Geologic Map of the As Sulayyil Quadrangle, Sheet 20H, with Explanatory Notes, Kingdom of Saudi Arabia

SGS-GM-141C Saleh M. Dini, Chester A. Wallace, Mohammed A. Halawani, Saleh A. Al-Sobhi, Wadee A. Kashghari, and Ali S. Al-Ghamdi
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The As Sulayyil quadrangle (sheet 20 H) is located in the east-central part of the Kingdom of Saudi Arabia about 500 km southwest of Ar Riy, and the rocks consist of Paleozoic, Mesozoic, and Cenozoic sedimentary units and nonconsolidated deposits that mantle older rocks.  The main structure is an east-dipping homocline that is bounded on the west by the prominent north-south-trending Ţuwaiq escarpment.  The quadrangle is bounded by lat 20º00N. and 21º00N., and by long 45º00E. and 46º30E.  Phanerozoic sedimentary rocks range in age from Permian to Eocene and were deposited in deltaic, strandline, supratidal, lagoonal, and shallow-marine environments.  The Phanerozoic rocks that underlie most of the As Sulayyil quadrangle form part of the west edge of the Arabian sedimentary basin.  These rock units were deposited unconformably over the east flank of the Proterozoic Arabian Shield.  Permian to Early Cretaceous rock units are designated the Dir supergroup, and in the map area the complete sequence, which includes post-Carboniferous subsurface rocks, is about 1,400 m thick.  About 740 m of this sequence is exposed at the surface.  The Buraydah group is represented by the Khuff Formation (Middle Permian and Early Triassic), the Jilh and Sudair? Formations (Early to Late Triassic), and the Minjr Formation (Late Triassic).  The Khuff Formation is composed of interbedded gray limestone and shale; the Jilh and Sudair? Formations are composed of shale, siltstone, and limestone; and the Minjr Formation is composed of fine- to coarse-grained sandstone.  The Shaqrā’ group (Early Jurassic to Late Jurassic) contains the Dhruma, Ţuwaiq, anfah, Jubailah, ‘Arab, and Ht Formations, in ascending order.  The Dhruma Formation is composed of crossbedded, medium-grained sandstone.  The Ţuwaiq Formation contains limestone, calcareous sandstone, and shale.  The anfah  Formation is composed of nodular limestone, laminated limestone, sandstone, and shale.  The Jubailah Formation is sandstone, conglomerate, and interbedded limestone.  The ‘Arab Formation is a cyclic sequence of limestone and gypsum.  The Ht Formation is composed of gypsum and anhydrite.  The Thammah Group (Late Jurassic? to Early Cretaceous) consists of the Sulaiy Formation, the combined Yammah and Buwaib Formations, and the Biyadh Formation, in ascending order.  The Sulaiy Formation consists of interbedded limestone, sandstone, and rare shale.  The combined Yammah and Buwaib Formations are composed of sandstone, limestone, shale, and gypsum beds.  The Biyadh Formation is composed of crossbedded, medium-grained sandstone and siltstone.  The Wasa‘ Formation (Early to Late Cretaceous) is composed of crossbedded, medium- and coarse-grained, conglomeratic sandstone.  Cenozoic rocks that unconformably overlie these Paleozoic and Mesozoic formations are isolated patches of the Paleogene Ru’s Formation (Eocene), which is composed of chert and silicified limestone, limestone, and gypsum.  Paleozoic and Mesozoic rock units form a gently eastward dipping homocline that terminates at the prominent west-facing Ţuwaiq escarpment in the western part of the map area.  The homocline underlies the upland surface of the Jibāl Ţuwaiq.  The escarpment has a local relief of 231 m to 300 m, and the cliffs generally trend at 020º (azimuth) across the map area.  The Middle Jurassic Ţuwaiq Formation forms the cap rock on the escarpment and an extensive east-facing dip slope, above which occur mesas of the anfah  and Jubailah Formations.  The ‘Arab and Ht Formations form a north-south-trending band, about 20 km wide, of hummocky terrain dominated by features formed by dissolution-induced subsidence.  The Sulaiy Formation, and to a lesser extent the Yammah and Buwaib Formations, are disrupted by dissolution-collapse, and these units include breccia masses above the former stratigraphic level of the Ht Formation.  The Biyadh Formation and Wasa‘ Formation form a northeast-trending band of low hills and a flat, poorly dissected plain that drains east.  Quaternary deposits overlie parts of all rock units in the quadrangle, and extend over a broad area to cover Late Triassic to Early Jurassic rocks west of the Ţuwaiq escarpment.  Alluvial fan deposits of three generations occur along the piedmont of the Ţuwaiq escarpment in the western part of the map area.  In the eastern part of the map area, three successive levels of alluvial gravel sheets overlie bedrock, but in the western part of the map area, only two levels of sheet gravels occur.  Alluvium occurs in modern drainages.  Khabras (playas) of three different ages occur in the western part of the quadrangle.  Extensive dune systems (ergs), both active and inactive, occupy the southeastern quadrant in the Rub al Khl, and dunes and sheet sands cover extensive tracts of the gravel plain in the west.  Faults are rare in the As Sulayyil quadrangle, and most folds are related to dissolution-induced subsidence phenomena caused by interstratal dissolution of evaporite beds in the ‘Arab and Ht Formations in the north-central part of the map area.  North-south-trending normal faults occur in the southwestern part of the map area, and two normal faults trend to the north in the ‘Arab Formation.  Low-relief basins and domes in Jurassic and Cretaceous rocks in the east-central part of the map area are dissolution-induced subsidence structures.  Dissolution-collapse breccias are common in rocks overlying evaporite deposits affected by dissolution in the east half of the quadrangle.  Two large-scale, parallel folds probably generated by differential dissolution of evaporite beds trend approximately 010º (azimuth) in the north-central part of the map area; these are the Sufayr imm synclinorium and the Jilh Rmn anticlinorium.  These folds extend southward about 30 km from the north boundary of the map area.  The youngest unit exposed in the center of the Sufayr imm synclinorium is the Dughum Member of the Biyadh Formation, and the synclinorium probably formed by subsurface dissolution of the ‘Arab and Ht Formations.  The axis of the Jilh Rmn anticlinorium is sinuous and the Sulaiy Formation forms the core of two culminations along the axial trace.  The anticlinorium probably formed by differential dissolution of evaporites in the ‘Arab and Ht Formations and subsequent collapse of overlying rocks into the accommodation space.  The general geometry of the synclinorium and anticlinorium has been modified by numerous small-scale synclines, anticlines, domes, and basins.  The synclinorium formed over the easternmost limit of maximum dissolution of evaporites from the ‘Arab and Ht Formations, and the west flank of the Jilh Rīmān anticlinorium probably represents the east edge of the subsurface dissolution front in the ‘Arab and Ht Formations.  A synclinal structure occurs in the southwestern part of the quadrangle where the ‘Ulayyah Member of the anfah  Formation and the lower member of the Jubailah Formation are downwarped into the awţah Member of the anfah  Formation.  Although similar synclinal structures have been ascribed to meteorite impacts and karst dissolution of limestone, and these synclines have a superficial resemblance to salt piercement structures, the evidence from the synclines in the map area fits none of these possible origins.  The origin of these synclines remains an enigma.  Mineral resources identified in the map area consist of industrial minerals and oil and gas.  Limestone and dolomitic limestone suitable for manufacture of cement, a fluxing agent, or aggregate are present in the map area.  Gypsum and anhydrite are common in the map area, and these minerals can be used for wallboard, plaster, roof tile, or fillers in paper or paint.  Silica sand can be used for manufacture of silica-lime bricks, glass, concrete production, or foundry sand.  Oil and gas resources were not systematically evaluated for this mapping project, but along the dissolution edge of the ‘Arab and Ht Formations is a shut-in unnamed oil field of unknown dimension.  Potential hydrocarbon reservoirs may occur at depth in the map area in the Cambrian to Ordovician Sq and Qam Formations, in the Silurian to Ordovician Zarq’ and rah Formations, in clastic rocks of Devonian and Carboniferous age, in members of the Permian to Carboniferous ‘Unayzah Formation, in sandstone of the Jurassic Dhruma, anfah , and Jubailah Formations, in carbonate of the Jurassic ‘Arab Formation, and in Cretaceous clastic rocks.

Dini, S.M., Wallace, C.A., Halawani, M.A., Al-Sobhi, S.A., Kashghari, W.A., and Al-Ghamdi, A.S., 2010, Geologic map of the As Sulayyil quadrangle, sheet 20H, with Explanatory Notes, Kingdom of Saudi Arabia: Saudi Geological Survey Geologic map GM-141C, 42 figs., 1 pl.