The Cretaceous/Paleogene (K/Pg) boundary marks the global extinction of many life forms, this boundary around Sulaimani city coincides with the boundary between Tanjero and Kolosh formations. These two units are flysch deposits of the Zagros Foreland Basin. This study focuses on petrography and clay mineralogy variations between these two units. Petrographic study, X-Ray Diffraction analysis, and Scanning Electron Microscope analysis were conducted on samples from different lithologies. The petrographic study of fine-grain lithologies shows that they are mainly wackestone without variation across the boundary. Sandstone petrography shows variation in framework components by increasing quartz, feldspar, and igneous fragments while argillaceous, carbonates, chert, and chalcedony fragments are decreased from Tanjero to Kolosh formations. Obtained results from XRD and SEM show that the clay mineral assemblage is dominated by corrensite (regularly interstratified chlorite/smectite) with discrete chlorite, smectite and traces of illite. The existence of these clay assemblage suggests that corrensite is the diagenetic product of smectite as it is an intermediate stage of smectite chloritization. Enrichment of precursor smectite and high content of unstable grains in sandstones across the K/Pg boundary is the result of low-intensity weathering in arid and seasonal climates of the source area. The smaller size of detrital smectite than other clay minerals is behind its enrichment in the deep marine basin for both Tanjero and Kolosh formations. Sandstone enrichment with sedimentary fragments in both formations related to uplifted sedimentary terrain in its hinterland, while variation across the boundary indicates gradual uplift of Zagros ophiolite zone in the early Paleogene.