TOWARDS CHARACTERIZATION OF LANCE FORMATION (MAASTRICHTIAN) STRATIGRAPHY IN THE TYPE AREA, NORTHERN NIOBRARA COUNTY, WYOMING

Because of discontinuous exposures across grassland of low-topographic relief, the Lance Formation lacks a type section. Its type area covers the roughly 90 mi² of Hatcher’s (1893) original description along the lower Lance Creek. Another challenge to characterizing the stratigraphy of the Lance Formation is the highly variable and discontinuous lithification of its sandstones. Apparently because of that, marker beds and internal stratigraphy have not been proposed for the roughly 3000’ of Lance Formation in its type area (based on Powder River Basin well logs: Connor 1992). Recent field work in the type area west of Lance Creek largely ignoring lithification has permitted preliminary mapping of greater than 30 mi² of the southeastern part of the type area. More than six sandstone beds exceeding 30 feet with distinctive thicknesses and sedimentary structures have been correlated over 6 miles along an east-west (strike) line at the southern boundary of the formation east of Lance Creek. One of those beds has been followed ten miles to the north on either side of the ridge running down the middle of the map area, indicating a simple layer-cake stratigraphy of nearly flat-lying beds upturned in the half mile along the southern and eastern edges of the Basin.

Although the Lance is often thought to be dominantly mudstone with discontinuous sandstone lenses, the correlations suggest that the lower 1300’ of the Lance Formation is composed of thick, sheet sandstones with discontinuous intra-sandstone mudstone lenses. And, although fossils of the Lance are exclusively terrestrial, strong cross-bedding (commonly 3’, not infrequently >10’) in the sandstones, thick convolutions (often >6’) in the sandstones, somewhat common claystone conglomerates and breccias (clasts up to 1’ diameter), and fine lamination in the shales between the sandstones suggests subaqueous deposition dominates the lower 1300’ of the Lance Formation.