Sumeini group, Oman—evolution of a Mesozoic carbonate slope on a South Tethyan continental margin

Abstract

Deposition of the mostly Mesozoic Sumeini Group occurred on the slope between the shallow-marine, Arabian carbonate platform and the deep-oceanic Hawasina Basin or South Tethys Sea. These strata record the evolution of the northeast Arabian continental margin from Permian(?) and Triassic rifting to ocean basin closing with the Late Cretaceous obduction of the Semail Ophiolite. Platform margin type and its evolution can be inferred from exposures of these slope sediments at Jebel Sumeini in the Western Oman Mountains. In the Lower Permian base of the sequence, bedded, spicular limestones suggest deep-marine conditions. This is followed by an enigmatic, thick sequence of bedded dolomite. In the Early to Middle Triassic (?), small carbonate submarine fans formed along a distally steepened slope. A subsequent interval of terrigenous, clastic sedimentation was followed by development of a steep escarpment margin during Ladinian to Norian time, with coralgal reefs at the shelf break and a debris apron at the base of the scarp. The Late Triassic is marked by another influx of terrigenous sediment which accompanied widespread emergence of the platform. In the Jurassic, a thick apron of thin-bedded limestone formed on the slope; this apron was cut by channelized gullies that now include blocks of older reefal material and oolitic sands. A well-documented submergence of the platform during the Tithonian and Early Cretaceous led to the deposition of radiolarian cherts on the slope and basin. Upper Lower Cretaceous megabreccias, up to > 200 m thick, suggest an interval of localized slope instability and/or tectonism. Continued deposition of bedded limestone and marl with lesser calcirudite marks the top of stratigraphic sequence at Jebel Sumeini. The Early to Middle Triassic (?) carbonate submarine fan deposits occur as discrete, lenticular sequences of bedded calciturbidite, modified grain flow and debris flow deposits which were derived from mostly slope sources. The Ladinian to Norian base-of-slope debris apron deposits were derived from both slope and shallow-marine sources (including reefs) and appear to form a laterally extensive but narrow belt of thick lenticular to sheet-like beds of debris which accumulated over a long interval of time. Coarse limestones in the Jurassic gullied slope deposits are distinct in that they occur randomly as individual channelized beds in a sequence dominated by thin-bedded lime mudstones. Thick Lower Cretaceous megabreccias are distinguished by their great lateral distribution and general lack of matrix.