ConcepciOn and 

Palma Escrita Formations, 

Weste~rn Puerto Rico 

GEOLOGICAL SURVEY BULLETIN 1394-D 

Prepared in cooperation with the 

Commonwealth of Puerto Rico 

Department of Natural Resources 
Concepcion and 

Palma Escrita Formations, 

Western Puerto Rico 

By DAVID H. MciNTYRE 

CONTRIBUTIONS TO STRATIGRAPHY 

GEOLOGICAL SURVEY BULLETIN 1394-D 

Prepared in cooperation with the 

Commonwealth of Puerto Rico 

Department of Natural Resources 

The Concepcion Formation of 

Late Cretaceous age is redefined, and the name 

Palma Escrita Formation is adopted for a 

sequence of early Tertiary age 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHI~GTON 1974 
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress catalog-card No. 73-600304 

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

Washington, D.C. 20402- Price 30c (paper cover) 

Stock Number 2401-02437 
CONTENTS 

Page 

~bstraet ------------------------------------------------------ D1 

Introduction --------------------------------------------------- 1 Concepci6n Formation of Mcintyre, Aaron, and Tobisch (1970) --- 3 

New data and revised nomenclature ----------------------------- 3 

Concepci6n Formation redefined --------------------------------- 4 

Palma Escrita Formation --------------------------------------- 5 

References cited ----------------------------------------------- 9 

ILLUSTRATIONS 

Page 

FIGURE 1. Index map of part of western Puerto Rico showing geographic and physiographic features cited in text ---- D2 

2. Generalized geologic map of part of western Puerto Rico 

showing stratigraphic units cited in text ----------- 6 

m 
CONTRIBUTIONS TO STRATIGRAPHY 

CONCEPCION AND PALMA ESCRITA 

FORMATIONS, WESTERN PUERTO RICO 

By DAvin H. MciNTYRE 

ABSTRACT 

The Concepcion Formation is redefined to include only greenish-gray 

plagioclase and pyroxene porphyry lavas and related volcaniclastic rocks 

of Late Cretaceous (Campanian-Maestrichtian) age that crop out in 

the San Sebastian and Maricao quadrangles, western Puerto Rico. Most 

of the formation has been somewhat altered. Analyses of rare unaltered 

samples show the rock to be a high-alumina basalt. 

The Palma Escrita Formation, of early Tertiary and probable Eocene 

age, consists chiefly of greenish-gray pumice-rich dacitic to rhyodacitic 

tuff that crops out in the Central la Plata and Maricao quadrangles. It 

conformably underlies lavas and volcaniclastic rocks of the Mal Paso Formation (lower middle Eocene). Rocks in the Central Ia Plata quadrangle 

formerly included in the Concepcion Formation now are considered part 

of the Palma Escrita Formation. 

INTRODUCTION 

The geology of western 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 Department of Natural Resources, Commonwealth of Puerto Rico. In past years, during which the results 

summarized in this report were obtained, the program also 

was supported by the Puerto Rico Economic Development 

Administration. 

The stratigraphic results of geologic mapping in the San 

Sebastian, Central la Plata, and Rincon quadrangles, northwestern Puerto Rico (fig. 1), have been reported earlier by Mcintyre, 

Aaron, and Tobisch (1970). (See also Tobisch and Turner, 1971; 

Mcintyre, 1971.) Further fieldwork in western Puerto Rico, prinDl 
Of UU 66° 52'30" --··- 18°22'30" 

BAHIA DE ANASCO 

15' 

RINCON 

2 CENTRAL LA PLATA 

3 SAN SEBASTIAN 

4 MARICAO 

0 10 20 30 40 MILES 0 1 2 3 4 5 

0 10 20 30 40 KILOMETERS 0 1 2 3 4 5 KILOMETERS 

I I I I I I I I I I 

'--------------------------------.L----------------' 18°07'30" 

FIGURE 1.-Index map of part of western Puerto Rico showing geographic and physiographic features cited in text. 

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CONCEPCION AND PALMA ESCRITA FORMATIONS D3 

cipally in the Maricao quadrangle (fig. 1), has clarified some of 

the stratigraphic relations and has indicated the need for some 

revision of the stratigraphic nomenclature proposed by Mcintyre, 

Aaron, and Tobisch (1970). This revision involves redefinition of 

the Concepcion Formation and proposal of a new formation, the 

Palma Escrita Formation, to include rocks excluded from the 

Concepcion as redefined. 

CONCEPCION FORMATION OF MciNTYRE, AARON, AND 

TOBISCH (1970) 

Mcintyre, Aaron, and Tobisch (1970, p. D10) defined the Concepcion Formation as "Near-vent dacitic and rhyodacitic pyroclastic rocks and lavas associated with two separate, but related 

vent areas in northwestern Puerto Rico * * * ." Type localities 

were defined in Barrio Purisima Concepcion in the San Sebastian 

quadrangle, where the formation consists of an intertonguing 

sequence of lavas and tuff-breccias that contain plagioclase and 

clinopyroxene phenocrysts and of thin-bedded tuff that contains 

crystals of plagioclase, clinopyroxene, and hornblende and fragments of plagioclase porphyry and pumice. All these rocks in the 

San Sebastian quadrangle are somewhat altered and contain epidote, chlorite, prehnite, or zeolites. This alteration seriously interfered with determination of original chemical composition and 

the proper classification of these rocks. No fossils or other age 

data were known for these rocks in the San Sebastian quadrangle. 

A sequence of dacitic and rhyodacitic · pyroclastic rocks and 

lavas in the Central la Plata quadrangle (fig. 1) were correlated 

on the basis of apparent similarity in stratigraphic position with 

those in Barrio Purisima Concepcion in the San Sebastian quadrangle. These rocks, less altered than those in Barrio Purisima 

Concepcion, were used to characterize the chemical composition 

of the Concepcion Formation as defined by Mcintyre, Aaron, and 

Tobisch (1970). An Eocene age for the Concepcion was suggested 

by the stratigraphic relations in the CentralIa Plata quadrangle. 

NEW DATA AND REVISED NOMENCLATURE 

New data discovered during fieldwork in the Maricao quadrangle have demonstrated that the correlation of the rocks in 
D4 CONTRIBUTIONS TO STRATIGRAPHY 

Barrio Purisima Concepcion, San Sebastian quadrangle, with 

those in the Central la Plata quadrangle is incorrect. The new 

data are as follows : 

1. Chemical analyses of rare samples of unaltered rocks in the 

area adjacent to Barrio Purisima Concepcion, San Sebastian 

quadrangle, show that the unit present there is a highalumina basalt, not dacite or rhyodacite. 

2. A fossil locality now known to be in the Barrio Purisima Concepcion rocks contains Foraminifera of CampanianMaestrichtian (Late Cretaceous) Age. 

3. Rocks identical in composition and stratigraphic position to 

the dacitic and rhyodacitic rocks of the Central la Plata 

quadrangle also are present in the Maricao quadrangle. 

These rocks contain early Tertiary Foraminifera and are 

not related to rocks of the Barrio Purisima Concepcion type 

that occur in the same area (fig. 2). 

In order to resolve the discrepancies now apparent, the following changes in stratigraphic nomenclature are made: 

1. The Concepcion Formation is redE!fined to include only those 

rocks of Late Cretaceous age irl Barrio Purisima Concepcion in the San Sebastian quadrangle and other directly 

related rocks in adjacent areas. 

2. Rocks of early Tertiary age in the CentralIa Plata quadrangle, 

formerly included in the Concepcion Formation, are grouped 

in a new unit, the Palma Escrita Formation. Rocks in the 

Maricao quadrangle identical in composition and stratigraphic position with those of the Central Ia Plata quadrangle also are included in the Palma Escrita Formation. 

CONCEPCION FORMATION REDEFINED 

The Concepcion Formation, as redefined, consists of an interbedded sequence of gray and greenish-gray porphyritic lavas, tuffbreccias, and thin-bedded tuffs that crop out in the San Sebastian 

and Maricao quadrangles. The type localities (along the Rio Afiasco near the Route 119 bridge, along the road between Barrios 

Purisima Concepcion and Buena Vista, and in Quebrada Velez) 

described in Mcintyre, Aaron, and Tobisch (1970, p. D10) are 

retained. 

The lavas and tuff-breccias in the Concepcion Formation contain phenocrysts of plagioclase and clinopyroxene and often are 

amygdaloidal. The thin-bedded tuffs contain abundant crystals 
CONCEPCION AND PALMA ESCRITA FORMATIONS D5 

of plagioclase and clinopyroxene or hornblende in addition to porphyritic non vesicular rock fragments and pumice or scoria. Nearly 

all these rocks have been somewhat altered, so that chlorite, epidote, prehnite, or zeolites are now present in varying amounts. 

Many samples also contain pyrite. The alteration tends to obscure 

the original textures and compositions. Analyses of rare unaltered 

parts of the formation show that it is a high-alumina basalt. 

The contact relations of the Concepcion Formation and surrounding units are obscured chiefly by faulting. The Concepcion 

Formation is bordered on the southwest by the Rio Blanco Formation (Campanian-Maestrichtian); the contact is a fault (fig. 2). 

To the northeast, a fault also separates the Concepcion from the 

Palma Escrita and Mal Paso Formations (lower Tertiary). Because of the faulting, there is no physical evidence concerning the 

stratigraphic position of the Concepcion Formation relative to 

adjacent formations. 

Evidence for the age of the Concepcion Formation is provided 

by Foraminifera in a thin limestone lens near the northwest border of the Maricao quadrangle (this area of Concepcion not shown 

in fig. 2). This locality (USGS locality f-35157) earlier was believed to be in the Rio Blanco Formation, but mapping in the 

Maricao quadrangle has shown conclusively that the limestone 

lens is, instead, in the Concepcion. The limestone is interbedded 

with green thin-bedded tuff and dark-greenish-gray plagioclase 

porphyry lava of the Concepcion. According to K. N. Sachs, Jr., 

the Foraminifera are indicative of a Campanian-Maestrichtian 

Age (Mcintyre, Aaron, and Tobisch, 1970, p. D9). 

PALMA ESCRITA FORMATION 

The name Palma Escrita Formation is herein adopted for a 

sequence consisting chiefly of pumice-rich dacitic to rhyodacitic 

volcaniclastic rocks of early Tertiary age that crop out in the 

Centralia Plata and Maricao quadrangles. The type area for the 

formation is in Barrio Palma Escrita, Municipio de Las Marias, in 

the Maricao quadrangle (fig. 2). The typical exposures are along 

the unimproved road along the east flank of the ridge that extends 

northward from Escuela Palma Escrita (95,000E; 42,060N to 

95,840E; 43,020N (Puerto Rico Meter Grid System coordinates)). 

Reference exposures are those along Route 405, Barrio Cerro 

Gordo, Municipio de Afiasco, in the CentralIa Plata quadrangle. 
D6 CONTRIBUTIONS 'TO STRATIGRAPHY 

58,000 

METERS 

48,000 

METERS 

BAHIA DE ANASCO 

Gill . 

. 

Alluvium 

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EXPLANATION 

FAULT AND (ORJ 

UNCONFORMITY 

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Rio Blanco Concepcion 

Formation Formation ~ 

~ ;§ Units young~r than "'!;;l Mal Paso Formation, 

§ undivided 

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~ Mal Paso Formation 

J{ B Palma Escrita 

Formation 

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FAULT 

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Upper Cretaceous 

units, undivided 

(includes serpentinite) 

FIGURE 2.-Generalized geologic map of part of weste.rn Puerto Rico showing 

Rincon quadrangle), Mcintyre (1971, 

Pumice lapilli tuffs and pumice-rich tuffs in the Palma Escrita 

Formation usually are greenish gray, well indurated, and thick to 

thin bedded. These rocks consist chiefly of pumice and glass shards 

altered to chlorite or to intergrowths of quartz and alkali feldspar. Plagioclase is the only phenocryst mineral ; other minerals 

that may be present in very small amounts are hornblende, clinopyroxene, quartz, pyrite, magnetite, and sphene. A few red non0 

w 

u 

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1-

w 

a:: 

u 
CONCEPCION AND PALMA ESCRITA FORMATIONS D7 

,000 MET~E~RS _______ 6_7",--00_' ______ ,_9_:8,_00_0_M_E_T_E_:RS _____ 66---,•52~!~; 30 ,. 

Contact 

Fault 

Dotted where concealed. 

Arrows indicate relative 

horizontal movement 

Anticline 

Showing crestline 

Syncline 

Showing troughline 

xxxxx 

Type locality and type 

area for formations 

defined in this report 

0 2 3 4 5 MILES 

2 3 4 5 KILOMETERS 

58,000 

METERS 

48,000 

METERS 

15' 

METERS 

stratigraphic units cited in text. Generalized from J.M. Aaron (unpub. data, 

1973), and Tobisch and Turner ( 1971). 

vesicular lava fragments locally are present, })articularly near the 

contact with the overlying Mal Paso Formation. 

The Palma Escrita Formation weathers readily to a reddishbrown, yellowish-brown, and yellow saprolite. The altered pumice 

fragments, often recognizable in saprolite exposures, appear as 

lenticular or irregular patches that are either green or red, depending on the degree of weathering. 
D8 CONTRIBUTIONS TO STRATIGRAPHY 

Locally interbedded with the tuff of the Palma Escrita in the 

Maricao quadrangle are dark-gray, nonvesicular, nonporphyritic 

lavas of probable basaltic composition. The lava flows increase in 

abundance toward the east border of the Maricao quadrangle. It 

is conceivable that the stratigraphic interval occupied by the 

Palma Escrita may be occupied chiefly by these lavas (toward the 

east). 

The Palma Escrita Formation is exposed on both noses of a 

much-faulted, north-northwest-trending doubly plunging anticline (fig. 2). The unit may be as much as 2,200 m thick in the 

northeastern part of the Maricao quadrangle, where it crops out 

on the southeast-plunging anticlinal nose. Extensive cover in that 

area, however, may conceal structural complications that would 

invalidate this estimate. A thickness of about 800 m is exposed 

near Cerro Gordo in the Central la Plata quadrangle, where the 

Palma Escrita is exposed on the northwest-plunging anticlinal 

nose. 

Systematic changes in thickness or grain size, which would indicate the position of the formation relative to source vent(s), are 

not apparent in the Palma Escrita Formation in the Maricao 

quadrangle. Such changes are, however, very noticeable within the 

formation in the Centralia Plata quadrangle. Grain size and proportion of pumice to glass shards diminish southwest and east 

from the area of the granodiorite porphyry pluton north of Cerro 

Gordo. The direction to the source vent is also shown by the rapid 

thinning of the formation toward the east; however, at least part 

of this thinning may be the result of faulting. These relations 

suggest that the source vent for the rocks in the Central Ia Plata 

quadrangle was at a site now occupied by the granodiorite porphyry body. 

The Palma Escrita Formation is overlain concordantly, and 

probably conformably, by basaltic lavas and pyroclastic rocks of 

the Mal Paso Formation. The contact of the Palma Escrita with 
CONCEPCION AND PALMA ESCRITA FORMATIONS D9 

adjacent older formations is, at most places, a fault. About 1 km 

northwest of Escuela Palma Escrita, the contact with the Rio 

Blanco Formation (Campanian-Maestrichtian) does not appear to 

be a fault contact (fig. 2). If not a fault plane, the contact must 

be an unconformity because of the discordance in attitude between 

the two formations and because of the probable time interval 

separating them. 

The exact age of the Palma Escrita Formation is not known. 

Planktonic Foraminifera have been found at scattered localities, 

but preservation has been poor. A sample containing well-preserved foraminifers (location: 96,200E; 41,680N) was sent to 

E. A. Pessagno, Jr., who stated (written commun., May 13, 1971) 

that "The planktonic forams in MM-248 appear to be late Paleocene to Eocene in age. There is a possibility of reworking of 

Upper Cretaceous (middle Cenomanian to lower Turonian).'' 

This approximate age assignment is in accord with the physical 

stratigraphic relations. The Mal Paso Formation, which conformably overlies the Palma Escrita, is of early middle Eocene 

age (Mcintyre, Aaron, and Tobisch, 1970, p. D12). It is unlikely, 

therefore, that the underlying relatively thin, rapidly deposited 

Palma Escrita Formation is older than Eocene. 

REFERENCES CITED 

Mcintyre, D. H. 1971, Geologic map of the Central la Plata quadrangle, 

Puerto Rico: U.S. Geol. Survey Misc. Geol. Inv. Map I-660. 

-- 1973, Geology of the Maricao quadrangle, Puerto Rico: U.S. Geol. 

Survey open-file report, 36 p., map. 

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

lower Tertiary stratigraphy in northwestern Puerto Rico: U.S. Geol. 

Survey Bull. 1294-D, p. Dl-D16. 

Tobisch, 0. T., and Turner, M. D., 1971, Geologic map of the San Sebastian 

quadrangle, Puerto Rico: U.S. Geol. Survey Misc. Geol. Inv. Marp I661. 

*U.S. GOVERNMENT PRINTING OFFICE: 1973 0-543-579/34