Hirnantian?-Early Silurian brachiopods from the Sierra Grande Formation (North Patagonian Massif, Río Negro Province, Argentina)

SICCARDI, ARÓN; URIZ, NORBERTO JAVIER; RUSTÁN, JUAN JOSÉ; CINGOLANI, CARLOS ALBERTO

Mendoza

Congreso; 4th International Palaeontologial Congress The history of life: A view from the Southern Hemisphere; 2014

The Sierra Grande Formation is composed of mature quartz-rich sandstones interlayered with shales, subordinated wackes, conglomerates, and rich oolithic ironstone horizons.The unit has been interpreted as deposited in an open marine shelf dominated by processes of waves and storms. The majority of the outcrops are located in the eastern margin of the North Patagonian Massif, to the southeast of the Río Negro Province, Argentina. Containing the only one (not re-sedimented) Early Paleozoic fossiliferous record from Patagonia, the Sierra Grande Formation provides relevant insights for the understanding of the palaeogeographic context of Western Gondwana during Paleozoic times. In addition, its paleontological content allows an adjustment of the regional stratigraphic frame among related basins; particularly regarding equivalent units from South America and South Africa. Fossiliferous localities of the Sierra Grande Formation include Loma de los Fósiles and Loma de los Guanacos. This contribution deals with new information on the brachiopods of the first locality, which had been previously considered as old as Wenlockian, based on the tentative presence of Clarkeia antisiensis. New fossils come from a dark grey-bluish mudstone bed in Loma de los Fósiles, yet former collections housed in the Museo de La Plata have also been reviewed. The recognized brachiopod association is composed by Eostropheodonta aff. chilcaensis parvula, heterorthellasp., resselerids (Dedzetina? and Resserella sp.), rhyncotrematoids (cf. Rostricellula, Rhynchotrema), Dalmanella sp. and leptaenids. After the revision of the previously reported specimens, the presence of Clarkeia could not be confirmed. These elements are accompanied by trilobites such as Eoleonaspis sp., orthoconic nautiloids, conularids, eotomarids and holopeids gastropods, bivalves, crinoids ossicles, hyolitids and a rugose coral. Preliminary, the fauna seems to be closer to the Heterorthella precordillerana assemblage (middle Rhuddanian-early Aeronian) known in the basal levels of the La Chilca Formation (Precordillera of western Argentina), rather than the Hirnantian brachiopod assemblages of the Cedarberg Formation in South Africa. However, the overall faunal composition shows a typical assemblage from the Hirnantian-early Silurian boundary