Latest Ordovician-earliest Silurian chitinozoans from Puna, western Gondwana

DE LA PUENTE, G. SUSANA; RUBINSTEIN, CLAUDIA V.; VACCARI, N. EMILIO; PARIS, FLORENTIN

Harrisonburg

Simposio; 12th International Symposium on the Ordovician System; 2015

James Madison University

Chitinozoans from Late Ordovician-early Silurian Upper Member of the Salar del Rincón Formation from Puna of northwestern Argentina are analyzed. This area belongs to the Central Andean Basin situated on the western Gondwanan margin during the early Palaeozoic. The Upper Member of the Salar del Rincón Formation could include the postglacial stage of the late Hirnantian glaciation, even when direct evidence of the glacially-related deposits are not recorded in this part of the basin. Although its contact with the Lower Member has been considered transitional, the marine to mixed Upper Memberdeposits could represent a transgressive event developed on conglomerate and cross-beded sandstones of the Lower Member deposits that involves an erosive contact. Strongly bioturbated grey to greenish-grey shales characterize the lower and middle part of the Upper Member of the unit. This part of the section (around 20 m thick) contains a chitinozoan association, mainly composed of Spinachitina verniersi Vandenbroucke in Vandenbroucke et al., 2009, Cyathochitina kuckersiana (Eisenack, 1934) and Cyathochitina latipatagium (Jenkins, 1969). Spinachitina oulebsiri Paris et al., 2000, Eisenackitina sp. cf. ripae Soufiane & Achab, 2000, Ancyrochitina corniculans Jenkins, 1969, Angochitina spp. and  Ancyrochitina spp. are present in a lower proportion. The chitinozoan association indicates a late Hirnantian age for these deposits. A different chitinozoan association is observed in the base of a lenticular coarse-grained sandstone bed (15 m thick) upward in the section(around 20 m above the preceding chitinozoan-bearing sample), which is deposited over the shaley to silty part of the member. Spinachitina is practically absent in this level. A single incomplete specimen bearing comparatively longer processes is observed. Cyatochitina caputoi group Da Costa, 1971, Belonechitina pseudarabiensis Butcher, 2009, Angochitina hansonica Soufiane and Achab, 2000, Ordochitina sp. cf. nevadensis Soufiane and Achab, 2000 and Plectochitina spp. are the main components. Cy. caputoi and B. pseudarabiensis indicate the earliest Rhuddanian age. The last chitinozoan-bearing sample is from a shaley level deposited above the coarse- grained sandstone bed. These specimens are poorly-preserved. Cy. kuckersiana group, Cy. caputoi group, O. sp. cf. nevadensis, A. hansonica?, Angochitina spp. and Ancyrochitina spp. are observed. The upper part of this section shows an increasing sediment grain size and finishes with a ferruginous and chitinozoan-barren level. All chitinozoan-bearing levels also contain acritarchs and cryptospores. The Salar del Rincón Formation is succeeded by Late Carboniferous deposits (Cerro Oscuro Formation) through an angular discordance. According to palynomorphs the Upper Member of the Salar del Rincón unit records latest Ordovician earliest Silurian deposits, which are usually absent in other parts of the Central Andean Basin. Latest Hirnantian-earliest Rhuddanian chitinozoan associations from western Puna allow correlating these western Gondwanan deposits with other glacially- related regions of northern Gondwana, such as North Africa and Arabian Peninsula, where the records of the glacial and postglacial events, which occurred in the Ordovician-Silurian boundary, are well-known.