Lake Magadi, Kenya: a model for rift valley hydrochemistry and sedimentation?

Abstract

Summary Previous studies on Lake Magadi, Kenya, and the Green River Formation of Wyoming, are reviewed in order to delineate the differences between these two settings, to determine whether Lake Magadi can serve as a reference for rift valley environments and to define a set of diagnostic criteria for recognizing saline alkaline lake deposits in the geologic column. Magadi inflow waters are influenced strongly by weathering of volcanic rocks and are characterized by Na≈Ca and (Ca + Mg)/HCO 3 ≪1. Compared to the Green River Formation, calcite production is low to moderate and occurs largely in the soil zone. Sediments are dominated by volcaniclastic material transported during flood events. In contrast, Eocene Lake Gosiute and its mudflats had more gentle gradients, and inflow chemistry was such that vast amounts of calcite and dolomite were produced before brines became alkaline. Sedimentation in both cases is represented by the ephemeral floodplain-saline lake complex; a preliminary subfacies map for Magadi has been constructed. Magadi possesses the typical attributes of a rift valley basin in terms of hydrology, hydrochemistry and sedimentation, as well as the indices characterizing saline alkaline lakes: sodium carbonate evaporites or their pseudomorphs, Magadi-type chert, and tuffaceous rocks altered to zeolites and K-feldspar. Many similar environments have been recognized in the Pleistocene lake beds of North America. Magadi-type cherts were found from the Pleistocene to the Jurassic, and are also present in the Cambrian Officer Basin of Australia. Proterozoic Magadi-Green River-type environments have been suggested for the Duruchaus Formation of Namibia and HYC shale of the McArthur River area of North Australia. Many more Proterozoic examples remain to be discovered.