Cretaceous and Tertiary Rocks 

In the Corozal Quadrangle 

Northern Puerto Rico

By ARTHUR E. NELSON

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1244-C

Prepared in cooperation with the 

Economic Development Administration 

of Puerto Rico
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1966

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

Washington, D.C. 20402 - Price 15 cents (paper cover)
CONTENTS

Page

Abstract________________________________________________________ Cl

Introduction. _____________________________________________________ 1

Stratified rocks __________________________________________________ 2

Cretaceous rocks._____________________________________________ 5

Volcanic breccia.._________________________________________ 5

Andesitelava._____________________________________________ 5

Volcanic sandstone and siltstone __________________________ 6

Rio Orocovis Group. _______-_-_-_-___________-____-_-__- 6

Magueyes Formation._________________________________ 8

Perchas Formation.___________________________________ 9

Avispa Formation.____________________________________ 9

Cuchillas Member.________________________________ 10

Los Negros Formation.________________________________ 10

Stratigraphic relations___-_____-_-_-____--___---___-_- 11

Undivided rocks_________________________________________ 11

Pozas Formation._________________________________________ 11

Cretaceous or lower Tertiary rocks____________________________ 14

Cibuco Formation.____-________-___-_-_-___-_____-_-__--_- 14

Carreras Siltstone-__----___-----___-----__------_-_----__- 14

Hornblende tuff.__________________________________________ 14

Lower Tertiary rocks._________________________________________ 15

Palmarejo Formation._.________________________-_------__- 15

Probable stratigraphic relations of Cibuco Formation, Carreras

Siltstone, hornblende tuff, and Palmarejo Formation.________ 15

Corozal Limestone_______________________________________ 16

Ortiz Formation_________________________________________ 17

Middle Tertiary rocks_________-_______-______----___-_-___-_- 17

San Sebastidn Formation.__________________________________ 17

Mucarabones Sand-_______________________________________ 18

Lares Limestone_________________________________________ 18

Cibao Formation._________________________________________ 19

References cited.__________________________________________________ 20

ILLUSTRATIONS

Page 

FIGURE 1. Index map of the Corozal quadrangle and part of the Naranjito

q uadrangle __ ________________________________________ C2

2. Generalized geologic map of the Corozal quadrangle._________ 4

3. Schematic diagram showing stratigraphic relations between 

formations of the Rfo Orocovis Group and rock types in each 

formation___________________________________________ 12

4. Generalized correlation diagram of the Rfo Orocovis Group. __ 13

m
IV CONTENTS

TABLE

Page

TABLE 1. Stratigraphic succession of consolidated rocks presently recognized in the Corozal quadrangle.-_-_-----___---_-_-_-___ C3
CONTRIBUTIONS TO STRATIGRAPHY

CRETACEOUS AND TERTIARY ROCKS IN THE COROZAL 

QUADRANGLE, NORTHERN PUERTO RICO

By ARTHUR E. NELSON

ABSTRACT

Approximately 11,600 meters of Cretaceous and lower Tertiary volcanogenic 

rocks are exposed in the Corozal quadrangle, and these rocks are unconformably 

overlain in the northern fifth of the quadrangle by about 470 meters of middle 

Tertiary marine deposits. Faulting has divided the Cretaceous and lower 

Tertiary rocks into three distinct stratified sequences. An unnamed Cretaceous 

volcanic sequence, probably the oldest group of rocks in the quadrangle, is in the 

northeast quadrant. The next youngest sequence is the Rio Orocovis Formation 

of earlier workers and other Cretaceous rocks that are exposed in the south half 

of the quadrangle. Upper Cretaceous and lower Tertiary rocks, the youngest 

sequence, are exposed in a westerly to northwesterly trending band near the 

central part of the quadrangle. Stratigraphic relations between these rock 

sequences are obscure.

Detailed mapping and recognition of a greater number of units have made it 

necessary to raise the Bio Orocovis Formation to group rank and its former 

members to formation rank. Two newly divided stratigraphic units in the 

group are herein named the Los Negros Formation and the Cuchillas Member (of 

the Avispa Formation).

In addition it is necessary to name and define or to adopt several formations in 

the Upper Cretaceous and lower Tertiary sequence: a unit of Late Cretaceous or 

early Tertiary age is named the Cibuco Formation; three lower Tertiary units 

are named the Palmarejo Formation, the Corozal Limestone, and the Ortiz Formation. A unit of middle Oligocene age is herein designated the Mucarabones 

Sand.

INTRODUCTION

This report presents changes in stratigraphic nomenclature in a succession of Cretaceous and Tertiary rocks in the Corozal quadrangle in 

northern Puerto Rico. Six stratigraphic units are formally named; 

one previously used unit is redefined and adopted for use by the U.S. 

Geological Survey and one previously named formation has been 

raised to group rank and its members to formation rank. Stratigraphic studies in the Corozal quadrangle (fig. 1) are part of a larger 

program of geologic mapping and mineral resource studies of Puerto

Cl
C2 CONTRIBUTIONS TO STRATIGRAPHY

160,000 

meters

18°22'30

18°15

(BARRANQUITAS QUAD)

FIGURE 1. Map of the Corozal quadrangle and part of the Naranjito quadrangle.

Kico by the U.S. Geological Survey in cooperation with the Economic 

Development Administration of Puerto Rico.

Many of the stratigraphic units occurring in the Corozal quadrangle 

are named herein. To present their relations to the other formations 

more clearly, all Cretaceous and Tertiary stratigraphic units in the 

quadrangle are described. Table 1 shows the stratigraphic succession 

of these rocks in the Corozal quadrangle.

The proposed classification of Fisher (1961) is used in the description of the volcanic rocks.

STRATIFIED ROCKS

The rocks of the Corozal quadrangle constitute a complex of Cretaceous to lower Tertiary volcanogenic deposits. These rocks have 

been intruded by diorite and gabbro and are unconf ormably overlain 

by middle Tertiary marine calcareous and related deposits in the 

northern fifth of the quadrangle. The complex, consisting of approximately 11,600 meters of marine volcanogenic rocks, has been divided by faulting into three distinct stratified sequences. The locations of these sequences are shown on figure 2. Area 1 is underlain
CRETACEOUS AND TERTIARY ROCKS, COROZAL QUADRANGLE C3

TABLE 1. Stratigraphic succession of consolidated rocks presently recognized in the

Corozal quadrangle

Formation Member

Upper member

Upper Oligocene and 

lower Miocene

Miranda Sand 

Member

Cibao Formation Quebrada Arenas 

Limestone Member

Rio Indio Limestone 

Member

Middle and upper 

Oligocene Lares Limestone

Middle Oligocene

Mucarabones Sand*

San Sebastidn Formation 

-Unconformity 

Ortiz Formation*

Paleocene or Eocene Corozal Limestone*

Palmarejo Formation*

Hornblende tuff

Upper Cretaceous or 

lower Tertiary

-Break in sequenceCarreras Siltstone (equivalent? 

to Palmarejo Formation)

Cibuco Formation*

(equivalent? to hornblende tuff)

-Break in sequenceUpper (?) and Upper 

Cretaceous

Pozas Formation (equivalent? 

to undivided rocks)

Undivided rocks

-Break in sequenceLower and Upper 

Cretaceous

Los Negros Formation*

Avispa Formation* Cuchillas Member*

Perchas Formation*

Magueyes Formation* 

-Break in sequence 

Volcanic sandstone

Cretaceous Andesite lava

Volcanic breccia

'Redefined, adopted, or named and defined in this report.
C4 CONTRIBUTIONS TO STRATIGRAPHY

M

 O
CRETACEOUS AND TERTIARY ROCKS, COROZAL QUADRANGLE C5

by an unnamed Cretaceous volcanic sequence, probably the oldest 

group of rocks in the quadrangle; area 2 is underlain mostly by the 

Rio Orocovis Group, and area 3 by Upper Cretaceous and lower Tertiary rocks, the youngest sequence in the complex. Area 4 shows the 

distribution of middle Tertiary deposits that have a maximum thickness of 470 m and that unconformably overlie rock units in areas 1 

and 3.

CRETACEOUS ROCKS

The oldest rocks in the Corozal quadrangle are probably those in 

the northeastern part (area 1, fig. 2). They include volcanic breccia, lava, and well-bedded volcanic sandstone and siltstone. Because 

this unnamed sequence is bounded by faults, its stratigraphic relations to the other stratified units in the quadrangle could not be 

determined. The stratigraphic units in this sequence are described 

but are not named; they are of Cretaceous age.

VOLCANIC BRKCCIA

Volcanic breccia and associated rocks occupy a small area in the 

northeastern part of the quadrangle (fig. 2). In the Corozal quadrangle the unit consists mostly of volcanic breccia with some interstratified discontinuous beds of volcanic sandstone and a lava lens. 

The volcanic breccia, generally poorly exposed and partly weathered, 

is a characteristic reddish purple and contains numerous fragments 

of andesite. The rocks strike generally west-northwest and dip moderately to the south. Only the upper 300 m of the volcanic breccia 

is exposed, but in the adjacent Naranjito quadrangle this sequence is 

at least 2,000 m thick (M. H. Pease, Jr., written commun., 1964). 

On the north and northwest, the volcanic breccia is overlapped by 

middle Tertiary deposits; to the south and southwest, it is overlain 

by andesite, but because of poor exposures in this part of the Corozal 

quadrangle the nature of the contact could not be established.

ANDESITE LAVA

The next younger unit is composed of porphyritic andesite and 

associated rocks exposed in the northeastern part of the quadrangle 

(fig. 2). This unit contains lenticular flows of porphyritic andesite 

interstratified with andesitic volcanic breccia, volcanic sandstone and 

siltstone. The lava is dark greenish gray, weathers brownish gray, 

and commonly contains pillow structures and amygdules. Plagioclase phenocrysts, which characteristically have a glomeroporphyritic 

habit, compose about 18-24 percent of a sample, by volume, but the 

percentage varies greatly from bed to bed. The groundmass is 

usually minutely crystalline. Interstratified with the flows are massive layers of volcanic breccia and lenticular units of distinctly

228-591 66 t 2
C6 CONTRIBUTIONS TO STRATIGRAPHY

bedded volcanic sandstone and siltstoiie. The breccia contains angular to subrounded andesite fragments lithologically similar to the 

lava; the matrix contains fragments and crystals of plagioclase and 

pyroxene and tiny pumiceous particles. Hydrothermal alteration 

has affected much of the andesite unit so that in some areas the original texture and lithology cannot be determined.

In the Corozal quadrangle, the andesite unit has a maximum thickness of 1,800 m. Like the older volcanic breccia, this unit is bounded 

on the northwest by overlapping middle Tertiary deposits; to the 

south and southwest these rocks are conformably overlain by the volcanic sandstone and siltstone unit (fig. 2).

VOLCANIC SANDSTONE AND SILTSTONE

The next younger unit consists of interbedded volcanic sandstone 

and siltstone; the unit forms a narrow belt of south-dipping rocks 

that extends from near the middle of the east border of the quadrangle 

to a short distance east of the town of Corozal (figs, 1, 2). Part of 

this unit is also present in a fault wedge along the east edge of the 

quadrangle 3.5 kilometers south of the north border. The volcanic 

sandstone and siltstone are commonly tuffaceous and less commonly 

calcareous. The unit also contains some massive layers of conglomerate and several andesite flows. The cobbles and pebbles in the conglomerate are mostly andesite, and the flows are similar to those previously described in the underlying unit. Only about 500 in of the 

lower part of this clastic unit is present in the quadrangle, but in the 

Naranjito quadrangle, this sequence has a maximum thickness of 

1,100 m (M. H. Pease, Jr., written commun., 1964). In. the Corozal 

quadrangle, the upper part of the volcanic sandstone and siltstone 

has been removed by faulting, and the lower part rests conformably 

on the older andesite lava unit.

BIO OROCOVIS GROUP

The term Rio Orocovis Formation (Berryhill, 1965, p. 19) was first 

used for a sequence of volcanic rocks in the next quadrangle to the 

west, the Ciales quadrangle. Four members, comprising 4,500 m of 

rock, wy ere included in the formation: the Magueyes Member at the 

base, the Perchas Lava Member, the Avispa Lava Member, and a 

basalt tuff member. Because some of these members were divided 

into additional mappable units in the Corozal quadrangle, the Rio 

Orocovis Formation is here raised to the rank of group and the former 

members are here raised to formations. In addition, basaltic tuff 

that is contiguous with the basalt tuff member of the Rio Orocovis 

Formation of Berryhill (1965) is here named the Los Negros 

Formation.
CRETACEOUS AND TERTIARY ROCKS, COROZAL QUADRANGLE C7

The Rio Orocovis Group, a major stratigraphic division of the 

Cretaceous System in east-central Puerto Rico, occupies most of the 

south half of the quadrangle (fig. 2). It comprises a thick accumulation of basalt, andesite, basaltic tuff, sandstone, siltstone, and breccia 

primarily of volcanic origin. The Magueyes Formation is composed 

chiefly of epiclastic rock, the Perchas Formation of basalt, the Avispa 

Formation of andesite, and the Los Negros Formation of basaltic 

tuff. Although one lithologic type is the principal constituent in 

each formation, the Magueyes Formation, for example, contains, in 

addition to volcanic sandstone and siltstone, members of basalt and 

andesite and basaltic tuff that are similar to the Perchas, Avispa, and 

Los Negros Formations, respectively.

Owing to the complex depositional history, the stratigraphy of the 

Rio Orocovis Group is characterized, as is the stratigraphy of the 

entire Cretaceous System in Puerto Rico, by abrupt lateral and vertical facies changes. In places, parts of one formation in the Rio 

Orocovis Group are time equivalents of parts of another formation 

in the group. In general, however, most of the Perchas Formation 

is younger than the Magueyes Formation; most of the Avispa Formation is younger than the Perchas Formation; and the Los Negros 

Formation, which has the greatest stratigraphic range, is for the most 

part the youngest formation in the group.

Rocks of the Rio Orocovis Group apparently accumulated in an 

active volcanic terrain where predominantly volcanic material of 

various types and chemical compositions was derived from several 

different sources. Although each formation represents an interval of 

fairly continuous deposition of one type of rock, there were repeated 

instances when, from different sources and for relatively short intervals, other lithologic types of this volcanic suite were simultaneously 

deposited. This volcanic material apparently erupted from vents 

and long fissures on the ocean floor. Thick lava and coarse pyroclastic 

debris probably accumulated near source areas; ash and reworked 

volcanic debris were deposited over a much broader area and interfingered with the near-vent deposits.

Approximately 3,000 m of the Rio Orocovis Group is present in the 

Corozal quadrangle. Relations with older and younger rocks are not 

known, and the cumulative thickness of the group is uncertain. The 

base of the group is not exposed, the top is faulted out, and other parts 

may be repeated by folding and faulting.

Diagnostic fossils have not been found in the rocks of the Rio 

Orocovis Group in the Corozal quadrangle. On the basis of stratigraphic relations to younger fossil-bearing rocks elsewhere in Puerto 

Rico, the Rio Orocovis Formation was assigned to a probable Turo-
C8 CONTRIBUTIONS TO STRATIGRAPHY

nian(?) to late Santonian(?) or early Oampanian age by Berryhill 

(1965, p. 45). Fossils in rocks equivalent to the lower part of the Rio 

Orocovis Group in the Barranquitas quadrangle, south of the Corozal 

quadrangle, have been dated by N". F. Sohl (written commun., 1963) as 

Albian. Hence, the lower part of the Rio Orocovis Group is probably 

Early Cretaceous, and the upper part apparently is Late Cretaceous.

MAGUEYES FORMATION

The Magueyes Formation (Berryhill, 1965, p. 21) occupies most of 

the southwestern part of the quadrangle (fig. 2). There it consists 

chiefly of alternating beds of volcanic sandstone and siltstone and 

interstratified amygdaloidal pillow basalt, pillow andesite, basaltic 

flow breccia, basaltic tuff, volcanic breccia, and lapilli tuff.

Good exposures of volcanic sandstone and siltstone, basaltic tuff, and 

andesite can be seen in discontinuous outcrops on the westerly trending section of Route 568 west of Berio in barrios Magueyes and Palmarito. The best exposures of basalt are on Route 568 approximately 1.4 

km north of the south boundary of the quadrangle.

The dark-greenish-gray volcanic sandstone and dark-gray volcanic 

siltstone are composed mostly of tuffaceous material. The sandstone 

ranges from massive and thick bedded to thin bedded; some beds are 

graded. It is fine to coarse grained and is characterized by crystals 

and fragments of dark-green clinopyroxene. Chlorite and epidote 

commonly are present, and in some places calcite has been introduced 

and has made the rock decidedly calcareous.

The volcanic breccia contains fragments of andesite in a matrix of 

small crystals and fragments of plagioclase and pyroxene. Lapilli 

tuff contains pale-green pumice fragments, some lithic fragments of 

lava, and small grains of pyroxene and plagioclase in a matrix of glass 

and microcrystalline material. The basaltic flow breccia is an accumulation of large fragments of basalt, and individual samples are 

similar in composition to basalt.

Some of the thicker basalt flows, andesite flows, and basaltic tuff 

deposits have been mapped locally in the Corozal quadrangle as unnamed members.

A maximum of 1,400 m of the Magueyes is exposed in the southwestern part of the quadrangle, but the formation thins to the east. 

Although a part of the Magueyes is conformably underlain by rocks 

equivalent to the Perchas, some of the lower part of the formation is 

faulted out in the adjoining Barranquitas quadrangle to the south 

(Briggs and Gelabert, 1962), and the lower contact relations are not 

known. The upper part of the formation is conformably overlain by 

the Perchas Formation. The Magueyes in the Corozal quadrangle 

was said to be contiguous with strata mapped as Cariblanco Forma-
CRETACEOUS AND TERTIARY ROCKS, COROZAL QUADRANGLE C9

tion in the northern Barranquitas quadrangle (Briggs and Gelabert, 

1962). Later work, however, has shown that the strata mapped as 

Cariblanco Formation in the northern part of the Barranquitas quadrangle are a part of the Magueyes Formation (-K. P. Briggs, written 

commun., 1965).

PERCHAS FORMATION

The Perchas Formation (Berryhill, 1965, p. 25), named for exposures in the Ciales quadrangle, extends in a narrow belt eastsoutheastward across the Corozal quadrangle into the adjoining Naranjito and Barranquitas quadrangles (fig. 2). The Perchas consists 

mostly of lenticular flows of dark-greenish-gray to dark-grayish-black 

pyroxene-rich basalt. Individual flows, ranging from 20 to 50 m in 

thickness, commonly are pillowed; some grade into highly vesicular 

flow breccia. Scattered discrete beds of volcanic sandstone and siltstone and lenses of basaltic tuff and andesite are also present. The 

sandstone and siltstone are simliar to the rocks composing most of the 

underlying Magueyes Formation. The tuff and andesite are similar 

to the rocks of the younger Los Negros and Avispa Formations. 

Some of the thicker units of volcanic sandstone and siltstone, andesite, 

and basaltic tuff have been mapped as unnamed members in the 

Perchas Formation.

Approximately 800 m of the formation is exposed in the Corozal 

quadrangle. Contacts with the underlying Magueyes and overlying 

Avispa and, locally, Los Negros Formations are conformable.

AVISPA FORMATION

The Avispa Formation (Berryhill, 1965, p. 28) occupies a crudely 

lenticular area north of the Perchas Formation in the southern part 

of the quadrangle (fig. 2). It extends westward into the Ciales 

quadrangle but thins abruptly toward the east and pinches out just 

west of the east edge of the Corozal quadrangle. The formation is 

a heterogeneous unit consisting primarily of lenticular pillow andesite interbedded with volcanic breccia, intraformational conglomerate, discrete beds of volcanic sandstone, siltstone, and tuff, and 

basalt flows. The lava ranges from greenish gray to dark gray and 

has a purplish cast locally; it weathers dull brown. Several unnamed members of the various lithologic types have been mapped. A 

distinctive lenticular unit of interbedded volcanic breccia and volcanic sandstone and siltstone is present near the top of the lower 

third of the formation. This unit, which separates the lava flows in 

the lower part from those in the upper part, is named the Cuchillas 

Member and is described in the following paragraph. The maximum 

thickness of the formation in the Corozal quadrangle is estimated to
CIO CONTRIBUTIONS TO STRATIGRAPHY

be 1,800 m. Contacts with the underlying Perchas and overlying Los 

Negros Formations are conformable.

Cuchillas Member. The Cuchillas Member of the Avispa Formation is exposed in the southern part of the Corozal quadrangle (fig. 

2) and is here named after barrio Cuchillas in Mimicipio de Corozal 

where it is best exposed. The type locality is along the banks of the 

Rio Grande de Manati between coordinates 50,000-159,000 and 

50,800-158,660 (reference to meter grid based on Puerto Rico coordinate system shown by ticks at margin of fig. 1). The Cuchillas 

Member consists of dark-grayish-grcen massive volcanic breccia and 

lapilli tuff interstratified with fine-grained dark-greenish-gray to 

dark-gray volcanic sandstone, siltstone, and crystal tuff. The breccia facies of the member occurs as large, discontinuous, somewhat 

lenticular bodies separated by the finer grained discretely bedded 

facies. The breccia and lapilli tuff, which are poorly sorted, are 

made up of andesite fragments in a fine-grained matrix of rock fragments, plagioclase and pyroxene crystals, some pumice, and some 

small unidentified mineral fragments. The Cuchillas Member is 

about 500 m thick in the type locality, but eastward along strike it is 

quite variable in thickness where it intertongues with andesite lava of 

the Avispa Formation. Contacts with the enclosing lava are poorly 

exposed. The Cuchillas Member is considered to be of Late Cretaceous age.

LOS NEGROS FORMATION

A unit of basaltic tuff trends eastward across the middle of the 

Corozal quadrangle (fig. 2). Berryhill (1965) mapped contiguous 

rocks in the Ciales quadrangle as the basalt tuff member of the Rio 

Orocovis Formation. This pyroclastic unit is here named the Los 

Negros Formation after the Rio Los Negros along which the formation 

is well exposed. The type area is along the Rio Los Negros and the 

Camino Del Banco road between coordinate 51,680 and 53/T10, and the 

type locality is in the Rio Los Negros valley between coordinate 51,800 

and 52,140. Excellent easily accessible exposures of the formation 

also occur along Route 814 from 200 m west of the bridge over the Rio 

Guadiana to 500 m east of the bridge.

The Los Negros Formation consists predominantly of numerous 

layers of massive crystal-lithic basaltic tuff but includes some basalt 

flow breccia and lapilli tuff. In fresh exposures, the formation is 

mostly dark greenish gray to dark greenish black, but in weathered 

exposures it is characteristically light olive green. Rounded somewhat globular masses of basalt, averaging about 10 centimeters in 

diameter, are randomly scattered throughout the tuff. These globular 

masses are similar in lithology to the basalt of the Perchas Formation.
CRETACEOUS AND TERTIARY ROCKS, COROZAL QUADRANGLE Cll

Several unnamed members and small lenses of basalt and andesite 

similar to those of the Perchas and Avispa Formations, respectively, 

are included in the formation. Lenses of volcanic sandstone and 

siltstone, ranging from less than 30 cm to more than 6 m in thickness, 

are also irregularly distributed throughout the massive tuff.

As much as 1,000 m of the formation is exposed in the Corozal quadrangle and 1,800 m in the Naranjito quadrangle (M. H. Pease, Jr., 

written commun., 1964). Throughout most of its extent in the Corozal 

quadrangle, the Los Negros conformably overlies the Avispa Formation. In the southeastern part of the quadrangle, however, it overlies 

the Perchas Formation. Because of faulting, the upper part of the 

Los Negros is not exposed in either the Corozal or adjacent quadrangles. The formation is considered to be of Late Cretaceous age.

STRATIGRAPHIC RELATIONS

The stratigraphy of the Rio Orocovis Group is complicated by much 

and unusually abrupt interfingering of units; each formation contains 

interstratified units that are lithologically similar to the principal 

rock types of the other formations in the group (figs. 3, 4), and interpretation of the stratigraphy is further complicated by intense 

deformation. Generally, the upper parts of a formation intertongue 

with the lower parts of an overlying formation. Commonly, formations and members are lenticular; the f ormations thin abruptly 'and, in 

places, pinch out in other formations of the group. Thicknesses of 

stratigraphic units are, therefore, variable.

UNDIVIDED ROCKS

Unidentified rocks occur in the southern half of the quadrangle in 

several small slices in fault zones along the easterly trending faults 

(fig. 2). The rocks are weathered to a deep-reddish-purple saprolite 

whose texture resembles that of highly weathered volcanic breccia of 

the Upper Cretaceous Pozas Formation exposed in the Ciales quadrangle (Berryhill, 1965). Presumably these rocks are of Late Cretaceous age.

The Blacho Tuff Member of the Pozas Formation (Berryhill, 

1965, p. 51) is present in a small area in the southwestern part of the 

quadrangle (fig. 2). It consists of partly welded tuff composed of 

pumice and dense lava fragments and is underlain by pillowed 

andesite. The tuff is reddish gray and on weathering turns to a 

characteristic deep reddish purple; the lava is a greenish gray to dark 

gray. Less than 50 m of the Blacho is present in the quadrangle.
FIGTJEE 3. Stratigraphic relations between formations of 

in each formation
CRETACEOUS AND TERTIARY ROCKS, COROZAL QUADRANGLE C13

Breccia in float along the contact between the Blacho Tuff Member 

and the Magueyes Formation suggests that the contact is a fault. The 

lower part of the Pozas Formation is not exposed in the Corozal quadrangle, but in the Ciales quadrangle (Berryhill, 1965, p. 49), the Pozas 

conformably overlies rocks that are underlain by the Rio Orocovis 

Group.

Fossils have not been found in the Blacho Tuff Member in the 

Corozal quadrangle. In the Florida quadrangle (Nelson and Monroe, 

1966), the Pozas Formation contains Campanian to Maestrichtian 

fossils (N. F. Sohl, written commun., 1963).

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Approximate location of 

section lines in the Corozal 

quadrangle (south half)

METERS

r-600

- 500 

400

- 300

- 200

- 100

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Principal lithologies

Basaltic EH] Andesite 

n

tuff lava

FIGTJBE 4. Generalized correlation diagram of the Rio Orocovis Group. 

Only major lithologic types are shown.
C14 CONTRIBUTIONS TO STRATIGRAPHY

CRETACEOUS OB LOWER TERTIARY ROCKS 

CIBTJCO FORMATION

A conspicuous mappable unit of massive conglomerate and thinner 

bedded clastic rocks that is well exposed in the west-central part of the 

quadrangle (fig. 2) is here named the Cibuco Formation for exposures 

in the Rio Cibuco. The type locality is in the valley of the Rio Cibuco 

between coordinates 55,560 and 55,180, about 3 km west-southwest of 

Corozal. The Cibuco Formation is composed mostly of massive conglomerate or breccia-conglomerate. Pebbles and some angular fragments of porphyritic lava, tuff-breccia, tuff, and diorite are present; 

detrital quartz and plagioclase grains are common in the finely crystalline matrix. Massive conglomerate in the lower part of the formation 

grades upward into thinner bedded conglomerate interstratified with 

volcanic sandstone, siltstone, and bedded tuff, and in the upper part 

of the formation finer grained clastic rocks predominate. The conglomerate is dark gray where fresh, but where weathered it has a 

medium-reddish cast.

About 2,000 m of the Cibuco is exposed in the quadrangle, but because of faulting the lower contact is not exposed. The Cibuco is 

overlain by and interfingers^with the lower strata of the Carreras 

Siltstone (Berryhill, 1965, p. 63-65). The Carreras as defined by 

Berryhill is Late Cretaceous or early Tertiary in age, and presumably 

so is the Cibuco.

CARRERAS SILTSTONE

The Carreras Siltstone is exposed in the west-central part of the 

Corozal quadrangle (fig. 2). It consists of thin-bedded dark-gray to 

olive-black locally calcareous volcanic siltstone and some thin beds of 

volcanic sandstone and tuff. A maximum thickness of about 600 m 

is exposed in the quadrangle. The contact with the underlying Cibuco 

Formation is placed at the top of the uppermost conglomerate bed. 

Overlapping middle Tertiary deposits conceal the upper part of the 

formation.

The Carreras Siltstone is Late Cretaceous or early Tertiary in age 

(Berryhill, 1965, p. 63-65).

HORNBLENDE TUFF

A distinctive unit of hornblende-bearing tuff and associated epiclastic rocks in the east-central part of the quadrangle extends northwestward to the town of Corozal (figs. 1, 2), where it is concealed by 

overlying middle Tertiary rocks. These reddish-brown to grayishpurple hornblende-bearing rocks include beds of massive hornblende 

crystal tuff, tuffaceous conglomerate with some diorite fragments,
CRETACEOUS AND TERTIARY ROCKS, COROZAL QUADRANGLE C15

laharic deposits, and volcanic sandstone. This hornblende tuff ranges 

in thickness from 50 to 450 m.

Owing to faulting, the lower part of this unit is not exposed in the 

quadrangle. Massive crystal tuff at the top of the sequence is conformably overlain by fine-grained volcanic sandstone of the Palmarejo 

Formation. Lithologic similarities and stratigraphic position suggest that the hornblende tuff and associated rocks may be equivalent to 

the Cibuco Formation in the western part of the quadrangle, and they 

are, therefore, also probably Late Cretaceous or early Tertiary in age.

LOWER TERTIARY ROCKS 

PA.LMAREJO FORMATION

An alternating sequence of thin-bedded volcanic sandstone and siltstone is here named the Palmarejo Formation for exposures in barrio 

Palmarejo (figs. 1, 2). The type locality is designated as the valley 

of the Rio Mavilla between coordinates 166,960 and 167,220 about 750 

m south-southeast of Escuela Segunda Unidad de Palmarejo. The 

formation is exposed almost continuously from the middle of the 

"east border of the quadrangle westward to the town of Corozal, 

where it pinches out between the hornblende tuff and the Corozal Limestone. Both sandstone and siltstone are gray to dark gray. The 

sandstone is medium to fine grained and in places calcareous. Its 

maximum thickness is almost 370 m in the area near Escuela Segunda 

Unidad de Palmarejo. In the eastern part of the outcrop belt, much 

of the upper part of the formation is faulted out along the Quebrada 

San Francisco fault. The Palmarejo conformably overlies the hornblende tuff, and a fine-grained sandstone at the top of the formation is 

conformably overlain by a fairly thick bed of buff limestone in the 

base of the Corozal Limestone.

Because of lithologic similarities and stratigraphic position, the 

Palmarejo Formation may be equivalent to the Carreras Siltstone of 

Cretaceous or Tertiary age in the western part of the quadrangle. 

The Palmarejo Formation contains fossils that range from Paleocene 

to Eocene in age (E. A. Pessagno, written commun., 1965). Pessagno 

identified Foraminifera collected by V. M. Seiders as Pseudophraginina sp.; the specimens were from an outcrop across the road from 

the cemetery on Route 164 in the eastern part of the Corozal quadrangle.

PROBABLE STRATIGRAPHIC RELATIONS OP CIBUCO FORMATION, 

CARRERAS SILTSTONE, HORNBLENDE TUFF, AND PALMAREJO 

FORMATION

The Cibuco Formation and the Carreras Siltstone in the west-central 

part of the Corozal quadrangle may be equivalent in age to the horn-
C16 CONTRIBUTIONS TO STRATIGRAPHY

blende tuff and Palmarejo Formation, respectively, in the east-central 

part. Although these two pairs of formations are separated from 

each other by a fault, they are similar in several respects. They 

occupy the same relative stratigraphic position, and in both sets of 

formations the older unite are coarse elastics, whereas the younger 

rocks are fine well-bedded volcanic siltstone and sandstone. In addition, fragments of diorite occur in both the Cibuco Formation and 

parts of the hornblende tuff.

It may be significant that fragments of intrusive rock, even though 

they are common in the Cibuco and only rarely occur in the hornblende tuff, are present in both formations. Dioritic intrusive rocks 

were not emplaced in rocks of the Corozal quadrangle until the later 

part of the Cretaceous Period, and dioritic fragments have not been 

observed in the older rocks of the Corozal quadrangle. It is suggested, 

therefore, that both the Cibuco Formation and hornblende tuff were 

deposited after the emplacement of dioritic intrusive rocks, possibly 

about the same time. If the Cibuco Formation and the hornblende 

tuff are correlative, then the Carreras Siltstone might well be equivalent to the Palmarejo Formation.

COROZAX, LIMESTONE

A conspicuous unit of limestone exposed near the town of Corozal 

is here adopted as the Corozal Limestone (figs. 1, 2), as originally 

proposed by Berkey (1915, p. 23). The type locality is in the area 

from coordinates 54,620-165,720 north to coordinate 54,750. Northwest of the town of Corozal, the formation is overlapped by middle 

Tertiary rocks; 2 km southeast of Corozal the formation pinches out 

between the Palmarejo and Ortiz Formations, but a small lens 

reappears a little farther southeast along strike.

The formation is made up mostly of lenticular beds of light-gray 

limestone and grayish-red limestone breccia. In the lower part, above 

a basal limestone bed, thin beds of fine-grained calcareous sandstone, 

siltstone, and a bentonitic clay bed are interstratified with lenses of 

limestone. Higher in the section the limestone is more massive, commonly fluted and pitted, and in places weathers to a rubbly mass. A 

nodular limestone underlies a f ragmental limestone in the upper part 

of the formation. Massive coarse breccia is well exposed in a quarry 

just west of the town of Corozal. Fragments consist mostly of limestone, but small amounts of volcanic rocks are also present. The 

matrix is somewhat calcareous and consists mostly of grayish-red shale 

and sandy material. Thin interbeds of shale and limestone are also 

present in the breccia. The thickness of the formation is variable; it 

has a maximum thickness of 150 m, and its average thickness is about 

80 m.
CRETACEOUS AND TERTIARY ROCKS, COROZAL QUADRANGLE C17

The Corozal Limestone rests on the Palmarejo Formation and is 

conformably overlain by the Ortiz Formation. The contact with the 

Ortiz is locally indistinct; it is placed at the base of a highly calcareous 

coarse sandstone at the base of the Ortiz.

Kaye (1956, p. 115-117) listed fossils collected from the Corozal 

quarry and elsewhere in the Corozal Limestone; on the basis of these 

fossils he considered the formation to be Paleocene or Eocene in age.

ORTIZ FORMATION

A sequence of well-bedded volcanic sandstone, siltstone, and tuff 

exposed near the settlement of Ortiz (figs. 1, 2) is here named the 

Ortiz Formation. The type locality is in the Rio Corozal between 

coordinates 55,320 and 55,720. The formation strikes northwest and 

dips southwest; it is present west, south, and southeast of the town of 

Corozal. To the southeast it is faulted out by the Quebrada San 

Francisco fault, and to the northwest it is unconformably overlain by 

middle Tertiary deposits.

Highly calcareous thick-bedded coarse sandstone with interbeds of 

dusky-red shale occurs at the base of the formation. This sandstone is 

overlain by a thin conglomerate bed containing many limestone pebbles. The upper part of the exposed section consists mostly of interstratified somewhat varicolored, red, green, and dark-gray volcanic 

sandstone and siltstone and pale-green tuff. A thin gray limestone 

lens is mapped in the upper part of the exposed formation. In places 

bentonitic clay occurs. The formation has a maximum thickness of 

600m.

The Ortiz overlies the Corozal Limestone conformably and is, therefore, presumed to be of Paleocene or Eocene age. The upper part of 

the Ortiz is not exposed because of faulting.

MIDDLE TERTIARY ROCKS 

SAN SEBASTIAN FORMATION

The San Sebastian Formation (Zapp and others, 1948) occurs in 

an irregular westerly trending band in the northern part of the quadrangle (fig. 2). It consists predominantly of reddish-brown and gray 

sandy carbonaceous clay, locally containing pebbles and cobbles derived 

from the underlying volcanic rocks. At some places it contains large 

lenses of clayey and silty fine to very fine sand and thin beds of calcareous sandy clay. Its thickness varies but is generally about 70 m. 

The San Sebastian rests with angular unconformity on the Cretaceous 

and lower Tertiary volcanic rocks and is of middle Oligocene age 

(Zapp and others, 1948).
CIS CONTRIBUTIONS TO STRATIGRAPHY

MUCARABONES SAND

A distinctive unit of quartz sand extends across the northern part 

of the quadrangle (fig. 2). It is here named the Mucarabones Sand 

after barrio Mucarabones in the northwest corner of the adjacent 

Naranjito quadrangle where the formation is well exposed along the 

Quebrada Piiia. The type locality is along Highway 861, 700-2,000 

in northwest of its intersection with Highway 827, near the northwest 

corner of the Naranjito quadrangle. The formation is grayish-orange 

to grayish-yellow generally crossbedded quartz sand interbedded with 

glauconitic calcareous sandstone and thin beds of calcareous clay. 

Locally the sand contains a few persistent layers of hard sandy lime- -* 

stone. Although texture is varied, most of the sand is medium 

grained. The Mucarabones ranges from 5 to 50 m in thickness in the 

Corozal quadrangle.

The Mucarabones Sand clearly overlies the San Sebastian Formation in the eastern part of the quadrangle. Toward the west the 

formation grades laterally into the middle and upper Oligocene Lares 

Limestone. As far west as the longitude of Corozal, the sand is 40-50 

m thick and is overlain by an increasingly thick sequence of LaresLimestone. West of Corozal, successively lower parts of the sand 

grade laterally into the Lares until near the western boundary of the 

quadrangle only 5 m of calcareous glauconitic sandstone separates the 

San Sebastian from the overlying Lares.

In barrio Quebrada Arenas near the northeast corner of the quadrangle, the sand contains at the top a persistent bed of very glauconitic 

sandy clay rich in tests of the Foraminifera Lepidocyclina undosa. 

The Mucarabones is of middle Oligocene age.

LARES LIMESTONE

The Lares Limestone (Zapp and others, 1948) crops out in a westerly trending band in the northern part of the quadrangle (fig. 2). 

It is mostly a pale-grayish-orange fairly pure limestone that is locally 

fragmental. Beds in the upper part are commonly about 20-30 cm 

thick. The limestone is about 130 m thick in the western part of the 

quadrangle but thins eastward and grades into the Mucarabones Sand. 

In barrio Quebrada Arenas and Contorno near the northeast corner 

of the quadrangle, only the upper 20 m of the Lares is present, and 

this part of the Lares consists of nontypical grayish-orange thinbedded calcareous clay. The Lares has a sharp contact with the conformably overlying Cibao Formation.

The Lares Limestone is of Oligocene age (Zapp and others, 1948).
CRETACEOUS AND TERTIARY ROCKS, COROZAL QUADRANGLE C19 

CIBAO FORMATION

The Cibao Formation (Monroe, 1962), which is present in a band 

across the northern part of the Corozal quadrangle (fig. 2), is divided 

into four members. At the base is the Rio Indio Limestone Member, 

whose basal beds of dark-yellowish-orange fragmental limestone are 

in striking contrast to the underlying grayish-orange limestone of the 

Lares. Higher in the Rio Indio Member are some thin beds of hard 

grayish-orange limestone that resemble the Lares closely and may be 

confused with it. The Rio Indio, which is about 100 m thick west of 

the Rio Cibuco, gradually thins eastward to about 50 m in the barrio 

rContorno and rests on the upper clay of the Lares Limestone. 

" The Rio Indio Member grades upward into the overlying Quebrada 

Arenas Limestone Member, which consists of pale-orange to grayishorange finely crystalline to granular limestone in beds ^-2 m thick 

interbedded with grayish-orange chalky limestone in beds averaging 

one-half meter in thickness. The member contains a local 6-m bed 

of sandy limestone 1 km east of Cerro Santa Barbara. The Quebrada 

Arenas ranges from 60 to TO m in thickness.

v^At two places in the quadrangle, channels about 10 m deep were 

eroded in the top of the Quebrada Arenas Limestone Member and 

filled with alluvium. In the extreme northwest corner, this alluvium, 

known as the Miranda Sand Member, consists of clayey red and gray 

mottled medium to coarse sand; in the area near Cerro Santa Barbara, 

, a similar sand contains a basal unit of coarse gravel.

Resting in sharp contact with the Quebrada Arenas Limestone Member and gradationally with the Miranda Sand Member, where it is 

present, is about 50 m of locally sandy calcareous clay and clayey sand 

mapped as an upper unnamed member of the Cibao Formation.

The Cibao Formation is of Oligocene and Miocene age (Monroe, 

1962).
C20 CONTRIBUTIONS TO STRATIGRAPHY 

REFERENCES CITED

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

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

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

Geol. Survey Bull. 1184,116 p. 

Briggs, R. P., and Gelabert, P. A., 1962, Preliminary report of 4 he geology of the

Barranquitas quadrangle, Puerto Rico: U.S. Geol. Survey Misc. Geol. Inv.

Map 1-336. 

Fisher, R. V., 1961, Proposed classification of volcanoclastic sediments and rocks:

Geol. Soc. America Bull., v. 72, p. 1412. 

Kaye, C. A., 1956, The lower Tertiary of Puerto Rico: Am. Assoc. Petroleum

Geologists Bull., v. 40, p. 108-121. 

Monroe, W. H., 1962, Geology of the Manati quadrangle, Puerto Rico: U^

Geol. Survey Misc. Geol. Inv. Map 1-334. 

Nelson, A. E., and Monroe, W. H., 1966, Geology of the Florida quadrangle,

Puerto Rico : U.S. Geol. Survey Bull. 1221-C, 22 p. 

Zapp, A. D., Bergquist, H. R., and Thomas, C. R., 1948, Tertiary geology of the

coastal plains of Puerto Rico: U.S. Geol. Survey Oil and Gas Inv. Prelim.

Map 85, 2 sheets.

U.S. GOVERNMENT PRINTING OFFICE:1966