Tertiary Fluvial Systems Within the Bear Creek Coal Field, Northern Big Horn Basin, Montana

WEAVER, JEAN N., U.S. Geological Survey, Denver, CO, and JAMES R. GRUBER, JR., Bureau of Land Management, Billings, MT

The Bear Creek coal field contains the 250-m-thick coal-bearing Paludal Member of the Paleocene Fort Union Formation in the northern Big Horn Basin, Montana. Detailed field and subsurface data show two contrasting geometries in alluvial strata, each bounded by an economic coal bed.

The lower 50 m of the Paludal Member is dominated by sheet and ribbon sandstones. The sheet sandstones are as long as 1.5 km and fine upwards from medium to fine grained. Some sandstones are multistory with sharp upper and lower contacts. The upper portion has convolute bedding, ripple lamination, and some horizontal and tabular crossbeds. The overlying ribbon sandstones are 0.6-1 m thick and extend up to 1.5 m laterally. They are lenticular in cross section, are found en echelon, and can either fine or coarsen upward. Ripple lamination and plant fragments are common. Bounding both sandstone types are fine-grained sequences of interbedded siltstones and mudstones, most of which are carbonaceous. An absence of well-developed plant and root horizons suggests a marshy environment. This l wer 50-m interval is interpreted as a series of distal lobes of crevasse splays. The thickness of the sheet sandstones and the absence of large-scale bed forms imply a shallow stream bed with upper flow regime conditions. The ribbon sandstones may represent small abandoned channels associated with the splay complex.

Stratigraphically higher is a 12-m-thick fine-grained sequence, containing large tree trunks in growth position and extensively rooted mud rocks. Sandstone bodies, 6 m thick and 10 m wide, are enclosed within mudstones and siltstones. The sandstones are primarily ripple laminated and have stepped bases and internal erosion surfaces. This interval has previously been interpreted as deposits of an anastomosed fluvial system. The sandstones show little evidence of significant lateral migration. In contrast to the lower interval, the environment here consisted of well-developed vegetated islands separating fluvial channels.

Subsurface data show that the major coal beds are laterally continuous within the study area. The cyclic development of the coals reflects intermittent periods of long-term basin stability. Alternating dominance of the sandstones suggests that influx and distribution were controlled through episodic uplift of the nearby Beartooth Mountains.

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)