Petrology of the Beulah-Zap lignite bed, Sentinel Butte Formation (Paleocene), Mercer County, North Dakota

The Beulah-Zap lignite bed is one of several economically important lignites in the Sentinel Butte Formation (Paleocene). This lignite occurs as a subsurface unit in the Knife River Basin of Mercer and I Oliver counties of North Dakota. The Beulah-Zap lignite is a heterogeneous carbonaceous rock unit composed of megascopic and microscopic I components, which are lithotypes and macerals, respectively. Relationships between the maceral contents of the lithotypes follow patterns similar to those of bituminous coals. Coal petrology, the study of maceral and lithotype occurrence and I distribution, was applied to determine: 1) mode of coalification, and 2) depositional environment of the Beulah-Zap lign1te. The lignite seams were divided into lithologic units (lithobodies) I on the basis of petrographic features within the layers. These lithobodies range in thick Jess from 5 to BS cm. Lithotypes, the petrographic units within the lithobodies, were separated for various analyses and their relative abundances were estimated. Three distinct lithotypes in the Beulah-Zap lignite have been identified and average I abundances estimated as: vitrain (501.l, fusain (5%), and attritus (45%). Macerals were identified by reflected light microscopy. I Maceral groups were classified into three major groups on the basis of their relative reflectances. Individual maceral types within the same group I were differentiated by morphology. The morphology is related to the i maceral precursor and mode of coalification. The lithotype fusain is I rich in inertinite macerals such as fusinite and semifusinite. The attritus lithotype is composed of all three maceral groups in variable percentages: huminite (41-82%), inertinite (10-65%), and liptinite (4 13%). The identified liptinite macerals include sporinte, resinite, cutinite, and suberinite. The huminite maceral group, consisting mainly of ulminite, is the major constituent of vitrain. The lithotype and maceral occurrence and distribution suggests that the depositional environment of the Beulah-Zap lignite was a lacustrine marsh complex with a steady rate of subsidence and peat accumulation. The initial peat deposition occurred in water 1.5 to 2.0 m deep. Later stages of deposition occurred in shallow water ( Three processes are responsible for the coalification of the Beulah Zap lignite. These processes are: 1) humification, 2) gelification, and 3) fusinitization. The principal mode of coalification was humification. This process was the dominant type of diagenetic alteration in the middle of the seams. Gelification, the complete biochemical maceration, has also altered portions of the Beulah-Zap lignite. The gelification process is most pronounced immediately above the clay partings. Fusinitization, indicative of oxidizing conditions, was more frequent towards the uppermost part of the seams. Several local fusinized horizons are present in the middle section of the Beulah- Zap lignite