New lithostratigraphic framework proposal for the Neoproterozoic sedimentary succession of Sierras Bayas, Tandilia System, Argentina.

Abstract

In the Alicia Quarry (S 37º 2´ 75”´´ and W 60º 20´ 25´´), located in the southern area of the Sierras Bayas, Olavarria region, Tandilia System, Buenos Aires Province, Argentina, a 150 m thick sedimentary succession was recognized exclusively through by vertical wells. This subsurface Neoproterozoic succession is composed by black shales, siltstones and gray-greenish-gray fine sandstones. There is a profuse bibliography about sedimentary units of Sierras Bayas Group, in contrast with Cerro Negro Formation. Moreover, a thick black shale and tuff layer was previously assigned to Sierras Bayas Formation, which could be similar with the black shales found by drilling. So that it was the unique contribution published in reference to the sedimentary unit described. An Ediacaran acritarch assemblage was reported from these black shales, which is characterized by high reflection index, indicating thermal maturity with temperatures higher than 250ºC. The discovery of this important new unit modifies the existing stratigraphic scheme for sedimentary units located above the Loma Negra Formation (top unit of the Sierras Bayas Group). We propose the designation of a new group named La Providencia Group, which comprises three different formations, Avellaneda Formation (shales and marls), Alicia Formation (black shales, siltstones and gray-greenish-gray fine sandstones) and the already existing Cerro Negro Formation.