Cretaceous and Lower 

Tertiary Stratigraphy in 

Northwestern Puerto Rico

By DAVID H. McINTYRE, JOHN M. AARON, and OTHMAR T. TOBISCH

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1294-D

Prepared in cooperation with the 

Commonwealth of Puerto Rico 

Economic Development Administration 

Industrial Research Department
UNITED STATES DEPARTMENT OF THE INTERIOR 

WALTER J. HICKEL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

U.S. GOVERNMENT FEINTING OFFICE, WASHINGTON : 1970

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

Washington, D.C. 20402 - Price 20 cents
CONTENTS

Page

Abstract__ ______________________________________________________ Dl

Introduction._____________________________________________________ 1

Cretaceous System._______________________________________________ 6

Yauco Mudstone___-_____-_______________________-___-___-_-_- 6

Maricao Basalt______________________________________________ 7

Rio Blanco Formation____________________________________-____ 7

Tertiary System...________________________________________________ 10

Concepci6n Formation_________________________________________ 10

Mal Paso Formation________-_-_-__-______________-___-_-__---- 11

Rio Culebrinas Formation___________________________________-_- 12

Discussion._______________________________________________________ 15

References cited._____._____-______________________________________ 16

ILLUSTEATIONS

Page 

FIGURE 1. Index map showing location of report area_______-___----__- D2

2. Geographic and physiographic map of northwestern Puerto

Rico showing features cited in text______-^_________-___- 3

3. Generalized geologic map of northwestern Puerto Rico______ 5

CONTRIBUTIONS TO STRATIGRAPHY

CRETACEOUS AND LOWER TERTIARY STRATIGRAPHY 

IN NORTHWESTERN PUERTO RICO

By DAVID H. MCINTTRE, JOHN M. AARON, and OTHMAR T. TOBISCH

ABSTRACT

Clastic products of submarine volcanism constitute the bulk of the upper 

Cretaceous and lower Tertiary stratified rocks of northwestern Puerto Rico. 

The rocks are separated into two contrasting suites by a major northwesttrending wrench fault. Southwest of this fault, at least 500 m (meters) of 

thin-bedded sandstone and calcareous mudstone, the Yauco Mudstone, and 

more than 1,900 m of coarse basaltic tuff and tuff breccia, the Maricao Basalt, 

are exposed. Both formations are of Late Cretaceous (Campanian-Maestrichtian) age. Northeast of the major fault, an indeterminate thickness of intricately faulted oxidized basalt and andesite tuff breccia and coarse tuff of Late 

Cretaceous (Campanian-Maestrichtian) age, the Rio Blanco Formation, is in 

fault contact with rocks of Eocene age. The oldest unit of the Eocene sequence, 

as much as 1,100 m thick, is the Concepcion Formation, which contains plagioclase porphyry lavas and pyroclastic rocks and rhyodacitic vitric and pumice 

tuffs. The Concepci6n is overlain conformably by as much as 1,000 m of amygdaloidal basalt lavas and pyroclastic rocks of the Mal Paso Formation. Conformably overlying the Mal Paso Formation is the Rio Culebrinas Formation, which 

contains as much as 3,000 m of andesitic and dacitic tuff breccias, coarse tuffs, 

^nd thin-bedded volcanic sandstone and mudstone. The Rio Culebrinas has a distinctive thin-bedded siliceous and calcareous mudstone, the Guacio Member, at 

its base. Middle Eocene Foraminifera have been collected from the Mal Paso and 

Rio Culebrinas Formations.

The unconformity between Upper Cretaceous and Eocene rocks recognized in 

west-central Puerto Rico may also be present in northwestern Puerto Rico.

INTRODUCTION

The geology of northwestern Puerto Rico is being studied as part of 

a program of geologic mapping and mineral resource investigation 

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

with the Economic Development Administration, Commonwealth of 

Puerto Rico.

Dl
D2 CONTRIBUTIONS TO STRATIGRAPHY

Upper Cretaceous and lower Tertiary stratified volcanic rocks of 

the San Sebastian, Central la Plata, and Rincon quadrangles of northwestern Puerto Rico (figs, land2) are chiefly the products of submarine volcanic eruptions. They include both volcaniclastic rocks and 

lavas, although volcaniclastic rocks are far more abundant. These 

rocks range in composition from basalt to rhyodacite and they range 

in age from Campanian-Maestrichtian to middle Eocene.

The formations exposed in the report area are separated into two 

contrasting suites by a major northwest-trending sinistral wrench 

fault (fig. 3), the Cerro Goden fault. The Yauco Mudstone and Maricao Basalt (Campanian-Maestrichtian) are southwest and south of 

the fault. Northeast and north of the fault are the Rio Blanco Formation (Campanian-Maestrichtian), Concepcion Formation (Eocene), 

Mal Paso Formation (middle Eocene), Rio Culebrinas Formation 

(middle Eocene), and the Milagros Formation (middle Eocene).

Special acknowledgement is due E. A. Pessagno, Jr., K. N. Sachs, 

Jr., and N. F. Sohl for their fossil identifications and biostratigraphic 

interpretations.

In this report, locations of specific points are referred to the Puerto 

Rico Meter Grid System coordinates in this fashion: (82,640 E.; 

62,7901ST.). Grid system reference marks can be found on the margins 

of all quadrangle maps of the island. The coordinate numbers that 

appear on the west and east margins of the 1958 edition of the San 

Sebastian T1/^-minute quadrangle are in error. Coordinates of points 

in the San Sebastian quadrangle therefore have been adjusted to conform to the correct values shown on adjacent quadrangles.

ATLANTIC OCEAN

SAN JUAN nn,

PUERTO RICO

SEA

20

0 10 20 30 40 KILOMETERS 

I I I I I

FIGURE 1. Location of the report area (shaded).
66°52'30"

Finca 

San Miguel - T ^ , Las Marias J ( BUENA .QneKrada

01 2345 MILES

012345 KILOMETERS

FIGURE 2. Map of northwestern Puerto Rico showing geographic and physiographic features cited in text.
CONTRIBUTIONS TO STRATIGRAPHY
EXPLANATION

 e s 3 

I I Alluvium

;Tg

Rio Guatemala 

Group \ w' h

ANGULAR UNCONFORMITY

LltrH

Rio Culebrinas 

Formation

Mal Paso 

Formation

Tc

Concepcion 

Formation

Milagros 

Formation

NE OF CERRO GOD EN FAULT

SW OF CERRO 

GODEN FAULT

Maricao 

Basalt

Yauco 

Mudstone

FIGURE 3. Generalized geologic map of northwestern Puerto Rico.

Contact

Fault 

Dotted where concealed

Anticline

Showing c.restline

\- Syncline

rf Showing troughline

CERRO GORDO

X X X X X

Type locality and 

type area for formations defined in 

this report
D6 CONTRIBUTIONS TO STRATIGRAPHY

Eock classifications used in this report are based on thin-section 

study and chemical analyses as well as field study; they generally 

follow Wentworth and Williams (1932) and Williams, Turner, and 

Gilbert (1954), with reference to the average analyses of Nockolds 

(1954).

CRETACEOUS SYSTEM

YAUCO MUDSTONE

The Yauco Mudstone (Mattson, 1960,1967) in the Central la Plata 

and Eincon. quadrangles consists chiefly of dark-gray pyritic thinbedded calcareous mudstoiie and calcilutite containing Radiolaria and 

abundant planktonic Foraminifera, but it also has some thin-bedded 

chert and thick- to thin-bedded volcanic sandstone containing plagioclase, clinopyroxene, aphyric rock fragments, and pumice. In the area 

2 km (kilometers) east of Aiiasco, thin- to thick-bedded coarse tuff and 

massive tuff breccia containing abundant altered pumice are prominent constituents of the formation. A pillow lava containing clinopyroxene phenocrysts was seen 400 m (meters) southwest of the Cerro 

Goden fault on Highway 108.

The Yauco Mudstone is the most widespread formation now recognized in western Puerto Eico; outcrops of this unit extend somewhat 

continuously at least from the southwest corner of the Adjuntas quadrangle (Mattson, 1967) to Punta Cadena in the Eincon quadrangle a 

distance of 55 km.

Foraminifera from samples collected along the Eio Casey north of 

Finca San Miguel were examined and identified by E. A. Pessagno, Jr. 

(written commun., 1968).

Locality MOP-325 (83,000 E.; 47,760 N.) : 

Foraminifera:

Globotruncana linneiana (d'Orbigny) ?; preservation poor. 

Abundant Radiolaria ; mostly Spongodiscacea. 

Age: Probably early Campanian-early Maestrichtian; Globotruncana fornicata stuartiformis Assemblage Zone. 

Locality MCP-328 (83,340 E.; 47,380 N.) :

Foraminifera:

Globotruncana elevata (Brotzen) 

GloMgerlnelloides sp. 

Heterohelix sp.

Age: Late Campanian to late Maestrichtian; Globotruncana fornicata 

stuartiformis Assemblage Zone, G. elevata, Subzone; G. contusa stuartiformis Assemblage Zone.

These ages agree with many other Campaniaii-Maestrichtian age determinations for the Yauco Mudstone elsewhere in Puerto Eico (Mattson, 1967).
NORTHWEST PUERTO RICO D7

A thickness of at least 500 m of Yauco Mudstone is present in the 

area, but little confidence can be placed even in this approximate figure 

because of small-scale folding.

MARICAO BASALT

The name Maricao Basalt of Mattson (1960) is herein adopted for 

a sequence of thick-bedded to massive coarse basaltic tuff and tuff breccia and thin-bedded coarse tuff exposed southeast of Anasco. Mattson 

named the Maricao Basalt for exposures "on the Rio Maricao north of 

Maricao" (1960, p. 337), but he did not formally propose a type locality. Better exposures of the formation occur along Route 120 northwest of Hacienda Balare, about 400 m north of the Rio Maricao (93,600 

E.; 40,000 N.), and these exposures are herein designated as the type 

locality. The formation is at least 1,400 m thick in the Central la Plata 

quadrangle.

The tuff breccias generally contain angular to subangular lava blocks 

that have clinopyroxene and plagioclase phenocrysts, although one 

sample contains hornblende rather than clinopyroxene. The blocks generally are scoriaceous and have amygdule fillings of chlorite. Some 

samples of tuff-breccia matrix contain pumice fragments, which are 

now altered to chlorite.

The coarse tuff and the tuffaceous matrix of the tuff breccias contain 

clinopyroxene and plagioclase crystal fragments, small fragments of 

lava and pumice, and recrystallized fossil debris that are chiefly tests 

of larger Foraminifera. Finely disseminated pyrite is common. These 

rocks generally range from light gray to dark greenish gay, but oxidized fragments impart a dark-red color to a few samples. Incipient 

alteration to secondary epidote, chlorite, and prehnite was observed 

in some samples.

The Maricao Basalt overlies the Yauco Mudstone in the Central la 

Plata quadrangle. The two formations are concordant and are probably conformable. Mattson (1960) indicated that the two formations 

intertongue in the area immediately to the south. If this interpretation 

is correct, the Maricao Basalt should be approximately the same age 

as the Yauco Mudstone (Campanian-Maestrichtian).

RIO BLANCO FORMATION

The name Rio Blanco Formation is herein adopted for rocks of Late 

Cretaceous (Campanian-Maestrichtian) age that form a major part of 

the Rio Blanco Series of Hubbard (1923). Hubbard proposed no type 

locality for this unit. More recently, Mattson (1960) used the name 

Rio Blanco Formation for rocks in the northeastern part of the Mayagiiez area (fig. 1), but he also did not designate a type locality.
D8 CONTRIBUTIONS TO STRATIGRAPHY

Hubbard (1923, p. 28) noted that "The best exposures of the Rio 

B]anco Series are found along the sugar railroad east of Aiiasco * * *" 

that is, according to his map, along the north side of the Rio Anasco 

valley in Barrios Miraflores and Corcovada. The type area we have 

chosen for the Rio Blanco Formation includes exposures corresponding to those "best exposures" mentioned by Hubbard. The exposures 

occur intermittently along Route 109, the Rio Anasco, and from Quebrada Alto Sano (88,700 E.; 51,300 N.) to about 200 m west of Quebrada Cerro Gordo (85,000 E.; 50,200 N.).

Outcrops of the Rio Blanco Formation are in an area of deep tropical weathering; thus, no extensive outcrops of fresh rock were found 

even in deep roadcuts or along major streams. Most exposures consist 

of reddish-orange, red, reddish-purple, or purple saprolite in which 

only the gross textural characteristics of the parent rock generally can 

be discerned.

The Rio Blanco Formation consists chiefly of massive tuff breccia 

containing angular to well-rounded blocks of basalt and pyroxene or 

hornblende andesite. Amygdaloidal blocks are present, but they seldom 

can be recognized as such in saprolite exposures. Iron-bearing minerals 

in the tuffaceous matrix generally are oxidized, giving fresh samples 

of the rock their characteristic dark-red or reddish-purple color. The 

blocks generally are not oxidized.

Coarse tuff, either massive or thin bedded, contains basalt and 

pyroxene andesite rock fragments and crystal fragments of plagioclase and clinopyroxene or hornblende.

One lava flow was seen in this unit in a roadcut on Highway 109 

southwest of Quebradala Balza.

Over much of its known outcrop area, particularly along the Rio 

Anasco, the formation has been cut by many faults of unknown displacement, intruded by many dikes of granodioritic composition, and 

partly metamorphosed to the albite-epidote hornfels facies. However, 

in some areas Barrio Miraflores in the Municipio de Anasco and a 

small area in Barrio Anones, Municipio de Las Marias the metamorphism has only reached zeolite grade.

Three limestone exposures were found in the Rio Blanco Formation, 

all in the Maricao quadrangle south of the report area (fig. 1). Fossils 

collected from these localities provide the only direct evidence concerning the age of the formation.

USGS locality 27009 (93,900 E.; 43.900 N.) : Quarry immediately west of Route 

120 on Sierra de Naranjal about 3 km south of Las Marias, Barrio Maravilla 

Sur, Municipio de Las Marias. Fossils identified by N. F. Sohl (written commun., 1959).
NORTHWEST PUERTO RICO D9

Pelecypoda:

Durania sp.

Durania nicholasi (Whitfield)

Radiolitids

Indeterminate fusifonn gastropod 

Foraminifera abundant 

Age: Oampanian-Maestrichtian.

*JSGS locality 27010 (92,520 E.; 43,350 N.) : Cut on Route 120, 0.8 km south of 

USGS locality 27009. Fossils identified by N. F. Sohl (written commun., 1959) 

Pelecypoda:

Radiolitid indeterminate 

Antillocaprina sp. 

Durania sp.

Age: Campanian-Maestrichtian.

CTSGS locality f-35157 (94,630 E.; 46,120 N.) : Quebrada Mayagiiecillo, Barrio 

Buena Vista, Municipio de Las Marias. Fossils identified by K. N. Sacks. Jr. 

(written commun., 1967). 

Foraminifera:

Pseudorbitoides sp. cf. P. israelsJcyi Vaughan and Cole 

Age: Campanian Maestrichtian.

Mattson (1960, p. 345) reported that Foraminifera identified as

 Sulcoperculina sp. aff. S. vermunti and aff. 8. dickersoni were collected 

lay T. R. Slodowski from "the quarry south of Las Marias on Route 

120." All these fossils indicate a Campanian-Maestrichtian age for the 

Hio Blanco Formation.

According to N. F. Sohl (written commun., 1968), age ranges for the 

rudists suggest that the limestones in the Rio Blanco should be restricted to the late Campanian or early Maestrichtian.

The Rio Blanco Formation is separated from the Yauco Mudstone 

and Maricao Basalt by the Cerro Goden fault. The contact between the 

Rio Blanco and younger, lower Tertiary formations to the north also 

is faulted (fig. 3). Attitudes of well-bedded rocks in the Rio Blanco 

indicate that the formation has been complexly deformed, probably for 

the most part by movement along the many faults that cut it.

Because no marker horizons were noted that could be used in measuring the amount of deformation within the unit, and because the bedding, rarely seen, varies erratically in attitude, 110 meaningful estimate 

of the stratigraphic thickness can be made at present. The estimates 

previously made by Mattson and by Turner (Mattson, 1960, p. 344) 

have no validity because they were based on the incorrect assumption 

that the formation was part of a simple southwest-dipping homocline.
D10 CONTRIBUTIONS TO STRATIGRAPHY 

TERTIARY SYSTEM 

CONCEPCION FORMATION

Near-vent dacitic and rhyodacitic pyroclastic rocks and lavas associated with two separate, but related vent areas in northwestern Puerto 

Rico are herein named the Concepcion Formation for exposures in 

Barrio Purisima Concepcion.

Typical exposures of the Concepcion Formation are found along the 

Rio Aiiasco near the Pioute 119 bridge (94,620 E.; 49,270 N.), along 

the road that follows the boundary between Barrios Purisima Concepcion and Buena Vista (97,380 E.; 47,420 N. to 98,265 E.; 48,400 

N.) and in Quebrada Velez (98,170 E.; 46,200 N. to 98,460 E.; 47,570 

N.). At the Rio Anasco locality thin-bedded tuff is well exposed. That 

part of the formation dominated by lavas is exposed along the Purisima 

Concepcion road. The intimate relation of lava and the intrusive rocks 

of the vent area can be seen in Quebrada Velez. The maximum thickness of the formation in this area is about 1,100 m.

The Concepcion Formation exposed in Barrios Purisima Concepcion and Buena Vista, Municipio de Las Marias, contains both volcaniclastic rocks and lavas. The western part of the formation in this 

area is chiefly gray to green thin-bedded tuff containing altered plagioclase, pyroxene, or hornblende crystal fragments, fragments of plagioclase porphyry, and some pumice. Toward the east, however, lavas and 

closely related small intrusive bodies predominate. Two types of lavas 

are present: pale-gray to greenish-gray lava that contains abundant 

plagioclase, commonly as phenocrysts, and dark-green to gray lava 

that contains abundant clinopyroxene phenocrysts. Tuff breccias occur 

with the lavas. All these rocks have been somewhat altered, and many 

samples now contain secondary epidote, chlorite, prehnite, or zeolites. 

Pyrite is a common accessory mineral in many samples.

Other exposures of the Concepcion Formation are related to another 

vent area about 14 km west-northwest of the area discussed above. 

Rocks immediately surrounding this second vent, the position of which 

is marked by the intrusive body north of Cerro Gordo, consist chiefly 

of greenish-gray rhyodacitic pumice lapilli tuff and coarse crystalvitric-pumice tuff. The formation becomes progressively finer grained 

away from the vent area. Toward the southwest the lapilli tuff gives 

way to thin-bedded crystal-vitric tuff. Exposures 4 km east of the vent 

area reveal thin-bedded volcanic sandstone and mudstone. An exposure 

of thin-bedded tuff 2 km northwest of the vent contains poorly preserved planktonic Foraminifera. The Concepci6n is more than 800 m 

thick near Cerro Gordo.
NORTHWEST PUERTO RICO Dll

In addition to abundant pumice and altered glass shards, the Concepcion Formation in this area also contains plagioclase (andesine); 

very small amounts of hornblende, pyroxene, quartz, pyrite, and magnetite; and a few red dense oxidized lava fragments. The lava fragments are more common near the contact with the overlying Mal Paso 

Formation. Glass is replaced by chlorite or by microscopic intergrowths 

of quartz and potash feldspar.

In the area northeast of Afiasco, the contact between the Conception 

Formation and the overlying Mal Paso Formation is conformable; 

some interbedding of lithologic types characteristic of the two formations was observed near the contact. The contact is drawn at the base 

of the first amygdaloidal lava or dominantly purple pyroclastic rock 

typical of the Mal Paso Formation. Toward the east, however, the 

contact is a fault.

The Mal Paso Formation is middle Eocene in age; therefore, the 

conformably underlying, rather thin, rapidly deposited Concepcion 

Formation surely is no older than Eocene.

MAL PASO FORMATION

The Mal Paso Formation, herein named for Barrio Mal Paso in the 

Rincon quadrangle, is a widespread and readily recognizable unit in 

northwestern Puerto Rico. The type area for the Mal Paso Formation 

is in Barrios Mal Paso and Piedras Blancas, Municipio de Aguada, in 

the northeast part of the Rincon quadrangle. Typical exposures can 

be seen along Route 416 for 2.7 km from the northern border of the 

Rincon quadrangle. In addition, excellent exposures are found along 

several secondary roads in Barrio Piedras Blancas, especially the one 

that joins Route 416 about 1.4 km south of the junction of Routes 416 

and 417 (73,800 E.; 59,130 N.) and follows a ridge southwest of the 

Rio Culebra. The formation is estimated to be at least 1.000 m thick 

in the type area.

The most distinctive rock type of the Mal Paso Formation is grayish-red-purple amygdaloidal basalt lava in which the amygdule fillings 

are white calcite and zeolites. The lavas generally are pillowed and 

commonly contain olivine pseudomorphs. Locally, pale-red-purple 

impure limestone containing planktonic Foraminifera occurs between 

lava pillows as lenses a few centimeters thick. Amygdaloidal basalt 

breccias commonly are associated with the pillowed flows. In addition, 

the formation contains crystal and crystal-vitric tuffs that are various 

shades of reddish purple, green, and gray; some contain abundant 

silicic glass and pumice in addition to basaltic detritus and resemble 

tuffs of the underlying Concepcion Formation. These rocks indicate
D12 CONTRIBUTIONS TO STRATIGRAPHY

that vents supplying silicic detritus still were active during a time 

dominated by basaltic volcanism.

Lava flows, or possibly intrusive rocks, of keratophyric plagioclase 

porphyry crop out for about 200 m along Route 119 about 1.2 km 

northwest of the Guacio Mission (93,500 E.; 49,630 N.). Chemical 

analyses of these rocks resemble one analysis of lava in the Concepcion Formation exposed a few kilometers to the southeast.

The Mal Paso Formation conformably overlies the Conception 

Formation and is, in turn, overlain conformably by the Rio Culebrinas 

Formation. Direct evidence concerning the age of the Mal Paso Formation is provided by Foraminifera in calcareous tuff that outcrops 

at La Tosca, Barrio Caguabo, Municipio de Aiiasco; these were identified by E. A. Pessagno, Jr. (written commtm., 1968).

Sample 255-A-54 (70,350 E.; 51,840 N.) : 

Foraminifera:

Globorotalia, s.s. sp. (keeled) 

G. aragonensis Nuttall 

G. densa (Cushman) 

Globigerina sp. 

Age: lower middle Eocene

RIO CULEBRINAS FORMATION

During his pioneer study in northwestern Puerto Rico, Hubbard 

(1923) used the name Rio Culebrinas Series for rocks that crop out 

in the valley of the Rio Culebrinas. This name has been in common use 

to the present day (Turner, 1958; Mattson, 1960, 1967), although no 

type locality has been formally proposed. The name is very useful and 

is herein adopted and redefined. As redefined, virtually all rocks included in the Rio Culebrinas Series of previous reports are included 

in the Rio Culebrinas Formation.

The Rio Culebrinas Formation contains four principal lithic types. 

The base of the formation is marked by a remarkably persistent thinbedded siliceous and calcareous mudstone and impure limestone, 

herein designated the Guacio Member. The Guacio Member is overlain 

by very thick bedded tuff breccia, very thick to thick-bedded coarse 

crystal-lithic tuff, and thin-bedded volcanic sandstone and mudstone 

that form the bulk of the formation. The relative proportions of these 

three rock types vary greatly from place to place.

The superb exposures of the Rio Culebrinas Formation for 8.6 km 

-along Route 2 in the Rincon quadrangle, from the intersection with 

Route 417 (75,700 E.; 59,600 N.) to the intersection with Route 402 

(75,900 E.; 52,100 N.), are designated the type area for the formation. 

All lithologies, except that of the Guacio Member, are present in these 

exposures. Thin-bedded rocks predominate, forming approximately
NORTHWEST PUERTO RICO D13

60 percent of the rocks exposed. Coarse tuff forms about 35 percent 

of the exposures, and tuff breccia, about 5 percent.

The type locality for the Guacio Member of the Rio Culebrinas 

Formation is in Barrio Guacio, Municipio de San Sebastian, and consists of the exposures along Route 433, immediately east of its junction 

with Route 119 (93,200 E.; 50,600 N.). Partly weathered thin-bedded 

mudstone typical of most outcrops of the member is exposed in roadcuts along the northwest-trending ridgecrest. In addition, fresh rock 

can be collected in the borrow pit beside the road on the inside of the 

bend at the northwest end of the ridge. The member is about 100 m 

thick at the type locality. Use of the name Guacio for this unit conforms to earlier informal usage by M. D. Turner (unpub. data). Fresh 

outcrops of the Guacio Member located closer to the type locality of 

the Rio Culebrinas Formation are those in Quebrada Caguabo, 5 km 

southwest of the Route 2 exposures.

The tuff breccias of the Rio Culebrinas Formation are very thick 

bedded and poorly sorted; they contain both subangular and rounded 

blocks ranging from 3 cm (centimeters) to more than 1 m in diameter 

in a matrix of poorly sorted coarse crystal-lithic tuff. The blocks are 

chiefly andesite or dacitic andesite that contain phenocrysts of plagioclase and hornblende. However, some contain clinopyroxene phenocrysts and others contain rare quartz phenocrysts. Algal fragments 

and larger Foraminifera locally are conspicuous constituents of the 

tuffaceous matrix. Blocks of coarse tuff and limestone are also present 

locally.

The very thick to thick-bedded coarse tuff contains volcanic rock 

fragments that have plagioclase and hornblende, or plagioclase and 

clinopyroxene, phenocrysts. Altered green pumice fragments are common, as are single crystals of plagioclase, clinopyroxene, hornblende, 

quartz, magnetite, and diagenetic pyrite. Algal fragments and broken 

tests of larger Foraminifera locally are abundant.

The thin-bedded volcanic sandstone and mudstone contains the same 

clastic constituents as the coarse tuff. In addition, planktonic Foraminifera are conspicuous in some samples.

These three lithologic types commonly grade into one another and 

are interbedded at all scales. Crude large-scale grading in size of 

blocks is apparent in many outcrops of the tuff breccia. Graded bedding is exceedingly common in the thin-bedded volcanic sandstone 

and mudstone.

The thin-bedded siliceous and calcareous mudstone that is the dominant lithology in the Guacio Member contains variable amounts of 

silt-sized volcanic debris and organic detritus, chiefly tests of planktonic Foraminifera and Radiolaria. With decreasing proportion of 

volcanic debris, this lithology grades into siliceous limestone. Most
D14 CONTRIBUTIONS TO STRATIGRAPHY

outcrops of the member are deeply weathered and consist of brittle, 

blocky, chertlike material interbedded with clay.

The Rio Culebrinas Formation conformably overlies the Mal Paso 

Formation, though previous workers (Hubbard, 1923; Turner, 1958 

and unpub. data; Mitchell, 1954) believed that the Rio Culebrinas was 

overlain by the rocks we now are including in the Mal Paso and Rio 

Blanco Formations. Turner (oral cominun., 1967; unpub. data) believed the Guacio to be the youngest unit of the Rio Culebrinas Formation. However, our detailed mapping in the Central la Plata quadrangle has shown that the section near the Rio Culebrinas-Mal Paso 

contact southwest of San Sebastian is overturned. Many examples of 

overturned graded bedding were seen in that area and areas to the 

southeast in the San Sebastian quadrangle. Mapping farther west in 

the Central la Plata and Rincon quadrangles and paleontologic data 

provide ample corroboration for this interpretation.

The Rio Culebrinas Formation is overlain with angular unconformity by rocks of the middle Tertiary Rio Guatemala Group. The middle 

Oligocene San Sebastian Formation (Zapp and others, 1948) overlies 

the Rio Culebrinas in most of the area. North of Rincon, however, the 

Rio Culebrinas is overlain by the Cibao Formation (Monroe, 1968). 

The Rio Culebrinas Formation is about 3,000 m thick in the Rincon 

quadrangle and is about 2,800 m thick in the western part of the Central la Plata quadrangle. Only about 1,700 m are preserved in the area 

southwest of San Sebastian.

Many outcrops of the Rio Culebrinas Formation contain well-preserved tests of larger Foraminifera. Most determinations indicate a 

middle Eocene age for the formation, although some of the taxa do 

range into the late Eocene. The following is a list of fossils from an 

outcrop in the lower part of the formation in Barrio Cruz, Municipio 

de Moca, near the northwest corner of the Central la Plata quadrangle 

(K. N. Sachs, Jr., written commun., 1967).

USGS locality f-35158 (81,420 E.; 59,530 N.) : 

Foraminifera:

Nummulites ivilcoxi (Heilprin), abundant

Amphistegina sp., common

Discocyclina (Discocyclina) marginata (Cushman), abundant

Asterocyclina habanensis Cole and Bermudez, abundant

Lepidocyclina (Polylepidina) antillea Cushman, rare

Sphaerogypsina sp., rare

Eorupertia sp. cf. E. lermudezi Ansigard, rare 

Algal fragments, abundant

Sachs regards this as a typical middle Eocene fauna, closely resembling that described by Cole and Gravell (1952) from Peiion Seep, 

Cuba.
NORTHWEST PUERTO RICO D15 

DISCUSSION

Many uncertainties remain concerning correlation of formations discussed in this report with those exposed elsewhere in Puerto Eico. The 

problems involving the early Tertiary formations are particularly 

acute; at present no detailed correlations between northwestern and 

west-central Puerto Rico are possible, chiefly because the intervening 

area has not been studied in detail. Mattson (1967), however, has suggested some correlations, based upon his detailed work in west-central 

Puerto Rico and reconnaissance to the northwest. According to Mattson (1967, p. B23), the rocks we here define as the Mal Paso Formation 

may correlate with the Anon Formation of west-central Puerto Rico. 

Available paleontologic evidence shows that the ages of the two formations are indeed similar. However, the lithologic characters of the two 

formations contrast sharply; the Mal Paso is dominated by basalt 

lavas and pyroclastic rocks, whereas the Anon Formation is chiefly 

dacite, according to the Rittman definition (Mattson, 1967, p. B30).

At another point in his discussion Mattson (1967, p. B31) correlated the Rio Culebrinas with the Anon Formation. These two units 

may well be genetically related. However, the Anon contains much 

lava, whereas no lava has been recognized in the Rio Culebrinas. The 

Anon may prove to be material deposited close to one of the vents that 

supplied detritus for the Rio Culebrinas. Detailed work in the Monte 

Guilarte quadrangle will be necessary before this relation can be considered more than a possibility.

Closer at hand, a correlation problem exists between the Rio Culebrinas Formation and the Milagros Formation (Nelson and Tobisch, 

1967) that crops out in the eastern part of the San Sebastian quadrangle (fig. 3) and in the Bayaney quadrangle. Exposures northwest 

of Quebrada Negrito in Barrio Espino, Municipio de Lares, show that 

the contact between rocks of the Rio Culebrinas Formation and those 

of the underlying Milagros are conformable and gradational. Moreover, the lithologies of the two formations are similar. The Guacio 

Member of the Rio Culebrinas, always present at the base of the Rio 

Culebrinas in the area to the southwest, is not present where the Rio 

Culebrinas rests on the Milagros. This contact may not be equivalent 

to the basal contact of the Rio Culebrinas that is recognized in the area 

to the southwest, and the Milagros may be in part equivalent to the 

lower part of the Rio Culebrinas of the southern area.

Foraminifera recently collected from the Matilde Formation, which 

conformably underlies the Milagros. tend to support this interpre-
D16 CONTRIBUTIONS TO STRATIGRAPHY

tation. The fossils were identified by E. A. Pessagno, Jr. ( written? 

commun., 1969).

Locality B-5 (106,540 E. ; 50,800 N. ) : 

Foraminifera :

Globorotalia lelineri Cushman and Jarvis

G. aragonensis Nuttall 

Age : early middle Eocene

This determination indicates that the overlying Milagros Formationmost probably is middle Eocene in age, the same age as the Rio Culebrinas Formation.

A very significant result of Mattson's work in west-central PuertoRico was the recognition of an important unconformity between Cretaceous and middle Eocene rocks in that area (Mattson, 1966, 1967) . 

Paleocene rocks and possibly also lower Eocene rocks are missing in 

northwestern Puerto Rico, so the unconformity may be present in this 

area as well.

REFERENCES CITED

Cole, W. S., and Gravell, D. W., 1952, Middle Eocene Foraminifera from

Seep, Matanzas Province, Cuba : Jour. Paleontology, v. 26, p. 708-727. 

Hubbard, Bela, 1923, The geology of the Lares district, Porto Rico: New Yort

Acad. Sci. Scientific Survey of Porto Rico and the Virgin Islands, v. 2, pt. 1^

p. 1-115. 

Mattson, P. H., 1960, Geology of the Mayagiiez area, Puerto Rico: Geol. Soc.

America Bull., v. 71, p. 317-361.

 1966, Unconformity between Cretaceous and Eocene rocks in central PuertoRico : Caribbean Geol. Conf., 3d., Jamaica 1962, Trans., p. 49-53.

 1967, Cretaceous and lower Tertiary stratigraphy in west-central PuertoRico : U.S. Geol. Survey Bull. 1254-B, 35 p.

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

Agr. Expt. Sta. Tech. Paper 13,167 p.

Monroe, W. H., 1968, The Aguada Limestone in northwestern Puerto Rico: U.S. 

Geol. Survey Bull. 1274-G, 12 p.

Nelson, A. E., and Tobisch, O. T., 1967, The Matilde and Milagros Formations of 

early Tertiary age in northwest Puerto Rico: U.S. Geol. Survey Bull. 1254-Ar 

p. A19-A23.

Nockolds, S. R., 1954, Average chemical compositions of some igneous rocks: Geol. 

Soc. America Bull., v. 65, p. 1007-1032.

Turner, M. D., 1958, Geology of the San Sebastian area, Puerto Rico [abs.] : 

Caribbean Geol. Conf., 1st, Antigua 1955, Trans., p. 25.

Wentworth, C. K., and Williams, Howel, 1932, The classification and terminology 

of pyroclastic rocks: Natl. Research Council Bull. 89, Rept. Comm. Sedimentation 1930-32, p. 19-53.

Williams, Howel, Turner, F. J., and Gilbert, C. M., 1954, Petrography an introduction to the study of rocks in thin section: San Francisco, W. H. Freeman 

and Co., 406 p.

Zapp, A. D., Bergquist, H. R., and Thomas, C. R., 1948, Tertiary geology of thecoastal plains of Puerto Rico: U.S. Geol. Survey Oil and Gas Inv. PrelimMap 85.

O