THE AGE OF THE HAWASINA AND OTHER PROBLEMS OF OMAN MOUNTAIN GEOLOGY

It is now popular to interpret the geological evolution of the Oman Mountains within the framework of the plate-tectonic paradigm. Current interpretations hold that the deep-water Hawasina sediments, huge exotic limestone blocks and the overlying Semail serpentinite originated in a Tethyan Ocean to the north of the present-day mountains in Permian to Senonian time. In the Late Cretaceous, continental collision telescoped these oceanic rocks and thrust them southward over the autochthonous carbonate platform where they are now exposed. The origin of turbidites, debris flows and exotic blocks in the Hawasina complex is interpreted to have been a north-facing continental margin that bordered the Tethys Ocean. However, Permian to Cenomanian limestones that are exposed all along the axis of the Oman Mountains show no sign of penecontemporaneous rifting and erosion. If the supposed shelfedge ever existed, it is now hidden from view. There are some serious questions that counter the popular model. The long duration of deepwater deposition is predicated upon the assumption that cleanly-reworked shallow-water benthonic fauna in the Hawasina turbidites were derived penecontemporaneously from shelf sediments and are therefore age indicative. This is a subjective judgment that can be challenged, because clean, recycled fossils from older consolidated formations are common constituents of deep-water turbidites worldwide. Also, the dissolution of contemporaneous pelagic microfauna below the Carbonate Compensation Depth is supported by present-day analogues. Where faunal inversions occur in a bedded succession, the palaeontological assumptions force a structural interpretation of repetition by thrusting. However, in the Oman Mountains, massive thrusts interpreted to have horizontal displacements of over 100 km from a supposed Tethyan ocean basin appear to lack the expected disruption in the field. For example, in Wadi Mi’aidin, supposedly Jurassic sandstone turbidites of the Guwayza Formation (part of the Hawasina) appear to rest on the autochthonous Coniacian Muti Formation without major structural disturbance. This lack of deformation across what is interpreted to be a major thrust plane casts serious doubt upon dating criteria and demands detailed structural analyses to resolve both structural and palaeontological assumptions.