Geology of Isla Desecheo, 

Puerto Rico, With Notes on the 

Great Southern Puerto Rico 

Fault Zone and Quaternary 

Stillstands of the Sea

GEOLOGICAL SURVEY PROFESSIONAL PAPER 739

Prepared in cooperation with the Industrial 

Research Department, Puerto Rico Economic 

Development Administration
GEOLOGY OF ISLA DESECHEO,

PUERTO RICO
Aerial view of Isla Desecheo from the north. Photograph by Pedro Sola Amadeo, Laboratory of Perinatal Physiology, U.S. 

National Institutes of Health, San Juan, Puerto Rico. Permission to publish the photograph granted by Dr. Ronald E. 

Myers, Chief, Laboratory of Perinatal Physiology.
Geology of Isla Desecheo, 

Puerto Rico, With Notes on the 

Great Southern Puerto Rico 

Fault Zone and Quaternary 

Stillstands of the Sea

By VICTOR M. SEIDERS, REGINALD P. BRIGGS, and LYNN GLOVER III 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 739

Prepared in cooperation with the Industrial 

Research Department:, Puerto Rico Economic 

Development Administration

The surjicial deposits and volcanic bedrock of 

an island in Mona Passage with comments on 

regional structure and higher sea levels

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1972

459-607 O - 72 - 2
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress catalog-card No. 72-600060

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402 - Price 45 cents (paper cover)

Stock Number 2401-2106
CONTENTS

Abstract................................................................................

Introduction.........................................................................

Previous work......................................................................

Geography............................................................................

Stratigraphy..... ..... .. .. ........................... .......

Tertiary System............................................ ....

Rio Culebrinas Formation(?)..........................

Volcaniclastic rocks....................................

Calcareous mudstone and sandstone........

Calcarenite...................................................

Penecontemporaneous deformational

structures.........................................................

Conditions of deposition.....................................

Alteration..--.... - --........................................

Age.............................. ...................................

Page 

1 

1 

4

4

5

5

5

5

7

7

10

10

Page

Stratigraphy Continued

Quaternary System...................... ........................... 11

Marine-terrace deposits.........................- ........ 11

Deposits chiefly between 6- and 12-meter

elevation.................................................... 11

Deposits at about 2-meter elevation.... .. 13

Colluvium.................................................--..--..... 13

Beach deposits.........-....---..-.--. ....................... 14

Structural geology........ ... .. . .-- 15

The great southern Puerto Rico fault zone............ -... 17

Quaternary stillstands of the sea..................................... 19

Upper Desecheo stillstand......................................... 19

Lower Desecheo stillstand......................................... 20

Regional relations and age........................................ 20

References cited............................................................ .. 21

ILLUSTRATIONS

2.

3.

4-6.

FRONTISPIECE. Aerial view of Isla Desecheo from the north.

FIGURE 1. Geologic map and sections of Isla Desecheo............................................................. . ................

Bathymetric map of Mona Passage.--.... ..-.- . ..---.-- ........................ ..... ........... .... .....

Photograph showing mudstone slabs oriented parallel to bedding in volcanic sandstone,

Rio Culebrinas Formation (?)............................... ..... ............................................... ..-. 

Photomicrographs:

4. Volcanic sandstone............................................................................................................... ......

5. Volcanic sandstone............................................................................................... ......................

6. Calcareous foraminiferal mudstone...........................................................................................

Sketch showing eroded and upturned mudstone beds in lower part of volcanic sandstone bed. 

Photographs:

8. Recumbent slump fold and slide breccia.............. ....................................................... ......

Zeolite mottling in zeolitized volcanic sandstone.............. .......... ................. . . .

Marine-terrace deposits at Puerto de los Botes....................................................................

Marine-terrace deposits at Puerto Canoas............ .. .............................. .... ..........

Marine-terrace deposits in irregular depression in volcanic bedrock just west of 

Puerto de los Botes......................................................................................................... ..-

Overturned syncline near cable house. ......-- ............. ..... .................... .. ...... 

Puerto Manglillo fault..... .......................................................................................................

Joints filled with calcite and zeolites.......................................................................................

9.

10.

11.

12.

13.

14.

15.

16.

17.

Map of the Greater Antilles showing major fractures........................................................................... . .. .

Photograph of northwest coast from near helicopter pad showing lower Desecheo stillstand platform.

Page

2

4

10

11

12

12

12

15

16

17

18

20

TABLE

TABLE 1. Chemical analysis and molecular norm of cemented-colluvium sample from south cut of helicopter pad, 

Isla Desecheo.................................................................................................................................. . .. .. 

Page

13

GEOLOGY OF ISLA DESECHEO, PUERTO RICO,

WITH NOTES ON THE GREAT SOUTHERN PUERTO RICO FAULT ZONE 

AND QUATERNARY STILLSTANDS OF THE SEA

By VICTOR M. SEIDERS, REGINALD P. BRIGGS, and LYNN GLOVER III

ABSTRACT

Isla Desecheo is in Mona Passage between Puerto Rico and 

Hispaniola. The bedrock, 700-1,100 meters of middle Eocene 

volcaniclastic rocks, consists chiefly of dacitic volcanic sandstone, fine-grained volcanic breccia, and mudstone. Deposition 

in at least moderately deep water is indicated by the abundance of graded beds and by the fact that interbedded mudstone contains exclusively planktonic faunas. Sparse rounded 

grains and shallow-water fossil debris in the graded beds indicate that the volcaniclastic debris came from volcanoes in 

shallow seas. Steep submarine slopes are suggested by submarine-slide deposits and penecontemporaneous deformation 

structures. The rocks are extensively zeolitized.

The southern two-thirds of the island, south of an eastwest vertical fault, has west-trending nearly isoclinal folds 

which are overturned to the north. North of the fault, the 

main structural feature is a gentle anticline which plunges 

south. The contrasting structural features are believed to 

represent different tectonic levels that are separated by a 

low-angle thrust fault. The strongly deformed beds may be 

the remnant of a gravity-glide sheet that was eroded from 

the northern part of the island.

Isla Desecheo lies along the seaward projection of the great 

southern Puerto Rico fault zone, which is, perhaps, the major 

fracture zone of Puerto Rico. Large-scale transcurrent and 

vertical faults and associated gravity-glide structures seem 

closely related in age and geometry to similar features in 

Hispaniola and Cuba.

Surficial deposits include marine terrace deposits, colluvium 

(in part phosphate cemented), and beach deposits. There is 

evidence of two stillstands of the sea, one at 12-13 meters and 

the other at about 2 meters higher than present sea level. 

The higher level is probably of Pleistocene age and the lower, 

of Holocene age, perhaps about 3,300 years before the present. 

Tectonic uplift is believed responsible for the 2-meter level 

and, therefore, at least in part for the 12-13-meter level.

INTRODUCTION

Since 1955, Puerto Rico has been the subject of a 

detailed geologic investigation by the U.S. Geological

Survey in cooperation with the Puerto Rico Economic Development Administration. Although most 

efforts have been concentrated on geologic mapping 

of the main island of Puerto Rico, the goal of this 

program is knowledge not only of the main island, 

but also of the many smaller adjacent islands.

In this report, Seiders and Glover are chiefly 

responsible for data and discussion on bedrock 

geology, and Briggs, for surficial geology. The fieldwork was done during two periods: Seiders, Briggs, 

and Glover were on Isla Desecheo April 15-19, 1966, 

and Seiders and Glover returned to the island May 

8-10, 1967, accompanied by Eduardo Aguilar-Cortes, 

geologist with the Puerto Rico Economic Development Administration.

For the first fieldwork period, transportation to 

Isla Desecheo was by RV Shimada of the Puerto 

Rico Nuclear Center; the return trip was made in 

a small craft of the National Institutes of Health. 

For the second period, transportation was by a 

helicopter of the U.S. Navy Fleet Composite Squadron 8 (VC-8), Commander M. E. Smith commanding. Arrangements for transportation to and from 

Isla Desecheo were made through the cooperation 

of the staff of Dr. R. E. Myers, Chief, Laboratory 

of Perinatal Physiology, San Juan, and J. A. Morrison, Acting Chief, Section on Primate Ecology, 

Punta Santiago, Puerto Rico, both of the National 

Institutes of Health. Food and water were supplied 

to us by National Institutes of Health personnel.

The topographic map used as the base for the investigation (fig. 1) was compiled from 1951 U.S. 

Air Force photographs and from other data by 

Marshall Mannello of the U.S. Geological Survey 

and by Briggs during January 1966.
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from 

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Base map by M. Mannello and R. P. Bri 

U.S. Geological Survey, 1966. Compiled 1951 U.S. Air Force aerial photographs, u 

ER-55 equipment.

The~re is only one horizontal control point (lat 

18°23'02" .014 N., long 67°28'46" .761 W), 

so scale and orientation are approximate. 

Scale error probably is less than 5 percent. 

The azimuth of a line through easternmost and 

westernmost islets is shown as 96.5°, probably within 3 ° of correct,

Geographic names are those in common usage, 

as reported by personnel of the U.S. National 

Institutes of Health in-1966

1500 FEET

400 METERS

Punta Culebron

CONTOUR INTERVAL 10 METERS WITH

SUPPLEMENTARY CONTOURS AT 5 METERS

DATUM IS MEAN SEA LEVEL

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GEOLOGY OF ISLA DESECHEO, PUERTO RICO

PREVIOUS WORK

This is the first detailed study of the geology of 

Isla Desecheo. Brief descriptions and sketch maps 

of the geology of the island were included in reports of larger areas by Hubbard (1923) and 

Mitchell (1954). Shoreline features were mentioned 

by Lobeck (1922).

GEOGRAPHY

Isla Desecheo is in the northeastern part of Mona 

Passage (fig. 2), the broad shallow strait between 

the islands of Puerto Rico and Hispaniola that connects the Atlantic Ocean with the Caribbean Sea. 

The island is about 21 kilometers (km) west of 

Puerto Rico and about 100 km east of Hispaniola. 

A low submarine ridge extends between Isla 

Desecheo and the western tip of Puerto Rico. The 

head of Mona Canyon, a deep steep-walled submarine depression extending northward to the 

Puerto Rico Trench, is just north of this ridge. Isla 

de Mona, the only other island of significant size 

in Mona Passage, is 51 km southwest of Isla 

Desecheo, at the southern entrance to the passage

and about midway between Puerto Rico and 

Hispaniola.

In plan, Isla Desecheo is roughly a northwesttrending rectangle having an area of about 1.2 

square kilometers. On three sides the island rises 

abruptly from a narrow wave-cut bench to ridge 

crests 100-200 meters above sea level (frontispiece). 

Average slopes range from 20° to 35°. From the 

southwestern coast, three valleys and the intervening ridges rise northward to a ridge near the northeast coast. Most of the drainage, therefore, is 

toward the southwest.

Although the island probably has an annual rainfall of about 1,020 millimeters (40 in.), the rapid 

runoff and high evaporation rate combine to reduce 

available moisture. As a result, the island has no 

permanent springs or streams and has a semiarid 

type of vegetation. Distribution of vegetation is influenced by the prevailing easterly trade winds. On 

east-facing slopes, where the evaporation rate is 

highest, is a sparse to dense cover of cactus and 

thorny shrubs and few small trees. West-facing 

slopes and sheltered valleys support a moderate 

growth of small- and medium-sized trees and much 

less abundant cactus and shrubs.

Bathymetric data from U.S.C. and G.S. 

chart 920 and H.O. chart 2650. Contoured 

by C. A. Kaye; Mona Canyon modified from 

Northrop (1954)

FIGURE 2. Bathymetric map of Mona Passage (from Kaye, 1959b, fig. 68).
TERTIARY SYSTEM

Isla Desecheo can be reached by small boat or by 

helicopter. At several places narrow inlets indent 

the rocky shoreline and give access to small beaches 

where light boats can land in good weather. The 

best landing place is Puerto de los Botes, followed 

by Puerto Canoas and Puerto Manglillo. A concrete 

helicopter pad is on the western tip of the island.

The best rock exposures are along the coast, and 

most of the coast can be traversed on foot with 

little difficulty. However, at several places southeast 

of Puerto Canoas and west of Dos Piedras, passage 

is blocked by cliffs.

STRATIGRAPHY

TERTIARY SYSTEM

The bulk of Isla Desecheo is underlain by a 

strongly deformed sequence of dacitic volcaniclastic 

rocks and less abundant calcareous mudstone. Hubbard (1923) correlated these rocks with his Rio 

Yauco Series of Late Cretaceous age (renamed 

the Yauco Mudstone by Mattson, 1960). The present study has shown that the rocks on Desecheo are 

of Eocene rather than Cretaceous age. They are 

clearly correlative with part of the sequence of 

lower Tertiary formations in northwestern Puerto 

Rico described by Mclntyre and others (1970). The 

most probable correlation is with the widespread 

Rio Culebrinas Formation (Mclntyre and others, 

p. D12) but correlation with the Conception Formation (Mclntyre and others, p. D12) is also possible. 

The Isla Desecheo rocks are herein assigned to the 

Rio Culebrinas Formation of the Jacaguas Group 

(Glover and Mattson, 1967), a thick sequence of 

uppermost Cretaceous and lower Tertiary beds 

characterized by abundant dacitic volcaniclastic 

rocks.

RIO CULEBRINAS FORMATIONS)

The exposed thickness of the Rio Culebrinas 

Formation (?) on Isla Desecheo is estimated to be 

700-1,100 m. The most abundant rock types are 

dacitic volcaniclastic rocks, chiefly volcanic sandstone and subordinate volcanic mudstone and volcanic breccia.1 These rocks are interbedded on a 

scale of a few centimeters to a few meters with 

generally less abundant calcareous foraminiferal 

mudstone, calcareous sandstone, and minor calcarenite. In the northern part of the island, part of 

the section is composed chiefly of calcareous mudirThe classification of volcaniclastic rocks proposed by Fisher (1961) is 

used here.

stone and calcareous sandstone and is mapped 

separately.

Volcaniclastic rocks

Volcaniclastic rocks are medium gray to greenish 

gray; less commonly, grayish red.2 Beds range from 

a few millimeters to as much as 10 m in thickness 

and, generally, grain size increases as bed thickness 

increases. Many of the thinnest beds are composed 

of a pale-green aphanitic rock resembling an impure chert, probably an altered very fine grained 

volcanic ash. Volcanic sandstone beds as much as 

about 1 m thick are moderately sorted and mostly 

graded; grain size ranges from silt to coarse sand. 

Load or flute casts are abundant. Many thin beds 

and upper parts of thicker beds are crossbedded 

on a small scale, and the upper parts of some beds 

are convoluted.

Volcanic sandstone and volcanic breccia beds 

thicker than about 1 m have a variety of textures. 

Some beds have an intact framework and are 

moderately sorted; the grain size ranges from about 

2 centimeters down to fine sand and silt. Some beds 

as much as about 5 m thick are graded, but many 

very thick beds seem to lack grading. Some otherwise moderately sorted beds contain scattered chips 

and slabs of mudstone. In some, the mudstone slabs 

are oriented roughly parallel to bedding (fig. 3), 

but in others the orientation is random. Other very

2Colors correspond to the "Rock-Color Chart" of the National Research 

Council (Goddard and others, 1948).

r-:

FIGURE 3. Mudstone slabs oriented parallel to bedding in 

volcanic sandstone, Rio Culebrinas Formation (?), just 

northwest of Puerto de los Botes.
6 GEOLOGY OF ISLA DESECHEO, PUERTO RICO

thick beds are very poorly sorted and have a disrupted framework of angular to rounded pebbles, 

cobbles, and boulders as large as 2 m mixed with 

sand and silt. The coarse fragments include dacitic 

or andesitic lava, volcanic sandstone, and mudstone.

ular, and most consist of a very fine grained aggregate of minerals, probably devitrified glass, with 

or without phenocrysts of plagioclase, clinopyroxene, hornblende, and quartz. Some fragments have

FIGURE 4. Photomicrograph of volcanic sandstone. Note larger Foraminifera (F). Plane-polarized light.

In thin section, the volcanic sandstone and finegrained volcanic breccia are composed chiefly of 

volcanic rock fragments and plagioclase, having 

lesser amounts of hornblende, clinopyroxene, quartz, 

opaque minerals, and calcareous fossil debris (figs. 

4 and 5). Most grains are angular, but some lava 

fragments are rounded (fig. 5). The rock has a 

fairly compact framework, and grains commonly 

interpenetrate. The matrix consists of recrystallized 

calcite, zeolites, or very fine grained yellowishbrown material, possibly clay minerals and calcite.

Pumice is absent. The volcanic rock fragments 

are nonvesicular or, less commonly, sparsely vesica trachytic texture, and a few grains are intergranular or intersertal; a few fragments are oxidized.

Much plagioclase is fresh and zoned from andesine to sodic labradorite (An32-6o), but some crystals are clouded and albitized. Hornblende is generally 

green, rarely brown. Quartz in both euhedral and 

broken grains is unstrained and is commonly embayed.

Calcareous fossil debris, sparse in most volcanic 

sandstone, locally makes up as much as 10 percent 

of the rock. Fragments of calcareous algae and 

whole and broken larger Foraminifera are the main 

components of the debris; planktonic Foraminifera 

are rare.
TERTIARY SYSTEM

FIGURE 5. Photomicrograph of volcanic sandstone. Note rounding of some fragments. Plane-polarized light.

Calcareous mudstone and sandstone

Calcareous mudstone is generally medium to light 

gray, less commonly grayish red. Beds are thin to 

medium. The mudstone has an aphanitic calciterich groundmass in which are abundant planktonic 

Foraminifera, less abundant small grains of 

plagioclase, quartz, volcanic-rock fragments and 

Radiolaria, and rare microcorals and benthonic 

Foraminifera (fig. 6). One sample of calcareous 

mudstone that was broken down with acid was 59 

percent calcite; some samples are probably somewhat richer in calcite. The calcareous mudstone 

grades continuously to noncalcareous aphanitic volcanic mudstone.

The calcareous sandstone differs from the typical 

volcanic sandstone only in being richer in calcite 

and poorer in volcanic-rock fragments. Calcareous 

sandstone is medium gray, medium to thick bedded, 

and medium grained and is composed of about equal 

parts of volcanic rock fragments, plagioclase and 

quartz grains, and recrystallized calcite.

Calcarenite

This rock was found only at two places on Isla Desecheo on the ridge crest southwest of the 143-m 

peak as float, and near the western tip of the island 

in a bed 25 cm thick. The rock is light gray and is 

composed of well-sorted 0.5- to 3-mm angular to 

well-rounded grains of calcareous fossil debris, less 

abundant lava fragments, and plagioclase, hornblende, and volcanic quartz grains. A single grain 

of multigranular strained quartz was found. The 

fossils are chiefly larger Foraminifera and coralline 

algae.

PENECONTEMPORANEOUS DEFORMATIONAL STRUCTURES

The rocks on Isla Desecheo show some features 

attributable to deformation during or soon after 

deposition. On a small scale these features are convolute bedding and load and flute casts. On a larger 

scale some very thick slide and turbidity-current 

deposits are broadly channeled at the base and contain fragments eroded from the underlying beds.
GEOLOGY OF ISLA DESECHEO, PUERTO RICO

FIGURE 6. Photomicrograph of calcareous f oraminif eral mudstone. Plane-polarized light.

Locally, the eroded edges of the underlying beds are 

bent upward and into the overlying massive deposit 

(%. 7).

Small tight folds a few centimeters to about a 

meter in wavelength, locally accompanied by small 

low-angle faults, are seen in a few exposures. The 

fold axes diverge and bear no relationship to major 

structures, suggesting that the deformation took 

place by slumping soon after deposition. At one 

locality, the deformation preceded deposition of the 

immediately overlying beds. Figure 8 shows a recumbent fold in thin-bedded mudstone and volcanic 

sandstone. The fold (at hammer) rests on a lowangle fault that is in contact with moderately deformed beds below. In the upper part of the fold, 

the beds are broken and pulled apart. The fold is 

unconformably overlain by a poorly sorted breccia 

conglomerate about 1 m thick, probably a submarine-slide deposit. The breccia conglomerate contains fragments of mudstone similar to those in

the recumbent fold as well as angular to rounded 

exotic fragments of lava and volcaniclastic rock. 

The breccia conglomerate is overlain by gently dipping beds (upper right) that were not affected by 

the deformation that acted on the lower beds. The 

folding and faulting were probably nearly synchronous with the slide. The deformation may have 

resulted from slumping which was caused by the 

same event that triggered the slide, or from passage 

of the slide itself.

CONDITIONS OF DEPOSITION

That angular volcanic rock fragments of fairly 

uniform texture and composition are the dominant 

constituents of the volcaniclastic rocks suggests that 

these rocks are chiefly of pyroclastic, or slightly reworked pyroclastic, origin. The abundance of graded 

beds and the exclusively planktonic fauna of the 

interbedded mudstone indicate that the rocks were 

deposited in rather deep water below the base of
TERTIARY SYSTEM

the surge zone. Coarse poorly sorted submarine 

slide deposits and penecontemporaneous deformavolcanoes. Explosive volcanic eruptions and accompanying earthquakes generated submarine pyro-

'. ' '& T^T-j^' : ', "^ ' " "' X'K

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^ _ " .^ » r *^ Jt -\ * ^" x ^ X

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. Volcanic ^> jf^ ..-.- :/:--^V\X *x\ ^t/ / 

sand^pne-n^-"^^/^"-:''*''/^/ X x*,-

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FIGURE 7. Eroded and upturned mudstone beds in lower part of volcanic sandstone bed which is more than 3 m 

thick and is locally cut out by Pleistocene terrace deposits. On the northeast coast, 700 m northwest of Dos Piedras.

tion features suggest that at least some of the rocks 

were deposited on an unstable submarine slope.

The source was probably a shallow-sea volcanic 

area, as is shown by the small amount of rounded 

volcanic rock fragments and shallow-water fossil 

debris in the volcaniclastic rocks. Pyroclastic debris, 

detritus from the active volcanoes, and marine shells 

were deposited in the shallow water near the

clastic ash flows, turbidity currents, and submarine 

slides. The newly formed pyroclastic debris was 

mixed with the unconsolidated shallow-water deposits and transported to deep water. Fine ash, 

transported by winds and surface currents, and the 

tests of planktonic Foraminifera and Radiolaria 

settled to the bottom continuously to form the thin 

beds of volcanic and calcareous mudstone.
10 GEOLOGY OF ISLA DESECHEO, PUERTO RICO

FIGURE 8. Recumbent slump fold and slide breccia on the northeast coast, 575 m northwest of Dos Piedras.

ALTERATION

Secondary minerals are in nearly all the rocks 

as vein fillings in the numerous joints, as void fillings or replacements of the matrix in the volcaniclastic rocks, and, less commonly, as replacements of 

clastic grains. In most rocks the secondary minerals 

are not abundant, but locally the alteration is intense. Highly altered volcanic sandstone is mottled 

with light-gray zeolite-rich blebs about 0.25-1 cm 

in diameter; these give the rock the appearance of 

being composed of clastic grains of that size (fig. 

9). The most abundant secondary mineral is calcite, 

followed by zeolites, phyllosilicates, and lesser 

amounts of albite, prehnite, quartz, and epidote.

Except for a single reconnaissance run, X-ray 

identifications were not used, but in thin section, 

optical properties suggest that the most abundant 

zeolite is laumontite. It occurs both in joint fillings 

and as a pervasive replacement. One vein contains 

both laumontite and heulandite. Analcime occurs in

some veins together with epidote and turbid prehnite. An analcime-calcite vein in calcarenite contains a zeolite with optical properties consistent 

with those of mordenite.

AGE

Three samples of calcareous mudstone collected 

north of the Puerto Manglillo fault were examined 

by E. A. Pessagno, Jr., for planktonic Foraminifera. 

Pessagno identified Globigerina sp., Globorotalia s.s. 

(keeled), and Tuborotalia densa (Cushman) in each 

sample and assigned a middle Eocene age to the 

samples. A sample of massive volcanic sandstone 

from north of the Puerto Manglillo fault was 

examined by K. N. Sachs, Jr., who identified 

Discocyclina (Discocyclina) weaveri and a microspheritic Asterocyclina; he also gave a middle 

Eocene age to this sample.

Sachs examined a specimen of calcarenite from 

between the Puerto Manglillo and Helicopter Pad 

faults and reported the following:
QUATERNARY SYSTEM 11

This sample has moderately abundant specimens of Asterocyclina minima, known typically from the upper Eocene of 

numerous Caribbean localities. However, there is another, 

middle Eocene, species of this genus which I consider to be 

a synonym of A. minima. Therefore, my opinion of the age 

of this sample is probably upper Eocene, but possibly middle 

Eocene.

Two samples of calcareous mudstone from near 

Puerto de los Botes on the southwest coast were 

examined by Pessagno, who found Globigerina sp. 

and Globorotalia s.s. (sharply keeled). He assigned 

an age of late Paleocene to middle Eocene to the 

samples. From one sample of sandy mudstone collected near the cable house, Pessagno reported 

Globotruncana sp. cf. G. linneiana (d'Orbigny) and 

indeterminate planktonic Foraminifera and Radiolaria. He suggested a possible Late Cretaceous age 

but pointed out that the Foraminifera may be reworked and are in large part badly recrystallized.

The above determinations show that some rocks 

are definitely of middle Eocene age and, except for 

one questioned Late Cretaceous determination, determinations with broader ranges include the middle 

Eocene. All the Rio Culebrinas Formation (?) on 

Isla Desecheo is probably of middle Eocene age.

FIGURE 9. Zeolite mottling in zeolitized volcanic sandstone. 

Note scattered large clasts. Just northwest of Puerto de los 

Botes.

QUATERNARY SYSTEM

MARINE-TERRACE DEPOSITS 

Deposits chiefly between 6- and 12-meter elevation

Discontinuously exposed around the coast of Isla 

Desecheo and unconformably overlying the volcanic 

bedrock is an elevated seaward-sloping marine terrace composed of well-indurated calcite-cemented 

sand and gravel (figs. 10 and 11). At most places, 

the terrace deposits are about 1 m thick (fig. 10), 

but, locally, depressions in the preterrace surface, 

which are channels or zones of plucking, are filled to 

depths of as much as 3 m (fig. 11). At other places, 

the deposits abut ramparts of volcanic rock.

The terrace surface dips seaward 5° or less at 

most points. Landward, the terrace deposits are 

overlain in many places by colluvium, but it appears 

likely that no presently exposed terrace surfaces 

ever were thickly covered. Landward, elevations are 

about 8-9 m above sea level where terrace surfaces 

are not extensively covered by colluvium (fig. 10). 

The highest terrace level on Isla Desecheo is 12 m 

above sea level at the head of Puerto Canoas and 

in a small cove 50 m east-northeast of the helicopter pad.

The terrace has been extensively eroded; its 

widest exposure is about 50 m, but at most places 

it is less than 10 m wide or is absent. Its upper 

surface has been pitted and fretted, like the eolianite surfaces in Puerto Rico described by Kaye 

(1959a, p. 83-88) and like the marine-terrace surfaces seen by Seiders and Briggs on Isla de Mona; 

this has produced a microkarst having relief ranging from 1 or 2 cm to about 50 cm. Dense cementation, though, has rendered the terrace deposits more 

resistant to erosion than the underlying weathered 

volcanic rock. Thus, erosion of the terrace deposits 

has been largely by undercutting; at many places 

they now form roofs of sea caves cut in the volcanic 

rock (fig. 10), and locally they cap arches and 

pedestals. This ledge-forming characteristic contributes greatly to the assurance that there is only 

one marine terrace at this general level; despite 

zones in which the terrace deposits are concealed 

or have been completely eroded away, the essential 

continuity can be seen readily (frontispiece).

Distinct layers within the terrace deposits range 

from 5 cm to about 50 cm thick, and most have 

gradational lower and upper contacts. At some 

places, however, the entire thickness of the deposit is formed by one bed only (fig. 12). Locally, 

snail shells about 6 cm in diameter form layers 

within the deposits.
12 GEOLOGY OF ISLA DESECHEO, PUERTO RICO

FIGURE 10. Marine-terrace deposits 1 m thick resting on steeply dipping volcanic rocks at Puerto de los Botes. Colluvium, whose apparent great thickness is misleading, rests on upper terrace surface at about 9-m elevation. Sloping volcanic 

bedrock probably is considerably less than 10 m beneath surface of colluvial deposits in most of the area shown. Note 

resistant character of terrace relative to volcanic bedrock and colluvium.

FIGURE 11. Marine-terrace deposits resting on steeply dipping volcanic rocks at Puerto Canoas (lower left).

FIGURE 12. Marine-terrace deposits in irregular depression 

in volcanic bedrock just west of Puerto de los Botes. Note 

boulders and cobbles of volcanic rock, coral heads, subsidiary 

channel (lower left), and cactus (above outcrop). Hammer 

45 cm long.
QUATERNARY SYSTEM 13

Sand grains in the terrace deposits are chiefly 

coarse subround and round fragments of coral, 

coralline algae, and other marine organisms, but 

medium and coarse subround and round volcanicrock fragments are also fairly abundant. The calcareous fragments are very light gray and yellowish 

gray. Most volcanic rock fragments are moderately 

weathered and are yellowish brown.

Subangular to subround boulders, cobbles, and 

pebbles of volcanic rock are the principal large 

constituents, but there also are large coral heads 

and coral fragments (fig. 12). At a few places, 

blocks of volcanic rock are as large as 2 m in diameter. In one such place only a few centimeters 

separate a block from its preerosion position.

This marine terrace was first recognized by 

Lobeck (1922, p. 367-368), who believed that its 

upper surface sloped seaward continuously from 

6 m down to and below sea level. Hubbard (1923, 

p. 97) considered the terrace, which he referred 

to the "Upper Desecheo Stage," to be in the range 

of 6-7.5 m. Both early estimates of elevation are 

appreciably less than the highest elevation, 12 m, 

of the upper terrace surface measured in this study.

Deposits at about 2-meter elevation

In a few places between the helicopter pad and 

Puerto Manglillo, beachrock (calcite-cemented beach 

sand) adheres to bedrock at elevations of about 2 m 

above sea level. The presence of beachrock, because 

it forms beneath the surface of beaches from about 

the low-tide mark to the normal limit of wave uprush, demonstrates that beaches once formed at 

these places; thus, at one time, sea level stood 

slightly higher than the present stand. These terrace remnants are too small to show at the scale 

of the geologic map (fig. 1).

The significance of the Isla Desecheo terrace deposits, evidence for sea stands higher than the 

present stand, regional relations, and possible ages 

are discussed in a following section.

COLLUVIUM

Poorly sorted colluvium forms a thin scree over 

most of the slopes of Isla Desecheo; thicker deposits are at the bases of slopes and in the valleys 

leading inland from the coast. These deposits are 

shown on the geologic map (fig. 1) only where they 

are believed to be 1 m or more thick.

The colluvium is composed largely of angular and 

subangular volcanic rock fragments that range in 

size from small pebbles to boulders and are set in 

a moderately abundant matrix of finer volcanic 

grains ranging from fine sand down to silt and clay.

Locally, the larger fragments are texturally intact 

and the matrix is sparse. Most colluvium is the result of natural fragmentation and mass movement. 

However, for many years the island was a practice 

target for naval and aerial bombardment, evidence 

of which can be found in the form of metal fragments and, occasionally, unexploded ordnance. Bedrock at some places was extensively fragmented 

and pulverized by explosions, most notably along 

the northwest-trending part of the central highland 

between elevations of 143 and 206 m (fig. 1). The 

amount of colluvial cover due to target practice is 

not known.

The colluvium within a few centimeters of the 

surface commonly is loose or friable, but below this 

zone it may be moderately to densely cemented. The 

cement apparently is largely phosphatic; a yellowishbrown specimen of rather fine densely cemented 

colluvium collected from a 1.5-m-high cut at the 

south edge of the helicopter pad yielded the analysis 

shown in table 1. No definitive mineralogical study 

was done on this sample but, on the basis of a comparison of refractive indices, Virginia S. Mclntyre 

(oral commun., 1968) suggested that some volcanic 

ash and a zeolite mineral may be present.

TABLE 1. Chemical analysis and 'molecular norm of cementedcolluvium sample from cut at south edge of helicopter pad, 

Isla Desecheo

[Percentage: rapid-rock analysis W167676 by P. Elmore, H. Smith, L. Artis, 

S. Botts, and J. Glenn, U.S. Geological Survey]

Chemical analysis

Oxide; cation Percent

Si02 ; Si f4 ......................................... 47.2; 51.6

Alo03 ; Al+3 ........................................ 15.9; 20.5

FeoQ3 ; Fe+3 ........................................ 4.4; 3.6

FeO; Fe+2........................................ .36; .33

MgO; Mg+2....................................... 1.7; 2.8

CaO; Ca+2 ........................................ 5.3; 6.2

NaoO; Na+ ......................................... 2.4; 5.0

KoO; K+ ........................................... 1.4; 2.0

HoO............................................................ 5.9

HoO+ ............................................................ 6.3

..................................... .44; .39

..................................... 7.6; 7.0

MnO; Mn+2....................................... .33; .33

C02 ; C+4 ..... .................................... .14; .20

____Total............................................... 99.4; 99.9

________________Molecular norm_________________ 

n,. , Total phosphate minerals,

_________Mlneral________________in percent_____

Calcite......................................... 0.4

"Hydroxylapatite".................... 4.8)

"Brushite".................................. 6.0 >15.2

"Variscite"---------.-...----------------... 4.4;

Ilmenite...................................... .7

Rutile.......................................... .1

Pyrolusite................................... .3

Orthoclase.................................. 10.0

Albite.......................................... 25.0

Hematite.................................... 3.6

Enstatite.................................... 5.6

Quartz......................................... 27.8

"Boehmite"................................ 11.3

Total............................... 100.0
14 GEOLOGY OF ISLA DESECHEO, PUERTO RICO

Study of the cation percentages in table 1 reveals that the phosphorus-calcium ratio is too high 

for the standard normative "apatite." The last section of table 1, therefore, contains a speculative 

molecular norm based on minerals known to be in 

rocks from similar geological environments in the 

Caribbean area (Altschuler, in Kaye, 1959b, p. 157- 

162; Kaye, 1959b, p. 162-164; Palache and others, 

1951, p. 660-661, 684-686, 704-705, 756-760, 878- 

887). In this norm, brushite [CaHP04 : 2H20] may 

include monetite [CaHP04 ], and hydroxylapatite [Ca5 (P04) 3OH] may include whitlockite 

[Ca3 (P04 )2]. The other unusual normative minerals are variscite [A1PCV 2H20] and boehmite 

[Al2 03 'HoO], the latter having been reported in 

surficial deposits in Puerto Rico by Hildebrand 

(1960).

In calculating this norm, half the calcium remaining after the calcite calculations was assumed 

to be contained in hydroxylapatite and the other 

half, in brushite. Phosphorus remaining after the 

brushite and hydroxylapatite calculations was assumed to be in variscite. The remainder of the norm 

calculation was essentially standard, varying chiefly 

in the substitution of boehmite for corundum.

The chief value of this speculative norm is that 

it shows that the sample analyzed contained on the 

order of 15 percent phosphate minerals and about 

15 percent oxides of aluminum and iron, for the 

most part probably hydrous, considering the relatively high percentage of combined water. Other 

combinations of the possible minerals yield similar 

or higher grouped percentages. The remainder of 

the norm tends to confirm the dacitic character of 

the volcanic rock from which the colluvium was 

derived.

Hubbard (1923, p. 100-111) stated that bird 

guano was the chief cementing agent in the marineterrace deposits. In this he was mistaken, for this 

investigation shows that the terrace cement is 

largely calcite. We conclude on the basis of the 

colluvium analysis in table 1 that the sample that 

led Hubbard to his conclusion came rather from 

the cemented phosphatic colluvium, the cement in 

which may have been derived from the bird guano, 

as he suggested.

At the heads of Puerto Canoas and Puerto de 

los Botes and at many places along the northwest 

and northeast coasts, colluvium rests on the higher 

marine terrace (fig. 10). Small ravines cut in the 

colluvium reveal crude stratification; beds about 0.5 

m thick dip seaward at angles as great as 25° and 

terminate abruptly against the much less steeply dipping marine terrace. Nowhere was the colluvium seen

to interfinger with the terrace deposits, and nowhere were terrace deposits seen resting on colluvium. Most, perhaps all, colluvium is therefore 

younger than the higher marine terrace. Perhaps 

preterrace colluvium was largely removed or reworked by the sea at the time of deposition of the 

terrace deposits, or perhaps a wetter climate prevailed prior to the deposition of the high terrace, 

promoting much denser vegetation than now occurs 

and thus inhibiting downslope mass movement.

The thickest colluvium exposed in a vertical cut 

is about 3 m. However, just northeast of Puerto 

Manglillo, a large mass of colluvium rests on the 

upper surface of the higher marine terrace, which 

is there about 6.5 m in elevation. If the seawardsloping bedrock surface uphill from the colluvium 

is projected beneath the colluvium and the surface 

of the marine terrace is projected landward beneath the colluvium at a positive angle of 5°, the 

surfaces intersect along a line about 15 m below 

the present upper surface of the colluvium. The 

colluvium, therefore, may be as much as 15 m thick 

in this area, and it may approach this thickness 

elsewhere as well (fig. 10). ' >

BEACH DEPOSITS

Beach deposits of waterworn sand and gravel 

are few; the largest are at the heads of coves and 

reentrants (fig. 10). Elsewhere, waterworn sand 

and gravel are only in scattered small patches. Only 

the larger beach deposits are shown on the geologic 

map (fig. 1).

The larger beach deposits are composed of medium 

and coarse sand consisting largely of volcanic rock 

and calcium carbonate fragments. The carbonate 

fraction is composed principally of first-cycle organic 

remains, but probably also includes reworked fragments from caved parts of the marine terraces. Pebbles and cobbles are minor constituents. In smaller 

patches of sand, pebbles and cobbles are abundant, 

and calcium carbonate grains are fairly rare.

At Puerto Manglillo, exposed beachrock is surrounded by uncemented sand of similar composition. 

This beachrock is believed related to the present 

beach and present sea level, unlike the somewhat 

higher nearby beachrock outcrops referred to earlier. 

This is the only exposure of recent beachrock seen 

on Isla Desecheo.

The sparseness of recent beach deposits on Isla 

Desecheo probably is due chiefly to the presence of 

deep water that is fairly close to shore all around 

the island and to the fine particle size of most of 

the materials available to form beaches. The island 

is surrounded by waters more than 20 fathoms
STRUCTURAL GEOLOGY 15

deep, at no place more than 700 m offshore (as at 

Corona del Norte) and closer than 200 m offshore 

southwest of El Muro. The volcanic rocks around 

the shores are slightly to moderately weathered and 

tend to disintegrate to particles which, for the most 

part, are finer than the sand sizes, as is indicated 

by the appreciable silt and clay fraction of the colluvium. In addition, the deep water just offshore 

has inhibited establishment of large shallow organic 

reefs which are known to be excellent suppliers of 

sand-size material to beaches elsewhere.

STRUCTURAL GEOLOGY

The island is divided by the west-trending Helicopter Pad fault into two areas with sharply contrasting fold types and trends. North of this fault, 

the folds are gentle. Most beds dip 20°-40° W., 

though a few beds dip to the southeast in the southeast corner of the area. This area north of the Helicopter Pad fault may be the faulted remnant of a 

broad southwest-plunging anticline.

Folds south of the Helicopter Pad fault are west 

trending, nearly isoclinal, and overturned to the 

north. One fold axis was observed near Puerto de 

los Botes (fig. 13), but elsewhere axes were located

only by reversals in the direction of tops of graded 

beds. Determinations of the directions of tops, although very sparse in some areas, suggest the 

presence of two anticlines and an intervening syncline (fig. 1). There is considerable variation in the 

inclination of axial planes. In the southwestern part 

of the island (fig. 1, section B-B'), near Puerto 

Canoas, beds dip steeply southward, but to the 

north the axial planes are more gently inclined; on 

the 140-m hill the folds are recumbent. No smallscale or drag folds were observed except near 

faults and in penecontemporaneous deformation 

structures.

The faults observed on Isla Desecheo are highangle faults that have slight to moderate amounts 

of vertical displacement. A low-angle detachment 

fault or decollement zone, not exposed on land, is 

inferred beneath the overturned folds of the southern part of the island.

The Helicopter Pad fault is the principal highangle fault on the island. The fault strikes N. 

75° E. from the western tip to the north-central part 

of the island where it merges with the Puerto 

Manglillo fault and turns, striking S. 75° E. to the 

coast. Nearshore topography suggests that it continues eastward under water, probably after slight

FIGURE 13. Overturned syncline in Jacaguas Group rocks near cable house.
16 GEOLOGY OF ISLA DESECHEO, PUERTO RICO

offset along the Northeast Coast fault. The Northwest Coast fault was not observed on land, but its 

trace can be seen in shallow water on aerial photographs. It seems to cut both the Puerto Manglillo 

and Northeast Coast faults and, therefore, probably 

also cuts the Helicopter Pad fault.

The vertical displacement downward on the south 

side of the Helicopter Pad fault is at least 130 m 

and could be much more. Considerable displacement 

probably also took place on the Puerto Manglillo 

fault. Displacement on other faults, with the possible 

exception of the Northwest Coast fault, is probably 

slight.

Nearly all the faults shown on the geologic map 

were either observed in outcrop at one or more 

places along the coast or are shown on aerial photographs in the shallow waters just offshore. Most 

faults dip more steeply than 60°. They commonly 

have a central gouge zone a few centimeters to 3 m 

thick with an outer zone of fractured rock locally 

showing drag. The total width of most fault zones 

is 2-10 m. The zone of fracturing along the Puerto

Manglillo fault at Puerto Manglillo is about 30 m 

wide. Drag indicates that the south side moved 

down relative to the north side (fig. 14). At the 

western tip of the island, where the Helicopter Pad 

and Northwest Coast faults converge, a zone of 

fracturing extends along the coast for about 200 m.

Joints are well developed, but cleavage is absent. 

Most joints are filled with veins of calcite and zeolites. At least two sets of joints are present at most 

localities (fig. 15). Orientations of 25 joints are 

markedly diverse. On the south coast the most conspicuous joints strike north-northeast and dip 

steeply, approximately perpendicular to fold axes.

The most striking structural feature of Isla 

Desecheo is the sharp contrast in fold type and 

trend on either side of the Helicopter Pad fault. 

This contrast, the variable inclination of axial 

planes, and the inverted structure on the 140-m hill 

strongly suggest that deformation in the southern 

part of the island was by gravity-gliding tectonics 

(de Sitter, 1956, p. 289-290). We believe the most 

probable interpretation of the observed structures

FIGURE 14. Puerto Manglillo fault, looking east at Puerto Manglillo. Central gouge zone is to left of man. Note drag of

beds behind man.
THE GREAT SOUTHERN PUERTO RICO FAULT ZONE 17

is that the rocks north and south of the Helicopter 

Pad fault represent different tectonic levels that 

are separated by a low-angle detachment fault or 

decollement zone. The isoclinally folded beds south 

of the Helicopter Pad fault are probably a remnant 

of a gravity-glide sheet that moved northward and 

folded above essentially undeformed strata. Subsequent movement on the Helicopter Pad fault 

raised and gently folded the northern part of the 

island; the strongly deformed upper sheet was removed by erosion in the north but was preserved 

in the south.

The age of the major deformation on Isla 

Desecheo is middle Eocene or younger, as indicated 

by the middle Eocene age of the rocks on the island. 

Elsewhere in Puerto Rico, gravity-glide structures 

in middle Eocene and older rocks are unconformably overlain by essentially undeformed early Oligocene and younger beds (Glover, 1971). By 

extrapolation, the age of the deformation on Isla 

Desecheo probably is middle Eocene to early 

Oligocene.

THE GREAT SOUTHERN PUERTO RICO 

FAULT ZONE

In south-central Puerto Rico, gravity-glide structures have been described by Glover and Mattson 

(1960) and Glover (1971). These structures are 

restricted to rocks of the Jacaquas Group, which 

are largely restricted to the proximity of a major 

fault zone named the "great southern Puerto Rico 

fault zone" by Glover (1971). Mapping (Briggs 

and Akers, 1965; Mattson, 1960, 1967a, b; Mclntyre 

and others, 1970; Glover, 1971) indicates that the 

fault zone (fig. 16) trends about N. 70° W. from 

the central part of the south coast to the northern 

part of the west coast of Puerto Rico. Its continuation at least as far west as Isla Desecheo is suggested by submarine topography (fig. 2).

The great southern Puerto Rico fault zone divides 

areas of strongly contrasting structure and stratigraphy. In southwestern Puerto Rico (Mattson, 1960), 

a sedimentary-volcanic sequence largely no older than

|tv^r- -w \ V^%^^T^r%^^%*^^y>'^^^.^v *: * ., *

-:^\>s*- "\.rr »?*> v' i, V"n^^<^%i-^ ^ \""l.^' *J«^ ^'-- '*

^% ^vCV^$^*^P^ 

^S^v ^ %^?*f :^ '^%v -r*^x< -^ V-^*

'- .*., *S ' -.- :*

FIGURE 15. Joints filled with calcite and zeolites, just northwest of Puerto de los Botes.
18 GEOLOGY OF ISLA DESECHEO, PUERTO RICO

Campanian (Late Cretaceous) in age rests on a basement composed chiefly of serpentinite. North of the 

fault zone, a much thicker sedimentary-volcanic 

sequence ranges down to at least the Albian (Early 

Cretaceous) (Douglas, 1961), and no basement is 

exposed. The upper part of the Cretaceous section is 

correlative with rocks in southwestern Puerto Rico, 

but the lithofacies are quite different (Glover, 1971). 

The structure of southwestern Puerto Rico is dominated by west- and northwest-trending folds, asymmetric or overturned toward the south-southwest. 

North of the fault zone, folding is quite gentle, and 

fold axes trend east, locally northeast (Briggs and 

Akers, 1965).

Details of the great southern Puerto Rico fault 

zone in south-central Puerto Rico have been described by Glover and Mattson (1960) and by 

Glover (1971). The fault zone is a west-northwesttrending belt of horsts and graben. Within the fault 

zone, rocks of the Jacaguas Group are highly deformed by folding and faulting. In addition to the 

high-angle faults, the Jacaguas Group is cut by 

low-angle imbricate thrust faults. Beds in some of the 

thrust plates show tight, locally almost isoclinal, 

folds. A giant slide breccia within the fault zone, 

the Guayabal megabreccia, contains blocks from the

Jacaguas Group as well as blocks from nearly all 

older formations. The complex structures in the 

Jacaguas Group were formed by submarine sliding 

of recently deposited sediments. The sliding was 

probably initiated by both vertical and left-lateral 

movements along the great southern Puerto Rico 

fault zone. Various lines of evidence show that the 

sliding was directed to the north-northeast. The 

great southern Puerto Rico fault zone may have 

been intermittently active since as far back as the 

Early Cretaceous, with a major episode of movement and most of the gravity gliding concentrated 

in the interval between the middle Eocene and 

early Oligocene.

Just west of the area described by Glover (1971), 

rocks of the Jacaguas Group are generally more 

massive and, although they are in part strongly 

folded, no gravity-glide structures have been described (Mattson, 1967a, b). In northwestern 

Puerto Rico, the Rio Culebrinas Formation is in 

part strongly folded (Mclntyre and others, 1970, 

fig. 3), and J. M. Aaron (written commun., 1969) 

indicated that gravity-glide structures are present 

in three places and are of the same type and scale 

as those on Isla Desecheo.

Elsewhere in the northern Caribbean region, de-

&REAT SO UTH ERW f UERTO JRIC" -w ,

 " ' '

FIGURE 16. Major fractures of the Greater Antilles. Patterned areas show approximate limits of the Cayman Trough and

Puerto Rico Trench.
QUATERNARY STILLSTANDS OF THE SEA 19

scriptions of several areas suggest a relationship 

between submarine sliding and fault zones. Nagle 

(1966, 1968) described a giant breccia or olistostrome near Puerto Plata, on the north coast of the 

Dominican Republic (fig. 16). The breccia has an 

outcrop area of about 90 sq km and was emplaced 

during the middle Eocene as a great submarine 

slide. The geology of the surrounding region is not 

yet known in detail. South of the slide deposit 

large faults trend N. 70° W. (Nagle, 1966, p. 139- 

140), and, to the north, a major submarine fault 

may separate the volcanic and plutonic rocks of 

Hispaniola from the Bahama Platform.

In Cuba, Furrazola-Bermudez and others (1964) 

and Khudoley (1967a) have described a major 

lineament, the Las Villas fault, known to extend 

about 800 km along the north coast of Cuba. The 

Las Villas fault marks an abrupt facies change. 

To the south are large ultramafic masses and Cretaceous eugeosynclinal deposits, whereas to the 

north are carbonate deposits of equivalent age. 

Both thrust faults and high-angle faults have been 

described along the Las Villas lineament, but there 

is little agreement among workers as to which type 

of fault predominates (Hatten, 1967; Khudoley, 

1967a, b). Thayer and Guild (1947) have described 

two areas in eastern Cuba, near Central Senado and 

Gibara (fig. 16), where large low-angle thrust 

faults are associated with giant submarine breccias. 

In central Cuba, Georges Pardo (unpub. data, 1965) 

has proposed large-scale northward gravity gliding 

followed by left-lateral wrench faulting. The Las 

Villas fault may have been active intermittently 

since the Early Cretaceous (Furrazola-Bermudez 

and others, 1964, p. 197), and major movements 

and thrusting occurred during the middle Eocene 

(Thayer and Guild, 1947, p. 929; Hatten, 1967, p. 

785).

The submarine-slide deposits and gravity-glide 

structures of Puerto Rico, Hispaniola, and Cuba are 

remarkably similar in age and in their position 

along boundaries between contrasting geologic provinces. Although they define a roughly straight lineament nearly 2,000 km long extending N. 70° W. 

through the Greater Antilles (fig. 16), the apparent 

linearity may well be fortuitous. South of Puerto 

Rico, the great southern Puerto Rico fault zone 

abuts a major east-west fault and may be a secondorder shear subordinate to first-order east-west 

shears (Glover, 1971). North of Hispaniola, the 

Las Villas-great southern Puerto Rico fault lineament crosses the lineament defined by the north

walls of the Cayman Trough and the Puerto Rico 

Trench at a sharp angle. The nature of the junction of these two major lineaments still remains to 

be investigated by marine geophysicists.

QUATERNARY STILLSTANDS OF THE SEA

On Isla Desecheo there is widespread evidence 

for two stillstands of the sea higher than present 

sea level. Hubbard's terms (1923, p. 97), the "Upper Desecheo Stage" and the "Lower Desecheo 

Stage," no longer can be used completely because 

the term "Stage" is now restricted (American 

Commission on Stratigraphic Nomenclature, 1961, 

art. 31 [a]). In this report no formal names are 

applied.

UPPER DESECHEO STILLSTAND

The marine terrace deposits which constitute the 

chief evidence for this stillstand are described in a 

preceding section. These deposits rest unconformably upon what was apparently a solid rock apron 

having only moderate relief and completely surrounding the island. Quite plainly this apron was 

formed by wave attack as sea level rose gradually 

relative to the land, for the apron rises landward 

at slopes commonly approaching 5°. The highest 

measured level of this terrace basement is about 

11 m higher than present sea level, where the upper surface of the terrace deposit is at 12 m. To 

have eroded materials at the 11-m level, the sea 

must have stood somewhat higher, probably in the 

range of 12-13 m. This estimate is considerably 

higher than the range of 6-7.5 m suggested by 

Hubbard (1923, p. 97).

Sea level probably remained at 12-13 m for some 

time, for the associated terrace deposits contain 

appreciable quantities of calcium carbonate detritus 

that must have been derived from nearby organic 

reefs. Time must have been sufficient for the formation and erosion of reefs on the rather shallow 

wave-cut apron in order to supply the sand in the 

marine terraces. Later erosion has removed all 

vestiges of these organic reefs.

The retreat of the sea from this 12-13-m stillstand was probably rather rapid, because the terrace deposits remain. Had the retreat been slow, 

most of these deposits probably would have been 

removed by wave-reworking and surface runoff. 

Rapid accumulation of colluvium may have rendered 

the marine-terrace deposits closest to the island 

more resistant to subaerial erosion.
20 GEOLOGY OF ISLA DESECHEO, PUERTO RICO

LOWER DESECHEO STILLSTAND

The chief evidence for a stillstand of the sea at 

about 2 m is the broad irregular wave-cut platform 

that surrounds much of Isla Desecheo (frontispiece). At Corona del Norte and on the northwest 

coast, this platform extends seaward as far as 100 

m (fig. 17). On the geologic map (fig. 1) the seaward edge of the platform is shown by a dotted 

line partly surrounding the island. The shoreline 

shown represents the approximate limit of the 

highest tides. Local relief on the platform is about 

2 m, so many rocks are above sea level most of the 

time, and unusually low tides probably bare much 

of the area between shoreline and dotted line. Tides 

in this area commonly range from about 0.3 to 0.8 

m, so the elevation of this wave-cut platform may be 

considered as approximately zero, or present sea 

level.

FIGURE 17. View of northwest coast from near helicopter 

pad. Lower Desecheo stillstand platform extends seaward 

as far as 100 m to zone of breaking waves on left. Note 

local relief on platform. Resistant marine-terrace deposits 

crop out in right center (at Puerto Manglillo) and in right 

foreground. The suggestion of two terrace levels at Puerto 

Manglillo is due to the seaward dip of marine-terrace deposits. On the horizon in right center, land to right of break 

in slope is underlain by volcanic bedrock; land to left underlain by colluvium.

Unlike the sloping apron of the 12- to 13-m stillstand, this lower platform is nearly horizontal. The 

lack of significant seaward slope indicates that the 

platform was shaped during an extensive stillstand, 

not during a period of gradually rising sea level.

At present the sea is eroding the edge of the platform, and water of only moderate energy flows over 

the shallows to the shore. Plainly then, the platform 

cannot have been formed during the present regime. 

For the platform to have been cut, sea level must 

have been from 1 to 2 m higher than present sea 

level, enabling energetic surf to travel the whole 

width of the platform.

This estimate of sea-level difference from the 

present stand coincides with elevations of floors of 

some sea caves and the 2-m beachrock and is of the 

same order as the range estimated by Hubbard 

(1923, p. 97), 0.6-1.5 m.

REGIONAL RELATIONS AND AGE

Kaye (1959a, p. 131), Williams (1965, p. 162- 

182), and Monroe (1968) cited evidence for raised 

marine terraces in Puerto Rico. Evidence for a stillstand at 1.6-2 m is found at many places around 

the Puerto Rican coast, and at least five higher 

stillstands are suggested, including one in the range 

from 12 to 15 m.

On Isla Desecheo, no evidence gives strong support to stillstands of the sea higher than 12-13 m. 

There is no firm evidence for stillstands between 

the upper and lower stillstands, and bathymetry in 

the area is too sparse to allow meaningful comment 

on possible submarine terraces.

Hubbard (1923, p. 103) considered the upper 

Desecheo stillstand as "post-Pleistocene" in age. 

Kaye's (1959a, p. 135) analysis, however, related 

the Puerto Rican equivalent of the upper Desecheo 

stillstand to either upper Yarmouth or lower Sangamon Interglaciations of the Pleistocene. Williams 

(1965, p. 176) recognized no equivalent to the upper Desecheo stillstand, but he (p. 163) considered 

all stillstands higher than the 2-m level as Pleistocene in age.

No new evidence on the age of the upper Desecheo 

stillstand was found during the investigations reported herein. We can say only that the work of 

Kaye and Williams strongly suggests that the upper 

Desecheo stillstand took place during the Pleistocene Epoch, rather than during the Holocene Epoch 

as was suggested by Hubbard.

The lower Desecheo stillstand was believed Holocene in age, and perhaps historic, by Hubbard 

(1923, p. 103) ; Williams (1965, p. 163) also concluded that the 2-m stillstand took place in Holocene time. Kaye, on the other hand, reasoned that
REFERENCES CITED 21

this stand of the sea took place in the Pleistocene 

Epoch, in a Wisconsin interstade or perhaps in the 

latter part of the Sangamon Interglaciation.

Probably the strongest evidence for the age of 

this 2-m stand is the carbon-14 age determination 

made on a sample of carbonized wood collected by 

Briggs from a swamp deposit in north-central 

Puerto Rico (71,090 m N.; 142,242 m E.; Puerto 

Rico coordinate system). The relations of this deposit to the present shoreline and sea level indicate 

that the swamp originated as a coastal lagoon during a sea-level stand about 2 m higher than the 

present. The age reported was 3,300 ± 300 years 

before the present (USGS lab. No. W-1036), thus 

Holocene.

Kaye (1959a, p. 131-135) and, for the most part, 

Williams (1965, p. 162-163) ascribed the terraces 

to the advance and retreat of the sea due to glaciation. However, Williams (p. 171-175) considered 

the 2-m stand to have been caused by tectonic 

elevation of the land which was accompanied by 

tilting, the uplift being greater to the west than 

to the east. We concur with Williams that the 2-m 

terrace results from tectonic uplift, for the reason 

given in the following paragraph, but the concept 

of west-to-east tilting we consider unproven. Briggs 

has seen raised beachrock, fossiliferous terrace deposits, and other features at elevations commensurate with a stand at about 2 m in the Fajardo area 

of extreme eastern Puerto Rico, along the central 

and western north coast of Puerto Rico, and on Isla 

Desecheo. Therefore, it appears that any regional 

tilting from west to east after the 2-m stillstand 

cannot have exceeded 1 m in the 200-km distance 

that separates Isla Desecheo from extreme eastern 

Puerto Rico.

The evidence for the 3,300-year age of the 2-m 

stand of the sea appears to conflict with the consensus of most investigators of late Quaternary sea 

levels that about 3,300 years ago, sea level stood 

about 2 m lower, not higher, than it does at present 

(Curray and others, 1970, p. 1878, fig. 6). Thus, if 

the age assigned to the 2-m stand in Puerto Rico 

is correct, about 4 m of tectonic uplift in the past 

3,300 years is indicated for northern Puerto Rico 

and Isla Descheo. This conclusion appears reasonable in the light of the present seismicity of the 

region (Sykes and Ewing, 1965).

Holocene tectonic activity also would have had an 

effect upon the present levels of other terraces. If 

the elevation of the terrace of the lower Desecheo 

stillstand is controlled tectonically, then it follows

that the upper Desecheo stillstand level, and other 

earlier terrace levels in this area, no longer can be 

considered as having been controlled solely by 

glacioeustatic rises and falls of sea level.

REFERENCES CITED

American Commission on Stratigraphic Nomenclature, 1961, 

Code of Stratigraphic nomenclature: Am. Assoc. Petroleum Geologists Bull., v. 45, no. 5, p. 645-665.

Briggs, R. P., and Akers, J. P., 1965, Hydrogeologic map of 

Puerto Rico and adjacent islands: U.S. Geol. Survey 

Hydrol. Inv. Atlas HA-197.

Curray, J. R., Shepard, F. P., and Veeh, H. H., 1970, Late 

Quaternary sea-level studies in Micronesia: CARMARSEL expedition: Geol. Soc. America Bull., v. 81, no. 7, 

p. 1865-1880.

Douglas, R. C., 1961, Orbitolinas from Caribbean islands: 

Jour. Paleontology, v. 35, no. 3, p. 475-479.

Fisher, R. V., 1961, Proposed classification of volcaniclastic 

sediments and rocks: Geol. Soc. America Bull., v. 72, no. 

9, p. 1409-1414.

Furrazola-Bermudez, Gustavo, and others, 1964, Geologia de 

Cuba: Havana, Inst. Cubano Recursos Minerales, 239 p.

Glover, Lynn, III, 1971, Geology of the Coamo area, Puerto 

Rico, and its relation to the volcanic arc-trench association : U.S. Geol. Survey Prof. Paper 636, 102 p.

Glover, Lynn, III, and Mattson, P. H., 1960, Successive thrust 

and transcurrent faulting during the early Tertiary in 

south-central Puerto Rico, in Short papers in the geological sciences: U.S. Geol. Survey Prof. Paper 400-B, 

p. B357, B363-B365.

_____1967, The Jacaguas Group in central-southern Puerto 

Rico, in Cohee, G. V., West, W. S., and Wilkie, L. C., 

Changes in Stratigraphic nomenclature by the U.S. Geological Survey, 1966: U.S. Geol. Survey Bull. 1254-A, 

p. A29-A39.

Goddard, E. N., chm., and others, 1948, Rock-color chart: 

Washington, D.C., Natl. Research Council (repub. by 

Geol. Soc. America, 1951), 6 p.

Hatten, C. W., 1967, Principal features of Cuban geology: 

discussion [of paper by K. M. Khudoley, 1967]: Am. 

Assoc. Petroleum Geologists Bull., v. 51, no. 5, p. 780-789.

Hildebrand, F. A., 1960, Occurrences of bauxitic clay in the 

karst area of north-central Puerto Rico, in Short papers 

in the geological sciences: U.S. Geol. Survey Prof. Paper 

400-B, p. B368-B371.

Hubbard, Bela, 1923, The geology of the Lares district, Porto 

Rico: New York Acad. Sci., Scientific Survey of Porto 

Rico and the Virgin Islands, v. 2, pt. 1, p. 1-115.

Kaye, C. A., 1959a, Shoreline features and Quaternary shoreline changes, Puerto Rico: U.S. Geol. Survey Prof. Paper 

317-B, p. 49-140.

_____1959b, Geology of Isla Mona, Puerto Rico, and notes 

on age of Mona Passage: U.S. Geol. Survey Prof. Paper 

317-C, p. 141-178.
22 GEOLOGY OF ISLA DESECHEO, PUERTO RICO

Khudoley, K. M., 1967a, Principal features of Cuban geology: 

Am. Assoc. Petroleum Geologists Bull., v. 51, no. 5, p. 

668-677.

_____1967b, Reply [to article by C. W. Hatten on "Principal features of Cuban geology" discussion, 1967] : Am. 

Assoc. Petroleum Geologists Bull., v. 51, no. 5, p. 789-791.

Lobeck, A. K., 1922, The physiography of Porto Rico: New 

York Acad. Sci., Scientific Survey of Porto Rico and the 

Virgin Islands, v. 1, pt. 4, p. 301-379.

Mclntyre, D. H., Aaron, J. M., and Tobisch, 0. T., 1970, 

Cretaceous and lower Tertiary stratigraphy in northwestern Puerto Rico: U.S. Geol. Survey Bull. 1294-D, 16 p.

Mattson, P. H., 1960, Geology of the Mayagiiez area, Puerto 

Rico: Geol. Soc. America Bull., v. 71, no. 3, p. 319-361.

 1967a, Geologic map of the Adjuntas quadrangle, 

Puerto Rico: U.S. Geol. Survey Misc. Geol. Inv. Map 1-519. 

-1967b, Geologic map of the Jayuya quadrangle, Puerto

Rico: U.S. Geol. Survey Misc. Geol. Inv. Map 1-520. 

Mitchell, R. C., 1954, A survey of the geology of Puerto Rico:

Puerto Rico Univ., Agr. Expt. Sta. Tech. Paper 13, 167 p. 

Monroe, W. H., 1968, High-level Quaternary beach deposits in

northwestern Puerto Rico, in Geological Survey Research

1968: U.S. Geol. Survey Prof. Paper 600-C, p. C140-C143.

Nagle, F., Jr., 1966, Geology of the Puerto Plata area, 

Dominican Republic: Princeton, N.J., Princeton Univ., 

unpub. Ph.D. dissert., 171 p.

_____1968, Chaotic sedimentation in north-central Dominican Republic [abs.] : Geol. Soc. America Spec. Paper 115, 

p. 280-281.

Northrop, John, 1954, Bathymetry of the Puerto Rico Trench: 

Am. Geophys. Union Trans., v. 35, no. 2, p. 221-225.

Palache, Charles, Berman, Harry, and Frondel, Clifford, 1951, 

The system of mineralogy of James Dwight Dana and 

Edward Salisbury Dana, Yale University, 1837-1892 

[7th ed.] : New York, John Wiley and Sons, v. 2, 1123 p.

Sitter, L. U. de, 1956, Structural geology: New York, McGrawHill Book Co., 552 p.

Sykes, L. R., and Ewing, Maurice, 1965, The seismicity of the 

Caribbean region: Jour. Geophys. Research, v. 70, no. 20, 

p. 5065-5074.

Thayer, T. P., and Guild, P. W., 1947, Thrust faults and 

related structures in eastern Cuba: Am. Geophys. Union 

Trans., v. 28, no. 6, 919-930.

Williams, R. S., Jr., 1965, Geomorphology of a portion of the 

northern coastal plain of Puerto Rico: University Park, 

Pa., Pennsylvania State Univ., unpub. Ph.D. dissert., 

191 p.

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