Origin and morphosedimentary evolution of fine-grained modern continental shelf deposits: the Gironde mud fields (Bay of Biscay, France)

The Gironde mud fields are thin blankets (<4 m) that extend from the inner to mid-shelf over a restricted area (600 km 2) and overlie palimpsest sands and gravels seaward of the modern Gironde estuary, south-western France. The bulk of the mud has accumulated over the last two millennia and reflects the complex interactions of: (1) an increasing estuarine supply of suspended particulate matter; (2) resuspension of substrate material by storm events; and (3) biological mixing. The combination of these processes has produced various features within the deposits, related to: (1) the water depth and distance from source of the fine-grained material, which are manifested as a proximal-distal cross-shelf trend, from an area of alternating mud sedimentation (temporary after large river flows) with resuspension by storm events to an area of mud accumulation in deeper water where bioturbation predominates; (2) expansions of the mud depocentres over time, such that they become more remote from sources of coarser shelf sediment, resulting in an upward-fining trend. The lithofacies of the Gironde shelf mud fields exhibit a number of similarities to those of larger scale modern muddy shelves and ancient shallow-water mudrocks; this work provides a detailed description of a facies model for fine-grained shelf sediments deposited under relatively constant sea level.