Geologic Map of the ‘Ar‘Ar Quadrangle, Sheet 30E, with Explanatory Notes, Kingdom of Saudi Arabia

SGS-GM-142C Abdullah F. Al-Khattabi, Saleh M. Dini, Chester A. Wallace, Ahmed S. Banakhar, Mohammed H. Al-Kaff, and Ali M. Al-Zahrani
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ABSTRACT
The ‘Ar‘Ar quadrangle (sheet 30 E), in the northwestern part of Saudi Arabia, is underlain by Late Cretaceous sedimentary rocks of the ‘Aruma Group and by Paleogene and Neogene sedimentary rocks.  Devonian, Silurian, Ordovician, and possibly Ca mbrian sedimentary units occur in the subsurface.  Nonconsolidated gravel, sand, and silt occupy most wadis and dissolution-subsidence depressions.  The ‘Ar‘Ar arch crosses the northwest corner of the map area, where it trends northeast-southwest.  The city of ‘Ar‘Ar is located in the northwest quadrant of the map area, along the north border of the quadrangle.  Sedimentary rocks exposed at the surface in the map area consist of the Hudayb, Zallm, and Badanah Formations of the ‘Aruma Group (Late Cretaceous), the Umm Radhamah Formation (Paleocene to Eocene), and the Ajfar Formation (middle Miocene?), in ascending order.  Rocks of the ‘Aruma Group form a homoclinal structure that dips gently toward the northeast.  The Hudayb Formation (unit Kah), which is the oldest unit exposed at the surface in the southwestern part of the map area, is a fine-grained dolomitic limestone that was deposited on a near-shore carbonate tidal flat that had ephemeral supratidal lagoons.  The Zallm Formation (unit Kaz) is composed of sandstone in the clastic lithofacies and of argillaceous and bioclastic, fine-grained dolomitic limestone in the carbonate lithofacies.  The upper part of the Zallm Formation contains sandy limestone, oolitic limestone, and sparse pelecypod shells.  The clastic lithofacies of the Zallm Formation was deposited in a near-shore, tropical, sandy tidal environment in which barrier bars may have caused local supratidal conditions.  The carbonate lithofacies of the Zallm Formation was deposited on a shallow-marine carbonate platform that contained ephemeral supratidal basins.  West of the ‘Ar‘Ar arch, the Badanah Formation is divided into two informal members:  (1)  The base of the lower member (unit Kabl) is a dolomitic limestone that contains Loftusia sp., which is overlain by interbedded gypsiferous, chalky limestone and dense dolomitic limestone that grades laterally into carbonate mounds composed of fine-grained and microcrystalline dolomitic limestone, bioturbated dolomitic limestone, and thin beds of algal-laminated, silicified dolomitic limestone.  (2)  The base of the upper member (unit Kabu) is a crossbedded limestone composed of pelloidal oolite that is overlain by phosphatic and dolomitic limestone, and dense, calcite-cemented, bioclastic dolomitic limestone.  East of the ‘Ar‘Ar arch, the Badanah Formation (unit Kab) is composed of bioturbated, sparsely fossiliferous, fine-grained dolomitic limestone, limestone, silicified limestone, and interbedded gypsum.  Locally, the Badanah Formation contains layers and concretions of black and grayish-orange, banded chert; geodes occur at some levels.  West of the ‘Ar‘Ar arch, the Badanah Formation was deposited in a tropical, shallow-marine environment on a carbonate platform at, and below, wave base, but east of the ‘Ar‘Ar arch periodic restriction of water circulation on the shallow-marine, carbonate platform favored evaporite deposition.  Dissolution-collapse breccias, recrystallized calcite, boxwork structures, residual interbedded gypsum, caves, and sinkholes are common features in units of the ‘Aruma Group.  The Umm Radhamah Formation (unit PAur)is composed of light-gray and moderate-gray, laminated, fine-grained dolomitic limestone.  Some beds are bioturbated and contain burrows, and other beds display algal lamination.  Large, vertical burrows are common.  Chert and phosphatic chert nodules occur in some beds.  The Umm Radhamah Formation was deposited on a tropical, shallow-marine carbonate platform associated with lagoons and ephemeral supratidal basins in which evaporite minerals accumulated.  Evaporite karst consists of layered breccia and breccia dikes, crackle breccia, dissolution pits on outcrops, and common karren and krillen.  Recrystallized limestone is common.  At the top of the sedimentary sequence, the Ajfar Formation (unit Na) is composed of sandstone, conglomeratic sandstone, and conglomerate at the base, and limestone, dolomitic limestone, gypsiferous limestone, and gypsum interbeds upward in the sequence.  The Ajfar Formation occurs along the east edge of the map area. The basal part of the Ajfar Formation was deposited in a fluvial environment, and the upper part records a shallow, near-shore supratidal and lacustrine environments.  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 are common in subsidence basins that formed from differential interstratal dissolution of evaporite beds and consequent subsidence of the overlying strata.  Bedrock- and caprock-collapse sinkholes and karst topography are common where the Aruma Group and Umm Radhamah and Ajfar Formations are exposed.  Deposits in khabras and sinkholes consist of silt and clay and commonly are cemented by evaporite minerals.  The principal structure in the map area is the north- and northeast-trending ‘Ar‘Ar arch in the northwestern part of the map area.  Rocks of the ‘Aruma Group dip gently toward the northeast and form an extensive homocline over most of the map area.  The axis of the ‘Ar‘Ar arch generally coincides with the northeast trend of Wd ‘Ar‘Ar.  West of Wd ‘Ar‘Ar, Cretaceous rocks dip gently toward the northwest, and east of the wadi Cretaceous and Cenozoic rocks dip gently southeastward.  The Ab al Qr dissolution-collapse depression is located north of this map area, and the prominent escarpment that defines the west edge of this morphostructure disappears at the northern border of the ‘Ar‘Ar quadrangle (sheet 30 E); the morphological subdivisions within Ab al Qr disappear as well.  The main periods of dissolution of evaporite beds and the consequent subsidence of overlying limestone strata are likely to have occurred during Oligocene and perhaps early Miocene time. A conjugate set of linear features, most likely a joint system, trends at about N.45E. and N.45W., and it probably resulted from regional compression that created the ’il arch.  The N.45E. orientation of linear features parallels the axial surface of the ‘Ar‘Ar arch.  Linear features that trend north-south and east-west occur mainly in the southwestern part of the map area.  A few linear features trend at N. 30W, and these features have no obvious relation to regional structures.  Some wadis show linear courses that probably exploit more erodible bedrock along fractures or even narrow half-grabens. Geologic hazards in the map area are mainly related to extensive evaporite karst.  Subsidence related to subjacent evaporite dissolution may severely affect any human structure, including roads, buildings or pipelines.  Moreover, karstic depressions are prone to flood.  Fragile evaporite-karst environments can be easily degraded by human activities.  Karst aquifers, which have high hydraulic conductivity, are very susceptible to contamination and over-exploitation. Potential resources in the ‘Ar‘Ar quadrangle (30 E) include limestone and dolostone, evaporite minerals, silica sand, phosphorite, and possibly hydrocarbons.  Industrial minerals in the map area include dolostone and high-calcium limestone.  The dolostone resource is estimated at 7.29 billion tonnes.  High-calcium limestone was identified in chemical analyses, but this resource has not been evaluated.  Evaporite minerals are known from surface exposures in the ‘Aruma Group, Umm Radhamah Formation, and the Ajfar Formation, but the quality and quantity of this resource has not been determined.  Gypsum is the common mineral exposed at the surface.  The likelihood for the occurrences of oil and gas in the map area was not analyzed, although potential source rocks exist at depth in this map area, and hydrocarbon maturation models suggest that gas or oil could occur in structural and stratigraphic traps. 

Al-Khattabi, A.F., Dini, M.S., Wallace, C.A., Banakhar, A.S., Al-Kaff, M.H., and Al-Zahrani, A.M., 2010, Geologic Map of the ‘Ar‘Ar quadrangle, Sheet 30E, with Explanatory Notes, Kingdom of Saudi Arabia: Saudi Geological Survey Geoscience Map GM-142C, scale 1:250,000, 50 p., 29 figs, 2 tables, 1 pl.