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 GEOCIENCIAS HACIA LA COMUNIDAD; 2018

Universidad de Concepción

The late Paleozoic ice age (LPIA) had a significant impact on the development of Gondwanan biotas. In particular, the invertebrate faunal assemblages associated to the Paleo-Pacific transgressions that affected the west central Argentina during the Early Carboniferous -Early Permian, are long well known. Researches of the last decade mostly focused on brachiopods and bivalves, the main components of these assemblages, suggest that three faunas particularly relevant by their biostratigraphic importance in regional correlations can be distinguished. The oldest, 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 & Cisterna, 2010), and it is characterized by the predominance of bivalves over brachiopods. However, studies realized in the last years in Las Minitas locality (Bolsón de Jagüel Area, La Rioja, province), allowed to identify new taxa that suggest an increase of the diversity (particularly in the brachiopods) for the Early Carboniferous faunas of the Precordillera (Sterren et al., 2013).The second faunal assemblage appears 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, and it was widely studied because its biostratigraphical, paleogeographical and paleocological implications (Cisterna &Sterren, 2010). Nevertheless, more recent studies have allowed us to recognize a coeval early postglacial fauna, the Aseptella-Tuberculatella/Rhipidomella Micraphelia fauna (AT/RM), which would have inhabited a relatively isolated or restricted part of the basin. In this sense, the coastal configuration 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 Fauna (Cisterna & 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, and include the Tivertonia-Streptorhynchus (TS) Fauna, widely extended in the Río Blanco and western Paganzo basins (Fig. 1), and the Costatumulus Fauna, restricted to the south of the Calingasta-Uspallata Basin. A climatic amelioration suggested to western margin of Gondwana from the Late Carboniferous to the Early Permian, as well as, the volcanic activity 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 & Cisterna, 2010). The TS Fauna that integrates the Tivertonia jachalensis-Streptorhynchus inaequiornatus Zone, 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). 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 Argentinean basins.