Geologic Map of the Jibal at Tubayq Quadrangle, Sheet 29B with Explanatory Notes, Kingdom of Saudi Arabia

SGS-GM-133C Chester A. Wallace, Saleh M. Dini, and Anwar A. Al-Farasani
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ABSTRACT
The Jibal at Tubayq quadrangle (29B), in the northwestern part of the Kingdom of Saudi Arabia about 900 km north of Jeddah, is located between lat 29o00'N. and 30o00'N. and long 36°00'E. and 37°30'E.  The quadrangle contains Paleozoic, upper Mesozoic, and Tertiary sedimentary rocks that are offset by dip-slip faults; nonconsolidated Quaternary deposits locally form mantles over older rock units.  Sandstone, shale, and limestone of Paleozoic age dip gently to the northeast.  In the central part of the quadrangle, isolated exposures of Cretaceous sandstone and Tertiary phosphorite-bearing rocks are preserved on downthrown fault blocks.  The Phanerozoic sedimentary rocks range in age from Ordovician to Eocene and were deposited in glacial, glaciomarine, shallow-marine, deltaic, strandline, and continental environments.  Ordovician and Silurian rocks are exposed mainly in the southwestern part of the quadrangle.  The Ordovician Qasim Formation (Tayma Group) is composed mainly of siltstone and fine-grained sandstone that were deposited in a shallow-marine environment.  The Ordovician and Silurian Sarah Formation (Tabuk Group) is composed of sandstone, siltstone, and conglomerate of glacial origin that were deposited in a marine proglacial environment.  Late Silurian siltstone and sandstone of the Sharawra Formation (Qalibah Group), widely exposed in the southwestern part of the map area, were deposited in a shallow-marine environment.  The Early Devonian Tawil Formation (Huj Group) disconformably overlies the Sharawra.  The Tawil was deposited in a deltaic and strandline environment.  Shallow-marine shale, siltstone, and limestone of the Early Devonian Jauf Formation (Huj Group) overlie the Tawil.  During diagenetic alteration, extensive lenses and beds of authigenic kaolin replaced large volumes of quartzose sandstone of the Tufayhah Member of the Tawil Formation, and authigenic hematite or pyrite replaced quartzose sandstone beds of the Juraniyat Member of the Tawil Formation.  The Zallum formation, a deltaic and continental sandstone unit in the Late Cretaceous Aruma Group, unconformably overlies rocks of the Jauf and Tawil Formations.  Unconformably overlying this Cretaceous sandstone are Paleocene shallow-marine sandstone, shale, chert, and phosphorite of the Jalamid formation of the Turayf group.  Small, isolated exposures of basalt of probable Miocene age occur in the southwestern corner of the Jibal at Tubayq quadrangle.  Alluvial deposits of Quaternary age occur in modern drainages and cut older duricrust and older chert-bearing gravel deposits.  Isolated tracts of eolian sand mantle bedrock in parts of the map area.  Faults are common in the Jibal at Tubayq quadrangle and most trend north-northwest.  The prominent Wadi al Fihah graben, in the east-central part of the quadrangle, extends southward into the adjoining Tabuk quadrangle (28B) and northward into Jordan.  The Jibal ash Shuhayba fault bounds the graben on the east, and the Sanam fault bounds the graben on the west.  Near the eastern border of the quadrangle the Widyan ash Shuttiyat fault truncates Devonian rock units, and in the central part of the map area the Jabal Anz fault truncates Silurian and Devonian rock units.  In the southwestern part of the map area the prominent Wadi Dabl fault corresponds to the “Great Jada’ah dike,” which is a prominent magnetic anomaly that represents a buried basaltic dike.  Several small doubly plunging folds trend nearly parallel to the Wadi Dabl fault.  West of the Wadi Dabl fault, the Qa al Aliqah fault offsets Silurian rocks, the Ayn al Kubraytiyah fault separates Silurian units from Ordovician rocks, and the Jibal al Badiah fault displaced Ordovician rock units.  Tertiary basalt erupted near the fracture plane of the Jibal al Badiah fault.  Numerous unnamed faults of small separation occur throughout the map area.  Additional linear features may repre sent joints or faults that show little offset.  Extensional slip in the Wadi al Fihah graben postdates lithification of the Jalamid formation, and the graben and other dip-slip faults probably formed during Miocene and younger extensional faulting that affected the Arabian plate.  Mineral resources in the area of the Jibal at Tubayq quadrangle are mainly phosphorite, oil and gas, and metallic minerals.  Phosphorite occurs in the Jalamid formation, and an inferred resource of 23,000,000 tons of friable, sandy phosphate at a concentration of about 17 percent P2O5 has been identified.  The potential for the occurrence of oil and gas in the map area remains poor, but the Qusaiba Formation is known to have generated oil and gas during Late Devonian time, so stratigraphic or structural closures could have trapped hydrocarbons.  A poorly known potential for the occurrence of low-grade stratabound copper, lead, zinc, or silver deposits exists in lower Paleozoic rock units. Kaolin replacement bodies are impure, and kaolin is not likely to be an exploitable resource.  Resource potential for silica sand, gravel, limestone, and dolomite was not evaluated in this study.

Wallace, C.A., Dini, S.M., and Al-Farasani, A.A., 2001, Geologic Map of the Jibal at Tubayq Quadrangle, Sheet 29B with Explanatory Notes, Kingdom of Saudi Arabia: Saudi Geological Survey Geologic Map GM-133C, scale 1:250,000, 36 p., 47 figs., 1 table, 1 pl.