Geology Reference
In-Depth Information
should have a signifi cant impact on the action of
deep-water tides. The strongest tides in the deep oceans
may therefore generally occur during sea-level low-
stands (greatly modifi ed, no doubt, by any given basin's
bathymetry and geometry), when preservation may be
more of an issue, especially within submarine canyons.
For the present, however, we have a very poor under-
standing of deep marine tidal deposits and their
relationship with sea-level or energy cycles.
demonstrated that internal tidal energy can become
concentrated in submarine canyons, which may aid in
enlargement of the canyons by erosion of the walls.
Much of the current surface tidal energy is dissi-
pated on continental shelves and in shallow seas
(Egbert and Ray
2003
). Many of these were smaller or
absent of water during the Last Glacial Maximum, and
therefore the excess tidal energy, both of the surface
and the internal tides, had to be directed elsewhere.
The fi rst effect of this is that the conversion of surface
to internal tidal energy would have been greater, mean-
ing there was signifi cantly more energy available in
the deep-water via internal tides during periods of low-
ered sea-level (Egbert et al.
2004
) . One would expect,
therefore, that any effect of erosion or non-deposition
such as that proposed by Cacchione et al. (
2002
) should
be more of a factor during times of lowered sea-level,
and that internal tides would have more of or a slightly
different morphological impact on continental slopes
during those times. While the morphological impact of
wave, surface, and internal tides has been examined by
trying to forward model seismic profi les of some upper
continental slopes using the gravity effect (Mitchell
and Huthnance
2008
), the same has not been done
looking at really deep-water settings, thus the sensitiv-
ity of seafl oor morphology to internal tidal forcing is
very poorly understood.
14.9
Morphological Impact of Tides
in Deep-Water Setting
Altimetry analysis of the oceans have demonstrated
that a huge amount of tidal energy is dissipated in the
deep oceans (about 10
12
W (1 TW) - one-quarter to
one-third of the total tidal energy) (Egbert and Ray
2000
). This energy is dissipated in areas of rough
seafl oor topography such as seamounts, continental
slopes, and ridges, and may be a major formative agent
in the morphology of these features, and a major factor
in abyssal circulation (Cacchione et al.
2002
; Mitchell
and Huthnance
2008
; Munk and Wunsch
1998
) .
Cacchione et al. (
2002
) calculated that refl ected semi-
diurnal internal tides may provide enough energy to
continental slopes to inhibit deposition of very fi ne-
grained sediment over a characteristic depth range
dependant on the physiography of the slope, oceano-
graphic conditions, and the orientation and strength of
incident internal tidal waves. This inhibition of deposi-
tion, they suggest, helps to create the morphology of
the continental slopes on the large-scale by creating a
zone of nondeposition or resuspension on the slope.
While their formulation suggested that a particular
angle of incidence of the internal wave to the angle of
the slope is required to cause shear coupling with the
substrate, which is in agreement with other analytical
solutions (Cacchione et al.
2002
; Garrett and Kunze
2007
; Thorpe
1992
), nonlinear analyses suggest that in
fact the angle of incidence relative to the slope plays
little role in energy transfer (Legg and Adcroft
2003
) ,
and therefore shear should be generated at any bound-
ary acted upon by internal waves. Observational data
from offshore Oregon show that turbulent mixing can
occur quite strongly due to internal tidal forcing, and
appears to be aided signifi cantly by seafl oor topogra-
phy, such as rugosity above a submarine landslide
(Moum et al.
2002
) . Hotchkiss and Wunsch (
1982
)
14.10
Summary
Internal tidal currents have been well documented in
the modern oceans, from the upper slope into deep
ocean basins. The strength of internal tides is com-
monly enough to remobilize coarse sediment and form
bedforms in sand and gravel, and may be a signifi cant
factor in sediment movement over long time periods in
some deep-water settings (Fig.
14.2
). Recognition cri-
teria for deep marine tidal deposits include evidence of
cyclicity, including short-term (semi-diurnal, diurnal)
and long-term cyclicities (bi-monthly, monthly, and
longer), commonly refl ected in tidal couplets or in
inverse-to-normally graded intervals (Fig.
14.7
).
Additionally, the extremely asymmetrical current ener-
gies involved in tidal regimes often leave evidence in
the way of mud-draped ripples or cross-strata with evi-
dence of reactivation surfaces, sometimes in different
or even opposing directions. Although some work has
been done on internal tidal deposits in outcrop,
