Stratigraphical and palaeoenvironmental studies in the Lower Fars Formation (Miocene) of northern Iraq

Abstract

The stratigraphy of two facies within the Lower Fars Formation (Miocene) of northern Iraq is redescribed: a basin-centre facies represented by three sections near Fathah, Sheikh Ibrahim and Butma, and a basin-margin facies represented by a section near Shaqlawa. Two members are defined within the basin-centre facies. Asymmetrical cyclic deposition of marls, carbonates, and nodular gypsum characterises the Lower Member, whilst similar cycles, but with the addition of brownish red mudstones at the top of each cycle, characterise the Upper Member.From field observation, petrology and X-ray diffraction analysis, the sediments of this Formation are described in terms of four lithofacies: mudrocks, carbonates, sulphates, and sandstones. X-ray diffraction analysis shows that the clay minerals of the grey-green marls are mainly illite and chlorite whilst the red mudstones are mainly chlorite. The carbonates of the Lower Member are mainly dolomite and predominantly skeletal wackestones, whilst those of the Upper Member are mainly calcite and predominantly skeletal packstones. The sulphates are gypsum secondary after anhydrite.Several species and genera of molluscs (including Ostrea lati-mavginata, O. subangulata and Clementia papyracea), foraminifera (including Borelis melo), and ostracods (including marine and brackish water genera) are diagnosed and illustrated. The faunal evidence seemingly indicates that the Lower Fars Formation is probably late Lower Miocene "Burdigalian" rather than later in strati graphic age.Structural and computer-based isopachyte studies show that the Lower Fars basin is separated into two main (Kirkuk and Sinjar) basins. The nature and thickness of the Formation reflect a tectonic control.Sedimentological, palaeontological, structural and isopachyte studies show the Lower Fars to have been deposited in a broad butrelatively shallow NW-SE trending basin with SE marine connection during a major Miocene transgressive cycle. Fluctuation in sea level is inferred to be responsible for the deposition of the evaporite cycles, desiccation taking place in a closed (desiccated) basin. Cycles represent periodic transgressions of the sea, initiated by marine marl, with subsequent deposition of shallow warm v/ater carbonate, then sulphate through desiccation, terminated by a regressive phase with brackish water red mudstones.Lithological and palaeontological evidence and facies distribution are consistent in indicating that the basin-margin facies, at least that represented at Shaqlawa, is equivalent to part, possibly the whole of, the Upper Member of the basin-centre Lower Fars. Also they indicate that the evaporitic facies in Iraq is equivalent to the evaporitic facies (Gachsaran Formation) in Iraq, and possibly to the so-called Lower Fars Formation in Syria.