MIDDLE JURASSIC NONMARINE SILICICLASTIC FACIES IN SOUTHERN CENTRAL SAUDI-ARABIA

Middle Jurassic deposits of Saudi Arabia show distinct lateral fades variation, where the carbonates in the north are replaced by siliciclastic rocks in the south. In southern central Saudi Arabia, the Middle Jurassic deposits are composed exclusively of siliciclastic rocks, mainly sandstone with minor shale and siltstone, and rare conglomerate. Five stratigraphical sections were measured and studied along the southern Tuwaiq escarpments, where these rocks are well-exposed. A fluvial depositional environment is proposed here for the Middle Jurassic rocks in southern central Saudi Arabia based on sedimentological and petrological evidence. Eight different lithofacies were recognized in these deposits and each is described and interpreted. Cyclic sedimentation is prominent in all the sections studied; the most frequent transition is from medium-coarse pebbly sand to fine sands/silts/muds indicating a transition from sand flat or channel bar deposition, in which the upper flow regime dominates, to overbank deposition, where the lower flow regime prevails. Sandy bedform and downstream-accretion macroforms are the most common fluvial architectural elements. A braided stream system is the most appropriate model for these rocks, but the geomorphology of the streams was not constant in either space or time. However, the streams were braided most of the time, especially in the vicinity of the upstream source area in the southern highlands, and they changed their morphology gradually northward to become more sinuous, but never reached the stage where they become true meandering rivers. The change in stream morphology affected the distribution of fluviual fades and resulted in the presence of a proximal fluvial fades dominating in the south and a distal fades prevailing in the north. The streams flowed from south to north with little or no contribution from east-flowing tributaries. At this time, the Central Arabian Arch was a negative, submerged structural feature subsiding under the accumulating shelf carbonate sediments, whereas the Hadramaut and Mukalla Arches were positive features controlling the sedimentation pattern in southern central Arabia. Although the Middle Jurassic rocks, both in the north and in the south, form a slightly diachronous, contiguous sedimentary body, they differ from each other significantly. Therefore, it is recommended here that the Middle Jurassic siliciclastic facies in southern central Saudi Arabia be separated from the Dhruma Formation and be recognized as a new stratigraphical unit with formational rank.