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)
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