Uma nova hipótese de empilhamento para a Formação Santa Maria (Triássico) na região da cidade de Santa Maria (RS)

Abstract

The most expressive Triassic strata in Brazil correspond to the Santa Maria Formation, which occurs in the State of Rio Grande do Sul. This layer has been lithologically divided into two members (Passo da Tropas, sandstone, and Alemoa, mudstone). The Formation has also been biostratigraphycally divided into four assemblage-zones (AZ, = cenozones): the Dicroidium AZ, which comprises the entire Passo das Tropas Member, and the Dinodontosaurus AZ (lower), Santacruzodon (middle) and Hyperodapedon AZ (upper) comprising the entire Alemoa Member. In the region of the town of Santa Maria only the Hyperodapedon AZ is effectively recorded and there are currently two hypothesis for the vertical position of the Dinodontosaurus AZ: (1) it underlies the Passo das Tropas Member but is absent due to erosion before this Member deposition; or (2) it overlies the Passo das Tropas Member. In this case, it is not recognized due to the absence of any indicative fossil or it is absent due to erosion, although there is no indication of discordance. Recently, the Passo das Tropas Member was recognized as comprising two different sandstone units: São Valentim (lower) and Sarandi (upper). The presence of mudstone intraclasts at the erosional base of the Sarandi sandstone suggests a third hypothesis. These intraclasts could represent the deposited relics of the eroded Dinodontosaurus AZ. This hypothesis recognizes at least two alternating sandstone-mudstone depositional cycles partially recorded in the region of the city of Santa Maria. This model is in agreement with the three (at least) alternating sandstone-mudstone depositional cycles found elsewhere outside this region.