Abstract

Utilizing the approach of Winczewski (1982), subsurface lithologic and geophysical information was used to identify and study four sedimentation intervals in the Paleocene lower Sentinel Butte Formation of the upper Fort Union Group in the Knife River area of west-central North Dakota (east-central Williston Basin). Each of the four intervals (from base to top - Kinneman Creek, Antelope Creek, Spaer, and Beulah-Zap) is mappable over the 4,290 sq km (1,650 sq mi) study area. Each interval consists of a medium- to fine-grained clastic component capped by a lignite or group of related lignites (lignitic component) for which the interval is named.

Primarily on the basis of sediment distribution patterns, coupled with results of studies elsewhere in the basin, the lower Sentinel Butte Formation in the study area is interpreted as a record of swamp (lignitic components) and fluvio-lacustrine (clastic components) conditions that alternately dominated a broad, flat, alluvial plain with drainage east to southeast, perhaps to the distal Cannonball sea. Neither the marine-deltaic model (Jacob, 1976a) nor the tectonic-fluvial model (Winczewski, 1982) adequately address all of the stratigraphic and sedimentologic relationships observed either in the study area or regionally in the Williston Basin for the upper Fort Union Group.

The development of a comprehensive depositional model for the alluvial-plain setting of the upper Fort Union Group in the Williston basin including an explanation for the occurrence of widespread lignites, is contingent on the following, all of which are presently lacking: (1) the delineation of the stratigraphic and areal distribution of lignites in outcrop and in the subsurface to provide regional stratigraphic framework; (2) integration of surface and subsurface information on a regional basis to establish facies relationships for environmental interpretation; and (3) the establishment of a geologic time framework for the Paleocene in the Williston basin. Such a model for the upper Paleocene in the Williston Basin must take into consideration the possible effects of tectonics within the Basin, Paleocene eustatic sea-level changes, and climate.