A submarine fan in the Mesa Central, Mexico

A submarine fan in the Mesa Central, Mexico

G. Silva-Romo*, J. Arellano-Gil, C. Mendoza-Rosales, J. Nieto-ObregoÂn

DivisioÂn de IngenierõÂa en Ciencias de la Tierra, Facultad de IngenierõÂa, Universidad Nacional AutoÂnoma de MeÂxico, Cuidad Universitaria,

DelegacioÂn CoyoacaÂn, MeÂxico, D.F. c.p. 04510, Mexico

Abstract

The contact between the Guerrero and Sierra Madre tectonostratigraphic terranes has been proposed to lie in the Mesa Central, east of the

city of Zacatecas. Marine Triassic units have been assigned to the Guerrero Terrane. It is here proposed that this contact occurs to the west of

the city of Zacatecas and the Triassic marine sequence assigned to the Sierra Madre Terrane.

We analyzed the stratigraphic record and structural features of pre-Late Jurassic sequences at four localities in the Mesa Central. They

contain a marine turbiditic Triassic unit, which includes La Bellena, Taray, and Zacatecas Formations, and a continental unit of probable

Middle Jurassic age. Triassic sandstones were derived from a cratonic area, without the in¯uence of arc volcanism. The sequences were

affected by two phases of deformation. The Triassic formations are unconformably overlain by a continental volcano-sedimentary sequence

that contains fragments of sandstones derived from the underlying unit. Sedimentologic characteristics of the Triassic unit ®t a submarine fan

model. The submarine fan developed at the continental margin of Pangaea during Triassic times. Turbidite associations in the San Rafael

Area indicate a middle fan depositional environment, while in the Real de Catorce Area, they correspond to the distal part (basin plain facies).

At La Ballena and Zacatecas the turbidite associations occur in the middle part and perhaps the external part of the fan. q 2000 Elsevier

Science Ltd. All rights reserved.

Keywords: Submarine fan; Mesa Central, Mexico; Stratigraphic record and structural features

1. Introduction

Mesozoic strata in Mexico have a dual character; to the

east, sequences are associated with the opening of the Gulf

of Mexico, while in the west, sequences are associated with

a convergent margin. The nature of the contact between

these sequences is not clear as it is typically covered by

Cenozoic rocks. In the Mesa Central (Fig. 1A), several

structures have been proposed such as: the Zacatecas-

Guanajuato Frontal Thrust of Early Jurassic age (DeCserna,

1971; the contact between the Sierra Madre and Guerrero

tectonostratigraphic terranes (Campa and Coney, 1983)

(Fig. 1B), and the contact between the Circum-Gulf and

Paci®c Provinces (Winker and Buf¯er, 1988). In these

proposals, the marine character of the Zacatecas Formation

has been emphasized and is assumed to be exotic. Marine

rocks of Triassic age (or attributed to this period) of central

and eastern Mexico are exceptional since most rocks are of a

continental nature.

Global reconstructions of the Early Mesozoic era illus-

trate the dynamics of the opening of the Gulf of Mexico

(Coney, 1983; Anderson and Schmidt, 1983; Pindell,

1985). These models have not fully considered the role of

the Triassic marine strata that occur in the Mesa Central of

Mexico, mainly due to the lack of information in the source,

environment of deposition, stratigraphic relations and

structure.

In the Mesa Central, rocks of pre-Late Jurassic age

consist of two lithologic suites: the oldest suite, of marine

aspect and Late Triassic age, has not been clearly documen-

ted in all cases, so the following stratigraphic units have

been proposed: Zacatecas Formation (Carrillo-Bravo,

1968), La Pimienta Phyllite (Ranson et al., 1982), Taray

Formation (Cordoba, 1964), El Bote and El Ahogado

Formations (Monod and Calvet, 1992) and La Ballena

Formation (Silva-Romo, 1994). The second lithologic

suite, the Nazas Formation (Pantoja-Alor, 1963; 1972) is

clearly controversial since its stratigraphic relations and

lithic nature have not been clearly recognized. It has

frequently been considered as part of the older marine

unit. The Nazas Formation has a continental character and

is clearly different from other units of pre-Late Jurassic age

(Arellano-Gil, 1988), and unconformably overlies the

marine Triassic rocks (Silva-Romo, 1994).

On the other hand, it has been proposed that the basement

of the region is exposed in the Caopas area in Zacatecas,

where Cordoba (1964) used the names Rodeo Formation

Journal of South American Earth Sciences 13 (2000) 429±442

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

PII: S0895-9811(00)00034-1

www.elsevier.nl/locate/jsames

* Corresponding author. Fax: 152-5-550-0040.

E-mail address: [email protected] (G. Silva-Romo).

and Caopas Schists for a sequence of volcanic origin with

metamorphic features. These units have been considered to

be younger in age by Ortega-Gutierrez (1984). In this

region, a radiometric date of 183 ^ 8 My (K/Ar date on

hornblende: LoÂpez-InfanzoÂn, 1986) has been reported for

andesites and tuffs affected by cataclasis, within the

Rodeo Formation (Rogers et al., 1963). For that reason we

propose that the Rodeo Formation must be considered as the

Nazas Formation of Middle Jurassic age.

In order to clarify the geological setting during the pre-

Late Jurassic of the Mesa Central, we tested the hypothesis

that the Triassic marine sequence was accumulated in a

submarine fan (Silva-Romo, 1994). We investigated the

areas of La Ballena and Zacatecas (Fig. 2), where Late

Triassic fossiliferous sequences have been recognized

(Burckhardt, 1905; Silva-Romo, 1994), and two other

regions (Real de Catorce, S.L.P., and San Rafael, Zac.)

(Fig. 2) where units of similar lithologic character but

non-fossiliferous have been reported (Cordoba, 1964;

Barboza-GuidinÄo, 1992).

Petrographic studies of sandstone provenance were

performed following the petrographic criteria of Dickinson

(1985), and the sequences were characterized according to

the criteria of Mutti and Ricci-Lucchi (1972). Although we

could not con®rm that the marine pre-Late Jurassic

sequences of Real de Catorce, S.L.P., and San Rafael,

Zac., correspond to Late Triassic Zacatecas Formation,

their sedimentologic characteristics suggest that they equate

to the latter unit in the La Ballena and Zacatecas areas.

2. La Ballena area

The La Ballena area is located to the southeast of Salinas

de Hidalgo, S.L.P. (Fig. 2), the central part of the Sierra de

Salinas reaches its maximum elevation (2740 m) at PenÄon

Blanco. The rural community of La Ballena is located 7 km

SSE of PenÄon Blanco. Four structural sectors bounded by

normal faults striking N548±708W are de®ned (Silva-Romo,

1994) in terms of constraining structural and stratigraphic

features. Two major intrusive bodies are PenÄon Blanco and

Cerro Verde (Fig. 3).

The lowermost part of the Mesozoic sequence is a

turbiditic unit, which contains Late-Triassic fossils. This

G. Silva-Romo et al. / Journal of South American Earth Sciences 13 (2000) 429±442430

Fig. 1. (A) Mesa Central physiographic province in Mexico. (B) Tectonostratigraphic terranes of Mexico (After Campa and Coney, 1983). Abbreviations for

terranes: CHI� Chihuahua; CA� Caborca; COA� Coahuila; V� Vizcaino; S� Sonobari; R� Rusias; G� Guerrero; SM� Sierra Madre; A�Alistos;

MI�Mixteca; O� Oaxaca; J� Juarez; M�Maya; XO�Xolapa; Overlap Terranes: SMO� Sierra Madre Occidental, and TMV� Trans-Mexico Volcanic

Axis. It is here proposed that the northeastern border between the Guerrero and Sierra Madre Terranes is located west of the marine triassic outcrops of Central

Mexico, implying that the Sierra Madre Terrane extends up to that limit. In the ®gure, the previous and proposed borders of the Guerrero Terrane are outlined.

unit is informally designated by Silva-Romo (1994) as the

La Ballena Formation. It is unconformably overlain by

conglomerates and volcanic rocks of the Nazas Formation

(Silva-Romo, 1994). These two pre-Late Jurassic units are

covered by a marine sequence more than 1500 m thick, that

includes Jurassic to Late Cretaceous calcareous rocks and

Late Cretaceous turbidites (Arellano-Gil, 1988).

2.1. La Ballena Formation

The La Ballena Formation consists of a quartz turbiditic

sequence containing Late Triassic ammonoids and

pelecypods. These are partially, affected by greenschist

metamorphism. In general, the sedimentary character of

these rocks is evident, even in the northeast portion,

where sericite phyllites are exposed and the strata are

broken and transposed. Deformation inhibits the study of

the overall stratigraphic sequence. Nevertheless, based on

structural information and the geometry of the angular

unconformity between La Ballena and Nazas Formations,

a structural thickness of more than 2500 m is estimated

(Fig. 4). The lower contact, however, remains concealed.

Structurally, the outcropping Triassic sequence is a horst

with inverse relief (ChaÂvez-Aguirre, 1968).

ChaÂvez-Aguirre (1968) reported the ammonoid Sirenites

sp. To the northeast of La Ballena. In La Huerta Creek

southeast of La Ballena, specimens of Beyrichitidae family

were also reported (Gallo-P et al., 1993). In the same creek,

Sirenites and Clionites genus were collected by the authors

at the base of the beds; pelecypods molds were also seen that

may correspond with the Palaeoneilo genus and some

poorly preserved Halobia sp. This fauna is indicative of a

middle to Late Triassic age, and therefore are older than

those of Late Triassic age, recognized near Zacatecas City

(Burckhardt, 1905).

2.1.1. Sedimentologic Characteristics

The turbidites are lithic wackes, medium to ®ne grained,

and disposed in strata with thicknesses between 1 and

100 cm. Frequently, the beds have a whole Bouma

G. Silva-Romo et al. / Journal of South American Earth Sciences 13 (2000) 429±442 431

Fig. 2. Location map showing areas where geologic maps are reported in Figs. 3, 6±8. Main localities referred to in the text are outlined here. Zac� Zacatecas

State; SLP� San Luis PotosõÂ State.

sequence, and sole marks as ¯ow and load casts. Some of

the strata contain ammonoids at their base. Thick beds have

mud chips. The unit contains ®ne to medium grained ortho-

quartzites, with quartz fragments mainly of metamorphic

origin. The formation also includes some conglomeritic

lenses with subangular quartzite gravels and fossiliferous

calcareous exotic deformed blocks in a matrix of coarse

grained sandstone. In outcrops with the lowest grade of

metamorphism, bedding characteristics are preserved and

the proportion between sandy and pelitic fractions can be

identi®ed. Such features allow the de®nition of a lithofacies

for a submarine fan as proposed by Mutti and Ricci-Lucchi


Fig. 3. Geological map of La Ballena Area, located northwest of the city of San Luis PotosõÂ (Modi®ed from Silva-Romo, 1994). The horst with inverted relief,

represented by the outcrops of turbiditic triassic rocks of the La Ballena Formation is truncated by faults striking W±NW, and is therefore in tectonic contact

with Cretaceous and Late Jurassic rocks. The Nazas Formation discordantly overlies the La Ballena Formation, and is shown by sandstone fragments of the La

Ballena Formation in the conglomerates of the Nazas Formation (see Fig. 5). The La Ballena Formation presents two phases of deformation: D1 is recognized

by the foliation S1, by the development of cleavage C1, and by the development of minor folds with axial planes AP1 and fold axis F1. While D2 is recognized

by the development of an overprinted cleavage C2 and in the refolding of the minor folds with axial planes AP2 and fold axis F2 (Schmidt net, lower

hemisphere). The two stages of deformation also affect the Nazas Formation.

(1972). Facies C, D, and E were clearly recognized while

some observed lenses could correspond to facies B.

The lithofacies in the area characterizes the associations

as a middle to mostly external (basin plain) submarine fan

according to the model of Mutti and Ricci-Lucchi (1972).

On the other hand, the presence of carried fossils at the base

of the beds is typical of the association of middle fan facies

(Howell and Normark, 1982).

2.1.2. Sediment Source

Sandstones of the La Ballena Formation consist mainly of

quartz grains and some feldspar of metamorphic origin, the

feldspar grains being more abundant in thick mica-bearing

strata. The composition of sandstones according to

Dickinson, 1985 criteria are shown in Table 1 and indicate

derivation from a recycled orogen (Fig. 5). In this ®gure,

sandstone proportions from the La Ballena Formation are

compared with other Triassic or presumed Triassic units.

2.1.3. Structure

The La Ballena Formation is cut by low-angle thrusts

with a northwesterly vergence and observed structures,

such as horses and pods of metric scale, are frequently

present. In most outcrops, the sequence is inverted. As is

shown in Fig. 3, at least two phases of deformation are

registered in this unit. D1 is recognized by the deformation

of foliation S1, also by a cleavage with an ENE strike and

verging to the NW (S2), and in minor folds with axial planes

dipping to the SE (S3). Foliation is folded and minor folds

are refolded, and this de®nes the second deformation (D2);

the axial planes of the latter dip toward the SW (S4). D2 also

develops an overprint cleavage with a NNW strike and dip

to the SW.

2.2. Nazas Formation

In the La Ballena area, the Nazas Formation is 310 m


Fig. 4. Correlation chart of studied areas: (B) La Ballena Area (after Silva-Romo, 1994); (R) San Rafael Area (this paper); (C) Real de Catorce Area (this

paper), and (Z) Zacatecas Area (modi®ed from Monod and Calvet, 1992 and Monod, 1993). In these areas it is recognized as a marine turbiditic sequence,

dated as Triassic in the La Ballena Area (La Ballena Formation) and in Zacatecas (Zacatecas Formation), while in the San Rafael Area (Taray Formation) and

Real de Catorce Area (Zacatecas Formation?) we consider those sequences of Triassic age by their sedimentologic characteristics and structural features. A

unit that is discordantly overlying the turbiditic units in the four areas is recognized as one consiting of volcanic and continental clastic rocks (Nazas Formation

of Middle Jurassic age).

thick and contains a lower volcanic member (173 m thick),

and an upper clastic member (137 m thick) as observed

south of PenÄon Blanco Mountain (Fig. 3).

The volcanic member mainly consists of intercalated

lavas, with pyroclastic breccias and some conglomerates.

The lavas are of andesitic and basaltic composition, and

are hydrothermally altered with epidote and chlorite. They

also contain quartz veins and hematized pyrite pseudo-

morphs 0.5 cm in diameter. The clastic member consists

of intercalated siltstones with crystal tuffs and polymictic

conglomerate with sandstone fragments, similar to those

found in the La Ballena Formation (Fig. 4). The upper

part of the clastic member contains two gray ignimbrite

¯ows with greenstones.

On the southern side of PenÄon Blanco the two members of

the Nazas Formation are exposed but its base is not

observed, whereas to the north of the village of La Ballena,

the upper clastic member unconformably overlies the La

Ballena Formation, in a stratigraphic window which

outcrops in the Comanja Creek. The wedging out of the

volcanic member may be due to the ®lling in of the pre-

existing relief of the La Ballena Formation. Nevertheless,


Table 1

Average modal compositions (n� 1500) of sandstone from the La Ballena, Taray, Zacatecas, and Nazas Formations (abbreviations used: Qm�monocrystalline quartz, Qp� polycrystalline quartz, Qz� quartzite, Ch� chert, L� lithics, Lv� volcanics, F� feldspar, M�mica, Mt�matrix, Qs�secondary quartz, and O� opaques)

Sample Location (UTM) Formation Qm Q Qz Ch L Lv F M Mt Qs O

227SRB 14QKV24288606 La Ballena 307 211 372 115 48 65 379 1

233SRA 14QKV24738674 La Ballena 440 59 110 49 70 38 734

230SRA 14QKV24958666 La Ballena 356 195 218 25 4 563 127 18

237SR 14QKV24308663 La Ballena 513 38 295 69 39 3 500 38 5

PT-8 14QGT84952096 Taray 458 14 346 36 15 33 49 3 546

PT-19 14QGT90161899 Taray 663 101 344 54 30 23 9 244 32

PT-21 14QGT91151864 Taray 730 207 143 2 29 36 5 25 270 53

PT-27 14QGT88731820 Taray 645 156 230 1 31 70 20 291 56

PT-46A 14QGT82202195 Taray 634 37 235 10 37 1 6 528 17

PT-47B 14QGT82182193 Taray 631 397 20 3 385 64

CAT-77 14QLB01471421 Zacatecas 732 196 153 25 5 325 64

CAT-93 14QLB01441421 Zacatecas 754 1 246 54 14 11 21 372 27

54ECE 14QKV23149158 In Nazas 512 102 191 85 164 12 402 19 13

54ECC 14QKV23149158 In Nazas 531 52 258 55 50 14 434 100 6

54ECA 14QKV23149158 In Nazas 334 46 323 79 73 7 556 66 13

PT-45 14QGT83162275 In Nazas 504 111 125 32 27 60 10 581 50

PT-45A 14QGT83162275 In Nazas 643 188 28 114 391 106

Fig. 5. Sandstone provenance of Triassic sequences. (Ternary diagrams after Dickinson, 1985); QT� Total Quartz; F� Total Feldspar, and L� Lithics.

Provenance in diagrams is de®ned by ornamented areas as follows: CB� Continental Blocks; RO� Recycled Orogens; MA�Magmatic Arcs. Symbols

represent the following: Large empty circle� La Ballena Formation; Empty square� Taray Formation; Large ®lled circle� Zacatecas Formation; Small

®lled circle� Pebbles in Nazas Formation.

the Nazas Formation is recognized as discordant over the La

Ballena Formation, as is also shown by the presence of

Triassic sandstone fragments in its clastic member.

The Nazas Formation accumulated in a continental envir-

onment. The interbedded tuffaceous units in the volcanic

member demonstrate subaereal accumulation mainly

because it contains volcanic bombs, some of which have

scoriaceous crusts. Channel ®lling structures, lenticular stra-

ti®cation, conglomeritic lenses with load cast, cross-

bedding and ®ner-grained fractions with carbonate nodules

from the clastic member, all suggest that the clastic member

accumulated in a meandering river system which carried

products of the early denuded volcanic member. Alterna-

tively, the interbedded calcareous horizons may represent

lacustrine accumulations as proposed by Cuevas-PeÂrez

(1983) and Arellano-Gil (1988).

The Nazas Formation accumulated between Late Triassic

and Kimmeridgian, probably in the Middle Jurassic, over an

unconformity, that later served as a decollement surface

during post-Jurassic deformation. Late Jurassic transgres-

sion is expressed by calcareous and sandy platform rocks

of the Zuloaga Formation.

3. San Rafael area

This area is located in northern Zacatecas State, south of

Caopas (Fig. 2). The Pico de Teyra is the most conspicuous


Fig. 6. Geological map of San Rafael Area, northern Zacatecas. The mesozoic units form an anticline oriented to the northwest. The Taray Formation of

Triassic age outcrops in the central part of the map area. It is intruded by a granitic stock, and is discordantly covered by the Nazas Formation. The Taray

Formation presents two phases of deformation identical to those described for the La Ballena Area in Fig. 3.

orographic feature in the region and consists of a Tertiary

granitoid body emplaced in the Mesozoic sequence. The

mountain range is formed by an anticlinorium with a

NW±SE general orientation, dislocated by normal faults

at its southwestern border in such a way that the older

exposed units on that ¯ank are Juxtaposed against a

Quaternary basaltic ¯ow.

The pre-Late Jurassic rocks of the area consist of a turbi-

ditic marine sequence, designated as the Taray Formation

by Cordoba (1964), that was deformed together with a

continental unit of conglomerate and lavas during the

beginning of the Late Jurassic transgression. This is repre-

sented in northeast Mexico by a mainly carbonate marine

sequence of Late Oxfordian to Late Cretaceous age

(Anderson et al., 1991).

3.1. Taray Formation

This is the oldest unit of the Sierra de Teyra and it is

found in the surroundings of Pico de Teyra along the

southeast ¯ank of the range (Fig. 6). The Taray Formation

consists of a turbiditic sequence with quartz graywackes.

Intercalated greenstones were observed that might corre-

spond to pillow lavas.

In the San Rafael area the lower contact of the unit was

not observed. A structural thickness of 4500 m is estimated.

Cordoba (1964) assigned a late Paleozoic age to this unit

based on the lithological similarity between the Taray

Formation and the Tesnus Formation of the Marathon

region. We reject such an age because the novaculite

described by Cordoba (1964) most probably belongs to


Fig. 7. Geological map of Real de Catorce Area, northern San Luis PotosõÂ. The Zacatecas Formation outcrops in the nucleus of an asymmetric anticline,

oriented in a N±S direction and discordantly overlain by the Nazas Formation. The Zacatecas Formation and the Nazas Formation have structural features that

correspond with two phases of deformation: D1 is recognized by the development of cleavage C1, oriented in an E±W direction, and a foliation S1 striking W±

NW; D2 is recognized by overprinting of a cleavage C2 and a foliation S2, both with a NE strike; and S0� Strati®cation.

cherts of the Late Jurassic La Caja Formation (Silva-Romo

et al., 1994). Therefore, the Taray Formation must be

restricted only to the turbiditic sequence. The Taray

Formation Ð although without fossils Ð has sedimentolo-

gic and structural characteristics that are similar to the La

Ballena Formation (Silva-Romo, 1994) or the Zacatecas

Formation (Carrillo-Bravo, 1968). Therefore, a Triassic

age is assigned to the Taray Formation (Fig. 4).

3.1.1. Sedimentologic characteristics

The sandstones are arranged in beds 30±60 cm thick,

sometimes forming entire Bouma Sequences, intercalated

with low grade phyllite, siltstone, and shale in different

proportions. It has ¯ow casts, rip casts, graded bedding,

and some worm tracks. Toward the top of the unit, a

conglomerate is observed with rounded pebbles and cobbles

in a sandy±mud matrix. Lithofacies A, B, C, D, and G were

recognized which correspond to those of a submarine fan

(Mutti and Ricci-Lucchi, 1972). Facies A, B, and in some

places C, suggest a medium fan environment with some

shallow channels and inter-channel deposits. Facies C, D,

and G characterize a typical external fan environment.

3.1.2. Sediment source

The conglomeratic clasts of this unit consist of schist,

sandstone, shale, chert, milky quartz, and two different

granites. Sandstones of the Taray Formation (Table 1) are

composed of craton-derived quartz fragments which,

following Dickinson's criteria, are similar to those found

in the La Ballena Formation (Fig. 5).

3.1.3. Structure

The unit contains broken beds and in places, the sandy

fraction of turbidites has mixed chaotically as boudins and

pods in an incipient foliation. In the majority of outcrops it

was observed that the sequence is inverted. The unit has

been affected by two phases of deformation which are

manifested in refolded minor folds and by the development

of two cleavages (Fig. 6).

3.2. Nazas formation

This unit unconformably overlies the Taray Formation

and outcrops in the middle of the Sierra de Teyra. It consists

of two members: a basal member formed by alternating

sandstone and conglomerate in beds 35±60 cm thick, with

moderately sorted and rounded clasts of quartz, sandstones,

chert, and intermediate volcanics ranging in size from 3 mm

to 7 cm in a sandy matrix. Clasts from the conglomerate

(Table 1) include Taray Formation sandstones showing a

clear cratonic af®nity, as depicted in the source ternary

diagram (Fig. 5).

The upper member is formed by a variety of volcanic

rocks, such as dark gray phyllitized tuffs, air-fall tuffs

with impact marks, ignimbrites, light green lithic tuffs,

with shale and rounded quartz clasts, vitric tuffs with

abundant and deformed spherulites, and some intercalated

andesitic lavas. A total structural thickness of 500 m (Fig. 4)

is estimated.

The Nazas Formation has two ductile±brittle deformation

phases, one with foliation and cleavage and an E±W strike.

A second one with numerous reverse faults of little

displacement oriented to the N088E, 308SE and an overprint

cleavage.

4. Real De Catorce area

The Real de Catorce area is located north of San Luis

PotosõÂ State in the northern sector of the Sierra de Catorce

(Fig. 7). Located in the nucleus of the range is the formerly

resplendent Real de Catorce City, an abandoned XVIII

century mining center. The Sierra de Catorce consists of

an asymmetrical anticlinorium trending north, exposing a

sedimentary sequence more than 2500 m thick. The pre-

Late Jurassic rocks exposed in the core of the sierra consist

of: the Zacatecas Formation of Late Triassic age (Palazue-

los, 1970; Barboza-GuidinÄo, 1992) and overlying the

Zacatecas Formation with an angular unconformity lies

the Nazas Formation, consisting of continental red beds,

volcanic ¯ows, and intercalations of volcaniclastic rocks

(ZaÂrate, 1982; Barboza-GuidinÄo, 1992).

4.1. Zacatecas formation

This is composed of a thin bedded turbiditic sequence,

whose sandstone clasts are mainly quartz in an abundant

matrix. It contains shale horizons and has low grade meta-

morphism. It outcrops west of Real de Catorce (Fig. 7),

where it is estimated to have a 600 m structural thickness.


Strata at the base of the sequence are characterized by

®ne-grained quartz-rich graywackes that are rhythmically

intercalated with shale beds 10 to 15 cm thick. They contain

sole marks, graded beds, and parallel laminations. Higher up

in the sequence laminated and foliated shales occur in beds

60±80 m thick, with some isolated beds of ®ne-grained

sandstone. Toward the middle part, another ®ne-grained

sandstone is observed rhythmically intercalated with shales

10 and 40 cm thick. Some beds present sole marks and

lamination. The top is characterized by a package of foliated

shale beds 40±80 cm thick. Following the criteria discussed

by Mutti and Ricci-Lucchi (1972), lithofacies C and G were

identi®ed. These occur in the most distal fart of a fan in the

basin plain.


Sandstones are quartz wackes with mainly reworked

igneous and metamorphic quartz and some fragments of

quartzite. According to Dickinson's ternary diagram

(1985), the sandstones have a continental provenance

(Fig. 5).


4.1.3. Structure

The sequence is in an upright position with evident low

grade metamorphism in the pelitic fraction. Two phases of

superposed deformation were recognized. The ®rst one

developed a cleavage and a foliation with E±W and

WNW strike. The second phase is recognized by an

overprint cleavage and foliation with a NE trend (Fig. 7).


This unit outcrops extensively on the eastern side of the

Sierra de Catorce and is characterized by a steep relief. Its

thickness is estimated at 350 m and it is composed of

conglomerate, sandstones with lenticular strati®cation, and

andesitic ¯ows (Fig. 4). Toward the base it is composed of

lithic arenite with some siltstone intercalations. In the

middle and upper parts of the unit, polymictic conglomer-

ates were observed with conglomeratic sandstone horizons.

The sandstones contain crossed and lenticular bedding,

graded beds and ®lling channel deposits. The clasts are

subrounded and consist of sandstones, quartz, and andesites

up to 5.5 cm in diameter. At the top, the formation is

composed of beds of graywacke 80 cm±1 m thick, interca-

lated with thinner bedded siltstones. The unit was accumu-

lated in a continental environment related to a volcanic arc,

with intense denudation in the Zacatecas Formation as well

as in the volcanic rocks of the same formation. The Nazas

Formation is folded parallel to the general strike N±NW of

the Sierra Madre Terrane structures. It also has a cleavage

similar to the Zacatecas Formation.

5. Zacatecas area

In the area west of Zacatecas City there are outcrops

where the Carnian fossils were ®rst recognized at the

beginning of this century (Burckhardt, 1905). However,

their meaning has been unclear because the structural

complexity of the area has obscured stratigraphic relations

and sometimes the very nature of the rocks as well. In this

controversial locality we recognized the units, but not the

sequence, proposed by Monod and Calvet (1992), who


Fig. 8. Geological map of the Zacatecas Area, central Zacatecas (modi®ed from Monod and Calvet, 1992). We propose here to include within the Zacatecas

Formation the El Bote and El Ahogado Formations of Monod and Calvet (1992), and within the Nazas Formation the Pimienta Formation of the same authors.

According to the structural information of Monod and Calvet (1992), the units are affected by two phases of deformation. The ®rst one is recognized by the

development of an S1 foliation; while the second one is manifested by the folding of the foliation as open folds with a NW strike, and the development of a

mineral stretching L1 lineation. De acuerdo a la informacioÂn estructural de Monod and Calvet (1992). Las unidades estaÂn afectadas por dos fases de

deformacion. La primera se reconoce en el desarrollo de foliacioÂn S1; en tanto que la segunda se mani®esta en el plegamiento de la foliacion de acuerdo

a pliegues abiertos con rumbo al NW y en el desarrollo de lineacion (L1).

reinterpreted the work of Ranson et al. (1982) and McGehee

(1976). Comparing the units with those of La Ballena

area, we make the following consideration: the Pimienta

phyllite Ð considered to be the oldest by Monod and Calvet

(1992) Ð corresponds to the Nazas Formation and overlies

the fossiliferous rocks of the Late Triassic.

5.1. Zacatecas formation

Northeast of Zacatecas City in El Bote Creek (Fig. 8),

outcrops the ®rst marine sequence of Late Triassic age

recognized in Mexican outcrops. It was studied by Burc-

khardt and Scalia (1906), and by Carrillo-Bravo (1968)

who assigned it the informal name of Zacatecas Formation.

Monod and Calvet (1992) used the name El Bote Formation

(¯ysch) for a monotonous turbiditic sequence of black

phyllite and light brown beds of quartzite with thicknesses

between 1 and 10 cm. This is overlain by the El Ahogado

Formation which consists of black slates with some thick

beds of highly tectonized quartzite; both sequences are of

Late Triassic age. Monod (1993) estimated a total thickness

of more than 180 m for both units (Fig. 4). We agree with

Centeno-GarcõÂa et al. (1993a) who considered that both

units correspond to the Zacatecas Formation.


McGehee (1976) characterized the sequence as a metase-

dimentary unit of greenschist facies and recognized relict

sedimentary structures such as strati®cation, sole marks,

graded bedding, and probable cross bedding.


Centeno-GarcõÂa et al. (1993a) reported a continental

provenance for the siliciclastic rocks of the Zacatecas and

indicated similarities between these rocks and those of

Artega and Placeres in MichoacaÂn and Guerrero, based on

trace element concentrations and Nd isotopic ratios.

5.1.3. Structure

Internally, the Triassic sequence contains foliation and

kink bands identi®ed by McGehee (1976) who also recog-

nized transposition of the strati®cation and two deformation

phases expressed in cleavage overprint. Monod and Calvet

(1992) interpreted thrust±slip faulting between the exposed

units. Nevertheless, given the stratigraphic relations and the

contrast in the lithologic character of the units, we consider

that the structural features developed at their contacts are

caused by the rheologic differences between them.


The Zacatecas Formation is overlain, above a faulted

contact, by the Nazas Formation, which is characterized

by the abundance of metavolcaniclastic material as well as

sandstones and well-bedded conglomerates, containing

sandstone pebbles. This unit was described by Monod and

Calvet (1992) as the La Pimienta Formation. Monod (1993)

assumes a thickness of more than 300 m.

5.3. Greenstones

In the area, a ªgreenstoneº sequence occurs whose

genesis and age are controversial. Burckhardt and Scalia

(1906) considered them Late Triassic age spillites, whereas

PeÂrez-MartõÂnez et al. (1961) assigned them a Tertiary age

and regarded them as sub-volcanic. Ranson et al. (1982)

considered that they are post-sedimentary and are of an

intrusive origin. Alternatively, Servais et al. (1986)

suggested that the greenstones from Zacatecas correspond

to the pillow-lava sequence of the Chilitos Formation from

Fresnillo, Zac., (DeCserna, 1976), which they consider to be

of Late Jurassic age. Monod and Calvet (1992) considered

them as submarine lavas of possible Early Cretaceous age

and called them the ªZacatecas Pillow Lavas.º

6. Triassic marine rocks in the Guerrero Terrane

The eastern limit of the Guerrero Terrane was proposed

on the basis of the outcrops of the Triassic marine rocks

(Campa and Coney, 1983). In addition to the outcrops

described herein, other outcrops have been reported: in the

area of Charcas, S.L.P., CantuÂ-Chapa (1969) reported

marine Late Triassic rocks based on a specimen of Juvavites

(Anatomites) sp. Those rocks have a ¯yschoidal character

(TristaÂn-GonzaÂlez and Torres-HernaÂndez, 1992). A struc-

tural thickness of 4640 m was reported in the Tapona

Well drilled by the Mexican oil company (PEMEX) to the

NW of Charcas (LoÂpez-InfanzoÂn, 1986).

In the southwestern part of the Guerrero Terrane a

Triassic marine sequence outcrops. According to Campa

et al. (1982), the Artega Schist of the type locality consists

of basaltic pillow lavas, chert, and intercalated sandstones

with shales. This sequence is metamorphosed to greenschist

facies and has been affected by two phases of deformation

(Centeno-GarcõÂa et al., 1991). Centeno-GarcõÂa et al.

(1993b), considers that the Arteaga Complex may represent

the basement of the Guerrero Terrane, and consists of a

terrigenous sequence (Varales Formation) with a continen-

tal source which accumulated on an oceanic crust.

7. Paleoenvironmental and Tectonic interpretation

The stratigraphic characteristics and spatial relations of

the recognized units constrain paleogeographic reconstruc-

tions of the Late Triassic of Mexico. Identifying the corre-

spondence of the Middle Jurassic Nazas Formation to the

Pimienta Formation of Monod and Calvet (1992) removes

any suggestion of a volcanic in¯uence in the Triassic

stratigraphic record of the Mesa Central.

The age of the Zacatecas Greenstones remains unsolved.

However, based on the greenstones recognized in the San


Rafael Area, we consider the existence of two units of

pillow lavas possible: a Triassic one as a substrate in

which external facies of turbidites were accumulated

(Centeno-GarcõÂa et al., 1993a), and another one of Late

Jurassic±Cretaceous age. The latter is structurally juxta-

posed with the Triassic sequence as considered by Monod

(1993). Based on these considerations we outline the follow-

ing Mesozoic paleographic evolution, with emphasis on the

Late Triassic to Middle Jurassic.

During the Late Triassic a submarine fan accumulated on

the periphery of a cratonic area. In the analyzed areas (Fig.

9) and according to the model of Ricci-Lucchi (1975), the

following submarine fan environments are recognized: the

Middle Fan (San Rafael Area), the Middle and External Fan

(La Ballena and Zacatecas Area), and the most External

Facies associations Ð Basin Plain Facies association Ð

(Sierra de Catorce Area).

The dimensions and orientation of the fan are uncertain

due to the unconnected nature of the outcrops. The present

distribution of the outcrops is controlled by major structural

features formed in two phases of deformation and prevents

any accurate palinspastic reconstruction. The fan accumu-

lated at a continental margin of non-collisional type, devel-

oped to the west of Pangaea (Fig. 10). Most likely the

turbiditic sequence represents denudation of the North

American Craton, and indicates that a major ¯uvial system

existed. (see Potter, 1978). Such a ¯uvial system transported

an enormous quantity of sediment to the paleo-Paci®c

border of Pangaea. The clastics perhaps originated from

highlands at Appalachian and Grenvillian Belts. If the

greenstones in Zacatecas are of a Triassic age, they might

represent slivers of the oceanic crust on which the most

distal facies of the fan were deposited.

The sequence of the fan was incorporated into the

orogenic belt during a deformation phase in the Middle to

Late Jurassic. The shortening that is seen in the Triassic

sequence in the SE±NW direction can be explained by a

transpression phase probably associated with a lateral slip

similar to the Mojave±Sonora±Megashear (M±S±M)

(Silver and Anderson, 1974). The sequence, already

deformed or in the process of deformation, was transported

following such structural lineament from a septentrional

position (Fig. 10). According to Anderson et al. (1991),

the lateral displacement on the megashear accurred in the

Late Jurassic (Oxfordian?), while for Sedlock et al. (1993),

the structure has had two stages of movement, one in the

Paleozoic and one in the Late Jurassic. We consider that the

tectonic transport along the M±S±M may have occurred in

an intermittent manner between the Paleozoic and the Late

Jurassic, and in the shear environment the Triassic marine

sequence accumulated. Later, as the tectonic transport of

blocks occurred, a convergent margin was initiated in the

west, which generated a volcanic arc represented by the

Nazas Formation, which presents dynamic metamorphism

(LoÂpez-InfanzoÂn, 1986; Anderson et al., 1991).

Tectonic transport of the Triassic sequence at the western

border or Pangaea, occurred at the same time as the rifting

between America and Africa. According to Hay et al.

(1982), marine sedimentation in the proto-Atlantic rift was

initiated during Carnian times and communication between

Tethyan and Paci®c waters did not occur until the Late

Jurassic. As Atlantic divergence accentuated, opening of

the Gulf of Mexico was initiated. With this rifting process


Fig. 9. Proposed location of paleoenvironments for the turbiditic Triassic

sequences in the submarine fan model for the Mesa Central, Mexico. (R)

Tentative position for the Taray Formation in the San Rafael Area; (B) La

Ballena Formation in the La Ballena Area; and (Z) Zacatecas Formation in

the Real de Catorce Area (adapted from Ricci-Lucchi (1975)).

Fig. 10. Late Triassic paleographic reconstruction, (non-palinspastic,

modi®ed from Pindell, 1985 and Sedlock et al., 1993). The analyzed

Triassic sequence accumulated in the western border of Pangaea, without

any volcanic in¯uence, somewhere northwest of its present position. Later

modi®cations of its tectonic position were in¯uenced by the Mojave-Sonora

megashear system.

the northeastern region of Mexico acquired a con®guration

of continental blocks with contrasted reliefs (Buf¯er et al.,

1980; Pindell, 1985). The region was affected by a gradual

marine transgression accompanied by a subsidence in the

Late Jurassic to Late Cretaceous interval (Arellano-Gil,

1988).

As a consequence of the reorganization of tectonic plates,

a Late Jurassic to Early Cretaceous volcanic arc (Guerrero

Terrane) was initiated which subsequently migrated east-

ward until it collided against the North American craton.

Border Triassic turbidites were ®rst accumulated on the

craton, followed by the arc sequence (Nazas Formation)

and still later the calcareous and shale sequence of Late

Jurassic to Early Cretaceous, and ®nally an Upper

Cretaceous turbiditic sequence with volcanic detritus from

the approaching arc.

This scenario for the evolution of the Guerrero Terrane

requires con®rmation from the analysis of green stones of

the Mesa Central, for which we propose the following

hypothesis: the greenstones of the Mesa Central (arc-

assemblage of the Fresnillo region (Centeno-GarcõÂa and

Silva-Romo, 1993) are the limit expression between the

Guerrero and Sierra Madre Terranes. If this hypothesis is

con®rmed, all of the Triassic turbiditic sequence would

correspond to the Sierra Madre Terrane, without excluding

the sequence of the Zacatecas Area as proposed by Centeno-

GarcõÂa and Silva-Romo (1993).

8. Conclusions

(1) The pre-Late Jurassic rocks exposed in the Mesa

Central consist of two units. The ®rst is a marine unit of

turbidites known locally as the La Ballena, Zacatecas, and

Taray Formations. The other unit is continental and consists

of volcanic and clastic rocks and encompasses the Nazas

Formation of Early Jurassic to Oxfordian age. Such units are

separated by a discordance expressed in the geometric rela-

tions between the units and by the presence of clastics of the

turbidites within the conglomerates of the Nazas Formation.

(2) The supposedly Triassic Pimienta Formation (Monod

and Calvet, 1992), corresponds to Middle Jurassic Nazas

Formation and therefore the Triassic sequence at Zacatecas

does not include pyroclastic components.

(3) The sedimentologic characteristics of the Triassic

sequence suggest that it accumulated in a submarine fan.

The shape and extension of the fan cannot be reconstructed

due to intense deformation despite the fact that submarine

fan environments such as the Middle, External, and Basin

Plain were recognized (Ricci-Lucchi, 1975)

(4) The provenance of the sandstones of the La Ballena,

Taray, and Zacatecas Formations is clearly cratonic (Dick-

inson, 1985). These sequences indicate a prolonged denuda-

tion of a cratonic area and the existence of a ¯uvial system

that probably built a delta, which fed an extensive submar-

ine fan at the Western margin of Pangaea. Consequently, the

Sierra Madre Terrane includes the turbiditic sequence of the

Zacatecas Area (Fig. 1).

(5) Although the La Ballena Formation is deformed and

overthrusted, its facies indicate that the continental margin

was located along central Mexico during Triassic time. This

point supports the model that the Guerrero Terrane formed

outboard the continental margin.

(6) The Triassic marine sequence has been subjected to at

least two phases of tectonic compressive deformation,

which are manifested in the development of foliation,

refolded folds, folded foliation and the development of

two cleavage plane orientations.

Acknowledgements

We thank Barbara Martiny Kramer and Marco A. CarreoÂn

MeÂndez for his helpful review. The writing of this

manuscript was improved thanks to the keen observations

of Peter Coney, Elena Centeno-GarcõÂa, and an anonymous

referee. We thank all of them for their valuable comments,

but above all their patience and indulgence.

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A submarine fan in the Mesa Central, Mexico

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