Barrier-Island Coastline Deposition and Paleogeographic Implications of the Upper Cretaceous Horsethief Formation, Northern Disturbed Belt, Montana

The Horsethief Formation (upper Campanian) of the Northern Disturbed Belt and western Sweetgrass Arch area of Montana, was deposited along the western margin of the Western Interior Cretaceous Seaway. Investigation of the Horsethief in an area extending northwest of Augusta, Montana and southwest of Choteau, Montana indicates that deposition occurred along a barrier-island coastline. Deposits in the lower part of the Horsethief (Horsethief-Bearpaw Transition Unit) are transgressive in nature whl1e those in the upper part are regressive. Several distinct lithofacies within the Horsethief Formation lend themselves to paleoenvironmental interpretation on the basis of texture, sedimentary structures, paleocurrent indicators, and faunal content. Lower shoreface sediments consist of a thick sequence of interbedded mudstone, siltstone, and horizontally stratified and ripple cross-laminated sandstone. Predominantly trough cross-bedded sandstone with abundant Ophiomorpha characterizes the upper shoreface. Sandstones of the foreshore and backshore are horizontally to subhorizontally bedded. Marsh-tidal flat sediments consist of carbonaceous siltstone, claystone, sandstone and shale; the siltstone and sandstone is commonly ripple cross-laminated. Shoreward-dipping wedge-planar cross-stratification represents deposition within flood-tidal deltas. Horsethief Formation sandstones include: 1) volcarenites, 2) chertarenites, 3) feldspathic volcarenites, and 4) feldspathic chertarenites. Abundance of volcanic rock fragments increases upward in the Horsethief, suggesting that the regressive nature of the Upper Horsethief may be due in part to influx of volcanogenic sediment from the west.