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.