Facies associations and sequence stratigraphy of the Middle-Late Eocene Pila Spi Formation in the High Folded Zone, Kurdistan Region, northeastern Iraq

Abstract

Facies analysis and sequence stratigraphy of the Pila Spi Formation (Middle-Late Eocene) was studied from two outcrop sections (Bekhma and Gomaspan) within the High Folded Zone, Iraqi Kurdistan Region. The formation consists of light brown dolomitic, crystallized, chalky, limestones, and dolostone, alternating with multicolored marl and marly limestones. Chert nodules are present in the upper part of the formation. The petrographic analysis was based on 65 thin sections, and the results showed that the carbonate rocks are partially dolomitized. The distinguished bioclasts consist of benthic and planktonic foraminifera, micro-gastropods, calcispheres, ostracods, pelecypods, and bryozoans. Non-skeletal grains such as peloids, intraclasts, and extraclasts were also observed. X-ray diffraction analysis revealed that abundant clay minerals are palygorskite with the rare existence of chlorite. Non-clay minerals include dolomite, calcite, and quartz with low pyrite contents. According to field observations, four different lithofacies were recognized. They are dolomitic limestone, chalky limestone, marl-marly limestone, and cherty limestone. Based on detailed microfacies analysis of limestones, four main microfacies and eleven submicrofacies types are observed. These microfacies are subdivided into two groups of facies associations; they are semi-restricted and open lagoon. The results of stable carbon and oxygen isotopes of the studied carbonate samples show negative values of δ18O, and this indicates that the seawater was warm with lower salinity. However the positive δ13C values reflect the deposition of organic-rich sediments with transgression of the sea and deepening of the basin. Two third-order depositional sequences are observed. Each of them is subdivided into a TST and HST, reflecting the transgression and stillstands of the relative sea level during Middle-Late Eocene.