AAPG Middle East Region Geoscience Technology Workshop

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Setting the Scene for a Paleogene Clastic Basin; Obduction of the Masirah Ophiolite, Oman

Abstract

The Paleocene emplacement of the Samail and Masirah ophiolites in Oman marks a major plate tectonic event which separates earlier production of thick carbonate shelves from the later deposition of continental and shallow marine clastics. Thick foreland basins of clastics occur in front of each ophiolite in tertiary basins. The first offshore Oil discovery in Oman was found in 2014 by a well drilled within the Masirah tertiary basin and several wells in this margin have seen shows. This poster draws together some insights into the Masirah margin geometry, the obduction process, and how these two interact to produce the present day tertiary clastic basin which is an emerging hydrocarbon province. The Masirah ophiolite outcrops along the eastern margin of Oman on Masirah island and the Ras Madrakah headland. It is a Jurassic aged unit containing an oceanic crustal sequence overlain by Cretaceous deepwater pelagic facies including late Cretaceous turbidites. It was emplaced in the Paleocene on top of the Masirah Cretaceous passive margin, forming a ‘tertiary’ basin between the ophiolite and the Paleozoic Huqf high onshore. A wedge of weathered ‘melange’ partially infilled the basin along with tertiary aged deepwater clastics and carbonates. 5 broad tectonic phases are interpreted associated with the present-day geometry of this margin based primarily on legacy well penetrations along the Masirah margin and plate tectonic reconstructions using GPlate. 1) ESE directed extension occurred in the Jurassic as India-Pakistan rifted from Oman resulting in Tithonian aged oceanic crust. This left behind a passive margin sequence of carbonates within a normally faulted geometry (the Shuaiba, Nahr Umr, Natih formations). 2) The India-Pakistan block rifts from Antarctica/Australia in the early Cretaceous which results in India moving northwards towards Oman. This is interpreted to have buckled the Tithonian aged oceanic crust offshore Oman leading to intraplate magmatism as seen onshore Masirah island within the ophiolite sequence. 3) As Madagascar separates from the India-Pakistan block in the late Cretaceous, the rate of northwards movement for India-Pakistan becomes extremely high. Most of this movement is taken up on the Owen fracture zone offshore Oman. It is possible that a new dextral transform fault (the Masirah transform) develops at this time, shearing off the outer edge of the Masirah passive margin and displacing it northwards parallel to India. 4) In the latest Cretaceous the Seychelles rifts from India-Pakistan resulting in a counter-clockwise Indian-Pakistan plate motion. At this time the Muslim Bagh Ophiolite is obducted onshore Pakistan and contemporaneously the Masirah ophiolite is obducted onshore Oman (ie both conjugates). The carbonate platforms of the Masirah margin are rapidly drowned and shallow water clastics enter the basin (the Aruma formation). During the obduction event the ophiolite is heavily weathered and thick piles of shales with ophiolitic debris prograde inboard within the new Tertiary foreland basin (the Fiqa shales). 5) In the Eocene (Hadramaut formation) the Gulf of Aden opens. With the Indian subcontinent away to the north, the NE oriented Gulf of Aden extension is accommodated on the Masirah transform fault which deforms the outer edge of the obducted ophiolite in a transpressional manner forming large anticlines around the Sawquirah bay area. The interaction of phase 1 rift transforms and phase 5 Gulf of Aden transforms generates the distinctive sawtooth geometry seen along to the Masirah margin present day. A model for obduction is presented involving uplift of the oceanic crust offshore Masirah to sealevel in the latest Cretaceous (phase 4) coupled with transpression as Jurassic rift geometries (phase 1) were reactivated. The Maradi fault zone maybe also activated, offsetting the ophiolite melange. The method and timing of ophiolite obduction are crucial to understand when considering burial and uplift rates associated with hydrocarbon plays along this margin. In addition was the ophiolite thrusted and ‘bulldozed’ onto the margin as with the Samail ophiolite, or was it ‘gently placed’ and then weathered to cover a broad area? This work suggests the latter, allowing for the preservation of Cretaceous carbonate and lower Tertiary clastic plays beneath the advancing mélange.