1-Introduction
The Lower Khuzestan plain, located in southwestern Iran, forms the southeastern extension of the Lower Mesopotamian plain. It borders to the west one of the most important estuaries of the Middle-east region, i.e. the Shatt-el Arab. Three major rivers (Tigris, Euphrates and Karun) flow into the Shatt-el Arab which debouches into the Persian Gulf nearby Fao (Fig. 1). The study of palaeoenvironmental changes in the Lower Khuzestan plain is challenging, because of its rich archaeological heritage. One of the most ancient civilizations, the Elamite Kingdom (2700–539 BC) was primarily centred in the province of what is now called modern-day Khuzestan. Changes in coastal configuration could have had a profound effect on the population occupying the Khuzestan region from Elamite time to present. In this study, the Holocene sedimentary sequence of the Lower Khuzestan plain is investigated to reconstruct the Holocene evolution of the coastline and plain.
The Lower Khuzestan coastline is today being shaped by a semi-diurnial mesotidal regime. The tidal range averages ca. 3–4 m along the coastline, increasing to 5–6 m in the Khawr-e Musa tidal embayment. Because of the gentle offshore slope, wave energy is very low. The tide-dominated coastline is fringed by a large tidal flat with a width up to 15 km, extending landward along the Khawr-e Musa tidal embayment. The vast intertidal area is bordered by supratidal salt marshes and clastic coastal sabkha''''s with saltpans. There is no freshwater inflow into the intertidal area, except in case of extreme river flood events. In the south, the Jarrahi river distributary fan system feeds the Shadegan marshes. The vegetation density (Typha and Phragmites) and extension of these marshes vary greatly, as a function of the seasonal rainfall regime. The Holocene sequence of the Lower Khuzestan plain in southwest Iran has been investigated in the context of coastal evolution and relative sea-level change.

2-Materials and methods
For the stratigraphical investigation, 11 cores were collected manually. The coring was carried out using a gouge auger to obtain undisturbed and continuous cores to a depth of 5–10 m below the surface. A spiral auger was used to penetrate the compact clay layers. Borehole locations were registered using GPS. The surface elevation of the boreholes was inferred from topographic maps and sitespecific measurements obtained at the regional topographic institute. In the field, the cores and outcrops were described on lithology, sedimentary structures and macrofossils and preliminary facies identification was made. Samples were taken for laboratory analyses when significant changes in color, texture or lithology were observed. A further interpretation of the different depositional (sub) environments was carried out on the basis of the integration of lithological and palaeoecological (foraminifera and diatom) analyses and is discussed. In this study, radiocarbon dating 14C-AMS (Accelerator Mass Spectrometry) was performed using organic materials and bulk of 6 samples. The sample age was calibrated by the OxCal software (Bronk Ramesy and Lee 2013) with a 2-Sigma error range and a reliability coefficient of more than 95%.

3-Results and discussion
The sedimentary succession in undisturbed hand-operated cores and temporary outcrops is described and facies are identified on the basis of lithology, sedimentary structures and macrofossils. Three main sedimentary environments are interpreted from the Holocene sedimentary record of the plain: tidal flat and coastal sabkha, brackish–freshwater marsh and fluvial plain.  This study shows that during the early and middle Holocene, the Lower Khuzestan plain was a low-energy tidal embayment under estuarine conditions. During the initial sea-level rise of the early Holocene, the coastline rapidly transgressed across the shelf, and drowning of a major valley resulted in the development of extended tidal flats. Deceleration of sea-level rise after approximately 5500 cal BP, together with probably more arid conditions, allowed coastal sabkhas to extend widely and to aggrade while the position of the coastline remained relatively stable. Continued deceleration of sea-level rise initiated the progradation of the coastline from ca. 2500 cal BP. The effect of sediment supply by the rivers became more important than the effect of relative sea-level rise. The Karun megafan developed under a descelerating rate of sea-level rise, controlling the avulsive shifting of the river Jarrahi and their loci of sediment input. The development of the Jarahi River megafan has occurred in the study area with a decrease in the rate of sea level rise and stabilization of the river course in the upstream sections of the river, as well as an increase in sediment input from a depth of about 3 to 5 meters depending on the location of the core.

4- Conclusion
As the Jarahi River alluvial fan advances from east to west, environmental changes from coastal and tidal sediments to wetlands and then alluvial sediments appear in the upper part of the cores. The presence of alluvial and wetland sediments in the wetland marginal cores can be affected by changes in the level of the wetland, migration of the Jarahi River channel at the level of the alluvial fan, and human factors and manipulations.