Depositional and erosional features of Oligocene paleokarsts in the Kirkuk Group, Kurdistan Region, Northeast Iraq

Paleokarst features are rarely preserved in geologic history due to terrestrial erosion and later burial diagenetic changes. In rare cases, where karstification was intense it can be preserved by burial in areas where erosion was slow. In Iraq, one of the ages that preserved clear features of the paleokarsts is the Oligocene. During this age, a thick succession of shallow (platform) carbonates were deposited and assigned stratigraphically as Kirkuk Group. During the late Oligocene, the inner shelf of the platform exposed for long time and an unconformity was developed and associated with well-developed epi- and endokarst features. The present study focused on the documentation of the erosional and depositional features of paleokarstification during the latter age for the first time in Iraq and Zagros Orogenic Belt. In the northeast Iraq, many Oligocene paleokarst features are discovered in the medium beds of Anah and Bajwan Formations (as two units of the Kirkuk Group) in the Sulaimani and Erbil Governorates. These features are such as vertical and inclined shafts filled with brown pebbly sandstone, horizontal cavities, large planoconcave cavities filled with conglomerates, solution breccias, underground solution networks, travertine, and palaeosoil. All these features are discussed to prove their attribution to the Oligocene by highlighting many field and petrographic evidences. The documented paleokarst can indicate accurately depositional extent, paleogeography, and paleoshoreline of Oligocene succession in Iraq and part of Western Zagros. Therefore, this study is significant for paleo-mapping of the northwestern margin of the Arabian plate and applicable for paleoenvironment concerns and oil explorations in the studied area.