Tertiary fluvial systems within the Bear Creek coal field, northern Big Horn basin, Montana

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 upoper and lower contacts. The upper portion has convolute bedding, ripple lamination, and some horizontal and tabular crossbeds. 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 studymore » 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.« less