Microfacis and sequence stratigraphy of the Oligocene-Miocene sequence at Golan Mountian, Kurdistan, Iraq

The Oligocene-Lower Miocene carbonate sequence of northeast Iraq is a potential reservoir rock in many oil fields of the Foot Hill Zone. A comparatively thin section of this sequence is studied at the high folded zone from Golan Mountain near Derbandikhan Town, Kurdistan region of northeast Iraq. The study aimed at revealing the stratigraphic status and differentiation of this sequence through microfacies and sequence stratigraphic analyses. The section is 27-m thick and bounded by sequence boundaries of type 1 (top) and type 2 (bottom). The underlying unit is the Pila Spi limestone Formation of Late Eocene, and the overlying unit is the Fatha Formation of Middle Miocene. Microfacies analysis (nine basic microfacies), and biostratigraphic and sequence stratigraphic investigations of this sequence reveal the occurrence of the following formations with their common microfacies (from bottom to top): the Bajwan Formation (Rupelian) with 18-m-thick massive, fragmented to rubbly lagoonal limestone. The microfacies include miliolid-bioclastic wackestone to packstone. Index forams include Archaias kirkukensis, Praerhapydionina delicate, and Peneroplis thomasi. The Anah Formation (Chattian) is thin (3 m) and characterized by grayish, rubbly to massive, lagoonal limestone horizon with foraminiferal-bioclastic packstone as the dominating microfacies. Index foraminiferas include Meandropsina anahensis, Rotalia vienoti and Miliolidae. The uppermost 6 m of the sequence represents a cyclic alternation of limestone (calcarenite) with common microfacies type of oolitic grainstone and pelloidal-bioclastic packstone, and sandy dolomitic limestone of arenaceous bioclastic dolopackstone microfacies. Three complete cycles were recognized, the lower two represent the Aquitanian Euphrates Formation, and the upper cycle seems to represent the Burdigalian Jeribe Formation. A reconstruction of the depositional environments indicates transition from shallow marine lagoonal and carbonate shoals to coastal depositional system. These shallow marine environments and the comparatively thin section of the sequence emphasizes marginal location of the studied section within the sedimentary basin. The depositional history of the section is reviewed within a sequence stratigraphic framework which shows the occurrences of four shallowing upward third-order sequences.