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 Carbonif erous age were not buried more than 1.5 km until the Late Cretaceous a nd younger, when the lowermost rocks reached a depth of 2.5 km. The fi rst diagenetic event was cementation by a trace to 4% of normal quartz overgrowths. Petrographic and O isotope data of the overgrowths are c ompatible with their precipitation from meteoric water at the surface to depths of a few hundred metres. These nonsilcrete sandstones lack t he 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 (Ea rly Carboniferous). Other formations contain incipient silcretes chara cterised by discontinuous pore spaces (between detrital grains and syn taxial cement) inferred to represent dissolved opal rinds. Oxygen isot ope 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 cemen ting waters were thermal fluids; thus, the silcretes are groundwater s ilcretes and not pedogenic silcretes. The more than 15 cathodoluminesc ence zones in the Naqus Silcrete attests to variations in the temperat ure and/or composition of groundwater that precipitated the quartz. Af ter 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-Ear ly Jurassic age in the region may have been the heat source of thermal waters for the silcretes. (C) 1998 Elsevier Science Limited.