The age of the so-called Sakaka Sandstone exposed in the al-Jawf area, northern Saudi Arabia, has in the past been disputed. It is assigned here to the Middle -Lute Devonian and Middle Cretaceous. This age has been established from correlations of sugacesubsugace sections utilising palynological and lithological data, and also from the regional geological framework. The sequence is divided into two units. The lower unit, composed of non-marine clastic sediments. is Middle-Lute Devonian. The upper unit, consisting of clastic and mixdjinec h i c and calcareous shoreline-shallow marine deposits, is considered to be part of the Middle Cretaceous Wasia Formation.The name "Sakaka Formation" is now restricted to the lower unit of the Middle -Upper Devonian sequence. This Formation forms a well-exposed and lithologically-distinct mappable unit. The type section of the Formation can be designatedfrom a traverse in the al-Jawf area, where a thickness of 215.5 m is exposed. The Sakaka Formation conformably overlies the Lower Devonian Jauf Formation, and unconfonnably underlies the Middle Cretaceous Wasia Formation. At depth, in the subsu$ace. the Sakaka Formation underlies conformably the Lower Carboniferous Benvath Formation.Reliable relative age-dating of the Sakaka Formation has contributed to investigations of both the histoy of the Hail Arch, and the economic prospects of the area. This Arch extends throughout the al-Jawf area, and is probably of ReCambrian age; it was rejuvenated in pre-Middle Devonian and pre-Middle Cretaceous times. The Arch separates the Widyan Basin to the east from the Tabuk Basin to the west. The former basin is promising in terms of petroleum exploration owing to its great possible thickness. and the presence of favourable lateral facies changes. and potential source and reservoir rocks; moreoveE tectonic activity has resulted in the generation of many traps, including stratigraphic traps. The Sakaka Formation and the overlying Berwnth Formation could serve as potential reservoir rocks. capped unconformably by .fine, argillaceous rocks of the Upper Carboniferous -Lower F'ermian Unayzah Formation.* "al-Jawf' refers here to both the town of this name, recentb known as Dumat al-Jandal, and the surrounding region. The Jauf Formation takes its namefrom this town, despite the dagerent transliterated spelling.
Eight clastic and carbonate lithofacies of Lower and Middle Jurassic age (the Marrat and Dhruma Formations respectively) have been identified. These lithofacies are mainly shales, shales‐and‐siltstones, siltstones‐and‐shales, sandstones, argillaceous limestones, calcarenitic limestones, calcarenites and dolomites. Intraclasts, pellets, oolites, gypsum, algae, and coral reefs were also found to be dominant among these lithofacies. Furthermore, these beds contained either restricted or diversifed biota, with a few sedimentary structures such as lamination, cross‐bedding and common bioturbation. Thus, it is presumed that the Lower and Middle Jurassic rocks in Central Saudi Arabia were deposited in very shallow (i.e. tidal flat and lagoon), shallow‐neritic, and deep‐marine conditions of the Tethys Sea. During Toarcian (Lower Jurassic) time, the Tethys Sea extended towards Arabia, forming an arcuate shoreline around the Pre‐Jurassic sediments. The greater part of the Arabian Peninsula was under very shallow marine (Tethys) water as is shown by the presence of tidal‐flat and lagoon deposits in northern Oman, Rub al‐Khali, parts of the Aden Protectorate and Yemen, central, eastern and NNW Saudi Arabia, the northern edge of Sinai, NW Jordan, Lebanon, central and northern Syria, and west, central and eastern Iraq and Iran (Fig. 5). On the other hand, east and north of this area, very shallow marine deposits overlaid the shallow‐neritic Tethys deposits. Further expansion of the Tethys Sea transgression occurred during Bajocian‐Bathonian (Middle Jurassic) time, when major parts of the Arabian Peninsula and its neighbouring regions were submerged below shallow‐neritic and deepmarine Tethys conditions (Fig. 6). During this time, shallow‐nentic Tethys conditions extended to include the Oman Mountains, as far west as western Rub al‐Khali, north to include Central Saudi Arabia, running through parts of its northern area and passing under the Saudi Arabia‐Iraq Neutral Zone and reaching SW Iraq, turning west to include part of the area of NW Arabia and reaching NW Jordan. Northern Oman, most of central Rub al‐Khali, eastern Saudi Arabia, western Iran, central and eastern Iraq, and eastern and northern Jordan, were covered by deepmarine Tethys waters (F&. 6). However, simultaneously, SW Arabia, Yemen and the Aden Protectorate display a marked continuous sedimentation of continental and neritic conditions, as neritic deposits narrow toward the north.
The Hanifa Carbonate lithofacies (Upper Jurassic) started to develop during Early Kimmeridgian time, when an extensive area in central Arabia was covered by relatively deep marine waters. These rock units were formed along a shoreline, resulting in sucessive lagoonal and tidal‐flat belts, the deposition of pelleted and laminated calcilutites and gypsum, and a restricted fauna. The open shelf lay east of the lagoon zone, accounting for the development of oolites, coral reef and calcarenites, and a diversified fauna on the deeper shelf. At its maximum extent, the “Hanifa Sea” reached west nearly as far as the city of Riyadh in central Saudia Arabia, and extended NE and east, covering Kuwait, Iraq, Oman and Iran. South of Riyadh, the Hanifa lithofacies may be traced towards the SE over the Rub‐al‐Khali area, and SW over Yemen and the Aden Protectorates. North and NW of Riyadh, these same Upper Jurassic shallow‐marine rock units developed over Jordan, Syria, Lebanon, Palestine and northern Sinai, and most probably extended over northern Africa. Although the sediments forming the lower part of the Hanifa suggest that during Early Kimmeridgian tima a transgressive phase began, and most likely continued during part of the Middle Kimmeridgian, a regressive stage ensued towards the end of th Kimmeridgian, when the upper part of the Hanifa was deposited.
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