Abstract
This study investigates the nature of the contact between the two upper Cretaceous carbonate Qamchuqa and Bekhme formations in Bekhme and Kalate sections of northern Iraq through detailed field and laboratory analyses including petrographic, mineralogical, geochemical and stable isotopic analyses in addition to calcareous nannofossils investigations. Lenticular shape bed of polymictic carbonate conglomerate, rhythmically laminated, microbial, dark grey and beige colors carbonate clasts existed. Microbial clasts are calcite with sporadic dolomite and with sharp edges within the micritic matrix. The technique of X-ray diffraction and SEM-EDs analysis show that the Qamchuqa Formation in the Bekhme section consists of calcite in addition to dolomite, ankerite, quartz and hematite and becomes pure dolomite in the upper part while it is pure calcite in Kalate section. Conversely, the Bekhme Formation in the Bekhme section consists mainly of dolomite, while in the Kalate section, calcite is the main mineral. Major elemental data support this mineralogical composition, where CaO dominates major oxides followed by MgO with varied composition of SiO2, Al2O3, Fe2O3, K2O, Na2O, and TiO2. Notable variations are observed in the concentrations of trace and rare earth elemental data. Based on nannofossils investigations, two biozones were recognized and are dated Early Coniacian and Late Campanian in the Bekhme section while in Kalate section the age is Late Aptian and Late Campanian for the Qamchuqa and Bekhme formations respectively with a missing of Cenomanian, Turonian, Coniacian, and Early Campanian. Based on stable oxygen isotopic data and authigenic U concentration, the studied rocks were deposited under warm marine and dysoxic conditions. Thus, the contact between the two formations, and with solid evidence, is unconformable.