Lithofacies and Clay Mineral Analysis of the Upper Cenomanian Ms'ad Formation, Rutbah Area, Western Iraq

Abstract

The Cenomanian succession in Rutbah City, western Iraq, is composed of siliciclastic carbonate with multicolored paleosoil layers that refer to unconformity surfaces within the Ms'ad Formation. The current study will examine the clay mineralogy and facies to determine the source of these clay minerals and depositional settings. Fifteen carbonate facies have been identified as three distinct facies associations (FA1–FA3). These FAs are supratidal to intertidal (FA1), semi-restricted lagoon (FA2), and shallow open subtidal ramp with patches of a rudist-bivalve reef (FA3). The reported Multispirina Iranica-Nezzat simplex-Chrysalidina gradata assemblage Zone within the Ms'ad Formation indicated the Cenomanian age. Clay mineral analysis displays three distinctive clay mineral assemblages; kaolinite-dominated with illite (assemblage A), R0-dominated (smectite-rich illite/smectite mixed layers) with kaolinite and illite (assemblage B), and illite-dominated (illite and illite-rich illite/smectite mixed layers) with kaolinite and R0 (assemblage C). Most of these clays are detrital, generated by weathering source rocks (igneous, metamorphic, and sedimentary) under various paleoclimatic circumstances. The Kaolinite-dominated assemblage A represents intense chemical weathering from hot, humid circumstances. Smectite-dominated R0 assemblages were created by weathering alkaline rocks in flat pedogenic soils with seasonal humidity. The source of clay minerals in the studied region was subject to a dry climate towards the end of the Cenomanian period, which encouraged physical weathering and the production of illite-rich assemblages.