ABSTRACT: High-resolution biostratigraphic and paleoenvironmental investigations have been integrated with sedimentology and wireline log data to establish a biostratigraphically constrained depositional sequence stratigraphic framework. The Rumaila and Mishrif formations are early to middle Cenomanian to early Turonian in age. The Rumaila Formation was deposited in distal inner ramp to outer ramp settings. The Mishrif Formation was deposited in inner to middle ramp settings comprising transitional, tidal flats, lagoonal, shoal, backshoal and foreshoal sub-settings. The two formations are coincident with 3rd order depositional sequences K130 and K140. The K130 sequence boundary lies on the contact between the lagoonal, non-calcareous shale of the upper Ahmadi Formation and the carbonates and highly calcareous shale of the Rumaila Formation. The K130 transgressive systems tract is composed of a highly calcareous marine shale at the base of Rumaila Formation, which contains abundant to common nannofossils, common to rare foraminifera and ostracods. The K130 MFS is characterized by occurrence of abundant foraminifera and ostracods. The K130 highstand systems tract is comprised of interbeded mudstones, wackestones, packstones and dolomitic mudstone. The early highstand is fossiliferous, becoming progressively less fossiliferous in the late highstand. The K140 sequence boundary coincides with the Rumaila and Mishrif formational contact. The incremental increase in the abundance of deeper marine foraminiferal assemblages within the mudstone and wackestone interbeds overlying this contact indicates initiation of a marine transgressive event. A thin, regionally extensive, dark-grey, non-calcareous shale which is largely barren of microfossils and high gamma ray is considered a proxy of the global OAE 2 anoxic event of late Cenomanian to earliest Turonian age. The early highstand of K140 is composed of middle ramp mudstone to wackestone interbeds enriched in open marine planktonic and benthic foraminifera. The late highstand is represented by shoal, proximal shoal deposits enriched in coralline and rudist wackestone and packstone overlain by miliolid lagoonal wackestones underneath the K150 sequence boundary, known as pre-Aruma unconformity.


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