Late Mississippian-Cisuralian Brachiopods from Patagonia and western Argentina: a tool for higher paleolatitud correlations.

TABOADA, ARTURO CÉSAR

Melbourne

Workshop; The Permian of Gondwana: Stratigraphy, Sedimentology and Paleaeontology –An International symposium and field workshop on the Permian of Gondwana, as exemplified in the southern Sydney Basin, South-east Australia-; 2008

Deakin University

Upper Palaeozoic faunas of Argentina include brachiopods, mollusks, bryozoans, cnidarians, echinoderms, ostracods and scarce trilobites, warm-water fossils such as foraminifers and conodonts being absent. The oldest Carboniferous marine fauna occurs in western Argentina (Uspallata-Iglesia Basin), carrying the temperate late Tournaisian-early Visean brachiopods Yagonia and Azurduya. Afterwards since the late Visean-Serpukhovian, the cold Gondwanic Levipustula fauna and its local Rugosochonetes-Bulahdelia relative association appear. By this time, the basin was located at a subpolar setting. Other significant brachiopods associated with these cold-water brachiopod faunas are Absenticosta, Aseptella, Costuloplica, Spiriferellina, Kitakamithyris, Torynifer, Syringothyris and Septosyringothyris. Epeirogenic movements affected the region in the middle Pennsylvanian, causing the intrabasinal expansion of depositional areas and a regional and mostly erosive disconformity between the glacimarine deposits bearing the Levipustula fauna and the overlying short-lived transgressive marine sequence with the late Balakhonia-Maemia fauna with the brachiopods Neochonetes, Reticularia and Leiorhynchus, among others. The Balakhonia-Maemia fauna was linked to a regional climatic change, most likely related to volcanism. This fauna postdates the extensive glacial conditions faunistically characterised by the Levipustula fauna, hence indicating the onset of ameliorated paleoclimatic conditions in western Argentina. Following a mainly regressive depositional interval during the Late Carboniferous, an extensive marine incursion, one of the largest Late Paleozoic marine transgression in Argentina, occurred during the middle to late Asselian in western Argentina, marked by a diverse brachiopod fauna named the Tivertonia-Streptorhynchus assemblage. This fauna also contains such other brachiopod genera as Kochiproductus, Coolkilella, Coronalosia, Tupelosia, Guadalupelosia, Pericospira, Saltospirifer, Septosyringothyris and Neochonetes. The marine depositional condition was disrupted by another short-lived regressive interval before it returned in the late Asselian-Sakmarian. This last phase of marine deposition was geographically restricted to an embayment located in the south border of the Uspallata-Iglesia Basin. Associated with this final stage of marine incursion was the cool-water Costatumulus fauna which additionally also includes Coolkilella, Attenuatella, Tivertonia, Streptorhynchus and Septosyringothyris. The Costatumulus biozone enclosed within largely glacimarine sediments thus indicates a brief interruption of the temperate paleoclimatic conditions established since the middle Pennsylvanian. In Patagonia (Tepuel-Genoa Basin), the older marine fossil assemblages of a continuous and uninterrupted thick sequence of more than 5000 m thick are the Lanipustula and Tuberculatella faunas. The first, regarded of Bashkirian-Moscovian age, also has yielded Kitakamithyris, Spiriferellina and also probably Aseptella and Alispirifer. The second, characterised with Amosia and Verchojania, ranges from a possible Moscovian to early Asselian age.  Both Lanipustula and Tuberculatella faunal associations are intercalated with diamictite levels deposited mainly by direct action of glacier ice (as glacial pavements indicate). This scenario thus supports the position of Patagonia near the south paleopole covered by ice sheet (peripherical satellite ice sheet of the main polar Gondwana ice cap), at least during the early Late Carboniferous and earliest Asselian. Apparently, an interglacial episode would be linked with the first ocurrence of the Tuberculatella fauna. Latest records of Verchojania and Tuberculatella are little above the start of Asselian deglaciation. The overlying levels contain a relatively more diversified fauna with Rhynchopora, Costatumulus, Cimmeriella, Neochonetes, Sulciplica, Coolkilella, Jakutoproductus, Brachythyrinella and Spirelytha, among others. This last stratigraphic interval appears to represent a postglacial sea level rise that is probably synchronous with the major global sea level rise across Gondwana and beyond which occurred during late Asselian-Sakmarian times. First levels with the Glossopteris flora are intercalated with those bearing Cimmeriella. The Cimmeriella faunal association from Patagonia may have coexisted partially with the cold Gondwanic Eurydesma and may be contemporaneous with the cool Cimmeriella-bearing brachiopod faunas in Western Australia and Southeast Asia, as well as with the Jakutoproductus assemblages of the Boreal Realm. All of these brachiopod faunas appeared during the initial global climatic amelioration of the early Permian in Gondwana and thus could probably serve as a tool for the correlation of lower Permian marine sequences from the high to middle paleolatidudinal settings of both hemispheres. Finally, a terminal short-lived glacimarine succession (probably equivalent to those enclosed by the Costatumulus biozone in western Argentina), occurs in Patagonia intercalated between the underlying faunas with Cimmeriella and the overlying late Cisuralian assemblages with Costatumulus, Attenuatella, Trigonotreta, Kochiproductus, Jakutoproductus, Piatnitzkya, Rhynchopora, etc. The Upper Cisuralian deposits are not associated with direct evidences of glacial conditions (except scarce hollow moulds of glendonite).