Geology Reference
In-Depth Information
creating a tendency for the flood-tidal currents to be
faster than the ebb. A similar distortion occurs if the
tidal wave enters an embayment, because the progres-
sive, offshore tidal wave cannot continue to propagate
freely. Interference of M2 and M4 harmonics of the tide
brings about either tidal-phase asymmetry if the M2 and
M4 are 90° out of phase or tidal-current inequality if the
M2 and M4 are in phase. The tidal motions will be
asymmetric in either case: in the first case, the flow in
one direction will last longer than in the other, and, in
the second, the flow in one direction will be faster
although of shorter duration.
Since bedload transport is approximately propor-
tional to the cube of the current speed, any asymmetry
in ebb and flood currents will generate inequalities in
the sediment transport in the two directions. The result
is the creation of a residual sediment transport in one
direction (either the ebb or flood). Such inequalities
extend over large areas and are referred to as tidal-
transport pathways, which are discussed at length later
in this chapter.
supplied by rivers. Consequently, older deposits have
been reworked by waves and tidal currents that have
winnowed away the fine-grained material, leaving
behind tidal deposits that are composed predominantly
of medium to coarse sand. Such is the case around the
British Isles. For the reasons discussed at length by
Dalrymple (
2010a
), the sand becomes finer in the
direction of sediment transport, such that coarser sedi-
ment, including gravel, can be present at the up-current
end of tidal-transport paths, where the currents are
fastest, passing down the transport path to fine and
very fine sand and even muddy deposits. Tidal currents
are an effective sorting agent, and the sorting index is
generally high, and increases along the pathway (Gao
et al.
1994
).
On shelves that are not supplied by large mud-rich
rivers, carbonate grains can be an important constitu-
ent of the deposits because tidal currents favor the
supply and mixing of nutrients coming from the open
sea, thereby promoting carbonate production. In cases
where there is little or no siliciclastic material, the
tidal-shelf deposits can be composed entirely of car-
bonate grains. In tropical settings, such tidal deposits
are commonly composed of ooids, which are believed
to be a type of grain formed almost exclusively in tidal
settings (e.g. the Bahama Banks; see Chap. 20). In
cool- to cold-water settings, herterozoan benthic com-
munities generate abundant bioclastic debris that is
particularly prone to reworking by tidal processes
(cf. Anastas et al.
1997
; James
1997
). Tidal-transport
pathways exist in carbonate environments (e.g. Harris
1988
), but, in such settings, sediment grain size is more
strongly controlled by the biota present than by the
speed of the tidal currents.
Along a tidal-transport pathway, the nature of the
substrate and the strength of the currents control the
nature of the benthic biota. Areas scoured by strong
currents, where the sea floor consists of exposed bed-
rock, are dominated by epibenthic, encrusting faunas,
whereas depositional tracts with mobile sand are domi-
nated by endobenthic faunas (Wilson
1982
); in general,
however, the more mobile the substrate, the less diverse
the fauna will be. In the modern, relatively little study
has been devoted toward linking the fauna with position
along a transport pathway, although spatial variations
in the composition of small bryozoan particles (Bouysse
et al.
1979
) or molluscan species (Reynaud et al.
1999c
)
have been noted. Physical and biogenic destruction of
particles occurs during transport. On modern shelves,
13.3
Sediment Types on Tidal Shelves
Tidal currents are fast enough on many shelves to
transport sand and finer-grained sediment. Much of the
existing literature concentrates on sandy deposits, but
muddy tidal-shelf deposits are important in areas sup-
plied with large quantities of mud by rivers (e.g. the
Amazon and Guyana shelf, the Gulf of Bengal and the
Andaman Sea, and the inner portion of the East China
Sea). On these shelves, tidal currents contribute sig-
nificantly to the resuspension of mud (e.g. Viana et al.
1998
; Yang and Liu
2007
). For example, one of the
largest turbid plumes in the world occurs in the
Andaman Sea as a result of tidal-current activity
(Ramaswamy et al.
2004
) with the resulting export of
mud to deep water (Rao et al.
2005
). On the Amazon
shelf, the tidally resuspended mud is advected to the
north by wind-driven currents and forms a near-coast
nepheloid layer that reduces the bottom friction; con-
sequently, the tide that reaches the coast is larger than
would be the case otherwise (Gabioux et al.
2005
;
Bourret et al.
2008
). In the Yellow Sea, tidal resuspen-
sion of mud from offshore deposits is responsible for
the creation of sandy lags.
Most modern shelves that experience significant
tidal-current action are beyond the influence of sediment
