Quantitative microfacies analyses of a carbonate ramp in the upper Cretaceous Mishrif Formation, A oilfield in Iraq, and paleogeography reconstruction of the North-eastern Arabian Plate

Abstract

Sediment composition and microfacies (MF) types in the Cretaceous Mishrif Formation of the A oilfield in Iraq were quantitatively analyzed to identify changes in facies belts. Nine major constitutes were recognized via point-count group analysis and thin sections. The hierarchical cluster analysis of point-count groups indicated that the Mishrif Formation contained 12 MF types that were clustered into seven facies belts: protected-, open inner ramp, and shallow shoal, proximal and distal middle ramp, agitated outer ramp, and distal outer ramp. The quantitative analysis of MF enabled the interpretation of the water energy and continuity of the sub-environments. A low-energy homoclinal ramp was established by integrating the significant volumes of fine-grained carbonates that were present in almost all MF and low-relief green algal build-ups. Diagenetic discrepancy along Cenomanian-Turonian (C-T) unconformity is presented by extensive fabric-selective dissolution, classical karstification of dissolution-collapse breccia and paleosol deposits, and incisions and channels. Finally, a comprehensive comparison of the carbonate developments and diagenetic processes along unconformities in the Mishrif and its time-equivalent platform implied that the configuration of platforms associated intrabasins, and related diagenesis along unconformities were affected by the far-field effect of the tectonic activities that occurred in the northeast margin of the Arabian Plate. This study demonstrated that the quantitative analysis of carbonate MF is a powerful tool that can improve the reconstruction of a model of a low-energy carbonate ramp and provided important implications for the study of the paleogeography of the northeastern margin of the Arabian Plate during the Cenomanian–Middle Turonian.