Geology Reference
In-Depth Information
Currently an active area of research, much of the
state of knowledge on internal waves and internal tides
comes from a lot of numerical modeling and a geo-
graphic scattering of in-situ measurements at arbitrary
depths, most commonly at least a few meters above the
sea fl oor, which although numerous are far from a
comprehensive look at the deep oceans.
amplitude and 20-75 m wavelength in the Monterey
submarine canyon which they interpreted as the prod-
ucts of internal tidal currents, although whether these
were generated by internal tidal currents or simply
modifi ed by them is unclear. Cacchione et al. (
1988
)
showed that strong internal tides are present near
Horizon Guyot in the Pacifi c Ocean at depths of
1,100 m, where large-wavelength (30 m) sand dunes
with rippled stoss sides had previously been observed
(Lonsdale et al.
1972
). These dunes appear to migrate
up-slope on both sides of the Guyot (Lonsdale et al.
1972
), possibly due to transport by different phases of
the internal tides, equivalent to fl ood and ebb barotro-
pic tides in nearshore environments.
In modern fjord settings, water depths can easily
reach several hundred meters, and not uncommonly
can exceed 500 m (Benn and Evans
1998
; Eyles et al.
1990
) . The infl uence of internal waves generated by
the interaction of the surface tides with topography
(especially sills) has long been recognized (Allen and
Simpson
1998
; Hein and Syvitski
1992
; Stigebrandt
1976
; Stigebrandt
1979
), and appears to be a major
factor in water exchange (Vlasenko et al.
2002
) .
Generation and propagation of internal tides in fjords
is greatly enhanced by strong vertical density gradients
due to temperature, sediment concentration, and salin-
ity changes. Bottom current velocities due to internal
tides in fjords can exceed several tens of centimeter per
second (Inall et al.
2004
; Stashchuk et al.
2007
) , suffi -
cient not only to cause vertical mixing, but to create and
move signifi cant bedforms (Fig.
14.2
). Direct observa-
tion of internal tidal bedforms or deposits in modern
fjords are not well documented, however, and most
tidal rhythmites in fjords are attributed to the action of
the surface and not internal tides (Cowan et al.
1997
;
Cowan et al.
1998
). Like many other deep-water phe-
nomena, however, observation is often the most diffi -
cult part.
14.3
Modern Examples of Deep-Water
Tidal Deposits
Excellent but fairly short-term records of currents in
deep-water settings are plentiful, and clear and con-
vincing correlation of many deep-water currents with
tidal periods exist (Fig.
14.4
). Observations include the
direction and strength of currents, and resulting bed-
forms (Figs.
14.5
and
14.6
) (Cacchione et al.
1988
;
Noble et al.
1988
; Shanmugam et al.
2009
; Shanmugam
et al.
1993
; Xu et al.
2008
). Common bedforms on the
modern seafl oor that can be related to tidal currents
include plane beds, furrows and scours, asymmetric
and symmetric ripples, and small to large dunes.
A major problem in modern deep-water settings is
direct assignment of formative currents to bedforms,
as most of the photographs of bedforms are taken when
the currents are not strong or are absent. However, the
common presence of bedforms such as these in areas
where internal tidal currents appear to dominate over
contour currents strongly suggests internal tides are a
major factor in their formation (Heezen and Rawson
1977
; Okada and Ohta
1993
) . Heezen and Rawson
(
1977
) reported observations of internal tide and other
bottom-current driven erosion and deposition on the
Cocos Ridge in 650-2,000 m water depth. The ero-
sional channels they reported range from tens of meters
to several kilometers wide, and hundreds of meters
deep. They interpreted the common occurrence of
symmetrical ripples in foraminiferal sand along the
seamounts they examined as a result of internal-tide
reworking. Other areas of major erosion attributed to
tidal currents include canyons 100-300 m deep and
several km wide on the Ecuadoran slope, and local
scours hundreds of meters wide and deep in calcareous
sediment on ridges in the Panama Basin, the latter of
which are apparently carved by dissolution aided by
tidal currents (within the scours tidal currents are con-
sistently fast, >15 cm/s) (Lonsdale
1976
) . Xu et al.
(
2008
) documented large-scale dunes with 1-2.3 m
14.4
Ancient Examples
Ancient deposits reported to be of deep-water tidal ori-
gin include the Ordovician Bays Formation, Virginia,
U.S.A. (Zhenzhong and Eriksson
1991
) , the Ordovician
of Tonglu and the Tarim Basin, the Lower Cambrian in
Hunan, the Devonian to Triassic in Western Qingling,
and the Mesoproterozoic in Xiushui, China (He et al.
2008
), the Devonian Greenland Group, New Zealand
