Recent high resolution sequence stratigraphic studies of the time-equivalent Blackhawk Formation in Utah and the Eagle Formation in Wyoming, allow the comparison of changes in architecture within depositional systems of the same age, 700km apart within the same basin. These comparisons are made on a number of scales from individual facies, through parasequence scale to large-scale clastic wedge architecture and have important implications for understanding controls on stratal geometries.

Both systems comprise wave dominated shallow marine shoreface facies with associated coastal plain and offshore deposits. However, the facies proportions of individual shoaling upward successions is significantly different. The Utah succession is dominated by hummocky cross-stratified, storm/wave-dominated parasequences, characteristic of a linear strandplain while the Wyoming parasequences record a mixed storm/wave/current-influenced deltaic environment, interpreted as due to closer proximity to fluvial input points. In addition, Utah successions are usually gradationally based whilst those in Wyoming are dominantly sharp-based.

High frequency sequence boundaries in Utah are typically expressed as incised valley-fills with associated detached lowstands deposited up to 40 km seaward of the preceding highstand shorelines. In contrast, the Wyoming examples are predominately characterized by attached lowstand shorelines. On the scale of the overall clastic wedge, the Blackhawk and Eagle Formations exhibit comparable gross thicknesses. They contain a similar number of high frequency sequences and significant low order sequence boundaries and flooding surfaces occur in similar stratigraphic positions but the progradational extent of the Blackhawk is approximately double that of the Eagle system. The comparable thickness and duration indicates similar subsidence rates with both systems responding to long wavelength basin scale driving mechanisms. Differences in the systems are attributed to variations in sediment supply proximity to sediment input points and possible local structural/topographic variations.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah