Sedimentology and paleohydraulics of the meandering river systems of the Fort Union Formation, southeastern Montana

Abstract

The Fort Union Formation in southeastern Montana is composed of four lithofacies associations: sandstone bodies, sandstone-mudstone interbeds, carbonaceous shales to coals, and sheet sandstones. The sandstone bodies are meters thick, fine upward and comprise cross-stratified, horizontally stratified, and ripple cross-laminated sandstones. Bedsets are typically arranged into erosively based storeys with lateral accretion bedding. Basal breccias above erosion surfaces locally contain turtle, fish and reptile bone fragments of Torrejonian age. Most sandstone bodies are laterally extensive but channel initiation and abandonment margins occur locally. Sandstone bodies are either single- or multi-storeyed and are generally tabular. Some sandstone bodies are locally thickened where the basal erosion surface cuts down several meters (commonly through an immediately underlying coal seam to a subjacent coal seam). The laterally extensive sandstone bodies are interpreted as point-bar deposits of laterally migrating and aggrading single-channel rivers. Single-storey sandstone bodies record deposition from single lateral migration events. Most multi-storey sandstone bodies probably record downvalley migration of meander bends and accumulation of point-bar deposits within an aggrading channel belt prior to an avulsion event. A few multi-storey sandstone bodies are likely the product of superimposed channel belts which returned to an area of the alluvial plain as a result of avulsion events. The locally thickened portions of sandstone bodies are interpreted as the product of scouring during episodic major flood events. The sandstone-mudstone interbeds, carbonaceous shales to coals, and sheet sandstones are interpreted as overbank, swamp and possibly lacustrine deposits accumulating next to the channels on an alluvial plain. Paleohydraulic reconstructions of point-bar deposits suggest three distinct channel sizes. Large channels (Lebo Member only) flowed easterly and had sinuosities, discharges and slopes of about 1.22 to 1.35, 460 to 600 cm, and 0.000026 to 0.000051, respectively. Intermediate channels show an upsection change in paleoflow from southeasterly in the Tullock Member, to easterly in the Lebo Member, to northerly in the lowest Tongue River Member, possibly influenced by the contemporary uplift of the Black Hills. Intermediate channels had sinuosities, discharges and slopes of 1.25 to 1.38, 208 to 351 cm, and 0.000038 to 0.000057, respectively. Small channels flowed in a variety of easterly directions (ranging from N 350° to N 110°), and had sinuosities, discharges and slopes of about 1.2 to 1.35, 140 to 240 cm, and 0.000038 to 0.000057, respectively. Outcrop relationships suggest the smaller channels were tributaries and/or distributaries of the larger channels. Regional paleodrainage studies also indicate a shift in drainage direction from southeast to northeast probably due to uplift of the Black Hills. The change in paleodrainage direction was accompanied by a shift in depocenters from the Black Hills region to the Williston Basin. In middle Tongue River times, the depocenter shifted to the Powder River Basin causing paleodrainage directions to change to a southwesterly direction. During times when paleodrainages were changing orientation, widespread coal swamps developed.