Transgressive Facies and Sequence Architecture in Mixed Tide- and Wave-Dominated Incised ValleysExample from the Gironde Estuary, France

Publisher Website
Google Scholar
Abstract
A study of the Holocene deposits within the Gironde estuary incised valley indicates that a significant proportion of the valley fill has accumulated during transgression in the form of backstepping estuarine deposits. This incised valley was subjected to a mixed wave- and tidal-energy regime, and the transgressive fill was deposited in three major depositional environments: (1) the tide-dominated inner estuary, extending between the estuary mouth and the bayline, (2) the mixed tide and wave estuary mouth, and (3) the landward-migrating wave-dominated shoreface. The sediments deposited within each environment are bounded by valley-wide stratigraphic discontinuities and together comprise the transgressive systems tract. Each depositional environment develops a specific facies assemblage, and the transgressive fill occurs in three successive phases. Phase I occurs at the onset of transgression, when the incised alluvial valley is converted into an estuary by marine flooding. At this time, inner-estuary tidal sands and muds onlap the lowstand alluvial profile at the bayline. Phase 2 occurs when the spit-constricted estuary-mouth tidal channel migrates landward over the inner-estuary sands and muds. This results in tidal currents scouring deeply into the underlying phase 1 estuarine sediments, forming a highly erosional tidal ravinement surface overlain by massive estuary-mouth sands. As the wave-dominated shoreface migrates landward across the incised valley and its interfluves, the wave-ravinement surface is formed. The transgressive shoreface sands and marine muds overlying this surface constitute phase 3 of the transgressive incised-valley fill. These deposits subsequently are capped by the Maximum Flooding Surface which marks the upper bounding surface of the Transgressive Systems Tract. The shoreline at the time of maximum flooding forms a regional facies limit within the incised valley. Landward of this limit, the transgressive tract comprises primarily inner-estuary tidal sands and muds. Seaward of this limit the valley fill is more sand-prone and contain thick estuary-mouth sands.

This publication has 0 references indexed in Scilit: