Climate, sediment supply and tectonics as controls on the deposition and preservation of the aeolian-fluvial Etjo Sandstone Formation, Namibia

Abstract

Deposition and subsequent preservation of the Jurassic-Cretaceous Etjo Sandstone Formation of Namibia represents a complex interplay between climatic and tectonic factors and related variations in extrabasinal sediment supply. The aeolian and fluvial deposits indicate semi-arid to arid climatic conditions throughout the deposition of four distinct sedimentary units. The succession records either an upward increase in aridity or an upward increase in aeolian sediment supply, represented by a transition from a fluvially dominated basal unit, through a marginal fluvial-aeolian unit to an exclusively aeolian unit. A combination of inherited palaeotopography and syndepositional extensional faulting provided the space necessary for the accumulation of much of the succession. A basinwide unconformity (super surface) divides the succession. This hiatus resulted partly from a lack of available preservation space and partly from a shutdown in aeolian activity related to a regional climatic reorganization. A subsequent shift in the palaeowind direction from northwesterly to southwesterly exploited sand reserves in the Paraná Basin of South America and led to the resumption of aeolian sedimentation across the region. Variations in preserved bedform thickness were directly controlled by differential amounts of tectonic subsidence across the basin. A second major super surface towards the top of the succession resulted from the regional shutdown of large tracts of the aeolian system following the eruption of Etendeka flood basalts across the region.