Fusulínidos del Paleozoico tardío de Sonora, México: importancia para la interpretación de los ambientes de depósito, la biogeografía y la paleotectónica
Calvin H. Stevens1*, Forrest G. Poole2, and Ricardo Amaya-Martínez3
1 Geology Department, San José State University, San José, California 95192-0102, USA. *calvin.stevens@sjsu.edu.
2 U.S. Geological Survey, Box 25046, Federal Center, Denver, Colorado 80225-0046, USA.
3 Departamento de Geología, Universidad de Sonora, Hermosillo, Sonora 83000, México.
]]> Manuscript received: January 10, 2014
ABSTRACT
Three sets of fusulinid faunas in Sonora, Mexico, discussed herein, record different depositional and paleotectonic settings along the southwestern margin of Laurentia (North America) during Pennsylvanian and Permian time. The settings include: offshelf continental rise and ocean basin (Rancho Nuevo Formation in the Sonora allochthon), shallow continental shelf (La Cueva Limestone), and foredeep basin on the continental shelf (Mina México Formation). Our data represent 41 fusulinid collections from 23 localities with each locality providing one to eight collections.
Reworked fusulinids in the Middle and Upper Pennsylvanian part of the Rancho Nuevo Formation range in age from Desmoinesian into Virgilian (Moscovian-Gzhelian). Indigenous Permian fusulinids in the La Cueva Limestone range in age from middle or late Wolfcampian to middle Leonardian (late Sakmarian-late Artinskian), and reworked Permian fusulinids in the Mina México Formation range in age from early to middle Leonardian (middle-late Artinskian). Conodonts of Guadalupian age occur in some turbidites in the Mina México Formation, indicating the youngest foredeep deposit is at least Middle Permian in age. Our fusulinid collections indicate a hiatus of at least 10 m.y. between the youngest Pennsylvanian (Virgilian) rocks in the Sonora allochthon and the oldest Permian (middle Wolfcampian) rocks in the region.
Most fusulinid faunas in Sonora show affinities to those of West Texas, New Mexico, and Arizona; however, some genera and species are similar to those in southeastern California. As most species are similar to those east of the southwest-trending Transcontinental arch in New Mexico and Arizona, this arch may have formed a barrier preventing large-scale migration and mixing of faunas between the southern shelf of Laurentia in northwestern Mexico and the western shelf in the southwestern United States.
The Sonora allochthon, consisting of pre-Permian (Lower Ordovician to Upper Pennsylvanian) deep-water continental-rise and ocean-basin rocks, was thrust northward 50-200 km over Permian and older shallow-water carbonate-shelf rocks and Permian deep-water foredeep rocks of southern Laurentia. As Triassic rocks unconformably overlie the Sonora allochthon, we conclude that terminal movement of the allochthon was in Late Permian time.
Key words: fusulinids, sedimentation, stratigraphy, biogeography, tectonic framework, Pennsylvanian, Permian, Sonora, Mexico
]]>RESUMEN
Tres grupos de faunas de fusulínidos discutidos en este artículo documentan diferentes ambientes de depósito y tectónicos a lo largo de la margen suroeste de Laurentia (Norte América) durante el Pérmico y Pensilvánico. Estos ambientes incluyen: talud continental y cuenca oceánica (Formación Rancho Nuevo en el alóctono de Sonora); plataforma de agua somera continental (Caliza La Cueva) y cuenca del tipo "foredeep" sobre la plataforma continental (Formación Mina México). La información obtenida representa 41 colecciones de fusulínidos provenientes de 23 localidades, en las cuales se obtuvieron de una a ocho colecciones por localidad.
Los fusulínidos retrabajados contenidos dentro del Pensilvánico Medio y Superior de la Formación Rancho Nuevo varían en edad desde el Desmoinesiano hasta el Virgiliano (Moscoviano-Gzheliano). Los fusulínidos in situ del Pérmico en la Caliza La Cueva de la plataforma varían en edad desde al menos el Wolfcampiano medio o tardío hasta el Leonardiano medio (Sakmariano tardío-Artinskiano tardío) y los fusulínidos retrabajados dentro de la Formación Mina México indican rangos de edad entre el Leonardiano temprano a medio (Artinskiano medio-tardío). Conodontos de edad Guadalupiano contenidos en estratos de turbiditas en la Formación Mina México indican que la edad de depósito más joven dentro de la cuenca "foredeep" es al menos Pérmico Medio. Las colecciones de fusulínidos indican un hiatus de al menos 10 Ma entre las rocas más jóvenes del Pensilvánico (Virgiliano) en el alóctono de Sonora y las rocas más antiguas del Pérmico (Wolcampiano medio) en la región.
La mayoría de las faunas fusulínidos en Sonora muestran afinidades con aquellas del oeste de Texas, Nuevo México y Arizona; sin embargo, algunos géneros y especies son similares a aquellos de la porción sureste de California. Debido a que estas especies son similares a las reportadas hacia el este del arco Transcontinental, que muestra una orientación suroeste en Nuevo México y Arizona, se interpreta que este arco puede haber formado una barrera que impidió la migración a gran escala y la mezcla de faunas entre el extremo sur de la plataforma de Laurentia en el noroeste de México y el extremo oeste de esta misma plataforma en el suroeste de Estados Unidos.
El alóctono de Sonora, constituido por rocas anteriores al Pérmico (Ordovícico Inferior a Pensilvánico Superior)y depositadas en ambientes de talud continental de aguas profundas y cuenca oceánica, fue transportado tectónicamente hacia el norte una distancia estimada entre 50 y 200 km, cabalgando a secuencias de la plataforma carbonatada de aguas someras del Pérmico y más antiguas; así como también a las rocas pérmicas de aguas profundas, depositadas en las cuencas "foredeep" del sur de Laurentia. Debido a que rocas triásicas sobreyacen discordantemente al alóctono de Sonora, se concluye que el movimiento final del alóctono fue en el Pérmico Tardío.
Palabras clave: fósiles, fusulínidos, estratigrafía, marco tectónico, Pensilvánico, Pérmico, Sonora, México.
INTRODUCTION
Pennsylvanian and Permian fusulinids from the southwestern margin of cratonal North America in the State of Sonora, Mexico, occur in three principal depositional settings—continental rise-ocean basin (Sonora allochthon), continental shelf (Laurentian carbonate shelf), and foredeep (Mina México basin). Figure 1 shows the present distribution of the three depositional and tectonic settings. Figure 2 is a schematic north-south cross section showing pre-Mesozoic stratigraphic and structural relationships of rock units in the study area. The fusulinids occur in strata of Pennsylvanian and Permian ages (Figure 3). Of the fusulinid faunas discussed here, one set occupied shallow-marine environments on the continental shelf; the two other sets consist of displaced and reworked specimens, one deposited in a deep-marine foredeep developed on the continental shelf, and the other in deep-marine environments in continental-rise and ocean-basin settings along the southern margin of Laurentia.
]]> Fusulinid samples collected by Poole and co-workers in the 1980s and 1990s, and by Poole and Amaya-Martínez in the early 2000s were taken during geologic mapping and stratigraphic studies. Raymond C. Douglass (U.S. Geological Survey) identified the fusulinids collected in the 1980s and Stevens identified the fusulinids collected after 1990. For this report, Stevens re-examined the collections studied by Douglass. Fusulinid faunas reported herein are listed in Tables 1 and 2 and illustrated in Figures 4-8 (5, 6, 7).The stratigraphic position of the fusulinid collections from the shelfal La Cueva Limestone can be determined, but collections from the Sonora allochthon and most of the foredeep unit (Mina México Formation) cannot because of severe disruption, and limited exposures and structural complications, respectively. Investigations of the fusulinid faunas provide significant information concerning biogeographic affinities and depositional and paleotectonic settings on the southwest margin of Laurentia (Figures 1, 2, and 3). Distribution of our fusulinid samples shown on Figure 1 reflects the scattered exposures of upper Paleozoic strata in Sonora.
Fusulinid faunas and their depositional environments are discussed in ascending stratigraphic age, that is, from allochthon to shelf, to foredeep.
LATE PALEOZOIC FUSULINID FAUNAS IN CENTRAL SONORA
Previous work
Fusulinids have been known in Sonora since the 1930s when King (1939) and Dunbar (1939a) reported species generally resembling those in West Texas. In more recent times, Pérez-Ramos (1992) studied Permian fusulinids near Hermosillo and Arivechi in central and east-central Sonora, and compared them to species in southeastern Arizona. Pérez-Ramos and Nestell (2002) described Permian fusulinids near Cobachi in central Sonora, and Gomez-Espinosa et al. (2008) and Buitrón-Sánchez et al. (2012) described Pennsylvanian fusulinids at Cerro El Tule in northernmost Sonora and Sierra Agua Verde in east-central Sonora. The Cerro El Tule section contains fusulinids representing all stages of the Pennsylvanian.
Fusulinid faunas reported herein are listed in Tables 1 and 2 and illustrated in Figures 4-8 (5, 6, 7).
Pennsylvanian fusulinids in Sonora allochthon
The eugeoclinal deep-marine Ordovician through Pennsylvanian rocks in the Sonora allochthon (Figure 3), deposited in a deep-water environment along the southern margin of Laurentia, consist of turbiditic mudstone, siltstone, sandstone, conglomerate, and limestone, containing Nereites (e.g., Cosmorhaphe and Scalarituba) and Zoophycos ichnofacies fossils, which indicate deposition in bathyal water depths (200-2000 m). The Pennsylvanian fusulinid faunas within the Sonora allochthon occur within lime-grainstone turbidites in the Rancho Nuevo Formation.
]]> Permian fusulinids in Laurentian carbonate shelfPermian fusulinid faunas within the autochthonous Laurentian carbonate shelf (La Cueva Limestone) occur within shallow-marine lime mudstones to packstones that commonly contain pelmatozoans, brachiopods, and other megafossils. Locally, peritidal limestone units occur in shallowing-upward cycles beginning with fossiliferous shallow subtidal limestone and ending with supratidal algal laminites.
Permian fusulinids in Mina México foredeep
Permian fusulinid faunas in the autochthonous Mina México Formation occur in synorogenic lime-grainstone turbidites overlying the carbonate shelf (Figures 2 and 3). The Mina México Formation intertongues with and has a gradational contact with the subjacent carbonate shelf La Cueva Limestone (Figure 3), and contains fusulinids transported from the shelf.
This deep-marine turbiditic sequence contains abundant Nereites (e.g., Cosmorhaphe, Scalarituba, and Lophoctenium) and Zoophycos ichnofacies fossils.
FUSULINID IDENTIFICATIONS AND AGES
Fusulinid identifications and ages are listed in Tables 1 and 2, and the geographic locations of samples are shown in Figure 1. Stratigraphic positions of formations from which samples were collected are shown in Figure 3. North American Series names for both the Pennsylvanian and Permian are employed herein. The depositional sequence nomenclature of Ross and Ross (2003) for the Lower Permian Series in the Glass Mountains of West Texas also is used. These sequences include lower Wolfcampian Nealian Substage (NR-1 through NR-16); upper Wolfcampian Lenoxian Substage (LH-1 through LH-3); lower Leonardian lower Hessian Substage (H-1 through H-4); middle Leonardian upper Hessian Substage (H-5 through H-7), and upper Leonardian Cathedralian Substage (C-1 and C-2).
Samples from the Rancho Nuevo Formation within the Sonora allochthon range in age from Desmoinesian into the Virgilian. Most samples are Missourian in age. Species represented in these samples previously have been reported primarily from the southern United States, especially the cratonal platform of southern New Mexico.
Fusulinids in the autochthonous Permian Laurentian carbonate shelf range in age from middle Wolfcampian through Leonardian, although most of the samples studied are Leonardian. Three carbonate-shelf samples contain Eoparafusulina linearis, a late Wolfcampian species. About one-half of the species in the carbonate-shelf rocks also occur in Texas, with most other forms described previously from northern Mexico or southeastern California.
]]> Samples from the foredeep fill (Mina México Formation) contain fusulinids of Leonardian age. This unit probably is as young as Middle Permian (Guadalupian), an age based on conodonts found in some lime-grainstone turbidite beds in the foredeep (C.A. Sandberg and F.G. Poole, work in progress). In the foredeep, an equal representation of Texas and California species is present, with slightly fewer species described previously from northern Mexico and Nevada.The lack of typically Wolfcampian genera, especially Pseudoschwagerina and Stewartina, suggest that strata of this age, except for late Wolfcampian strata, as indicated by the presence of Eoparafusulina linearis, are rare or the appropriate facies are lacking in this region. Initial studies of redeposited conodonts in some lime-grainstone turbidites in the Rancho Nuevo Formation may be as young as late Virgilian in age (C.A. Sandberg and F.G. Poole, work in progress). The lack of early Wolfcampian samples may reflect incomplete sampling of the complexly deformed calciclastic turbidites in the region. However, numerous unconformities have been documented within the carbonate-shelf sequence in many Paleozoic sections in Sonora (Figures 2 and 3), so the apparent hiatus may represent a major stratigraphic gap within the upper shelf section.
DEPOSITIONAL ENVIRONMENTS
Continental rise - ocean basin
The Rancho Nuevo Formation consists of a deep-marine Upper Mississippian and Pennsylvanian turbiditic sequence within the upper part of the Sonora allochthon, which was emplaced during late Paleozoic time along the southern margin of Laurentia (Figures 2 and 3). It includes many complexly deformed beds of argillite, quartzite, conglomerate, limestone, and barite deposited in a deep-water continental rise-ocean basin setting. Trace fossils indicating deposition in bathyal water depths are common throughout the allochthon. Several interbedded lime grainstone turbidites were sampled for fusulinids and conodonts. The lime grainstones contain transported fossils representing all epochs of the Pennsylvanian Period. Individual turbidites contain reworked fusulinids derived mainly from exposed limestones on the carbonate shelf to the north, and the reworked conodonts in many turbidites were derived from many different-aged Paleozoic rocks also exposed on the shelf to the north. Therefore, it is likely that the fusulinids do not always represent the depositional age of the turbidite bed sampled, but rather an older source.
Carbonate shelf
The name La Cueva Limestone is used herein for the widespread Early Permian limestone in central and east-central Sonora that forms the highest unit in the Laurentian carbonate shelf (Figure 3). This name was first applied by Hewett (1978) and Schmidt (1978) to the upper member of their El Tigre Formation in east-central Sonora; however, their El Tigre Formation includes strata of the thick Cerro Santo Domingo Group of Poole et al. (2005, tab. 1, locality 40), which ranges in age from Ordovician to Permian. Poole et al. (2005) proposed abandoning the name El Tigre and retaining the name La Cueva Limestone, but restricting it to the Lower Permian fossiliferous shallow-water limestone at the top of the carbonate shelf.
The La Cueva Limestone consists mainly of shallow-marine shelfal lime mudstones to packstones containing fusulinids and other megafossils. Locally the La Cueva consists of shallowing-upward cycles composed of units deposited sequentially in subtidal, intertidal, and supratidal environments. For example, outcrops of La Cueva Limestone at the south end of Cerro Las Rastras expose a series of shallowing-upward cycles composed of lower units of shallow-subtidal limestone (0.2-8 m thick), containing fusulinids and other fossils; middle units of intertidal limestone (90± cm thick); and upper units of supratidal gypsiferous limestone (15± cm thick). Calcite has replaced gypsum crystals in the original gypsiferous limestone. The interval of cycles studied at Cerro Las Rastras is estimated to be 150-170 m below the top of the formation.
Foredeep on carbonate shelf
]]> The Mina México foredeep developed on the carbonate shelf of Laurentia as a result of crustal f1exing and isostatic loading associated with the collision of the Gondwanan and Laurentian continents (Figures 1-3 (2) and Poole et al., 2005). It was filled with turbiditic siltstones, fine sandstones and sparse turbiditic lime grainstones containing transported fusulinids, pelmatozoans and other megafossil fragments. The foredeep-fill rocks in east-central Sonora were informally named Mina México Formation by Hewett (1978) and Schmidt (1978). The name Mina México Formation is now an accepted formal name for sediments filling the Permian foredeep. The predominately siliciclastic turbidite sequence contains abundant deep-marine trace fossils.The contact between the La Cueva and Mina México formations is gradational and the two units intertongue through a transitional interval as thick as 50 m (Figure 3; Poole et al., 2005). Our fusulinid data indicate that intertonguing between the two formations occurred during a 2-3 m.y. period ranging from early to middle Leonardian time.
Dating the time of deposition of the bioclastic lime-grainstone turbidites of the Mina México Formation is difficult because of the unknown source of the sediment transported into the foredeep. In some cases, the age of the transported fusulinids and other fossils in the turbidites may ref1ect the age of contemporaneous deposits on the shelf, but it is likely that many of the fossils came from a variety of source rocks as indicated by a mixture of different-age fossils, especially conodonts.
U-Pb isotopic dating of detrital zircons (Mesoarchean-Neoproterozoic and subordinate Cambrian-Pennsylvanian) from turbiditic sandstones filling the Mina México foredeep in central Sonora suggests that most of the detritus was derived from Laurentia, but some detritus clearly came from the evolving Sonora allochthon and/or peri-Gondwana terranes to the south (Poole et al., 2008; Amaya-Martínez et al., 2010).
PALEOTECTONIC SETTING
Previous work (summarized in Stewart, 2005, and Poole et al., 2005) showed that Sonora comprised the southern part of Laurentia. The remainder of Central America south of Sonora, however, is composed of allochthonous terranes, which according to Vachard et al. (2000a) were located somewhere between Laurentia and Gondwana in the Pennsylvanian with the exception of the Chortis block, which was still drifting towards Pangaea in the Early Permian. By the end of the Middle Permian, most of the Mexican terranes were amalgamated to one another and /or accreted to Laurentia (Vachard et al., 2004). These Central American terranes probably represent fragments of northwest Gondwana.
The above scenario seems to be validated by various studies of Mexican fusulinids. The similarity of most of the Pennsylvanian and Permian fusulinid faunas on the Laurentian carbonate shelf in Sonora to those of the cratonal platform to the north (i.e., in Texas, New Mexico, and Arizona), as indicated by the work of Pérez-Ramos (1992), Pérez-Ramos and Nestell (2002), and Buitrón-Sánchez et al. (2012), demonstrates a close geographic proximity. The fusulinids in the Mina México Formation are also similar to those on the Laurentian carbonate shelf. Therefore, it seems unlikely that many, if any, fusulinids were derived from the Central American terranes. However, it must be recognized that fusulinid faunas similar to those in Sonora occur elsewhere in Central America, for instance in the Missourian through middle Leonardian sequence in the Mixteco terrane in southwestern Mexico (Vachard et al., 2000b). Farther south in Guatemala in rocks associated with the Chortis terrane, the Leonardian-Roadian fusulinids are similar to those in West Texas (Vachard et al., 1997). Also from the earliest Pennsylvanian (Bashkirian) to Early Permian late Wolfcampian faunal similarities exist as far south as Lake Titicaca (Bolivia-Peru border) indicating communion between these areas. Similar Pennsylvanian and Permian faunas in Central America and Sonora indicate proximity of Gondwana and Laurentia in late Paleozoic time.
CONCLUSIONS
]]> Although most fusulinid faunas along the southern margin of Laurentia show affinities to those of West Texas, New Mexico, and Arizona, some genera and species are similar to fusulinids in southeastern California (e.g., Magginetti et al., 1988). Stevens (2010) showed that some distinctive late Wolfcampian and Leonardian fusulinids and corals were able to disperse or migrate presumably from NevadaCalifornia to Sonora and West Texas. Even so, most species in Sonora are similar to those east of the southwest-trending Transcontinental arch in New Mexico and Arizona (position of arch shown on fig. 1 in Poole et al., 2005), suggesting the arch may have formed a barrier preventing large-scale migration and mixing of faunas between the southern shelf of Laurentia in Sonora and the western shelf in the southwestern United States.The Permian Sonora allochthon (Figures 1 and 2), consisting of Ordovician to Pennsylvanian deep-water continental-rise and ocean-basin rocks (Figure 3), was thrust 50-200 km onto shallow-water carbonate-shelf rocks (La Cueva Limestone) and overlying Permian foredeep f1ysch (Mina México Formation) of southern Laurentia (Poole et al., 2005). No evidence of major strike-slip offset of Paleozoic rocks has been observed in the study area, as advocated by proponents of several hypothetical megashears in Sonora (see papers in Anderson et al., 2005, for other interpretations).
Our fusulinid collections indicate a hiatus of at least 10 m.y. (see time scale of Walker and Geissman, 2009) between the youngest Pennsylvanian (Virgilian) rocks in the Sonora allochthon and the oldest Permian (middle Wolfcampian) rocks in the La Cueva Limestone. In addition, neither detrital fusulinids nor conodonts of early Wolfcampian age have been recognized in the lime-grainstone turbidites of the Mina México Formation. Thus far, the youngest transported fossils identified in Mina México turbidites are conodonts of Middle Permian (Guadalupian) age. The Sonora allochthon overrides part of the Mina México foredeep deposits, but as Triassic deposits depositionally overlie the Sonora allochthon, movement of the allochthon in Sonora probably ceased in the Late Permian. Therefore, the Sonora allochthon evidently was actively moving onto the Laurentian continental margin leaving no conspicuous depositional record. These data are interpreted as dating emplacement of the allochthon from Early to Middle Permian (a time period of 30-40 m.y.).
ACKNOWLEDGMENTS
The Geology Department of San José State University supported the laboratory work of Stevens, the U.S. Geological Survey supported the fieldwork of Poole, and the University of Sonora supported the fieldwork of Amaya-Martínez. Stevens identified the fusulinids and interpreted their biogeographic significance and Poole and Amaya-Martínez are responsible for interpreting their depositional and tectonic setting.
The Minas de Barita mapping project and regional stratigraphic studies in Sonora have greatly benefited from fusulinid identification and age determinations provided by Raymond C. Douglass (deceased) of the U.S. Geological Survey. We are grateful to Vladimir I. Davydov (Boise State University) and Charles H. Thorman (U.S. Geological Survey) for their careful reviews and suggestions, which significantly improved an earlier version of the manuscript. We also are very grateful to Carlos M. González-León (Instituto de Geología, UNAM) and Daniel Vachard (Université des Sciences et Technologies de Lille) for their very careful reviews of the final version of the manuscript. We thank Jeremy C. Havens and Emily M. Taylor (both U.S. Geological Survey) for computer preparation of the figures.
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