Process Change
from the Inner to Outer Reaches of the Western Interior Clastic Platform:
Examples from SW Wyoming, USA
Uroza, Carlos A.1, Ronald J.
Steel2 (1) The University of Texas, at Austin, Austin, TX (2) University of Texas-Austin, Austin, TX
The Western Interior Seaway provides good
examples of prograding clastic tongues that show the variable interplay of
fluvial, wave, and tidal processes from the proximal to the distal reaches of
their platform transits. We consider here the Mc Court and Brooks Tongues
(Rock Springs Formation) as examples sited on the inner platform, and the
O'Brien Springs and Seminoe Members of the Haystack Mt. Formation as sited
farther into the Seaway on the same platform. On the inner platform, and when
relative sea level is high, wave swell across the wider basin dominantly molded
the sand-bodies on the shelf. This produced hummocky to swaley-bedded
sandstones and strike-extensive, homogeneous, sheet-like sandbodies, though
there can be estuarine/tidal channels towards the top of the tongues. Later in
the same cycles and farther into the basin, when the depocenter narrowed as
relative sea level fell, a much stronger tidal influence is seen in units like
the O'Brien Springs and Seminoe Members, where stacked, current-generated dunes
and interbedded mudstones resulted from southerly prograding tide-dominated
deltas, partly reworked by north-south oriented tidal seaway currents. This
change in the character of the sandbodies across the platform transits is
important for hydrocarbon reservoir understanding as these changes will not
necessarily be clear in the subsurface log or seismic data. The process regime
may thus change significantly across the same clastic platform eventhough
within the same depositional cycle.