Geology Reference
In-Depth Information
the larger clastic wedge on which they are perched
built out irregularly into the Western Interior Seaway
(Fig.
17.18
). From the oldest (Blair) to the youngest
(Gunsight Pass) of these Campanian sandbodies, the
clastic wedges had cumulatively prograded some
600 km into the WIS during 10 My. Because these
tidal sandbodies systematically changed their location
in the basin (migrated southeastwards) through time,
their occurrence is less likely to be related to tectonic
lineaments, sediment supply pulses, subsidence rate
changes or even to tidal resonance related to particular
shelf widths. Their changing location but constant
occurrence near the tip of high-frequency sandstone
tongues rather suggests that their lowstand position in
the seaway was critical. Sea-level falls of only a few
tens of meters (in an already very shallow seaway), as
likely in late Cretaceous Greenhouse times (see Miller
et al.
2004
) during the observed regressive shelf tran-
sits, would likely have caused some narrowing and
northward constriction of the WIS at lowstand inter-
vals. This scenario somewhat resembles the southern
North Sea setting where there is a narrowing towards
the English Channel-Straits of Dover. In this case the
tide comes in as a Kelvin wave between Scotland and
Norway, and Coriolis acceleration forces it to bend to
the right in the direction of propagation, so that it
`bunches-up' on the southeast UK coast, before turn-
ing all the way around the N Sea Basin.
sion surfaces (Table
17.2
). Estuarine incised valley
fi lls in the WIS Campanian strata were fi rst recognized
by Van Wagoner (
1991
) in the Lower Sego Sandstone
and by Taylor and Lowell (
1991
) in the Kenilworth
Member of the Blackhawk Formation, as conceptual
models for incised valleys were being developed. A
third type of transgressive occurrence
and the most
unusual in the Campanian WIS, is a thick, vertically
stacked estuarine accumulation that is not constrained
to a valley, recognized only in Lower Campanian
Chimney Rock clastic wedge of the Utah-Wyoming
border area (Plink-Bjorklund
2008
) .
17.4.1 Thin, Widespread
Transgressive Tidal Strata
These thin tidal accumulations cap most regressive
clastic tongues and wedges throughout the Campanian
succession (Fig.
17.21
). There are especially many
examples from the Lower Campanian succession in
Utah and New Mexico, and from Upper Campanian
strata of NW Colorado. The main occurrences are
listed in Table
17.2
. Most examples of these transgres-
sive deposits are <5 m thick, but occasionally exceed
10 m. (Table
17.1
). Figure
17.21
shows the early
example where Devine (
1991
) clearly pointed to trans-
gressive tidal deposits occupying a relatively thick
capping to a larger-scale regressive unit, all of which
could be interpreted as regressive without facies infor-
mation. All of the transgressive accumulations of this
type have been interpreted in terms of a back-barrier
setting as lagoonal or non valley-confi ned estuarine
accumulations with tidally infl uenced fl uvial channel
sandstones, bayhead delta and lagoonal sediments, as
well as fl ood-tidal delta and tidal-inlet deposits.
17.4
Tidal Setting 2: Tidal Deposits
in Transgressive Settings
Tide-infl uenced transgressive strata are the best known
tidal accumulation in the WIS and they occur as three
types of succession.
Thin transgressive tidal deposits
are widespread and well documented throughout
Campanian statigraphy (Table
17.1
), normally forming
the linkage between successive regressive shoreline
units. There has been some tendency to misinterpret
these coarse-grained, transgressive estuarine or barrier-
remnant sandstones on the tops of regressive shorelines
as regressive distributary channels or upper shoreface
units, as was early pointed out by Land (
1972
) , Devine
(
1991
) , and Cross (
1998
). The latter two authors
emphasized the importance of preserved tidal signals
in the transgressive systems tract of sequences.
The
second type of transgressive occurrence
involves the
accumulation of thick, estuarine deposits within incised
valleys, or at least occurring above basal fl uvial ero-
17.4.2 Tidal Incised Valley Fills
Incised valley-fi ll tidal accumulations are less well
known in Upper Cretaceous strata than in the classic
Lower Cretaceous Dakota Formation valleys in the
Front Range and Denver Basin area (Weimer
1983
)
and the Muddy Sandstone valleys of NE Wyoming
(Weimer
1984
; Martinsen
2003b
) . Nevertheless,
Campanian incised valleys are fairly common
(Table
17.2
, Fig.
17.22
). In most cases incision depths
are 10-30 m deep and they are fi lled with fl uvial to estu-
arine sediments. The valley fi lls are based by regional
