Geologic Map of the Rumā Quadrangle Sheet 25I, with Explanatory Notes, Kingdom of Saudi Arabia

SGS-GM-145 C Abdullah M. Memesh, Saleh M. Dini, Abdullah O. Bamousa, Saeed A. Al-Amoudi, and Abdulrahman J. Al-Juaid
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The Rumā quadrangle (sheet 25I) is located in the east-central part of the Kingdom of Saudi Arabia north of Ar Riya city. The rocks consist of Mesozoic to Cenozoic sedimentary units and unconsolidated deposits of Quaternary age. Nearly 50 percent of the map area is covered by Quaternary eolian deposits. The main structure is the east-dipping Interior Homocline. It is interrupted in the southwest part of the quadrangle by Ath Thumāmah depression, a large structural element that was modified within the map area by dissolution-collapse features.
Phanerozoic sedimentary rocks that crop out within the Rumā quadrangle range in age from Late Jurassic to Pliocene(?); they were deposited in deltaic, strandline, supratidal, lagoonal, and shallow-marine environments. The youngest rocks are of continental and lacustrine origin. From a lithostratigraphic point of view, the portion of the Shaqrā’ group (Late Jurassic) exposed within the quadrangle consists of the ‘Arab Formation and the Hīt Formation. The ‘Arab and Hīt formations represent a cyclic deposition of carbonate and anhydrite, which is commonly hydrated to gypsum near the surface. Above the Shaqrā’ group, the Thumāmah Group (Late Jurassic(?) to Early Cretaceous) consists of the Silayy, Yamāmah, Buwaib, and Biyadh formations, in ascending order. The Silayy Formation is composed of argillaceous and bioclastic limestone, the Yamāmah Formation of bioclastic and argillaceous limestone, and the Buwaib Formation of interbedded sandstone, limestone, and siltstone. The Biyadh Formation is a crossbedded, medium-grained sandstone, with minor siltstone and shale. On top of the Ath Thumāmah Group is the Wasīa‘ Formation (Early to Late Cretaceous), which is composed of crossbedded, medium- and coarse-grained conglomeratic sandstone. The ‘Aruma Formation (late Cretaceous) is separated from the older rocks by a major unconformity. It is a richly fossiliferous rock unit composed of bioclastic limestone and calcareous claystone. The fossiliferous, nodular limestone of the Umm Radhamah Formation (early Paleocene(?) to early Eocene(?)) overlies the ‘Aruma Formation. Neogene deposits rest on older rocks topped by a major unconformity. The Ajfar Formation (Miocene(?)) is composed of sandstone, conglomeratic sandstone, and conglomerate at the base, and limestone, dolomitic limestone, and gypsiferous limestone above. The youngest rocks exposed in the quadrangle are Neogene conglomerate, sandstone, and limestone of the Umm ash Sha‘āl formation (Pliocene(?) to Pleistocene(?)).
The Silayy Formation, and to a lesser extent the Yamāmah and Buwaib formations, are disrupted by dissolution-collapse structures that formed when overlying units collapsed into voids left by dissolution of evaporite beds in the ‘Arab and Hīt formations. The Biyadh Formation forms a northeast-trending band of low hills, and a flat, poorly dissected plain drains southwest across these units. The Wasīa‘ and ‘Aruma formations form a northwest-southeast-trending escarpment (the ‘Aruma escarpment). The Ajfar Formation crops out only in the northeastern part of the map area. The Umm ash Sha‘āl formation (Pliocene(?)) forms prominent mesas along the Ath Thumāmah depression.
Quaternary deposits of probable Pleistocene to Holocene age overlie parts of all rock units in the quadrangle, and throughout a broad area east of the ‘Aruma escarpment they also cover Paleogene formations. Two generations of alluvial gravel mantle bedrock in the eastern part of the map area and in the Ath Thumāmah depression. Alluvium corresponds with modern drainages, and khabras occur in the western part of the quadrangle. Extensive eolian dune systems, such as ‘Irq Banbān, occupy the Ath Thumāmah depression in the southwest part of the map area, and elongated dune systems such as ‘ Irq Abā ath Thimām, ‘Irq al umrānī, and ‘Irq ar Ruwaykib occupy the northeast part of the map area. Active and inactive sand sheets cover an integrated drainage system, developed on the oldest alluvial gravel, that drained toward the northeast. The domal structure of Silayy reflects an elongate anticlinorium-synclinorium pair in the southwestern part of the quadrangle where the Yamāmah Formation and the Buwaib Formation are downwarped into the Silayy Formation.
Mineral resources in the map area consist of industrial minerals, oil, and gas. Limestone and dolomitic limestone are suitable for manufacture of cement and lime production, or use as fluxing agents, aggregate, and building stone. Gypsum is common in the map area and can be used for wallboard, plaster, roof tile, or filler in paper or paint. Silica sand can be used for manufacture of glass and bricks, as foundry sand, and to produce concrete. Oil and gas reservoirs are exploited in the Rumā quadrangle (sheet 25I); exploration is active east of the map area.

Memesh, A.M., Dini, S.M., Bamousa, A.O., Al-Amoudi, S.A., and Al-Juaid, A.J., 2017,  Geologic map of Rumāh quadrangle, sheet 25I, with explanatory notes, Kingdom of Saudi Arabia: Saudi Geological Survey Geoscience Map GM-145 C, 45 p, 32 fig., 1 pl.