ABSTRACT

Fluvial facies in the Sentinel Butte Formation include meandering-channel, channel-plug, natural-levee, floodbasin, and swamp facies. The channel facies comprises elongate, tabular, fining-upward sand beds that exhibit gently dipping point-bar surfaces (sigmoidal cross bedding). Trough cross bedding in the lower part of the sand beds grades upward to planar and ripple cross bedding. Paleocurrent data indicate current directions generally parallel to bed trends. Channel-plug facies consist of organic silty clay filling arcuate, asymmetrically trough-shaped abandoned meanders. Levee facies grade upward from channel facies and consist of ripple cross-bedded sandy silt and silty clay. Floodbasin facies comprise dark-gray, organic, bioturbated silty clay, and grade upward to woody lignite f the swamp facies. Petrified tree stumps are present in levee, floodbasin, and swamp facies.

Deltaic facies include distributary-channel, channel-plug, natural-levee, marsh, delta-front, prodelta, interdistributary-bay, and crevasse-splay facies. The channel facies consists of elongate, narrow, trough-shaped, fining-upward sand beds. Planar cross bedding grades upward to ripple and climbing-ripple cross bedding. Paleocurrent indicators are parallel to sand bed trends. Most channel deposits grade upward to organic, disturbed clay and lignite of the marsh facies, but others grade upward to ripple cross-bedded sandy silt and silty clay of the channel-plug facies. Small levee facies of silty clay border the upper part of the channel facies and grade downward to planar cross-bedded silty sand and silty clay of the delta-front facies. The delta-front facies overlies laminated, spar ely fossiliferous silty clay of the prodelta facies and grades laterally to fissile silty clay of the interdistributary-bay facies. Thin, lens-shaped beds of silty sand within the interdistributary-bay facies represent the crevasse-splay facies.