Stratigraphic assessment of the Arcelia±Teloloapan area, southern

Journal of South American Earth Sciences 13 (2000) 443±457
www.elsevier.nl/locate/jsames
Stratigraphic assessment of the Arcelia±Teloloapan area,
southern Mexico: implications for southern Mexico's post-Neocomian
tectonic evolution
E. Cabral-Cano a,*, H.R. Lang b, C.G.A. Harrison c
a
Instituto de GeofõÂsica, Universidad Nacional AutoÂnoma de MeÂxico, Ciudad Universitaria, MeÂxico , DF 04510, Mexico
b
Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
c
Marine 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 stratigraphy 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 Chilpancingo; Fig. 1) of similar mid-Cretaceous age and depositional 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 surroundings 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 contradictory 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 stratigraphic 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
* Corresponding author. Tel.: 152-5-622-4027; fax: 152-5-550-2486.
E-mail address: [email protected] (E. Cabral-Cano).
The approach used for this study was that described by
Lang et al. (1987) and Lang and Paylor (1994) in which
0895-9811/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved.
PII: S 0895-981 1(00)00035-3
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E. Cabral-Cano et al. / Journal of South American Earth Sciences 13 (2000) 443±457
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 ˆ 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.
E. Cabral-Cano et al. / Journal of South American Earth Sciences 13 (2000) 443±457
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), encompasses approximately 1600 km 2 of greenschist facies metamorphic rocks known as the TCMC. In 1981, OrtegaGutierrez 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 greenstone over mid-Cretaceous Morelos limestone west of Teloloapan. This fault was later interpreted by Campa and Coney
(1983) and Centeno-GarcõÂa et al. (1993) as a tectonostratigraphic 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 lowgrade, greenschist facies metamorphosed units that occasionally retain primary structure and texture. The disconformable 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 juxtapose 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 metamorphic rocks onto Cretaceous limestones.
3.2. Taxco Schist Formation
The Taxco Schist Formation is composed primarily of
445
®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 Morelos 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 resistance 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 consistently 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 estimate 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 EliasHerrera and Sanchez-Zavala (1990) include ®ne- and
medium-grained sericite and quartz schist, as well as chlorite schist, graphite schist, quartzite, and black slate.
3.3. Roca Verde Taxco Viejo Formation
Fries (1960) assigned foliated, low-grade metamorphosed
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E. Cabral-Cano et al. / Journal of South American Earth Sciences 13 (2000) 443±457
Fig. 3. Geologic map and cross sections of the Arcelia±Teloloapan area. Geology east of Pachivia is modi®ed after Barros (1996).
E. Cabral-Cano et al. / Journal of South American Earth Sciences 13 (2000) 443±457
447
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.
andesitic tuffs, breccias, lavas, and associated sandstones
west of Taxco Viejo to the Roca Verde Taxco Viejo Formation. 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 metasandstones. All of these rocks show greenschist facies chloritization 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 northcentral portion of the area (Fig. 3). The upper contact with
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E. Cabral-Cano et al. / Journal of South American Earth Sciences 13 (2000) 443±457
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.
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 metaconglomerate 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 phyllite 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) juxtaposes 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 undetermined 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 Zacatlancillo±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
E. Cabral-Cano et al. / Journal of South American Earth Sciences 13 (2000) 443±457
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 Archaeodictyomitra 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 discordant, 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 interface. PEMEX (1979) reported the existence of conglomerate 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
449
compositional variation may be responsible for the differential response to Late Cretaceous±Eartly Tertiary deformation.
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 recognized 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 globigerinids 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 bioturbating 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 original 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; GonzalezPacheco, 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 dolomitized. Based on Wilson (1975), this suite of rocks indicates deposition in environments that span toe-of-the-slope
to open platform facies. Limestones of the Morelos Formation have traditionally been assigned an Albian±Cenomanian age (Fries, 1960; Bonet, 1971; Ontiveros-Tarango,
1973). Equivalent age carbonates exist elsewhere in
Mexico, such as those of the El Abra Formation in
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E. Cabral-Cano et al. / Journal of South American Earth Sciences 13 (2000) 443±457
northeastern Mexico (Wilson and Ward, 1993; BasanÄezLoyola 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 Teloloapan 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 boundstone and calcareous conglomerate beds crop out at the base
of the unit, resting discordantly on Roca Verde metapyroclastics.
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 thickness 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 thickness of the Morelos Formation is partly the result of postdepositional 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 Chilacachapa range (de Cserna, 1978).
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 Chilacachapa 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., Quinqueloculina sp, Pyrgo sp., c.f.; ammonites include Peroniceras
a.f. tricarinatum, Peroniceras c.f. P. subtricarinatun
sturn, Peroniceras c.f. P. subtricarinatum Drescher, Peroniceras sp., Barroisiceras c.f. P. alstadenense, and Barroisiceras 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).
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 otherwise 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 depositional contact with Morelos limestone. The western limb, in
the vicinity of Pipicantla and Tenanguillo, is cut by a westdipping 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 limestone beds and volcanic ¯ows. We found no volcanic ¯ows
4. Discussion
4.1. Age of the metamorphic rocks
Sparse radiometric dating (Fig. 5) and limited biostratigraphic evidence have allowed a wide range of interpretations 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 sedimentary source terrain. Urrutia-Fucugauchi and Valencio
(1986) reported 108 ^ 5 and 125 ^ 5 Ma (Barremian±
Albian) whole-rock K/Ar ages from schist and metaandesite, respectively, collected near Ixtapan de la Sal. These
ages were attributed to the regional tectonic event that metamorphosed 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
E. Cabral-Cano et al. / Journal of South American Earth Sciences 13 (2000) 443±457
451
Fig. 6. Pre-Neocomian to Early Tertiary simpli®ed depositional and tectonic conditions for the study area and vicinity (see text for detailed explanation).
area to Ixtapan de la Sal (de Cserna and Fries, 1981; de
Cserna, 1983).
Pb ages from Tizapa massive sul®de deposits, near Zacazonapan, 15 km north of Tejupilco (Fig. 2) show singlestage 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) recalculated 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 concordant 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) reported
40
Ar/ 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 Teloloapan and Taxco, are incompatible with stratigraphic relationships. 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
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E. Cabral-Cano et al. / Journal of South American Earth Sciences 13 (2000) 443±457
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 siliciclastic deposition of the Mexcala Formation.
Biostratigraphic evidence regarding the age of the metamorphic 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 deposition between the Huetamo and Chilacachapa±Taxco platforms. This discontinuity misled Campa (1978) and Campa
and Ramirez (1979) to propose that metamorphic rocks in
the region are allochthonous. Instead, based on the stratigraphic and depositional evidence of the rocks in the studied
area, we interpret the Late Cretaceous volcanic rocks northwest 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 carbonate sediments was dependent on local conditions of sediment 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 Microcanthoceras sp. (Tithonian) and Wichmaniceras sp. (Valanginian) 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 disconformably underlie unmetamorphosed but locally highly tectonized 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 Teloloapan±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 (Gonzalez-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 carbonate 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 adjacent 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 Formations cropping out near Huetamo (Fig. 2) (Guerrero-Suastegui et al., 1992). Vidal-Serratos (1991a,b), Vidal-Serratos et
al. (1991) and Elias-Herrera and Sanchez-Zavala (1990)
E. Cabral-Cano et al. / Journal of South American Earth Sciences 13 (2000) 443±457
453
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.
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 oceanic crust, as has been suggested by Monod et al. (1990).
According to Moran-Zenteno et al. (1991), MoranZenteno 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 supporting 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 established, 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 subduction 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 volcaniclastic 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 Teloloapan 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 identi®ed radiolaria (see discussion in DaÂvila-Alcocer and Guerrero-SuaÂstegui, 1990). That a lack of volcanic rocks within
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E. Cabral-Cano et al. / Journal of South American Earth Sciences 13 (2000) 443±457
the Late Cretaceous carbonate platform sequence proves
that the volcaniclastic sequences, such as that of Teloloapan±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 metamorphic belts next to the volcanic arc from which the polarity of subduction may be deduced precludes a de®nitive
conclusion. Evidence necessary to substantiate the geodynamic model of Tardy et al. (1994), showing westward
subduction polarity, is circumstantial. We propose an alternative 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 undergone attenuation and plutonic activity during Early Cretaceous 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 Teloloapan and Chilacachapa are different entities (Guerrero-SuaÂstegui 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 similar lithology, stratigraphic position, and age on both sides of
the alleged western Guerrero terrane boundary does not
support this interpretation. Using a simple carbonate platform model, the sedimentary rocks mapped in the Teloloapan 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 environment.
Based on similarities of lithology, radiometric ages,
protoliths, and metamorphic grade, we correlate the metamorphic 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 metamorphic 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 lowgrade 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 deformation 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 America's Student Research Grant Program, as well as a PhD
E. Cabral-Cano et al. / Journal of South American Earth Sciences 13 (2000) 443±457
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, Universidad Nacional AutoÂnoma de MeÂxico) is gratefully acknowledged. 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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