Quartz cement of meteoric origin in silcrete and nonsilcrete sandstones, Lower Carboniferous, western Sinai, Egypt

Sandstones from western Sinai which are entirely or partly of Carboniferous age were not buried more than 1.5 km until the Late Cretaceous and younger, when the lowermost rocks reached a depth of 2.5 km. The first diagenetic event was cementation by a trace to 4% of normal quartz overgrowths. Petrographic and O isotope data of the overgrowths are compatible with their precipitation from meteoric water at the surface to depths of a few hundred metres. These nonsilcrete sandstones lack the strong silicification typical of pedogenic or groundwater silcretes. Three anomalous beds are strongly cemented either by megaquartz and cryptocrystalline quartz (15-36%) or by microcrystalline quartz (8%); these beds have characteristics of silcretes. The silcretes, from 1-4 m thick, occur at the top of the Naqus Formation (Early Carboniferous?) and at or near the top of the Abu Thora and Abu Durba Formations (Early Carboniferous). Other formations contain incipient silcretes characterised by discontinuous pore spaces (between detrital grains and syntaxial cement) inferred to represent dissolved opal rinds. Oxygen isotope data for quartz cement in silcretes is compatible with the quartz precipitating from meteoric water between 75 degrees C and 88 degrees C. Evidence favouring nearsurface cementation indicates that the cementing waters were thermal fluids; thus, the silcretes are groundwater silcretes and not pedogenic silcretes. The more than 15 cathodoluminescence zones in the Naqus Silcrete attests to variations in the temperature and/or composition of groundwater that precipitated the quartz. After partial cementation of the Abu Durba Silcrete by syntaxial quartz, groundwater flow through the sandstone was strong enough to introduce silt-sized quartz and heavy minerals, The silcretes are probably pre-Cretaceous in age. An episode of igneous activity of Late Triassic-Early Jurassic age in the region may have been the heat source of thermal waters for the silcretes. (C) 1998 Elsevier Science Limited.