The An Nabk and Turayf quadrangles lie at the northern border of the Kingdom of Saudi Arabia. Middle and upper Cenozoic sedimentary and volcanic rocks form the surface of the quadrangles, and sedimentary rocks of the Paleozoic, Mesozoic, and lower Cenozoic are found in the subsurface. The Paleozoic and Mesozoic rocks, described from drill hole records, include the Tabuk, Upper Sudair, Lower Jilh and Aruma formations which are mostly of marine origin.
The map area is part of the Sirhan-Turayf Basin and contains the Thaniyat, China, and Arqah Phosphorite members of the Upper Cretaceous-Tertiary Turayf group. Only the Arqah Phosphorite, the uppermost of the three members, crops out at the surface. From its outcrop along the east border of the area, the Arqah dips westward into the subsurface and passes into an argillaceous deeper water fades. The Thaniyat and China also dip westward in the subsurface and pinch out into a deeper water fades.
Upper Eocene calcareous claystone and chalk mapped in the Wadi As Sirhan depression on the west side of the map area are named the Rashrashiyah formation, and may be partly equivalent to Eocene bioclastic, chert-bearing carbonate rocks of the Turayf group mapped to the east.
The Sirhan formation is a new name for rocks of Miocene(?) age occurring in the Wadi As Sirhan area. These rocks are mostly sandstone of probable continental origin, and contain thin beds of fresh water limestone. However, marine limestone beds also occur, indicating periodic incursions by the sea.
Al Harrah, a large Quaternary-Tertiary basalt field in the Sirhan-Turayf Basin, occupies the central part of the map area. The flows consist of silicic alkali olivine basalt, typical of the Cenozoic lava flows of Saudi Arabia. Volcanism appears to be of the continental rift-type and is related to Red Sea rifting.
Quaternary calcareous and gypsiferous duricrust is found discontinuously throughout the western part of the map area. At one time it apparently formed a carapace-like cover over all surface rocks, and indicates a much wetter in northern Saudi Arabia than the present climate.
The Arqah Phosphorite member is exposed in the northeast corner of the map area and contains thin beds of phosphorite grading from 18 to 30 percent P2O5-
Oil shows have been reported in Cretaceous rocks of the Wadi As Sirhan depression, and oil has been discovered recently nearby in Jordan. Geologic and geophysical conditions are favorable for the entrapment of oil in the Wadi As Sirhan area and additional exploration is recommended.
