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Detailed high magnification scanning electron microscopic (SEM) study supported by petrographic, mineralogical, and geochemical (major and trace element) analysis of the Gercus Formation clastics (sandstones and mudstones) from northern Iraq shows that the main authigenic components are carbonate (dolomite and calcite), clay and haematite minerals with rare quartz, feldspar, and gypsum. The carbonates and sulphate minerals were the main early diagenetic phase in the Gercus Formation and were produced from the post-depositional breakdown of ferromagnesian minerals. These latter are unstable in the oxidizing conditions of the interstitial waters below the desert surface. The main authigenic clay minerals are illite, illite-smectite, and palygorskite. The presence of tangential illite coating and illite-smectite in the sandstones can be related to their desert origin. Palygorskite is associated with dolomite, calcite, and haematite. These mineral associations may indicate early diagenesis in evaporitic and saline conditions. It also confirms the syndepositional and diagenetic origin for the clayey-haematitic matrix and pigment of the Gercus Formation. Haematite occurs as extremely fine-grained crystals mixed with authigenic clays and exhibits a range of shapes (e.g., fibrous to lath-shaped aggregates, small spherical grains, specularite crystals, blade-shaped, rosette-like clusters, and a lump of bright spheroidshaped haematite/goethite). The arid to semi-arid conditions together with the oxidizing and alkaline interstitial water favoured the noted mineral assemblage within the Gercus Formation.

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.

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