MODELO DE TAFOFÁCIES PARA OS MOLUSCOS BIVALVES DO GRUPO PASSA DOIS (FORMAÇÕES SERRA ALTA, TERESINA E CORUMBATAÍ), PERMIANO SUPERIOR, BACIA DO PARANÁ, BRASIL

Abstract

TAPHOFACIES MODEL TO BIVALVE-DOMINATED CONCENTRATIONS OF THE PASSA DOIS GROUP (SERRA ALTA, TERESINA AND CORUMBATAÍ FORMATIONS), LATE PERMIAN, PARANÁ BASIN, BRAZIL A taphofacies analysis was performed for bivalve-dominated concentrations of the Passa Dois Group (Upper Permian), Anhembia froesi-Barbosaia angulata, Pinzonella illusa and Pinzonella neotropica biozones, Serra Alta, Teresina and Corumbataí formations, São Paulo State. Taphofacies were defined on the basis of quantifiable taphonomic signatures and the internal complexity of fossil concentrations. Five distinct taphofacies were recognized along a bathymetric gradient (onshore-offshore, below the storm wave base). Taphofacies 1, includes densely pocket, amalgamated coquinas, including high proportion of fragmented and chaotically oriented shells. The taphonomic signatures are indicative of long term exposure of shells at the sediment water interface, but its final deposition was related to storm flows. Taphofacies 2, includes 50 cm thick bioclastic sandstones with erosive base, sometimes including basal lags. These bioclastic sandstones are also the final product of storm events. Taphofacies 3, includes coquina-like deposits (similar to taphofacies 1), but with bivalve shells preserved in situ, at its top. This noteworthy feature indicates that its final deposition occurred below storm wave base. Taphofacies 4, encompass thin (20 cm) fossil concentrations with disperse shells, that are intercalated in a thick sequence of laminated, sometimes densely bioturbated mudstones. The rate of shell fragmentation is low and some closed articulated shells are filled by sediments that are distinct of the matrix. It is indicating that pre-fossilized elements were mixed with younger shells, possible by distal storm flows or by means of biological activity within the substrate. Taphofacies 5, includes fossil that are disperse in the matrix, high rates of articulation, and the lack of transportation signs. They were generated below storm wave base. The taphofacies above (1 to 5), are not randomly distributed on the Passa Dois Group. The more distal taphofacies (Taphofacies 4 and 5), are common in the Serra Alta Formation deposits and in the base of Corumbataí Formation, when relatively deep and calm neritic conditions prevail. The more proximal taphofacies (taphofacies 1 and 2), are common in the middle and upper portions of Teresina and Corumbataí Formations. Taphofacies analysis are thus useful as a confirmatory, independent tool, adding new and important evidences to the data furnished by litho-and biofacies analysis.