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Abstract


Clastic Tidal Sedimentology — Memoir 16, 1991
Pages 29-39
Recognition Criteria and Facies Models

Sedimentary Processes and Facies in the Gironde Estuary: A Recent Model for Macrotidal Estuarine Systems

George P. Allen

Abstract

The Gironde estuary was formed by the Holocene drowning of a Pleistocene incised fluvial valley and is presently filling with a regressive wedge of fluvially-derived sediment, organized in a longitudinal facies pattern reflecting the downstream evolution from fluvial, to tidal, to mixed tidal-wave dominant environments. The estuary exhibits a tripartite geomorphological zonation comprising: 1) a mixed fluvial and tidal upper estuary with meandering channels and tidal point bars; 2) a tide-dominated, funnel-shaped lower estuary with nonerosional muddy channels and elongate sandy tidal bars; and 3) a wave and tide-affected inlet with sandy coastal barriers and tidal delta shoals.

Tide amplitudes range from 2.5 to 6 m, and tidal current reversal can occur as far as 130 km from the estuary mouth. A large fluvial sediment load is introduced into the estuary where it is reworked by a combination of density circulation and tidal currents, and deposited in the upper estuary channels and funnel. Suspended sediment is trapped within a turbidity maximum that migrates longitudinally in the estuary, extending landward up to the limit of tidal currents during low river flow. The coarse fluvial sands and gravels are blocked from entering the estuary by tide current asymmetry.

These processes result in a distinct longitudinal facies zonation that parallels the morphological pattern of the estuary. The fluvial-tidal facies transition occurs in the vicinity of the bayline, where the fluvial profile reaches tidal base level, and is marked by an evolution from coarse sand and gravel point bars, to sand and mud estuarine point bars. These are characteristically devoid of alluvial levee and crevasse splay deposits, since the large tidal prism in the estuary damps out alluvial floods. Farther downstream, the limit between the upper estuarine channel and the funnel is marked by a transition from point bars to upward coarsening tidal bars that prograde seaward over estuarine mud. The estuary inlet contains coarse sand with tide and wave structures. These sands are of marine origin and introduced into the estuary from the coast by longshore drift and flood tide currents. They form the transgressive Holocene substratum over which accumulates the present regressive estuarine wedge.

These facies patterns and limits are easily recognizable in cores and outcrops and furnish criteria to delimit paleogeographic boundaries. They also form stratigraphic markers to identify and correlate the different phases of an estuarine valley fill.


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