ABSTRACT

The Belfry Member comprises a distinctive part of the Paleocene Fort Union Formation in the northern Bighorn Basin. Recognized in the field by the prominence of tabular sandstones, additional field work has shown that extensive lateral continuity on the order of several kilometers is restricted to a few major beds. These sandstones eventually thin and change into micritic limestones or silty calcareous mudstones. Thinner tabular sandstones show the same lateral changes, but over a more restricted geographic extent, and are often traceable to thicker channel sandstones. Rapid facies changes between the major sandstones is demonstrated by alternating thin beds of siltstone, mudstone, lignite and limestone.

The Belfry Member is thickest near the town of Belfry and thins in all directions, intertonguing with facies dominated by mudstones and nodular limestones with only scattered, thin lenticular sandstones. The "lower mudstone facies" contains prominent channel and valley-fill sandstones. Consistent paleocurrent directions and the absence of lateral accretion deposits point towards low sinuosity, perhaps braided, streams. Sandstones in the "upper mudstone facies" are much smaller and thinner. Paleocurrent directions are more variable, and epsilon cross-bedding is present. Consequently, higher sinuosity meandering streams seem more likely here.

A shallow lacustrine environment was originally proposed for all facies of the Belfry Member. The present data indicate that fluvial and palustnne conditions were intimately associated with lakes that varied in size and depth. The major tabular sandstones record widespread basinal subsidence that initiated the deposition of sheet sands, either along a delta front or as crevasse splays into a large shallow lake or pond. In between these major events, the lakes filled in by autocyclic switching of delta lobes, giving rise to the rapidly changing stratigraphy and ultimately, return to a fluvial depositional system.