Abstract
The Miocene Barreiras Formation in the Middle Rio Capim area records an incised valley system for which facies analysis and ichnology ( Skolithos, Ophiomorpha, Planolites, Gyrolithes, Taenidium) suggest an estuarine character. Three stratigraphic units are recognized (from bottom to top): Unit 1 includes an inner estuarine tidal channel complex and tidal flat/salt marsh deposits; Unit 2 consists of estuarine bay/lagoon and flood tidal delta deposits related to the estuary mouth; and Unit 3 includes a tidal channel with a tidal point bar, as well as tidal flat/salt marsh deposits similar to those from Unit 1. These units and their bounding surfaces record the history of relative sea level changes in the estuary. After a sea level drop, the valley was inundated and formed an amalgamated sequence boundary and transgressive surface. Transgression (Unit 1) promoted the landward shift of flood tidal deltas and lagoon settings (Unit 2). The system then moved seaward, with the superposition of inner estuarine deposits (Unit 3) over Unit 2. Facies architecture seems to have been controlled by tectonics, as shown by: the paleovalley orientation according to the main tectonic structures of the basin; the presence of faults and fractures that displace the basal unconformity; and the abundance of soft sediment deformation.
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