Depositional Sequences and Sand Distribution in the Postglacial Rhone Delta Complex1

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Abstract
An analysis of the lithological relationships in the postglacial Rhone delta complex shows that the deltaic sediments can be grouped in one of the following depositional sequences: (a) transgressive sequences, (b) regressive sequences, and (c) channel-fill sequences. Transgressive sequences are due to a process whereby the rate of subsidence exceeds that of sedimentary buildup. This process leads to deposition of a sediment body in which coastal plain deposits are overlain by coarse-grained coastal barrier deposits that are in turn overlain by marine deposits. The sand member in such sequences may vary in thickness, but in the Rhone delta complex it is seldom more than 2 meters thick. The sand commonly has a gradational contact with the underlying coastal plain clays and is deposited in sheets. Regressive sequences are due to a process whereby sediment-laden river water enters a basin that is filled with water of higher density (for example, sea or lagoon). This process produces a sediment body that contains fine-grained sediment at its base and dominantly coarse-grained sediment at its top. The topmost sand member may attain a thickness of 10 meters and has a gradational contact with the underlying marine or lagoonal clay. The sand is deposited in sheets hundreds of square kilometers in extent. Channel-fill sequences are due to a process whereby a decrease in current velocity occurs in a channel. This process produces a sediment body that contains coarse-grained sediment at its base and dominantly fine-grained sediment at its top. The basal sand member varies widely in thickness, but in the Rhone delta complex it is seldom more than 5 meters thick. Commonly the sand body is elongate and has a width/thickness ratio of as much as 1000. This volume was based on a symposium, Deltaic Sedimentation, which was held at the AAPG/SEPM Annual Meeting in New Orleans, Louisiana on April 1965. Many geologists have become involved in studies of deltaic sediments and sedimentation processes. Some of the papers in this volume are based on detailed local studies of modern deltaic sedimentary sequences, on processes of deposition, and on physical and biological characteristics of the deltaic environments.

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