Late Oligocene lacustrine deposition of the Sodmin formation, Abu Hammad Basin, Red Sea, Egypt: sedimentology and factors controlling palustrine carbonates

Abstract

The Late Oligocene lacustrine sequence of the Sodmin Formation is exposed in the Abu Hammad Basin, 27 km west of Hamrawein. The Sodmin Formation comprises two main lithostratigraphical units that are interpreted to represent the infilling of a closed lake located within a dextral shear setting that developed in association with the earliest stage of rifting of the Red Sea. The lower unit consists of grey lime-mudstone, marl and micritic limestone intercalated towards the top with fine siliciclastics, which reflect two stages of palagolake evolution. Initially, the lake was relatively deep with steep margins, experienced occasional periodic emergence, and had lacustrine carbonates deposited. During the second stage, the lake was generally shallower, had gently sloping margins and had become a palustrine setting in which fluctuations of the water level caused extensive emergence. Pedogenic processes affected most of the lake deposits, and modification was very intense in the marginal facies. The upper unit includes dolostone and cherts, which are indicative of lake desiccation and evaporitic conditions. Evidence from geochemical, petrographical and mineralogical analyses clearly indicates a gradual shift from open, freshwater lake conditions during deposition of the lower unit to closed, saline and alkaline waters during deposition of the upper unit. Climate and topography were the main controls on vertical facies variations of the palustrine carbonates. Climate influenced episodic lake water level fluctuations, whereas topography of the basin, which changed through time in response to tectonic uplift of the lake margins, influenced water run-off as well as the frequency and length of subaerial exposure.