The Maastrichtian transgression onto the northwestern flank of the Proto-Oman Mountains: sequences of rudist-bearing beach to open shelf facies

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Abstract The Maastrichtian Qahlah and Simsima Formations, the first autochthonous deposits on the obducted Oman ophiolite complex, crop out in a gridle of jebels around the Oman Mountains. Those around the NW flank, between Al Dhayd (Sharjah) and Al Ayn-Buraymi (Dubai-Oman) are documented to illustrate: (1) an exposed regional reference section for the widespread Simsima Formation, at Jebel Faiyah (Sharjah); (2) the marked transgressive nature of the sequence onto the obducted complex; and (3) the rich rudist faunas there, associated with various beach to shelf facies, and their palaeoecological and palaeobiogeographical implications. ‘Basement’ (serpentinites in the north and sedimentary nappes in the south) is overlain by locally developed laterites and the dominantly siliciclastic, marginal continental to marine deposits of the Qahlah Formation, followed, with local erosion and overlap, by the carbonate-dominated Simsima Formation. The latter continues the deepening trend, passing from basal rudist-rich inshore facies to open shelf foraminiferal packstones to wackestones. The whole sequence onlaps eastwards onto the obducted complex. Its truncated top is overlain by Palaeogene slope deposits. In the north (Jebel Faiyah area) the Maastrichtian sequence has an open, relatively high energy aspect. A shelf margin of Simsima Formation rudist limestones briefly developed in the early Maastrichtian around Qarn Murrah. Further transgression to the southeast drowned this and, by ‘mid’-Maastrichtian times, a bouldery beach conglomerate of the Qahlah Formation, with in situ Durania , was forming at Jebel Faiyah itself, along a cliff-backed shoreline with sandy embayments. Over this came tidally influenced rudist-rich facies and foraminiferal limestones of the Simsima Formation. In the south (Jebel Huwayyah area), in contrast, marginal fan-delta gravels and sandstones, of early Maastrichtian age, were followed by more restricted shallow marine silty marls and limestones with Loftusia , rudists and corals, of early to mid-Maastrichian age. These Qahlah deposits were overlapped by a relatively rudist-poor Simsima Formation sequence. The obducted complex was thus strongly emergent at the start of the Maastrichtian, and the Aruma Sea transgressed onto its western flank, as the ophiolite subsided through that stage, forming a relatively open shelf in the north, becoming broader and more restricted further south. The rudist assemblages found on this shelf are similar to those of other open Tethyan shelves in the eastern and central Mediterranean and Middle East.