Environmental Engineering Reference
In-Depth Information
Direct tidal controls (i.e., tide-influenced shorefaces or TIS) are observed in set-
tings with strong tidal currents and intermittent slack-water periods, wherein
strong tidal currents, running parallel to the coastline, dominate sediment trans-
port and deposition in the lower shoreface and, to a lesser extent, the middle
shoreface (
Dashtgard et al., in press; Frey and Dashtgard, 2011
). In the upper
shoreface and foreshore of TIS, the wave-orbital velocities and wave-generated
current velocities exceed the tidal current velocities, such that the upper shore-
face and foreshore are sedimentologically and ichnologically similar to their
fair-weather wave-dominated and storm-dominated (non-tidal) counterparts.
For TIS, tidal controls on sediment deposition are best expressed in the lower
shoreface and offshore. Across offshore through middle shoreface zones, grain
sizes either remain constant or increase slightly in the offshore direction, and
the mud content of the sediment is very low (
Frey and Dashtgard, 2011
). In the
lower shoreface and offshore, sand accumulates as current-generated structures
and plane beds, although these structures are commonly obliterated by pervasive
bioturbation. Burrows are predominantly vertical (
Fig. 14
) and form suites attrib-
utable to the archetypal
Skolithos
Ichnofacies. Toward the middle shoreface, there
is increased preservation of wave-generated sedimentary structures (e.g., SCS and
HCS), with relatively limited preservation of bioturbated fair-weather beds
(
Fig. 14
). Burrows observed in modern middle shoreface deposits of TIS are also
typical elements of the
Skolithos
Ichnofacies and include
Arenicolites
,
Planolites
,
Skolithos
,and
Conichnus
(
Fig. 14
). The upper shoreface and foreshore of TIS are
sedimentologically and ichnologically similar to their wave-dominated and storm-
dominated counterparts and commonly possess the
Macaronichnus segregatis
“toe-of-the-beach” assemblage (cf.
Saunders et al., 1994
), developed between
the upper shoreface and foreshore (
Vakarelov et al., in press
).
Tidal modulation, unlike direct tidal controls on sediment deposition, refers
to the lateral translation of wave zones across the shoreface profile in response
to the rising and falling of tides. In microtidal and lower mesotidal settings, the
translation of wave zones along the depositional dip is minimal and probably
falls within the range of variability imposed by changing wave conditions. With
increasing tidal range (macrotidal and megatidal settings), however, the proxi-
mal-distal displacement of wave zones is pronounced, and it is not uncommon
to subaerially expose upper, middle and possibly lower shoreface sediments at
low tide (
Dashtgard and Gingras, 2007; Masselink and Hegge, 1995; Masselink
and Short, 1993; Masselink and Turner, 1999; Short, 1999
). The sedimentologi-
cal and ichnological expression of such significant tidal ranges are distinct and
the resulting shoreface character is referred to as a “tidally modulated shore-
face” (
Dashtgard et al., 2009, in press
).
TMS constitute the tidal end member of the wave-tidal spectrum for shore-
face settings. TMS are identified by (1) common interbedding of tidal sedimen-
tary structures down the shoreface profile, (2) the notable absence of an obvious
middle shoreface, (3) an anomalously thick foreshore, and (4) the presence of a
low-diversity and low-density trace-fossil suite of simple vertical and
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