Geology of Saudi Arabia
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Saudi Arabia is readily divisible into two geological provinces. The western area consists of crystalline igneous and metamorphic rocks, of presumed Pre-Cambrian age, topographically and structurally sloping to the northeast, east and southeast. The eastern area has easterly dipping sedimentary rocks overlying the basal igneous and metamorphic complex. These sediments consist of Paleozoic, Mesozoic (nearly complete section), Tertiary, and Recent deposits. The area is subdivided into Nejd and Hassa (Persian Gulf) sub-provinces. The eastern parts of Nejd subprovince consist of sedimentary rocks, ranging in age from early Paleozoic to early Eocene. These sediments consist of alternating calcareous and clastic facies. Differences in the relative resistance of these rocks are responsible for smooth, gently dipping slopes to the east, on the one hand, and truncated west-facing escarpments on the other. Examples of these resistant rocks are found in the Jurassic strata of the Tuwaiq Mountains which extend in a north-south direction. Another example is the Aruma Plateau, capped by resistant limestone of Upper Cretaceous age. There is an absence of marked altitudinal variations in the Nejd sedimentary rocks, and, hence, the rather uniform homoclinal dip has been preserved. In the Hassa subprovince the outcropping rocks are limited to the Tertiary and later; these include the lower and middle Eocene, and Miocene to Pliocene. The Eocene rocks are almost entirely calcareous, except one local anhydrite member on the coast of the Persian Gulf. The Miocene and Pliocene strata are primarily continental deposits, except for some intercalated marine beds. The latter are composed of marls, clays, sands, and thin limestone, and they cover the Eocene sediments in most of the Hassa Province.