SHEETFLOOD SAND PLAINS IN THE REVETT FORMATION, MIDDLE PROTEROZOIC BELT SUPERGROUP, NORTWESTERN MONTANA

The Revett Formation is a gray, silty and muddy quartzite unit in northwestern Montana. It fines eastward to purple, mudcracked argillite of the Grinnell Formation in Glacier National Park, forming a lithosome. Within the lithosome of the lower Revett member are eight through-going lithic units (lithostromes) numbered from the base up. Lithostromes 1, 3, 5, and 7 contain cycles. The bases of most cycles have silt-to-clay mudcracked couplets deposited on expanding playa lakebeds. Above the couplets in westernmost Montana are muddy and sandy antidune beds that were deposited by upper flow regime sheetfloods with wavy surfaces. The muddy antidune sheetfloods were sourced in a muddy sand area that lay west of the Belt basin, and the sheetfloods built a huge muddy sand plain over the retreating playa lake flats. Chutes with surging hydraulic jumps deposited inclined beds above some muddy antidune beds in the west. Higher in the cycles the muddy antidunes and chute beds continue up into sandier tabular antidune beds deposited by shallow sheetfloods. These are overlain by sorted, flat-laminated, tabular arenite beds up to a meter thick that extend across most outcrops. They were deposited by flat, upper regime plane bed flow and are locally cut by trough crossbeds where the sheetfloods deepened and shifted to the lower flow regime. Sand and silt in these beds were eroded from a sorted sand and silt source that lay southwest or farther west of the Belt basin than the muddy antidune sources.

These kinds of flat-laminated arenite beds also make up most of lithostromes 2, 4, 6, and 8, and their distal siltite beds extend eastward into the Grinnell Formation. The processes and scale of the sheetfloods that built these vast recurrent sand plains of lithostromes 2, 4, 6, and 8 across northwestern Montana appear to have no counterparts in the Phanerozoic.