Calcretes and palustrine carbonates in the Oligo-Miocene clastic-carbonate unit of the Farafra Oasis, Western Desert, Egypt: Their origin and paleoenvironmental significance

摘要

The origin and paleoenvironmental significance of the calcretes and palustrine carbonates in the Oligo-Miocene clastic-carbonate unit that outcrops at the eastern sector of the Farafra Depression (Western Desert, Egypt) have been discussed based on field and petrographic investigations. The calcretes-palus-trine carbonates assemblage occurs above a siliciclastic/distal alluvial-floodplain facies. The calcretes represent the transition from the underlying siliciclastic/distal alluvial-floodplain facies to the overlying palustrine carbonates. The calcrete-host rocks are muddy sandstones and sandy mudrocks. This study reveals the occurrence of groundwater calcretes with an upward gradational maturity pattern, ranging from incipient to nodular and to massive calcretes. The calcretes micromorphological analysis suggests that they were originated in vadose and phreatic diagenetic environments by groundwater through evaporation, degassing with no biological activity. The palustrine carbonates are also recognized above the calcrete horizons. They occur in the form of micritic limestones displaying different features that indicate their modifications during pedogenesis and subaerial exposure. These features include clot-ted-peloidal texture, fenestral fabric, mottling, pseudo-brecciation, desiccation cracks, pseudomicrokarst, root traces and silicification of the lime mud. The calcretes-palustrine carbonates assemblage records a progressive decrease in the terrigenous supply and a continuous rise of the groundwater table associated with local subsidence in a semi-arid to sub-humid climate. A model is suggested for the development of calcretes and palustrine carbonates in the study area.