UPPER FLOW REGIME PLANE BED, STANDING WAVE, AND BREAKING WAVE, SHEETFLOOD DEPOSITS IN THE MIDDLE PROTEROZOIC REVETT FORMATION, BELT SUPERGROUP, NORTHWESTERN MONTANA

Feldspathic quartzite of the approximately 680 m thick, fluvial Revett Formation of northwestern Montana forms a lithosome that pinches eastward into rippled, mudcracked argillite of the Grinnell Formation of Glacier National Park. The Revett Formation is subdivided into lower, middle, and upper members. Within the approximately 500 m thick lower Revett member are six alternating quartzitic and argillitic tabular lithostromes, averaging 84 m thick. And within them are upward-fining and -thinning stratigraphic successions, about 11 m thick. Within these are cm to meter-thick, upward-fining and -thinning event beds that are classified into sediment types on the basis of their grain sizes, layer thicknesses and sedimentary structures. The sediment types record sedimentary processes, and their facies relationships within the upward-fining and -thinning successions and within the lithostromes places the processes into inferred interrelated depositional environments. Three sediment types that characterize proximal fluvial deposits of the Revett in northwestern Montana include the trough crossbedded sand sediment type that records sheetflood flow in the upper part of the lower flow regime. Tabular, flat-laminated sand beds capped by ripples and mudcracks form the flat-laminated sand sediment type and record upper plane bed sheetflood flows that decelerated, and were followed by exposure. Antidunes in the lenticular sand and mud sediment type record upper flow regime standing wave deposition, and scoured sandy and muddy lenses with rippled ball and pillow structure indicate deposition from breaking standing waves in the upper flow regime.

Sandy sediment types in the west fine eastward to silty, and rippled mudcracked silt-to-clay couplet sediment types in the east. They reflect eastward flow and ponding of the floodwaters in ephemeral lakes. Upward-fining and -thinning successions from sandy to silt, to silt-to-clay couplet sediment types record abrupt eastward advances of proximal sheetflood flows followed by retreat and westward overstepping of more distal flows. This depositional pattern shows the Belt basin was terrestrial and internally drained during deposition of the Revett Formation.