Petrography and geochemistry of the Cambro-Ordovician Wajid Sandstone, southwest Saudi Arabia: Implications for provenance and tectonic setting

Abstract

The Wajid Sandstone, of probable Cambro-Ordovician age, rests unconformably on peneplained Precambrian crystalline rocks on the southeastern margin of the Arabian Shield. The sandstone is composed of varicolored siliciclastic rocks, mainly coarse-to fine-grained sandstones with granule conglomerate and siltstone interbeds, displaying diverse sedimentary primary structures such as planar- and trough-cross bedding, and flat bedding. They form repeated fining-upward cycles, typical of deposition from braided streams. Current indicators indicate that flow was from SW to NNE. Petrographically, all samples of the Wajid Sandstone are of quartz arenite type, highly enriched in quartz, but poor in heavy minerals, feldspar and lithic fragments. The provenance and tectonic setting of the Wajid Sandstone have been assessed using integrated petrographic and geochemical studies. Petrographic analysis reveals that mono-and poly-crystalline quartz grains and heavy minerals from metamorphic and igneous rocks of a craton interior setting were the dominant sources. Chemically, major and trace element concentrations in the rocks of the Wajid Sandstone indicate deposition in a passive continental margin setting. Petrographic and geochemical data suggest that the sediments were derived from metamorphic and igneous rocks forming the adjacent Precambrian basement rocks of the Arabian Shield, and were deposited on a passive continental margin. Deposition took place after stabilization of the Arabian Shield following the Late Precambrian Pan-African Orogeny. The Wajid Sandstone can be correlated with similar deposits elsewhere in Arabia and North Africa which were deposited on a low-lying landmass forming the stable continental margin of the Arabian-Nubian Shield, flanking the southern margin of the Paleo-Tethys Ocean.