| Summary: | The Lower Mesopotamian fluvial plains crossed by the Tigris-Euphrates Rivers are occupied by fresh-brackish water lakes and extensive reed marshes dominated by Phragmites sp. and Typha sp. (together locally called Ahwar). The main modern allochthonous sediment sources are: the Tigris and Euphrates seasonal floodwaters, together with aeolian dust and sand driven by storms from the western deserts and northwestern areas during summer. Other contributions to the sediment budget are made by autochthonous biochemical activity, and chemical processes within the sediments. The same sources and processes were responsible for sedimentation during older periods of Holocene as reflected in the examination of several borehole sequences in Lower Mesopotamia. The surface and subsurface sediments are dominantly silty with silt comprising about 55% of the sediment mainly as clayey-silt and clayey-sandy-silt; the remainder consists of varying quantities of clayey and sandy sediments. Clay is more abundant in the lower parts of the surface cores analysed (30-80 cm in length) and in the upper pedogenic fluvial-plain sediments of the borehole sequences in addition to the brackish/marine sediments of the Holocene Hammar Formation. Well developed stratification is found only on the surface sections of the river levees and in the clayey-rich sediments of the modern lakes and marshes. The remainder of the sediments of both surface and subsurface are generally poorly stratified with some irregular stratification and considerable bioturbation, particularly in shelly horizons. Mineralogically, the sediments of the Ahwar consist of calcite, quartz, dolomite and feldspar (particularly albite) . Aragonite is present as a result of "in situ" production by macrofauna. Some authigenic dolomite and Mg-calcite are detected in specific playa and brackish/marine units which may be accompanied by other minerals such as gypsum and palygorskite clays, particularly in the evaporitic sediments. Smectite, illite, palygorskite, kaolinite and chlorite are the main clay minerals present and those are mostly detrital in origin. Palygorskite is the most interesting clay mineral present. It is both detrital and authigenic in origin. The formation of authigenic palygorskite reflects an alkaline, Mg-rich and relatively saline environment found accompanying arid climatological conditions. It is usually found associated with authigenic dolomite and gypsum. Detrital palygorskite is here reported in the suspended sediments of the Tigris-Euphrates rivers of this area for the first time. The calcium carbonate content of the Ahwar sediments varies from 20-80%. The latter high values are only found in skeletal-rich horizons of the near-surface and subsurface sediments. The inorganic geochemistry of the sediments reflects their high calcium carbonate content. The values of most of the analysed elements were within the limits of standard mudrocks except for Ni and Cr which are surprisingly high, presumably mainly due to the high contents of these elements in the suspended materials of the major rivers and of the aeolian deposits. The total organic content of the sediments is usually < 5% TOC . Organic-rich horizons (of > 5% TOC) are mainly restricted to the upper surface layers of the surface sections and at some specific depths of a few borehole sections. The latter has been proved to be an ancient Ahwar (lake/marsh) sedimentary unit. The preservation of the organic matter has mainly taken place during the mid-Holocene transgression and particularly along the depositional axis areas of the main Mesopotamian Basin. Pyrolysis, gas chromatography and petrography of the organic matter have revealed that many reed-originated biomarkers were present in the gas chromatograms which supported the conclusions from the petrographic analysis of organic matter indicating that reeds were the main source of this material in both ancient and present times. Peat is not forming at the present time in most parts of the area studied, except in some limited northwestern parts such as Lake Zechri and surrounded marshes. The examination of the fauna revealed the dominance of aragonitic molluscs of various species, in addition to some calcareous microfauna mainly as foraminifers and ostracods. The dominant species of the latter two were Ammonia beccarii and Cyperidles torosa respectively. These two species indicated the influence of the sea in the area as various brackish/marine coastal environments. Both surface and subsurface sequences have been subdivided into various sedimentary units. The units have been proved useful for regional correlation; particularly the three surface units (i.e. upper organic-rich, intermediate shelly and lower clayey brackish/marine units), while the five borehole units (i.e. modern fluvial-plain/Ahwar, older brackish/marine, ancient lake/marsh, ancient playa and ancient fluvial/gypcretes units) proved to be correlated throughout Lower Mesopotamia. These sedimentary units have been differentiated geochemically for both surface and subsurface sections using a multivariate statistical package (MVSP). The reconstruction of the Holocene geological history and the calculation of the sedimentation rates have been made using and ^^"^Cs dates of subsurface peats, near-surface shelly sediments, and the upper 20 cm of the surface sections respectively. Sea-level fluctuation during the Holocene has been the main factor in the evolution of the deltaic area with climatological changes, local neotectonics and differential sedimentation rates during Holocene might be additional factors.
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