Oligocene stratigraphy in the Sinjar Basin, northwestern Iraq

Abstract

The distribution of the Oligocene succession (basinal and reef-back-reef deposits) in Iraq closely follows the pattern set in Middle to Late Eocene times. Reef-back-reef limestone has a linear outcrop across northern Iraq from northwest (the eastern part of the studied area) to southeast. In the Sinjar Basin, the Oligocene succession is unconformable on the Middle Eocene Jaddala Formation and unconformably to relative conformably underlies the Lower Miocene Anah or Ibrahim formations. Two surface sections on the Sinjar Anticline and one cored borehole in the Butmah Anticline formed the basis for our study. Biostratigraphic analysis indicated four planktonic and three benthonic foraminiferal biozones. The planktonic biozones are Pseudohastigerina micra and Globigerina ampliapertura Partial-Range zones, Globorotalia opima opima Total-Range Zone and Globigerina ciperonesis ciperoensis Partial-Range Zone; the three benthonic biozones are Nummulites fichteli-Nummulites intermedins Assemblage Zone, Borelis pygmaeus Total-Range Zone, and Praerhapydionina delicata-Austrotrillina howchini-Peneroplis evolutus Assemblage Zone. Three depositional sequences are present in the sections and borehole and comprise two third-order sedimentary cycles. The first consists of the Palani, Sheikh Alas and Shurau formations and the lower part of the Tarjil Formation deposited in upper-bathyal to intertidal environments; the second is the upper Tarjil Formation and Bajwan Formation. The Baba Formation (a barrier deposit) was not seen in the studied sections, but probably reflects lateral stacking development of the barrier to the west. The upper Tarjil Formation was deposited in upper-bathyal to middle-shelf environments whereas the Bajwan Formation consists of subtidal to tidal-flat deposits. Sequence-stratigraphic analysis, as calibrated by sedimentary facies and biostratigraphy, delineated the two third-order depositional cycles as the Oligocene First Cycle of Rupelian age and the Chattian Oligocene Second Cycle. This suggests that the studied Oligocene succession was deposited over a period of about 9 million years and shows good correlation of the northeastern Arabian Platform with other parts of the Platform and with European Oligocene sequences.