Geology Reference
In-Depth Information
Fig. 11.14
A simplifi ed sketch of the cut-and-fi ll succession produced on a tidal fl at by lateral migration of a channel meander/
pointbar. Note the healing of a slump mid-succession (Adapted from Reineck (
1967
) in Eisma (
1998
) )
The base of a channel can be recognized by a con-
cave upward erosional bounding surface, which indi-
cates confi ned fl ows (Santos and Rossetti
2006
) . In
intertidal channels, the base is often identifi able by a
lag of coarse sediment or shell, although in very
muddy systems this may be more diffi cult to distin-
guish (Klein
1977
; Barwis and Hayes
1979
; Terwindt
1988
; Rieu et al.
2005
; Pearson and Gingras
2006
) .
The thalweg of salt marsh creeks may present only as
increase in sand content. In larger channels, lag depos-
its range in thickness from a few decimeters to a few
meters and in intertidal channels shell lags of a few
centimeters in thickness are expected (Barwis
1978
;
Terwindt
1988
). Similarly, mud blocks (breccia) from
bank slumping and channel edge erosion may form
part of a channel lag, creating lithologies such as mud
chip conglomerates (Klein
1977
; Terwindt
1988
;
Santos and Rossetti
2006
) . In mesotidal back-barrier
environments, bank-margin slump blocks up to a
meter in diameter and containing preserved rhizomes
and burrows can occur (Barwis
1978
) . Large-scale
slumping has also been observed on the meter scale in
regions of the Bay of Fundy (Pearson and Gingras
2006
) in areas on pointbars that are dissected by
tributaries.
In regions experiencing seasonal variation in tem-
perature, where ice periodically forms in channel beds
(such as the north east coast of the USA and the east
coast of Canada), ice rafts may also produce patchy
granule and pebble lags and deposits of marsh peats
across pointbars (Pearson and Gingras
2006
) . In the
Bay of Fundy these were observed to form part of a
repetitive set of bedding associated with seasonal vari-
ability, rather than occurring over a clear erosional
contact as would be expected in a channel base.
Using the known relationship between cross-sec-
tional area and peak discharge, reasonable estimates
of maximum paleo-current velocity and historical
variation in tidal prism may be estimated from pre-
served channel cross-sections. Rieu et al. (
2005
)
examine a preserved tidal channel, offshore of the
western Netherlands (Fig.
11.15a
). The channel fi ll is
characterized by alternating sub-parallel high- and
low-amplitude seismic refl ectors. A clear lateral
accretion unit can be seen proximal to the channel,
indicating channel migration. The thickening of these
lateral units toward the fi nal channel position is inter-
preted to indicate an increasing tidal prism, followed
by channel fi ll related to a decrease in tidal prism
(Fig.
11.15b
). In salt marshes where meandering
channels are stable and lateral migration is close to
zero, no such lateral accretion would be expected,
accretion would occur purely in the vertical (Redfi eld
1972
; Gabet
1998
) .
Common indicators of tidal infl uence in a channel
include: reactivation surfaces (formed as the tidal fl ow
changes direction) and mud drapes in cross-sets, and
low angle dipping cross-sets with alternating thicker
and thinner packages of sands and mud, or muds and
silts (Santos and Rossetti
2006
) . Tidal deposits typically