Cyclic cross-bedding in the eolian dunes of the Sergi Formation (Upper Jurassic), Recôncavo Basin: Inferences about the wind regime

Abstract

The Upper Jurassic Sergi Formation displays eolian cyclic cross-bedding, composed of a regular intercalation of grain-flow and wind-ripple packages, forming 1–3 m thick cycles. Each cycle is bound by erosive downwind surfaces, and it comprises a sequence of lower angle, wind-ripple laminae followed by steeper grain-flow strata that built out upon the wind-ripple wedges in the migration direction. The cross-strata mean vectors dip consistently towards SW. These cycles are related to seasonal changes in wind direction. The cyclic stratification pattern suggests an alternation of northeasterly transverse winds responsible for slipface evolution and dune migration, followed by reverse transversal or oblique winds (overall southerly winds). These winds caused dune degradation and reworking and led to the formation of reactivation surfaces and the related deposition of wind-ripple wedges. The wind pattern inferred from the eolian cyclic cross-bedding is consistent with the quantitative atmospheric circulation model that indicates that a monsoonal wind regime operated in the low latitudes of Gondwana during the Late Jurassic.