Sedimentary Environment of Naokelekan Formation (Late Oxfordian – Early Kimmeridgian) of Selected Surface Sections from Northern-East of Iraq

Abstract

The Naokelekan Formation (Late Oxfordian - Early Kimmeridgian) was studied at three surface sections (Rowanduz, Ranya and Sargelu), in northeastern Iraq. The Formation is composed mainly of alternating sequence of thin- bedded limestone, dolomitic limestone and bituminous black shales. Streaks of calcite veins, chert nodules and bituminous stylolite are present. Traces of Ammonite molds, flat, wavy to domal stromatolites are also present. The Petrographic study showed that the formation contains various petrographic components represented by Ammonites, pelecypods, gastropods, calcispheres, planktonic and benthonic foraminifera, ostracods, stromatolite, radiolaria and bryozoans, pellets and intraclasts, in addition to the groundmass of micrite and microspars. The rock successions were extensively affected by many diagenetic processes and their effect is most pronounced pronounced on textural properties of these rocks. The dolomitization seems to be one of the most important diagenetic processes recognized. The other processes are; compaction, micritization, bioturbation, authigenic minerals, cementation, neomorphism, dissolution, fractures and veins, silicifaction, replacement and dedolomitization. The detailed facies analysis reveals that the rock succession consists mainly of three microfacies and two lithofacies type. The main microfacies include lime mudstone,wackestone and packstone. Where the lithofacies comprise limestones and shales which are represented by four sub lithofacies. The overall characters of these facies indicate that the formation was deposited in two different environments; (1) shallow marine sub tidal environment at the lower part and upper part respectively. (2) Outer shelf in a euxinic environment at its middle part.