Holocene sedimentary evolution and palaeocoastlines of the Lower Khuzestan plain (southwest Iran)

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. A literature review about the coastal evolution of the Shatt-el Arab region with respect to relative sea-level changes is provided. 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. Four main sedimentary environments are interpreted from the Holocene sedimentary record of the plain: brackish tidal flat, elastic coastal sabkha, brackish-freshwater marsh and fluvial plain. The study of the vertical and spatial distribution of sedimentary facies, their environmental interpretation especially with respect to the relationship to tide levels is done for five different zones in the plain. Chronological control is provided by radiocarbon dates. On the basis of this analysis, palaeogeographical reconstructions of the Lower Khuzestan plain, and the northern extension of the Persian Gulf were made for different points in time between 8000 and 450 cal BP. 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 rivers Karkheh and Jarrahi and their loci of sediment input. Moreover, in this study it is suggested that a Holocene RSL highstand, above the present-day sea level, as suggested by previous workers, did not occur. (c) 2007 Elsevier B.v. All rights reserved.