Carboniferous-Permian glacial-deglacial events and their effects on the brachiopod faunas from Argentina and Australia

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

Milán

Congreso; 8th International Brachiopod Congress; 2018

Università degli studi di Milano

Late Palaeozoic sequences from Argentina and Australia provide a complete record of the glacial-deglacial events and their effects on the marine benthic faunas can be evaluated in several key sections. In Argentina, two different scenarios for the postglacial transgressions are possible to identify according to the glacial episodes suggested in the early Late Carboniferous-Early Permian interval (López-Gamundí, 1997): to the west, the Late Carboniferous paleopacific transgression, related to the Glacial Episode II represented in the Andean basins, is characterized by a well diversified brachiopod fauna developed in arc-related and retroarc basins; to the east, the Late Carboniferous-Early Permian transgression associated with the Glacial Episode III, the most widespread, is documented in the Sauce Grande Basin. This transgression can be distinguished by fossil assemblages dominated by bivalves (Eurydesma Fauna) with a low-diversity brachiopod fauna, which inhabited epeiric seas generated by flooding over an intraplate basin.Carboniferous postglacial faunas of the late Serpukhovian-early Bashkirian age (Levipustula and Aseptella-Tuberculatella/Rhipidomella Micraphelia faunas) from western Argentina have been widely studied in their type sections of the Calingasta-Uspallata Basin (Cisterna & Sterren, 2016; Cisterna et al., 2017). In this basin, the postglacial mudstones with the marine invertebrate faunas can be present in fjords systems and in open shelf. The marine flooding over different coast configurations and the availability of sediments would have favoured the development of coeval faunas with differences in their taxonomic composition and paleoecological structure. Our study is now focused on brachiopods of the postglacial Eurydesma Fauna from East and West Gondwana, particularly those from the Sauce Grande basin and southern Sydney Basin in Australia.Fossil assemblages of the Eurydesma Fauna from the Sauce Grande basin are identified in the lower part of the Bonete Formation (Pillahuincó Group, Sierras Australes, Buenos Aires province), which records the transgression related with the retreat of the South African glaciers and would has been deposited at low slope platform (Andreis & Ribeiro, 2003). This fauna is composed of more than 80% of bivalves, many of them species of the key genus Eurydesma, accompanied by the brachiopods Tivertonia and Tomiopsis. This last genus has often been recognized in the Eurydesma Fauna of several Gondwana basins. In particular, the postglacial brachiopod faunas of the Wasp Head Formation, a shallow marine sandstone dominated unit in the southern Sydney basin, is characterized by the abundant occurrences of the spiriferids Tomiopsis and Trigonotreta, and bivalves are dominant with more than 60%. This succession encloses records of the first Permian glacial interval suggested for eastern Australia (Fielding et al., 2008), and its brachiopod assemblages are characterized by a group of diagnostic species whose diversity and stratigraphic occurrences in relation to sedimentary facies reflect the transition from an intra-glacial interval to post-glacial conditions (Cisterna & Shi, 2014). The predominance of the bivalves in relation to the brachiopods has also been observed in the postglacial faunas of the Itararé Group in the Parana Basin, Brazil (Neves et al., 2014; Taboada et al., 2016). Further analytic studies will be necessary to understand the paleoenvironmental conditions (i.e. substrate stability, turbidity, nutrient availability, variation in oxygen levels, poor circulation and salinity variations in the water column), directly related to glacial retreat dynamics that would have controlled the composition and distribution patterns of the postglacial Eurydesma fauna.