The Paleogene Carbonate Banks of the Zagros Foreland Basin’s Forebulge, Northern Iraq: Stratigraphic Architectures and Facies Associations

The Paleogene sequence of the Zagros foreland basin of NE Arabiais punctuated by a linear chain of carbonate bank-ramp associations. These carbonate banks had developed over the NW-SE trending basin forebulge. Continued convergence and compressional tectonics of the basin made the forebulge become incipient and shallow enough to commence carbonate sedimentation. The porpose of this work is to investigate the origin, facies and regional stratigraphic relations of these carbonates to the forebulge and other basin depozones. Stratigraphic data of this research are collected from (15), measured outcrop sections distributed over the basin forebulge zone and surroundings. In addition, re-examination and reinterpretation of (30) previously studied surface and subsurface sections are compiled into the study. This done to integrate the regional facies distribution and basin-wide sequence stratigraphic architecture. Stratigraphic investigations include lithostratigraphic logging and correlation, microfacies analysis, and facies modeling. Four major banks were recognized, three of these banks (informally named: Dohuk, Kirkuk, and Sulaimaniyah) were developed over the forebulge zone. The other one (Sinjar bank) of the Sinjar area, is related to basininversion mechanism. The forebulge carbonate banks were developed as the reefal limestone of the Sinjar Formation, and the lagoonal carbonate and shale of the Khurmala Formation. The Sinjar Formation is dominated by coralgal reef and foraminiferal-shoals with facies spectrum forming the basic body of the bank. The ramp setting was developed on both sides of the bank. The bank limestone represents the inner ramp facies, whilea windward ramp is developed as foraminiferal bioclastic limestone fingers into the back-bulge basinal marlstone of the Aaliji Formation. A leeward ramp facies was developed into the foredeep depozone, with bioclastic carbonate intercalations that characterize the upper part of the Kolosh Formation. At a later stage of basin shoaling (Early Eocene), a carbonate back-bank lagoon was developed over the foredeep area. It became gradually, isolated at certain parts of the basin, and finally replaced by the dolostone of the Khurmala Formation.