The sedimentary record of ophiolite obduction in North Oman

The Late Cretaceous emplacement of the Semail ophiolite and underlying thrust sheets onto the Arabian continental margin is well constrained by geochronological data. The stratigraphy and the development of the foreland basin in front of the advancing nappes is still poorly understood. This study aims to unravel the tectonostratigraphy of the foreland basin and link this to different stages of the nappe emplacement.

Obduction is associated with a major regional Turonian (92Ma) unconformity that ended the middle Cretaceous shallow water carbonate deposition on the passive margin. Locally this is related to the development of a foreland bulge in front of the southward advancing nappes. It caused collapse and erosional recession of the platform margin; the platform top was subaerially exposed and was incised by fluvial valley systems some 150 m deep.

Along the collapsed margin, a sedimentary mélange formed with re-deposited platform sediments and blocks. These were later incorporated into the thrust complex and returned tectonically onto the margin. The subaerial unconformity on the exposed carbonate platform was onlapped during the initial phase of foredeep development by a thin (150 m thick 150km wide) transgressive carbonate ramp as it subsided into a starved foredeep. Further forebulge onlap was by fine-grained coastal clastics that were sourced laterally from local uplift of the Huqf Basement along the eastern margin of the Arabian Plate.

Transgression ended in the Santonian (85Ma) and is followed by some 250 km northward progradation of a mud-prone delta complex of more than 1 km thick into the southeastern part of the foredeep. This deltaic wedge has been incised by deep canyons and slump scars suggesting slope collapse and sediment by-pass during the Early Campanian (83Ma). The eroded slope is onlapped by a sequence of laterally-derived siliciclastic turbidite siltstones and sands which onlap the nappes to the north demonstrating that nappe emplacement ended in the early Campanian. The clastics are sourced from exposed Upper Paleozoic clastics in the Huqf area. There is very little detritus from the orogen.

The Late Campanian to Maastrichtian deposition in the foredeep consists of hemipelagic chalks and marls which can be more than 1300 m thick. Influx of detrital sediments from the orogen is restricted to a strip just a few kilometers wide in front of the thrusts.  This sequence is affected by a latest Campanian tectonic uplift associated with incisions up to 150 m deep filled with redeposited hemipelagic carbonates. This event may be related to inversion in eastern Oman coinciding with slab break-off (ca 75Ma) and exhumation of the subducted continental margin in the northeast.

The foredeep along the ophiolite obduction complex was a persistently underfilled basin:  filled by hemipelagic carbonates and local clastic detritus from the forebulge. Lack of sediment input from the orogen suggest that this was mostly subaqueous. Limited uplift may be related the high density of the ophiolite slab and could be a general feature of obduction orogens. Erosion by dissolution could help explain the lack of sediment input from both the forebulge and the orogen.