Sedimentary facies and paleoenvironmental interpretation of a Holocene marsh in the Gironde Estuary in France

The Monards Marsh is located on the northeastern bank of the Gironde Estuary in France.Lithological, sedimentological and micropalaeontological investigations were made on four cores to determine the evolution of Holocene sedimentary environments and processes in this area over the last 6 000 a.Three main lithological facies are distinguished from bottom to top:(1) grey laminated silty-sandy clay; (2) homogeneous dark grey silty clay; and (3) compact silty clay.About 26 benthic foraminifera species are identified and divided into six groups according to their ecological characteristics.In association with lithology, sedimentary structures and grainulometry, the distribution of foraminifera group is used to define external slikke, internal slikke, external schorre, internal schorre, and continental marsh facies.Combined with ¹⁴C (AMS) dating, these data indicate four successive paleoenvironments in the Monards Marsh:(1) Holocene transgression resulted in the development of a basal schorre facies overlying fluvial deposits that transformed to slikke facies sedimentation; the transgression maximum occurred around 5 600 to 5 400 a BP and was inferred to be associated with the last phase of the rapid Holocene sea-level rise; (2) post-trangressive maximum sedimentation resulted in a regressive sequence of deposits prograding towards the estuary, corresponding to the stabilisation of sea level after 5 400 a BP; (3) a slight positive tendency in the sea level around 2 800 a BP recorded in the central part of the marsh; and (4) the wetland to a continental marsh environment.The sequential pattern for the evolution of wetlands in this estuarine area during the Holocene is fluvial facies-blackish schorre facies-slikke facies-blackish schorre facies-continental marsh facies.Characteristics of sedimentary facies distribution and evolution reveal that the development of Holocene salt marsh in this area was controlled by the sea-level change and tidal range.The sedimentary facies show an obvious surficial and vertical banding distribution in which the texture and structure, authigenic mineral, foraminifera distribution and bioturbation indicate many distinct difference characteristics.