The Wadi as Sirhan (30C) quadrangle, which is in the northwestern part of Saudi Arabia adjacent to Jordan, is underlain by Devonian sandstone, lower Tertiary sedimentary rocks of the Turayf group, and by Tertiary and Quaternary basalt and nonconsolidated sedimentary deposits. The Wadi as Sirhan graben is the principal structural element in the quadrangle. Sedimentary rocks exposed at the surface in the map area consist of the Tawil Formation (Late Silurian? to Early Devonian), the Jalamid, Mira, and Umm Wu'al formations of the Turayf group (Paleocene and Eocene), and the Sirhan Formation (Miocene to Pliocene?), in ascending order. The Tawil Formation is represented by only the Ghuwar Member, a crossbedded sandstone that is the third member from the base. The Jalamid formation, at the base of the Turayf group, is divided into two members, the Thaniyat phosphorite member at the base and the Kuwaykabah member at the top. These members are composed of phosphorite, shale, limestone, cherty limestone, and dolomitic limestone. The Jalamid formation was deposited in a shallow-marine environment. In this region the Mira formation is divided into the Ghinah phosphorite member at the base, and the Hawsa, Mindassah, and Sib members, in ascending order, but the Mindassah is absent at the surface in the Wadi as Sirhan quadrangle. This sequence is composed mostly of pelletal phosphorite, dolomitic limestone, limestone, and argillaceous silicified limestone. The Mira was deposited in shallow-marine conditions. The Umm Wu'al formation is divided into the Arqah phosphorite member at the base, and the Amud, Hamad, Tarbah, and Shihiyah members, in ascending order. This sequence is composed mostly of phosphorite, limestone, coquinoid limestone, cherty limestone, argillaceous limestone, claystone, and marl. The Umm Wu'al formation was deposited in an open-water marine environment. At the top of the sedimentary sequence, the Sirhan Formation is mostly an interbedded calcareous friable sandstone, limestone, and shale that contains some beds of chert and claystone. The Sirhan Formation is interbedded with basalt at some places. The Sirhan Formation occurs only in the central part of the map area in the Wadi as Sirhan graben and adjacent to the graben. The Sirhan Formation was deposited in a restricted marine and continental setting. Harrat al Harrah, a large Tertiary to Quaternary basalt field in the Sirhan-Turayf basin, is exposed in the north-central part of the map area where lava flows and pyroclastic material are composed of alkali-olivine basalt. Volcanic cones form the highest peaks in the map area. Quaternary calcareous and gypsiferous duricrust forms discontinuous mantles on bedrock in much of the map area. Duricrust was probably much more widespread, and it may have mantled most rocks exposed at the surface earlier in Quaternary time. The occurrence of duricrust suggests that a wetter environment existed earlier in Quaternary time than at the present time in northern Saudi Arabia. Alluvial deposits form nonconsolidated gravel mantles over bedrock and fill wadis. Khabras contain silt and clay and commonly are cemented by evaporite minerals. Extensive sabkhas occur in the northwestern part of the quadrangle in closed depressions in the Wadi as Sirhan graben. The principal structure in the map area is the Wadi as Sirhan graben in the central part of the map area. The graben trends northwest across the quadrangle and bounding faults are not precisely located. The Wadi as Sirhan graben extends northward into Jordan. Late Cretaceous left slip has been documented from adjacent quadrangles, but dip slip on bounding faults of the graben may have been initiated during Late Paleocene? to Early Eocene time during an extensional tectonic regime that predated the actual rifting of the Arabian plate to the northwest as the Red Sea rift opened. The eastern part of the map area is dominated by rocks that dip gently toward the west where a large-scale homocline extends from the adjacent Ash Shuwayhitiyah quadrangle (30D) into this map area. A small-scale, northwest-trending anticline occurs along the eastern border of the map area, and a northeast-trending syncline occurs in the northwestern part of the map area. Northwest-striking linear features are common and a few northwest-striking faults and linear features occur in the map area, which are probably related to the Miocene and younger extensional regime that accompanied rifting of the Arabian plate. Potential resources in the Wadi as Sirhan quadrangle include phosphorite, uranium, limestone, opaline claystone, lignite, and oil and gas. Phosphorite resources occur in the Thaniyat member of the Jalamid formation, in the Ghinah and Sib members of the Mira formation, and in the Arqah phosphorite member of the Umm Wu'al formation. The Thaniyat phosphorite member had a phosphate concentration of 26.3 percent P2O5 and an estimated resource of 344 million metric tons in the western part of Wadi al Ghinah, based on data from two drill holes. The Ghinah member contains thin beds of phosphorite of relatively low concentration, so these phosphorite occurrences may not be economic resources at this time. The Sib member contains a phosphorite zone that is 3 m thick, and that has an average P2O5 concentration of 19 percent. Along the eastern border of the map area the Sib contains an estimated 1,550 million metric tons of phosphate resources. The Arqah phosphorite member of the Umm Wu'al formation, exposed in the eastern part of the map area, contains beds of phosphorite between 2.0 and 2.5 m thick. The phosphate resource has a P2O5 concentration of about 20 percent and the resource is estimated at 3,250 million metric tons. The great thickness of overburden makes it unlikely that these beds are exploitable at this time. Uranium oxide (U3O8) occurs as a trace element in phosphorite deposits of the Ghinah and Arqah phosphorite members, but the economic potential for uranium oxide recovery as a by-product of phosphorite production has not been evaluated. High-purity limestone occurs in the northeastern part of the quadrangle, where the Tarbah member of the Umm Wu'al formation is exposed. Opaline claystone that occurs in the Amud member of the Umm Wu'al formation (Turayf group) has a resource potential as building stone, abrasive, refractory insulation, filtration medium, and as an absorbent. Lignite, a sub-bituminous coal, occurs in Late Cretaceous rocks of the Aruma Group, but the resource potential of this lignite has not been determined. Oil and gas was generated during Paleozoic time in the region of northwestern Saudi Arabia, and probably during Mesozoic time in the Wadi as Sirhan graben, so an unknown potential exists for the occurrence of oil and gas in structural and stratigraphic traps in this map area. Oil shows have been reported in Cretaceous rocks of the Wadi as Sirhan graben, and oil has been discovered recently in the northern extension of the Wadi as Sirhan graben in adjacent Jordan.
Wallace, C.A., Dini, S.M., and Al-Farasani, A.A., 2000, Geologic Map of Wadi As Sirhan Quadrangle, Sheet 30C, Kingdom of Saudi Arabia with Explanatory Notes: Saudi Geological Survey Geologic Map GM-127C, 27 p.16 figs., 1 pl.