Distal fluvial to lacustrine/palustrine carbonate facies of the El-Ris Formation (Late Miocene?) at the Bahariya Depression, Western Desert, Egypt: Depositional model and controls

Abstract

This article aims to discuss the distal fluvial to lacustrine/palustrine carbonate facies of the El-Ris Formation (Late Miocene?) in the Bahariya Depression (Western Desert, Egypt), in terms of their depositional evolution and factors (base-level, climate, tectonics) controlling deposition. The El-Ris Formation disconformably overlies the Lower Cenomanian Bahariya Formation. It is up to 11 m thick, and includes siliciclastic facies, followed upward by carbonate facies. The siliciclastic facies consist of greenish-white, fine-grained calcareous sandstones interbedded with subordinate yellowish-brown mudrocks having insects ichnofossils reflecting deposition in a distal fluvial distributary system. The carbonate facies include the fossiliferous (mostly ostracods and a few dispersed gastropods, Lanistes carinatus,) fine crystalline lacustrine/palustrine carbonates, and calcretes-dolocretes. The lacustrine/palustrine beds are grey to pale-yellow colored massive dolomitic beds, with exposure features, reflecting deposition in a shallow, low-gradient (marginal) lake with frequent base-level changes. Calcretes-dolocretes are nodular to massive orange beds, developed at the expense of siliciclastic facies, that formed a fringe between the distal alluvial and central lacustrine-palustrine environments. Palustrine carbonate and duricrust (calcrete/dolocrete) facies increase progressively toward the top of the studied rock unit. The development of silica nodules in the palustrine facies is a result of intense evaporation just above the water table. Silica could have been released by silicate dissolution from the host alluvial deposits into the groundwater. δ13C and δ18O data indicate a hydrologically closed palustrine/lacustrine environment fed by meteoric waters, that originally precipitated calcite. Progressive evaporation of freshwater in this closed system led to dolomitization. The upward increase of carbonates in the El-Ris Formation reflects a progressive decrease of a terrigenous clastic supply in the source area and a dominance of carbonate development in the study area. During the deposition of the El-Ris Formation, the climate had marked more arid seasons with fewer intermittent humid periods. Also, the tectonic influence cannot be ruled out, since tectonic instability led to the creation of the basin depression and subsequent accommodation space that made the deposition of the El-Ris Formation.