programme tuesday, july, 31 - Université de Caen Basse Normandie

Tidalites 2012, 8th International Conference on tidal environments, Caen, France – Abstract book
TIDAL AND CLIMATE CONTROLS ON THE MORPHOLOGICAL EVOLUTIONS AND
THE INTERNAL ARCHITECTURE OF A TIDAL BAR: THE PLASSAC TIDAL BAR IN
THE BAY-HEAD DELTA OF THE GIRONDE ESTUARY
Eric CHAUMILLON*, Hugues FENIES**, Julie BILLY***, Jean-François BREILH*
*UMR CNRS 7266 LIENSS, 2 rue Olympe de Gouges, 17000, La Rochelle, France,
eric.chaumillon@univ-lr.fr, jbreil01@univ-lr.fr
**CV ASSOCIES ENGINEERING, 7 Chemin de la Marouette, 64100, Bordeaux, France,
hugues.fenies@orange.fr
***CEFREM, UMR 5110, 52 Avenue Paul Alduy, 66860, Perpignan, France, juliebilly@gmail.com
Estuarine tidal bars occur in the mouth and in the bay-head deltas of tide-dominated
estuaries or mixed tide-and-wave dominated estuaries. Tidal bars deposited in estuary mouths
are composed of relatively clean sands whereas tidal bars deposited in bay-head deltas can
contain large quantities of mud. Estuarine tidal bars have an elongated or a lobate morphology,
but relatively few studies have been conducted on the lobate category. This study is focused on a
lobate tidal bar, the Plassac tidal bar, located in the bay-head delta of the Gironde Estuary.
This work is based on: (1) successive bathymetric data (1905-2010) which allow to observe
the geomorphological evolution of the bar (century scale), (2) detailed imaging of the bedforms
that cover the surface of the bar (over a few days in 2010, Figure 1) which allow to analyze the
sediment dynamic. Those well-constrained evolutions and governing processes, combined with
very high resolution seismic and core data are then used to understand the internal architecture
of the tidal bar and the fluvial sediment influx within the bay-head delta at a time scale of a
century.
The sediment transport pattern, inferred from the lee face orientations of subaqueous dunes
observed on very high resolution bathymetric data acquired in 2010 (Figure 1), together with
results from previous studies, allows explaining the five main mechanisms for the tidal bar
evolution identified from 29 bathymetric maps since 1905: flood ramp infill, partial ebb shield
breaching, lateral accretion of the ebb spits and ebb shield lengthening, generated by the merging
of mini-flood lobes on the outer sides of the ebb spits. Lateral accretion seems to be a
key-process of sediment accretion for lobate tidal sand bars. Most of the evolutions are explained
by tidal processes, but fluvial influence is evidenced by correlating the presence of mini flood
lobes (migrating seaward from the upper reaches of the bay-head delta) and the lengthening of
the tidal bar with periods of high fluvial discharge. A 25 years periodicity in both the fluvial
discharge and the tidal bar width, length and volume variations is evidenced and suggest the
climate control on the tidal bar evolution.
Seismic and core data show that the bar was deposited onto a basal bounding surface
which consist of a discontinuous sub horizontal reflector correlated with sand and clay
alternations and which may represent the main flooding surface within the Gironde Estuary.
Within the sandbar, the master bedding corresponds to strong amplitude reflectors (clinoforms)
correlated with continuous inclined mud-rich strata. Correlation with bathymetric data shows that
clinoform orientations are not indicative of tide-induced sand transport direction but record the
progressive lateral accretion of the sandbar generated by fluvial sand influx. Thus it appears that
the internal architecture of the bar is dominated by lateral accretion of sand packages, isolated
from one another by extensive inclined mud layers. A major strong amplitude inclined reflector,
observed in the eastern ebb spit of the sandbar, is correlated with a 20 cm thick bed made of
angular mud pebbles and was deposited between 1991 and 1993. Considering the period of time
between 1958 and 2008, the years 1991 to 1993 were characterized by the largest number of
autumn river floods and preceded by the years 1989 to 1991, characterized by the largest number
of days of low river stage. A 3-step depositional process is proposed to explain the heterogeneity
present within the internal architecture of the tidal bar: (1) during periods of low river stage, the
turbidity maximum of the Gironde Estuary is located upstream, in the bay head delta, leading to
mud deposition on the sandbars; (2) subsequent periods of floods (especially autumn floods
occurring immediately after the dry summer season), lead to erosion of the mud drape lying on
the sandbar and to deposition of a mud clasts layer rapidly buried by massive sand transport.
From this study it appears that sandbars emplaced in bay head deltas of estuaries are
heterolithic bodies dominated by tides and influenced by fluvial processes. The fluvial influence
implies that they have a good potential to record high frequency climate changes. During periods
of low rainfall and low river stage, sand supply to the tidal bar is reduced and mud deposition
occurs on the tidal bar, whereas during high rainfall and associated floods, seaward sand
transport increases and leads to tidal bar lengthening, lateral accretion processes and rapid
burying of antecedent morphological features and strata.
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