Tidal and fluvial controls on the internal architecture and sedimentary facies of a lobate estuarine tidal bar (The Plassac Tidal Bar in the Gironde Estuary, France)

摘要

Estuarine tidal bars emplaced in estuaries are complex sediment bodies composed of sand and mud provided by rivers. This study focuses on the Plassac Tidal Bar, which is the unique lobate-shape tidal bar of the Gironde Estuary, located at the very upstream extremity of the bay-head delta, because no detailed sedimentological studies have been made on such geomorphological category of tidal bar. Its stratigraphy was investigated using a dense grid of 65 very high resolution seismic profiles, ground-truthed by 6 to 9 meter-long cores and time-controlled by historical bathymetric surveys. The base of the tidal bar consists of a tidal erosional surface. This surface has eroded muddy tidalites, corresponding to the maximum flooding surface of the Gironde Estuary. The bar is composed of two vertical sequences of facies, composed of thickening-up cross stratified fine- to medium-grained sand beds at the top and decimeter-thick mud-layers at the base. Those mud layers constitute the master bedding of the bar and are dipping in a channelward direction, perpendicularly to the tidal flows and to the sediment transport directions. One phase of lateral accretion of the Plassac Tidal Bar is correlated with variations in fluvial discharge of the last decades and suggests a climate control on the internal architecture of this sandbar. Low water periods lead to the upstream migration of the turbidity maximum and to the deposition and consolidation of the muddy master bedding planes. During the subsequent flood periods, the muddy master bedding planes are partially eroded and then buried by the fluvially-sourced sandy mini-flood lobes that merge with the tidal bar. The internal architecture of the Plassac Tidal Bar is controlled by a lateral accretion process, its master bedding surfaces which record this process are dipping perpendicularly to the axis of the tidal flows. All the other tidal bars of the bay-head delta (around a dozen) exhibit an elongated morphology and prograde longitudinally by seaward frontal accretion occurring in the axis of the tidal flows. The master bedding surfaces which record this process are seaward dipping along the axis of the tidal flows. Those variations in internal accretion surfaces orientation of tidal bars are of considerable importance and must be taken into account for studies of ancient tidal deposits, paleoenvironment reconstructions and for heterogeneities characterization and quantification of reservoir properties in these types of sandstones.