Geochemistry of sandstones from the Pliocene Gabir Formation, north Marsa Alam, Red Sea, Egypt: Implication for provenance, weathering and tectonic setting

摘要

Petrographic, major and trace element compositions of sandstones from the Pliocene Gabir Formation, Central Eastern Desert, Egypt have been investigated to determine their provenance, intensity of paleo-weathering of the source rocks and their depositional tectonic setting. Gabir Formation is composed mainly of sandstones alternating with limestone and shale beds. The Gabir sandstone is yellowish gray to yellowish brown color, calcareous and fossiliferous. The composition of this formation refers to shallow warm agitated marine conditions. Texturally, Gabir sandstones are immature, poorly sorted and grain supported. Abundance of feldspars indicates rapid deposition of sediments from a nearby source rocks. Their average modal composition (Q(71.35)F(16.6)L(12.05)), classifies them as sublitharenite and arkose with subordinate litharenite and subarkose, which is also supported by geochemical study. Chemical analyses revealed that sandstones have high SiO2, K2O > Na2O, and low Fe2O3 values, which are consistent with the modal data. Also, sandstone samples are enriched in most trace elements such as Ba, Sr, Ni, Cr and Zr and depleted in U and Th. The petrography and geochemistry suggest that Gabir sandstones were deposited in an active continental margin basin. They were mainly derived from granitic and low grade metamorphic sources. The CIA values (41.69-74.84) of the Gabir sandstones indicate low to moderate degree of chemical weathering, which may reflect cold and/or arid climate conditions in the source area. The source rocks are probably identified to be Proterozoic granites, metagabbros and metavolcanics, which must have been exposed during rifting, initiated during Oligocene and continued till post Miocene. (C) 2014 Elsevier Ltd. All rights reserved.