Iraqi National Journal of Earth Science

Abstract

The Middle Eocene Red Beds known as typical facies of Gercus Formation in north and northeastern Iraq. In the field they cropout as red succession of claystone, lensoidal sandstones and conglomerates. Tectonically the beds are exposed in the folded area of the Zagros Fold-Thrust belt of northeastern Iraq. The whole formation consists of major low stands system tract within stratigraphic record of Tertiary which belongs to 2nd order sea level change. This major lowstand system tract is divided into two depositional sequences named upper and lower sequences which are modulated within 3rd order sea level change. About 90% of its thickness is deposited in the lower sequence, and 10% in the upper one. The lower sequence bounded by SB1 and SB2 from the bottom to the top respectively. It is comprised of SMST, TST and HST. Only the LST of the upper sequence is represented in the Gercus Formation, while the other systems tracts are possibly included in the carbonate of Pila Spi Formation. The SMST consists of alternation of red claystone and marl which are locate at the top of Khurmala Formation.
The TST (middle part of Gercus Formation) is composed of grey marl and laminated marly limestone. The HST consists of alternation of red claystone, laminated sandstone and calcareous shale.
The LST consists of 1-4m of rounded and sorted conglomerate located at the upper part of Gercus Formation. This system tract is underlain by an erosional unconformity. The inferred depositional environment is most possibly braided delta plain which is prevailed by fresh water influx and flooding by marine water. This is shown in the field by mud cracks, marl marine sediments and laminated sandstone which deposited in upper flow regime.