Late Cretaceous Nappes in Oman Mountains and Their Geologic Evolution

Abstract

The rocks of the central Oman Mountains can be divided into five main units. These comprise, from bottom to top: a pre-Permian basement that stratigraphically is overlain by mostly shallow-marine carbonate rocks ranging in age from Middle Permian to Late Cretaceous (Senonian). Above this, several thin and imbricated nappes, together with several large (mountain-size) exotics, form the allochthonous Hawasina unit. The age range of the strata within each of these nappes covers a part of the time span from Permian to Cenomanian. The sediments vary from the Permian and Triassic shallow-marine limestones of the exotics that were deposited over a substrate of pillow lavas, to limestone turbidites and cherts which locally are associated with minor amounts of basaltic pillow avas. In the northern mountains the Hawasina tectonically overlies Permian to Cenomanian sequences of reefal limestones, slump conglomerates, turbidites, and calcareous mudstone of the Sumeini Group. The Hawasina is overlain by the Semail nappe, an enormous mass of ophiolites. Minor amounts of pelagic sediment dated as Cenomanian and early Senonian are associated with the submarine lavas of the ophiolite suite. Both the Hawasina and the Semail are overlain unconformably by Maestrichtian and early Tertiary sedimentary rocks. From the sedimentary and structural complexities of this mountain range, the following simplified interpretations have been made. 1. The autochthonous Permian to Senonian shallow-marine carbonate sediments were deposited on the continental margin of Arabia. 2. The Sumeini Group was deposited on the northeastern edge and continental slope of Arabia. 3. The age range of each of the Hawasina sedimentary sequences is based partly on the recognition that many of the microfossil assemblages from turbidite sequences were derived penecontemporaneously (rather than reworked into a younger sequence) and form part of a normal stratigraphic succession. 4. The sedimentary rocks of the Hawasina nappes were deposited northeast of the present Oman Mountains between the Permian and the Cenomanian. 5. The rocks that now constitute the Semail nappe were generated in an upwelling part of the earth's mantle (peridotites and gabbros) and on the ocean floor (lavas). The upwelling is thought to have been associated with ocean-floor spreading and to have given rise to an oceanic ridge which formed the distal limit to that part of the basin in which the Hawasina sediments were deposited. 6. During the Late Cretaceous (late Campanian) the Sumeini and Hawasina sedimentary rocks and the Semail ophiolites were emplaced tectonically into their present location on the outer part of the Arabian continental margin. 7. After emplacement, the Hawasina and Semail nappes were covered unconformably by shallow-marine Maestrichtian and early Tertiary limestones. 8. The outer part of the Arabian continental margin (together with its cover of nappes and younger marine limestones) was uplifted during the Oligocene and early Miocene to form the present Oman Mountain range.