Late Paleozoic marine faunal assemblages from the central western Argentinian basins: an update based on the recent studies

CISTERNA, G.A.; STERREN A.F,

Concepción

Congreso; XV Congreso Geológico Chileno; 2018

The late Paleozoic ice age (LPIA) had a significant impact on the development of Gondwanan biotas. In particular, the invertebrate faunal assemblages related to the Paleo-Pacific transgressions that affected the central western Argentina during the Carboniferous -early Permian, are long well known. The marine sequences containing these assemblages appear disconnected, showing an irregular pattern along western Argentinian basins, as a consequence of a complex history of interaction among tectonism, sea-level changes and climatic conditions (Limarino et al., 2006). Abundant and diverse benthic invertebrates (i.e. brachiopods, bivalves, gastropods, nautiloids, crionids, briozoans, conularids, corals and ostracods), characterize the assemblages studied, usually associated with plant remains and palynomorphs. The taxonomic composition of these faunas has been extensively studied, as well as, their paleogeographic and bioestratigraphic implications (Césari et al., 2007 and references therein). However, the researches of the last decade mostly focused on brachiopods and bivalves, have allowed to evaluate the importance of these clades to understand the dynamics of ecosystems during the glacial mega-events and their biostratigraphic value in regional correlations (Cisterna and Sterren, 2010; 2016; Balseiro et al., 2014; Balseiro, 2016; Balseiro and Halpern, 2016; Cisterna et al., 2017).The marine faunal assemblages from central western Argentina have been particularly studied in three time intervals: Mississippian (Tournaisian-Visean), early Pensylvanian (late Serpukhovian-Bashkirian) and late Pensylvanian-early Cisuralian? (Moscovian-Asselian?). The oldest assemblage, that integrates the Michiganites (Protocanites) scalabrinii-Azurduya chavelensis Zone (Tournaisian-Visean), defined in Malimán area and usually recognized in the Río Blanco Basin (Fig. 1), is an impoverished fauna compared with the youngest faunal assemblages (Sterren and Cisterna, 2010), and it is characterized by the predominance of bivalves (Palaeoneilo subquadratum González, Malimania triangularis González, Phestia sp., Streblochondria sp., Cypricardinia sp., Edmondia sp., Myofossa sp., Malletioidea indet., Paralellodontidae indet., Aviculopectinidae indet. and Anomalodesmata indet.) over brachiopods (Azurduya chavelensis (Amos), Azurduya cingolanii Cisterna and Isaacson, Yagonia furquei Taboada and Shi and Septosyringothyridine new genus?). However, studies realized in the last years in Las Minitas locality (Bolsón de Jagüel Area, La Rioja, province) (Fig. 1), allowed to identify new taxa that suggest an increase of the diversity, particularly in the brachiopods (Anopliidae indet., Productidae indet., Spiriferellinidae indet., Syringothyridinae indet. and terebratulids that are currently being studied) for the early Carboniferous faunas of the Precordillera (Sterren et al., 2013). Two early Pensylvanian faunas appear typically associated to an important postglacial transgression related with the Carboniferous glacial event (Glacial Episode II, López-Gamundí, 1997). The Levipustula Fauna, included in the Levipustula levis Zone (late Serpukhovian-Bashkirian), well known in the Calingasta-Uspallata Basin (Fig. 1), characterizes this early postglacial faunal assemblage in several localities of the basin. Brachiopods (Levipustula levis Maxwell, Costuloplica leoncitensis (Harrington), Kitakamithyris booralensis Campbell, Kitakamithyris immensa Campbell, Torynifer tigrensis Taboada and Cisterna, Spiriferellina octoplicata (Sowerby), Septosyringothryis keideli (Harrington) and Beecheria sp.) and bivalves (Palaeolima retifera (Shumard), Aviculopecten barrealensis Reed, Streblochondria sanjuanensis Sterren, and Streblochondria stappenbecki Reed, Oriocrassatella andina González, Myofossa calingastensis González, Promytilus grandis González, Leptodesma (Leiopteria) aredesi González, among others), were widely studied because their biostratigraphical, paleogeographical and paleocological implications (Cisterna and Sterren, 2010). In particular, the paleocological analysis of the communities that integrate the postglacial Levipustula Fauna in its type section indicate an exposed open marine coast with a gently sloping shelf (Cisterna et al., 2017).More recent studies have allowed us to recognize a coeval early postglacial fauna, the Aseptella-Tuberculatella/Rhipidomella-Micraphelia fauna (AT/RM), characterized by the brachiopods Rhipidomella discreta Cisterna, Micraphelia indianae Simanauskas and Cisterna, Micraphelia? sp., Tuberculatella peregrina (Reed), Overtoniinae indet., Aseptella sp. aff. A. patriciae Simanauskas, Linoproductoidea indet., Beecheria patagonica Amos, athyrids indet., ?inarticulates?, and the bivalves Nuculanella camachoi González, Quadratonucula? sp., Nuculopsis? sp., which would have inhabited a relatively isolated or restricted part of the basin. In this sense, the coastal configuration (palaeofjord type) and abiotic factors, mainly the nutrient availability, related to glacial retreat dynamics, have been proposed to explain the important compositional and paleoecological differences between the contemporaneous AT/RM and Levipustula faunas (Cisterna and Sterren, 2016; Cisterna et al., 2017). The youngest faunal assemblages are associated to a transgressive marine event recorded in the three Late Paleozoic basins of Precordillera, during the late Pensylvanian- early Cisuralian and include the Tivertonia-Streptorhynchus (TS) Fauna, widely extended in the Río Blanco and western Paganzo basins, and the Costatumulus Fauna, restricted to the south of the Calingasta-Uspallata Basin (Fig. 1). A climatic amelioration suggested to this part of Gondwana for this time interval, as well as, the volcanic activity in the western margin and the action of relatively warmer marine currents, would have been important local factors that controlled the bivalve and brachiopod distribution in these faunas (Sterren and Cisterna, 2010). The TS Fauna that integrates the Tivertonia jachalensis-Streptorhynchus inaequiornatus Zone, composed of brachiopods such as Tivertonia jachalensis (Amos), Streptorhynchus inaequiornatus Leanza, Kochiproductus riojanus (Leanza), Costatumulus spp., Coronalosia argentinensis Archbold and Simanauskas, Tupelosia paganzoensis Archbold and Simanauskas, Crurithyris? sp., Pericospira pericoensis (Leanza), Pericospira riojanensis (Lech and Aceñolaza), Saltospirifer guevarii Cisterna and Archbold, Septosyringothyris (Precosyringothyris) jaguelensis Lech, Rhynchopora sp., Neochonetes pegnonensis Cisterna and Simanauskas, and the bivalves Nuculavus levatiformis (Walcott), Leptodesma veladerensis González, Ptychopteria (Ptychopteria) liagracielae (Leanza), Pterinopectinella ramacionii González, Heteropecten anteloi González, Septimyalina sp., Acanthopecten jaguelensis González, Schizodus arrondoi González, Vacunella aff. argentinensis (Harrington), Wilkingia riojana González, among the most characteristic. This fauna was considered in the definition of the Carboniferous-Permian boundary in the central west Argentina (Cisterna, 2010; Cisterna et al., 2011), on the base of its brachiopods and the palynological information associated. However, the radiometric data from several sections bearing this fauna suggest a Moscovian age (Gulbranson et al., 2010). The Costatumulus Fauna is mainly characterized by the brachiopods Costatumulus amosi Taboada, Coolkilella keideli Taboada, Tivertonia leanzai Taboada, Crurithyris sp. and the bivalves Oriocrassatella sanjuanensis González, Cypricardinia? sp., Streblochondria? sp., Sanguinolites sp. and Schizodus sp.Although the Costatumulus fauna that integrates the Costatumulus amosi Zone (early Cisuralian?) is considered to be younger than the TS Fauna (Taboada, 2014), the chronologic relationship between both is not clear to date. Particularly considering that a number of brachiopod genera that characterize the TS fauna have also been identified in association with the Costatumulus Fauna in the classical sections (Cisterna, 2010). In absence of most precise biostratigraphic markers such as conodonts or fusulinid foraminiferids, the brachiopods have been considered to be an important tool in the Late Paleozoic biostratigraphic schemes of the Precordillera. However, in some cases, this group lacks well-resolved indices to determine a precise age and it can be also strongly controlled by the paleoenvironmental conditions, particularly in glacial sequences where the composition and distribution of coeval faunas are related with the glacial dynamics. Hence, it is emphasized the need for radiometric data, as well as, of interdisciplinary studies combining the diagnostic mega/microflora and marine invertebrate paleontological evidences, to improve the biostratigraphical framework for the Late Paleozoic central western Argentinian basins