Sedimentary facies and architectural elements of the Serra do Apertado and Pedra Pintada Formations in the Minas do Camaquã Region - RS, Brazil

Abstract

The Guaritas Group (Cambrian, RS, Southern Brazil) encompasses sedimentary deposits of bedload or mixed load rivers, eolian dune fields and alluvial fans, and constitutes a rare sedimentary record of Cambrian age in Eastern South America. In this context, its uppermost unit, named Serra do Apertado Formation, contains the last record of stabilization phase of the Gondwana Continent in south and southeastern Brazil. Nevertheless, there are no published detailed studies on its facies and depositional architecture, and this unit has been considered as similar to the Guarda Velha Formation since its original definition. Based on sedimentary facies and depositional architecture studies of the Serra do Apertado Formation and the upper portion of its underlying unit, the Pedra Pintada Formation, the depositional environments were characterized and a descriptive basis was established to distinguish them from the other units of the Guaritas Group. The Serra do Apertado Formation is characterized by the architectural element EL (Sheet floods), which encompasses facies associations that points to a fluvial system characterized by large discharge changes and occasional eolian reworking of bar tops. The upper part of the Pedra Pintada Formation intercalates the architectural elements DB (barchanoid dunes) and IF (Fluvial interdunes). The exclusivity of EL in the Serra do Apertado Formation contrasts with previous descriptions of the Guarda Velha Formation, which show elements of channel-fill and frequent conglomeratic lenses. Those differences reveal more eolian reworking in the Serra do Apertado Formation and possibly more episodic flow, suggesting, preliminarily, a dryer climate than the one recorded in the Guarda Velha Formation.