A zone of intense soft-sediment deformation, with associated spherules and shocked quartz grains, is identified over an area of ∼1000 km 2 in southwestern South Dakota. This Disturbed Zone (DZ) is up to 5 m thick and is preserved within distal deltaic deposits of the Upper Cretaceous Fox Hills Formation. Localized structural development caused thinning and eventual subaerial exposure of several sections within Badlands National Park, whereas sections to the north of the park were unaffected. Although previously interpreted as an intense period of soil formation under tropical conditions, the degree of ancient soil overprinting of these sections is minimal, with the exception of bright coloration of the strata, and appears not to have had any effect on ejecta preservation. Biostratigraphic data suggest a middle to late Maastrichtian age for the DZ. When compared to other Cretaceous impactites, our study sections in southwestern South Dakota are most similar to, and may correlate with, the recently documented 68 Ma impactite within the Vermejo Formation of Berwind Canyon in southeastern Colorado. If this correlation is correct, the size of the ejecta within the Fox Hills and Vermejo Formations suggests that the sections in South Dakota represent distal deposits.

In 1907, Barnum Brown named the Hell Creek beds (Formation) for the strata exposed in the Hell Creek Valley and other downstream tributaries of the Missouri River. In the absence of a stratotype section, a lectostratotype is herein proposed for the Hell Creek Formation based on 84.2-m-thick exposures at Flag Butte (local name) in Ried Coulee (archaic use; East Fork of Hell Creek) and East Ried Coulee, tributaries of Hell Creek, Garfield County, Montana (sec. 29, T. 21 N., R. 38 W., NAD27 CONUS; base 47.55931°N, 106.88111°W; top 47.55533°N, 106.86810°W). The formation is underlain with general conformity by sandstone beds of the Fox Hills Formation (as characteristically known, the Colgate Member is absent) and is for the most part conformably overlain locally by the Tullock Member of the Fort Union Formation. The upper contact at Flag Butte is demarcated at the base of the IrZ lignite bed (above an iridium anomaly). The boundary has been demonstrated to be somewhat unconformable in areas to the west. The IrZ bed is also missing at Bug Creek in McCone County. In its type section, the Hell Creek Formation is subdivided (simply and informally) into Ried Coulee (lower Hell Creek), East Ried Coulee (middle Hell Creek), and Flag Butte (upper Hell Creek) units, each containing a sandstone and a mudstone lithofacies. Formational thickness varies with local depositional and erosional history of various coastal-deltaic environments across the Williston Basin and a trend of overall thinning to the east and northeast.