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The AAPG/Datapages Combined Publications Database

North Dakota Geological Society

Abstract

NDGS-AAPG

Symposium on the Geology of Rocky Mountain Coal, October 2-4, 1984

Pages 41 - 60

PETROGRAPHIC CHARACTERISTICS AND DEPOSITIONAL ENVIRONMENT OF THE PALEOCENE 61-M-THICK SUBBITUMINOUS BIG GEORGE COAL BED, POWDER RIVER BASIN, WYOMING.

E.C.T. Chao1, J.A. Minkin1, J.M. Back1, F.W. Pierce2

1U.S. Geological Survey, Reston, Virginia 22092
2U.S. Geological Survey, Denver, Colorado 80225

ABSTRACT

The flat-lying Big George coal bed of the Tongue River Member of the Paleocene Fort Union Formation is located in the central part of the Powder River Basin in Wyoming. The coal bed is approximately 335 m below surface where a USGS exploratory drill hole penetrated its projected thickest part (61 m, 200 ft). Coal samples recovered from this drill hole are the subject of this study.

The Big George coal is an outstanding example of a coal bed showing characteristics the authors regard as indicating an allochthonous origin. Most samples are crudely lenticularly banded, with chunky fragments of vitrain. Steep to vertically inclined wrinkle-fold bands are common.

Petrographically, the Big George coal is subbituminous in rank, and consists dominantly of macerals of the vitrinite and inertinite groups. Macerals of the vitrinite group are dominated by telinite. Exinites are generally less than 5 volume percent.

Petrographic data closely reflect the chemical composition of the Big George coal. Forty samples representing depth intervals from top to bottom of the drill core have been chemically analyzed. On an as- received basis, the averages are: 3.60 percent ash; 0.26 percent total sulfur; and 9850 BTU/pound.

The Big George coal precursor materials were deposited in a flood-plain environment with locally aggraded major streams in a subsiding intermontane basin. The plant materials were transported and deposited in clear water of a marsh-lake complex surrounded by forest swamps between two major north-south-trending meander fluvial channel belts.

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