Geology Reference
In-Depth Information
Fig. 5.10
Morphology and facies zonation in the Cobequid
Bay—Salmon River estuary, Bay of Fundy, Nova Scotia. (
a
)
Elongate sand bar in the outer part of the estuary, covered by
large compound and simple dunes. The featureless area to the
south of the bar (
at bottom
) is an erosional, wave-dominated
foreshore/shoreface. (
b
) Upper-flow-regime sand flats that lie
landward of the elongate sand bars, flanked on the south (
fore-
ground
) by mudflats and salt-marsh. Note the dendritic tidal-
gully networks that dissect the muddy deposits. Until the 1950s,
the main ebb channel lay along this south shore. It then abruptly
switched to its present course along the north shore, allowing
7-8 m of mudflat and salt-marsh deposits to fill the old channel.
(
c
) Subtle elongate bar and flood barb (Fig.
5.9b
) on the seaward
side of a gentle point bar (to the
left
of the image) in the outer
'straight' portion of the Salmon River. The surface sediment in
the channel is fine sand. A narrow band of mudflat separates the
channel—bar sands from the salt-marsh, most of which has been
reclaimed for agriculture. (
d
) Mudflat terraces, separated by for-
mer cutbank cliffs, near the transition from the outer 'straight' to
the tightly meandering zone in the Salmon River (Fig.
5.1a
,
inset
). The dashed line is the former cutbank location of the
channel
(Fig.
5.1d
). Bars are commonly asymmetric, with the
steeper side facing in the direction of the stronger of
the ebb and flood currents; because of the overall flood
dominance that characterizes the outer estuary, this is
generally the flood current. Bar crests vary from rela-
tively narrow and sharp-crested to broad and flat. As
described first by Harris (
1988
), and noted subse-
quently by other workers (Dalrymple et al.
1990
; Ryan
et al.
2007
), the sharp-crested bar form represents situ-
ations that are underfilled, whereas the flat-topped
form occurs in situations where the bar has aggraded
as high as it can, and has expanded laterally, through
deposition on one or both flanks. It is invariably the
case that the broad, flat-topped bars occur in the inner
part of sand-bar complexes, whereas the narrow, sharp-
crested forms occur at the seaward end (unless wave
action prevents this). For this reason, the crest of indi-
vidual bars, and of the bar complex as a whole, rises in
a landward direction.
The rate of morphologic change of the channels that
separate the elongate tidal bars is not known with con-
fidence. The most dramatic and frequent changes occur
as a result of 'tidal avulsions' whereby a swatchway
becomes large enough that it captures the main ebb
flow, causing an abrupt change in the path of the main
channel. This appears to have occurred repeatedly in
the outer part of the Ribble Estuary, Great Britain
(Van der Wal et al.
2002
), and has been documented in
the Cobequid Bay (Bay of Fundy) estuary (Dalrymple
et al.
1990
). Major storms might play an important role
in triggering such channel switches. Sediment then
fills the abandoned channel (Van der Wal et al.
2002
),
provided there is not enough tidal flux to maintain
the channel. Slow, progressive shifting of the gentle
