The Middle Miocene Fatha (Lower Fars) Formation, Iraq

Abstract

ABSTRACT The Middle Miocene Fatha Formation (previously Lower Fars Formation) in northern Iraq was deposited in a broad and shallow foreland basin adjacent to the Zagros and Taurus Mountains. It forms a transgressive-regressive sequence comprising numerous shallowing-upward cycles of alternating mudrocks, limestones, gypsum and/or anhydrite and halite. These cycles reflect rapid changes in accommodation space in settings that ranged from open and restricted hypersaline marine to continental (sabkha and fluvio-deltaic). In the marginal parts of the basin, continental siliciclastics (red and variegated marls, silts and fine sandstones) represent either aeolian deposition or a combined lagoonal- and/or fluvial-dominated delta system. Eustasy, rather than tectonics, caused the high-frequency cyclicity seen in the Fatha Formation. We present twelve sections dominated by evaporites from the Sinjar and Fatha sub-basins to represent the main lithologic constituents of the formation. Our detailed analysis of the sedimentary succession focuses on the three main lithofacies (siliciclastics, carbonates and evaporites). Petrographic, geochemical and scanning electron microscope analysis of these units are presented. We identified a range of carbonate lithotypes: marly, arenaceous (detrital), organic-rich (fossiliferous), dolomitic limestone and dolomite. Dolomitic limestones occur mostly in the lower part of the lower member of the formation, which was deposited in a barred lagoonal environment with high salinity. The presence of peloidal lime-wackestone with bioclasts, particularly in the upper part of the lower member of the formation, may reflect quiet, shallow-water marine conditions with moderate depths and low energy. The bioclastic-peloidal grainstone-packstone microfacies, with a common and diverse fossil assemblage, may reflect high to moderate energy, shallow-water environments. Evaporites comprise the main sediments of the Fatha Formation. Nodular gypsum is the dominant gypsum type, although laminated, thick-bedded, and secondary gypsum (selenite and satin spar) also occur. In the subsurface, anhydrite and halite are the principal minerals.