and distal part of the fan-delta

plain range from less than 10 to 900 m2/d (fig. 53). If a direct

correlation between estimates of specific capacity and

hydraulic conductivity is assumed, coarse-grained deposits

within the Guayanilla and Macaná fan deltas are probably

less permeable. Specific-capacity estimates in the Guayanilla

and Macaná alluvial valleys range from 1 to 10 liters per second per meter [(L/s)/m] drawdown as compared to the

upstream parts of the Yauco which ranged from 1 to more

than 21 [(L/s)/m] of drawdown. Higher conductivities within

the Yauco alluvial valley as compared to the Guayanilla and

Macaná alluvial valleys are attributed to a slightly larger

drainage basin size and the narrower upland channel width.

Such features may result in higher floodflow velocities and

transport of coarser bedload. The transmissivity of alluvium

within the Guánica alluvial valley is reported to range from

100 to 7,500 m2/d; the hydraulic conductivity of the aquifer is

reported to range from 20 to 220 m/d (McClymonds, 1967).

THE PONCE-JUANA DÍAZ AQUIFER

The Ponce-Juana Díaz aquifer is most productive where

buried beneath the fan-delta plain near Ponce or beneath alluvium in the Tallaboa-Guánica area. In these areas, alluvium is

relatively permeable and recharged with ground water, low in

dissolved solids and undersaturated with respect to calcite.

Monroe (1976, p. 14) reports that the only part of the Juana

Díaz Formation that exhibits karstic dissolution and cave

development is within the reef tract sediments of the formation

that are south and southwest of Peñuelas. Cavernous limestone was reportedly encountered in wells drilled in proximal

and midfan areas of the Portugúez-Bucaná and Capitanejo fan

deltas; these permeable limestone units are probably associated with fracturing and jointing of beds by nearby faults.

Cavernous limestone porosity also was reported in the Guayanilla-Yauco area; wells that penetrate the limestone strata

reportedly yield 8 to 63 L/s (42 L/s average) (Crooks and others, 1968).

Although there are limited occurrences of cavernous limestone reported in some localized areas, extensive dissolution

within the Ponce-Juana Díaz aquifer has not developed notwithstanding periods of subaerial exposure from the end of the

Oligocene to Holocene time. Frost and others (1983, p. 82)

suggest three factors that could have effectively reduced the

opportunity for solution diagenesis with the reef deposits of

the Juana Díaz Formation: (1) meteoric waters were captured

by sandy alluvial and nearshore deposits as they moved downgradient; (2) reef deposits were overlain and diagenetically

sheltered by poorly permeable forereef and island slope mud;

and (3) lime mud, internal sediment, and cementation reduced

the original depositional porosity. Arid to semiarid climatic

conditions that have prevailed on Puerto Rico’s south coast

since Pliocene time could have also contributed to the lack of

dissolution within the reef tract deposits of the Juana Díaz Formation. Conglomeratic beds in the Juana Díaz Formation,

probably deposited as part of an early Oligocene fan-delta

sequence, consist of bedded cobble- to sand-size material.

Despite their conglomeratic nature, they are not considered a

reliable source of ground water. Intergranular voids of the

conglomeratic Juana Díaz Formation are infilled with partially-cemented sand and silt, effectively reducing porosity and

permeability within the aquifer. For the most part, limestone

strata within the Ponce-Juana Díaz aquifer also do not represent important water-bearing units (K<1.5 m/d). Secondary

permeability within the aquifer is only locally well-developed

due to dissolution along fracture and fault zones that lie buried

beneath unconsolidated clastic deposits of Quaternary age.

THE WEST COAST GROUND-WATER PROVINCE

Sandier beds within silt- and clay-dominated alluvium and

minor underlying limestone beds represent the principal waterbearing units within Lajas Valley. Igneous and volcaniclastic

bedrock units that extend beneath the valley are poorly permeable; wells located on the valley sides and tapping weathered

bedrock are reported to yield about 1 L/s. Irregularly-distributed limestone rock of Cretaceous to Tertiary age exposed in

the foothill areas is believed to extend beneath the Lajas Valley
104 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS YAUCO GUÁNICA GUAYANILLA YAUCO GUÁNICA GUAYANILLA

Río

CARIBBEAN SEA

PresadaLoco

Loco

Río

Yauco

Río

Guayanilla

Río

CARIBBEAN SEA

PresadaLoco

Loco

Río

Yauco

Río

Guayanilla

Guayanilla

Yauco

Guayanilla

Yauco

18°02'30"

18°00'

17°57'30"

18°02'30"

18°00'

17°57'30"

66°52'30" 66°50' 66°47'30''

Base from U.S. Geological Survey, Guánica, Punta Verraco, Saban Grande, and Yauco, Puerto Rico, 1:20,000

66°52'30" 66°50' 66°47'30''

Base from U.S. Geological Survey, Guánica, Punta Verraco, Saban Grande, and Yauco, Puerto Rico, 1:20,000

ATLANTIC OCEAN

CARIBBEAN SEA

IslaMona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

0 4 KILOMETERS 20 1 4 MILES

EXPLANATION

Transmissivity, in meters squared per day  (based on simulation) Less than 10 10 to 50 50 to 900 900 to 1,400

EXPLANATION

Percentage sand and gravel Less than 20 20 to 40 Greater than 40 Insufficient dataControl point—Well

 1,400 to 1,900 1,900 to 2,300 Greater than 2,300 Not estimated

FIGURE 53.—Distribution of transmissivity (A) and percentage sand and gravel (

B) in the Yauco alluvial valley, western south-central Puerto Rico

(modified from Quiñones-Aponte, 1986 b).

A

B
HYDROLOGIC FRAMEWORK OF THE U.S. CARIBBEAN ISLANDS 105

alluvial cover (Anderson, 1977). This limestone aquifer,

which is as much as 12 to 15 m thick, underlies a large area in

the central part of Lajas Valley. Eighteen wells tapped the

limestone bedrock during the 1920’s and 1930’s and were

reported to yield 30 to 130 L/s; however, these wells were later

abandoned due to the toxic effects of mineralized water

(greater than 300 mg/L chloride concentration) on crops

(Anderson, 1977, p. 32). Ground water within the Lajas Valley aquifer occurs under confined and unconfined conditions.

Unconfined conditions generally occur along valley sides

where alluvial deposits are most permeable, and the outcropping limestone bedrock has been exposed to dissolution processes. Ground water within the central part of the valley is

confined by clayey, poorly permeable, distal fan or lacustrine

deposits, or both. Transmissivity is reported to range from 60

to 3,000 m2/d. Well yields from the alluvial deposits are

reported to range from 0.32 to 43 L/s (Graves, 1991), but may

yield 10 to 60 L/s where screened in both alluvial and underlying limestone units (Anderson, 1977). 

Water-bearing strata within the Añasco alluvial valley

aquifer are comprised of sand and minor gravel. However, the

Añasco alluvial valley aquifer is poorly permeable and dominated by fine sand- and silt-sized sediment, reportedly yielding

less than 9.5 L/s (Díaz and Jordan, 1987). However, an underlying, interlayered limestone-clay sequence also serves as a

source of ground water. One well, developed in a 15-m-thick

limestone bed having an estimated transmissivity of 200 m2/d,

reportedly yields 32 L/s. 

Bedded sand and gravel alluvium and an underlying limestone sequence compose the Guanajibo alluvial valley aquifer.

However, fractured andesitic tuff and tuff-breccia along the

southwestern edge of the valley also function as important

local sources of ground water. The reported transmissivity for

wells penetrating both alluvium and limestone ranges from 60

to 560 m2/d (Colón-Dieppa and Quiñones-Marquez, 1985).

Two wells screened in the fractured bedrock aquifer have

transmissivities that range from 150 to 1,020 m2/d. Well performance data indicate that the maximum yield of 7 wells

screened in alluvium and limestone ranges from 5 to 70 L/s.

Two wells screened in an adjoining fractured bedrock aquifer

reportedly yield 12 to 25 L/s.

ALLUVIAL VALLEY AQUIFERS OF THE EAST 

COAST GROUND-WATER PROVINCE

Coarse-grained (sand- to boulder-size) fanglomerates and

alluvial plain deposits function as principal water-bearing

units of the East Coast ground-water province. For the most

part, East Coast ground-water province alluvial aquifers are

unconfined with recharge to the aquifer originating by stream

seepage or entering the system through buried, weathered, or

fractured bedrock. In coastal areas, discontinuous lenses of

coarse sediment are interbedded with less-permeable silt beds.

Assuming that areas of greatest hydraulic conductivity reflect

highest concentrations of sand and gravel, coarse-grained

deposits are probably centered along the axis of the principal

streams that drained the valleys.

In the Fajardo area, the most prolific aquifers are located in

the upper part of the alluvial valley, where recharge can be

induced from flowing streams (Gómez-Gómez, 1987). In the

Humacao-Naguabo area, the aquifer is characterized by

coarse-grained alluvial valley-fill sediment and an underlying

6-m-thick weathered bedrock zone (Graves, 1989). A weathered bedrock zone underlies alluvial deposits in the Maunabo

area, but is relatively thin and not a reliable source of water. 

The hydraulic conductivity of Maunabo Valley alluvium

reportedly ranges from 30 m/d along the course of the Río

Maunabo, to 15 to 23 m/d along its major tributaries (Adolphson and others, 1977). In the Yabucoa Valley, estimated

transmissivity of alluvial deposits ranges from 93 to 650 m2/d

with hydraulic conductivity ranging from 3 to 9 m/d. Interstream and colluvial deposits are least permeable (3 m/d). In

the Humacao-Naguabo area, aquifer test data collected from

two wells screened in the alluvial aquifer indicate that transmissivity of the alluvial aquifer ranges from 56 to 74 m2/d.

The transmissivity of alluvium and underlying fracture

weathered bedrock is reported to be 186 m2/d (Graves, 1989).

Transmissivity values were calculated also from specific

capacity data for three wells completed in alluvium; on the

basis of these data, transmissivity of the aquifer is reported to

range from 65 to 140 m2/d. Specific capacity data were collected from two wells completed in weathered bedrock; transmissivity of the weathered-bedrock aquifer is reported to

range from 56 to 84 m2/d. The hydraulic conductivity of

weathered bedrock underlying Maunabo Valley is approximately 1.5 m/d. The clay-and-silt dominated lithology of East

Coast ground-water alluvial valley aquifers limits the yield of

water to wells. Wells screened in the alluvium of the Humacoa-Naguabo are reported to yield only 2 to 6 L/s with higher

yields (as much as 11 L/s) reported in wells completed in alluvium and underlying fractured bedrock. Wells screened in the

alluvium of the Maunabo alluvial valley are reported to yield

1.3 to 38 L/s.

ALLUVIAL VALLEY AQUIFERS OF THE INTERIOR 

GROUND-WATER PROVINCE

Ground water within the Caguas-Juncos alluvial aquifer

occurs largely under unconfined conditions and is hydraulically interconnected with streams that cut across the valley

(Puig and Rodríguez, 1993). Coarse-grained beds of boulderto sand-sized deposits function as the principal water-bearing zones in the Caguas-Juncos aquifer. Well yields in the

Caguas-Juncos alluvial valley are reported to range from 0.6 to
106 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

20 L/s (Puig and Rodríguez, 1993). The transmissivity and

hydraulic conductivity of the Juncos valley aquifer is highest

along a narrow zone that extends westward from the city of

Juncos to the northern edge of the Caguas alluvial basin

(fig. 54). The transmissivity of the Caguas aquifer is, in general, less than the Juncos aquifer and is partly be attributed to

the aquifer’s silt and clay-dominated lithology, but also to its

limited thickness. The transmissivity of the Caguas alluvial

valley aquifer is reported to average less than 100 m2/d; the

transmissivity of sand- and boulder-size detritus that dominate

the Juncos valley aquifer, reportedly ranges from less than 100

to 450 m2/d.

NORTH COAST GROUND-WATER PROVINCE: THE 

NORTH COAST LIMESTONE AQUIFER SYSTEM

Platform carbonates and minor clastic rocks that underlie

Puerto Rico’s northern coastal plain make up the North

Coast limestone aquifer system. River alluvium consisting of sand, silt, and clay underlies northward-flowing rivers,

forms broad alluvial plains near the coast, and functions as an

important local sources of ground water. A minor confining

unit comprised of unconsolidated marsh and swamp surficial

deposits consisting of clay, silt and fine sand overlies the

North Coast limestone aquifer system in some restricted areas

near the coast.

The North Coast limestone aquifer system comprises four

principal hydrogeologic units that include an upper aquifer, a

middle confining unit, a lower aquifer, and a basal confining

unit (fig. 55; pl. 5A). The highly permeable Aymamón Limestone and the Aguada (Los Puertos) Limestone make up the

upper aquifer, whereas the middle confining unit consists

largely of the clay and marl contained within the upper part of

the Cibao Formation. A lower aquifer, that contains water

under confined conditions where buried, consists of carbonate

and some clastic rocks of the Montebello Limestone Member,

the Lares Formation, Mucarabones Sand, and in updip areas,

water-bearing sand and conglomeratic beds of the San

Sebastián Formation and the Quebrada Arenas and Río Indio

Limestone Members. The lower confining unit consists of

mudstone, siltstone, marl, and poorly permeable limestone of

the San Sebastián Formation buried in the deep subsurface.

In updip areas and to the east, where the lower aquifer grades

to more permeable clastic beds of the San Sebastián Formation and Mucarabones Sand, lithified volcaniclastic rocks and

minor igneous rocks of Cretaceous and early Tertiary age

form the basal confining unit. 

The first regional evaluation of ground-water resources in

the North Coast limestone aquifer system was conducted by

McGuinness (1948) as a part of a reconnaissance study of the

island. With the exception of work by Guisti (1978) and

Heisel and others (1983), subsequent studies of Puerto Rico’s

North Coast Limestone aquifer system have focused primarily on hydrologic conditions in localized areas. Region wide

pre-development and development (1980-1990) potentiometric maps showing hydrologic conditions within the upper and

lower aquifer were only recently published (Renken and

Gómez-Gómez, 1994).

Exploratory drilling near Barceloneta in 1968 first confirmed the occurrence of a lower limestone aquifer that contains freshwater under artesian conditions. Discovery well

Abbott 1 was drilled principally for the purpose of disposing

industrial wastes into underlying limestone strata thought to

contain saltwater; however, a fresh water-bearing limestone

unit was encountered at 350 m below land surface. Other

wells have been completed in the lower aquifer, primarily to

provide water for Puerto Rico’s pharmaceutical industry.

Twelve industrial wells and 3 public supply wells were withdrawing water from the confined system by 1987. In the shallower, updip, unconfined parts of the lower aquifer near the

town of Florida and barrio Montebello, 21 public supply

wells were withdrawing water in 1987. Twenty-three wells

located west of the Río Grande de Arecibo and east of the Río

Grande de Manatí areas were also withdrawing water from

the lower aquifer.

In 1986, an exploratory drilling program was initiated as

part of a cooperative program between the U.S. Geological

Survey and the Commonwealth of Puerto Rico (TorresGonzález and Wolansky, 1984). A major purpose of the drilling project was to evaluate the lithologic and hydraulic character of the limestone sequence, to delineate the geographic

extent of confined conditions within the lower aquifer, and to

define the water-quality of water-bearing units. As part of

this exploration program, seven wells were cored continuously, penetrating the upper and lower aquifer; subsequently,

eight additional wells were drilled.

OCCURRENCE AND MOVEMENT OF GROUND WATER

Diffuse fluid flow is the primary mechanism of groundwater movement within the poorly consolidated to unconsolidated clastic deposits that compose the principal aquifers of

the southern, eastern, western, and interior ground-water

provinces of Puerto Rico. In diffuse fluid flow, the movement

of water is contained within, and flows through, the small

intergranular pores that exist within the rock. The movement

of ground water within carbonate rocks is not, however,

restricted to diffuse movement. Flow within carbonate aquifers can range between the two end members, diffuse and

conduit (or concentrated) flow. The movement of ground

water within conduit-flow aquifers occurs within large openings formed along bedding planes, fractures, joints, and

faults.

Bennett and Guisti (1972) and Guisti (1978) inferred that

ground-water flow within limestone rocks of northern Puerto

Rico largely occurred through a system of closely spaced
HYDROLOGIC FRAMEWORK OF THE U.S. CARIBBEAN ISLANDS 107

conduits. Their interpretation was based on previously published geologic work, their own study of outcropping limestone rocks that compose principal water-bearing zones, and

the observation that numerous springs served as primary

points of discharge from the North Coast limestone aquifer

system. According to their conceptual model, ground-water

movement occurred along preferential flow paths that

included bedding planes, vertical fractures, joints, faults (if

present), and “pipes”, all of which had expanded in varying

degrees by chemical dissolution. Thus, according to their

interpretation, the North Coast limestone aquifer system can

be viewed as a honey-combed system of solution openings

and subterranean caves that combine to form a highly permeable, conduit flow system. Bennett and Guisti (1972) envisioned a hydrologic framework in which solution

passageways that served as conduits were of a sufficient density and close spacing so that, for the purpose of simulating

ground-water flow, assumptions of diffuse flow were acceptable.

The movement of ground-water within karstic aquifer

systems of relatively young rock (in a geologic sense) commonly is not limited to conduit flow. Diffuse and conduit

flow occurs within the Floridan aquifer system (U.S.A.) and

the Yucatan aquifer (Mexico) that are comprised largely or

entirely of Tertiary rocks (Thrailkill, 1976, p. 759). Like the

Floridan and Yucatan aquifers, conduit and diffuse-carbonate

ground-water flow occurs within the North Coast limestone

aquifer system. Diffuse-carbonate flow occurs largely occurs

 Lago

Loíza

de

Gurabo

Turabo

Río

Río

Loíza

Grande

Río

Juncos

Caguas

CAGUAS

RÍO GRANDE

GURABO

AGUAS

BUENAS

JUNCOS

GUAYNABO

CAROLINA

LAS PIEDRAS

HUMACAO

SAN LORENZO

SAN JUAN TRUJILLO

 ALTO

CANÓVANAS

ATLANTIC OCEAN

CARIBBEAN SEA

Isla

Mona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

0 4 1 3 2 KILOMETERS

0 1 2 MILES

18 ̊18'45"

18 ̊11'15"

18 ̊15'

66 ̊ 65 ̊52'30"

Base modified from U.S. Geological Survey digital data

Transmissivity, in meters squared per day

 0 to 100

 100 to 200

 200 to 300

 300 to 450

Well control point

EXPLANATION

FIGURE 54.—Distribution of transmissivity in the Caguas-Juncos alluvial valley aquifer (modified from Puig and Rodríguez, 1993).
108 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

within intergranular pores contained within bedded grainstone-packstone layers, moldic and intercrystalline pores of

thin, dolomitized, coral-rich grainstone-packstone and boundstone layers, and coral-patch reef limestone (Ward and others,

1990, this report). In northern Puerto Rico, conduit flow is

generally associated with a poorly defined system of solutionenhanced vugs and fractures within the upper aquifer

(Aymamón Limestone and Aguada (Los Puertos) Limestone)

and large cave systems contained within the lower aquifer

(Lares Limestone). Large cave systems represent major

ground-water drains, especially in areas where the lower

aquifer crops out.

Topographic relief and incision of carbonate coastal plain

rocks by streams are the principal factors that control the

direction of ground-water flow in areas of outcrop of the

upper and lower aquifer. Areas that have a high potentiometric surface generally correspond to areas of elevated topography and are principal sites for ground-water recharge. Areas

that have a low potentiometric surface generally correspond

to areas of low topography and commonly are sites of

ground-water discharge. Under predevelopment conditions,

large rivers, subterranean cave systems, springs, and coastal

wetland areas were the principal areas of ground-water discharge. Major rivers of Puerto Rico’s North Coast groundwater province that gain flow due to regional ground-water

discharge include, from west to east, the Ríos Culebrinas,

Guajataca, Camuy, Tanamá, Grande de Arecibo, Grande de

Manatí, Indio, Cibuco, and de la Plata. Under present-day

conditions (1980-90), well withdrawals and agricultural

drainage also represent important mechanisms of groundwater discharge.

Well-completion records and other geologic data indicate

that northward-flowing rivers were deeply incised during lowstands of Pleistocene sea level. Mechanical erosion of the

coastal plain by fluvial processes may have been extensive

enough to have breached middip parts of the middle confining

unit. These deeply incised areas are covered by thick alluvial

deposits that were deposited by channel aggradation in



  

  

250

500

Sea level

250

500

750

1,000

Aymamón Limestone Aymamón Limestone Aguada (Los Puertos) Limestone

Aguada (Los Puertos) Limestone

Cibao Formation Cibao Formation

Mudstone unit

Montebello Limestone Member

Lares Limestone

Río Indio-Quebrada Arenas 

Limestone Members San Sebastián Formation

Pre-middle Oligocene bedrock 

Mucarabones Sand

Alluvium

ManatíDoradoSan Juan

Quebradillas Limestone

IsabelaArecibo

San Sebastián Formation

Surficial deposits 

(alluvium, beach, 

swamp, marsh, 

artificial fill)

METERS

WEST

A

FEET

EAST

A'

1,000

Sea level

1,000

2,000

3,000

NOT TO SCALE Geology modified from Ward and others, 1981.

Hydrogeology modified from Renken and Gómez-Gómez, 1994

CORDILLERA CENTRAL

A A'

Mayaguez

Ponce Patillas

San Juan

0 30 60 KILOMETERS

0 30 MILES

ATLANTIC OCEAN

CARIBBEAN SEA



EXPLANATION

Alluvial valley aquifer

Local confining unit

Unsaturated—Not an aquifer

North Coast limestone aquifer

 Upper aquifer

 Confining unit

 Lower aquifer

 Basal confining unit

Insufficient data

Contact—Dashed where

 approximately located

FIGURE 55.—Diagrammatic section A–A' showing principal aquifers and confining units of the North Coast limestone aquifer system

(geology modified from Ward and others, 1991; this report).
HYDROLOGIC FRAMEWORK OF THE U.S. CARIBBEAN ISLANDS 109

response to interglacial sea level rise (fig. 56). The large rivers of the North Coast ground-water province are the principal sites of discharge from the lower aquifer. It is postulated

that ground water is probably discharged from the lower aquifer upward to large springs within these valleys, or into overlying alluvium that covers the breached part of the middle

confining unit, or both (Renken and Gómez-Gómez, 1994)

(fig. 57).

Major springs that produce water from the North Coast

limestone aquifer system are described by Guzmán-Ríos

(1988). These and other North Coast springs are listed in

table 1; however, this table is not considered a complete list

of all North Coast springs. Other springs could be present,

but are difficult to locate as a result of extensive vegetative

cover and limited access. No first-order springs (discharge

more than 2,800 L/s) are in the North Coast limestone aquifer

system, but numerous second- (280 to 2,800 L/s) and thirdorder (30 to 280 L/s) springs are. One of the two largest

springs that produce from the lower aquifer, the Aguas Fría, is

the point where discharge from the Río Encantado cave system surfaces. Between December 1982 and January 1984,

instantaneous discharge measurements at this spring ranged

from 150 to 312 L/s. The other large spring, the San Pedro, is

a second- to third-order spring along the Río Grande de

Arecibo. Discharge measurements at this spring ranged from

135 to 1,530 L/s between June 1983 and June 1984 (GuzmánRíos, 1988). The San Pedro spring probably discharges from

the Montebello Limestone Member, which is the most permeable part of the lower aquifer. A dye-trace study in the Río

Tanamá Basin area indicates there is loss of the water from

that river to the aquifer with subsurface movement of the

water to the San Pedro spring located on Río Grande de

Arecibo (Jordan, 1970, p. 17). Two large springs (la Cambija

and Zanja Fría) that discharge from the upper aquifer are

located in the Caño Tiburones area. These two springs are

considered to be second- to third-order springs with discharge

from la Cambija reported to be as much as 566 L/s (Zack and

Class-Cacho, 1984). However, most springs that discharge

water from the upper aquifer are third- (30 to 280 L/s) or

fourth-order (6 to 30 L/s) springs. Similarly, most freshwater

and saltwater springs and seeps located in the Caño Tiburones, which is an area of upper aquifer ground-water discharge, are classified as third- or fourth-order springs.

ALLUVIAL VALLEY AQUIFERS AND LOCAL CONFINING UNITS

Wide, low-lying alluvial plains have formed near the

mouths of the large, perennial, northward-flowing rivers that

transverse the North Coast ground-water province. Large

alluvial plains are associated with the Ríos Grande de

Arecibo, Grande de Manatí, de la Plata, and Bayamón. These

alluvial deposits are comprised largely of sand, silt, clay, and

minor gravel deposits and function as important local aquifers. However, only limited information is available

regarding the extent and the distribution of coarser sediment

within these aquifers. In the lower reaches of the Río Grande

de Arecibo Valley, the average thickness of alluvial deposits

is 40 m and an extensive clay layer within the alluvial aquifer

tends to separate it into upper and lower water-bearing units.

Deposits exceed 90 m at the river’s entrance into a narrow,

incised upland channel (fig. 58). Coastal reaches of the Río

Grande de Manatí alluvial valley also contain relatively thick

deposits of alluvium. Deposits locally exceed 80 m in thickness and consist of fine sand, silt, clay, and minor lenses of

fine to medium gravel. 

Marsh and swamp deposits are in many coastal areas of

the North Coast ground-water province and consist of sandy

clay, sandy mud, and clayey sand. Although the thickness of

these deposits has not been well-mapped, they function

locally as a confining unit. 

Ground water occurs largely under unconfined conditions

within alluvial valley aquifers of Puerto Rico’s North Coast

and often the aquifer is hydraulically interconnected to underlying carbonate strata of the upper limestone aquifer.

Recharge to the alluvial aquifer is possible by stream seepage,

infiltration of precipitation, or from the movement of ground

water from the underlying and adjoining upper limestone

aquifer. In most circumstances, the streams act as a drain for

the regional flow system; however, some recent investigations

indicate that, in places, the Ríos Grande de Arecibo, Tanamá,

and Cibuco lose surface flow to the aquifer (QuiñonesAponte, 1986a; Gómez-Gómez and Torres-Sierra, 1988).

Discharge from the alluvial aquifer occurs by ground-water

flow to the sea and nearby coastal lagoons, by well withdrawals, and by evapotranspiration. 

The hydraulic conductivity of the Arecibo and Manatí

alluvial valley aquifers are reported to range from 6 to 12 m/d

(Quiñones-Aponte, 1986a; Gómez-Gómez and Torres-Sierra,

1988). The transmissivity of the Río Cibuco and Río de la

Plata alluvial valley aquifers is estimated to range from 10 to

1,000 m2/d (Torres-González and Díaz, 1984).

UPPER AQUIFER

The upper aquifer is made up of carbonate strata that are

part of the Aymamón Limestone, the underlying Aguada (Los

Puertos) Limestone, and in some areas of the North Coast

ground-water province, the uppermost clastic beds of the

Cibao Formation. The Quebradillas Limestone, a foraminiferal-algal limestone, crops out and overlies the Aymamón

Limestone along a narrow belt that extends 60 km westward

from the Río Grande de Arecibo to Punta Borinquen in the

northwestern corner of Puerto Rico. The Quebradillas Limestone is not considered an important part of the upper aquifer,

because its lower contact generally lies above the water table. 

The upper aquifer is subaerially exposed in most of the

study area. It has been subject to extensive karstification and

is riddled with numerous solution passages. Precipitation,
110 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

which is the ultimate source of water to the North Coast limestone aquifer system, enters the upper aquifer by infiltrating

through the overlying surficial deposits or as runoff that

enters the system directly through karstic conduits (sinkholes,

dolines, dry valleys, and zanjones). Unconfined groundwater conditions within the upper aquifer are pervasive in

most of the North Coast area with confined conditions present

where the aquifer extends beneath poorly permeable clayey

sand, silt, clay, and clayey mud deposited along marshy

coastal areas. Water that enters the unconfined flow system of

the upper aquifer flows northward toward the Atlantic Ocean

and discharges to wetlands along the coast; to coastal, nearshore and offshore springs; or to the ocean as diffuse seabed

seepage. Ground water is discharged also along the inland

reach of north-flowing rivers. 

Regional synoptic measurement of water levels in the

upper and lower aquifers has never been conducted within the

North Coast ground-water province. However, a potentiometric surface map that shows the ground-water conditions in the

late 1960’s was presented by Guisti (1978). Unfortunately,

Guisti’s maps did not separate the flow system into upper and

lower aquifers. 

The reader is cautioned to interpret potentiometric maps

of the North Coast limestone aquifer system shown herein

and in Renken and Gómez-Gómez (1994) with care; some of

the maps are based on water-level measurements made over a

long period of time and, in some instances, in wells installed

in different stratigraphic horizons within the aquifer. Despite

these limitations, the maps are considered to be reasonable

estimates of regional predevelopment hydrologic conditions

for the lower aquifer and postdevelopment conditions in for

the upper and lower aquifers. Seasonal water-level fluctua-

      Montebello Limestone Member

SOUTH

Montebello Limestone Member

Aguada (Los Puertos) Limestone

Aymamón Limestone

Cibao Formation

NORTH

Cibao Formation

Aguada (Los Puertos) Limestone

Aymamón Limestone

Alluvial deposits

Alluvial deposits

San 

Pedro

Spring

Río

Grande

de

Arecibo

Río Tanamá

NOT TO SCALE





EXPLANATION

Alluvial valley aquifer

North Coast limestone aquifer

 Upper aquifer

 Confining unit

 Lower aquifer

Discharge from lower aquifer

Surficial extent of underlying hydrogeologic

 unit—Approximately located

Spring

FIGURE 56.—Area of probable discharge from lower aquifer to the alluvial aquifer in the Río Grande de Arecibo valley, northern

Puerto Rico (modified from Renken and Gómez-Gómez, 1994).
HYDROLOGIC FRAMEWORK OF THE U.S. CARIBBEAN ISLANDS 111

tions are small in many wells that are cased in the aquifer system (Torres-Gonzáles, 1991), and stress on the system due to

pumping is largely restricted to certain areas of the North

Coast. Water-level measurements used to draw the development potentiometric-surface map for the upper aquifer

(pl. 5B) were collected between 1980 and 1990 and are considered to be a reasonable representation of conditions during

1987, when most of the measurements were collected for the

lower aquifer. Data were not available to construct a

regionwide predevelopment map of the upper aquifer

because development of the upper aquifer preceded hydrologic data collection efforts in Puerto Rico by the U.S.G.S. In

  

Alluvial aquifer

Upper aquifer

Confining unit North Coast

limestone 

aquifer 

system

Lower aquifer

SOUTH

NORTH NOT TO SCALE

EXPLANATION

Direction of ground-water flow

Line of equal hydralic potential

Spring

FIGURE 57.—Exploded view of aquifers in the Rio Grande de Arecibo valley and patterns of ground-water flow, northern Puerto Rico

(modified from Renken and Gómez-Gómez, 1994).
112 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

Table 1. Principal springs within the North Coast limeston aquifer system

[Data from Persious (1971), Gómez-Gómez (1984), Quiñones-Aponte (1986), Gómez-Gómez and Torres-Sierra (1988),

and Guzmán-Ríos (1988). Spring order (Meinzer, 1927) refers to discharge; 2nd order, 280 to 2,800 L/s; 3rd order, 30 to

280 L/s; 4th order, 6 to 30 L/s; 5th order, .6 to 6 L/s]

Maguayo spring near Dorado

Ojo de Agua spring at Vega Baja

Ojo de Agua Guillo spring near Manatí

Palo de Pana and Mamey spring

Isadora spring

La Cambija spring at Caño Tiburones

Zanja Fría at Caño Tiburones

El Dique spring near Río Tanamá

Sonadora spring near Camuy

Tiburón spring near Camuy

Sumbadora spring at Los Puertos near Camuy

Ojo de Agua spring at Aguadilla

Nearshore, offshore, and shoreline springs

 Borinquen Beach

 U.S. Coast Guard lighthouse

 Colonel B. Smith's house

Unnamed spring near Río Cibuco

Ojo de Valencia near Municipio de Moca

Spring

number Spring name Spring order

Upper aquifer

Lower aquifer

1

2

3

4

5

6

7

8

9

10

11

12

13

a

b

c

14

15

3rd-4th.

 Do.

3rd.

4th.

 Do.

2nd.

2nd-3rd.

4th.

4th-5th.

4th.

 Do.

3rd.

 Do.

No information.

Do.

5th.

3rd-4th.

2nd-3rd.

 Do.

4th.

3rd-4th.

 Do.

3rd.

 Do.

 Do.

No information.

1

2

3

4

5

6

7

8

a

b

9

Ojo de Agua at Torrecillas near Morovis

Repressa Sonadora de Ciales at Ciales

Aguas Frías spring near Ciales resurgence of Río Encantado

San Pedro spring near Río Arecibo

Hato Viejo spring near Río Arecibo

Zanja spring

Los Chorros spring near Dos Bocas dam

Unnamed spring along Río Tanamá

Salto Collazo spring near San Sebastián

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HYDROLOGIC FRAMEWORK OF THE U.S. CARIBBEAN ISLANDS 113

some areas, agricultural dewatering and pumping of wells

caused change in upper aquifer water levels as early as 1909

(Zack and Class-Cacho, 1984). 

A potentiometric map of the upper aquifer (Renken and

Gómez-Gómez, 1994), which is based on water-level measurements made between 1980 and 1990, shows that principal

recharge areas are in topographically elevated, inland parts of

the aquifer (pl. 5B). Limited water level data are available for

the southernmost areas underlain by the upper aquifer, which

creates some uncertainty regarding the altitude of the potentiometric surface in these areas (pl. 5B). East of Río Grande de

Arecibo, the landwardmost contour is 30 m above sea level.

Additional data are needed to accurately assess the occurrence of water in the upper aquifer within upgradient areas;

however, ground water may occur at higher altitudes as a thin

freshwater lens or saturated zone located at the base of the

aquifer.

Only limited water-level data are available for the upper

aquifer in areas east of the Río Bayamón. Future groundwater development in the Metropolitan San Juan area is limited by an upper aquifer that is thin or may be entirely absent

in some areas (fig. 55; pl. 5A) and is reported to mostly contain brackish or saline water. 

Marl, mudstone, and clayey wackestone of the middle

confining unit that separates the upper and lower aquifers

grades by facies change to terrigenous sediments in the Metropolitan San Juan area. These sediments, which include

sandstone and sandy limestone, are more permeable and permit greater hydraulic interconnection between the two aquifers. Sparse hydrologic and geologic control in Metropolitan

San Juan limit detailed mapping of water levels in the area.

The configuration of the lower and upper potentiometric surfaces in this area were assumed to be similar in form; it seems

likely that there is potential for upward ground water discharge from the lower aquifer to upper aquifer (compare pls.

5B and 5C). 

East of the Río Grande de Arecibo, the upper aquifer discharges ground water mainly to springs and coastal wetland

areas. West of the Río Grande de Arecibo, the aquifer discharges to wetland areas near the shoreline; to nearshore, offshore, and shoreline springs such as those located along the

northwest coast of the island (Percious, 1971); or to the ocean

as seabed seepage. Future investigations could focus on identifying other coastal freshwater springs that may lie offshore.

Percious (1971) suggested the possibility of offshore spring

discharge at three site—one near the western end of Punta

Peñón, one near the inlet at Hatillo, one near and off the point

near the Río Guajataca (known locally as El tunel). 

Contours on the upper aquifer potentiometric-surface

map (pl. 5B) curve upstream in many interstream areas;

ground water discharges along inland, upstream reaches of rivers and streams that include the Ríos Guajataca, Camuy, Tanamá, Grande de Arecibo, Grande Manatí, Cibuco, and de la

Arecibo

Río Grande d e Ar ecibo

Río Tanamá

Río Santiago

Caño Vieque

Quebrada Regadera

ATLANTIC OCEAN

Tiburones

–100

–75

–60

–45 –30–15

–45

–15–30

–15

–30

–30

–15

–45

66°44' 66°42' 66°40'

18°28'

18°26'

18°24 '

Base from unpublished U.S. Geological Survey,

Arecibo, 1:100,000, 1980

0 1 2 KILOMETERS

0 0.5 1 MILE

ATLANTIC OCEAN

CARIBBEAN SEA

Isla

Mona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

EXPLANATION

Alluvium

Beach deposits

Swamp deposits

Lagoonal deposits

Eolianite deposits

Undifferentiated surficial deposits

Rocks of Tertiary age—

 Bedrock contour—Line of equal altitude of underlying

 rocks of Tertiary age. Dashed where approximately

 located. Datum is sea level. Interval 15 meters

Geologic contact

–30

FIGURE 58.—Basal configuration of Arecibo alluvial valley aquifer,

northern Puerto Rico (modified from Quiñones-Aponte, 1986a).
114 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

Plata. Near the coast, however, some rivers lose water to the

upper aquifer during most of the year (Quiñones-Aponte,

1986, p. 26). Simulation of ground-water movement in the

lower Grande de Arecibo valley indicates that ground water

discharges from the upper aquifer along the western limestone

channel bank and moves across a highly stratified alluvial

aquifer. Ground water reenters the upper aquifer along the

eastern margin of the alluvial valley (Quiñones-Aponte, 1986,

p. 24). Stratification of the alluvial aquifer by clay and silt in

the lower reach of the Río Grande de Arecibo valley has

resulted in local confinement of the upper limestone aquifer.

Movement of water from the aquifer to the river and back into

the aquifer and the northward and northeastward direction of

regional ground-water flow toward Caño Tiburones is illustrated in plate 5B. Local confinement of alluvium and the

underlying upper aquifer also helps explain why lines of equal

hydraulic head on the potentiometric-surface maps do not

show strong hydraulic interconnection between the upper

aquifer and the river. Simulation of ground-water flow near

the Río Cibuco indicates that the river could contribute as

much as 15 percent of the water that enters the aquifer in that

area (Gómez-Gómez and Torres-Sierra, 1988).

Ground water does not discharge to the upper reaches of

all major rivers that traverse the upper aquifer. For example,

water-level measurements and streambed altitude data indicate that there is limited ground water discharge along a reach

of the Río Camuy. Similar hydrologic conditions could exist

along the Río Guajataca. The Quebrada de los Cedros, which

is in the northwestern part of the study area, is an ephemeral

stream. The water table in the basin lies below the

streambed much of the area. In areas of northwestern Puerto

Rico drained by such streams, regional ground-water discharge from the upper aquifer probably is to the ocean or

springs, rather than to streams (Guisti and Bennett, 1976, p.

27).

The potentiometric surface of the upper aquifer has one

large and one small potentiometric depression (pl. 5B). The

large depression, which extends over an area of 36 km2, is in

the swampy coastal depression at Caño Tiburones is located

between the Ríos Grande de Arecibo and Grande de Manatí.

The smaller, but locally significant potentiometric depression

(also shown in pl. 5B) is the Colonia Combate-Colonia Coto

Sur area, which is located about 6.1 km south of Laguna Tortuguero. 

Caño Tiburones is the site of a former shallow brackish

water coastal lagoon that accumulated freshwater from adjacent springs and rivers. Under natural conditions, Caño Tiburones drained to the ocean by subterranean caverns and

conduits. Before agricultural devel-opment of the area

(1909), saltwater entered the lagoon during high tides or by

up-stream migration during periods of low flow. The large

present-day (1980-90) potentiometric low at Caño Tiburones

is attributed to agricultural dewatering. Drainage of the area

for agriculture contributed to soil shrinkage and land subsidence and gradually lowered the water table to sea level. By

1949, gravity drainage became less effective and a drainage

system of canals, laterals, tidal gates, and coastal pump stations had been installed to lower water levels to below sea

level. Spring-head levels of 0.6 m below sea level in this area

have been reported by Zack and Class-Cacho (1984). One

major side-effect of lowering the water levels to below sea

level was that it reversed the local hydraulic gradient and

caused intrusion of seawater into Caño Tiburones. Much of

this seawater intrusion was the result of seawater moving into

the aquifer through caverns that had previously drained freshwater to the sea.

In 1987, ground-water withdrawals of near 7,570 m3/d

concentrated in a 5.2 km2 area at the Colonia Combate-Colonia Sur location locally lowered the potentiometric surface

below sea level (U.S. Geological Survey, unpub. data, 1993).

Three pumped wells in this area were abandoned in 1989

because they yielded water with a nitrate-nitrogen concentration that exceeded 10 mg/L. Before 1989, the depression in

the potentiometric surface at this site may have been as much

as 3 m below sea level. 

Freshwater within the upper aquifer is underlain by a

basal zone of salt water along the coastal margins of the

North Coast ground-water province. This saltwater zone can

extend up to 7 km inland of the Atlantic Ocean coastline

(pl. 5E). 

The thickness of the freshwater lens was, in part, determined following the approach described and applied to the

South Coast aquifer. However, the specific conductivity of

ground water in the upper aquifer was measured in selected

wells at different depth intervals (Gómez-Gómez, written

commun., 1993) and served as principal control points. The

thickness map was constructed by comparing the saltwater

interface depth map (ground-water with estimated dissolved

solids exceeding 35,000 mg/L) with the map that shows the

average potentiometric surface between 1980 and 1990 (Renken and Gómez-Gómez, 1994). 

Within the homoclinal, seaward-thickening wedge of permeable stratum, the thickest part of a freshwater lens generally lies along a zone where the landward limit of the

freshwater-saltwater interface intersects the base of the aquifer. In northern Puerto Rico, this area generally coincides

with the interstream areas that lie between several major rivers that include the Ríos Grande de Arecibo, Grande de

Manatí, and de la Plata (exceeding 100 m). The thickest parts

of the lens appear to lie between the Río Grande de Arecibo

and the westernmost coastline of Puerto Rico (exceeding 150

m thickness). East of the Río de la Plata, the freshwater lens

is thin or absent; the upper aquifer is reported to locally contain brackish water in metropolitan San Juan.
HYDROLOGIC FRAMEWORK OF THE U.S. CARIBBEAN ISLANDS 115

MIDDLE CONFINING UNIT

The middle confining unit extends throughout the North

Coast ground-water province (pl. 5F) and consists largely of

marl, calcareous mudstone, and clayey wackestone and wackestone-packstone of the Cibao Formation. The top of the

middle confining unit generally conforms the position of the

contact between the Aguada (Los Puertos) Limestone and the

Cibao Formation and is shown as such in figure 55 and plate

5A. However, the top of the confining unit may lie below a 5

to 30 m thick layer of dolomitic limestone and dolomite that

represent the uppermost beds of the Cibao Formation. In the

Isabella-Hatillo area, the top of the confining unit is similarly

difficult to define, but may coincide with the lowermost

boundary of a karstic zone that is located within the lowermost Aguada (Los Puertos) Limestone and generally considered to be the base of the upper aquifer. Additional

hydrologic data are needed to better define the relation

between the hydrogeologic and rock-stratigraphic boundaries.

West of the Río Grande de Arecibo, the Montebello Limestone Member and Lares Formation grade to poorly permeable, red algal wackestone and minor packstone that is

lithologically indistinguishable from the Cibao Formation.

Here, the base of the middle confining unit is difficult to

determine. Whereas underlying strata are poorly permeable,

they locally contain minor water-bearing zones separated by

less permeable strata. A minor water-bearing zone is contained within the confining unit in an area that extends from

Arecibo to Vega Alta and could be equivalent to the Quebrada

Arenas Limestone Member. Permeable wackestone-packstone of the Montebello Limestone grade to a less permeable

clay, marl, mudstone, and siltstone sequence in the Río

Grande de Manatí and Río Cibuco area. Here, the bottom of

the middle confining unit occurs at the base of the mudstone

unit. In the Manatí-Vega Baja area, the middle confining unit

includes subsurface rocks that are part of the Quebrada Arenas and Río Indio Limestone Members. The same limestone

strata could be part of the lower aquifer where they crop out

or lie in the shallow subsurface. The contact between the

Cibao Formation and the Lares Limestone conforms well

with the middle confining unit’s lower boundary in the areas

east of the Río Cibuco and with the Cibao Formation and

Mucarabones Sand Member east of the Río Bayamón. Additional well data are needed to more fully understand the character, thickness, and extent of the middle confining unit,

particularly in the western part of the North Coast groundwater province. 

Hydrologic data suggest that the middle confining unit is

leaky in restricted updip localities (Renken and GómezGómez, 1994). This is probably due to two factors. In these

areas, the confining unit, which is the Cibao Formation, consists primarily of poorly permeable clay, marl, and mudstone

that interfingers with comparatively permeable siliciclastic

and limestone strata, thereby increasing vertical hydraulic

conductivity. The second geologic factor relates to a poorly

defined fracture system (Ward and others, 1990; Ward, this

report). In areas where the Cibao Formation lies near the surface, fractures are more likely to be open and more numerous.

In the deeper subsurface, they tend to be closed due to lithostatic pressure caused by the weight of overlying rock and soil.

The confining unit is entirely absent at outcrop between the

Ríos Grande de Arecibo and Grande de Manatí due to a facies

change. Here, the clay, marl, and mudstone confining unit

grades to the Montebello Limestone Member that forms part

of the lower aquifer. However, the effectiveness of the confining unit can be readily demonstrated in middip and downdip areas where the difference between potentiometric heads

of the upper and lower aquifers locally exceeds 90 m.

LOWER AQUIFER

Ground water within the lower aquifer occurs under confined and unconfined conditions. Unconfined conditions

occur where the principal carbonate-rock units that make up

the aquifer crop out or lie in the shallow subsurface. Confined conditions occur in areas where the aquifer extends

northward and dips beneath the Cibao Formation’s poorly

permeable claystone, wackestone, and marl that are a major

part of the middle confining unit. 

The lower aquifer includes skeletal wackestone-packstone

and foraminiferal-red algal packstone-grainstone of the

Montebello Member, dolomitic red algal Lepidocyclina packstone and wackestone of the Lares Limestone, and in some

updip northwestern localities, conglomeratic sandstone of the

San Sebastián Formation. The lower aquifer may include

rocks of the Río Indio and Quebrada Arenas Limestone Members that crop out or lie in the shallow subsurface. To the

east, the lower aquifer includes sandstone, conglomerate, siltstone, and silty clay of the Mucarabones Sand. Marl and

claystone of the lower part of the Cibao Formation and the

upper Lares Limestone grade into the Mucarabones Sand,

which becomes part of the lower aquifer in those subsurface

areas that lie east of the Río de la Plata and where it crops out

east of Río Grande de Manatí. This change in lithofacies also

occurs to a limited extent in the upper part of the Cibao Formation and could account for greater upward leakage from

the lower aquifer. The lower aquifer’s freshwater lens probably does not extend much beyond the Río Piedras to Carolina

area of metropolitan San Juan where it becomes poorly permeable and contains water of poor quality farther eastward.

In deep subsurface areas west of the Río Grande de

Arecibo, the stratigraphic and hydraulic nature of the lower

aquifer is increasingly complex. East of the river, the lower

aquifer consists of the Montebello Limestone Member and

the Lares Limestone. West of the river, the aquifer grades by
116 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

facies change to poorly permeable wackestone, wackestonepackstone, and minor dolomite. Here, the lower aquifer is

discontinuous and probably is made up of several different

water-bearing zones that are restricted in areal extent. Therefore, the boundary of the lower aquifer can not confidently be

extended westward very far beyond the NC 6 deep test well in

section A–A ́ (fig. 55; pl. 5A). 

The Lares Limestone does crop out and is thought to

underlie the extreme updip, shallow subsurface areas west of

the Río Grande de Arecibo before it pinches out or grades

westward and northward to less permeable rocks, or both. In

updip areas, the lower aquifer includes water-bearing rocks of

the Lares Limestone, but also contains local beds of sand,

sandstone, and conglomerate of the San Sebastián Formation.

Downdip in this area, the Lares Limestone grades into the

lower part of the Cibao Formation. 

Water level measurements of wells completed in the lower

aquifer were used to construct potentiometric-surface maps

that show predevelopment (pl. 5C) and 1987 (pl. 5D) hydrologic conditions (Renken and Gómez-Gómez, 1994). Additional information regarding predevelopment conditions,

mostly within a small subbasin in the upper reach of the Río

Tanamá Basin, is presented by Jordan (1970, 1977). 

The lower aquifer is exposed as a 3- to 11-km wide band

along the southern one-third of the North Coast ground-water

province. The lower aquifer is recharged by infiltration of

precipitation through soil cover or as runoff that enters the

aquifer through numerous sinkholes, solution trenches (zanjones), and dry streambeds in the outcrop areas. The predevelopment and 1987 potentiometric-surface maps for the

lower aquifer indicate that the principal areas of recharge are

in topographically elevated interstream areas where the aquifer crops out. Much of the water that enters the shallow part

of the flow system moves to the principal north-flowing rivers

and some water enters deeper confined parts of the flow system. Many areas where the potentiometric surface is elevated

are located along the northern boundary of the outcrop belt.

Other areas where the potentiometric surface is elevated are

located downdip where the lower aquifer is in the shallow

subsurface (pls. 5C, 5D). The elevated potentiometric surface

in these areas indicates that the lower aquifer is probably

recharged by the downward leakage of water from the upper

aquifer through a leaky confining unit. 

Areas where the potentiometric surface of the lower aquifer is low generally coincide with principal stream valleys

and north-flowing rivers. The upstream V-contour pattern

of potentiometric-surface contours in predevelopment and

1987 potentiometric-surface maps (pls. 5C, 5D) indicates that

the lower aquifer discharges water upward to shallower units

in the major river valleys. Large potentiometric lows associated with major river valleys probably coincide with areas

where the middle confining unit was breached by deep channels during Pleistocene sea-level lowstands. Two of Puerto

Rico’s largest springs (San Pedro and Aguas Fría) are located

within the Ríos Grande de Arecibo and Grande de Manatí

potentiometric lows. Southwestward movement of ground

water to the Río Culebrinas indicates that this river is a major

area of discharge from the lower aquifer in the outcrop belt

(pl. 5C). Ground water from the lower aquifer also discharges to the Ríos Limon, Cialitos, Manvilla, and Mucarabones in the outcrop belt. Although these rivers receive

discharge from the lower aquifer, the amount of ground water

discharged is not considered to be significant in terms of the

total ground-water budget. 

Large cave systems represent major ground-water drains

in areas where the lower aquifer crops out. This is evident in

the Río Camuy cave system where potentiometric-surface

contours show a characteristic up-stream V-contour pattern

indicative of ground-water discharge (pl. 5C). Ground-water

also discharges to the Río Encantado cave system located

in the Florida-Montebello area (pls. 5C, 5D). A large number of water-level measurements in this area were used to

constrain the position of the lines of equal hydraulic head

(water-level contours) and help define the probable extent of

the subterranean passageway. Potentiometric surface data

also indicate the possible presence of another major cavernous drainage feature that lies to the north and appears to parallel the Río Encantado cave system. However, further study

is needed to confirm the presence of this possible subterranean drainage feature.

In the Ríos Grande de Arecibo-Grande de Manatí area,

ground-water movement in the deeper parts of the flow system is coastward along a circuitous path to the principal river

drains. East of the Río Grande de Manatí, some ground water

flows eastward toward the Levittown and Metropolitan San

Juan area. Test drilling indicates that the upper aquifer is thin

or absent in the Metropolitan San Juan area (fig. 55; pl. 5A)

and that the confining unit (Cibao Formation) overlying the

lower aquifer is leaky. The pattern of ground-water flow indicates that the San Juan area may be a site of regional discharge from the lower aquifer. However, surficial deposits in

the San Juan area form a local confining unit that tends to

impede upward leakage. West of the Río Grande de Arecibo,

ground-water flow is largely coastward within the lower aquifer; however, some upward ground-water discharge to northflowing streams is possible. In the westernmost part of the

island, ground-water flow is locally westward. Ground-water

flow in deep confined parts of the lower aquifer is poorly

understood because of a lack of data. As described above, test

drilling near Camuy and Isabella indicates that the stratigraphic and hydraulic nature of the lower aquifer is increasingly complex. Ground-water is contained under fragmented,

heterogeneous artesian conditions made up of different waterbearing and confining zones of limited extent. Freshwater

was encountered at depth in this western area, but facies relations within the Lares Limestone and Cibao Formation sug-
HYDROLOGIC FRAMEWORK OF THE U.S. CARIBBEAN ISLANDS 117

gest strata equivalent to the lower aquifer are poorly

permeable.

Ground-water development within confined parts of the

lower aquifer is limited to the north-central part of the coastal

plain, which extends eastward from the Ríos Grande de

Arecibo to the Laguna Tortuguero and southward to Florida

and Montebello. This area corresponds to the area where

water-level declines in the lower aquifer have been greatest.

Little has changed in water levels within the lower aquifer

elsewhere, although ground-water withdrawals and possible

leakage from wells southwest of the Laguna Tortuguero have

locally lowered water levels as much as 40 m in some places

(fig. 59). The greatest decline in the potentiometric surface

was in the area between the Ríos Grande de Arecibo and

Grande de Manatí. In 1987, the potentiometric surface in this

area was more than 45 m lower than predevelopment levels in

some wells. This is largely attributed to industrial groundwater withdrawals, mainly from the Montebello Limestone

Member near Barceloneta, and the pumping of public water

supply wells from equivalent strata near Florida. Upward

leakage of water from the lower aquifer around poorly constructed wells in the Barceloneta area have also contributed to

a decline in heads (Conde-Costas and Torres-González, written commun., 1990; Conde-Costas, written commun., 1991;

Hydro Geo Chem., 1991).

BASAL CONFINING UNIT

The basal confining unit of the North Coast limestone

aquifer system is made up of poorly permeable, lithified, volcaniclastic and igneous bedrock of Cretaceous and early Tertiary age that underlie the central mountains of the island and

extend beneath the northern coastal plain. In the deeper subsurface, mudstone, clay, marl, and silt of the San Sebastián

Formation compose the basal confining unit.

DISTRIBUTION OF TRANSMISSIVITY AND HYDRAULIC 

CONDUCTIVITY

A number of factors influence the distribution of transmissivity or hydraulic conductivity within the North Coast Limestone aquifer system. Some of the factors that govern the

regional distribution of hydraulic conductivity include the

original lithologic character of the carbonate rock, carbonate

depositional patterns, subsequent diagenesis, and the array of

fractures and joints that facilitate dissolution and karstification (see Ward, this report). Transmissivity patterns within

the upper aquifer are also regulated by a diminishing thickness of the freshwater lens where the aquifer extends seaward

or reaches southward and thins to a featheredge; and the

degree of interconnection between surface and subterranean

drainage systems. 

Estimates of transmissivity for the North Coast Limestone

aquifer system were determined by the analysis of aquifer test

and specific capacity data (Torres-Gonzáles, written commun., 1990). Specific capacity-derived transmissivity estimates were provided using the empirical equations developed

by Theis (1963) and Brown (1963) and parallels the approach

previously described for the South Coast aquifer. The distribution of apparent hydraulic conductivity (K  ́) was computed

by dividing apparent transmissivity estimates (T  ́) by the

thickness of the aquifer that was penetrated by the well.

These maps are subject to similar qualifications described earlier. They are based on specific-capacity methods that assume

100 percent well efficiency, full penetration of the aquifer, 30

cm well diameter, a storage coefficient of 1x10-4 for wells

subject to artesian conditions or 0.10 for wells subject to

water-table conditions, and a 24-hour pumping time. Two

important qualifications of the transmissivity and hydraulic

conductivity maps is that most wells only partly penetrate the

aquifer and represent only the freshwater part of the section.

It is assumed that the hydraulic properties of these partial penetrating wells were representative for the entire aquifer and,

therefore, estimated transmissivity values were not adjusted. 

In the region west of Río Guajataca, hydraulic data for the

upper aquifer are sparse, and transmissivity and hydraulic

conductivity patterns are, in general, poorly known or understood (pls. 5G, 5H). On the basis of two wells, the transmissivity of the upper aquifer in the northwestern Puerto Rico is

reported to range from 1,000 to more than 10,000 m2/d. However, hydraulic data collected for these two wells are not considered to be representative of regional transmissivity. One

well near the northwestern coast may be screened in coastal

deposits (1,499 to 2,360 m2/d). The other well, locally known

as the Monte Encantado well, is drilled into a subterranean

stream. Here, transmissivity is reported to range from 6,618

to 10,051 m2/d. The wide variability in transmissivity and

great distance between wells suggests that it is not possible to

map the regional distribution of transmissivity in this area.

Similarly, few data are available east of Río de la Plata for the

upper aquifer. The upper aquifer contains minor freshwater

within a small area east of the Río de la Plata; transmissivity

is reported to exceed 4,000 m2/d locally, but averages 75 to 90

m2/d (I. Padilla, USGS, written commun., 1990). 

The upper aquifer is most transmissive in middip regions

of the north coast that lie between the Río Camuy and Río de

la Plata (pl. 5G). The transmissivity of the upper aquifer is

reported to exceed 10,000 m2/d in several locations, including

areas that lie southwest of Arecibo, west of Barceloneta, eastsoutheast of Laguna Tortugero, and in the Los Puertos-Regadera area. Transmissivity of the upper aquifer is reported to be

as much as 68,000 m2/d. Highly transmissive rocks within the

upper aquifer are commonly, but not exclusively, associated

with a freshwater lens that is greater than 50 to 100 m thick.

As previously discussed, the thickest part of the freshwater

lens is in interstream areas between major stream valleys, but

is proximate to the landwardmost extent of saltwater and usually is the zone of highest transmissivity. Highly transmissive
118 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

rocks are also in coastward localities where the freshwater

lens is considerably thinner; extremely vuggy or cavernous

limestone can account for these high transmissivities. In general, transmissivity within the upper aquifer diminishes coastward (northward) and landward (southward), and in

interstream areas as a result of thinning of the freshwater lens.

In these areas, transmissivity within the upper aquifer is often

less than 1,000 m2/d. 

When compared to stratigraphic depth, there is wide variability in hydraulic conductivity within the upper aquifer

(Guisti, 1978) and is indicative of the aquifer’s heterogeneity.

Hydraulic conductivity within the Aymamón Limestone

appears to diminish with stratigraphic depth, a decrease in

permeability that is probably associated with a maximum

effective depth to which karstification will occur within the

aquifer. In some localities, values of hydraulic conductivity



ARECIBO

VEGA BAJA

BARCELONETA

MANATÍ

FLORIDA

MOROVIS

CIALES

COROZAL

UTUADO

JAYUYA

OROCOVIS

VEGA

ALTA

ATLANTIC OCEAN

Lago

Dos Bocas

Río Grande de Arecibo

Río Limón

Río Yunes

Río Grande de Manatí

Río Toro Negro

Río Bauta

Río Sana Muerto

Río Indio

Río Cibuco

Lago

Caonillas

Laguna

Tortuguero

Caño Tíburones

Coto

Norte

Pugnado

Afuera

Ciales

Florida Montebello

Arecibo

Barceloneta

Manatí

Vega

Baja

Base modified from U.S. Geological Survey

digital data

66 ̊45' 66 ̊37'30" 66 ̊30' 66 ̊22'30"

18 ̊30'

18 ̊22'30"

18 ̊15'

0 24 8 6 10 KILOMETERS

0 4 2 6 MILES

ATLANTIC OCEAN

CARIBBEAN SEA

Isla

Mona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

EXPLANATION

Drawdown in lower aquifer, North Coast 

 limestone aquifer system, in meters

 Less than 15

 15 to 30

 30 to 60

 Greater than 60

Area of outcrop  

FIGURE 59.—Drawdown in the lower aquifer between 1968 and 1987, north-central Puerto Rico.
HYDROLOGIC FRAMEWORK OF THE U.S. CARIBBEAN ISLANDS 119

or transmissivity may not appear to fit the regional pattern

and is probably due to the erratic distribution of cavernous

porosity.

On the basis of specific-capacity data, hydraulic conductivity of the upper aquifer is estimated to range from less than

1 to more than 2,000 m/d (pl. 5H). In general, hydraulic conductivity of the upper aquifer increases northward (coastward) with the least permeable parts of the aquifer inland

(less than 30 m/d) or west of the Río Camuy. Highest

hydraulic conductivities (greater than 300 m/d) occur along a

narrow coastal band that extends eastward from the Río

Grande de Arecibo to the Río Grande de Manatí. Highly permeable carbonate rocks are also near Laguna Tortugero. 

Several factors contribute to the pattern of permeability

within the upper aquifer. Ward and others (1990; this report)

suggest that porosity within the Aguada (Los Puertos) Limestone and the lower part of the Aymamón Limestone could be

lithologically controlled by the occurrence and distribution of

dolomite and of formerly aragonitic fossils contained within

these strata. Conversely, porosity within the upper 30 to

100 m of the Aymamón Limestones appear to be related more

to processes of karstification and the associated development

of cavernous porosity.

If uniform solubility within the carbonate rocks that make

up the aquifer is assumed, the coastward increase in permeability appears, at first glance, to be at odds with geochemical

data for the North Coast of Puerto Rico (Román-Mas and

Lee, 1987) that show carbonic acid is consumed as meteoric

waters that recharge the aquifer move downgradient. Under

this scenario, one would expect permeability to diminish

coastward. However, mixing of freshwater and seawater at

the coast may help enhance processes of dissolution (Plummer, 1975). Such mixing could help explain the apparent

coastward increase in porosity and permeability as well as the

occurrence of cavernous zones and subterranean conduits

near Caño Tiburones (Zack and Class-Cacho,1984).

The transmissivity of the lower aquifer is reported to

range from less than 1 to more than 1,300 m2/d (pl. 5I).

Reported transmissivity values that fall within the upper

range are unusual, however, and transmissivity within the

most productive part of the lower aquifer generally ranges

from 50 to 800 m2/d. The lower aquifer is most transmissive

in an area that includes outcropping rocks of the Montebello

Limestone Member and the Lares Limestone and equivalent

subsurface strata that extend coastward and lie between the

Río Grande de Arecibo and Río Grande de Manatí. Highest

transmissivity corresponds to areas of greatest aquifer thickness. Transmissivity of the lower aquifer diminishes to the

west and east in conjunction with facies changes occurring

within the rocks composing the aquifer. For example, the

Montebello Limestone Member grades eastward to the poorly

permeable “mudstone unit,” and the aquifer is comprised only

of the less transmissive Lares Limestone. In the easternmost

part of the study area, transmissivity within the lower aquifer

increases and is reported to be as much as 56 m2/d, largely

attributed to a clastic facies change to rocks of the Mucarabones Sand. As the lower aquifer extends westward, the

Lares Limestone and Montebello Limestone Member both

grade to a poorly permeable facies of either the San Sebastián

Formation or the Cibao Formation.

CONTROLS ON POROSITY AND PERMEABILITY IN CARBONATE 

AQUIFERS OF NORTHERN PUERTO RICO

BY W.C. WARD1

The principal aquifers of the North Coast limestone aquifer system consist of open-platform limestones and dolomites

of the Aymamón and Aguada Limestones and the Montebello

Limestone Member and Lares Limestone. The new subsurface data provided by core drilling demonstrate that the distribution and quality of the carbonate-rock aquifers are directly

related to (1) the regional stratigraphic framework, (2) local

lithologic and diagenetic facies, and (3) fracture systems

(Ward and others, 1990).

Stratigraphic Control

To a large degree, the principal hydrogeologic units of the

North Coast limestone aquifer system coincide closely with

major middle Tertiary geologic units of northern Puerto Rico.

The North Coast limestone aquifer system’s upper aquifer is

within the Aymamón Limestone and Aguada (Los Puertos)

Limestone carbonate rocks and the Montebello Limestone

Member and Lares Limestone rocks form the lower aquifer

(Renken and Gómez-Gómez, 1994; Renken, Gómez-Gómez,

and Rodríguez-Martínez, this report). 

From a hydrogeologic perspective, the upper aquifer is a

relatively uniform limestone sequence; the Aguada (Los Puertos) Limestone and Aymamón Limestone include bedded skeletal packstone-wackestone, and packstone-grainstone with

lesser dolomitic beds. The two limestone units combine to

form one aquifer that cannot be regionally separated on the

basis of potentiometric data. 

The lower aquifer is confined by less permeable marl,

limestone, and mudstone of the overlying Cibao Formation

and by poorly permeable mudstones of the underlying San

Sebastián Formation. The lower aquifer also is confined

below in easternmost areas by Upper Cretaceous and lower

Tertiary rocks where the San Sebastián Formation is either

missing or grades updip to permeable conglomeratic beds.

The Cibao confining unit that separates the upper and lower

aquifers appears to be hydraulically less effective in some

1

 Department of Geology and Geophysics, University of

New Orleans, 2000 Lakeshore Drive, New Orleans, LA 

70148, (retired)
120 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

updip localities (Renken and Gómez-Gómez, 1994). In the

western part of the basin, this may partly be due to interfingering of the Cibao with terrigenous sediment. 

The relatively high potentiometric head and low specific

conductance measured during drilling within Lares Limestone

and Montebello Limestone Member carbonate rocks (Renken

and Gómez-Gómez, 1994) indicate that these strata contain

freshwater under confined conditions in an area that lies north

of its outcrop belt (north of approximately latitude 18o23’;

fig. 3). The most productive part of the aquifer is in an area

between the Río Grande de Manatí and Río Grande de

Arecibo, an area that corresponds to where the Montebello

Limestone Member is relatively thick and widespread. On the

western side of the basin, the Lares and Montebello Limestones interfinger with poorly permeable and porous marl and

limestone of the undifferentiated Cibao Formation and San

Sebastián Formation. East of the Río Grande de Manatí,

porosity and permeability within the aquifer diminish as the

Montebello grades into undifferentiated Cibao and the mudstone unit. On the eastern side of the basin, however, sandstone and conglomerate of the Mucarabones Sand are more

permeable and may provide avenues for diffuse upward flow

of ground water from the lower aquifer.

Lithologic and Diagenetic Control 

Porosity and permeability within the major aquifers are

highly variable, largely because of lateral and vertical variations in depositional textures and in diagenetic features. In

general, porosity is highest in coarser and better-sorted grainstone-packstone and lowest in wackestone and marl (clayey

wackestone or highly calcareous claystone). Primary intergranular porosity is retained in some grainstone and packstone

layers. More commonly, however, porosity is secondary,

resulting from dissolution of original skeletal constituents or

from dissolution of calcium carbonate during dolomitization,

or both. The most abundant type of secondary porosity in

these rocks is moldic porosity, most of which occurs where

aragonitic fossils, particularly mollusk and corals, were dissolved. Although moldic porosity is abundant in some mollusk- and coral-rich wackestones, permeability in these rocks

is generally low. Moldic porosity is most effective in grainstone-packstone layers that retain intergranular porosity or that

have been dolomitized. Dolomitized zones commonly have

fossil-moldic porosity and intercrystalline porosity. Porosity

and permeability is significantly reduced where calcite cement

fills intergranular and moldic pores.

Upper Aquifer

Enhanced porosity and permeability in the upper part of

the Aymamón Limestone are predominantly the products of

late Miocene and Quaternary karstification. The upper 30 to

100 m of the Aymamón Limestone are dense recrystallized

limestone and chalky limestone with cavernous porosity. The

cavernous and vuggy porosity is concentrated in zones, which

become more numerous upward. Many of the large vugs,

however, are filled or partly filled with red-brown mud.

In the lower part of the Aymamón and in the Aguada (Los

Puertos) Limestone, porosity is highly variable, being controlled principally by the distribution of dolomite and of originally aragonitic fossils. Numerous thin units of dolomitized

coral-rich grainstone-packstone and coral boundstone have

up to 25 percent moldic and intercrystalline porosity (Hartley,

1989, Scharlach, 1990). 

Porosity in the lower Aymamón is locally as much as 15

percent (estimated in thin section) in mollusk- and coral-rich

layers, and where these are dolomitized, as much as 30 percent. Dolomite in the Aymamón Limestone increases downdip (Scharlach, 1990). In poorly indurated coarser

grainstone-packstone, intergranular and moldic porosity may

be up to 25 percent.

Porosity of the Aguada (Los Puertos) Limestone is best in

dolomitized units, which commonly have an estimated 10 to

20 percent moldic, intercrystalline and intergranular porosity. Dolomite in this formation also apparently increases

downdip. Some poorly cemented soritid-mollusk packstonegrainstone and coral-mollusk wackestone-packstone have

greater than 10 percent intergranular and moldic porosity.

Calichified zones and the upper karsted interval may have as

much as 20 percent vuggy and moldic porosity.

Middle Confining Unit

Coarser skeletal packstone-grainstones of the undifferentiated Cibao Formation generally have the highest porosity, up

to about 10 percent moldic and intergranular. Dolomitic layers in the upper and lower parts of this member have 5 to 10

percent moldic, intergranular, and intercrystalline porosity.

Porosity generally increases updip because of fewer clayey

intervals and more packstone-grainstone intervals. 

Porosity in the mudstone unit is low. Moldic porosity is

uncommon because of the scarcity of larger mollusks and

other originally aragonitic fossils. The finely crystalline

dolomite has less than 5 percent (estimated in thin section and

hand specimen) intercrystalline porosity. At the top and bottom of the unit, dolomitized skeletal wackestones have 5 to

10 percent moldic and intercrystalline porosity (Scharlach,

1990).

The fossiliferous coarser terrigenous rocks of the Quebrada Arenas and Río Indio Limestones cored in NC 2 have

estimated 5 to 7 percent intergranular porosity with lesser

moldic and intraparticle porosity. Sandy large-foraminifer

packstone-grainstone in NC 8 has 5 to 15 percent porosity,

mostly intergranular with moldic and intraparticle. The upper
HYDROLOGIC FRAMEWORK OF THE U.S. CARIBBEAN ISLANDS 121

15 m of this member has low porosity because the rocks are

better cemented.

Lower Aquifer

Porosity in the Montebello Limestone Member is highly

variable. Intergranular and moldic porosity in skeletal packstone commonly is 5 to 10 percent and may be as high as 15

percent (Hartley, 1989). Mollusk- and coral-rich units may

have up to 25 percent moldic porosity. Porosity decreases

downdip as the Montebello Limestone grades into clayey

wackestone-packstone and claystone. 

Porosity in the Lares Limestone also is highly variable,

depending on depositional texture, original mineralogy of the

skeletal fragments, and dolomitization. Mollusk- and coralrich layers of coarser packstone have moldic and intergranular porosity estimated to be as much as 15 percent. In largeforaminifer packstone-grainstone estimated porosity is 12

percent, mostly intergranular with lesser moldic and intraparticle. Dolomite has much as 10 to 15 percent moldic and

intercrystalline porosity. In the eastern part of the basin

(NC 2; fig.21), 65 to 70 percent of the Lares limestone and

dolomite has less than 5 percent porosity, the rest having 10

to 15 percent (Scharlach, 1990). 

Intergranular porosity of the Mucarabones Sand is generally less than 5 to 10 percent because many sandstones are

clayey. Some thin moderate- to well-sorted sandstones have

greater than 10 percent intergranular porosity (Scharlach,

1990). 

The most productive part of the lower aquifer lies largely

in an area between the Río Grande de Arecibo and the Río

Grande de Manatí. Porosity and permeability in the lower

aquifer in the vicinity of NC 5 is best developed in coarse

grainstone-packstone and coral-patch-reef limestone (Hartley, 1989). These rocks typically have about 10 to 15 percent

moldic and intergranular porosity with minor zones of mollusk- and coral-moldic porosity as high as 25 percent (Hartley, 1989). Patches of dolomitized grainstone in the middle

Lares have 5 to 10 percent intercrystalline-intergranular

porosity. An estimated 65 percent of the Lares section and

45 percent of the Montebello section in wells NC 5 and NC

10 have low permeability with less than 5 percent porosity.

Basal Confining Unit

Estimated porosity in San Sebastián Formation limestones and sandstones is variable, but generally no more than

a few percent. Characteristically, the carbonate rocks of the

San Sebastián Formation have less than 5 percent (estimated

in thin section) intergranular and moldic porosity.

Fracture Control

The Tertiary limestone outcrop belt in northern Puerto

Rico is a well-developed karst terrain (Monroe, 1976). Karst

features of the North Coast ground-water province has been

ascribed to a variety of tectonic, geomorphic, hydrologic (climatic), and geochemical factors. Some straight dry valley

segments have been attributed to dissolution along joint systems, whereas other dry valleys have been attributed to superposed drainage (Monroe, 1976, p. 27). Preferential alignment

of cones or cockpit karst has been alternatively ascribed to

preferential drainage patterns as the Puerto Rico platform

emerged with a supposed eastward tilt (Lobeck, 1922; Meyerhoff, 1933; Guisti, 1978) or to dissolution along joint or fractures (Lehmann, 1954). Monroe (1976, p. 38) believed there

was such variability within these alignments as to preclude

joint control. Joint control on the arrangement of mogotes

(steep-sided karst hills) was inferred by Hubbard (1923) and

Kaye (1957), whereas Monroe (1976, p. 47-48) believed this

also might be the result of trade winds effects, late Tertiary

drainage patterns, or fluvial flow during a Pleistocene interglacial stage. There does seem to be general agreement that

the linear solution trenches or zanjones are joint-related karst

features (Sweeting, 1972; Monroe, 1976, p. 48). 

Assuming that most straight segments of karst valleys in

this region represent dissolution along major fractures, the

topography in the limestone hills shows the strong influence

of several sets of regional fractures. For example, between

the Río Grande de Arecibo and the Río Grande de Manatí,

major trends of karst valleys (fracture systems) are N55°-

60°E, N10°-N5°W, N15°-20° W, N30°-40°E, and approximately E-W (fig. 60). Although the upper aquifer seems to be

best characterized as a diffuse-flow carbonate aquifer, it

seems likely that ground-water flow in the upper aquifer is

controlled, at least in part, by regional fractures. Groundwater flow in karst systems is often channeled through caverns and along solution-enhanced fractures (White, 1969,

1988).

The lower aquifer is characterized by vertically and laterally discontinuous porosity and permeability, suggesting that

porous zones may be connected by fractures. The comparatively low transmissivity of the lower aquifer (pl. 5I), however, suggests that permeability within Montebello Limestone

Member and Lares Limestone carbonate rocks is not substantially enhanced by fractures.

U.S. VIRGIN ISLANDS

BY R. A. RENKEN1

St. Croix is the only island of the three U.S. Virgin Islands

that is underlain by poorly lithified or unconsolidated sedi1

 U.S. Geological Survey
122 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

ARECIBO

BARCELONETA

MANATÍ

FLORIDA

CIALES

UTUADO

JAYUYA

ATLANTIC OCEAN

Lago

Dos Bocas

Río Grande de Arecibo

Río Limón

Río Yunes

Río Grande de Manatí

Río Toro Negro

Lago

Caonillas

Caño Tíburones



OUTCROP OF LOWER TERTIARY AND CRETACEOUS ROCKS

Ciales

Florida Montebello

Arecibo

Barceloneta

Manatí

Base modified from U.S. Geological Survey

digital data

18 ̊30'

18 ̊22'30"

18 ̊15'

66°37'30" 66°30'

0 24 8 6 10 KILOMETERS

0 4 2 6 MILES

ATLANTIC OCEAN

CARIBBEAN SEA

Isla

Mona

Isla de

Culebra

Isla de

Vieques

St.

Thomas

St.

 John

St. Croix

Puerto Rico

MAP AREA

NUMBER OF LINEAMENTS

0 5 10 15

A B

N

W E

S

N

W E

S

FIGURE 60.—Major fracture patterns inferred from strike of surface lineaments in karst topography between the Río Grande de Arecibo

and Río Grande de Manatí, northern Puerto Rico. Rose diagrams, constructed using 10-degree sectors, show the orientation of 

99 lineaments from the Aymamón and Aguada Limestones (A) and orientation of 100 lineaments from the Montebello Limestone

Member and the Lares Limestone (B) (reprinted from Ward and others, 1990, and published with permission).
HYDROLOGIC FRAMEWORK OF THE U.S. CARIBBEAN ISLANDS 123

mentary strata that function as a source of ground-water and

have “regionally-significant” areal extent. Even then, the

areal extent of the Kingshill aquifer is small, underlying a

total area of only 78 km2. The Kingshill aquifer, by definition, is comprised of the Kingshill Limestone and overlying

carbonate rocks of the Blessing Formation. Underlying rocks

of the Jealousy Formation are considered to form the hydrologic basement, in large part due to the progressively poorer

chemical quality of water with increasing depth (Robison,

1972, p.3). Combined with its poorly transmissive nature in

most places (less than 3 m2/d reported for wells that penetrate

only the Kingshill Limestone) (Torres-González, written

commun., 1990), the Kingshill aquifer’s importance is strictly

a function of the lack of an alternative source of groundwater. Most of the large-yield wells reported to be screened

in the Kingshill aquifer are also screened in more permeable

alluvium. The karstic Blessing Formation is more permeable,

but its thickest extent is restricted to St. Croix’s heavily

industrialized south-central coast area, an area subject to

industrial contamination or to salt-water intrusion if pumped

heavily. 

A veneer of alluvial sediment that locally overlies the

Kingshill aquifer and Cretaceous bedrock of St. Croix’s highland ranges forms a patchy, discontinuous alluvial aquifer.

Whereas alluvial deposits in St. Croix and in St. Thomas and

St. John are more permeable than most underlying strata,

their restricted extent limits their use as a reliable source of

ground water. Production from wells that penetrate fractured

bedrock in the U.S. Virgin Islands has met limited success,

and reported sustainable yields are usually less than 1.0 L/s.

DISTRIBUTION AND TYPES OF POROSITY WITHIN THE 

KINGSHILL AQUIFER

BY I.P. GILL1, D.K. HUBBARD2, P.P. MCLAUGHLIN3, 

AND C.H. MOORE4

Porosity types in carbonate rocks of St. Croix central

basin are varied and complex. This complexity is a result of

the range of primary pore types and partly a result of dissolution, cementation, and recrystallization. The distribution and

nature of the porosity is variable geographically and stratigraphically.

Kingshill Limestone in the Central and Northern Basin

The Kingshill Limestone, exclusive of the overlying Mannings Bay Member, is dominated by fine-grained, basinal

deposits, and is rich in planktonic foraminifera and other

pelagic biota. The unit is poorly lithified in the subsurface,

with lithification evident mainly in the coarser grained, shelfderived beds that interstratify the pelagic accumulations.

Because of the lack of lithification, primary porosity in much

of the Kingshill Limestone is preserved as intergranular pores

and as pore space within the grains themselves. Circulation

of ground water has leached the fine-grained sediments and

produced recrystallization and minor cementation. In general, however, permeability within the Kingshill Limestone is

commonly poor, with much of the pore space being microscopic.

In the coarser-grained sediment-gravity flows (sand- to

boulder-sized constituents) that are common throughout the

Kingshill Limestone, interspersed between the pelagic accumulations discussed above, much of the primary porosity has

been destroyed by cementation and recrystallization. These

beds generally show intergranular and intragranular pores

occluded by calcite cementation and are mostly impermeable.

Although the Kingshill Limestone is widespread in the

Central Plain region (fig. 5), water production is patchy and is

poor in large areas. Production in the Kingshill Limestone is

limited to areas where terrigenous components have allowed

porosity to be preserved, or production stems from incursion

of alluvial aquifers on top of the Kingshill Limestone. Fractures within the poorly lithified Kingshill Limestone are also

a potential source of water and have been developed in watersupply drilling during the 1980’s (Jesús Rodríguez, oral commun., 1989). The large number of wells in the Kingshill

Limestone attests mostly to its areal extent and to the limited

supply available from other sources rather than its production

capabilities.

The porosity of the Jealousy Formation is similar in many

respects to that in the Kingshill Limestone. In many cases,

the Jealousy Formation can be considered the hydrologic

basement, and is not a significant water producer.

Mannings Bay Member and the Blessing Formation along 

the southern coastline

The Mannings Bay Member of the Kingshill Limestone

and the overlying Blessing Formation are coarser-grained than

the La Reine Member and are far more affected by diagenetic

alteration. Both units are correspondingly more porous than

the lower Kingshill Limestone. These two units are thickest

and best exposed in the southeastern coastal section of the

Kingshill Basin area, where they contain permeable sections

that are the sources of both industrial and public water supply.

Because of their greater porosity and the wider variety of

diagenetic alteration affecting them, these units are important

to the discussion of this paper.

Porosity in the Mannings Bay Member and the Blessing

Formation ranges from microscopic intergranular pores to

large voids up to a meter across. The voids are evident in out1

 Department of Geology, University of Puerto Rico,

Mayagüez, PR 00681;

2

Department of Geology, Oberlin College, Oberlin, OH 44074

3

Delaware Geological Survey, Newark, DE 19716-7501

4

Department of Geology and Geophysics, Lousiana State

University, Baton Rouge, LA 70803, (retired)
124 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

crop and in the subsurface when the drill bit penetrates empty

space. Megascopic porosity in the post-Kingshill units is

restricted to the coastal region, and is in many cases represented by the wholesale removal of unstable minerals. In this

case, many of the voids observed in outcrop are external

molds of cobble- to boulder-sized coral heads, some with

molds of the coral calyxes preserved (Gill and others, 1989).

Corals, originally composed of aragonite, are easily dissolved by the circulation of waters undersaturated with

respect to calcium carbonate. Throughout the Blessing Formation rocks, large voids have been produced by selective dissolution of aragonite and high-magnesium calcite, or through

wholesale dissolution due to karsting or a similar process.

Non-fabric selective dissolution can be observed in outcrop as

caliche zones and solution pits (Lidz, 1982; Gill and Hubbard,

1987; Gill and others, 1989).

The Blessing Formation and the Mannings Bay Member

are the host rocks for localized dolomitization (fig. 49).

Accompanying the dolomitization process is a large-scale

increase in porosity due to the selective leaching of bioclasts

as well as an increase in intercrystalline porosity. In heavily

dolomitized strata, aragonitic and calcitic bioclasts originally

in the formation have dissolved, leaving a matrix of dolomite

rhombs with moldic porosity. The selective removal of calcitic and aragonitic bioclasts is due presumably to differential

solubility. The net result of the dissolution accompanying

dolomitization is high porosity and transmissivity as much as

370 m2/d in localized areas.

Dissolution and karsting

Large-scale dissolution in the Blessing Formation and

Mannings Bay Member has resulted in megascopic voids,

accompanied in outcrop by evidence of subaerial exposure of

the limestone and leaching by meteoric waters. This evidence

includes caliche layers and soil horizons (Behrens, 1976;

Lidz, 1982; Gill 1989), as well as significant shifts in the stable isotopic ratios of the remaining limestone (Gill and others,

1989; Gill, 1989). In the subsurface samples, insufficient data

have been obtained to judge the causes of cavernous porosity

production. However, the outcrop exposures and caves point

to modest karst development, and it is reasonable to extend

this interpretation to the subsurface as well.

The restriction of cavernous porosity to locations along

the coastline might indicate an alternative hypothesis to subaerial exposure and leaching: the dissolution and megascopic

porosity in the upper Kingshill and Blessing Formation rocks

is due to the circulation of fresh ground water and seawater in

a coastal mixing zone (Back and Hanshaw, 1983). 

In St. Croix, chemical equilibria calculations show that

some of the coastal ground waters are undersaturated with

respect to calcite, and the pattern of salinity in coastal zones

indicate that mixing is taking place today (Gill and Hubbard,

1986; Gill, 1989). Although equilibria calculations show that

modern hydrological conditions are presently favorable to

limestone dissolution within the coastal portion of the aquifers, this does not prove that this process was important during the geologic past. The coastal distribution of the porosity

could indicate a lithologic control, rather than a geochemical

control, on the dissolution of the coastal limestone units. In

short, although ground-water mixing is a viable hypothesis to

explain the development of porosity in the upper Kingshill

and Blessing Formation rock units, its effects are impossible

to separate from those of localized karsting and leaching by

meteoric water. Both of the latter processes are common in

exposed limestone areas, and both can be demonstrated to

have occurred in the upper Kingshill and Blessing Formation

limestones.

Dolomitization

The localized dolomitization of parts of the Blessing Formation and the Mannings Bay Member has produced a significant amount of porosity in these units. Although highly

localized on St. Croix, the dolomitization process is an

important mechanism of porosity development. 

Combining the evidence from both the oxygen and strontium isotopes (Gill and Hubbard, 1987; Gill 1989), the main

conclusions are that the highly porous zones of dolomitization were the result of a ground-water system that (1) allowed

the mixing of ground water and seawater to produce a fluid

with a strontium isotopic composition intermediate between

modern marine water and ground water and (2) allowed the

evaporation to produce a fluid with an oxygen isotopic composition enriched in 18O relative to normal seawater. 

This scenario is consistent with the geological distribution

of the dolomitized zones on St. Croix. The dolomite occurs

only in shallow marine strata very close to the paleocoastline

of St. Croix, and only within a block that has been downfaulted relative to the surrounding rock. The location of the

dolomitized strata is thus consistent with the requirement that

mixing takes place between the seawater and the ground

water; coastal regions are the only place on St. Croix where

mixing would be possible, and it can be shown that some

coastal mixing occurs today (Gill and Hubbard, 1987; Gill

1989). The dolomitized strata occur only in a structurally

controlled embayment or lagoon that could have allowed

extensive evaporation to take place. Similar lagoons and

embayments exist on St. Croix today, and the salinity and

oxygen isotopic composition of their waters shows that extensive evaporation is taking place (Gill, 1989).

More generalized porosity patterns, such as the karstrelated and selective dissolution porosity in other areas

underlying St. Croix’s central plain are lithologically and

geographically restricted to the Blessing Formation and Mannings Bay Member carbonates as well. The distribution of
HYDROLOGIC FRAMEWORK OF THE U.S. CARIBBEAN ISLANDS 125

these porosity types can be predicted to occur on the basis of

lithology, and can therefore be expected to occur in a narrow

band around the coastline from the southern to the western

end of St. Croix. The Mannings Bay Member and the Blessing Formation can be expected to contain these porosity types

because of their original mineralogies, their geographic location close to the coastline, and their higher initial porosity and

permeabilities

ALLUVIAL AQUIFERS

ROBERT A. RENKEN1

Unconsolidated clastic detritus in St. Croix deposited as

part of alluvial fan, slope wash, debris flow, lagoonal muds,

and stream alluvium are, for purposes of this discussion, collectively grouped as part of U.S. Virgin Islands alluvial aquifers. Restricted in its physical extent, the alluvial aquifer is

moderately permeable, serving as an aquifer and as a temporary storage zone for rainfall and stream runoff. Where these

deposits overlie the Kingshill aquifer, ground water is slowly

released to the underlying, poorly transmissive Kingshill

aquifer. 

Unconsolidated clastic detritus in the Central Limestone

Plain of St. Croix consist of moderately thin, alluvial fan and

slope wash deposits, or occur as stream alluvium that has

infilled overdeepened valleys now drained by ephemeral to

intermittent streams. Alluvial-fan deposits along the Northside Range appear to be dominated by silt- or clay-rich material and usually less than 10 m thick. Deposits of coarse

alluvial detritus (sand-size or larger) that exceed 20 m thick

lie within river guts that incise the Kingshill aquifer north of

Krause Lagoon (fig. 61). The alluvial aquifer is thickest along

the south-central coast of St. Croix (greater than 40 m) where

alluvium and lagoonal sediments underlie Krause Lagoon.

The thickness of alluvial deposits in eastern St. Croix is

poorly known due to limited well data, but is reported to be as

much as 25 m thick (Cederstrom, 1950, p. 28). Here, alluvial

deposits consist of alluvial fan and matrix-supported boulderconglomerate debris fan deposits. The poorly sorted nature of

the debris flow deposits probably limits their utility as an alluvial aquifer. Silt and clay make up a part of the alluvial section in many areas. Sand and gravel deposits are thin and

interbedded with silt and clay. Alluvial deposits are reported

to yield 2 to 10 L/s to some wells. Estimates of transmissivity

for alluvial sediments in St. Croix are difficult to obtain as

many wells screened in such deposits also tend to be screened

in the underlying Kingshill Limestone or older volcaniclastic

bedrock units. The transmissivity of the alluvial deposits contained within the River Gut area is reported to range from 20

to 450 m2/d. The transmissivity of wells screened in the alluvium and Kingshill Limestone in the Bethlehem Gut area

north of Krause Lagoon is reported to range from 20 to 650

m2/d (Torres-González, written commun., 1990). 

Alluvium and beach deposits of Holocene and Pleistocene

(?) age occur in a number of coastal embayments in St. Croix,

St. Thomas, and St. John. They are only of local importance

and not areally extensive (figs. 5, 6). The alluvial aquifer that

lies within the lower reach of Turpentine Run in eastern St.

Thomas, extends only as a narrow, 60 m-wide band that is no

more than 12 m thick. Alluvium in St. Thomas and St. John

consist largely of silt, clay, sand and gravel lenses that grade

coastward to sandy carbonate beach sands. Recharge to these

alluvial aquifers occurs by seepage of storm runoff or water

gained from the adjoining bedrock aquifer. The yield of the

alluvial aquifers of St. Thomas and St. John is small. Sustainable withdrawals from Turpentine Run’s alluvial aquifer

exceeding more than 0.4 L/s could cause an encroachment of

saltwater (Jordan and Cosner, 1973).

WEATHERED MANTLE-BEDROCK AQUIFER

Limited supplies of ground water are available in St.

Croix, St. Thomas, and St. John contained within the Cretaceous volcaniclastic bedrock and volcanic (gabbro and diorite) rocks of Tertiary and Cretaceous age. Movement of

ground water within these rocks is largely limited to weathered and open joints and fractures not subjected to secondary

mineralization, or along fault zones not sealed with gouge. In

St. Croix, the depth of the weathered fracture and joint bedrock aquifer does not extend more than 30 to 45 m below land

surface. The bedrock aquifer of St. John has been separated

into three hydrogeologic units and probably parallels the

physical character of bedrock aquifers in St. Thomas and St.

Croix: (1) a thin soil zone and saprolite zone capable of

absorbing large amounts of rainfall, (2) a weathered bedrock

zone (1 to 15m thick and rarely as much as 55 m thick), and

(3) unweathered bedrock containing joints that are more

numerous and more open at shallower depths (Cosner, 1972).

Bedrock fractures and joint weakness are best expressed in

valleys and provide the greatest opportunity for additional

ground-water supplies, particularly in the lower part of the

valley, where overlying alluvium often acts as a source of

recharge. In St. John and St. Thomas, drainage basins and

minor valleys have formed along fault and fracture zones that,

in general, function as conduits. Antecedent soil-moisture

conditions are important to recharge of the bedrock aquifers

on each of the U.S. Virgin Islands. The bedrock aquifer is

infrequently recharged and usually only after heavy rainfalls

or a series of storms that generate 50 mm of rain (Jordan,

1975). 

The hydraulic conductivity of the bedrock aquifer in St.

Thomas near Charlotte Amalie ranges from 0.3 to 2.7 m/d.

Hydraulic conductivity of a well located within a nearby fault

zone ranged from less than 21 to 46 m/d (Jordan and Cosner,

1

 U.S. Geological Survey
126 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS Christiansted Frederiksted CARIBBEAN SEA Krause Lagoon Salt River Bethlehem GutRiver Gut ATLANTIC OCEAN CARIBBEAN SEA Isla Mona Isla de Culebra Isla de Vieques St. ThomasSt. John St. Croix Puerto Rico Base modified from U.S. Geological Survey, Fredriksted, Chistiansted, East Point, 1:24,000, 1958 0 10 KILOMETERS 5 0 5 MILES 64 ̊48' 64 ̊55'30" 64 ̊40'30" 64 ̊30' 17°48' 17 ̊40'30" EXPLANATION Thickness of alluvial aquifer, in meters Less than 10 10 to 20 20 to 40

 Greater than 40 Insufficient data Alluvium not presentWell control point

FIGURE 61.—Thickness of the alluvial aquifer in St. Croix, U.S. Virgin Islands.
SUMMARY 127

1973). In St. Croix, the specific capacity of wells screened in

the bedrock aquifer is reported to range from 0.02 to 1 [(L/s/

m)] with highest values generally associated with bedrockvalley areas overlain by alluvium. Water obtained from the

Mahogany Road well field near Frederiksted is obtained

mostly from weathered bedrock underlying alluvium (Hendrickson, 1963). Transmissivity in this well field is estimated

to range from 4 to 150 m2/d (Torres-González, written commun., 1990).

VIEQUES ISLAND

Alluvial fan deposits and/or alluvium underlies Esperanza

alluvial valley in south-central Vieques and Resolución alluvial valley in northwestern Vieques. Consisting of largely

sand, silt, clay, and rock fragments, they represent the principal aquifers on the island (fig. 4).

The thickness of the Esperanza alluvial valley aquifer is

as much as 30 m. In much of Esperanza alluvial valley, a 3-

m-thick clay layer or weathered bedrock zone overlies dioritic bedrock (fig. 62). The clay layer is, in turn, overlain by a

18 m-thick coarsening upward zone of fine sand, rock fragments, and clay. In general, the amount of clay within the

aquifer increases with depth. A 1.5-m-thick clay layer occurs

less than 8 m below land surface. The clay layer functions as

a leaky confining bed and hydraulically separates a lower

confined zone from an upper water-table aquifer comprised of

sand and silt (Torres-González, 1989). The transmissivity of

the Esperanza alluvial valley aquifer reportedly ranges from

19 to 186 m2/d with hydraulic conductivity of the aquifer

ranging from less than 1 to 11 m/d (Torres-González, 1989).

The Resolución alluvial valley aquifer is a 9-m-thick alluvial aquifer which overlies a clay or weathered bedrock zone

similar to that underlying the Esperanza alluvial valley aquifer. Estimates of transmissivity and hydraulic conductivity

within the Resolución alluvial valley aquifer are not available

but probably are similar to the Esperanza alluvial valley aquifer.

SUMMARY

Poorly lithified to unlithified carbonate and clastic sedimentary rocks of late Tertiary and Quaternary age comprise an

aquifer system and two regional aquifers of the U.S. Caribbean Islands. The South Coast aquifer and the North Coast

limestone aquifer system underlies the southern and northern

coastal plains of Puerto Rico and accounts for approximately

80 percent of the total amount of ground-water withdrawn

from the U.S. Caribbean Island Regional Aquifer-System

Analysis (CI-RASA) study area. The Kingshill aquifer of St.

Croix is the most extensive aquifer of the U.S. Virgin Islands.

However, the importance of the Kingshill aquifer is more a

function of its physical extent and the lack of alternative water

supplies; it supplied less than 1 percent of the total amount of

ground water withdrawn from the U.S. Caribbean islands in

1985. 

Rocks that underlie the South Coast ground-water province of Puerto Rico range from Oligocene to Holocene in age.

Much of the South Coast ground-water province is underlain

by a southward thickening coastal plain wedge that has been

interrupted by fault movement; this coastal plain sequence

infills a large sedimentary basin that, in places, contains more

than 1,000 m of clastic and carbonate sediment. These sedimentary rocks and unconsolidated sediments are underlain by,

and in places fault-juxtaposed with, Cretaceous and early Tertiary bedrock that underlie the central mountainous core of

Puerto Rico. 

The Juana Díaz Formation consists of a basal clastic

sequence of late-early Oligocene age and grades upward to

shelf and island-slope carbonate rocks of late Oligocene age.

In places, the Juana Díaz Formation is, in turn, unconformably

overlain by an unnamed sequence of pelagic slope and deep

shelf chalk and limestone of early Miocene age. Elsewhere,

the Ponce Limestone of middle Miocene to early Pliocene (?)

age directly overlies the Juana Díaz Formation. The Ponce

Limestone is dominated by a shallow-water reef and carbonate

shelf limestone where it crops out west and north of the city of

Ponce and where it extends beneath the Quaternary-age fandelta plain south of the city. However, the Ponce Limestone

grades eastward to a thick sequence of clastic nearshore

marine, alluvial and fan-delta deposits. Clastic and minor

limestone deposits equivalent to the Ponce Limestone are

thickest (more than 800 m) where they lie buried beneath the

Quaternary fan-delta plain in south-central Puerto Rico. 

A 10- to greater than 100-m thick unlithified sequence

formed by five large coalescing fan-deltas and minor nearshore marine clastic sediment of Quaternary age (Pleistocene

to Holocene age) extends eastward 70 km from Ponce to Patillas. These fan-delta deposits make up the bulk of the stratigraphic sequence that functions as a freshwater aquifer. Fandelta deposits unconformably (?) overlie the Juana Díaz Formation and Ponce Limestone and equivalents in the central

part of the South Coast ground-water province, but directly

overlie Cretaceous and early Tertiary rocks in the eastern part

of this area. Geologic mapping indicates that the varied thickness of these Quaternary fan deposits is a result of pre-fan

preferential denudation and erosion along fractures and faults

of the great southern Puerto Rico fault zone. Oligocene sinistral and Miocene normal fault movement appears to have

played a significant role affecting the pattern of lithofacies

contained within the Juana Díaz Formation and the Ponce

Limestone. Paleofloods, Quaternary eustatic changes in base

level, and associated changes in paleoclimate played a more

important role in terms of controlling the cyclic distribution

and thickness of coarse- and fine-grained beds within the fan-
128 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

      

  

0 0.1 0.2 0.3 MILE

0 0.1 0.2 0.3 0.4 0.5 KILOMETER

20

10

Sea level

30

20

10

75

50

25

Sea level

25

50

75

100

METERS

SOUTH

A

FEET

NORTH

A'

CARIBBEAN SEA

Isla de Vieques

Esperanza alluvialvalley aquifer

MAP AREA

Colonia Puerto Real

Puerto Real Ensenada Sombe

CARIBBEAN SEA

A

A'

65 ̊29' 65 ̊28'

18 ̊06'

18 ̊05'

Esperanza

0 0.5 KILOMETER

0 0.5 MILE

EXPLANATION

Sand, silt, and clay

Clay and rock fragments Clay

Fine sand, some rock fragments,

 and clay Bedrock

Potentiometric surface

Well

Screened interval

FIGURE 62.—Section A–A' showing hydrogeology of the Esperanza alluvial valley aquifer, Isla de Vieques

(modified from Torres-Gonzáles, 1989).
SUMMARY 129

delta plain. Surficial and sand-and-gravel percentage maps

show that sediments within the fan-delta sequence are deposited as a series of depositional lobes where coarse detritus is

concentrated as a lobe and laterally separated from one

another by areas underlain by fine sediment. 

The South Coast ground-water province of Puerto Rico is

separated into three hydrogeologic units that include the South

Coast aquifer, alluvial valley aquifers located in the western

part of the province, and the Ponce-Juana Díaz aquifer. Boulder- to sand-size detritus constitute the major water-bearing

units within the South Coast fan-delta plain and alluvial valley

aquifers, and are intercalated with thick beds of silt that

impede vertical ground-water movement. Given limited well

data, separation of the numerous beds of coarse-grained detritus (1 to 20 m-thick) into distinct, mappable water-bearing

units is difficult, if not impossible. However, several intermediate-scale (30- to 40-m thick) depositional cycles containing

bedded sequences of sand, pebbles, and cobbles within the

distal parts the fan-delta sequence can be viewed as mappable

hydrogeologic units.

The pattern of deposition within the fan-delta and alluvial

valley sequence appears to directly control the distribution of

hydraulic conductivity within these aquifers. Diagenesis is

not important to the distribution of porosity or permeability

within this sequence. Sand-and-gravel lithofacies maps were

used to corroborate the distribution of hydraulic conductivity

based on specific capacity data. Highest conductivities are

associated with proximal and midfan parts of each fan delta

with smallest conductivities corresponding with the interfan

and distal parts of the fan. Transmissivity and hydraulic conductivity data of alluvial valley deposits that are present in the

western part of the South Coast ground-water province is

highest in upland channel areas; the permeability of these sediments diminishes considerably as they extend coastward. 

The Ponce-Juana Díaz aquifer is poorly permeable and not

considered to be a reliable source of water. Conglomeratic

beds are partially-cemented, and limestone beds, for the most

part, do not exhibit extensive dissolution features. Secondary

permeability within the limestone of the Ponce-Juana Díaz

aquifer is found only along fractures and faults that lie buried

beneath unconsolidated alluvial deposits. 

The South Coast aquifer contains water under unconfined

and confined conditions. Unconfined conditions generally

occur in the coarse-grained proximal and midfan areas where

the aquifer is recharged by irrigation water (surface and

ground water), infrequent rain, and streambed seepage from

streams entering the fan-delta plain. Regional discharge

occurs as seabed and coastal wetland seepage, base flow discharge near the coast, evapotranspiration from the shallow

water table, and by well withdrawals. Well withdrawals currently represent the principal mechanism of ground-water discharge. During the 1960’s and 1970’s, an increase in net

ground-water withdrawals led to a decrease in discharge to

coastal drains. From 1970 to 1980, a decrease in irrigation

deliveries and well withdrawals resulted in a decline in coastal

drain ground-water discharge to near predevelopment rates.

Regional ground-water flow is largely coastward from the fan

apex and foothills with local movement to well fields. Confined conditions are associated with more deeply buried sediments or those that lie in interfan or coastal areas. The

intercalated nature of sediments helps explain the confinement

in these areas where discontinuous fine-grained beds retard

vertical ground-water movement. 

The North Coast ground-water province of Puerto Rico is

underlain by a northward-dipping coastal plain wedge of carbonate and minor siliciclastic sediments that thicken to as

much as 1,700 m near the coast and infills the North Coast

Tertiary Basin. The sedimentary rocks of the north coast can

be separated into a relatively thin basal section of siliciclastic

sedimentary rocks of late Oligocene age and thick upper section of mostly carbonate rocks late Oligocene to middle

Miocene in age. The lowermost San Sebastián Formation

records a change from nonmarine to shallow-marine deposition in which the thickness of the formation reflects an irregular paleotopography and paralic depositional system. The San

Sebastián grades upward to the Lares Limestone, a unit that is

thickest in the central part of the basin between the Río

Grande de Arecibo and Río Cibuco. The lower and middle

parts of the Lares Limestone consist of a cyclic sequence of

skeletal wackestone, packstone, grainstone and record a deepening of the carbonate sea. The upper part of the formation

consists of dolomitic to clayey wackestone, skeletal wackestone and packstone, and reflects a shallowing of the sea. 

In the extreme western part of the basin, the Lares Limestone is absent. Equivalent rocks grade to clayey wackestone

and dolomitized wackestone that are similar to the Cibao Formation downdip or to mudstone strata that characterize the

San Sebastián Formation. In the eastern part of the basin, the

Lares Limestone and overlying Cibao Formation interfinger

with sandstone, conglomerate, siltstone, and claystone of the

Mucarabones Sand. The Cibao Formation is a complex facies

of carbonate, mixed carbonate-siliciclastic, and siliciclastic

rocks that reflect differential uplift or subsidence, or both, in

highland and marine platform areas. The Montebello Limestone Member of the Cibao Formation, the most permeable

part of the North Coast limestone aquifer system’s lower

aquifer, consists of a packstone-wackestone facies that contains large foraminifera and coral- and algal-rich layers. In

adjacent subsurface areas, the mudstone unit consists of calcareous mudstone that contains planktonic foraminifera and

may have been deposited within a restricted part of the basin

that had undergone preferential subsidence during the early

Miocene. At the same time, carbonates of the Montebello

Limestone Member built up directly over paleohighs that were

subsiding at a slower rate. To the east, carbonate and terrigenous sedimentary rocks of the Río Indio and Quebradas Are-
130 REGIONAL AQUIFER-SYSTEM ANALYSIS—CARIBBEAN ISLANDS

nas Limestone Members were deposited within the inner- to

middle-platform environments. An uppermost undifferentiated sequence of marl, calcareous mudstone, and clayey wackestone makes up the typical Cibao Formation lithology; this

sequence ranges from more than 300 m thick in the western

part of the basin to tens of meters thick in the eastern part of

the basin. 

The Aguada (Los Puertos) Limestone consists of shoalingupward cycles of skeletal wackestone to packstone and ranges

from 60 to nearly 180 m thick. The Aymamón Limestone consists of skeletal wackestone, packstone, lesser grainstone,

minor mudstone and coral-algal boundstone. The upper 60 to

90 m is highly karsted with large dissolution holes. The Quebradillas Limestone consists of a globergerinid packstone that

is restricted in extent, cropping out or lying in the shallow subsurface in northwestern part of the basin. This formation is

unimportant hydrologically as much of it lies above the water

table and is unsaturated.

Oligocene to middle Miocene sedimentary rocks of northern Puerto Rico are tentatively divided into five depositional

sequences and associated systems tracts. The oldest sequence

is not exposed but lies deeply buried and records an early

onlap of the basement complex; much of the strata that were

deposited as part of this sequence could have been removed by

erosion. A second depositional sequence that includes the San

Sebastián Formation and Lares Limestone records onlap during a rise in relative sea level. The overlying Cibao Formation

represents the third depositional sequence and shows evidence

of a transgression following a lowstand that marked the end of

Lares Limestone deposition. Uppermost Cibao Formation

rocks and the Aquada (Los Puertos) Limestone make up the

fourth sequence that include transgressive and high stand systems tracts. A karst surface separates the fourth sequence

from overlying fifth depositional sequence, the transgressive

Aymamón Limestone.

The North Coast limestone aquifer system can be separated into an upper aquifer, an intervening confining unit, a

lower aquifer, and a basal confining unit. The upper aquifer is

comprised of the karstic Aymamón Limestone and the underlying Aguada Limestone and contains a basal zone of saltwater that lies as much as 7 km inland from the coast. The

middle confining unit consists of clay, mudstone, and marl of

the Cibao Formation. The lower aquifer is made up of the

Montebello Limestone Member of the Cibao Formation, the

Lares Limestone, clastic sediments of the Mucarabones Sand,

and in the more inland, updip areas, conglomerate and sandstone beds of the San Sebastián Formation and clastic and carbonate beds of the Quebrada Arenas and Río Indio Limestone

Members. The North Coast limestone aquifer system is

underlain by a confining unit that consists of poorly permeable

mudstone and marl of the deeply buried San Sebastián Formation and lithified volcanogenic and igneous bedrock of early

Tertiary and Cretaceous age that underlie and form the mountainous core of Puerto Rico. A number of alluvial valley aquifers overlie the North Coast limestone aquifer system where

major rivers extend northward from the central highlands.

These alluvial valley aquifers consist of cobbles, pebbles, and

sand where present in the narrow, deeply incised upland valleys, but fine coastward to sand, silt, and clay forming wide

alluvial plains near the mouths of several major rivers. 

Topographic relief and incision of carbonate coastal plain

rocks by streams are the principal factors that control the

direction of ground-water flow where the upper and lower

aquifers crop out. Northward-flowing rivers were deeply

incised during Pleistocene sea level lowstands. Subsequently,

mechanical erosion breached middip parts of the confining

unit. These deeply incised areas are now covered by thick

alluvial deposits that were deposited in response to the interglacial rise in sea level. The occurrence of large springs and

low potentiometric surfaces within these valley-fill areas indicates that ground water from the lower aquifer probably discharges upward into the alluvium overlying the breached part

of the middle confining unit.

The North Coast limestone aquifer system is recharged in

topographically-elevated interstream areas and water flows

northward toward the Atlantic Ocean, discharging to the major

rivers, coastal wetlands, coastal, nearshore, or offshore

springs, or as seabed seepage. In the San Juan area, which is

likely an area of regional ground-water discharge from the

lower aquifer, the upper aquifer is thin or absent and mixed

siliciclastic sediments of the Cibao Formation provide an avenue for diffuse upward flow of ground water from the Mucarabones Sand. Ground-water withdrawals and agricultural

drainage have locally lowered water levels in the upper aquifer. Ground-water development within the confined part of the

lower aquifer is limited to the north-central part of the coastal

plain and corresponds to areas where water-level declines

have been greatest. This decline is attributed to industrial

withdrawals near Barceloneta, upward leakage from poorly

constructed wells, and pumping of public supply wells in

updip areas near Florida.

Factors that govern the distribution of permeability and

porosity within the North Coast limestone aquifer system

include the original character of the carbonate rocks, depositional patterns, diagenesis, dissolution along fractures, and

processes of karstification. Permeability of the upper aquifer

is controlled largely by the distribution of dolomite and formerly aragonitic fossils and, in part, by processes of karstification. It is also possible that recent mixing of meteoric and

saltwater lowered calcium carbonate saturation of ground

water near the coast causing additional dissolution of limestone and possibly promoting increased permeability in that

area. Dissolution along fractures may also have enhanced permeability along fractures within the North Coast limestone

aquifer system. Topographic relief and major segments of
REFERENCES 131

karst valleys show the strong influence of several regional

fracture sets.

Transmissivity of the upper aquifer is also controlled by

the thickness of the freshwater lens that thins landward and

coastward. The freshwater lens is thickest (100 to 150 m) in

regional interstream areas along a zone that roughly corresponds to the point where the saltwater interface intersects the

aquifer’s lowermost contact. The upper aquifer is most transmissive in these same interstream areas and locally may

exceed 10,000 m2/d. A hydraulic conductivity map of the

upper aquifer indicates that permeability of the aquifer

increase coastward, with least permeable parts of the aquifer

landward and west of Camuy. Transmissivity of the lower

aquifer is, in general, an order of magnitude or more lower

than that of the upper aquifer and usually ranges from 50 to

800 m2/d. Greatest transmissivity of the lower aquifer appears

to correspond to an area where the Montebello Limestone

Member crops out and lies in the subsurface. Transmissivity

diminishes to the east and west, paralleling facies changes

within these same rocks.

Porosity is highest in coarse grainstone-packstone and low

in wackestone and marl. Although moldic porosity is common in mollusk- and coral-rich wackestones, permeability of

these rocks is low. Dolomitized zones and moldic grainstonepackstones are some of the most porous and permeable carbonate strata. The dense, recrystallized Aymamón Limestone

(upper aquifer) is highly karstic within the upper 30 to 100 m

of land surface, and caverous zones and large mud-filled vugs

are common. Porosity and permeability of the lower aquifer is

developed best within grainstone-packstone and coral-patchreef limestone. However, these porous zones are usually no

more than a few meters thick and most of the section has a low

permeability. 

The island of St. Croix is bordered to the northwest and

east by low hills and mountains underlain by Cretaceous volcaniclastic and minor igneous rocks. These highland areas are

separated by a central graben structure largely infilled with

early Miocene to Pliocene age pelagic and hemipelagic limestone (Jealousy Formation and the Kingshill Limestone). The

Jealousy Formation is the oldest stratigraphic unit in the basin

and ranges from early Miocene to early-middle Miocene age

and is comprised of a planktonic foraminifera-rich mud. The

diachronous contact between the Kingshill Limestone and

Jealousy Formation is recognized by a distinctive, but unexplained change in color from light buff to blue-grey or gray.

The La Reine Member of Kingshill Limestone (early Miocene

to Pliocene age) is lithologically more diverse than the Jealousy Formation and includes packstone (planktonic foraminifera-rich carbonate mud with some lithic grains and pebbles),

wackestone, polymictic packstone facies, as well as debris

flow beds of coral and terrigenous gravel. Uppermost Kingshill Limestone of thinly-bedded limestone containing greater

amounts of shelf-derived carbonate clasts, and foraminiferal

faunal remains constitute the Manning Bay Member. 

The Mannings Bay Member is a limestone characterized

by large benthic foraminifera and skeletal debris, locally dolomitized, and leached by meteoric weathering. The Blessing

Formation unconformably overlies the Mannings Bay Member and is thickest in an area formed by a subsidiary Pliocene

age graben structure located along St. Croix’s south-central

coast near Krause Lagoon. Also occurring along the southwest and western coasts, the Blessing Formation of Pliocene

age differs considerably from older pelagic limestones of St.

Croix. The Blessing Formation consists of reef and shallow

lagoon limestone with minor patchy zones of dolomitization. 

The Kingshill aquifer consists of the Kingshill Limestone

and the Blessing Formation and contains ground water under

unconfined conditions. The Jealousy Formation is not considered to be part of the aquifer largely because of the poor

hydrochemical character of ground water it contains.

Although most of the primary porosity of the Kingshill Limestone aquifer is preserved, it is microscopic. In coarser sediments of the Kingshill Limestone, cementation and

recrystallization have destroyed original permeability. Limestone of the Blessing Formation and the Mannings Bay Member of the Kingshill Limestone are more permeable and porous

than the underlying units. Diagenesis directly controls permeability and porosity within the Mannings Bay Member and the

Blessing Formation and vertical tectonic movement near

Krause Lagoon during Pliocene time contributed to the limited

geographic distribution of dolomite within these two rock

units. Diagenetic factors affecting permeability of the upper

Kingshill aquifer include dissolution of unstable minerals,

karstification, and possible coastal mixing zone dissolution.

Localized dolomitization within the graben structure north of

Krause Lagoon has resulted in a localized increase in intercrystalline and moldic porosity. The dolomitization process is

attributed to the mixing of evaporated sea and ground water

within a structurally-controlled embayment. The moderately

permeable alluvial aquifer in St. Croix comprises alluvial fan,

debris flow, slope wash, stream alluvium, and lagoonal mud

deposits. Restricted in extent, the alluvial aquifer serves as a

temporary storage zone for rainfall and runoff, allowing

ground water to be slowly released to underlying carbonate

aquifers. Many of the large-yield wells reported to be tapping

in the Kingshill aquifer are also screened in the alluvium.

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