Abstract

Quartzite cobble and boulder conglomerates were episodically deposited in the western Bighorn Basin, Wyoming during early Tertiary time. Similar conglomerates, the Harebell and Pinyon Formations, which accumulated over 100 km (60 mi) to the west in the Jackson Hole region during Late Cretaceous-Paleocene time, gave rise to those in the Bighorn Basin. Paleocurrent patterns, together with known timing of early Tertiary orogenic events, indicate that periodic tectonism in the Jackson Hole region caused conglomerate recycling and initiated gravel transport to the Bighorn Basin.

During the earliest Paleocene, a coarse braided stream system carried recycled Harebell clasts to the western margin of the Bighorn Basin and deposited a conglomerate at the base of the Fort Union Formation. This unit is dominated by horizontally stratified conglomerate deposited as longitudinal bars. Thick sets of planar cross stratified conglomerate are also common and were probably deposited as transverse or medial bars. Set thicknesses indicate that floodstages were deep and prolonged, resulting from wet climates and valley confinement of flow.

The Bighorn Basin underwent at least four subsequent episodes of late Paleocene-early Eocene quartzite gravel deposition, reflecting renewed tectonic activity in the Jackson Hole area. Tectonic pulses along the western margin of the Bighorn Basin also controlled the periodicity of exotic gravel ingress. The final influx of exotic clasts in the late early Eocene was instigated by erosion of Pinyon Conglomerate from the rising Washakie Range. Deposition of this youngest quartzite conglomerate occurred on a vast braidplain that developed above a major syntectonic unconformity along the western basin margin.

Early Tertiary quartzite conglomerates in the Bighorn Basin provide examples of long distance fluvial transport of coarse gravel. Their study also demonstrates the utility of sedimentologic analysis in paleogeomorphologic reconstructions of tectonically active basin margins.