TheNubian Sandstone is a variety ofsedimentary rock deposited on thePrecambrianbasement in the easternSahara, north-east Africa andArabian Peninsula. It consists of continentalsandstone with thin beds ofmarinelimestones, andmarls. The Nubian Sandstone was deposited between the LowerPaleozoic and UpperCretaceous, with marine beds dating from theCarboniferous toLower Cretaceous.[1]
The Nubian Sandstone ranges in age from the Cambrian to Upper Cretaceous eras. Positioning of thepaleoequator andpaleolatitude at 20° S was derived frompaleomagnetic data showing the Nubian was originally deposited in the paleoequatorial to subequatorial zone. These paleomagnetic results corroborated previous studies suggesting that north Africa did not shift in latitude from 210 to 110 million years ago and extended this period to 85 million years. Nubian Sandstone was deposited under atropical to subtropical climate and formed under a variety of continental conditions, includingeolian merging intermittently into shallow marine.[2]
The Nubian Sandstone complex has a thickness varying from under 500 m to over 3000 m, resting on thePrecambrianbasement. This is complicated by various structuralfaults andfold axes traversing the region in a north-eastern direction. Maximum development occurs in the Ain Dalla basin, a downthrown structural block south-west of theBahariya oasis. Basement features present a dominant control on the complex's structural and sedimentological form. Despite many structural complications, the Nubian Sandstone likely constitutes a single hydrogeological system west of theSuez Gulf. To the east, on theSinai Peninsula, a second system might exist with some connection to the primary western system in the north. The main western system, extending intoLibya andSudan, consists of a multi-layeredartesian basin where massive groundwater reserves accumulated, principally duringpluvials of theQuaternary. Locally,carbonate rocks overlying complexkarst features and are recharged from the underlying majoraquifer.[3]Fluvial and structural interpretations from 2007 show the desert in western Egypt was induced by fluvial action, including recently mappedalluvial fans. In central areas,braided channels are spatially aligned to a north-east structural trend, suggesting preferential water flow paths. Alluvial fans and structurally enclosed channels with gentle slopes and optimal recharge conditions between 1 and 5%, indicating high groundwater potential.Synthetic Aperture Radar (SAR) interpretations correlated with anomalies from groundwater in 383 wells, suggest a connection between the spatial organization of fluvial and structural features with low-salinity groundwater, which exists adjacent to alluvial fans and the south-west reaches of structurally enclosed channels. Wells in the vicinity of structures contained low-salinity water.[4]
Nubian Sandstone exposures in sub-humid, semi-arid, andarid conditions producesoil that is red and sandy but very different in other aspects. Only in sub-humid zones do these soils contain a moderately developed profile, including a texturalB horizon lacking soluble salts andcarbonates. In semi-arid and arid zones, profile differentiation is either weak or does not exist. In arid zones soils are shallow and contain carbonates, and soluble salts, includinggypsum. The soleclay mineral common to all Nubian sandstone parent materials iskaolinite, which is the major clay mineral in sub-humid zone soil. In semi-arid soilssmectite is a second major clay component. In arid zones small amounts of smectite andpalygorskite accompany kaolinite. It is likely that smectite and palygorskite are products ofpedogenic neoformation.Aeolian material was probably introduced into thesilt and fine sand fractions from semi-arid and arid soils. It is also possible that some contamination of clay fractions occurred.[5]
When in contact with Upper Cretaceouslimestone, Nubian Sandstone underlies the latter conformably. InLebanon,Anti-Lebanon andHermon it is underlain byJurassic limestone. Its upper strata is likely from the Lower or Middle Cretaceous age. However, Jurassic limestone is absent in southern areas. In WesternSinai, Nubian Sandstone rests on Carboniferous limestone, and by theDead Sea on Cambrian limestone: atPetra and other locations it rests unconformably oncrystalline rocks. While age calculation for the Nubian Sandstone is relatively simple in Lebanon, Anti-Lebanon and Hermon, it is much more complicated in Western Sinai and the Dead Sea area. Since sandstone is assumed to form more rapidly than other rocks it is difficult to conceive that the 2,000 feet of sandstone in the southeastern Dead Sea was in the formative process from the Cambrian to the Cretaceous.
Nubian Sandstone is most commonly brown or reddish, but in places it shows a much wider variety of color. The ancient temples and tombs in Petra were carved from this rock. In certain places it is extremelyfriable, and in others compact and hard. Sand in the Arabian deserts was primarily derived from it, carried by prevailing western winds. Where it is covered by a sheet of eruptive rock (charrah), it is protected fromerosion. Nubian Sandstone frequently includes strata of clay andshale and thin seams ofcoal orlignite. This indicates that it was deposited in seas that were relatively shallow at the time.
The termNubian Sandstone was first introduced to the Egyptianstratigraphy by Joseph Rüssegger in 1837, who used the term "Sandstein von Nubien" to designate nonfossiliferous sandstone sections of Paleozoic or Mesozoic age. Rüssegger followed and studied this series of sandstone formations from theSudan,Egypt,Libya, and Arabia Petrsea (northeastern Arabia).[6]