Coal-Based Stratigraphy for Upper Fort Union Group (Paleocene), West-Central North Dakota: ABSTRACT

Abstract

On the basis of an evaluation of lithologic and geophysical logs from 2500 shallow drill holes and information from a limited number of natural outcrops and mine highwalls, a detailed stratigraphic framework was constructed for the Paleocene strata of the Sentinel Butte and upper Bullion Creek formations (upper Fort Union Group) in the Knife River area in west-central North Dakota. The study sequence was found to be characterized by at least seven major coals continuous over the 8000 km/sup 2/ (3089 mi/sup 2/) of the study area instead of a large number of locally persistent coals, as interpreted by previous investigations of surface sections. A series of geologic cross sections demonstrates the correlation of the lignite beds in the upper Bullion Creek and Sentinel Butte formations and together provide a three-dimensional picture of coal distribution in the study area. Each major lignite bed has been named and assigned both type and reference test holes. The recognition of coals continuous over the study area provides a means of correlating this complex terrestrial section. The resulting framework in turn provides a basis for (1) the development of a chronostratigraphy and biostratigraphy, (2) a more accurate evaluation of the magnitude and distribution ofmore » coal resources, and (3) an appraisal of depositional setting and history. The recognition of widespread coals in the upper Fort Union strata in other portions of the basin suggests the potential for establishing a basinwide stratigraphic framework based on these coals.« less