Geology Reference
In-Depth Information
which are here defined as open-mouthed indentations
of the coastline such as the North Sea and Yellow Sea.
In such epicontinental embayments, the distance from
the shoreline to the shelf margin can increase signifi-
cantly and the flow is partially confined. Broad shal-
low-marine basins can also occur within continental
interiors, far removed from a continental margin, with
only a narrow and/or circuitous connection with the
open ocean (e.g. Hudson Bay, Canada; the Baltic Sea).
Such water bodies are termed semi-enclosed epiconti-
nental seas here. Straits or seaways joining two larger
bodies of water commonly exhibit particularly strong
tidal water motions (e.g. the Strait of Dover in the
English Channel). In all of these offshore settings, cur-
rents generated by the tides interact with an array of
other processes, including waves, storm/wind-gener-
ated currents and geostrophic currents that are part of
the global-ocean circulation, giving the potential for the
creation of a complex variety of sedimentary deposits.
Because of the large geographic extent and substan-
tial water depth of modern shallow-marine areas, our
knowledge of the processes operating there, and of the
sedimentary facies generated by these processes, has
mostly been obtained by indirect observations. Our
understanding of tidal dynamics on shelves has
increased markedly over the last few decades, both as
a result of improved instrumentation and the applica-
tion of numerical-modeling approaches. Significant
advances have also occurred in our ability to obtain
detailed images of the sea floor (e.g. through the use of
swath bathymetry), but high-quality 3D seismic imag-
ing of subsurface deposits on modern shelves remains
beyond the capability of most academic institutions. In
addition, coring techniques have not evolved much
over the last several decades; consequently, informa-
tion on the nature of modern deposits is scanty,
although the available database is increasing. As a
result, facies models for the deposits of tidal shelves
remain poorly developed, as reflected by most text-
books (Stride
1982
; de Boer et al.
1988
; Suter
2006
).
This chapter begins by examining qualitatively
some aspects of the dynamics of tides as they progress
from the open ocean toward the coast. Then the range
of deposits that can occur in tidal settings is consid-
ered, namely their composition, surficial morphology
(i.e., the bedforms that are present) and internal struc-
ture. We focus on the origin and dynamic behavior of
compound dunes
(also called
sand waves
) and
tidal-
current ridges
(also called
banks
and
bars
) because
they are the largest and most distinctive of the tidally
generated bedforms in shallow-marine settings.
Reconstructing the Holocene evolution of the offshore
ridges in various tidal basins helps to define a model
for the transgressive evolution of these large sand bod-
ies, which might have application to the rock record.
Finally the potential response of shallow-marine tidal
systems to physiographic changes caused by variations
in relative sea level is examined briefly, taking exam-
ples from both the modern and the rock record. This is
coupled with the insights gained from paleotidal mod-
eling, in order to extend our understanding of where
tidal deposits are likely to occur in time and space.
13.2
Tidal Processes In shallow Seas
The ability of a basin to develop a large tide depends on
the possibility of an amphidromic system to be gener-
ated within the basin by the astronomic tide, and on
water motions to be amplified by co-oscillation within
the basin as the basin borders reflect the tidal wave (co-
oscillating tide). The minimum size of a sea (a basin)
where the tide is able to generate an amphidromic sys-
tem is determined by the Rossby radius of deformation
of the tidal wave (Pugh
1987
), which is the minimum
distance required for the Coriolis effect to cause a
motion to rotate through 360° (Fig.
13.1a
). The Rossby
radius decreases as the Coriolis effect increases with
increasing latitude. This implies that, for the same basin
depth, amphidromic cells are smaller at higher latitude.
Wave theory predicts an increase in the celerity of the
tidal wave, and hence a larger Rossby radius, as water
depth increases: because the perimeter of an amphidro-
mic cell has to be traversed by the tidal wave within one
tidal cycle, the larger the amphidromic cell, the higher
the celerity the wave must have at its periphery. This is
one reason why no shallow, semi-enclosed epeiric sea
has significant tides, even though its dimensions are
large enough to contain an amphidromic cell. This is,
for example, the case in Hudson Bay or the Baltic Sea.
Of course, semi-enclosed seas can have large tides even
if they are not able to develop tides by themselves, as
they can amplify an oceanic tide that enters them
through a wide oceanic connection. This is the case in
the North Sea and English Channel (Fig.
13.2
).
Water motion associated with a rotating Kelvin
wave involves two components: the main flow that is
perpendicular to the crests and troughs of the wave
(i.e., perpendicular to the cotidal lines that show the
location of the wave crest as a function of time;
