Genetic units and facies architecture of a Lower Cretaceous fluvial-aeolian succession, São Sebastião Formation, Jatobá Basin, Brazil

Abstract

The Lower Cretaceous São Sebastião Formation of Jatobá Basin consists of continental strata accumulated in an arid environment dominated by aeolian deposits. The best outcrops are located in the county of Ibimirim, in the Pernambuco State. Through the description of several stratigraphic sections, three aeolian and two fluvial facies associations characterize this formation. The aeolian facies represent (a) aeolian crescentic dunes, (b) aeolian sand sheets, and (c) blowouts; the fluvial facies record (d) sheet floods and (e) channelized ephemeral rivers. These facies associations are organized into three genetic units that are bounded by supersurfaces. The unit 1 is formed by the intercalation of sheet flood, aeolian dry sand sheets, and aeolian dunes facies associations, often with soft sediment structures. The cross-bedded strata ascribed to aeolian dunes become larger and more frequent towards the top of the unit, as the soft sediment deformation structures become scarcer. The Unit 2 comprises aeolian sand sheets facies scoured by fluvial channels, with erosive concave bases, and by blowouts, which are subsequently filled by aeolian dunes deposits. Unit 3 consists of cross-bedded strata related to medium-to-large, either simple or compound aeolian dunes (draas). Supersurfaces bound the genetic units recording depositional gaps, thus defining different episodes of accumulation.