The Lower Paleozoic rock units in Egypt: An overview

摘要

Lower Paleozoic rocks are exposed in various regions of Egypt(south central Sinai,north Eastern Desert and southwest Western Desert),in addition to occurring in the subsurface such as north Western Desert and the Gulf of Suez.The Lower Paleozoic rocks in Egypt include surface and subsurface rock units of formational status.The surface rock units are the Taba,Araba and Naqus formations. The subsurface rock units include the Shifa,Kohla and Basur formations. The Infracambrian Taba Formation has been discovered recently in the outcrops of the southeast central Sinai in the Taba-Ras El-Naqab area.It is missing and/or not recognized in the subsurface.The Taba Formation consists mainly of reddish brown,unfossiliferous gravelly fine-to medium-grained kaolinitic sandstones and subordinate horizons of paleosols.The Cambrian Araba Formation and its subsurface equivalent(the Shifa Formation) are essentially composed of reddish brown,line-grained laminated sandstone and siltstone with abundant Skolithos and Cruziana sp.In contrast,the Ordovician—Silurian Naqus Formation and its subsurface equivalents(the Kohla Formation and Basur Formation) are mainly composed of white,unfossiliferous,cross-bedded,medium-to coarse-grained sandstones with haphazardly distributed pebbles and cobbles.Sedimentological analysis indicates that the Araba Formation and its subsurface equivalent were deposited in a marginal-marine environment,whereas the Naqus Formation and its subsurface equivalents were laid down in a fluvio-glacial environment. Integrated stratigraphic and sedimentological studies of the Lower Paleozoic rocks permit reconstruction of the paleogeography of Egypt at that time.Egypt has been largely controlled since the Cambrian by the pre-existing structural framework of the pre-Phanerozoic basement rocks inherited from the Late Proterozoic Pan-African event.Additionally,sedimentation processes were controlled during Cambro-Ordovician times by tectonic movements,whereas glacio-eustatic control predominated during the Late Ordovician—Silurian Period.These studies suggest that most areas of Egypt were exposed lands with episodically transgression by epi-continental seas related to the paleo-Tethys.These lands formed a part of a stable subsiding shelf at the northern Gondwana margin.