The Markaz ‘Adhf’quadrangle (sheet 29 E) is located in the northwestern part of Saudi Arabia, about 1,300 km from Jeddah. The exposed rocks and deposits in the map area are Late Cretaceous and Quaternary in age. Triassic, Permian, Carboniferous, Early to Late Devonian, and possibly late Silurian sedimentary rocks occur in the subsurface. Nonconsolidated eolian dunes cover about 37 percent of the map area in An Nufd al Kabr. Nonlithified gravel, sand, and silt fill most wadis, and khabra deposits fill subsidence depressions formed by interstratal dissolution of evaporite beds. The exposed sedimentary rocks are the Hudayb, Zallm, and Badanah Formations of the ‘Aruma Group (Late Cretaceous in age), in ascending order. The Hudayb Formation (Kah) consists of light-gray and moderate-gray argillaceous limestone and bioclastic dolomitic limestone. Some limestone is bioturbated, and it contains gypsum and dissolution cavities. Fossils are rudists, algal lamination, and stromatolites. Caves and sinkholes are common, and layers of limestone breccia record the former presence of evaporite beds. Only the upper 30 m of the Hudayb Formation is exposed in the map area. The Zallm Formation (unit Kaz) is composed of interbedded argillaceous, dolomitic limestone; coarsely recrystallized, sparry limestone; and limestone breccia in a reddish-gray sandy matrix. Layers of dissolution breccia are light gray and are most common near the top of the formation, where dissolution cracks and boxwork structures are abundant. Fossils are mainly rudists in a matrix of bioclastic debris and coarsely crystalline calcite spar. About 60 m of the Zallm Formation remains at the surface, but dissolution of interbedded gypsum and limestone eliminated about 25 to 45 m of the original 85 to 105 m, determined from drill-hole data. The Badanah Formation (unit Kab) consists of shale at the base and a cyclic repetition of argillaceous limestone that passes upward into gypsiferous, dense, recrystallized limestone. A brecciated and recrystallized limestone occurs at the top. Three cycles are repeated in this unit. The basal shale is light gray, grayish green, and yellowish gray, and it is laminated and microlaminated. Argillaceous limestone is light gray to yellowish gray. Light-gray to brownish-gray, bioclastic limestone contains stromatolitic bioherms, algal-laminated limestone, and large gastropods in non-argillaceous limestone. Only about 25 m of the lower part of the Badanah Formation is exposed in the map area. The ‘Aruma Group is Late Cretaceous in age, and the paleoenvironment is characterized by a tropical, near-shore, lagoonal carbonate shelf that contained local, ephemeral, supratidal environments in which evaporite minerals were precipitated. Quaternary deposits consist of calcareous and gypsiferopus duricrust, eolian dune fields, khabra deposits, and alluvium in wadis. Duricrust is scattered across exposures of the ‘Aruma Group, but duricrust is not shown separately on the map. Duricrust occurs in alluvium and in dissolution-collapse breccia of the ‘Aruma Group. Eolian sand dunes of An Nufd al Kabr occupy the southern part of the quadrangle. Khabras are common in the northeast corner of the map area, where they occur in dissolution-induced basin structures developed in the Badanah Formation. Khabra deposits consist of silt and clay cemented by evaporite minerals. The principal tectonic structure in the map area is the southeast-trending Wd as Sirn fault, which is covered by dune fields of An Nufd al Kabr. Linear features form three families: (1) a possible joint system at 055 to 065 azimuth, which may represent part of a conjugate set identified in nearby quadrangles; (2) a general north-south trend of linear features at 345 to 360 azimuth, which may be related to north-south trending arches in the region; and (3) linear features at 290 and 300 azimuth, which may be the only evidence of the existence of the Wd as Sirn fault under An Nufd al Kabr in the southern part of the map area. Evaporite karst is widespread in this map area, and subsidence caused by interstratal dissolution had a significant impact on local structure and geomorphic features. Dissolution–induced subsidence structures occur throughout the Hudayb, Zallm, and Badanah Formations of the ‘Aruma Group (Late Cretaceous). Intrastratal dissolution of evaporite beds and accompanying subsidence of interbedded and overlying limestone of the ‘Aruma Group formed layers of pack and float breccia, boxwork structure, cavities in limestone, and sinkholes. Near the upper part of the Hudayb Formation is an extensive zone of sinkholes and caves. Caves and sinkholes occur throughout the Zallm Formation, as do closed depressions that resulted from downward flexure of limestone beds caused by differential interstratal dissolution of interbedded evaporite beds in the Badanah Formation. The map area is located at the edge of a large evaporite basin that formed in the Arabian Gulf during Late Cretaceous time. The Arabian Gulf sedimentary basin is separated from the Mediterranean sedimentary basin by the ‘Ar‘Ar arch. This recently discovered evaporite basin might underlie an area as large as 44,000 km2 of northern Saudi Arabia. Earlier mapping at 1:500,000-scale did not detect this large area of evaporite-karst, and the effects of dissolution and collapse processes in the evaporite basin confused definition of geologic map units. The main period of dissolution-induced subsidence may have occurred during Oligocene and Miocene time.Evaporite-karst terrains pose hazards to humans, buildings, and roads, and they can cause significant degradation of surface and underground water supplies. Ecosystems formed on evaporite karst are inherently fragile. The general lack of permanent human habitation sites in the map area reduces the potential impact of humans on the evaporite-karst terrain, as well as reducing geologic hazards that may affect humans. Potential resources include limestone, silica sand, and oil or gas. Some limestone is pure enough with adequate purity to be used for manufacture of glass and concrete. High-purity limestone has metallurgical applications and can be used as an extender in paper, plastics, and rubber products. High-purity limestone has been extensively recrystallized during dissolution and subsidence processes. Silica sand from the extensive dune fields of An Nufd al Kabr is pure enough and fine grained enough to be used for window glass. No systematic evaluation was made of the oil and gas potential, but source rocks exist at depth in this region. Oil and gas may have been expelled from the oil-prone Quaib’ Formation of the Qalbah Group (Silurian) in the Widyn basin of northern Saudi Arabia. Although the ‘Unayzah Formation is a potential reservoir in much of northern Saudi Arabia, lithofacies of this unit probably lack permeability at depth in the map area. Structural traps may have formed during Cretaceous folding, and stratigraphic traps may occur where permeable strata pinch out beneath impermeable strata. Gypsum and other evaporite minerals are not likely to be significant resources in the map area because evaporite beds were thin and discontinuous, and much near surface evaporite was dissolved, mostly during Oligocene and Miocene time. Dolomitic limestone is quarried for aggregate from the Zallm and Badanah Formations.
Banakhar, A.S., Al-Zahrani, A.M., Al-Juaid, A.J., Dini, S.M., Wallace, C.A., Al-Kaff, M.H., Gutiérrez, F., and Al-Tassan, A.A., 2010, Geologic Map of the Markaz ‘Adhfa quadrangle, sheet 29E, with Explanatory Notes, Kingdom of Saudi Arabia: Saudi Geological Survey Geoscience Map GM-144C, scale 1:250,000, 40 p., 29 figs., 1 table, 1 pl.