الملخص

Hard ground at the uppermost part of Bekhme Formation is studied at three outcrop sections located in the Dohuk area, northern Iraq, and it is found that it consists of two distinct facies associations. The first includes limestone beds that comprise the hard ground and its omission surfaces. The second consists of limestone, marly limestone, and marl, which is interbedded with successive hard ground. The overall characters of these two facies associations indicate that deposition took place in a carbonate ramp setting. Vertical variations of sedimentary components suggest that the hard ground was developed by three successive stages. These are the end of shallow carbonate sedimentation and the drowning of the platform, the formation and modification of hard grounds, and finally the deposition of pelagic carbonate sediments of the Shiranish Formation. The second stage is characterized by intercalations of continuous depositional and biological processes. These include bioturbation, bioencrustation, and secondary mineralization in addition to other early and late diagenetic processes. The depositional area had been affected by tectonic drowning of carbonate ramp with a wide gradual marine transgression associated with a period of local marine regression that is, in turn, responsible for deposition, transportation, redeposition, submarine erosion and formation of hard grounds. Thus, the hard ground zone represents drowning unconformity surfaces between Bekhme and Shiranish formations through the Campanian- Maastrichtian boundary.