Geology Reference
In-Depth Information
availability also influence the size to which dunes grow.
At the scale of an entire tidal-transport pathway,
the dunes decrease in size along the pathway due to
a decrease in tidal-current speed and grain size
(Fig.
13.18a
). Thus, the largest dunes are commonly
found a short distance down-flow from the point where
significant deposition occurs within the pathway,
because this is where the current speed, grain size and
sediment availability are greatest. At a smaller scale,
the dunes in sand sheets are commonly grouped in iso-
lated dune fields that are surrounded by an immobile
substrate (e.g. Reynaud et al.
1999a
). By contrast to
what happens at the larger scale, the dunes in a dune
field commonly increase in size in the down-flow direc-
tion, because of the increasing amount of available
sand. As the dunes get bigger, they also get less mobile,
with the larger ones tending to trap sediment brought to
them by more rapidly migrating smaller dunes. Sand
may not be able to escape from them; consequently, the
dune field can terminate abruptly with the largest dunes
near or at their leading edge. Within sand-ridge fields,
scour in the troughs between the ridges commonly
exposes older deposits that can be an internal source of
sediment. The coarse-grained, shelly lags that occur in
the troughs correlate to, and may be laterally continu-
ous with, the lag facies in the bedload-parting area.
tidal activity and areal expansion of the bedload parting
area because of the transfer of sand to the depositional
area (Harris et al.
1995
). The lower part of this succes-
sion should coarsen upward, with an upward increase
in the scale of the crossbeds, as a result of a 'prograda-
tion' of up-flow parts of the sand sheet over its more
distal portion (Fig.
13.19a
). As progressively larger
bedforms migrate into the area, the scour surfaces at
their base might become more prominent, potentially
removing significant a amount of the pre-existing suc-
cession. If migration of the sand sheet continues, the
succession will be overlain by an erosional surface
corresponding to the bypass zone at the upcurrent-end
of the transport path (Fig.
13.19a
). If, instead, flow
speeds decrease because of a change in the tidal regime,
then the dunes on the sand sheet should become smaller
and finer grained, generating an overall upward fining
succession at the top of the sand-sheet deposit.
Full development of such a succession, with a lower
upward coarsening/thickening part and an upper
upward fining thinning of crossbeds, requires a rela-
tively abundant supply of sediment. Mellere and Steel
(
1996
) and Blackwood et al. (
2004
) describe succes-
sions from a seaway setting that show strong similari-
ties to this model. Perhaps the closest match has been
described from a cool-water carbonate environment
(Anastas et al.
2006
; Fig.
13.19b
). In this case, the suc-
cession has been interpreted to reflect the migration of
an area where the tidal current is above the threshold
of dune formation rather than a change of current speed
as a result of sea-level rise, but the result of these two
processes may be difficult to distinguish. At the fringe
of that area, patch reefs are present and there is a ten-
dency for the development of hardgrounds (cf.
Fig.
13.19b
).
Strong tidal currents are commonly correlated with
the supply of nutrients, so that the deposits in tidal
sand sheets are likely to be bioturbated and, even in
siliciclastic settings, to contain shelly fauna (Wilson
1982
). If the dunes in a tidal-transport pathway migrate
rapidly, then the intensity of bioturbation will be low,
whereas dunes that migrate slowly (i.e. the larger ones,
or those toward the distal end of a tidal-transport
pathway) can be bioturbated more thoroughly. In gen-
eral, the bottomsets of the dunes will be more intensely
bioturbated than the foresets. The assemblage will be a
mixed
Skolithos
-
Cruziana
Ichnofacies (Seilacher
1967
;
Ekdale et al.
1984
; MacEachern et al.
2005
), with verti-
cal burrows subtending from the erosion surfaces that
13.6.3 Deposits
Evidence of the former existence of tidal-transport
pathways is likely to be preserved on flooding surfaces
in ancient successions that contain evidence of tidal
action. Over large areas, this evidence will consist of
a marine erosion or 'ravinement' surface (see more
below) that is mantled by a thin lag. In a down-transport
direction, the deposits on this surface will thicken,
potentially reaching a few tens of meters in thickness.
The most volumetrically significant facies will consist
of cross-bedded sands formed by dunes that were part
of isolated dune fields, or of more extensive sand sheets
and tidal-current ridges. The deposits of tidal-current
ridges have been discussed above; here, we examine
the deposits of sand sheets (Fig.
13.18a
).
Little is known about the organization of these
deposits, but we hypothesize that the vertical succes-
sion produced by a sand sheet consists of a stacked
succession of simple and compound-dune deposits
(Fig.
13.5
), produced during an episode of increased
