Geology Reference
In-Depth Information
Fig. 17.18
A Campanian transect from S Wyoming through N
Colorado showing a series of four clastic wedges reaching from
Rock Springs Uplift to the Denver Basin (Wedges modifi ed from
Crabaugh
2001
). The sandstones of Setting 1 (
in red
) include the
most basinal sands, on the distal fringes of basinward-stepping
clastic wedges. Note that these sands migrated eastwards through
time as the main clastic wedges built basinward
regression and associated sand deposition. For this rea-
son, the tidal deposits of Setting 1 have been referred
to as falling-stage or lowstand shorelines on the shelf
(Mellere and Steel
2000
). This is not to say that all
lowstand sands in any sequence were strongly tidally
infl uenced; in fact, there were coeval and along-strike
shoreline sands that were wave-dominated (Mellere
and Steel
1995a
; Hampson et al.
2008a
) . It is argued
that relative sea level is likely to have been lowered at
these times because it is diffi cult for comparatively
small rivers and deltas to maintain regression across a
slightly seaward-sloping, pre-existing shelf for such
great distances (up to 200 km) without the forcing aid
of falling relative sea level (Muto and Steel
1997
) .
At some stratigraphic levels the sandstones of Setting
1 extend many tens of km basinward into the muddy
seaway (e.g., into the Denver Basin). In these cases
some of the deltaic sandbodies have been partly or
extensively reworked (e.g., Krystinik
1995
) by south-
directed currents in the seaway (Martinsen
2003b
) .
Another aspect of these sandstones is that most of
them lack distributary channels so they appear to have
been dominantly subaqueous in character. The Blair
Sandstone is one of the few that does have a distribu-
tary channel.
Within the study region, the tidal sandstones of
Setting 1 occur in the following groups:
Fishtooth, Sussex and Shannon sandstones of the
•
Bighorn-Powder River basins in northern Wyoming
(Tillman and Martinsen
1987
; Walker and Bergman
1993
; Fitzsimmons
1994
,
1999
; Sullivan et al.
1997
;
Bergman
1999
; Fitzsimmons and Johnson
2000
)
Tapers Ranch, O'Brien Springs, Seminoe 1-3, and
•
Hatfi eld 1 sandstones in Hanna Basin of south-cen-
tral Wyoming (Smith et al.
1965
;
Gill et al. 1970
;
Tillman and Martinsen
1985
; McClurg
1990
;
Davies
1990
; Roehler
1990
; Mellere and Steel
1995a,
2000
; Uroza
2008
; Ahmed
2008
)
Hygiene, Terry, Rocky Ridge, Larimer and Richards
•
sandstones of the Denver Basin, Colorado (Kiteley
and Field
1984
)
Kremmling, Muddy Buttes, Carter, Hygiene and
•
Gunsight Pass sandstones of the Middle Park Basin,
Colorado (Izett et al.
1971
; Krystinik
1995
; A.
Petter, 2007, personal communication; J. Crabaugh,
2007, personal communication)