SHEETFLOODS ACCOMPANIED ARIDITY IN THE MIDDLE PROTEROZOIC BELT BASIN, MONTANA

Thick, extensive, alluvial sand wedges of the Revett, Mount Shields and Bonner formations contain episodic flood deposits recording huge floods derived from a large uplifted catchment area west and south of the block faulted Belt basin. This catchment area rifted and drifted in the Late Proterozoic. The more proximal Belt flood deposits are characterized by decimeters- to meters-thick layers of upward-fining and thinning trough crossbedded and flat-laminated arenite capped by discontinuously layered fine arenite and mudstone . These upward-fining layers form extensive, tabular deposits that record unconfined sheetflood flow across flat, featureless, gently sloping alluvial apron surfaces of the Belt. Their thicknesses record deposition of enormous quantities of sediment, inferentially deposited by huge floods, of a scale virtually unknown on earth in the Phanerozoic. Flood deposits thin and fine northeastward against Laurentian land surface and northward to decimeter-scale sandflat deposits and to more distal mudcracked silt-to-clay couplets and mudcracked mud of large playa lakes. The repeated flooding and drying of the playa deposits probably reflect arid conditions during deposition of the alluvial wedges. The shift toward less mudcracked, more carbonate-bearing deposits of the Helena, Wallace and Shepard formations may record more humid, perennially submerged conditions.