Abstract

Uplift of the Black Hills and development of the Powder River Basin is chronicled by the evolution of Late Cretaceous to early Eocene fluvial systems that existed in the eastern part of the basin. Stream directions are based primarily on vector means of trough-crossbedding dip directions calculated from crossbeds measured in the Upper Cretaceous Lance and Hell Creek Formations, the Paleocene Fort Union Formation, and the paleocene and lower Eocene Wasatch Formation.

North-northeasterly flow of Late Cretaceous streams in the southern part of the eastern Powder River Basin and southerly flow in the northern part suggest a northwest-southeast topographic low existed between the two areas. During deposition of the lower Paleocene Tullock Member of the Fort Union Formation, streams flowed northeast in Wyoming and east in Montana along the eastern flank of the basin. These data suggest that uplift of the Black Hills had not yet commenced. Early indications of basinal development are recorded by differential subsidence and stream flow to the north and northwest during deposition of the middle Paleocene Lebo Shale Member of the Fort Union Formation.

During deposition of the late Paleocene Tongue River Member of the Fort Union Formation, north-northwest-flowing streams along the eastern flank of the basin merged with east-flowing streams that crossed the northern basin and northern flank of the rising Black Hills uplift. The major late Paleocene trunk stream of the basin flowed north-north-west in the central basin, parallel with extensive raised peat swamps that are now the thick (as much as 60 m) "Big George"-Sussex and Wyodak-Anderson coal seams.

During deposition of the Paleocene and early Eocene Wasatch Formation, rapid uplift of the Black Hills and Laramie Range is documented by north-flowing streams in the southern basin and west-flowing streams in the eastern basin, all of which carried coarse-grained arkosic sandstone from the Black hills. These streams joined a north-flowing basin-axis trunk stream in the western basin.