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In-Depth Information
the falling-stage to lowstand basinal equivalents (albeit
southerly defl ected) of wave-dominated highstand
shorelines of the WIS (Mellere and Steel
1995a,
b,
2000
; Hampson
2010
) .
and preservation of tidal deposits (Klein and Ryer
1978
). Today there is general agreement that tides can
become signifi cant even in large epicontinental seas
due to localized funneling of tidal currents, local or
sub-regional shoaling, tidal resonance and the effects
of Coriolis acceleration and amphidromic circulation
patterns in large seaways (Dalrymple
2010
).
The present study area covers only the southwes-
tern reaches of the Campanian Western Interior Seaway,
a large epicontinental sea which covered much of the
foreland basin east of the Cordilleran fold-and-thrust
belt (Fig.
17.1
). Of main relevance for the study area
would have been the southerly incoming tide from the
Gulf of Mexico. The shallowness of the Campanian
seaway as well as its restricted oceanic opening to the
south (Fig.
17.1
) would predictably have dampened
tides and reduced the tidal range in the southern reaches
of the seaway (see also Wells et al.
2010
) . A modeling
of storm and tidal conditions within the entire seaway
was made by Ericksen and Slingerland (
1990
) , on the
basis of paleogeography and paleobathymetry (pro-
vided largely by Kauffman
1984
), as well as tides and
storm-wind stresses. Later modeling by Slingerland
et al. (1996) and by Slingerland and Keen (
1999
) dem-
onstrated that the normal surface circulation pattern in
the seaway was likely a counterclockwise gyre, and
that added storm conditions enhanced this to produce
dominantly southward-directed currents along the
western side of WIS. These models produced impor-
tant results, suggesting (1) that the seaway would have
been dominated by winter storm (passing west to east)
and hurricane (running northwards) conditions that
caused southerly longshore drift of sediment along the
studied west coast of the seaway, and southerly geo-
strophic currents on the shelf, and (2) that the overall
tidal regime is likely to have been microtidal (0-2 m tidal
range) on the southwestern seaway coastlines, though
on the southeastern side of the seaway there is likely to
have been a meso- to macrotidal regime.
17.1.2 Tectonic Setting
The Western Interior Seaway occupied a Cretaceous
retroarc foreland basin that extended at times from
Gulf of Mexico to the Arctic Ocean, on the eastern
fl ank of the Sevier fold-and-thrust belt. The fi ll of the
basin is characteristically asymmetric, a westward-
thickening Cretaceous sediment wedge. However,
from the point of view of the landscape and its tidal
infl uence, there was a key change in tectonic setting
around 77.5 Ma. At about this time fl exural loading by
the thin-skinned Sevier thrust-belt was being irregu-
larly replaced in places by basement-involved, steep
Laramide faulting and block uplifts, thought to be
related to sub-lithosphericic loading and cooling
induced by a shallowing of subducted oceanic
(Farallon) plate (Liu and Nummedal
2004
) . Cumulative
westward backtilting and thickening of strata in the
basin was being replaced by an irregular eastward
sloping landscape, but with a marked shallowing of
water locally above Laramide uplifts. As discussed
below, this tectonic shift that varied slightly in its timing
across the region, caused a major change in the mor-
phology of the WIS western shorelines, from an abun-
dance of straight, wave-dominated coasts to more highly
embayed, tide-infl uenced coasts, at least within the inter-
val 77.5 through 75 Ma. (Aschoff and Steel
in press
) .
17.1.3 Modelling of WIS Tides
Early opinion about the role of tides in the WIS was
that co-oscillating tides entering the epeiric WIS plat-
form from the southern oceanic area (Fig.
17.1
) could
not propagate large distances within the seaway
because of rapid attenuation of tidal wave energy
(Keulegan and Krumbein
1949
; Shaw
1964
; Irwin
1965
). However, as empirical evidence accumulated
from facies data, an opposing viewpoint emerged. This
new viewpoint was that ancient, shallow epeiric seas
and shelves, where there is often a correlation between
tidal amplitudes, tidal current velocities and shelf
width (Redfi eld
1958
), should have been dominated by
tidal action and were therefore an ideal setting for tides
17.1.4 Modelling Results and Field Data
Although we have no evidence to suggest a regional
tidal range greater than a microtidal one, it is clear that
local shoreline morphology varied greatly (not least
due to syn-sedimentary faulting, as emphasized by
Martinsen
2003b,
c
), with ample evidence of tidal cur-
rents locally strong enough to transport medium-
grained sand in simple dunes, compound dunes and bars.
