H. Hugh Wilson*
*formerly Chief Geologist, Petroleum Development Oman Ltd.
current address: 5929 Talbott Rd, Lothian, Maryland 20711, USA.
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 shelf-edge ever existed, it is now hidden from view.
There are some serious questions that counter the popular model. The long duration of deep-water 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 Miaidin, 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.
In the Al Ajal area on the north flank of the Oman Mountains, intra-volcanic sediments resting below the Semail Ophiolite, which have previously been dated as Triassic, have yielded Senonian Globotruncana. This also raises doubts about dating based upon reworked fauna in deep-water turbidites.
The tectofacies of the Hawasina sediments with ubiquitous turbidites and debris flows are typical of thick and rapidly deposited sediments in a tectonically active environment. However, the Hawasina succession is relatively thin for the assumed time span, and the Permian to Cenomanian autochthonous succession of monotonous platform carbonates is indicative of structural quiescence. On the other hand, regional extension and massive block faulting accompanied by rapid deep-water sedimentation typifies the Senonian depositional environment, which would be in concert with Hawasina tectofacies.
The huge Semail igneous mass is a layered rock sequence with dip slopes that conform to mid-Tertiary deformation. If the pre-Tertiary structure is restored, the Semail serpentinite becomes virtually horizontal at the time of emplacement. This structural simplicity belies the contention that the Semail was first ejected from a deep magma chamber and was then obducted onto continental crust. A sedimentary origin involving the erosion of rising hot simatic material and the monolithological deposition of disaggregated crystals by turbidity currents could explain the rhythmic bedding, pervasive serpentenisation and minimal deformation characteristic of the Semail.
Although a plate tectonic interpretation for the occurrence of the Steinmann trinity in the Oman Mountains is now generally accepted, the field evidence for extensional tectonics and gravity gliding appears to outweigh the evidence for regional compression and thrusting. Further work may provide more support for previously held autochthonist interpretations.