INCISED VALLEY FILLS IN THE JURASSIC SUNDANCE FORMATION, BIGHORN BASIN, WYOMING, USA

The mixed carbonate-siliciclastic Middle to Upper Jurassic Sundance Formation is well exposed along the eastern flank of the Bighorn Basin, yet correlation within the Sundance has been difficult. Our field studies resolve many of these problems by demonstrating dip-related facies variations along the outcrop belt, and by documenting the presence of incised valley systems at the J4 and J5 unconformities.

Along Sheep Mountain, the J4 unconformity separates the carbonate lower Hulett Member from the siliciclastic upper Hulett. Here, the upper Hulett fills distinctly channel-form erosional scours, which are up to 7 m deep and approximately 100 m wide. The incised valley fill consists of a basal bioturbated to cross-bedded very fine quartzose sand, and an upper wave-rippled to current-rippled very fine quartzose sand. The incised valley fill is capped by a thin (< 0.5 m) oyster-ooid grainstone, interpreted as a transgressive lag that underlies offshore mudstone of the Redwater Shale. Incised valley fills are not present to the north along Little Sheep Mountain or to the south near Shell and Tensleep.

Incised valley fill deposits above the J5 unconformity are recognized throughout the study area in the Windy Hill Member. The basal erosional surface displays up to several meters of relief in a single outcrop. In some places, shallow truncation preserves low-energy shoreface deposits in the upper Redwater Shale that were previously included within the Windy Hill Member. The incised valley fill is dominated by five lithofacies: 1) a shelly cross-bedded channel lag, 2) tidally bundled sigmoidally bedded sandstone, 3) trough cross-bedded sandstone, 4) wave-ripple to current-ripple laminated sandstone, and 5) bioturbated massive sandstone. Collectively, these are interpreted to reflect deposition in the outer portion of a wave-dominated estuary.

The presence of these incised valley fills, and the truncation of underlying deposits at the sequence boundary have hampered previous correlation attempts. Recognizing sequence-bounding erosional surfaces and the localized presence of incised valley fills permits confident correlation within the Sundance throughout the eastern flank of the Bighorn Basin.