Depositional setting and basin development of the Paleocene—Lower Eocene Sinjar and Khurmala formations, Northern Iraq

Abstract

Petrographic study of the Sinjar and Khurmala Formations at the Zagros and Taurus Folded Zones, in northern Iraq led to the recognition of 11 carbonate microfacies that can be grouped into seven facies associations. These facies associations deposited within outer shelf, fore-reef, reef, back-reef, lagoon, shoals/bars and channels, and tempestite subenvironments of a composite isolated carbonate platform. The digenetic imprints on the recognized microfacies are prominent and dominated by micritization, dissolution, cementation, neomorphism, dolomitization, compaction, stylolitization, and precipitation of some authigenic minerals such as pyrite, glauconite, and gypsum. Isopach and depositional facies maps of the Sinjar–Khurmala successions show three depocenters in two subbasins in the study area, two of them are elongated and coincide with the Zagroside and the Taurides trends, whereas the third accords with the trend of the Sinjar graben. The reconstruction of the history of the Sinjar and Khurmala carbonates revealed four stages of basin development within two subbasins. The first stage represents the initial deposition of the Sinjar and Khurmala Formations that was preceded by the deposition of the deep marine part of the underlying Kolosh Formation. This depositional phase is characterized by submarine fans and basinal sediments. Both subbasins were partially separated by a ridge or a series of paleohighs, which had been inherited from relieves of the Cretaceous paleosurface. During the second depositional stage, the Sinjar basin is developed by growth of another reef build-up to the southeast with a general direction from southwest near Kirkuk area to the northeast near Sulamani area. During the third depositional stage, the basin was developed through the growth of isolated carbonate platform extending from the Kirkuk to the Duhok. Such platform was drowned during the transgression that covered the northern part of the Kolosh basin and southwestern part of the Aaliji basin. The final depositional stage was represented by a still stand period, when the sea level was stilled, and led to the upgrading growth during the early Highstand system tract of the carbonate shelf . As a result, during this stage, the reef facies were prograded seaward and replaced by the back-reef facies to indicate the end of Highstand during the late regression stage.