Geology Reference
In-Depth Information
wedges of sediment that partly bury the ridge flanks
(Figs.
13.12
and
13.25
). Consequently, their outer
slopes are less steeply inclined than those of active
ridges, and commonly dip at less than 1° (Stride et al.
1982
). Dunes, which typically mantle active ridges,
are generally absent from their crest. A decrease in the
ability of the tidal currents to bring in more sand leads
to an increase in the carbonate content of the sediment,
forming a shelly lag on the ridge crest; the constituents
present in this bioclastic material should record the
increasing water depth (Wilson
1988
). The increase in
shell content can be either gradual or abrupt (Wilson
1982
; Davis et al.
1993
). The surface of the ridge can
also become pervasively bioturbated. In the ancient,
the post-ridge draping deposits might record a transi-
tion from tidal to wave dominance, as reported from
examples in the Western Interior Cretaceous of the
USA (Hein et al.
1991
; Mellere and Steel
1995
;
Yoshida et al.
2007
). As would be expected in a trans-
gressive succession (Fig.
13.20
), the first deposits
above the ridge crest are glauconitic shelf muds that
indicate sediment starvation and slow sedimentation
(Surlyk and Noe-Nygaard
1991
) and the formation of
the maximum flooding surface. The dominantly muddy
sedimentation that characterizes the overlying high-
stand systems tract may entirely bury the remnant
ridges, as is observed in the Pleistocene of the East
China Sea (Berné et al.
2002
; Fig.
13.21
) and the
Miocene of offshore Java (Posamentier
2002
).
there is no reason for tidal dominance to occur only
during transgressions (Yoshida et al.
2007
). The
following sections explore the timing of tidal domi-
nance, focusing primarily on the results of paleotidal
modeling of both modern and ancient basins.
13.8.1 High-Frequency Changes
Short-term, tectonic- or climate-driven sea-level varia-
tions with a period of less than about 100,000 years
(fourth-order or higher; Vail et al.
1977
) can bring
about rapid change in the nature of shelf sedimenta-
tion, potentially causing an alternation between tidal
and non-tidal deposits (Fig.
13.21
). Two contrasting
situations can be documented: (i) open shelves on
passive margins, and (ii) epicontinental seaways.
On open shelves, the tidal influence is expected to
increase with rising sea level, because the increasing
shelf width typically brings the system closer to reso-
nance, although the opposite can occur if the width at
lowstand is close to one-quarter of the tidal wave-
length. If the shelf has embayments, they might be the
most sensitive to changes in tidal influence, due to fun-
neling of the flow toward the head of the bay. The
increase in tidal influence can be geologically instanta-
neous in situations where the geomorphology changes
rapidly. This was the case in the Gulf of Maine-Bay of
Fundy system, which changed from microtidal to
extreme macrotidal over a period on only a few thou-
sand years (Greenberg
1979
; Dalrymple and Zaitlin
1994
; Shaw et al.
2010
). One possible ancient ana-
logue is provided by the Woburn Sands in the
Cretaceous Greensand Seaway of NW Europe, where
the strength of the tides increases upward through the
transgressive succession, being stronger in shelf sedi-
ments than it is in estuarine deposits at the base
(Yoshida et al.
2004
), a situation that is the reverse of
what might be expected because tidal currents are typi-
cally stronger within embayments than on the open
shelf. Once tidal resonance has been reached, however,
any further increase in sea level will result in the
decrease in tidal influence. This might be illustrated by
the sudden abandonment and preservation of fossil
tidal dune fields beneath Holocene offshore muds in
the Southern North Sea (Brew
1996
). This could also
be the case for the tidal sandbodies of the Devonian
Castkill Sea (Ericksen et al.
1990
).
It must be noted that different parts of the transgress-
ing sea can become resonant at different times, or
13.8
Sea-Level and Geomorphic
Interactions
Because the oceanic tidal wave interacts strongly
with the morphology of the shelf and coastline, changes
in the geomorphology of an area as a result of changes
in relative sea level can have a profound influence on
the occurrence of tidal deposits. As noted above, it is
generally believed that the formation of
sandy
tidal-
shelf deposits is restricted to transgressive situations.
The occurrence of
muddy
tidal-shelf deposits is not
well known because they have not been studied as sys-
tematically, but they appear to occur mainly in regres-
sive, delta-related settings (e.g. the Amazon River:
Gabioux et al.
2005
; Bourret et al.
2008
; the Irrawaddy
River: Viana et al.
1998
; Ramaswamy et al.
2004
; Rao
et al.
2005
; the Yellow River: Yang and Liu
2007
).
Such deposits are discussed elsewhere in this volume
(Chap. 7). From a theoretical point of view, however,
