Geology Reference
In-Depth Information
Fig. 17.26
Strike section through the Sego Sandstone showing
multiple deep channels and valleys (the V-units). Willis and
Gabel (
2001,
2003
) interpret many of these incisions as deep
tidal scours formed during the forced regression of tide-dominated
deltas, rather than as incised-valley fi lls (Figure is from Willis
and Gabel
2001
)
clastic wedges that built basinwards at rates of
50-80 km/My (Aschoff and Steel
in press
) . A sense
of the great basinward extent of individual shore-
line progradations can be seen in the lower half of
the Fig.
17.25
succession, in great geometric
contrast to the more limited progradational lengths
of individual shorelines in the overlying Rollins
Sandstone.
2. A highly amalgamated and incised sequence
architecture (Fig.
17.25
) with well-developed,
forced regressive sequence sets seen in the Sego/
Neslen/Corcoran/Cozzette interval in E. Utah
and W. Colorado (Aschoff
2008
), and in some of
the Iles shoreline complexes in N. Colorado
(Crabaugh
2001
). The frequent Sego Sandstone
incisions (Fig.
17.26
), some of which are inter-
preted as deep tidal scours during regressive delta
building (Willis and Gabel
2001
) , are among the
best-known examples of incised and amalgam-
ated architectures.
3. During regressions, the high-frequency shorelines
have the character of mixed tidal-wave shoreline
deposits, but with a signifi cant percentage of tide-
dominated segments. Regressive units are frequently
erosively based, as noted above.
4. During transgressions, the high-frequency shore-
lines of both Iles and Sego/Corcoran/Cozzette
intervals deposited thick tide- and wave-infl uenced
estuarine deposits (orange colored units in succession
of Fig.
17.25
) (Van Wagoner
1991
; Crabaugh
2001
;
Kirschbaum and Hettinger
2004
; Aschoff
2008
;
Gomez-Veroiza and Steel
2010
) making probably the
highest stratigraphic concentration of estuarine strata
within the entire Cretaceous succession of WIS.
17.5.2 Characteristics of Tide-Infl uenced
Strata Within the Low-
Accommodation Interval
There are no special or unusual sedimentary processes
for the deltas, estuaries and coastal plain systems that
developed during the 77.5-75 Ma stratigraphic inter-
val, but the particular geometric and stacking charac-
teristics referred to above, imparted by the low
accommodation setting, is emphasized. In addition to
the features already listed above, there is a notable
amalgamation of fl uvial and tidal-fl uvial channels in
lower alluvial plain deposits (Fig.
17.27
), a marked
clustering of bayhead delta deposits preserved at the
head of estuaries, great distances (80-100 km) of delta
regression with very thin (<10 m) prodelta deposits,
and a complex juxtaposition of upward-fi ning with
upward-coarsening units due to deep channel inci-
sions cutting through regressive shoreline tongues
(Figs.
17.25
and
17.26
).
