Stratigraphic evolution and depositional system of lower cretaceous Qishn formation, Dhofar, Oman

The Barremian(?)-Aptian Qishn Formation of Dhofar (Oman) is represented by eastward (landward) thinning strata that onlap Marbat palaeohigh basin margin. The formation includes Shabon, Hinna and Hasheer members, in ascending order. The Shabon Member consists of massive and cross-bedded, locally conglomeratic arkose and quartzarenite. The Hinna Member has a lower part of rhythmically arranged bioclastic wackestones/mudstones that grade to fine crystalline dolomudstone and an upper part of cyclic units of nodular, marly, rudistic floatstone/ rudstone lithofacies capped by normally grading, intraclastic, bioclastic packstone/grainstone lithofacies. Mudcracks, teepee structures and microbial laminations occur in the lower part. The member terminates with 10-m-thick discoidal orbitolinid-and rudist-bearing marls envisaged as the deepest facies of the formation. The Hasheer Member consists of partly dolomitized strata of bioclastic packstones, grainstones and rudstones. Cross-bedding and erosional surfaces are present. The overall depositional system of the formation evolved from high-energy clasticdominated, marginal marine environment through tidal carbonate mudflat and lagoonal setting to high-energy carbonate sand shoals. The overall stratigraphic arrangement of the formation suggests a third-order transgressive-regressive system with superimposed fourth-and fifth-order cycles. Shabon and Hinna members represent lowstand to transgressive systems tracts whereas Hasheer Member represents highstand to falling stage systems tracts. Terminal platform-wide exposure resulted in an unconformity at the Qishn-Kharfot contact.