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Fig. 13.30 Paleotidal modeling of the Burdigalian Peri-Alpine
Seaway. The colors show the velocity of residual tidal circula-
tion and the arrows the tidal-transport pathways. The first pale-
otidal modeling of this system by Martel et al. ( 1994 )
demonstrated that the Swiss Molasse basin, which hosts offshore
tidal deposits, would have been meso- to macrotidal only if the
tidal wave entered the seaway from both the southwest and east-
ern ends. More recent work by Bieg ( 2005 ) showed that the two
tidal waves had to be in phase to get the maximum resonance.
The numerically determined current patterns and strengths show
a relatively good agreement with the facies distribution and
tidal-transport pathways reconstructed from the bedforms in SE
France (e.g. Lesueur et al. 1988 ), Switzerland (e.g. Homewood
and Allen 1981 ), Germany (e.g. Hülsemann 1955 ) and Hungary
(e.g. Sztano and de Boer 1995 ) (After Bieg 2005 ). Inset map
modified from Martel et al. 1994 )
depths of approximately 200 m. These currents can
deposit large accumulations of well-sorted sand over
large areas, and are responsible for the resuspension
and redistribution large amounts of mud.
Interactions between various components of the
tide, and with the seafloor and coastal morphology,
cause the ebb and flood tidal currents to be unequal
over large areas. These inequalities generate tidal-
transport pathways along which bedload can be trans-
ported for distances of tens to hundreds of kilometers,
from high-energy erosional and by-pass zones, to lower
energy areas of sediment accumulation. The sandy tidal
sediments that accumulate in offshore areas are com-
monly coarser and less muddy than those deposited at
equivalent depths on storm-influenced shelves.
The most significant offshore tidal sediment accu-
mulations are sand sheets and ridges. These deposits
are made up predominantly of the crossbedded sand-
stone formed by tidal dunes, which can reach more
than 10 m in height. Most of these bedforms are
compound dunes, and generate compound crossbed-
ding in which a single the master bedding and smaller
cross beds dip in the direction of the residual trans-
port. Herringbone crossbedding can be present in
small amounts, but mud drapes, reactivation surfaces
and tidal bundles are not likely to be abundant. The
ichnology of these deposits reflects the mobile sandy
nature of the deposits, and the normal-marine salinity
of the water.
Most sandy tidal-shelf deposits are transgressive in
origin, with the sand supplied by erosion of the retreat-
ing coast or the offshore bedload parting area, and can
rest on an offshore tidal ravinement surface that com-
monly cuts into older coastal deposits. This scour sur-
face can be amalgamated with a sequence boundary, or
can be a more prominent distinct surface. The upper
boundary of the offshore tidal deposit is expected to be
a maximum flooding surface that reflects the decay of
tidal currents in the course of a sea-level rise. The large
size of some tidal dunes and tidal-current ridges means
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