Geology Reference
In-Depth Information
sediment supply (Plink-Bjorklund
2008
) . Because
the middle portion of the Chimney Rock Sandstone
was a mixed-energy estuary, and was then overlain by
vertically stacked tide-dominated estuarine units, it is
likely that the system entered a window of tidal
amplifi cation during transgressive widening of the
shelf (see also e.g. Dalrymple and Zaitlin
1994
;
Yoshida et al.
2007
) .
17.5
Widespread Tidal Deposits in the
Interval Bounded by Ammonite
Zones
B. perplexus
(late) Through
D. stevensoni
(ca 77.5-75 Ma)
There is an important third occurrence of WIS tidal
deposits. Although this occurrence contains the same
types of regressive and transgressive tidal deposits as
described above, the unusual aspect is that the tidal
deposits are ubiquitous and developed throughout high-
stand, lowstand and transgressive tracts of multiple
cycles within a particular short time interval from
Central Utah eastwards to Colorado and southern
Wyoming. Aschoff and Steel (
2010 and
in press )
referred to this as an 'anomalous clastic wedge' because
of its architectural contrast with other Cretaceous clas-
tic wedges in the Utah-Colorado segment of the sea-
way. The anomalous wedge is bounded by ammonite
zones
D.stevensoni
and
Baculites perplexus
(late),
though in Utah it begins earlier. The interval represents
about a 2.5 My period (ammonite zones and radiomet-
ric calibration of Cobban et al.
2006
) and is seen to be
anomalously amalgamated and highly progradational
on the regional well-log cross section of Kirschbaum
and Hettinger (
2004
) (Fig.
17.25
). It was studied in
detail in outcrop and subsurface by Aschoff (
2008
) ,
who extended the correlation 100 km westward to the
thrust-belt and illustrated the high-frequency sequence
architecture of this wedge.
Fig. 17.24
Sedimentary log through a 22 m-thick incised valley
succession of estuarine and tidal-fl uvial deposits that cut down
into a wave-dominated shoreface. Iles 3 Sandstone Tongue,
Horse Gulch in Sand Wash Basin, N. Colorado. The paleocur-
rents in the tidal-fl uvial part of the section record the southeasterly
regional paleoslope (From Crabaugh
2001
)
incision increases coastline irregularity, locally dissi-
pating wave energy and enhancing tidal currents as the
valleys are drowned during subsequent transgression.
Thus, in contrast to the thin tidal accumulations, the
valley-fi ll tidal accumulations are estuarine, some
wave-dominated with a wave-built barrier mouth, others
tide-dominated.
The thick non-valley accumulation in the upper-
most Chimney Rock Sandstone, resulting from verti-
cal stacking of four estuarine units, is attributed to
locally high subsidence rates, caused by an embry-
onic Laramide Uinta uplift, compensated by a high
17.5.1 Characteristics of the Widespread
Low-Accommodation Stratigraphic
Interval
The 400 km-long transect, referred to above, through
Campanian strata from the Wasatch Plateau of Utah to
western Colorado includes alluvial and estuarine depo-
sits of the Middle Castlegate sandstones and Neslen
