Geology Reference
In-Depth Information
Fig. 5.18
(
a
) Erosional foreshore along the margin of Cobequid
Bay, Bay of Fundy, with cliffs composed of Triassic sandstone,
with a beach at the high-tide level. (
b
) Gravel beach in Cobequid
Bay that has migrated in front of and is encroaching on salt-
marsh deposits. The gravel is sourced from coastal erosion of
Pleistocene till and glaciofluvial outwash
The nature of the contacts between the sand flats,
mudflats and salt-marsh can be either gradational
(Fig.
5.10b
) or erosional (Fig.
5.10d
). Lateral migra-
tion of a channel, or enlargement of a channel because
of increased fluvial discharge, causes frequent erosion
of the outer edge of the mudflat and/or salt-marsh
(Fig.
5.10c, d
). The cliffs created by these processes
generate steeply inclined or even vertical erosion sur-
faces that can be mantled by a mud-pebble conglomer-
ate. Once the channel migrates away, or the river flow
returns to a lower value, the previously erosional area
becomes depositional, and rapid vertical aggradation
occurs, producing a terraced margin to the channel
(Fig.
5.10d
). Such situations generate upward-fining
vertical successions with a thickness (before compac-
tion) that is equal to the channel depth, in which the
tidal deposits are essentially horizontal. In other cases,
the banks of the channel are more gently sloping, with
gradational facies contacts, and produce inclined het-
erolithic stratification (IHS; Thomas et al.
1987
) that
dips toward the channel with inclinations typically of
5-15°. The conditions under which each of these two
channel-bank morphologies exist are not known.
Smaller tidal channels, or the channels of tributary
streams, dissect the mudflats and salt marshes (Fig.
5.10b
;
Chap. 11). These channels become wider in a seaward
direction, and their banks become less steep as they
pass from the mudflats out into the sandflats. The floor
of these channels will consist of a patchy lag of mud
pebbles derived from erosion of the bank. Shell debris
can be present locally, but is typically monospecific in
character because of the reduced salinity. Sand is rarely
present in the channels that do not have terrestrial
drainage, but can be present in channels that have their
