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.

Abstract The Raton basin is bounded on the west by the ancestral San Luis highlands. The highlands were probably uplifted along a highangle reverse fault zone marginal to the basin. Coarseningupward megacycles and types of facies sequences and facies associations within the Upper Cretaceous Vermejo Formation and Upper Cretaceous and Tertiary Raton and Poison Canyon Formations indicate that both allocyclic and autocyclic mechanisms influenced alluvial sedimentation in the Raton basin. The megacycles comprise a coalrich, fine detritusdominated facies and an overlying coalpoor, sanddominated facies. The coalrich, fine detritusdominated facies was deposited by braided, meandering, and anastomosed fluvial systems during periods of increased basin subsidence associated with a stable source area. The sanddominated facies was deposited by alluvial fans and by braided and highbedload meandering fluvial systems during periods of tectonic uplift in the source area and reduced basin subsidence. The basin alluvial paleoarchitecture (spatial arrangement, patterns, interconnection, distribution, and evolution of the fluvial systems) was mainly controlled by allocyclic mechanisms of uplift in the source area and subsidence in the basin. Climatic change and the presence of vegetation played only minor roles in controlling the alluvial paleoarchitecture of the Raton basin. In this alluvial environment, basin subsidence (an allocyclic mechanism) and fluvial avulsion and abandonment (autocyclic mechanisms) served as the major controlling factors in the accumulation of thick coal beds.