Cretaceous and Lower 

Tertiary Stratigraphy in 

West-Central Puerto Rico

GEOLOGICAL SURVEY BULLETIN 1254-B

Prepared in cooperation with the 

Commonwealth of Puerto Rico 

Economic Development Administration, 

Industrial Research Department

Cretaceous and Lower 

Tertiary Stratigraphy in 

West-Central Puerto Rico

By PETER H. MATTSON

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1254-B

Prepared in cooperation with the 

Commonwealth of Puerto Rico 

Economic Development Administration, 

Industrial Research Department

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1967
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

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

Washington, D.C. 20402 - Price 20 cents (paper cover)
CONTENTS

Page

Abstract__________________________________________________ Bl

Introduction. _____________________________________________________ 1

The setting_____________________________________ 3

Cretaceous System_.____-__.__________________________ ________ 7

Jayuya Tuff__________ _______________________ 7

Robles Formation______-_______________-_______________-______ 8

Vista Alegre Formation_______________________________________ 11

Tetudn Formation.____________________________________________ 12

Malo Breccia.________________________________________________ 12

Cotorra Tuff_______________________________ 13

Achiote Conglomerate________________________________________ 13

Maravillas Formation..-._____________________________________ 16

Coamo Formation.____________________________________________ 18

Yauco Mudstone______________________________________ 20

Lago Garzas Formation_________________________..______________ 22

Tertiary System_____________________________________________ 24

Rio Prieto Formation____________________________________ 24

Monserrate Formation.________________________________________ 28

An6n Formation____________________________________________ 29

Stratigraphic summary____________________________________________ 32

ReferenceS-_______________________________________________________ 34

ILLUSTRATIONS

Page 

FIGURE 1. Map showing location of west-central Puerto Rico___________ B2

2. Simplified geologic map of west-central Puerto Rico.________ 4

TABLES

TABLE 1. Fossil collections in the Maravillas Formation, west-central

Puerto Rico____________________________ B18 

2. Fossil collections from the Rfo Prieto Formation.________ 27

m

CONTRIBUTIONS TO STRATIGRAPHY

CRETACEOUS AND LOWER TERTIARY STRATIGRAPHY 

IN WEST-CENTRAL PUERTO RICO

By PETER H. MATTSON

ABSTRACT

Volcanic rocks of Cretaceous and early Tertiary age underlie most of mountainous west-central Puerto Rico.

About 6,000 m of thin-bedded tuffaceous sandstone, siltstone, and mudstone 

and thick-bedded and massive lapilli tuff, coarse tuff, volcanic breccia, volcanic 

conglomerate, and lava of chiefly andesitic and basaltic compositions constitute 

the Cretaceous section in the eastern half of the area; in the western half the 

upper part of the section has been removed by erosion or faulted out. The oldest 

Cretaceous rocks exposed are probably Early Cretaceous, Albian, in age; the 

youngest are latest Cretaceous, Maestrichtian, in age. Except for possible local 

unconformities in the Albian interval and a probable disconformity or moderate 

unconformity in the Santonian (possibly Coniacian) interval, the record shows 

that deposition of fragmental volcanic rocks, accompanied by sporadic extrusion 

of lavas, was nearly continuous from the Albian to the Maestrichtian.

A profound unconformity separates the lower Tertiary sequence from the 

Cretaceous rocks; no strata yielded fossil collections referred to the Paleocene or 

lower Eocene, and middle Eocene rocks rest unconformably on rocks ranging 

from Albian to probably Oampanian in age, as well as on some plutonic rocks. 

About 3,000 m of middle and perhaps upper Eocene volcanic and epiclastic 

rocks are exposed. The Eocene volcanic rocks are chiefly dacite tuffs and lavas, 

as opposed to the andesites and basalts that predominate in the Cretaceous 

sequence.

Widespread intrusive igneous activity occurred from latest Cretaceous to 

Eocene time, and resulting metamorphism affected strata of all volcanic units 

from place to place. Part of the Utuado batholith underlies much of the northern part of west-central Puerto Rico. In addition, large faults have broken the 

area into a complex geological pattern.

INTRODUCTION

As used in this report, west-central Puerto Eico comprises the 

Adjuntas and Jayuya quadrangles (fig. 1). The chief purpose of the 

report is to describe in some detail the volcanic stratigraphy of these

Bl
B2 CONTRIBUTIONS TO STRATIGRAPHY

ATLANTIC 0 C £ A N

PUERTO RICO

CLP

riayoguez

SS

MS

Y

H

PE

F

mPO

CIA

0

COR

B

COA

COM

~18"00'

CARIBBEAN S £ A

40 MILES 

_J

40 KILOMETERS 

\

1. Map showing location of west-central Puerto Rico. The following 

7^-minute quadrangles referred to in the text are shown by abbreviations: 

CLP, Central La Plata; SS, San Sebastian; F, Florida; CIA, Ciales; COR, 

Corozal; MG, Monte Guilarte; A, Adjuntas; 3, Jayuya; O, Orocovis; B, 

Barranquitas; COM, Comerio; Y, Yauco; PE, Penuelas; PO, Ponce; COA, 

Coamo.

quadrangles, but reference also will be made to features in adjacent 

areas, and units in west-central Puerto Rico will be correlated with 

previously defined stratigraphic units elsewhere in Puerto Rico.

The geologic map (fig. 2) of this report is a simplified version of 

separately 1:20,000 scale geologic maps of the Adjuntas and Jayuya 

quadrangles (Mattson, 1967a and 1967b). Fossil and other data are 

keyed to localities shown on these maps, and most places referred to in 

this text are shown only on the 1:20,000 maps.

Places described in the text are located by means of the Puerto Rico 

Meter Grid System coordinates, tick marks for which are shown along 

the borders of the 1:20,000 scale geologic maps mentioned above. The 

east coordinate is given first and the entire coordinate is enclosed in 

parentheses for clarity, as (143,890 m E.; 34,Y80 m N.). Location 

generally are accurate to the nearest 10 meters.

Spanish words, other than place names, used in the text are: barrio, 

a political subdivision of a municipality; hacienda, estate, large farm, 

or plantation; lago, lake; quebrada, stream, rip, river.

The only two towns in the area are Adjuntas, 77 km southwest of 

San Juan and 20 km north of Ponce, and Jayuya, 16 km east-northeast 

of Adjuntas.
STRATIGRAPHY IN WEST^CENTRAL PUERTO RICO B3

The geology of west-central Puerto Rico is being studied as a program of geologic mapping and investigation of the mineral resources 

of Puerto Eico conducted by the U.S. Geological Survey in cooperation 

with the Economic Development Administration, Commonwealth of 

Puerto Rico. Field mapping for the report occupied 38 months in 

1960, 1961, 1962, and 1963.

Previous geologic studies in the area were by Kaye (1957), Kaye 

and Dunlap (1960), Moneymaker (1947a, b), and Weaver (1958).

Fossil identifications by W. S. Cole, E. A. Pessagno, Jr., Otto 

Renz, K. N. Sachs, and N. F. Sohl are gratefully acknowledged. I am 

also grateful for permission to study unpublished reconnaissance and 

detailed geologic maps of A. D. Fraser, Inc., San Juan, P.R., and 

Ponce Mining Co., Utuado, P. R.; for stimulating discussions with 

W. R. Bergey, R. Reynoso, J. Sugden, J. D. Weaver, and G. K. Williams; and for access to unpublished geologic maps of the Garzas 

hydroelectric tunnel by B. C. Moneymaker.

In descriptions of stratified rocks, bedding is described as thin 

if less than 5 cm, medium if between 5 and 50 cm, thick if between 

50 and 100 cm, and massive if larger than 100 cm. Laminations or 

laminae are planar zones less than 1 cm thick distinguished from 

adjacent zones by differences in texture, grain size, or composition 

but are not necessarily parallel to bedding. Stratified rock generally 

breaks along bedding planes but not along laminae.

Clastic and pyroclastic rocks are named in terms of their predominant grain size, not the maximum size found. If significant, the maximum size may be shown by an adjective, as "conglomeratic" or "pebbly 

mudstone."

The adjectives "clastic" and "epiclastic" as used herein indicate 

that such materials have been derived from another rock or another 

area by erosion and transportation. They do not necessarily imply 

that the source materials were lithified or are more than slightly 

older than the level in which the reworked material is deposited.

THE SETTING

West-central Puerto Rico is very mountainous. Nine of the 10 highest peaks in Puerto Rico are in the eastern half of the report area, the 

Jayuya quadrangle; the highest elevation in Puerto Rico Cerro de 

Punta (1,338 m) is in the east-central part of the report area; 

the lowest elevation is about 130 m at the southeast corner. The average local relief (the maximum difference in elevation within 3.2-km 

circles centered on a 1%-minute grid) is about 420 m.
B4 CONTRIBUTIONS TO STRATIGRAPHY
EXPLANAT

Intrusive rocks and

undivided metamorphic

rocks

Maravillas Formation

FIGURE 2. Simplified geologic map of wes
B6 CONTRIBUTIONS TO STRATIGRAPHY

The west-trending Cordillera Central crosses the southern part of 

the area and separates north-flowing tributaries of two of the master 

streams of Puerto Rico, the Rio Grande de Arecibo and Rio Grande 

do Manati, from the headwaters of the south-flowing Rfos Tallaboa, 

Canas, Portugues, San Patricio, Cerrillos, Anon, and Guayo, and 

minor streams. Rugged topography in the southwestern and eastern 

parts of the area is developed on bedded and massive volcanic rocks 

and small bodies of intrusive rock. In the southwest this topography 

is dominated by long linear valleys that have formed along large northwest-trending faults; in the east the topography has a less well defined 

northerly trend. In the extreme southwestern corner of the area and 

on the west side of Cerro de Punta, gently rolling upland on and near 

the Cordillera Central, at elevations of 750-900 m, is perhaps due to 

rapid erosion of soft volcanic rocks in a tropical climate with heavy 

rainfall. The north-central to northwestern part of the area contains 

only moderately rugged terrain underlain by easily weathered plutonic rocks.

West-central Puerto Rico contains a complex sequence of volcanic 

and sedimentary rocks cut by large bodies of igneous rock. There has 

been some metamorphism near the borders of the igneous intrusions. 

The stratified rocks range in age from Albian(?) to middle Eocene. 

The composite maximum thickness of Creaceous strata is about 6,000 

m, and the composite maximum stratigraphic thickness of lower Tertiary rocks is about 3,000 m. The rocks are folded and highly faulted, 

and Eocene strata rest unconformably on strata ranging in age from 

Albain to Campanian, as well as on plutonic rocks. Quartz monzonite, 

granodiorite, quartz diorite, and diorite were emplaced in Oampanian 

time or later, before deposition of the Eocene sequence, and form the 

central part of the Utuado batholith. A dacite stock and small stocks 

of quartz diorite and diorite cut the Eocene sequence. Copper, gold, 

iron, molybdenum, and manganese minerals occur in pods, small veins, 

or disseminated grains; two "porphyry copper" bodies and other copper occurrences are found in areas of hydrothermally altered and 

mineralized rock.

Fresh rock is generally well indurated. Except on the steepest slopes 

and in some stream beds, all rocks have been deeply weathered and 

are mantled by thick saprolitic or lateritic clays. Alluvial flood-plain 

deposits, alluvial fans, and terrace deposits occur in the major river 

valleys.
STRATIGRAPHY IN WEST-CENTRAL PUERTO RICO B7 

CRETACEOUS SYSTEM 

JAYTTYA TUFF

A sequence of pyroclastic volcanic rocks with some lava and finegrained sedimentary rocks is herein named the Jayuya Tuff, for exposures in the Jayuya quadrangle north and west of the town of 

Jayuya. The type locality is on a trail north from route 531 up the 

slope of Cerro Morales, from (132,730 m E.; 44,740 m N.) to (132,685 

m E.; 45,190 m N.). Exposed at the type locality are massive darkgreen coarse-grained crystal tuff and lapilli tuff with several lenses 

5-10 m thick of greenish-gray laminated medium-bedded silicified 

mudstone and thin-bedded, graded tuffaceous sandstone. The tuff contains abundant pyroxene and feldspar, lapilli of basalt, and sparse 

larger crystals.

Mafic volcanic rocks in the Jayuya Tuff occur along the north and 

east sides of the batholith, and the formation is also present south 

of the batholith in fault blocks between Hacienda Jauca and Hacienda 

El Banco. In this area the rocks are highly pyritized, epiodotized, and 

in part hydrothermally altered. Original textures that are preserved 

suggest that these deeply weathered gray and grayish-green rocks 

were feldspathic tuff and lava. In most places within the contact 

aureole of the intrusion, the rocks are metamorphosed to feldsparhornblende hornfels, gneiss, and schist; scattered exposures of less 

metamorphosed rocks occur in many of these areas and also to the 

south near Cerro Saliente. The less metamorphosed rocks are commonly massive coarse-grained dark pyroxene-rich crystal tuffs; pyroxene-rich lapilli tuffs; and uncommon basalt lavas. In some places, 

particularly north and northwest of Jayuya, feldspar predominates 

over pyroxene in the tuffs. The rocks generally form a knobby weathered surface decomposed to brownish-red soil and saprolite.

These mafic rocks are lithologically indistinguishable from rocks of 

the Cotorra Tuff and mafic volcanic rock lenses in the Vista Alegre 

Formation. They are also similar in lithology and stratigraphic position to pyroxene-feldspar crystal tuffs mapped to the east in Orocovis 

quadrangle as a lenticular unit at the top of pre-Eobles rocks (Briggs, 

1967).

East of Cerro Saliente, about 65 m of green to maroon medium- to 

thick-bedded laminated siltstone and mudstone, partly silicified, occurs as a lens between the mafic and feldspathic volcanic rocks. Maroon 

calcareous siltstone, partly bleached and silicified, also forms a small 

lens in the mafic volcanic rocks about 400 m southward at the north 

end of the small alluvial deposit in the Eio Saliente.
B8 CONTRIBUTIONS TO STRATIGRAPHY

In the valley of the Rio Saliente, feldspathic volcanic rocks lie 

above the mafic volcanic rocks. These rocks are green and gray coarsegrained feldspar crystal tuffs and feldspar porphyry lavas, metamorphosed to hornfels or gneiss in almost all exposures. Small bodies of 

mafic volcanic rocks occur within the predominantly feldspathic terrain, and vice versa.

The lower part of the Jayuya Tuff is cut out by the Utuado batholith; the upper contact appears to be conformable with the Robles 

Formation. The rocks south of the batholith are in fault contact with 

the Eobles on the north and west, are probably in fault contact with 

plutonic rocks on the southwest, and are unconformably overlain by 

Eocene rocks on the southeast. Massive pyroxene-rich tuff and lapilli 

tuff of the Jayuya are at least 900 m thick at Cerro Morales, and the 

feldspathic facies is as much as 600 m thick where it is exposed 2 km 

northwest of Cerro Saliente. No fossils have been found in the Jayuya 

Tuff.

ROBLES FORMATION

Bordering the eastern extension of the batholith and eastward in 

the Jayuya quadrangle, the Eobles Formation (defined by Pease 

and Briggs, 1960) consists of 500-600 m of massive to medium-bedded 

laminated green, greenish-purple, and blue-gray mudstone with some 

volcanic siltstone and sandstone. Good exposures are on route 141 

northwest of Jayuya. Scattered interbedded layers of fine-grained 

laminated dark pyroxene-bearing tuff and feldspathic tuff occur 

near the base of the unit, and there are occasional lenses of pryoxenebearing lithic lapilli tuff. Within the batholith contact aureole the 

mudstone is metamorphosed to a fine-grained impure siliceous rock 

and the coarse-grained volcanic and epiclastic rocks to hornfels. The 

impure siliceous rock and to a lesser extent the hornfels are the ridgeforming rocks in the central and western parts of the quadrangle.

Near Cerro de Punta and in the east-central part of the report 

area, the Robles Formation is 2,100-2,500 m thick and also contains 

conspicuous laminated mudstone, but has a larger proportion of 

massive laminated tuffaceous siltstone and sandstone, laminated finegrained pyroxene-bearing tuff, some fine-grained green sedimentary 

breccia, and basalt and andesite lava flows that are generally less 

than 10 m thick. Good exposures are on the dirt road from Hacienda 

Gripinas to route 143 at Cerro de Punta. Crossbedding and channeling occur at a few localities; one such exposure indicates a current moving towards the northeast. Several lenses, generally less than 

10 m thick, of friable coarsely crystalline pure white marble are interbedded with silicified siltstone northwest of Cerro de Punta near 

the lowest levels of the formation. Here, metamorphism has silici-
STRATIGRAPHY IN WEST-CENTRAL PUERTO RICO B9

fied, epidotized, and pyritized the volcanic rocks and formed hornfels 

and gneiss near the batholith.

The Eobles as mapped in the Cerro de Punta area may include 

parts or all of the Vista Alegre and Tetuan Formations, as it is 

overlain in that area by the Cotorra Tuff. This would account for the 

great difference in thickness between the Eobles in this area and the 

Eobles to the north and northeast. Poor exposures, deep weathering, and a probable westward decrease in the pyroclastic Vista Alegre 

component prevented further division of the Eobles in the Cerro 

de Punta area.

The Eobles Formation is also exposed in a northwest-trending horst 

near Adjuntas, in a belt along the southwest edge of the Utuado 

batholith, and in a small area northeast of the batholith. In the horst 

near Adjuntas are greenish-gray medium-bedded volcanic sandstone 

and some green and blue-gray laminated mudstone with interbedded 

feldspathic tuff and some andesite(?) lava. The relative amount 

of epiclastic and sedimentary rocks is greater, and the pyroclastic 

rocks are finer grained in the horst than in Eobles areas to the north. 

Epidotization, chloritization, and pyritization are more pronounced 

near the two small plutons than elsewhere in the horst, and some 

hornfels is exposed near the plutons.

Within the horst, near Guaraguao, are several exposures of a 

lenticular biostrome as much as 10m thick that contains abundant 

oysters and sparse calcareous algae. The matrix of the biostrome 

is generally dark limestone or limy mudstone, but in some places 

is a cream-colored lutite. Shale interbeds, in some places containing 

gastropod molds, are exposed within and bordering the biostrome.

Along the southwestern border of the batholith, the Eobles Formation is more volcanic in character than in the horst. Dark-green 

basalt lava with pyroxene phenocrysts is common andesite(?) lava 

is less common, and there are some pyroxene-bearing coarse-grained 

tuffs and lapilli tuffs. Medium-bedded volcanic sandstone and mudstone are interbedded with the extrusive rocks and underlie a large 

part of the area west of the Eio Grande de Arecibo, but the epiclastic 

facies could not be shown separately from the extrusive facies on 

the geologic map (Mattson, 1967a). Near the batholith the epiclastic 

rocks are epidotized and pyritized, the matrix of the effusive rocks 

is recrystallized to hornfels, and amphibole replaces some of the 

pyroxene phenocrysts in the lavas. Along the southwestern edge of 

the batholith, the Eobles is about 650 m thick, thickening westward 

to at least 1,300 m, whereas it is approximately 1,000-2,000 m thick 

in the horst near Adjuntas.

The Eobles overlies the Jayuya Tuff, and in the eastern half of 

the area it underlies the Vista Alegre Formation and Cotorra Tuff.
BIO CONTRIBUTIONS TO STRATIGRAPHY

Map relations indicate that both lower and upper contacts are gradational in the east, but in the west the Eobles is overlain unconformably 

by Eocene rocks along the south edge of the exposures that border 

the batholith. The only evidence for the unconformity in the west 

is the absence of the Achiote Conglomerate and other Cretaceous 

formations that overlie the Eobles in the east.

In the Ad juntas quadrangle the Eobles may be stratigraphically 

equivalent to the Jayuya, Vista Alegre, Tetuan, or Cotorra Formations 

in the Jayuya quadrangle and farther east, but rock units that could be 

correlated with these formations were not discovered. The Eobles 

and Tetuan are more epiclastic and contain less effusive volcanic material than the Jayuya, Vista Alegre, and Cotorra, which are dominantly pyroclastic and effusive volcanic rocks. The five formations 

are lithologically similar to each other and are gradational into or 

iiiterfinger with each other in part; hence it is difficult or impossible 

to map them across major structural breaks. Therefore, from Cerro de 

Punta westward, the entire stratigraphic unit is called the Eobles 

Formation.

The Eobles Formation is composed of sedimentary and volcanic 

rocks, probably largely deposited under shallow to moderately deep 

water marine conditions. This is shown by the interbedding of lavas 

with marine deposits, the presence of graded bedding, an oyster biostrome and other limestone lentils, and marine sedimentary directional 

features seen near Cerro de Punta and at three localities in the horst 

near Adjuntas. Crossbedding at (126,435 m E.; 32,550 m N.) in the 

Eio Cafias shows a north-northwest-directed current. Load casts at 

(123,635 m E.; 36,170 m N".) on the north side of Kio Saltillo show a 

northward-dipping slope, but fold axes of slumped beds at (125,370 m 

E.; 33,900 m N.) on route 10 show a west-southwest-dipping slope.

The only fossils found in the Eobles Formation in west-central 

Puerto Eico are abundant specimens identified by N. F. Sohl (written 

commun., 1963) as a new species of the oyster Crassostrea, and compressed unidentifiable internal molds of turriculate snails. The fossils 

are in an oyster bank and surrounding shales at USGS locality 28752 

(127,040 m E., 32,410 m N.) and USGS locality 28753 (126,710 m E., 

32,490 m N.) in the southern Adjuntas quadrangle, and at USGS 

locality 28754 (128,000 m E., 31,090 m N.) in northern Penuelas quadrangle about 1,200 m south of Adjuntas quadrangle on route 10. 

Foraminifera of Albian, Cenomanian, and possibly Santonian age 

have been described in the Eobles Formation about 25 km to the 

east (Douglass, 1961; Otalora, 1961).
STRATIGRAPHY IN WEST-CENTRAL PUERTO RICK) Bll 

VISTA ALEGRE FORMATION

Massive to medium-bedded crystal-lithic tuffs and subordinate volcanic sediments and lava are continuous with the Vista Alegre Formation (Nelson and Monroe, 1966) of the Florida quadrangle to the north. 

A reference locality within the Jayuya quadrangle is on the west bank 

of the Rio Toro Negro on route 149, from (143,530 m. E.; 43,020 m. N.) 

to (142,870 m. E.; 44,510 m. N.). The Vista Alegre is extensively exposed in the north-central to northeastern parts of the report area.

The formation at the reference locality consists predominantly of 

coarse-grained dark-green pyroxene-feldspar crystal tuff interbedded 

with coarse-grained greenish gray feldspar crystal-lithic tuff. There 

are also minor amounts of lapilli tuff and volcanic sandstone, siltstone, 

and mudstone. The rocks are medium bedded to massive, and there are 

some massive laminated mudstones similar to those in the Eobles Formation. Pyroxene, plagioclase, and volcanic rock fragments are abundant in the tuffs; hornblende, biotite, and quartz are sparse. In some 

places red volcanic scoria gives the rock a variegated appearance. 

Lenses of greenish-gray epiclastic breccia are exposed at a few localities; fragments are generally lavas and aphanitic volcanic rocks as 

much as 3 cm in size.

Crossbedding occurs at two localities in the tuff (Mattson, 1967b); 

one shows east- and the other northwest-directed current. These current directions, common regular bedding, and interbedded marine sedimentary rocks indicate that the formation was formed under marine 

conditions.

One large lens of mafic volcanic rocks was mapped north of Jayuya; 

another, in the northeast corner of the area. The lenses contain basalt 

and massive coarse-grained basaltic tuff. Some samples of the basalt 

contain pseudomorphs after olivine, but generally only pyroxene and 

plagioclase occur as phenocrysts. In the north-central part of the map 

area (Mattson, 1967b) andesite (?) lava forms at least one flow or 

series of flows. The basalt, basalt tuff, and andesite (?) are petrographically similar to rocks of the Rio Orocovis Group (Nelson, 1966) 

exposed northeast of the Jayuya quadrangle.

The Vista Alegre Formation is estimated to be 1,000-1,700 m thick in 

the eastern part of the Jayuya guadrangle, but thins to about 950 m 

in the western part of the Jayuya and in the southern part of the 

Florida quadrangles. Near Cerro de Pu-nta the Vista Alegre, if present, 

is included in the Eobles Formation for reasons given above. Contacts 

of the Vista Alegre with the overlying Tetuan Formation and the underlying Robles Formation are both gradational. No identifiable fossils were found in the formation.
B12 CONTRIBUTIONS TO STRATIGRAPHY 

TETITAN FORMATION

Rocks assigned to the Tetuan Formation (Nelson and Monroe, 1966) 

are exposed on both sides of the valley of the Rio Toro Negro in the 

eastern part of the area; the Tetuan was not recognized in the Adjuntas 

quadrangle. Good exposures are on a jeep road from Hacienda Ralate 

toward Los Tres Picachos, and also on the lower part of another jeep 

road extending up the next spur to the south.

The Tetuan Formation contains thin- to medium-bedded, laminated 

volcanic mudstone and sandstone interbedded with medium-bedded to 

massive coarse-grained feldspar crystal-lithic tuff. The rocks are 

greenish gray, gray and lavender. Graded bedding was rarely observed. 

Conglomeratic volcanic sandstone layers are common in the upper part 

of the formation west of the Rio Toro Negro and may be equivalent to 

the Malo Breccia to the east. Two thin andesite (?) flows were mapped; 

one olivine basalt flow was seen but not mapped about 800 m north of 

Hacienda Figueroa.

The formation overlies the Vista Alegre Formation gradationally 

and is overlain, probably conformably, by the Cotorra Tuff west of 

the Rio Toro Negro. East of the river, stratigraphic relations and 

thicknesses are approximate because of difficult access.

The Tetuan Formation is approximately 1,200 m thick west of the 

Rio Toro Negro, but only about 460 m thick east of the river where the 

upper part of the Tetuan apparently interfingers with the Malo 

Breccia. No fossils have been found in the formation.

MALO BRECCIA

The Malo Breccia (Briggs, 1967) occurs only on the east-central 

boundary of the report area east of the Rio Toro Negro. It is composed of green and gray volcanic breccia, lapilli tuff, and coarsegrained tuff with sparse volcanic sandstone, siltstone, and conglomerate. The tuffs and breccias contain fragments of volcanic porphyries, 

pumice, common feldspar, some pyroxene, and sparse hornblende and 

biotite, in a matrix of secondary chlorite, zeolite, and small pyroclastic 

or epiclastic feldspar crystals. Rocks are commonly epidotized, 

pyritized, and calcified, particularly in the westernmost exposures.

The Malo is very poorly exposed in the report area. It overlies the 

Tetuan Formation gradationally and is overlain by the Achiote Conglomerate. The contact with the Achiote is only known approximately; 

it could be either conformable or unconformable with small angular 

discordance. The Malo is 250-600 m thick in this area, thinning 

abruptly westward until it disappears at the Rio Toro Negro. No fossils 

were found in the formation.
STRATIGRAPHY IN WEST-CENTRAL PUERTO RICO B13

COTORRA TTTFF

Pyroclastic rocks and basalt lava, in part continuous with the 

Cotorra Tuff of the Orocovis quadrangle (Briggs, 1967), are exposed 

in the southeastern part of the report area and east and south of Cerro 

de Punta. It was not recognized in the Ad juntas quadrangle. Good 

exposures are in the Eio Toro Negro at Toro Negro Central Hidroelectrica Num. 2 and in and near Quebrada Achiote.

In the Jayuya quadrangle the Cotorra Tuff consists largely of 

dark-green and greenish-black coarse-grained tuff and lapilli tuff with 

minor volcanic breccia and basalt lava. The rocks are rudely bedded 

or sheeted in some places but are massive, sparsely laminated, at most 

localities. Pyroclastic components of the tuff are predominantly monoclinic pyroxene and fragments of pyroxene porphyries. Feldspar and 

hornblende crystals, fragments of hornblende porphyries, and fragments of hornblende-pyroxene porphyries are less common. Fragments 

are cemented by calcite, chlorite, and zeolite; some chlorite bodies 

probably represent altered volcanic glass.

Basalt lava and pyroclastic rocks containing fragments of basalt 

lava predominate in the Quebrada Achiote area. The lava commonly 

has pillow structures averaging 1 meter in longest dimension. It 

contains phenocrysts of augite and altered olivine(?) in a felty or 

trachytic matrix of feldspar, chlorite, and pyroxene. The presence of 

pillow structures and calcareous cement suggests that the Cotorra is 

a marine deposit.

Eock types in the Cotorra Tuff are similar to those found in lenses 

of volcanic rock in the Vista Alegre Formation, in the Jayuya Tuff, and 

in the Los Negros and Perchas Formations exposed to the northeast 

(Nelson, 1966). North of Quebrada La Mina in the Jayuya quadrangle 

the Cotorra overlies the Tetuan Formation, probably conformably; 

it overlies the Eobles Formation gradationally near Cerro de Punta. 

The formation is overlain by the Achiote Conglomerate conformably 

or by an angular discordance too small to be measured by available 

data. North of Quebrada La Mina the Cotorra is about 90 m thick; it 

thickens to more than 250 m to the south and to about 460 m to the 

west near Cerro de Punta. No fossils have been found in the formation.

ACHIOTE CONGLOMERATE

Distinctive red and purple tuffaceous conglomerate and associated 

sandstone are herein named the Achiote Conglomerate after good 

exposures on the west side of the valley of Quebrada Achiote in the 

southeastern part of the Jayuya quadrangle. The type locality is on 

route 149 from (144,220 m E.; 35,200 m N.) to (144,145 m E.; 34,060 m

264-418 67 3
B14 [CONTRIBUTIONS TO STRATIGRAPHY

N.). The Achiote is extensively exposed near the type locality; it is 

also exposed on and north of the Cordillera Central, in several areas a 

few kilometers north of the southern border of the quadrangle, and 

in a graben in the northeastern part of the quadrangle. It does not 

crop out in the Ad juntas quadrangle.

At the type locality the Achiote consists of massive red volcanic 

conglomerate with well-rounded clasts as large as 150 cm in diameter 

but generally no larger than 10-20 cm. The clasts are red and green 

tandesites and some chert; the matrix contains feldspar and red 

hematitic pellets in a red mudstone. Interbedded with the red conglomerate are green tuffaceous conglomeratic sandstones and conglomerate. The green rocks contain the same clasts as the red conglomerate 

but have a f eldspar-chlorite matrix. There also are interbeds of thinand medium-bedded red and greenish-gray tuffaceous sandstone and 

red mudstone. Graded bedding and channeling are present in the red 

conglomerate, and the exposures are cut by thin dikes and sills of 

blue-green pyroxene-feldspar porphyry.

Greenish-gray tuffaceous conglomerate and finer grained clastic and 

tuffaceous rocks are found throughout the exposure area in the Jayuya 

quadrangle, but are less common than the red more epiclastic tuffaceous 

conglomerate in most areas. In the extreme southeast corner of the 

quadrangle east of El Semil, however, green rocks are common at all 

exposed levels of the formation.

The clasts in both conglomerates are generally well rounded but in 

some places are elongate and alined parallel to bedding; they are as 

large as 150 cm in diameter but the maximum size in a single locality 

is about 12 cm. The larger cobbles and boulders in many places "float" 

in the matrix, not touching each other. Volcanic porphyries are the 

most common clasts; these include basalt and andesite similar to lavas 

in the Jayuya, Vista Algre, Cotorra, and other formations in the 

area, and less commonly feldspar-quartz, feldspar-hornblende, and 

f eldspar-pyroxene-biotite porphyries, not common in the older rocks. 

The porphyries are red, purple, or greenish gray. Other less common 

clasts are tuff, siltstone, limestone, sparse fossils, and red chert.

The matrix of the conglomerates is tuffaceous sandstone and mudstone ; similar sandstone and mudstone are interbedded in the formation. It is largely pyroclastic but is sorted and reworked at some levels, 

particularly in the red facies. Feldspar and small chloritized porphyritic volcanic rock grains are most common; pyroxene, hornblende, 

and magnetite crystals and fresh porphyritic volcanic rocks form a 

minor part of the matrix. These clastic and pyroclastic grains are 

cemented by chlorite, zeolite, and calcite. The red facies commonly 

contains more calcite cement than the green facies.
STRATIGRAPHY IN WEST-CENTRAL PUERTO RICO B15

Common oxidized clasts and matrix, common calcite cement, good 

sorting and reworking, some graded bedding and channeling, large size 

of the clasts, and the abrupt variation in stratigraphic thickness 

(Mattson, 1967b, section E-E'} suggest a shallow marine or fluviatile, 

deltaic environment for the red conglomerate and sandstone facies of 

the Achiote.

The Achiote Conglomerate is 200-400 m thick in the southeastern 

part of the Jayuya quadrangle but thickens westward to perhaps as 

much as 1,800 m between Cerro Maravillas and Monte Jayuya on the 

Cordillera Central, where the obscure structure of the Achiote prevents 

accurate measurement. It overlies the Cotorra Tuff except east of the 

Rio Toro Negro, where it overlies the Malo Breccia. As the Cotorra is 

present again a few kilometers to the east, where it occurs between 

the Achiote and the Malo, it may have been removed by pre-Achiote 

erosion in the area east of the Rio Toro Negro. The Achiote contact 

is conformable on the Cotorra or, at most, unconformable with slight 

angular discordance; contact relations with the Malo are obscured by 

tropical weathering and lack of outcrops. The Achiote grades upward 

into the Maravillas Formation, and the contact is defined as the top 

of the highest massive conglomerate.

The Achiote is the approximate stratigraphic equivalent of the 

massive conglomerate f acies of the Cariblanco Formation of the northeastern part of Coamo quadrangle (Glover, 1961) : the green f acies 

of the Achiote is similar to marine volcanic conglomerates of the 

Cariblanco. A minimum thickness of 400 m of weathered conglomerate 

and sandstone, exposed in a graben near the Rio Cialitos in the northeastern corner of the report area, is correlated with the Achiote on the 

basis of lithologic similarity. The rocks are red and purple tuffaceous 

conglomerates and sandstones, are moderately well sorted, and contain 

clasts of andesites and red and gray chert. Red volcanic sandstone and 

conglomeratic sandstone on route 157 in the Damian Aba jo area of 

northwestern Orocovis quadrangle at (146,680 m E.; 45,070 m N.) and 

(147,165 m E.; 45,070 m N.) are also similar to rock types in the 

Achiote Conglomerate.

Fossil collection USGS 28761 (143,890 m E.; 34,780 m N.) in greenish-gray tuffaceous conglomerate in the upper part of the Achiote 

contains the pelecypods Cymella ~bella Conrad, Cardium (G-ranocardiwn) sp., and Veniella sp., and the gastropods Pugnellus sp. and 

Cantharulus sp. (N. F. Sohl, written commun., 1963). Sohl considers 

the collection Late Cretaceous, Santonian to Maestrichtian, in age. 

Otalora (1961) reports Coniacian or Santonian Foraminifera from 

the base of his Ildef onso Formation stratigraphically equivalent to the 

Cariblanco Formation of Glover (1961) and hence also to the Achiote
B16 (CONTRIBUTIONS TO STRATIGRAPHY

Conglomerate. However, Briggs and Gelabert (1962) consider the 

Ildefonso fossil locality to lie in the Kobles Formation. Pessagno (1962, 

1963, and written commun., 1961-64) has described numerous Santonian and Campanian assemblages from the overlying Maravillas 

Formation. Thus the Achiote conglomerate is of Late Cretaceous age, 

probably Santonian and perhaps in part as old as Coniacian.

MARAVILLAS FORMATION

The Maravillas Formation is here defined as a unit of well-bedded 

volcanic sandstone and siltstone, with some pyroclastic rocks, lying 

above poorly bedded Achiote Conglomerate and below poorly bedded 

coarse volcanic rocks of the Coamo Formation. It is named after Cerro 

Maravillas on the Cordillera Central, and its type locality is on route 

143 east from Cerro Maravillas between (139,450 m E.; 35,360 m N.) 

and (140,530 m E.; 35,180 m N.). It is also exposed near Lago El 

Guineo, in the valley of Quebrada Dona Juana, and near the southern 

border of the quadrangle.

The type locality contains about 80 m of massive brownish-gray 

coarse-grained lithic tuff, overlain by about 300 m of medium- to thickbedded volcanic siltstone and sandstone with common hornblendefeldspar porphyry dikes and sills. The bedded rocks are dark-gray and 

brown laminated tuffaceous siltstones and sandstones with some beds 

of crystal-lithic tuff similar to the tuff member at the base of the formation Channeling, slump structures, and graded bedding exposed at 

(140,320 m E.; 35,300 m N). indicate a south-southeast-facing depositional slope. Foraminifera and calcite cement, in addition to current 

features, suggest a marine environment for both the basal coarsegrained lithic tuff and the overylying well-bedded rocks.

Rock types common in the well-bedded part of the Maravillas 

Formation are muddy dark f eldspathic sandstone, siltstone, and mudstone ; green crystal-lithic coarse-grained tuff; and silty brown coarsegrained crystal tuff. Beds range from less than 5 cm to as much as 100 

cm in thickness, but are generally less than 50 cm thick. Pyroclastic 

fragments include crystals of feldspar, amphibole, pyroxene, and 

sparse quartz, and green finely trachytic- and felty-textured volcanic 

rocks. The green fragments, probably largely chloritized, devitrified 

glassy rocks, give the coarse-grained green tuff its color. The matrix 

of the pyroclastic rocks is calcite, chlorite, and zeolite, with some 

compressed perlitic material. The green tuff contains less reworked 

material than the other rock types and is more common near the base 

of the formation than elsewhere.

A thick lens of brownish-gray lapilli tuff and coarse-grained tuff occurs at the base of the formation in several areas and may be continu-
STRATIGRAPHY IN WEST-CENTRAL PUERTO RIOO B17

ous; however it could not be mapped south of the Cerro de Punta 

fault. It contains crystals and crystal fragments of feldspar, hornblende, and pyroxene, and fragments of green, gray-green, and brown 

volcanic rocks set in coarse-grained feldspathic tuff. The coarsegrained tuff matrix is well sorted and cemented by calcite in some localities but is poorly sorted and muddy in other places. Common green 

and brown aphanitic and sparsely porphyritic volcanic rock fragments 

produce a similarity to tuffs of the Eocene An6n Formation. East of 

the Quebrada Achiote, the lens is a well-sorted crystal-lithic coarsegrained tuff and lapilli tuff resembling the salt-and-pepper facies of 

the Coamo Formation (see below).

The sandstone, siltstone, and mudstone have a smaller proportion 

of pyroclastic fragments, similar to these described above, in a matrix 

of carbonate and mud. Both pyroclastic and sedimentary rocks contain scattered Foraminifera and occasional E-adiolaria. Thin lenses of 

medium- to dark-gray fragmental limestone occur at several levels 

generally in the lower parts of the formation; one lens is shown on 

the geologic map (Mattson, 1967b). Some reworked blocks of similar 

limestone occur near limestone lenses just above the coarse-grained 

tuff and lapilli tuff lens near route 149 and eastward.

South of Cerro Maravillas near a pluton of the Utuado batholith, 

strata of the Maravillas Formation have been metamorphosed to a 

moderate degree with the formation of epidote. Near the Rio Guayo 

and the Quebrada Chiquita, the formation has been silicified and 

pyritized.

The Maravillas Formation is at least 380 m thick at the type locality 

and thickens to at least 560 m, a few kilometers to the south. The 

lapilli tuff and coarse-grained tuff lens is generally 50-80 m thick 

but is at least 200 m thick east of the Quebrada Achiote.

The Maravillas Formation interfingers with and grades downward 

into the Achiote Conglomerate; it is overlain conformably by the 

Coamo Formation. It is a mappable unit in the Jayuya quadrangle and 

is stratigraphically equivalent to the upper bedded part of the Cariblanco Formation and the lower bedded part of the Coamo Formation, as defined in the Coamo quadrangle (Glover, 1961). Limestone 

blocks and lenses in the Maravillas near route 149, just above the coarsegrained tuff and lapilli tuff lens, may be correlative with the Santa Ana 

Limestone Member of the Coamo Formation (Glover, 1961). If so, the 

lapilli tuff and coarse-grained tuff lens of Jayuya quadrangle may be 

correlative with the San Diego Lapilli Tuff Member of the Coamo 

Formation; the discontinuous bedded part of the Maravillas that occurs below the lapilli tuff and coarse-grained tuff lens would then 

correlate with the Sabana Hoyos Limestone Member and perhaps
B18 (CONTRIBUTIONS TO STRATIGRAPHY

the unnamed siltstone and sandstone member of the Cariblanco Formation (Glover, 1961). As none of these thin units are mappable across 

the area of Maravillas exposures, no names are applied to the members in west-central Puerto Rico.

Three of the four fossil collections from the Maravillas Formation 

give useful age information (table 1). These data indicate that the 

Maravillas Formation is Late Cretaceous in age, probably late Santonian and Campanian and possibly as young as Maestrichtian in 

the upper parts. This age agrees with that of numerous Santonian, 

Campanian, and Maestrichtian assemblages in tihe Maravillas in the 

adjacent quadrangle to the east and in correlative rocks to the southeast in the Coamo and Rio Descalabrado quadrangles (Pessagno, 1962, 

1963; N. F. Sohl, written commun., 1957-1963).

TABLE 1. Fossil collections in the Maravillas Formation, west-central Puerto Rico

Turritella trilira Conrad __ _ ___

P. sp_ ____ __. ___ _ __. _ _ __________

Globigerinelloides sp _____ _ __ __ ___ _____ __

G. fornicata _____________ ______ __

(?.sp. (?)___________________________ ___________

US GS locality

28758

X

J45-317

X 

X 

X

X

X 

X

?

J47-347

X

J47-358

X 

X

?

X

USQS 28758. (142,925 m E.; 32,880 m N.); N. P. Sohl, written commun., 1963: Late Cretaceous Campantan-Maestrichtian. 

J45-317. (141,990m E.; 34,660m N.);E. A. Pessagno, written commun., 1963: Late Cretaceous late

Santonian-early Campanian.

J47-347. (140,870 m E.; 33,335 m N.); E. A. Pessagno, written commun., 1963 Late Cretaceous(?). 

J47-358. (140,625 m E.; 33,350 m N.); E. A. Pessagno, written commun., 1963: Late Cretaceous late

Santonian.

COAMO FORMATION

Glover (1961) defined the Coamo Formation in Coamo quadrangle 

stating that "tuff-breccia, lapilli tuff, and tuffaceous conglomerate 

make up the bulk of the Coamo Formation in the Coamo quadrangle." 

However, he included within the Coamo Formation several bedded 

units that lie below the massive volcanic rocks. These are the San 

Diego Lapilli Tuff Member, the Santa Ana Limestone Member, an 

unnamed conglomerate unit, and unnamed sandstone and siltstone 

units. In west-central Puerto Rico, the Coamo Formation has been 

restricted to the massive volcanic rocks, and the Maravillas Formation (see above) has been defined to include the sequence below the 

restricted Coamo and above the Achiote Conglomerate.

In the Jayuya quadrangle the Coamo Formation is exposed in a 

trapezoidal fault block on the Cordillera Central near Lago Guineo,
STRATIGRAPHY IN WEST-CENTRAL PUERTO RICO ,B19

on both sides of the valley of Quebrada Dofia Juana, and in a small 

area at the extreme southeast corner of the quadrangle. The Coamo 

does not report out in the Adjuntas quadrangle. It is deeply weathered 

almost everywhere; in the area south of Quebrada Dona Juana, the 

only fresh rocks generally exposed are porphyry dikes cutting the 

formation.

The Coamo Formation in west-central Puerto Eico consists of 

coarse-grained tuff, lapilli tuff, and fine-grained volcanic breccia 

with a small amount of limestone and lava or intrusive rock. The 

largest fragment seen in the breccia was 30 cm. The rock is generally 

massive, but some coarse-grained crystal tuffs show layering and 

graded bedding, as near Mision Noel on route 143. Three pyroclastic 

rock types are common in the formation: salt-and-pepper tuff, green 

tuff, and muddy tuff. All occur in the size range from coarse-grained 

tuff to fine-grained volcanic breccia. The salt-and-pepper tuff facies 

is generally a crystal-lithic coarse-grained tuff and lapilli tuff, containing fragments of common to abundant feldspar, common hornblende, scarce quartz, sparse corroded and altered biotite (?), 

and common porphyritic and aphanitic volcanic rocks. There is a 

minor amount of chlorite, zeolite, and calcite cement. Sorting is generally good, but some localities show graded beds. The rock presents 

a green and white or black and white speckled appearance, in places 

resembling hornblende diorite. Saprolitic soils formed on this facies 

are yellow-brown to white and resemble soils derived from plutonic 

rocks. The salt-and-pepper facies generally occurs in the lower part 

of the formation in this quadrangle. It is quite similar to rocks of 

the San Diego Lapilli Tuff Member (Glover, 1961) and its correlative, 

the unnamed lapilli tuff lens of the Maravillas Formation, but is at a 

higher stratigraphic level than those units.

The green tuff facies in general contains fewer crystals, less hornblende, and more green lithic fragments than the salt-and-pepper 

facies. Quartz was not seen; some samples contain fragmental 

pyroxene. The color of the rock is caused by common green chloritized 

porphyritic and aphanitic volcanic rock fragments. This facies is 

generally a lapilli tuff or fine-grained volcanic breccia; it is similar 

to but generally coarser grained than the green tuff facies of the 

Maravillas Formation. The facies occurs throughout the Coamo but 

is more common in the upper portions.

The muddy tuff facies contains as much as 40 percent chloritic 

clay and silt matrix. Pyroclastic fragments are of volcanic rocks 

and some feldspar. The facies occurs at various levels within the 

formation and is also similar to pyroclastic rocks in the underlying 

Maravillas Formation.
B20 CONTRIBUTIONS TO STRATIGRAPHY

One lava or sill is exposed at (142,600 m, E.; 34,360 m, N.)- It 

is a dark bluish-gray pyroxene andesite (?) having an intergramilar 

porphyritic texture, and contains chlorite pseudomorphs perhaps 

after a second type of pyroxene. Volcanic breccias in the formation 

contain fragments of similar rocks.

A lens of dark fragmental limestone as much as 5 m thick is exposed east of Hacienda San Clemente, and another thicker lens forms 

a dip slope west of the hacienda. The eastern lens has wavy bedding 

and contains fragments of rudists, bryozoa, and Foraminifera; the 

western lens is recrystallized and epidotized. The stratigraphic relation of the two lenses could not be determined because of an intervening igneous intrusion.

The Coamo Formation overlies the Maravillas Formation conformably; its upper contact has been removed by faulting and erosion. It has an exposed thickness of about 300m in west-central 

Puerto Kico. Only one fossil locality has been discovered within 

the formation in this area. In a collection from (140,990 mE.; 34,- 

500 m N.), in the eastern limestone lens near Hacienda San Clemente, 

E. A. Pessagno, Jr. (written commun., 1963), identified Globotruncana stuartiformis and Sulcoperculma dickersoni(^). His age assignment of the collection is Late Cretaceous, Campanian or Maestrichtian. The collection is probably from near the middle of the 

part of the formation exposed in the Juyuya quadrangle. As the 

underlying Maravillas Formation is Santonian and Campanian in 

age, possibly as young as Maestrichtian, the Coamo Formation is 

therefore Campanian or Maestrichtian in age. No fossils younger 

than Maestrichtian have been found in the Coamo in Puerto Rico.

YAUCO MUDSTONE

The Yauco Mudstone (Mattson, 1960) is exposed only southwest of 

the Garzas fault in the Adjuntas quadrangle in west-central Puerto 

Rico. Mitchell (1922, p. 249) named the formation for exposures 

along route 128 north of Yauco along the Rio Yauco, but did not 

specify a type locality. Slodowski (1956, p. 62 ff.) mentioned good exposures of the Yauco in the same area but did not describe a type 

locality either. For the purposes of reference, one of the localities mentioned by Slodowski is herein described as the type locality and two 

others as reference localities.

Exposures at (106,930 m E.; 31,220 m N.) on route 128 north of 

Escuela Arturo Lluberas in northwestern Yauco quadrangle are herein 

designated as the type locality for the Yauco Mudstone. About 5 m of 

rhythmically bedded calcareous mudstone and mudstone occurs in 

paired beds composed of a bed of fissile mudstone that grades upward
STRATIGRAPHY IN WEST-CENTRAL PUERTO RICO B21

into a bed of more calcareous mudstone. The calcareous mudstone is 

dark gray and forms 5- to 60-cm beds; the fissile mudstone is also dark 

gray and forms layers ranging from thin partings to beds as much 

as 15 cm in thickness. There are a few beds of dark-brownish-gray f ragmental limestone that contain some Foraminifera. The beds dip about 

20° ENE. and are gently deformed into folds of about 0.5 m amplitude 

and 2 or 3 m half wavelength; a small thrust fault dips 45° NE. and has 

an apparent throw of about 20 cm.

One reference locality is defined as the exposures in a quarry at 

(107,220 m E.; 30,970 m N.), on route 373 about 300 m east of the intersection with route 128, in northwestern Yauco quadrangle. Here about 

40 m of thick to massively and evenly bedded mudstone is exposed in 

beds 0.5-7 m thick, cut by joints and calcite veins. The mudstone here 

is a dark-bluish-gray feldspathic calcareous siltstone that, above the 

quarry, weathers to thin- or medium-bedded brown rock. There are 

some beds of light-gray feldspathic coarse-grained tuff with calcareous 

cement. In nearby areas on routes 128 and 373, a massive grayish-tan 

rock that forms abundant pencil-fractured chips is probably the 

weathering product of this rock type.

Another reference locality is at (106,730 m E.; 31,470 m N.), in the 

Yauco quadrangle on route 128 north of the type locality. Here about 

8 m of pencil-fractured calcareous mudstone, thin- to medium-bedded 

and blue gray, is interbedded with thin beds of black mudstone or 

argillaceous limestone, and with 20- to 50-cm beds of feldspathic coarsegrained tuff. Also present are a 1-m bed and a 70-cm bed of brownishtan fragmental foraminiferal limestone that grades upward into calcareous mudstone.

In the Adjuntas quadrangle the Yauco Mudstone is thin- to mediumbedded calcareous mudstone and feldspathic sandstone, blue-gray or 

brownish-gray; it weathers to brick-red saprolite and soil. There are 

some beds of brown and red-brown feldspathic coarse-grained tuff and 

lapilli tuff. Scattered glauconite (?) flakes and glauconitic (?) worm 

trails occur locally, and there is some graded bedding. One faultbounded area of massive purple-weathering crystal-lithic coarsegrained tuff is herein included in the Yauco because similar rocks intertongue with the Yauco to the southwest.

The Yauco Mudstone crops out more or less continuously westward, 

to the west coast of Puerto Kico. Slodowski (1956) estimated a minimum thickness of 1,500 m for the formation in the quadrangles adjacent to the present map area, and Mattson (1960) estimated a thickness 

of 2,400-2,800 m, thinning southward, about 30 km to the west. No 

thickness for the Yauco could be measured in the Adjuntas quadrangle 

because of structural complexity; the only contacts seen are faults. To
B22 .CONTRIBUTIONS TO STRATIGRAPHY

the west, the Yauco is the northern and eastern component of the 

Mayagiiez Group of Mattson (1960). Mattson's Mayagiiez Group overlies the Albian(?) Rio Loco Formation of Otalora (1961) and is 

overlain by the Maestrichtian San German Formation of Mattson 

(1960). The Yauco Mudstone is equivalent to the Rio Yauco Formation of Slodowski (1956) in the Yauco quadrangle, the Penuelas and 

Rio Yauco Shales of Mitchell (1922), and the Rio Yauco Series 

ofHubbard(1923).

No fossil collections have been made from the Yauco in west-centra] 

Puerto Rico. To the west, many Foraminifera of Late Cretaceous 

(Campanian and Maestrichtian) age have been reported by Slodowski 

(1956), Pessagno (1960,1962), and Mattson (1960). A few foraminiferal age ranges permit the existence of rocks as old as Turonian or 

Santonian within the formation, but no collections contain forms definitely older than Campanian. Therefore, the Yauco is considered of 

Campanian or Maestrichtian age in the Adjuntas quadrangle.

LAGO GARZAS FORMATION

Highly amygdaloidal purple lava and associated volcanic rocks are 

exposed in the graben between the Garzas and Cienaga faults in the 

southwestern part of west-central Puerto Rico, in a horst south of 

the San Patricio fault in the south-central part, and in a fault block 

on route 10 along the Cerrillos fault. These rocks are herein named 

the Lago Garzas Formation for good exposures in the vicinity of Lago 

Garzas and eastward from the lake along route 518.

The type locality is at (122,300 m E.; 33,980 m N.), at kilometer 

post 5.0 on route 518 on the east side of Cerro El Gigante. Exposed 

here is a 5- to 8-m thick lava that is dark purplish blue, granular, fine 

grained, and highly amygdular. The lava is andesite or dacite having 

sparse small feldspar phenocrysts and common calcite and chlorite 

amygdules, calcite viens, and some epidote veins. The lava is overlain 

and underlain by purple lapilli tuff and volcanic breccia that contain 

fragments of rock similar to the lava. Within 200 m west of the 

exposure there are several dikes of andesite or dacite containing 

crowded feldspar and hornblende phenocrysts.

The formation consists of a basal member predominantly of mudstone and volcanic sandstone, a middle member of tuff, and an upper 

member of lava.

Lava and autobreccia in the formation are sparsely to extremely 

amygdaloidal purple, red, or greenish-gray andesite or dacite. Phenocrysts are of feldspar and small monoclinic pyroxene; some lava contain few or no phenocrysts. The groundmass of the lava is composed 

of f elty plagioclase microlites and glass. Amygdules of chlorite, calcite,
STRATIGRAPHY IN WEST-CENTRAL PUERTO RICO B23

quartz, chalcedony, and epidote are sparse to abundant. At (120,550 m 

E.; 35,020 m N.), north of Cerro El Gigante, imbricate elongated 

amygdules in purple aphanitic lava suggest a northerly source for the 

lava.

The pyroclastic rocks include purple, red, and green coarse-grained 

tuff, lapilli tuff, and some volcanic breccia. Common ejecta are red and 

purple aphanitic volcanic rocks, feldspar porphyries, crystals of 

feldspar and pyroxene, and chloritized fragments that are probably 

devitrified glass. Some shard structures were seen. The fragments are 

cemented by calcite and zeolite.

The epiclastic rocks are mudstone, volcanic sandstone, and siltstone. 

They are purple, red, and greenish gray, in part thin to medium bedded, 

and in part massive and laminated. Fragments, where identifiable, are 

similar to those in the pyroclastic rocks and contain grains of magnetite 

and quartz.

The three members of the formation intertongue or interfinger with 

each other in the central part of the exposure area. In general the 

stratigraphic sequence is, from older to younger, epiclastic rocks, pyroclastic rocks, and lava. A minimum thickness of 1,700 m of the formation was measured near Lago Garzas. The lava member is restricted 

to the graben between the Garzas and Cienaga faults. The eastern 

horst and the fault block at the Cerrillos fault are composed predominantly of red and purple conglomeratic tuff and tuffaceous sandstone, thin-bedded or laminated volcanic sandstone and siltstone, and 

red and purple mudstone. In west-central Puerto Kico all contacts of 

the Lago Garzas are faults.

The Lago Garzas outcrop is part of a broad band of rocks that are 

exposed from Ponce to the west coast of Puerto Eico near Aguada. 

Hubbard (1923) called these rocks the "Kio Blanco Series," and 

several reports (Slodowski, 1956; Turner, 1958; Pessagno, 1960; 

Mattson, 1960) use the same name for parts of the exposure band. 

However, M. D. Turner (oral commun., 1963) and Eigoberto Eeynoso 

(Bear Creek Mining Co., oral commun., 1964) recently discovered that 

the band of exposures can be divided in most places into a northern 

zone of pyroclastic rocks and lava probably correlative with the Eocene 

Anon Formation (see below), and a complex southern zone that contains several mappable units. One of these mappable units correlates 

with the Lago Garzas, another with the Anon, and others perhaps with 

the Eobles Formation or with the Mayagiiez Group. Unfortunately, 

Hubbard's typical exposures of the "Eio Blanco," as well as most of the 

modern Eio Blanco river, are in the area of rocks probably at least in 

part correlative with the An6n. Therefore, it is herein suggested that
B24 CONTRIBUTIONS TO STRATIGRAPHY

the name "Rio Blanco" be abandoned in favor of Lago Garzas, Anon, 

or other names as appropriate to the specific areas.

One fossil collection has been described from the Lago Garzas 

Formation in west-central Puerto Rico. From locality AM 2-7 at 

(129,365 m E.; 33,530 m N.), on the dirt-road extension of route 503 in 

the horst near the San Patricio fault, Pessagno (written comnrnn., 

1962) has identified the following Foraminifera: G-labotruncana (G-.) 

lapparenti lapparenti Bolli; G. (G.) stuarti stuartiformis Dalbiez; 

G. (R.) tilevi (Bronnimann and Brown); G. (R.} gansseri dicarinata 

Pessagno, two new(?) species of Globigerinelloides; Heterohelix 

striata (Ehrenberg), H. sp.; and Praeglobotrmwana (H.) havanensis 

Voorwijk. His stratigraphic detrmination is "early Maestrichtian, 

Globotruncana (R.) tilevi subzone. Probably upper part of G. lapparenti ss. zonule." The locality is in dark medium-bedded calcareous 

siltstone. Thus the Lago Garzas is at least in part of Late Cretaceous 

(Maestrichtian) age, but may extend somewhat older or younger.

TERTIARY SYSTEM 

BIO PRIETO FORMATION

The Rio Prieto Formation is herein named for exposures in and near 

the Rio Prieto in the southwestern part of the Jayuya quadrangle. It 

extends westward into the Ad juntas quadrangle near the Rio Jauca 

and continues westward to Hacienda El Banco. Two small exposures 

of the formation are in Barrio Tanama in the northwest part of the 

report area. The formation is an interfingering complex of several 

members: lapilli tuff and volcanic sandstone, volcanic sandstone and 

mudstone, algal limestone, and red conglomerate and conglomeratic 

mudstone. Because of the heterogeneity of the formation, type localities are described for each of the members.

The red conglomerate and conglomeratic mudstone is the herein 

adopted Miramar Conglomerate Member (Pessagno, 1960; Mattson, 

1966). At its type locality south of Hacienda Miramar at (141,530 m 

E.; 32,520 m N.) in the southeast part of the report area, it is a red 

well-rounded poorly sorted conglomerate. Clasts are red and purple 

andesites derived from the underlying Achiote Conglomerate, in a 

soft red siltstone matrix. Lenses and beds of light reddish gray and 

pink algal limestone are exposed at the base of the unit. From Hacienda Miramar, the Miramar Conglomerate Member crops out discontinuously along a west-northwest strike almost as far west as Hacienda 

El Banco in the Ad juntas quadrangle, and the small Rio Prieto outcrops in Barrio Tanama are composed of the Miramar.

The Miramar Conglomerate Member is generally at the base of the
STRATIGRAPHY IN WEST-CENTRAL PUERTO RICO B25

Rio Prieto Formation and is as much as 130 m thick in the east. In 

the Barrio Tanama and Hacienda El Banco exposures, it is probably 

less than 20 m thick. It contains clasts that are derived from the nearby 

underlying rock formations and that are generally angular except 

where derived from the well-rounded clasts of the Achiote Conglomerate. The matrix is sparse to common red siltstone and mudstone, containing fragments of calcareous algae and grading into a pink algal 

limestone in some places.

The algal limestone member is as thick as 70 m where it occurs above 

the Miramar Conglomerate Member at and near the type locality for 

the limestone, at (141,590 m E.; 32,150 m N.) on route 512 in Ponce 

quadrangle, about 130 m south of the report area. At the type locality 

it is a massive light-gray calcarenite and calcirudite with abundant 

fragments of calcareous algae and scattered corals in a calcarenite 

matrix. It is gradational downward with the Miramar Member with 

the appearance of fragments of volcanic rocks and a change in color 

to pinkish-white and pink, due to small amounts of manganese (Ponce 

Cement Co., oral commun., 1963). Thin lenses of limestone in the 

Miramar Member are generally of this pinkish transitional facies. 

Included in the algal limestone member are small massive lenses, 

perhaps 10 m thick, of white or grayish-white fragmental limestone 

containing abundant fragments of calcareous algae. These lenses 

occur below the Miramar Member just east of the Bio Jauca but overlie 

the Miramar in the exposures west of the river. In most of these 

exposure areas, the limestone contains common manganese dendrites 

and is finely recrystallized owing to its proximity to the Jauca stock.

A limestone lens about 20 m thick exposed in the Rio Guayo also is 

included in the algal limestone member, although the lens contains 

volcanic debris and is not as rich in algal fragments as the rest of 

the member. This gray limestone contains common shell fragments, 

algal fragments, and Foraminif era; crystals and crystal fragments of 

feldspar, quartz, and hornblende; and fragments of chloritized volcanic rocks. It is medium to thick bedded near the contacts of the lens 

but massive in the center.

A member of volcanic sandstone and mudstone overlies the Miramar 

Conglomerate Member in the central and western parts of the exposure 

area of the Rio Prieto Formation. The type locality is on a trail from 

(133,660 m E.; 35,890 m N.) to route 143 at (133,640 m E.; 36,420 

m N.); greenish- to purplish-gray laminated thin- to medium-bedded 

tuffaceous sandstone and siltstone, weathering to purple saprolite, 

occur with some layers of lapilli tuff and volcanic breccia. Graded 

bedding occurs in some beds. Elsewhere greenish- to purplish-gray 

mudstone beds are intercalated with the sandstone and siltstone. The
B26 CONTRIBUTIONS TO STRATIGRAPHY

member is as much as 250 m thick in the Rio Jauca area but is absent 

or less than 20 m thick elsewhere; contacts with the underlying Miramar Member and the overlying tuff member are gradational and, 

locally, the volcanic sandstone and mudstone member is interbedded 

with the tuff member.

A member of lapilli tuff, coarse-grained tuff, and volcanic sandstone 

is the youngest unit in the Rio Prieto Formation. Exposures south of 

route 143 on a road from (133,160 m E.; 36,100 m N.) to (133,490 m 

E.; 35,710 m N.), designated the type locality, consist of massive bluegray lapilli tuff, weathering purple, interbedded with a smaller amount 

of thin- to medium-bedded tuffaceous sandstone and coarse-grained 

tuff. Pyroclastic fragments in the tuffs and sandstone are crystals of 

hornblende, feldspar, and some quartz, porphyritic and aphanitic volcanic rocks, and some greenish-gray chloritized volcanic rocks; plutonic rock fragments are rarely seen. West of the type area, the member of lapilli tuff, coarse-grained tuff, and volcanic sandstone is graygreen and greenish-brown when fresh, weathers to purple, red, green, 

and brown, and contains sparse fragments of diorite and gabbro in 

addition to the more typical fragments and crystals. Some limestone 

fragments and calcite cement occur near the eastern limit of exposures 

of the unit.

The tuff member is as much as 120 m thick in the type area, but in 

the area west and south of the Rio Jauca, where the Rio Prieto Formation is 400 m thick, the member may be 150 m thick.

The entire Rio Prieto Formation has a maximum thickness of 460 m 

in the central part, of the report area, but is no thicker than 120 m in 

the southeastern part. It is apparently missing entirely for about a 

kilometer along strike between the Rio An6n and the Rio Guayo, in an 

area of poor exposures.

In the Jayuya quadrangle the Rio Prieto Formation rests unconformably upon Cretaceous rocks and plutonic rocks, and is overlain 

conformably by the Monserrate Formation. Farther west, the Rio 

Prieto rests unconformably on chloritized and epidotized rocks of the 

Jayuya Tuff, and grades laterally (westward) and vertically into the 

Monserrate Formation. The Jauca stock has intruded the Rio Prieto 

Formation and has caused recrystallization of the algal limestone and 

shattering and small-scale contortions visible in the adjacent bedded 

rocks.

The Miramar Member and the algal limestone member extend to 

the southeast into Ponce and Rio Descalabrado quadrangles, where 

Pessagno (1960) considered them a part of his Naranjo Formation. 

However, in the Jayuya quadrangle the two units are in a heterogeneous sequence below the Monserrate Formation and are best mapped
STRATIGRAPHY IN WEST-CENTRAL PUERTO RICO B27

as a unit here called the Rio Prieto Formation. The Rio Prieto Formation is thus correlative with a part of the Naranjo Formation of 

Pessagno (1960).

The algal limestone member is the lithologic and approximate stratigraphic equivalent of the Cuevas Limestone (Glover, 1961), the 

Coamo Springs Limestone Member of the Naranjo Formation of 

Pessagno (1960), the Coamo Tuff Limestone of Berkey (1915), the 

Guayabal Limestone of Mitchell (1922), and the Coamo Springs 

Limestone of Hodge (1920). Coamo Springs Limestone may have 

priority, but it was not deemed necessary to name the limestone in 

west-central Puerto Rico.

Six fossil collections have been described from four different localities in the Rio Prieto Formation (table 2). The last two collections 

listed are from the algal limestone member; the other collections are 

from the limestone matrix of the Miramar Conglomerate Member. 

The specimens in collection B are considered of poor quality by Otto 

Renz (written commun., 1961), so in the interpretation more significance is given to the middle Eocene collections A and E. As these middle 

Eocene collections are both from near the base of the formation, and the 

overlying Monserrate Formation is of probable middle Eocene age, 

the entire Rio Prieto Formation is considered middle Eocene in age.

TABLE 2. Fossil collections from the Rio Prieto Formation

G. velascoensis (Cushman) ____ ___ ___

(?. sp___. ____________________________________

A. aster Woodring___ _______ _ _ __________

Eoconuloides sp _ _ __ _______ _____ _____ _

Fabiana cassis (Oppenheim)__ __ ____ _______ _

Pseudophragmina (Proporocyclina) flintensis Gushman).. ___________________________________

P. (P.) sp___________________________.________

A

X

X

?

X

B

X

X

X

C

X

D

?

E

X

?

X

X

F

?

A. J34-63A (133,830 m E.; 36,550 m N.); E. A. Pessagno, written commun., 1963: middle Eocene.

B. 164 (sameloc. as A); Otto Renz, Paleont. Lab., Bataafselnternationale Petroleum Mij. N.V., The Hague;

written commun., 1961: late Paleocene or early Eocene.

C. f-35092 (same loc. as A); K. N. Sachs and W. S. Cole, written commun. 1963: indeterminate. 

D. f-35104 (139,130 m E.; 32,360 m N.); Sachs and Cole, written commun., 1963: indeterminate. 

E. f-35103 (139,130 m E.; 32,350 m N.); Sachs and Cole, written commun., 1963: middle Eocene. 

F. f-35102 (140,630 m E.; 32,020 m Nj; Sachs and Cole, written commun, 1663: middle(?) Eocene. This

locality is in Ponce quadrangle, 260 m south of the Jayuya quadrangle border.
B28 iCONTRIBUTIONS TO STRATIGRAPHY 

MONSEKRATE FORMATION

The Monserrate Formation was named and described as the Monserrate Member of the Augustinillo Formation by Pessagno (1960). 

It is named after Hacienda Monserrate on the Rio Cerillos in the 

Ponce quadrangle and has a type locality near kilometer post 6.9 on 

route 139 (135,910 m E.; 27,190 m N.). Pessagno (1960, p. 96) describes 

the unit as follows: "The Monserrate Member * * * consists principally 

of brownish black calcareous mudstones which alternate with medium 

gray calcilutites and occasional buff calcirudites. Apple green vitric 

tuffs are present at some horizons." The herein adopted Monserrate is 

considered a formation in west-central Puerto Eico because of its 

wide areal extent in the Jayuya and Adjuntas quadrangles. It trends 

west-northwest in almost continuous outcrop from the southeastern 

part of the report area almost as far as Hacienda El Banco, and it 

occurs in fault blocks along the same trend northwest beyond Pellejas 

in the northwestern Adjuntas quadrangle. The Monserrate also is 

exposed in a faulted Y-shaped area extending from the south-central 

Adjuntas quadrangle southeastward to the southwestern corner of 

the Jayuya quadrangle.

The Monserrate Formation is about 160 m thick and apparently 

interfingers with the Rio Prieto Formation in the Rio Jauca-Hacienda 

El Banco area, but detailed relations are obscure because intrusion 

of the Jauca stock and Tertiary plutonic rocks has left little of the 

Monserrate exposed in that area. West of Hacienda El Banco, and 

locally in its eastern outcrops, the Monserrate is exposed in probable 

depositional, unconformable contact with Cretaceous rocks; but over 

most of the outcrop area east of the Rio Jauca, the Monserrate rests 

conformably on the Rio Prieto Formation. The Anon Formation overlies the Monserrate, with some interlayering in the contact zone. In 

southeastern Adjuntas quadrangle, a thickness of perhaps 1,300 m of 

Monserrate is exposed in an area bounded by faults except on the 

northwest, where the Monserrate is overlain by the Anon.

In west-central Puerto Rico the Monserrate Formation is thin- to 

medium-bedded laminated reddish-gray, greenish-gray, and purplishgray mudstone and tuffaceous sandstone, calcareous and foraminiferal 

in many places. Fine conglomerate, tuff, and lapilli tuff form a minor 

part of the unit. Feldspar, quartz, some pyroxene, and green chloritized 

rock fragments are the most common pyroclastic components; green 

and greenish-brown vitric tuff beds, similar to coarse-grained vitric 

tuff of the Anon Formation (see below), are present at most exposures. The Monserrate is silicified, pyritized, and hydrothermally 

altered in the central part of the Adjuntas quadrangle.

Four collections of Foraminifera have been described from the
STRATIGRAPHY IN WEST-CENTRAL PUERTO RIOO B29

Monserrate Formation in west-central Puerto Rico. Collection AM 2-6, 

at kilometer post 4.4, route 143 (127,290 m E., 35,650 m N.), contains 

the following silicified fauna: Globigerina, ~boweri Bolli; G. sp. aff. 

G. boweri Bolli; G. sp. aff. G. prolata Bolli; Globoratalia densa (Cushman); common Radiolaria, mostly Spumellina (E. A. Pessagno, Jr., 

written commun., 1962). On the basis of these fossils, the collection is 

in the Hantkenina aragonensis zone (of Pessagno) and is early middle 

Eocene. Collection A49-39B, south of the Rio San Patricio at (130,090 

m E.; 33,845 m N.) contains fragments of coralline algae (cf. Archaeolithothamnion) and Amphistegina sp., giving an age determination 

of Tertiary, probably Eocene (E. A. Pessagno, Jr., written commun., 

1963). Pessagno also identified Globigerina sp., keeled early Tertiary 

GloborotcHia sp., and possibly Globorotalia densa, (Cushman) in collection J39-195 from (134,800 m E.; 33,290 m N.), in a tongue of the 

Monserrate in the Anon Formation. He suggested an age of late 

Paleocene or Eocene, possibly middle Eocene, for the collection.

From the Ponce quadrangle, about 270m south of the Jayuya 

quadrangle, K. N. Sa^hs and W. S. Cole (written commun., 1963) 

identified Amphistegina parvula (Cushman), Asterocyclina monticeUensis (Cole and Ponton), and possibly Eoconuloides sp. in collection f-35101 from (140,640m E.; 32,010m N.); they suggest a 

middle Eocene age for that collection.

Weaver (1958, p. 1129) states that Pessagno collected poorly preserved Globotruncana calcarata (Cushman) and G, lapparenti (?) 

from a locality on route 143 that plots near kilometer post 1.6, at 

approximately (126,040 m E.; 35,420 m N.), and gives an age of 

Late Cretaceous, probably Campanian. The four Tertiary localities, 

lithologic similarity to lower Tertiary rocks, and mapping continuity 

with lower Tertiary rocks suggest that this Cretaceous determination 

be rejected, and an age of early Tertiary, probably middle Eocene, 

be accepted for the Monserrate in west-central Puerto Rico. This age 

is confirmed by several other Eocene localities in the formation in 

the Ponce quadrangle (Pessagno, 1960; Sachs and Cole, written 

commun., 1963).

AN6N FORMATION

The Anon Andesite was named by Pessagno (1960) for Hacienda 

Anon on the Rio Inabon in the Ponce quadrangle; the type locality 

is at kilometer post 9.0 on route 511 and below the road in the bed 

of the Rio Inabon, at (137,185m E.; 30,840 m N.). Pessagno considered the unit as a series of instrusive stocks, but further mapping 

north of his mapped area, extended in reconnaissance to his type 

locality, shows that the Anon is largely a complex of lava and pyroclastic rocks with only minor volcanic intrusive rocks. Stocks of rock
B30 ^CONTRIBUTIONS TO STRATIGRAPHY

similar to the Anon do occur in west-central Puerto Rico but are 

shown separately on the geologic maps (Mattson, 1967a, 1967b). Ten 

chemical analyses of the Anon volcanic rocks and associated intrusive 

rocks range in silica content from 52 to 61 percent and show that, 

according to the classification of Rittmann (1952), the rocks are dacite 

(with minor andesite and rhyodacite) rather than andesite as was 

believed by Pessagno. For this reason, and because of its mixed 

lithologies, the name of the unit is here changed to An6n Formation.

The Anon Formation consists of interbedded pyroclastic and flow 

rocks with minor mudstone and volcanic sandstone. It is exposed 

in a northwest-trending zone across west-central Puerto Rico from 

the south-central border of the Jayuya quadrangle to the middle of 

the western border of the Adjuntas quadrangle.

Lava is more abundant than pyroclastic rock in the Anon in the 

Jayuya quadrangle, but is found less frequently farther west. The 

lava is a bluish- or greenish-gray porphyry, containing sparse to 

common phenocrysts of hornblende, plagioclase, some pyroxene, and 

more rarely quartz. The groundmass of most lavas contains felty or 

trachytic feldspar microlites, with interstitial chloritic or glassy 

material, and appears granular in hand specimen, but the groundmass 

of other lavas is a feldspar mosaic that appears cloudy in hand specimen. Amygdules of quartz, chalcedony, and chlorite are common 

at several levels in the formation and are in places elongated parallel 

to trachytic flow directions. Some lava, generally glassy and containing only sparse phenocrysts, forms flow breccias or autobreccias. 

Lavas are shown on the geologic maps in areas where they predominate over pyroclastic rocks, but they are also exposed though less 

abundantly, throughout the outcrop area of the Anon.

Pyroclastic rocks include lapilli tuff, coarse-grained tuff, and 

coarse-grained vitric tuff. The rocks are green, greenish-brown, and 

bluish-gray, are massive, and contain fragments as large as 30 cm 

in diameter in some exposures. The color of the rock is generally controlled by the percentages of green, greenish-brown, reddish-brown, 

and brown fragments of glass containing trachytic or felted feldspar 

microlites. A high percentage of these glassy fragments, from 20 to 

40 percent, distinguishes the Anon pyroclastic rocks from most of the 

Cretaceous pyroclastic rocks. Other fragments in Anon rocks are 

andesites or dacites with phenocrysts of hornblende, pyroxene, feldspar, and sparse quartz; crystal fragments of feldspar, hornblende, 

and sparse pyroxene; and sparse diorite or gabbro. Shard structure 

is very uncommon; sparse cement is chlorite, zeolite, and chalcedony.

Tuffaceous mudstone and sandstone occur as scattered layers and 

thin lenses in the Anon, and conglomeratic tuff layers are exposed
STRATIGRAPHY IN WEST-CENTRAL PUERTO RICO B31

at some places near the top of the formation. Three areas of mediumbedded tuff aceous mudstone and volcanic sandstone occur in the western part of the report area north of Lago Adjuntas. The rocks are 

gray or blue gray, generally calcareous, and contain common Foraminifera and Radiolaria. The westernmost body, exposed on both sides 

of route 526, is medium-bedded laminated tuffaceous sandstone and 

feldspathic tuff; the other two areas are fault blocks of well-bedded 

mudstone that might conceivably be segments of the underlying 

Monserrate Formation.

Highly altered green and greenish-gray volcanic rocks north of 

the San Patricio fault in the Adjuntas quadrangle orginally were 

f eldspathic lavas and tuffs. They are now hydrothermally chloritized, 

epidotized, pyritized, silicified, and locally mineralized, and are here 

correlated with the Anon Formation. This correlation is based mainly 

on the fact that bedded cherts and silicified mudstones, stratigraphically below the volcanic rocks, have been correlated with the Monserrate 

Formation. The correlation is questionable because of the complex 

structure and the high degree of alteration of the volcanic rocks. Pyritization and alteration to calcite and epidote also are common near 

stocks and near some faults, and copper minerals locally are associated 

with the pyritized rocks.

The Anon overlies the Monserrate Formation gradationally; its 

upper contact has been removed by erosion. In the Jayuya quandrangle 

the Anon interfingers extensively with the Monserrate Formation, 

and near Hacienda Cortada it actually underlies the Monserrate, where 

the Monserrate formation pinches out.

A thickness of about 1,400 m of Anon is exposed in the Jayuya 

quadrangle. Complex structure in the Adjuntas quadrangle prevents 

accurate measurement of the thickness of the Anon.

Reconnaissance has established the presence of pyroclastic rocks 

similar to the Anon in a zone extending from the western Adjuntas 

exposures westward across the Monte Guilarte and San Sebastian 

quadrangles into Central La Plata quadrangle, a distance of about 

30 km. In the San Sebastian and Central La Plata quadrangles, Turner (1958) and Hubbard (1923) have described these rocks as the Rio 

Culebrinas "series," and Turner has estimated an exposed thickness 

of about 1,100m.

Two foraminiferal collections from the Anon in west-central Puerto 

Rico have been described by Pessagno (written commun., 1963). He 

found Gflobigerma sp. and keeled GloborotaHia sp. at locality A14-3, 

kilometer post 37.95 on route 10 (121,070 m E.; 41,940 m N.), and also 

at locality A15-31, kilometer post 38.05 on route 10 (121,210 m E.; 

41,770 m N.). The former locality also has common Radiolaria of
B32 CONTRIBUTIONS TO STRATIGRAPHY

probable Tertiary age. Pessagno considers the Foraminifera as probable of late Paleocene or Eocene age. In addition, Pessagno (written 

commun., 1963) has identified Globigerlna sp. and questionably Globorotalia densa in a collection from the Ponce quadrangle, about 500 

m south of the boundary of the Jayuya quadrangle at (134,470 m E.; 

31,780 m N.). According to Pessagno, the age of the collection is late 

Paleocene or Eocene, possibly middle Eocene. The collection is from a 

lens of foraminiferal siltstone in the upper part of the formation, 

probably a faulted extension of the lens shown on the geologic map 

between Rio San Patricio and Rio Cerrillos, in the southwestern corner of the Jayuya quadrangle. To the west, in Central La Plata and 

San Sebastian quadrangles, Mattson (1960, p. 346) described two 

Eocene forminiferal collections, identified by Paul Bronimann in the 

Rio Culebrinas "series" of Hubbard (1923), correlative with the Anon 

and perhaps a younger bedded unit.

The Anon Formation seems to be at about the same stratigraphic 

level across both the Ad juntas and Jayuya quadrangle, and the Monserrate Formation, of middle (?) Eocene age, interfingers with the Anon. 

The Anon can therefore be considered of middle to late Eocene age in 

west-central Puerto Rico; it cannot be as young as the early Oligocene post-orogenic deposits about 10 kilometers to the south in the 

Ponce and Pefiuelas quadrangles (Ruth Todd, written commun., 1963).

STRATIGRAPHIC SUMMARY

The continuous Cretaceous sequence exposed in the eastern half of 

the report area may be divided into three broad parts.

The lowest of these parts contains only the Jayuya Tuff. In westcentral Puerto Rico the contact of the Jayuya with the overlying 

Robles is gradational, but farther east the Robles rests with apparent 

unconformity on strata informally named pre-Robles rocks and here 

correlated, at least for the uppermost part, with the Jayuya Tuff 

(Berryhill and Glover, 1960; Pease and Briggs, 1960; Briggs and 

Gelabert, 1962). As basal Robles strata are Early Qretaceous, probably 

Albian, in age, then the Jayuya Tuff is probably Albian in age or older.

The Robles, Vista Alegre, Tetudn, Malo, and Cotorra Formations 

form a broadly homogeneous, vertically and laterally gradational 

sequence averaging more than 2,000 m in thickness over a large area. 

Mineralogically, these units are very similar, and lithologic types 

dominant in one unit are frequently found to be subordinate rock types 

in other units. This middle part of the Cretaceous sequence is directly 

correlative with the Robles Formation in the Comerio and Barranquitas quadrangles to the east (Pease and Briggs, 1960; Briggs and 

Gelabert, 1962) and is correlated, chiefly on mineralogic and litho-
STRATIGRAPHY IN WEST-CENTRAL PUERTO RICK) B33

logic grounds, with the Rio Orocovis Group (formerly the Rio 

Orocovis Formation) in the Ciales and Corozal quadrangles to the 

northeast (Berryhill, 1965; Nelson, 1966). This part of the sequence 

ranges from Early Cretaceous, probably Albian to Late Cretaceous, 

perhaps Santonian, in age.

The Achiote Conglomerate rests with apparent discomformity on 

strata of the middle part of the Cretaceous sequence. It grades upward 

into the well-stratified Maravillas Formation, which in turn grades 

upward into the massive Coamo Formation; these three form the upper 

part of the Cretaceous sequence. Their thicknesses vary from place to 

place, but in the south-central part of the Jayuya quadrangle, they may 

total more than 2,500 m in thickness. The top of the Coamo is not 

exposed. These formations are correlated with the Cariblanco and 

Coamo Formations of the Coamo quadrangle (Glover, 1961) and are 

Late Cretaceous in age, probably ranging from Santonian to Maestrichtian, but the Achiate may be in part as old as Coniacian.

The fault-separated Yauco Mudstone cannot be directly correlated 

with the continuous Cretaceous sequence in the eastern part of the 

report area. However, it is Late Cretaceous, probably Campanian or 

Maestrichtian, in age in western Puerto Rico and thus may be generally correlated with all or parts of the Maravillas and (or) the 

Coamo Formations.

The Lago Garzas Formation also cannot be directly correlated 

within west-central Puerto Rico. It is Late Cretaceous in age, probably 

Maestrichtian. A correlation with the Coamo Formation is reasonable, 

but the Lago Garzas may be younger than the Coamo and probably is 

younger than at least the lower part of the Yauco Mudstone.

A profound unconformity separates the Cretaceous rocks and the 

lower Tertiary rocks in west-central Puerto Rico (Mattson, 1966) : 

lower Tertiary strata rest unconformably on plutonic rocks and Lower 

and Upper Cretaceous strata. If the assumption is made that the continuous Cretaceous sequence now present in the eastern part of the 

report area once was present where the Rio Prieto Formation now 

overlies the Jayuya Tuff, then perhaps locally 5,000 m of section were 

removed by erosion prior to Rio Prieto time. However, the missing 

Cretaceous section may not have been so thick in these areas.

The lower Tertiary Rio Prieto, Monserrate, and Anon Formations 

are probably middle Eocene to perhaps late Eocene in age, so, on the 

basis of evidence in west-central Puerto Rico, the unconformity 

developed between latest Cretaceous, Maestrichtian, time and middle 

Eocene time; thus, erosion characterized Paleocene time in this area.

The volcanic sequence in west-central Puerto Rico thus represents 

perhaps four episodes of continuous deposition, separated by possible
B34 (CONTRIBUTIONS TO STRATIGRAPHY

local unconformities in Albian time (not seen in the report area but 

present elsewhere), a probable disconformity or moderate unconformity possibly in Coniacian but more probably in Santonian time, 

and a profound unconformity in the Maestrichtian to Eocene interval.

REFERENCES

Berkey, C. P., 1915, Geological reconnaissance of Porto Rico: New York Acad.

Sci. Annals, v. 26, p. 1-70. 

Berryhill, H. L., Jr., 1965, Geology of the Oiales quadrangle, Puerto Rico: U.S.

Geol. Survey Bull. 1184,116 p. 

Berryhill, H. L., Jr., and Glover, Lynn, 3d, 1960, Geology of the Cayey quadrangle,

Puerto Rico: U.S. Geol. Survey Misc. Geol. Inv. Map 1-319. 

Briggs, R. P., 1967, The Malo Breccia and Cotorra Tuff in the Cretaceous of central

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.

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