CONJECTURED GEOLOGY OF THE RIFTED, WESTERN PART OF THE BELT BASIN BASIN

Westward extrapolation from the Belt basin may help characterize the missing rifted, western part of the basin, providing it is extant. The rifted basement probably contains mostly Early Proterozoic crystalline rocks capped by a blanket of supermature, quartz sandstone (Neihart correlative), possibly with ventifacts, representing a period of pre-Belt tectonic stability with eolian and fluvial deposition. Blocks of the ca.1500 to 1400 Ma extensional or transpressional? Belt basin were buried by up to 18 km of sediment accompanied by mafic sills. Little sediment came from the tectonically stable North American side of the basin, whereas most came from the tectonically active rifted side, including feldspathic sand containing 1510 to 1610 Ma zircon, jasper, and reworked Neihart? quartz sand grains. The lowest Belt correlative (Prichard Formation) might be 2.5 kms of thick dark, pyrite-bearing, thinly laminated silt and graded sandstone and siltstone, representing subaqueous suspension settleout, mixed with turbidite and deltaic sand in a euxinic basin. Above this may be 1.5 kms of flat-laminated and trough crossbedded fine-grained feldspathic sandstone deposited by terrestrial sheetfloods (Ravalli Group). Next may be 1km of medium to thin beds of graded hummocky, guttered and loaded sandstone that grade up into dark mudstone, representing shallow water deposition in a vast, flat bottomed sea or lake (Wallace Formation). The Missoula Group may be represented by flat-laminated and trough crossbedded feldspathic sandstone units 100’s of meters thick recording sheetflood deposition alternating with equally thick units of mudcracked, rippled thin fine sand-to-mud couplets of playa and perennial lake deposits. The Mount Shields equivalent may contain halite casts and magnesite. Belt rocks were intruded by 1370 Ma granite possibly accompanied by hummocky cross stratified, micaceous sandstone (Garnet Range Formation). Late Proterozoic Windermere faults limit the Belt on the west.