A submarine fan in the Mesa Central, Mexico

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

www.elsevier.nl/locate/jsames

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 ZacatecasGuanajuato 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 illustrate the dynamics of the opening of the Gulf of Mexico (Coney, 1983; Anderson and Schmidt, 1983; Pindell, * Corresponding author. Fax: 152-5-550-0040. E-mail address: [email protected] (G. Silva-Romo).

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 documented 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

0895-9811/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S 0895-981 1(00)00034-1

430

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

Fig. 1. (A) Mesa Central physiographic province in Mexico. (B) Tectonostratigraphic terranes of Mexico (After Campa and Coney, 1983). Abbreviations for terranes: CHI ˆ Chihuahua; CA ˆ Caborca; COA ˆ Coahuila; V ˆ Vizcaino; S ˆ Sonobari; R ˆ Rusias; G ˆ Guerrero; SM ˆ Sierra Madre; A ˆ Alistos; MI ˆ Mixteca; O ˆ Oaxaca; J ˆ Juarez; M ˆ Maya; XO ˆ Xolapa; Overlap Terranes: SMO ˆ Sierra Madre Occidental, and TMV ˆ 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.

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 preLate 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±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 ˆ Zacatecas State; SLP ˆ San Luis Potosõ State.

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

432

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

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.

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 orthoquartzites, 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

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

433

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).

(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

434

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

Table 1 Average modal compositions (n ˆ 1500) of sandstone from the La Ballena, Taray, Zacatecas, and Nazas Formations (abbreviations used: Qm ˆ monocrystalline quartz, Qp ˆ polycrystalline quartz, Qz ˆ quartzite, Ch ˆ chert, L ˆ lithics, Lv ˆ volcanics, F ˆ feldspar, M ˆ mica, Mt ˆ matrix, Qs ˆ secondary quartz, and O ˆ opaques) Sample

Location (UTM)

Formation

Qm

Q

Qz

227SRB 233SRA 230SRA 237SR PT-8 PT-19 PT-21 PT-27 PT-46A PT-47B CAT-77 CAT-93 54ECE 54ECC 54ECA PT-45 PT-45A

14QKV24288606 14QKV24738674 14QKV24958666 14QKV24308663 14QGT84952096 14QGT90161899 14QGT91151864 14QGT88731820 14QGT82202195 14QGT82182193 14QLB01471421 14QLB01441421 14QKV23149158 14QKV23149158 14QKV23149158 14QGT83162275 14QGT83162275

La Ballena La Ballena La Ballena La Ballena Taray Taray Taray Taray Taray Taray Zacatecas Zacatecas In Nazas In Nazas In Nazas In Nazas In Nazas

307 440 356 513 458 663 730 645 634 631 732 754 512 531 334 504 643

211 59 195 38 14 101 207 156 37

372 110 218 295 346 344 143 230 235 397 196 246 191 258 323 125 188

1 102 52 46 111

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 pseudomorphs 0.5 cm in diameter. The clastic member consists of intercalated siltstones with crystal tuffs and polymictic conglomerate with sandstone fragments, similar to those

Ch

36 54 2 1 10

32

L 115 49 25 69 15 30 29 31 37 20 153 54 85 55 79 27 28

Lv

F 48 70

33 23 36 70 1 14

60 114

39 49 5 6 25 11 164 50 73

M

Mt

65 38 4 3 3 9 25 20

379 734 563 500 546 244 270 291 528 385 325 372 402 434 556 581 391

3 5 21 12 14 7 10

Qs

O 1

127 38

19 100 66

18 5 32 53 56 17 64 64 27 13 6 13 50 106

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 preexisting relief of the La Ballena Formation. Nevertheless,

Fig. 5. Sandstone provenance of Triassic sequences. (Ternary diagrams after Dickinson, 1985); QT ˆ Total Quartz; F ˆ Total Feldspar, and L ˆ Lithics. Provenance in diagrams is de®ned by ornamented areas as follows: CB ˆ Continental Blocks; RO ˆ Recycled Orogens; MA ˆ Magmatic Arcs. Symbols represent the following: Large empty circle ˆ La Ballena Formation; Empty square ˆ Taray Formation; Large ®lled circle ˆ Zacatecas Formation; Small ®lled circle ˆ Pebbles in Nazas Formation.

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

435

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.

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 environment. 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 strati®cation, conglomeritic lenses with load cast, crossbedding 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. Alternatively, 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 transgression 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

436

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

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 ˆ Strati®cation.

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 turbiditic 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 represented 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 correspond 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

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

437

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 sedimentologic 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).

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.

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.

4. Real De Catorce area

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

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 preLate Jurassic rocks exposed in the core of the sierra consist of: the Zacatecas Formation of Late Triassic age (Palazuelos, 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 metamorphism. It outcrops west of Real de Catorce (Fig. 7), where it is estimated to have a 600 m structural thickness. 4.1.1. Sedimentologic characteristics 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. 4.1.2. Sediment source 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).

438

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

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).

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). 4.2. Nazas formation 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 conglomerates 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, intercalated with thinner bedded siltstones. The unit was accumulated 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

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

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 Burckhardt 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. 5.1.1. Sedimentologic characteristics McGehee (1976) characterized the sequence as a metasedimentary unit of greenschist facies and recognized relict sedimentary structures such as strati®cation, sole marks, graded bedding, and probable cross bedding. 5.1.2. Sediment source 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 recognized 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. 5.2. Nazas formation 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

439

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 structural 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 continental source which accumulated on an oceanic crust. 7. Paleoenvironmental and Tectonic interpretation The stratigraphic characteristics and spatial relations of the recognized units constrain paleogeographic reconstructions of the Late Triassic of Mexico. Identifying the correspondence 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

440

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

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)).

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 juxtaposed with the Triassic sequence as considered by Monod (1993). Based on these considerations we outline the follow-

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.

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 accumulated at a continental margin of non-collisional type, developed 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

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

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 eastward 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 (arcassemblage 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 CentenoGarcõÂa 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 relations 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 (Dickinson, 1985). These sequences indicate a prolonged denudation of a cratonic area and the existence of a ¯uvial system that probably built a delta, which fed an extensive submarine fan at the Western margin of Pangaea. Consequently, the

441

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. References Anderson, T.H., Schmidt, V.A., 1983. The Evolution of Middle America and the Gulf of Mexico Ð Caribbean Sea Region during Mesozoic time. Geological Society of America Bulletin 94, 941±966. Anderson, T.H., McKee, J.W., Jones, N.W., 1991. A Northwest Trending, Jurassic Fold Nappe, Northernmost Zacatecas, MeÂxico. Tectonics 10 (2), 383±401. Arellano-Gil, J., 1988. GeologõÂa de la PorcioÂn Septentroinal de la Sierra de PenÄoÂn Blanco, Estados de San Luis Potosõ y Zacatecas (Geologal Engineer thesis): Universidad Nacional AutoÂnoma de MeÂxico, Facultad de IngenierõÂa, 115p. Barboza-GuidinÄo, J.R., 1992. GeologõÂa de la Sierra de Catorce San Luis PotosõÂ. Encuentro Hispano Mexicano Sobre GeologõÂa y MinerõÂa. Universidad Nacional AutoÂnoma de MeÂxico, Facultad de IngenierõÂa, MEMORIAS, vol. 4, pp. 87±95. Buf¯er, R.T., Watkins, J.S., Shaub, F.J., Worzel, J.L., 1980. Structure and early geologic history of the Deep Central Gulf of Mexico Basin. In: Pilger (Ed.), The Origin of the Gulf of Mexico and the Early Opening of the Central North Atlantic Ocean, pp. 3±17. Burckhardt, C., 1905. A Faune Marina du Trias Superior de Zacatecas. BoletõÂn del Instituto de GeologõÂa, MeÂxico 21, 5±38. Burckhardt, C., Scalia, S., 1906. Geologie des Environs de Zacatecas. Congreso GeoloÂgico Internacional X, guõÂa de excursiones 16, MeÂxico, 26pp. Campa, F., Coney, P., 1983. Tectono-stratigraphic Terranes and mineral resource distribution in Mexico. Canadian Journal of Earth Sciences 20, 1040±1051. Campa, M.F., RamõÂres, J., Bloome, C., 1982. La Secuencia VolcaÂnicoSedimentaria Metamor®zada del TriaÂsico (Ladiniano CaÂrnico) de la RegioÂn de Tumbiscatio, MichoacaÂn (abs). Sociedad GeoloÂgica de MeÂxico, VI ConvencioÂn Nacional, Abstracts, 48pp. CantuÂ-Chapa, C.M., 1969. Una Nueva Localidad TriaÂsico Superior en MeÂxico. Revista Instituto Mexicano del PetroÂleo 1 (2), 71±72. Carrillo-Bravo, J., 1968. Reconocimiento GeoloÂgico Preliminar de la PorcioÂn Central del Altiplano Mexicano: PetroÂleos Mexicanos, IneÂdito, Original no consultado Citado en MartõÂnez, A., Malpica, R., Estudio Estratigra®co SedimentoloÂgico de la FormacioÂn: Instituto Mexicano del PetroÂleo, Proyecto C-1134, (IneÂdito), 28pp.

442

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

Centeno-GarcõÂa, E., Ruiz, J., Coney, P., Patchett, J.P., 1991. Geology Sandstone Petrofacies, and Geochemistry of the Guerrero Terrane, Western Mexico. Comunicaciones 42, pp. 39±43. Centeno-GarcõÂa, E., Silva-Romo, G., Geology of the San Luis PotosõÂZacatecas Region, 1993. Northeastern Limit of the Guerrero Terrane. First Circum-Paci®c and Circum-Atlantiv Terrane Conference, Guidebook of Filed Trip A, Instituto de GeologõÂa, Universidad Nacional AutoÂnoma de MeÂxico, pp. 59±66. Centeno-GarcõÂa, E., Coney, P., Ruiz, J., Patchett, J., Ortega-GutieÂrrez, 1993a. Tectonic Signi®cance of the Sediments of the Guerrero Terrane From Petrographic. Trace Element, and Nd-isotopic studies. Proceedings, First Circum-Paci®c and Circum-Atlantic Terrane Conference, Guanajuato, Mexico, Instituto de GeologõÂa, Universidad Nacional AutoÂnoma de MeÂxico, pp. 30±33. Centeno-GarcõÂa, E., Ruiz, J., Coney, P., Patchett, J., Ortega-GutieÂrrez, 1993b. Guerrero Terrane of Mexico; its Role in the Southern Cordillera from New Geochemical Data. Geology 21, 419±422. ChaÂvez-Aguirre, R., 1968. Bosquejo GeoloÂgico de la Sierra de PenÄon Blanco, Zac. (Geologal Engineer thesis), MeÂxico, D.F., Facultad de IngenerõÂ, Universidad Nacional AutoÂnoma de MeÂxico, 67pp. Coney, P., 1983. Un modelo tectoÂnico de MeÂxico y sus relaciones con AmeÂrica del norte. AmeÂrica del Sur y el Caribe. Revista del Instituto Mexicano del PetroÂleo 15 (1), 6±15. Cordoba, D., 1964. Resumen de la GeologõÂa de la Hoja Apizolaya, Estados de Zacatecas y Durango, Hoja Apizolaya 13R-I(9). Carta GeoloÂgica de MeÂxico, Serie 1:100,000, Instituto GeologõÂa, Universidad Nacional AutoÂnoma de MeÂxico. Cuevas-PeÂrez, E., 1983. EvolucioÂn GeoloÂgica del Estado de Zacatecas, MeÂxico (The Geological evolution of the mesozoic in the state of Zacatecas, Mexico). Geol. Palaont. Teil 1983 (3/4), 190±201. DeCserna, Z., 1971. Mesozoic Sedimentation. Magmatic Activity and Deformation in Northern Mexico. The Geologic Framework of the Chihuahua Tectonic Belt, West Texas Geological Society, Midland, pp. 99±117. DeCserna, Z., 1976. Geology of the Fresnillo Area, Zacatecas, Mexico. Geological Society of America Bulletin 87 (8), 1191±1199. Dickinson, W.R., 1985. Interpreting provenance relations from detrital modes of sandstones. In: Zuffa, G.G. (Ed.). Provenance of Arenites, Dordrecht Holl. Reidel, Dordrecht, pp. 333±361. Gallo-P, I., GoÂmez-L, M.E., Contreras, B., Cedillo-P, E., 1993. Hallazgos PaleontoloÂgicos del TriaÂsico Marino en la RegioÂn Central de MeÂxico. Revista de la Sociedad Mexicana de PaleontologõÂa 6 (1), 1±9. Hay, W.W., Barron, E.J., Behensky Jr., J.F., Sloan, S.L., II, ., 1982. Triassic±Liassic Paleoclimatology and Sedimentation in Proto-Atlantic Rifts. Palaeogeography Paleocilmatology Paleoecology 40, 13±30. Howell, D.G., Normark, W.R., 1982. Sedimentology of submarine fans. In: Scholle, P., Spearing, D. (Eds.). Sandstone Depositional Environments, vol. 31. Association of Petroleum Geologists, Memoir, pp. 365±404. LoÂpez-InfanzoÂn, M., 1986. Estudio PetrogeneÂtico de las Rocas Igneas en las Formaciones Huizachal y Nazas. BoletõÂn de la Sociedad GeoloÂgica Mexicana 2, 1±42. McGehee, R., 1976. Las Rocas MetamoÂr®cas del Arroyo de la Pimienta, Zacatecas, Zac. Boletin Sociedad GeoloÂgica Mexicana 37 (1), 1±10. Monod, O., 1993. Pre-Eocene tectonics in the Zacatecas Area Ð an imbrication of Triassic and Cretaceous units. First Circum-Paci®c and Circum-Atlantic Terrand Conference, Guidebook of Field Trip A, Instituto de GeologõÂa, Universidad Nacional AutoÂnoma de MeÂxico, pp. 67±73. Monod, O., Calvet, P., 1992. Structural and stratigraphic re-interpretation of the Triassic units near Zacatecas (Zac.), Central Mexico: evidence of a Nappe Pile. Zbl. Geol. PalaÈont. I, H. 6, 1533±1544.

Mutti, E., Ricci-Lucchi, F., 1972. Le torbiditi dell `apennino settentrionale: introduzione all `analisi di facies. Memoir Society Geology Italy 11, 161±199 (1978, English translation in International Geology Review, 20, 125±166.). Ortega-Gutierrez, F., 1984. Relaciones Estratigra®cas del Basament PreOxfordiano de la RegioÂn de Caopas-Rodeo, Zacatecas y su Signi®cado TectoÂnico. Paper presented at the 7th National Conventio, Geological Society of Mexico, Mexico City, October 8±11, p. 56.  rea de Laguna Seca Ð Palazuelos, C.R., 1970. ExploracioÂn GeoloÂgica del A Real de Catorce, S.L.P., (Hojas MeÂxico J-8, J-9). Informe GeoloÂgico No. 534 ZN, PetroÂleos Mexicanos. Pantoja-Alor, J., 1963. Resumen de la GeologõÂa de la Hoja San Pedro del Gallo, Estado de Durango, Hoja San Pedro del Gallo 13R-k(3): Carta GeoloÂgica de MeÂxico, Serie 1:100,000, Instituto de GeologõÂa, Universidad Nacional AutoÂnoma de MeÂxico. Pantoja-Alor, J., 1972. Datos GeoloÂgicos-Estratigra®cos de la FormacioÂn Nazas (abs.). Memoria, II ConvencioÂn Nacional, Siciedad GeoloÂgica Mexicana. pp. 25±31 and 194±196. PeÂrez-MartõÂnez, J.J., Mapes-VaÂsquez, E., Pesquera-VelaÂzquez, R., 1961. Bosquejo GeoloÂgico del Distrito Minero de Zacatecas. Consejo de Recursos Naturales No Renovables, (MeÂxico), Boletin No. 52, 38pp. Pindell, J., 1985. Alleghenian Reconstruction and Subsequent Evolution of the Gulf of Mexico, Bahamas, and Proto-Caribbean. Tectonics 4 (1), 1±39. Potter, E.P., 1978. Signi®cance and origin of big rivers. Journal of Geology 86, 13±33. Ranson, W., Fernandez, L., Simmons Jr., W., Enciso de la Vega, S., 1982. Petrology of the Metamorphic Rocks of Zacatecas. Zac. Mexico. Sociedad GeoloÂgica Mexicana BoletõÂn 43 (1), 37±59. Ricci-Lucchi, 1975. Depositional Cycles in Two Turbidite Formations of Northern Apennines (Italy). Journal of Sedimentary Petrology 45, 3±43. Rogers, C.L., De Cserna, Z., Vloten, V., 1963. Plutonic Rocks of Northern Zacatecas and Adjacent Areas. Mexico. United States Geological Survey Professional Paper 475-C, C7±C10. Sedlock, R.L., Ortega-GutieÂrrez, F., Speed, R.S., 1993. Tectonostratigraphic Terranes and Tectonic Evolution of Mexico. Geological Society of America (Special Paper No. 278), 153. Servais, M., Cuevas, P.E., Monod, O., 1986. Une Section de Sinoloa aÁ San Luis PotosõÂ: Nouvelle Approche de l'eÂvolution du Mixique Nord-Occidental. Bulletin de la SocieÂte GeoÂlogique de France, Serie 8 6 (2), 1033±1047. Silva-Romo, G., 1994. Estudio de la EstratigrafõÂa y Estructuras TectoÂnicas de la Sierra de Salinas. Edos. De S.L.P. y Zac., (MS Thesis). Facultad de Ciencias, Universidad Nacional AutoÂnoma de MeÂxico, 144pp. Silva-Romo, G., Mendoza-Rosales, C., Arellano-Gil, J., 1994. ReinterpretacioÂn de la formacioÂn taray en el aÂrea de pico de teyra, edo. De Zacatecas (abs.) XII Congreso de la Sociedad GeoloÂgica Mexicana, Mexico, Abstracts, 172pp. Silver, L.T., Anderson, T.H., 1974. Possible left-lateral Early to Middle Mesozoic disruption of the Southwestern North America Craton Margin. Geological Society of America Abstracts with Programs, vol. 6, 955pp. TristaÂn-GonzaÂlez, M, Torres-HernaÂndez, R., 1992. CartografõÂa GeoloÂgica 1:50,000 de la Hoja Charcas, Estado de San Luis PotosõÂ, MeÂxico. Universidad AutoÂnoma de San Luis PotosõÂ, Instituto de GeologõÂa y Metalurgia, Folleto TeÂcnico 115, 94pp. Winker, Ch., Buf¯er, R., 1988. Paleographic evolution of early deep-water Gulf of Mexico and margins, Jurassic to Middle Cretaceous (Comancgaean). American Association of Petroleum Geologists Bulletin 72 (3), 318±346. ZaÂrate, V.P.F., 1982. GeologõÂa y anaÂlisis metalogeneÂtico de la sierra de catorce. S.L.P. Sociedad GeoloÂgica Mexicana BoletõÂn 43, 1±22.