Abstract

Carbonate slope sediments of the Sumeini Group mark the northeastern margin of the Arabian carbonate platform. Most slope sediments exposed in the Oman Mountains are of Jurassic or Cretaceous age. Older slope deposits at Jebel Sumeini indicate establishment of the Oman continental margin and subsidence accompanied by faulting in the Early Triassic (Dienerian), Middle Triassic (Ladinian) and possibly earlier (Permian).

Widespread Jurassic slope deposits indicate significant variations in declivity and facing direction of the continental margin slope in Oman. At Jebel Sumeini, a gullied bypass slope formed an apron along the NNW-trending slope, previously established as a Triassic block-faulted escarpment. In the Dibba Zone, a steep bypass slope formed along an inactive, NE-trending, transform lineament. Farther south near Wadi Qumayrah, a Middle to Late Jurassic base-of-slope apron containing megabreccias formed along a possible NNW-trending fault scarp. East of Wadi Qumayrah, an open-end embayment in the margin separated this area from coarse-grained slope apron deposits at Jebel Sham. Carbonate slope deposits at Saih Hatat indicate a complex history of subsidence culminating in Jurassic platform drowning and development of a long-lived submerged plateau bounded by gently inclined ramp slopes. This general geometry of the Oman passive continental margin was maintained through Jurassic and Early Cretaceous time.

Major changes in platform margin sedimentation occurred in the Late Cretaceous (Cenomanian to Coniacian), as recorded by the upper Mayhah Formation and overlying siliceous Qumayrah Formation. Thick Cenomanian(?) to Coniacian syn-orogenic megabreccias associated with pelagic sediments formed due to collapse of oversteepened slopes, possibly resulting from tectonic downbowing of the Oman margin toward a northward-dipping oceanic subduction zone. Later, the geometry of the passive continental margin may have had a significant effect on the emplacement of nappes and formation of the Sumeini culminations during the structural development of the Oman mountain belt.