Stratigraphic assessment of the Arcelia–Teloloapan area

Stratigraphic assessment of the Arcelia±Teloloapan area,southern Mexico: implications for southern Mexico's post-Neocomian

tectonic evolution

E. Cabral-Canoa,*, H.R. Langb, C.G.A. Harrisonc

aInstituto de GeofõÂsica, Universidad Nacional AutoÂnoma de MeÂxico, Ciudad Universitaria, MeÂxico , DF 04510, MexicobJet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA

cMarine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Cswy, Miami,

FL 33149, USA

Abstract

Stratigraphic assessment of the ªTierra Caliente Metamorphic Complexº (TCMC) between Arcelia and Teloloapan in southern Mexico,

based on photo interpretation of Landsat Thematic Mapper images and ®eld mapping at the 1:100,000 scale, tests different tectonic evolution

scenarios that bear directly on the evolution of the southern North American plate margin. The regional geology, emphasizing the strati-

graphy of a portion of the TCMC within the area between Arcelia and Teloloapan is presented. Stratigraphic relationships with units in

adjacent areas are also described. The base of the stratigraphic section is a chlorite grade metamorphic sequence that includes the Taxco

Schist, the Roca Verde Taxco Viejo Formation, and the Almoloya Phyllite Formation. These metamorphic units, as thick as 2.7 km, are

covered disconformably by a sedimentary sequence, 2.9 km thick, composed of the Cretaceous marine Pochote, Morelos, and Mexcala

Formations, as well as undifferentiated Tertiary continental red beds and volcanic rocks. The geology may be explained as the evolution of

Mesozoic volcanic and sedimentary environments developed upon attenuated continental crust. Our results do not support accretion of the

Guerrero terrane during Laramide (Late Cretaceous±Paleogene) time. q 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Tectonic evolution; Arcelia; Teloloapan; Stratigraphy

1. Introduction

The location of the study area between two extensive

carbonate platforms (Huetamo area and the Guerrero±

Morelos platform located between Mexico City and Chil-

pancingo; Fig. 1) of similar mid-Cretaceous age and deposi-

tional environment has resulted in the formulation of

contrasting evolutionary schemes for southern Mexico.

Some interpretations explain the non-continuity of the two

carbonate platforms as the result of deposition controlled by

topography (e.g. de Cserna et al., 1978). Others assert that

the metamorphic rocks of the ªTierra Caliente Metamorphic

Complexº (TCMC), upon which the carbonates sit, are

allochthonous, resulting from the tectonic accretion of an

island arc (e.g. Campa and Ramirez, 1979; Tardy et al.,

1991), with consequent dissimilar stratigraphic records

and geologic evolution from the rest of cratonic Mexico.

These contrasting tectonic scenarios can only be tested by

a tectonostratigraphic analysis carried out in the surround-

ings of the proposed tectonostratigraphic terrane boundary.

Ortega-Gutierrez (1981, p. 194) considered the TCMC in

southern Mexico to be a provisional designation, ªuntil

better geochronology and mapping establish their true

geological relationships.º A compilation and comparison

of published maps (Cabral-Cano, 1995) reveals contradic-

tory stratigraphic af®nity. This is a direct consequence of the

diverse criteria used to de®ne mapping units and the lack of

clear contact de®nitions and lithologic characterizations.

A tectonostratigraphic assessment on the metamorphic

rocks of the Tierra Caliente complex in the vicinity of the

alleged terrane boundary was precluded by the absence of a

reliable cartographic base. Thus, new mapping and strati-

graphic analyses serve as the basis to test opposing tectonic

scenarios and derive important constraints on the tectonic

evolution of the southern North American plate margin.

2. Approach

The approach used for this study was that described by

Lang et al. (1987) and Lang and Paylor (1994) in which

Journal of South American Earth Sciences 13 (2000) 443±457

0895-9811/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved.

PII: S0895-9811(00)00035-3

www.elsevier.nl/locate/jsames

* Corresponding author. Tel.: 152-5-622-4027; fax: 152-5-550-2486.

E-mail address: [email protected] (E. Cabral-Cano).

E. Cabral-Cano et al. / Journal of South American Earth Sciences 13 (2000) 443±457444

Fig. 1. Location of the Tierra Caliente Metamorphic Complex (TCMC) and other metamorphic complexes in southern Mexico, and Cretaceous carbonate

deposits. Study area is boxed. Modi®ed from Ortega-Gutierrez et al. (1992).

Fig. 2. Location of the Teloloapan±Arcelia study area in southern Mexico (boxed). Major roads and towns in the region are shown for reference. Landsat

Thematic Mapper satellite imagery was used for photointerpretion of the area. The background is a greyscale version of a principal component image

(RGB� PC1, PC2, and PC3) generated from a Landsat TM Image which shows an example of digital enhancement of the imagery to better discriminate

lithologies due to contrasting spectral characteristics of the different rock units, which are expressed as different shades of grey on this image. Compare with

Fig. 3 for correspondence to lithostratigraphic units.

geologic mapping and structural/stratigraphic analyses

using photogeology and spectral interpretations of Landsat

Thematic Mapper (TM) images are guided by published

mapping and ®eld work. This approach makes ®eld work

more ef®cient by remotely identifying localities where key

stratigraphic and structural relationships are well exposed

and are most accessible. Three digital TM scenes acquired

in the winter of 1985±1986 under essentially cloud-free

conditions provided image base maps. Color composites

using different band combinations, principal component,

decorrelation stretch and edge enhanced images (Moik,

1980; Gillespie et al., 1986) were registered to Universal

Transverse Mercator (UTM) geographical coordinates and

photographically enlarged to 1:250,000; 1:100,000, and

1:50,000 scales. Fig. 2 is an example of the images used.

3. Results

3.1. Overview

The study area in Guerrero State, Mexico (Fig. 1), encom-

passes approximately 1600 km2 of greenschist facies meta-

morphic rocks known as the TCMC. In 1981, Ortega-

Gutierrez de®ned the TCMC as a complex of low-grade

metamorphic rocks, including calc-alkaline andesites,

ignimbrites, tuffaceous shales, sandstones, and limestones

exposed ªmainly in the southern slopes of the Balsas River

Basin and beyond the southern limits of the Transmexican

Volcanic Beltº (1981, p. 194) (see Fig. 1). The earliest

formal lithostratigraphic description in the study area was

that of Fries (1960), with complementary work by de Cserna

(1965). In a later stratigraphic review, Ontiveros-Tarango

(1973) reported a thrust of Roca Verde Taxco Viejo green-

stone over mid-Cretaceous Morelos limestone west of Telo-

loapan. This fault was later interpreted by Campa and Coney

(1983) and Centeno-GarcõÂa et al. (1993) as a tectonostrati-

graphic terrane boundary. Mapping of this area has also

been published by Campa et al. (1974), de Cserna (1978),

Campa and Ramirez (1979) and INEGI (1981a,b).

Lithostratigraphic units used here are summarized in the

composite column (Fig. 4). The metamorphic sequence in

the Teloloapan±Arcelia area contains three pre-Aptian low-

grade, greenschist facies metamorphosed units that occa-

sionally retain primary structure and texture. The discon-

formable sedimentary cover comprises four units that span

Aptian through early Tertiary times. The western limit of

these metamorphic rocks is de®ned by the Arcelia fault

zone, a NW-trending system of high-angle faults that juxta-

pose Tierra Caliente metamorphic rocks on the east against

Tertiary volcanic and clastic rocks on the west. The eastern

limit is the Teloloapan thrust fault that carried the meta-

morphic rocks onto Cretaceous limestones.

3.2. Taxco Schist Formation

The Taxco Schist Formation is composed primarily of

®ne- and coarse-grained mica and/or chlorite pelitic schists

and phyllites (Fries, 1960). Outcrops are restricted to the

lowermost topographic areas, in the north-central portion

of the study area (Fig. 3). The schists have a well-developed

cleavage, which is folded to centimeter-scale chevron folds,

and crenulation cleavage in the case of the ®ne-grained

rocks.

The base of the Taxco Schist is not exposed in the study

area, but near Zacazonapan (90 km northwest of the study

area) it rests in a fault contact with Permian±Early Triassic

mylonitic granite of continental af®nity (Elias-Herrera and

Sanchez-Zavala, 1990). The upper contact with the Roca

Verde Taxco Viejo is poorly exposed near La Parota Lidice

(Fig. 3). All outcrops of the Taxco Schist we visited are

deeply weathered and thus inappropriate for radiometric

dating. The age can be constrained only as pre-Aptian±

Albian Ð that is, older than the base of the overlying

Roca Verde Taxco Viejo (94.4±82.8 my; Fig. 5) and More-

los Formations.

The Roca Verde Taxco Viejo and the Taxco Schist

Formations are composed of distinct lithostratigraphic

units (Fig. 3). Besides their lithological differences, the

Taxco Schist presents high drainage density and a low resis-

tance to erosion, in contrast with the more resistant

geomorphic expression of the Roca Verde Taxco Viejo.

Within the study area, the Taxco Schist outcrops are consis-

tently topographically below outcrops of the Roca Verde

Taxco Viejo, suggesting a lower stratigraphic position for

the former. The minimum exposed thickness of the Taxco

Schist in the study area is approximately 900 m. This was

calculated in the vicinity of La Parota Lidice by measuring

the relief between the lowest part of the schist exposure

along the Sultepec River and the highest point of the

upper contact with the Roca Verde Taxco Viejo. This esti-

mate is crude, because it is likely that this sequence is

duplicated by small thrust faults that are dif®cult to detect

at the 1:50,000 scale mapping.

We correlated rocks that we mapped as the Taxco Schist

with rocks mapped by Fries (1960) in the Taxco type area,

33 km northeast of the study area, and with the metamorphic

sequence mapped by Elias-Herrera and Sanchez-Zavala

(1990) in the Tejupilco area, 68 km northeast of the study

area. These correlations are based on the similarities in their

lithologies and stratigraphic position, the lateral continuity of

exposures that can be mapped on the TM images, and mapping

by de Cserna and Fries (1981) and ElõÂas-Herrera (1993).

The micaceous, pelitic composition of the Taxco Schist

implies a pelitic protolith of probable continental margin

origin. Lithologies reported in adjacent areas by Elias-

Herrera and Sanchez-Zavala (1990) include ®ne- and

medium-grained sericite and quartz schist, as well as chlor-

ite schist, graphite schist, quartzite, and black slate.

3.3. Roca Verde Taxco Viejo Formation

Fries (1960) assigned foliated, low-grade metamorphosed

E. Cabral-Cano et al. / Journal of South American Earth Sciences 13 (2000) 443±457 445

E.

Ca

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l-Ca

no

eta

l./

Jou

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South

Am

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Earth

Scien

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443

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6

Fig. 3. Geologic map and cross sections of the Arcelia±Teloloapan area. Geology east of Pachivia is modi®ed after Barros (1996).

andesitic tuffs, breccias, lavas, and associated sandstones

west of Taxco Viejo to the Roca Verde Taxco Viejo Forma-

tion. Probable equivalents in the study area are porphyritic

andesite metalavas, phyllitic andesitic metatuffs showing

cleavage, and monomictic metaconglomerates with andesite

clasts. Metaconglomerates locally grade into metasand-

stones. All of these rocks show greenschist facies chloritiza-

tion of the matrix. Epidote, sericite, biotite, plagioclase, and

quartz are common. In the vicinity of Agua Colorada (Fig.

3), sparse marble beds (3±6 m thick) occur in the metatuff

sequence.

The Roca Verde Taxco Viejo is exposed in two major

areas: between Acapetlahuaya and the Vicente Guerrero

reservoir, and between Villa de Ayala and Teloloapan

(Fig. 3). The base is exposed (poorly) only in the north-

central portion of the area (Fig. 3). The upper contact with


Fig. 4. Composite stratigraphic column for the study area. Stratigraphic locations of the whole rock K/Ar dated samples (see also Fig. 5) shown as sample

numbers, with ages in parentheses.

the Almoloya Phyllite is exposed along Highway 51 east of

Las Ceibitas where meta-andesite intervals (3±5 m thick)

inter®nger with the base of the Almoloya Phyllite. This

contact indicates volcanic activity during the deposition of

basal Almoloya beds. The margins of these thin ¯ows

appear chilled, based on the presence of 1±3 cm-thick

dark cryptocrystalline bands. In the eastern portion of the

area, the upper contact of the Roca Verde Taxco Viejo is

discordant with the overlying Morelos Formation and

equivalent Pochote strata as exposed on the road from

Pochote to La Yerbabuena and southeast of Acatlan de la

Cruz. The metavolcanic member of the Roca Verde Taxco

Viejo is approximately 620 m thick near the Neblinas Radio

Station. The 1330 m thickness for metatuff and metacon-

glomerate members was measured near Agua Colorado.

3.4. Almoloya Phyllite

The Almoloya Phyllite is an informal lithostratigraphic

unit that consists of monotonous well-foliated, black phyl-

lite that weathers to light brown. This unit also includes

intervals of black chert nodules and irregular beds 3±4 m

thick. The best exposures of the Almoloya Phyllite are along

Highway 51, near the village of Almoloya, south of the

Vicente Guerrero Reservoir, and along the Copaltepec±

Tejupilco road near the 29 km marker. Along the western

margin of the study area, the Arcelia fault (Fig. 3) juxta-

poses the Almoloya Phyllite against undifferentiated

Tertiary red beds and volcanic and intrusive rocks (Jansma

et al., 1991; Jansma and Lang, 1996).

West of Las Ceibas, the basal contact with the Roca

Verde Taxco Viejo is a concordant depositional surface.

In other areas, such as in the vicinity of Almoloya, this

relationship is not so clearly exposed because an undeter-

mined amount of slip has occurred along this surface. The

upper contact was not clearly observed in the ®eld but is

inferred to be disconformable with overlying Cretaceous

sediments of the Pochote Formation. Alternatively, the

lack of a clear exposure of this contact may be evidence

that the Almoloya Phyllite is a lateral equivalent of the

laminated mudstones of the Pochote Formation. If this is

the case, then the Almoloya Phyllite represents a deeper,

hemipelagic facies of the Pochote.

The Almoloya Phyllite is not exposed east of Tehuixtla

(Fig. 3). There, the Roca Verde is found in contact with

either the Morelos or Pochote Formations. This suggests a

disconformable upper contact. The extent of the Almoloya

Phyllite exposures may have been controlled by: (1) relief

on the Roca Verde; and/or (2) uplift and subsequent erosion

of the Almoloya Phyllite prior to deposition of the Morelos

Formation.

The absence of the Almoloya Phyllite west of Zacatlan-

cillo±Tehuixtla (Fig. 3) and the presence of a conglomeratic

member in the uppermost part of the Roca Verde Taxco

Viejo suggests that both the phyllite and part of the Roca

Verde may have been removed by erosion. The presence of

the phyllites may indicate substantial relief during or after

deposition of the Roca Verde Taxco Viejo, if the phyllites

are distal turbidities derived from the volcanic centers of the

Roca Verde Taxco Viejo metavolcanic rocks. We measured


Fig. 5. Reported radiometric dates for the study area and vicinity. Lightly shaded boxes are previously reported ages (see text). Darkly shaded boxes are new

whole rock K/Ar dated samples.

a thickness of 800 m for the Almoloya Phyllite east of

Arcelia.

The only reported fossils from the rocks that we mapped

as the Almoloya Phyllite are radiolaria (DaÂvila-Alcocer and

Guerrero-SuaÂstegui, 1990) in the vicinity of Arcelia.

DaÂvila-Alcocer and Guerrero-SuaÂstegui (1990) suggested

that the fossil assemblage is Albian±Cenomanian age,

possibly Cenomanian. However, based on ranges of the

genera as reported by Moore (1954) and the zonation

scheme of San®lippo and Riedel (1985), the co-occurrence

of Podobursa sp., Acanthocircus sp., and Archaeodictyomi-

tra sp. suggest a Valanginian±Aptian age. Alternatively,

using the zonation of Pessagno (1971), an assemblage

consisting of Crucella messinae Pessagno, Podobursa sp.,

Acanthocircus sp., Paronaella sp., Praeconocaryomma sp.,

and Archaeodictyomitra sp. is Tithonian±Cenomanian. This

is based on Pessagno's (1971) original description of C.

messinae from the Great Valley Sequence of the California

Coast Ranges, where the base of the range is not de®ned.

Therefore, fauna in the Almoloya Phyllite record a latest

Jurassic to early Late Cretaceous age.

3.5. Pochote Formation

The Pochote Formation is an informal lithostratigraphic

unit. This name was informally used by Baro-Santos (1959)

and PEMEX (1979) for a sequence of laminated mudstone

and shale near Campo Morado (Burckhardt, 1930) and El

Pochote (Campa, 1978).

In the study area, the Pochote Formation consists of dark

gray to black, thinly bedded, (.1 cm) and/or laminated

(,1 cm) lime mudstone, interbedded with dark gray shale

in centimeter-thick beds. The unit locally shows intense

penetrative deformation, evidenced by axial plane foliation

and boudinage. The best exposures are between Villa de

Ayala and Las Ceibitas, as well as near El Pochote and

north of La Yerbabuena (Fig. 3).

The Pochote±Roca Verde Taxco Viejo contact is discor-

dant, based on exposures along the road from El Pochote to

La Yerbabuena and southeast of Acatlan de la Cruz (Fig. 3).

In these exposures, the contact is a sharp depositional inter-

face. PEMEX (1979) reported the existence of conglomer-

ate beds at the base of the Pochote where it sits

disconformably on meta-andesites and meta-tuffs of the

Roca Verde Taxco Viejo Formation in the vicinity of El

Pochote. Exposures of the contact of the Pochote with the

Almoloya Phyllite were not found, due to deep weathering.

The Pochote and Morelos Formations, locally with basal

conglomerate, rest disconformably on the Roca Verde

Taxco Viejo, Taxco Schist and Almoloya Phyllite.

The upper contact of the Pochote Formation with the

Mexcala Formation was mapped north of Los Aguajes.

The Morelos±Pochote formational contact was not found

exposed. The gradual thickening of limestone beds and

decrease in shale intercalations near La Yerbabuena suggest

that Morelos and Pochote strata may be a facies change. The

compositional variation may be responsible for the differ-

ential response to Late Cretaceous±Eartly Tertiary defor-

mation.

The total thickness of the Pochote could not be measured

because no locality was found where both upper and lower

contacts are exposed. The minimum thickness is 420 m in

the El Pochote area, which is a tentative estimate because

thrust faults probably exist that are too small to be recog-

nized at the 1:50,000 mapping scale. Reported thicknesses

of similar lithologies measured near Ixtapan del Oro (Parga,

1976) and Ixcatepec (Gutierrez, 1975) are 2000 and 1600 m,

respectively. It is likely that these reported thicknesses also

include fault duplications.

Biostratigraphic data for the Pochote are better than

underlying units. Burckhardt (1930) found Dufrenoya aff.

furcata Sow. near Campo Morado. This fossil indicates a

Late Aptian age (Moore, 1957). Campa (1978) and Campa

and Ramirez (1979) reported Parahoplites sp., Hamites sp.,

Calcisphaerula innomiata, and Stomiosphaera sphaerica in

the vicinity of El Pochote and Los Aguajes. These fossils

also indicate Late Aptian age. PEMEX (1979) reported the

presence of Hamites sp. at the base of the unit, which dates it

as Late Aptian±Turonian according to Moore (1957),

although PEMEX (1979) assigned an Aptian age because

higher strata contain unidenti®ed Albian±Cenomanian

microfaunas. De Cserna and Fries (1981) noted the presence

of poorly preserved and unidenti®ed calpionelids and globi-

gerinids that they claim suggest an Albian±Cenomanian age

for beds of the Pochote Formation. The even laminations

observed in dark, carbonaceous Pochote limestones indicate

that they were deposited below wave base, in stagnant

waters with low oxygen content and containing few biotur-

bating organisms.

3.6. Morelos Formation

The Morelos Formation, de®ned by Fries (1960), includes

miliolid-rich, Albian limestones and dolostones. Similar

strata of the same age crop out extensively in Morelos,

northern Guerrero, and eastern Michoacan states. The origi-

nal de®nition included an evaporitic member that has not

been encountered in the study area. The rocks that are

mapped as Morelos include reefal facies that crop out east

of the study area (de Cserna and Fries, 1981; Gonzalez-

Pacheco, 1991).

Strata that we assign to the Morelos Formation include

massive wackestone, packestone, and grainstone that

contain miliolids, bioclasts, and intraclasts, as well as rudist

banks, occasionally with chert. Strata are commonly dolo-

mitized. Based on Wilson (1975), this suite of rocks indi-

cates deposition in environments that span toe-of-the-slope

to open platform facies. Limestones of the Morelos Forma-

tion have traditionally been assigned an Albian±Cenoma-

nian age (Fries, 1960; Bonet, 1971; Ontiveros-Tarango,

1973). Equivalent age carbonates exist elsewhere in

Mexico, such as those of the El Abra Formation in


northeastern Mexico (Wilson and Ward, 1993; BasanÄez-

Loyola et al., 1993) and throughout the Cretaceous Tethys

domain (Simo et al., 1993). The Morelos Formation is best

exposed near the eastern margin of the area, around Telo-

loapan and Acatempan (Fig. 3), and in thin faulted slices

near Simatel and Tenanguillo. It also de®nes most of the

north-trending Chilacachapa range. In the vicinity of La

Yerbabuena, small remnants of thin laminated algal bound-

stone and calcareous conglomerate beds crop out at the base

of the unit, resting discordantly on Roca Verde metapyro-

clastics.

The Morelos Formation rests disconformably on the Roca

Verde Taxco Viejo Formation. For example, in the vicinity

of Pipicantla (Fig. 3) the Teloloapan thrust fault exposes

imbricate fault slices that include the Roca Verde±Morelos

contact. There, a conglomerate at the base of the Morelos

Formation contains Roca Verde Taxco Viejo clasts. The

upper contact of the Morelos with clastic beds of the

Mexcala Formation is sharp but apparently conformable.

The Morelos Formation shows a marked increase in thick-

ness between its westernmost exposures at Acatempan,

where we determined a 380 m thickness, and its eastern

exposures near Chilacachapa, where Gonzalez-Pacheco

(1991) calculated a 1000 m thickness. The variable thick-

ness of the Morelos Formation is partly the result of post-

depositional erosion, as in the Teloloapan area, as well as

original depositional variations resulting from the existence

of a reefal facies along the western margin of the Chilaca-

chapa range (de Cserna, 1978).

3.7. Mexcala Formation

The Mexcala Formation, which was de®ned by Fries

(1960) in the Balsas River region north of the study area,

consists of a basal member of thinly bedded limestones or

limey siltstones grading into an upper member composed of

a sequence of shale and sandstone and minor conglomeratic

beds. In the eastern part of the study area, on both ¯anks of

the Chilacachapa range, the Mexcala consists of black shale

and wackestone or packestone, intercalated within other-

wise monotonous dark gray shale with minor ®ne sandstone

beds. Shale beds may be thin (,2±3 cm) or thick (up to

60 cm). Axial plane cleavage is conspicuous in more thickly

bedded intervals.

In the Pachivia region, the Mexcala Formation is exposed

in an elongate, north-trending, asymmetric, east-verging

synclinorium (Fig. 3; A-A 0 and C-C 0). On the eastern

¯ank of the synclinorium, the Mexcala is in sharp deposi-

tional contact with Morelos limestone. The western limb, in

the vicinity of Pipicantla and Tenanguillo, is cut by a west-

dipping thrust fault involving older Morelos strata as an

imbricate, as well as thin slices of the Roca Verde Taxco

Viejo. De Cserna (1978) mapped exposures in the Pachivia

area as the Xochipala Formation, based on the apparent

absence of sandstone beds and the occurrence of thin lime-

stone beds and volcanic ¯ows. We found no volcanic ¯ows

in the Mexcala Formation in the Pachivia area, although we

did ®nd probable Tertiary volcanic rocks near Chapa, which

are ¯at lying, probably of subaereal origin, and discordantly

above the steeply dipping Mexcala beds. East of the Chila-

cachapa range, the Mexcala Formation is in thrust contact

with overlying Morelos strata that form most of the range.

The minimum thickness of the Mexcala Formation in the

Pachivia area is 2000 m. However, many small thrust faults

and folds have probably increased the apparent thickness of

this unit. Thickness estimates elsewhere are 1300 m near

Iguala (Gonzalez-Pacheco, 1991) and approximately

2500 m near Mitepec, 100 km east of the study area (Lang

et al., 1996).

PEMEX (1979) reported that foraminifera found in the

Mexcala include: Marginotruncana sp., Globotruncana

rioensis, Praeglobotruncana rioensis, Globotruncana

rosseta, Globotruncana sp., Rotalipora sp., Quinquelocu-

lina sp, Pyrgo sp., c.f.; ammonites include Peroniceras

a.f. tricarinatum, Peroniceras c.f. P. subtricarinatun

sturn, Peroniceras c.f. P. subtricarinatum Drescher, Pero-

niceras sp., Barroisiceras c.f. P. alstadenense, and Barroi-

siceras sp. According to PEMEX (1979), these faunas

indicate a Turonian±Senonian, possibly Maastrichtian age

for the Mexcala Formation. However, according to Moore

(1957), the ammonite fauna is Coniacian. The ages of the

reported foraminifera do not overlap; therefore we assume

that the sampling represents the entire Mexcala section. The

presence of Marginotruncana sp., Globotruncana sp., and

Rotalipora sp. suggest an Albian to Maastrichtian age

(Caron, 1985).

4. Discussion

4.1. Age of the metamorphic rocks

Sparse radiometric dating (Fig. 5) and limited biostrati-

graphic evidence have allowed a wide range of interpreta-

tions regarding the age of the Taxco Schist and Roca Verde

Taxco Viejo in the study area and vicinity. De Cserna et al.

(1974) published a Grenvillian Pb-a age �1020 ^ 110 Ma�for the Taxco Schist near Taxco. This date was challenged

by Campa and Ramirez (1979), who asserted that zircons

dated by de Cserna et al. (1974) were not authigenic and that

the sample was not from the Taxco Schist but rather from

the Roca Verde Taxco Viejo. Thus, this date may show that,

during Roca Verde time, there was a Grenvillian-age sedi-

mentary source terrain. Urrutia-Fucugauchi and Valencio

(1986) reported 108 ^ 5 and 125 ^ 5 Ma (Barremian±

Albian) whole-rock K/Ar ages from schist and metaande-

site, respectively, collected near Ixtapan de la Sal. These

ages were attributed to the regional tectonic event that meta-

morphosed the rocks. We correlate these rocks with the

Taxco Schist and Roca Verde Taxco Viejo, respectively,

based on the similar lithologies and stratigraphic position

and the lateral continuity of the Taxco Schist from our study


area to Ixtapan de la Sal (de Cserna and Fries, 1981; de

Cserna, 1983).

Pb ages from Tizapa massive sul®de deposits, near Zaca-

zonapan, 15 km north of Tejupilco (Fig. 2) show single-

stage model ages of 156.3 Ma (Oxfordian), 128.7 Ma

(Barremian), and 103.4 Ma (Albian) (JICA-MMAJ, 1991,

in Sanchez-Zavala, 1993). Sanchez-Zavala (1993) recalcu-

lated these dates using a two-stage model that yielded ages

of 227.5 Ma (Carnian), 188.3 Ma (Pliensbachian), and

156.3 Ma (Oxfordian), respectively. Based on the concor-

dant relationships of these massive sul®des with the Taxco

Schist, it appears that these ages are equivalent to that of the

Taxco Schist protolith.

Elias-Herrera and Sanchez-Zavala (1990) reported40Ar/39Ar dates of 101 ^ 1 (Albian) and 93:4 ^ 0:4 Ma

(Cenomanian) from rocks equivalent to the Roca Verde

near Tejupilco (Fig. 5). The San Pedro Limon batholith

rocks (Delgado-Argote et al., 1990) located northwest of

Arcelia, have been dated by 40Ar/39Ar as 105 Ma (Albian).

In the same area, Ortiz and Lapierre (1991) reported Albian

(108 Ma, K/Ar on amphibole) tholeiitic dikes near Palmar

Grande, and that Campanian age dikes (79 Ma, K/Ar bulk

rock) intrude pillow basalts in the Arcelia area. All of these

samples are from intrusions and ma®c segregations that are

younger than the metamorphic rocks we mapped as the

Taxco Schist and Roca Verde Taxco Viejo.

Radiometric dates obtained from the metavolcanic rocks

of the Roca Verde Taxco Viejo, in the vicinity of Teloloa-

pan and Taxco, are incompatible with stratigraphic relation-

ships. Talavera-Mendoza et al. (1993) reported K/Ar dates

on amphiboles that indicate a 99±105 Ma age (Albian) for

metavolcanic rocks from the Teloloapan area. Whole-rock

K/Ar ages of six samples that we collected from the Roca

Verde Taxco Viejo and Taxco Schist in the Taxco and

Teloloapan areas show ages that span from Cenomanian

to Campanian time (Fig. 5). We interpret these dates as

recording the cooling age of a thermal event that could

not be responsible for the original metamorphism of rocks

in the Teloloapan±Arcelia region. This must be the case

because these radiometric ages are all younger than the

biostratigraphic ages for the overlying Morelos Formation

that contains Roca Verde Taxco Viejo clasts in a basal


Fig. 6. Pre-Neocomian to Early Tertiary simpli®ed depositional and tectonic conditions for the study area and vicinity (see text for detailed explanation).

conglomerate. The radiometric ages match very well the

biostratigraphic ages reported for the Mexcala Formation

that overlies the Morelos. Therefore, this Cenomanian±

Campanian thermal event may date tectonic uplift of the

region; probably corresponding to the initial stages of the

Laramide Orogeny in this part of Mexico. Relief associated

with this event may be responsible for the cessation of

carbonate deposition of the Morelos Formation due to sili-

ciclastic deposition of the Mexcala Formation.

Biostratigraphic evidence regarding the age of the meta-

morphic rocks is scarce and inconclusive. As described

above, radiolaria from cherts interbedded with the phyllites

in the Almoloya Phyllite, collected near Arcelia, indicate a

Valanginian±Aptian age. Volcanic activity northwest of

Arcelia was responsible for the absence of carbonate deposi-

tion between the Huetamo and Chilacachapa±Taxco plat-

forms. This discontinuity misled Campa (1978) and Campa

and Ramirez (1979) to propose that metamorphic rocks in

the region are allochthonous. Instead, based on the strati-

graphic and depositional evidence of the rocks in the studied

area, we interpret the Late Cretaceous volcanic rocks north-

west of Arcelia to be the remnants of a volcanic ®eld that

was fringed by shallow carbonate deposits of the Morelos

Formation on the Huetamo platform in the west and the

Taxco±Chilacachapa±Teloloapan platform in the east

(Fig. 6). Siliciclastic deposits, such as the Almoloya Phyllite

in the study area and San Lucas Formation in the Huetamo

area, as well as basinal carbonates such as the Pochote

Formation in the study area, separated these platforms

(Fig. 6). The predominance of either siliciclastic or carbo-

nate sediments was dependent on local conditions of sedi-

ment availability and transport. The wide extent and

uniformity of black shales that contain the Tithonian±

Berriasian ammonite Protancyloceras sp. in the Mastlacua

and Los Amoles area (Ortiz and Lapierre, 1991), and Micro-

canthoceras sp. (Tithonian) and Wichmaniceras sp. (Valan-

ginian) in the Ixtapan de la Sal area (Campa et al., 1974)

support the idea that uniform depositional conditions may

have existed in the region as early as Late Jurassic time.

Roca Verde Taxco Viejo metavolcanic rocks disconform-

ably underlie unmetamorphosed but locally highly tecto-

nized and sheared strata of the Morelos and Pochote

Formations. Following the inference of Elias-Herrera

(1989) and Elias-Herrera and Sanchez-Zavala (1990) that

the Roca Verde Taxco Viejo is equivalent to the Teloloa-

pan±Tejupilco sequence, this implies that the protolith age

for the Roca Verde Taxco Viejo in the Arcelia±Teloloapan

area is Late Triassic through Early Cretaceous.

4.2. Environments of deposition

The sedimentary sequence exposed in the study area can

be explained as a carbonate reef-platform built on the

surface of metamorphosed volcanic rocks of the Roca

Verde Taxco Viejo Formation. Morelos strata in the eastern

part of the study area record basinal and platform facies that

range from intertidal to calcareous slope deposits (Gonza-

lez-Pacheco, 1991). The rocks exposed elsewhere in the

study area are predominantly foreslope to shelf facies

mudstones with rudist fragments, or intertidal or channel

facies represented by algal mats and breccias (facies #4±8

of Wilson, 1975). Cessation of Aptian±Albian carbonate

depositional environments was controlled by contemporary

topography on the volcanic rock surface, where the carbo-

nate platform west of Teloloapan was fringed and possibly

restricted by volcanic activity of the Roca Verde Taxco

Viejo Formation. This interpretation agrees with previously

suggestions by Fries (1960), Ontiveros-Tarango (1973). de

Cserna et al. (1978) believed that an Albian±Cenomanian

transgression was responsible for the development of rudist

reefs fringing a shallow platform located east of the study

area. They recognized the lateral change of the Morelos

reefal and platform facies into the deeper basinal strata

that we mapped as the Pochote Formation. Our results

show that rocks of the Mexcala Formation were probably

deposited from turbidity currents bordering a carbonate

platform that underwent drastic changes in sedimentation

conditions Ð hence the sharp contact of the Morelos±

Mexcala Formations.

4.3. Tectonic interpretation

One of the most controversial aspects of the geology of

southern Mexico is the nature and age of the crystalline or

high-grade metamorphic rocks that constitute the substrate

upon which Late Mesozoic sediments accumulated. The

reason for this controversy is that there are few outcrops

of this basement (Fig. 1). According to Campa and Coney

(1983), the metamorphic rocks in the study area represent

the basement of the Guerrero terrane. They claimed that the

region is a tectonic entity separate from similar lithologies

exposed east of the study area and the alleged terrane

boundary of Laramide age in the vicinity of Teloloapan.

This interpretation assumes that the exposed rocks cannot

be subdivided nor tied lithostratigraphically to rocks in adja-

cent terranes. According to our mapping and stratigraphic

assessment (Figs. 3 and 4), units correlate across the terrane

boundary.

The nature of the crystalline basement below the Taxco

Schist, Roca Verde Taxco Viejo, and Almoloya Phyllite has

been only indirectly inferred using sparse geochemical

evidence. The deepest reported stratigraphic exposure in

the vicinity of the study area occurs near Zacazonapan

(Elias Herrera and Sanchez-Zavala, 1990). There, Permian

granite is found below a low-grade metamorphic sequence.

Granite clasts have been reported in an Aptian±Albian

conglomerate cropping out near Ixcateopan±Ixtapan de la

Sal (Fig. 2) (Vidal-Serratos et al., 1991) and in the Late

Jurassic±Early Cretaceous Angao and San Lucas Forma-

tions cropping out near Huetamo (Fig. 2) (Guerrero-Suaste-

gui et al., 1992). Vidal-Serratos (1991a,b), Vidal-Serratos et

al. (1991) and Elias-Herrera and Sanchez-Zavala (1990)


found granitic metamorphic rocks in the Zihuatanejo and

the Ixcateopan areas (Fig. 2).

The presence of these granitic rocks does not support

the interpretations of the geologic evolution of southern

Mexico as one or several volcanic arcs built upon ocea-

nic crust, as has been suggested by Monod et al. (1990).

According to Moran-Zenteno et al. (1991), Moran-

Zenteno et al. (1992) and Herrmann et al. (1994),

there is geochemical evidence from the southern Mexico

Mesozoic arc that suggests the in¯uence of an ancient

continental margin. Therefore, evidence exists support-

ing the idea that the protoliths of the metamorphic rocks

exposed in the study area and in much of Guerrero and

Michoacan states outside the study area (Fig. 1) were

deposited upon continental basement. We do not reject

the possibility that this basement could have been

previously stretched and attenuated. This would be

expected if the continental margin of southern Mexico

was within a convergent margin back-arc basin during

Triassic±Jurassic time, as it is today (Engebretson et al.,

1985; Coney, 1989).

Although the exact location of a Jurassic±Cretaceous

volcanic arc in southern Mexico is still not ®rmly estab-

lished, Ramirez-Espinoza et al. (1991) showed that at

least one volcanic arc, manifested by arc af®nity intrusive

and volcanic rocks as well as Kuroko-type massive sul®de

deposits such as those in the Tizapa area north of the study

area, existed between Teloloapan±Tejupilco in the east and

Zihuatanejo in the west.

A west-dipping subduction zone in southern Mexico has

been suggested by Urrutia-Fucugauchi and Valencio (1986)

and Tardy et al. (1991). Isotopic data from volcanic rocks

collected in southern Mexico (Lapierre et al., 1992; Ruiz et

al., 1991; Centeno-GarcõÂa et al., 1993; Tardy et al., 1994;

Talavera-Mendoza et al., 1995) apparently supports this

interpretation. Other evidence for a west-dipping subduc-

tion zone have been summarized by Tardy et al. (1994, p.

69) and include: (1) the apparent lack of Late Jurassic±Early

Cretaceous igneous arc rocks within the Guerrero±Morelos

carbonate platform of cratonic eastern Mexico; and (2) the

apparent younger Albian±Cenomanian age of the Arcelia

volcaniclastic sequence (which apparently includes the

Almoloya Phyllite). These observations led Ortiz and

Lapierre (1991) to conclude that (3) the tholeiitic volcani-

clastic strata of the Arcelia sequence are younger than the

Teloloapan calc-alkaline sequence. They assert that there is

a thrust contact between the Arcelia sequence and the Telo-

loapan sequence, which in turn is thrust over the Guerrero±

Morelos carbonate platform.

This assessment is ¯awed because of the erroneous age

assignment of the Arcelia tholeiitic sequence. The ages are

based on the use of poorly preserved and questionably iden-

ti®ed radiolaria (see discussion in DaÂvila-Alcocer and Guer-

rero-SuaÂstegui, 1990). That a lack of volcanic rocks within


Fig. 7. Locally measured stratigraphic columns (inset shows locations within the boxed area of Fig. 2) and relative distribution of mid-Cretaceous sedimentary

facies in the study area. Fill patterns are as used for the geologic map in Fig. 3. See Fig. 4 for ages of the individual lithostratigraphic units.

the Late Cretaceous carbonate platform sequence proves

that the volcaniclastic sequences, such as that of Teloloa-

pan±Arcelia, must have been located far away from the

North American continental margin is also incorrect.

A tectonic model that includes a west-dipping subduction

zone usually includes an accretionary forearc prism with

associated blueschist and related ophiolitic slivers east of

the volcanic arc. This is the case in the Vizcaino Peninsula

arc of Baja California (Moore, 1986). To our knowledge, no

such rocks have been mapped in southern Mexico. If the

edge of a Late Jurassic±Early Cretaceous arc were located

somewhere within the Teloloapan±Arteaga Zihuatanejo

region, then one would expect to ®nd ophiolites, blueschists,

or accretionary complexes within the study area or east of it.

The only blueschists reported along the Paci®c margin of

Mexico are on Santa Margarita Island, San Benitos Islands

(Cohen et al., 1963), Cedros Island (Klienast and Rangin,

1982), and the Vizcaino Peninsula (Moore, 1986) on the

west coast of Baja California. Although low-temperature

greenschist facies metamorphic rocks have been mapped

over large areas of southern Mexico (i.e. Arteaga, Placeres

del Oro, and our study area; see Fig. 1), no belt of Mesozoic

high-pressure metamorphic rocks has been reported. We

therefore interpret the study area to have been located east

of an east-dipping subduction zone and possibly within an

intra-arc (or back-arc) basin during Cretaceous time.

The lack of paired high-pressure/high-temperature meta-

morphic belts next to the volcanic arc from which the polar-

ity of subduction may be deduced precludes a de®nitive

conclusion. Evidence necessary to substantiate the geody-

namic model of Tardy et al. (1994), showing westward

subduction polarity, is circumstantial. We propose an alter-

native conceptual model for the tectonic setting of southern

Mexico (see Figs. 6 and 7): east-dipping subduction of an

oceanic plate under continental lithosphere occurred during

Cretaceous time, and the continental crust may have under-

gone attenuation and plutonic activity during Early Cretac-

eous time. The existence of nearby granitic outcrops that

probably underlie rocks in the study area support this model.

5. Conclusions

The idea that Albian age carbonates exposed at Teloloa-

pan and Chilacachapa are different entities (Guerrero-SuaÂs-

tegui et al., 1991), without a common geographic or genetic

link, is a necessary conclusion if the Teloloapan thrust is

interpreted as the Guerrero terrane boundary (e.g. Campa

and Coney, 1983). However, the presence of rocks of simi-

lar lithology, stratigraphic position, and age on both sides of

the alleged western Guerrero terrane boundary does not

support this interpretation. Using a simple carbonate plat-

form model, the sedimentary rocks mapped in the Teloloa-

pan and Chilacachapa areas record compatible facies. This

platform was built disconformably over metamorphic rocks;

the carbonates were deposited at depths controlled by pre-

existing relief on the metamorphic surface and were fringed

by contemporary volcanic activity in a back-arc basin envir-

onment.

Based on similarities of lithology, radiometric ages,

protoliths, and metamorphic grade, we correlate the meta-

morphic rocks that we mapped with the Taxco Schist and

Roca Verde Taxco Viejo exposed in the Taxco region

(30 km east) and the Teloloapan±Tejupilco sequence as

described by Elias-Herrera and Sanchez-Zavala (1993).

Available evidence shows that the protolith age spans the

Late Triassic to Early Cretaceous. These correlations also

suggests that carbonate deposits of the Pochote±Teloloapan

area and volcanics reported within the Arcelia±Otzoloapan

volcanic sequence (Elias-Herrera and Sanchez-Zavala,

1990; Delgado-Argote et al., 1990; Ortiz and Lapierre,

1991) are coeval.

We propose that the name TCMC be abandoned because

our mapping has established the geological relationships of

units exposed in the ªcomplex.º The study region contains a

sedimentary sequence that has undergone intense strain in

localized areas, such as within the Pochote Formation.

However, it still retains much of its primary sedimentary

structure. Moreover, there is now stratigraphic evidence that

shows lateral continuity and age correlation of the meta-

morphic rocks in the Teloloapan±Arcelia region with

rocks exposed east in the Guerrero±Morelos platform.

We therefore propose that the Teloloapan thrust is not a

terrane boundary, based on:

² the lithological and radiometric similarities of the low-

grade metamorphic rocks in the Taxco and Teloloapan±

Arcelia regions;

² the disconformable nature of the contact between the

metamorphic rocks and overlying marine rocks; and

² the absence of metamorphism of the Aptian±Albian

marine sedimentary cover in the Teloloapan area.

Instead, the Teloloapan thrust is one of many east-verging

thrust faults that have been mapped in the region (Lang et

al., 1996). This thrust faulting probably resulted from defor-

mation associated with the Late Cretaceous to Paleogene

Laramide Orogeny.

Acknowledgements

This paper presents the results of research conducted as

part of U.S. National Aeronautics and Space Administration

contracts NAGW-1678 and NAGW-2710 with the Marine

Geology and Geophysics Program at the Rosenstiel School

of Marine and Atmospheric Science, University of Miami,

and the Jet Propulsion Laboratory at the California Institute

of Technology. EC-C also received support for ®eld work

from the Gulf Coast Association of Geological Societies'

Student Aid Program and the Geological Society of Amer-

ica's Student Research Grant Program, as well as a PhD


Fellowship and PAPIIT research grants IN105596 and

IN105592 from Universidad Nacional AutoÂnoma de

MeÂxico. Comments and discussions with Chris A. Johnson

and Tony Barros were of great help during the early stages

of the project. Field assistance and other logistical support

from Francisco Correa-Mora, Teodoro Hernandez-TrevinÄo,

Esteban Hernandez-Quintero, Gerardo Cifuentes-Nava, and

Ricardo Becerril-Herrera (Instituto de GeofõÂsica, Universi-

dad Nacional AutoÂnoma de MeÂxico) is gratefully acknowl-

edged. Steve Adams (JPL) provided invaluable help with

the satellite image processing. We thank referees Thomas H.

Anderson, Gary Prost, Charles F. Kluth, Meghan Miller,

Richard Sedlock, Cynthia Dusel-Bacon, James Kellogg,

and two anonymous reviewers. Their comments improved

the manuscript, even though some of the reviewers do not

agree with all of our conclusions.

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Documents

Stratigraphic assessment of the Arcelia–Teloloapan area