Micropalaeontology and palaeoenvironments of the Miocene Wadi Waqb carbonate of the northern Saudi Arabian Red Sea

Abstract

ABSTRACTThe Saudi Arabian Red Sea stratigraphy consists of a variety of lithologies that range from evaporites, deep- and shallow-marine siliciclastics and carbonates, biostratigraphically constrained to range from the Late Cretaceous, Campanian, to Late Pliocene. The succession consists of pre-rift Mesozoic and Palaeogene sediments, and syn-rift and post-rift late Palaeogene and Neogene sediments. Three main episodes of shallow-marine carbonate deposition can be determined, including those of the earliest Early Miocene Musayr Formation of the Tayran Group later Early Miocene Wadi Waqb Member of the Jabal Kibrit Formation and of the Pliocene Badr Formation of the Lisan Group. The Midyan area of the northern Red Sea offers a unique window into the Cretaceous and Miocene succession that is otherwise only present in the deep subsurface. The sediments are of hydrocarbon interest because of the presence of source rocks, siliciclastic and carbonate reservoirs. The Wadi Waqb reservoir is hosted within the Wadi Waqb Member of the Jabal Kibrit Formation, and is of latest Early Miocene to possibly earliest Middle Miocene age. It is considered to have formed a fringing reef complex that formed a steep, fault-influenced margin to a narrow platform, similar to Recent coralgal reefs of the Red Sea.The Wadi Waqb Member is exposed on the east and west flanks of the Ifal Plain. The bioclasts are considered to have traveled as a submarine carbonate debris flow 25 km from their presumed source to the east and possibly the west, and consist mostly of rhodoliths, echinoid and coral fragments together with the benthonic larger foraminifera Operculinella venosa, Operculina complanata, Heterostegina depressa and Borelis melo. The planktonic foraminifera include species of Globigerina, Globigerinoides and Praeorbulina; no specimens of the Middle Miocene planktonic foraminiferal genus Orbulina have yet been encountered in the thin sections. The presence of Borelis melo melo, and of B. melo curdica within the region, indicates a latest Early Miocene age. No specimens of the age-equivalent larger benthonic foraminiferal genera Miogypsina or Lepidocyclina have been observed, and this is consistent with evidence from the Wadi Waqb equivalent carbonates elsewhere in the Red Sea and Gulf of Suez.In the east, the Wadi Waqb is represented by discontinuous fringing rhodolith and coral reefs that are welded to steep cliffs of granitic basement. In Wadi Waqb, located in hills that form the western margin to the Ifal Plain, the Wadi Waqb carbonates consist of packstones containing autochthonous planktonic foraminifera and abundant shallow-marine microfossils that are considered to have been derived from the basement-founded rhodolith and coral reefs in the east. The Wadi Waqb reservoir is located beneath the central part of the Ifal Plain, approximately midway down a ramp between the in situ rhodolith-coral reefs and the mixed allochthonous and autochthonous facies at Wadi Waqb. The reservoir contains biofacies similar to those exposed in Wadi Waqb, and indicative of a deep-marine environment, in excess of 50 m water depth. The Wadi Waqb carbonates display sedimentological and petrographic features that closely resemble those described from stratigraphically equivalent carbonates from the localities along the west coast of the Gulf of Suez, including Abu Shaar, where three depositional facies have been defined. It is apparent that these shallow-marine carbonates were established along the west and east rift margins of the Red Sea-Gulf of Suez rift complex prior to their dislocation during the Late Miocene and Pliocene by the left-lateral Aqaba faulting.