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The Hajar Supergroup (Middle Permian-Lower Cretaceous) of northeastern Oman records rifting and development of a passive margin along the edge of the Arabian platform facing Neo-Tethys. The Jurassic and Lower Cretaceous part, comprising the Sahtan, Kahmah, and Wasia groups, was deposited during the maximum extent of the broad epicontinental sea landward of this margin. These limestone units reach a total of 1500 m in thickness and they correlate with the principal hydrocarbon reservoirs of the Arabian Peninsula.
The trace of the Jurassic and Cretaceous margin in northeastern Oman followed a zigzag series of rift segments, resulting in promontories and reentrants that changed in position through time in response to the configuration and differential motion of underlying rift blocks in an overall extensional regime. Synsedimentary normal faulting with 600 m of throw occurred locally during the Middle Jurassic, whereas during the Late Jurassic, the margin was subjected to erosion from variable uplift of up to 300 m before subsiding to below storm wave base. This uplift may have been caused by compression from oceanic crust that obducted along the southeastern side of the platform. Subsequently, Lower Cretaceous strata document varying rates of block subsidence.
The Lower Cretaceous succession in the central Oman Mountains and adjacent subsurface began with regional drowning around the Jurassic-Cretaceous boundary. The succession in the east (Saih Hatat) records a single regressive sequence, culminating in the progradation of the shallow-water carbonate platform by the Cenomanian. However, the succession in the west (Jebel Akhdar and interior) is dominated by shallow-water carbonate facies, but punctuated by a second regional drowning in the late Aptian. A third, Late Cretaceous drowning terminated deposition of the Wasia Group in the Turonian and was caused by convergence of oceanic crust and foreland basin formation.
Although previously ascribed to eustatic sea level rises, the two Early Cretaceous drowning events can be explained as consequences of the tectonic behavior of the platform. The earlier event (Rayda Formation) occurred when first uplift tilted the eastern side of the platform and induced a relative sea level stillstand and widespread evaporative conditions in the interior (Hith anhydrite), and then the eastern side subsided rapidly to below storm wave base, forming a broad embayment starved of periplatform sediment. By contrast, the second drowning (Nahr Umr Formation) was due to detrital influx generated by uplift on the west side of the platform. Both tectonic episodes led to a reduction in the calcium carbonate generating capacity of the epicontinental sea, causing shallow-water fa ies to be abruptly overlain by deeper water facies. The effects from these two drownings eventually diminished, and shallow-water environments reestablished themselves. The overall succession appears independent of eustatic sea level change.
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