Sedimentology of the Red Beds in NE Iraq

The Red Beds exist in a narrow belt in the NW-SE oriented imbricated zone in northeastern Iraq. Field and laboratory studies have shown that the Red Beds are composed of clastic rocks including conglomerate, sandstone and mudstone, with a few thin dolomite limestone beds in the lower part of the succession. The Red Beds exhibit several cycles of upward coarsening deposits, however fining upward cycles are also present locally. This research deals with a sedimentological investigation of six exposed stratigraphic sections divided into four main units in the Kurdistan region of northern Iraq. Four sections were investigated in the Mawat-Chwarta area north of Sulaymaniya city while the Suwais and Merga sections lie the Qandel area in northwestern Iraq. The sedimentary facies show that the Red Beds were deposited in subaerial environments such as river channels, flood plains and tidal areas, with some shallow marine deposits. A detailed petrographic study was carried out on sandstone, siltstone and some conglomerate units. The clastic rocks consist mainly of calcite cemented litharenite with rock fragments (volcanic, metamorphic and sedimentary), quartz and minor feldspar. Based on the subangular shape of most detrital components, they were only transported a short distance from their source area. The petrographic components reflect the tectonic system in the source area which ranges laterally from a mixed orogenic sand and magmatic arc in Mawat-Chwarta area, to recycled orogenic material rich in sedimentary rock fragments in the Qandel area. Heavy minerals were separated by bromoform and consist of opaque minerals (magnetite, hematite, chromite and pyrite) with some amphiboles and pyroxenes, as well as epidote, garnet, zircon, rutile, tourmaline and flaky minerals (biotite, muscovite and chlorite). Three main cement types were determined in the Red Beds: carbonate, clay and quartz. Geochemical analyses of trace and rare earth elements have shown that the non-carbonate fraction in the lower part of the Red Beds (unit one) was derived mainly from mafic and ultramafic rocks containing a high abundance of ferromagnesian trace elements (Ni, Co, Cr) with low abundances of the HFSE and other lithophile and rare earth elements. The upward decrease in mafic and ultramafic components in the Red Bed sediments was accompanied by an increase in the felsic component especially toward the upper part of the Red Beds. This is reflected in the increase in the HFSE and other lithophile and rare earth elements. These results indicate the exposure of both felsic and intermediate igneous bodies in the source areas. Trace elements were also normalized to upper continental crust and confirmed the occurrence of a mafic and ultramafic source for the lower part of the Red Beds. Units two and four reflect another style similar to the felsic and mafic trends with transition elements being depleted in these parts. The overlying unit three shows various patterns partly similar to unit one and some similar to unit two. Diagrams of chondrite normalized trace element concentrations generally show enrichment in LREEs with depletion or flat HREE and negative Eu anomalies. The lower part of the Red Beds is mainly enriched in HREEs ascribed to mafic and ultramafic source rocks in contrast to the middle and upper parts which have less HREES and were probably affected by a felsic source. The depletion in LREEs in the lower part of the Red Beds normalized to average shale indicate a difference in the source rock of this part relative to the middle and upper parts that are rich in HREEs. This evidence confirms that the lower part mainly received ultramafic and mafic detritus. Microprobe analyses of selected samples representing the different units proved a different source area for the lower Red Bed units and a mixed source for the upper parts. According to their geochemical signatures the Red Bed sediments were deposited in a range of sedimentary environments and were derived from varied source rocks. The clay mineral analyses (XRD and SEM) showed that kaolinite is abundant, especially in the lower part where it is represented by halloysite. Illite is rare except for local areas in the Red Beds. Other clay mineral types determined in this research include smectite, chlorite, mixed layer smectite-chlorite, mixed layer illite-smectite and palygorskite. The Cretaceous-Paleogene foreland basin of northern Iraq formed to the southwest of the Zagros Suture Zone and the Sanandaj-Sirjan Zone of western Iran collectively making up the Zagros Orogen. In the late Cretaceous, a tectonic event in the adjoining Tethys resulted in ophiolite emplacement across the continental slope and carbonate platform (Shiranish Formation) producing an outer bulge uplift followed by rapid deepening and ophiolitederived deposition of the coarsening upwards Tanjero Formation. In the Paleogene deposition of the Red Beds was caused by renewed shortening in the thrust sheets overlying the Arabian margin with uplift of the radiolarites (Qulqula Formation) resulting in an influx of radiolarian debris in addition to continuing ophiolitic detritus. In the upper part of the Red Beds mixed sources including metamorphic, volcanic and sedimentary terranes reflect changing provenance which indicates initiation of the continental collision of the Arabian Peninsula with the southern margin of Eurasia (Sanandaj-Sirjan Zone)