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 the mountains and exposes a section of massive limestones which range in age from Permian (transgressive on a Precambrian nucleus) to late Tertiary; the latter fringe the uplift. Intercalated in this sequence, between well-dated latest Cretaceous sediments, is a thick contorted assemblage of rocks composed, from base to top, of turbidite limestone and radiolarian chert (Hawasina Group), massive exotic blocks of Permo-Triassic limestone, and a thick sheet of serpentinic igneous rock (Semail igneous series). This association of rocks, which is comparable to that known from many complex Laramide orogenic belts, can be studied with advantage in Oman where the sequence has been little disturbed since its emplacement during the latest Cre aceous (Campanian-Maestrichtian).

The absence of recognizable latest Cretaceous fossils in the Hawasina, the common occurrence of well-preserved Permian to middle Cretaceous species, and the contorted features 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 is concluded from field data presented herein that the Hawasina was deposited in its present location during latest Cretaceous time.

Regional stratigraphic correlations show that northeast Oman was situated far out on the Arabian platform where quiet carbonate sedimentation persisted from Permian to middle Cretaceous time during prolonged regional tectonic quiescence. Sedimentary and tectonic quiescence ended during latest Cretaceous time when the thick Aruma pelagic shale, originating from a northerly source, was deposited across northeastern Oman concomitant with major normal faulting.

Hawasina facies, north of the Aruma province, give evidence in the form of basal boulder beds, turbidites, and massive terminal gravity slides of sharp contemporaneous tectonism. Lower Hawasina deep-water turbidite sediments have yielded an inverted faunal sequence of shallow-water fossils, whereas the overlying cherty facies contains only pelagic Radiolaria. Thus the whole sequence cannot be older than the reworked youngest middle Cretaceous fossils present in the basal beds.

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 north of the Oman Mountains. The absence 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 and at times strong block-fault movement.

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 (latest 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 during descent to abyssal depths. The latest Cretaceous Aruma Shale is compared with Holocene pelagic globigerinal sediment occurring above 4,500-m depth.

The deep trough which received Hawasina sediments appears to have been bounded by a steep block-faulted northeastern limb whereas the southwestern limb became shallower gradually through the neritic Aruma belt to the Arabian carbonate platform. At the close of the time of Hawasina deposition, volcanism and catastrophic tension-relief faulting dislodged the remnants of Permo-Triassic limestone sedimentary rocks from the seamount, 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 magma which blanketed the abyssal landscape and cooled slowly, under great hydrostatic pressure, along the axi of the trough.

According to this interpretation, the sedimentation of Hawasina beds is considered to have been contemporaneous with the deposition of the latest Cretaceous Aruma Shale, and major block-fault tectonism in the orogenic belt is correlated with known latest Cretaceous tension faulting on the southwest basin slope. Compressional features in the Hawasina are attributed to contemporaneous gravity slumping of the beds on the steep northeast limb of the trough, combined with the subsequent

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"bulldozing" effect which massive slides of exotic blocks and the thick Semail eruptives had on the incompetent sediments.

Oil prospects in the Hawasina belt are difficult to explore and assess for several reasons, not the least of which is the masking effect which contorted chert has on seismic and gravity exploration for more simple structures in the competent sub-Hawasina limestone succession, which is hydrocarbon-bearing below Aruma Shale at Fahud, Natih, and Yibal oil fields.