The Oman Mountains border the Gulf of Oman from the Arabian Sea to the mouth of the Persian Gulf. The Jebel Akhdar anticline forms the central core of these mountains and exposes a section from the Precambrian nucleus to late Tertiary aged strata on the flanks. Intercalated in this sequence between well-dated late Cretaceous sediments, is a melange of rocks composed of turbidite limestone, radiolarian chert Hawasina Group and massive exotic blocks of Permo-Triassic limestone overlain by a thick sheet of serpentinic igneous rock (Semail Igneous series). This association of rocks, known as Steinmann's Trinity has been little disturbed since its emplacement during the Late Cretaceous. The scarcity of Late Cretaceous fossils in the Hawasina, the common occurrence of well-preserved Permian to middle Cretaceous species, and the contorted nature of the strata have led some geologists to postulate that these sediments were deposited outside of their present location during a prolonged pre-Late Cretaceous interval and then tectonically emplaced during the latest Cretaceous However, it also can be interpreted from field data that the Hawasina is para-autochthonous and was deposited near its present location during Late Cretaceous time.

Regional correlation of autochthonous sections show that northeast Oman was situated far out on the arabian platform where carbonate sedimentation persisted from Permian to Cenomanian time during prolonged regional tectonic quiescence Sedimentary and tectonic quiescence ended during Late Cretaceous time when the thick Aruma pelagic shale was deposited across northeastern Oman concomitant with major normal faulting.

The distribution, grading, and constitution of the carbonate clastic material in the Hawasina, and the alignment of exotic limestone blocks indicate that the sediment source area was a northwest-southeast-trending uplift of Permian to middle Cretaceous carbonate rocks. Scarcity of terrigenous clastic material and terminal submarine volcanism suggest that the source area was a submerged seamount. Erosion from this high is believed to have been by means of turbidity currents activated by repetitive block-fault movement over a rising mantle diapir.

Hawasina sediments compare with present deep-water sediments in the Puerto Rico Trench where faunally barren siliceous oozes of abyssal facies are interbedded with calcareous turbidites rich in reworked older and contemporaneous shallow water fauna. The absence of contemporaneous (Late Cretaceous) shelf fauna in the Hawasina is attributed to the seamount source area being deeper than neritic, whereas the absence of contemporaneous pelagic calcareous fauna was the result of dissolution below the carbonate compensation depth. One of the most controversial problems in Oman is the dating of melange sediments which are heavily contaminated with reworked fossils.

The deep trough which received Hawasina sediments appears lo have been bounded by a steep block-faulted northeastern limb whereas the southwestern limb became shallower gradually through the Aruma belt to the Arabian carbonate platform. At the close of Hawasina time volcanism and catastrophic tension relief faulting dislodged the remnants of Permo-Triassic limestone from the roof of a mantle diapir, and these descended into the trough as huge gravity slides (e.g. Jebel Kawr, 250 sq mi), leaving the seamount as a denuded basement uplift. Regional tension relief was accomplished finally by crustal separation and flood eruption of Semail ultrabasic pillow lavas onto the abyssal landscape

Exploration for simple structures in the autochthonous Cretaceous limestones which subcrop the Hawasina is hampered by seismic energy attenuation and velocity problems. It is probable that simple block faulted structures such as the Fahud and Natih fields may underlie the contorted Hawasina cherts.

End_of_Record - Last_Page 3---------------