The Age of the Hawasina and Other Problems of Oman Mountain Geology

Abstract

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. Howevel; Permian to Cenomanian limestones that are exposed all along the axis of the Oman Mountains show no sign of penecontemporaneous rifing 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. Howevel; 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. Howevel; the ffawasina 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 ex...

The Jurassic–Cretaceous depositional and tectonic evolution of the southernwestern margin of the Neotethys Ocean, Northern Oman and United Arab Emirates

“…However, it can be clearly demonstrated that rather than being allochthonous, the upper part of the Muti Formation in Wadi Mi'aidin records continuous deposition, as argued by Wilson (2000). The tectonic boundary was inferred by Glennie et al (1974) because of an apparent juxtaposition of Jurassic strata above the late Cretaceous Muti Formation.…”

“…Globotruncana fauna have been recorded in this succession (Wilson 2000); and  An upper unit of coarse-grained and thick-bedded, locally ooidal and cross-bedded, shallowwater high-energy carbonates (Fig. 12b).…”

“…This relationship was proposed in large part because of the presence of radiolarian assemblages of Toarcian to earliest Bathonian age (Rousseau et al 2005) and because of an apparent similarity with the lithologies present in the Jurassic Guwayza Formation, despite an absence of evidence of a tectonic boundary. Wilson (2000) has proposed that the recorded fauna are all reworked, whereas contemporaneous pelagic fauna are absent through dissolution below the CCD in abyssal depths. Although we support the reworked nature of the fauna, the upper unit of the Muti Formation appears to have been deposited above storm wave-base and lacks the deep-water turbidite deposits typical of the Guwayza Formation (Fig.…”

“…Hamrat Duru Mountains) (Glennie et al 1974;Searle et al 1983;Robertson et al 1990a, b;Wilson 2000). The Lower Jurassic to Upper Cretaceous Hamrat Duru Group, deposited on the slope of the ocean basin, comprises a sequence of turbiditic clastic limestones and conglomerates that interdigitate with deepwater, basinal facies rocks.…”

“…2) crops out in the Hamrat Duru range of hills in Oman (Glennie et al 1974;Wilson 2000) where it comprises a lower unit of medium-grained, parallel and cross-lamination sandstones and sandy calcarenites ( Fig. 7a), overlain by a sequence of ooidal to peloidal calcarenites, which typically contain convolute lamination ( Fig.…”