LATE MESOZOIC TO EARLY CENOZOIC FORELAND SEDIMENTATION IN SOUTHWEST MONTANA

ABSTRACT Stratigraphic variations in the composition of Upper Jurassic through Lower Tertiary sandstones (Morrison, Kootenai, Blackleaf Formations and the Bozeman Croup) from southwest Montana, and inferred dispersal patterns and depositional environments, reflect the tectonic evolution of a foreland basin in response to gradually increasing orogenic activity. Upper Jurassic to Lower Cretaceous (Aptian) litharenites were largely derived from older miogeoclinal sedimentary rocks exposed to erosion by a combination of epeirogenic uplift over the embryonic Idaho batholith and foreland fold-thrusting in Idaho. Deposition was on a broad coastal plain. Although an active magmatic arc was present in western Idaho, first-cycle sand-size detritus derived from the arc was trapped in intra-arc basins and not transported into southwest Montana. Within the foreland basin the Belt Arch exerted progessively less influence on sedimentation from near the middle of the Jurassic to the start of the Cretaceous. By the end of the Early Cretaceous the active magmatic arc had migrated farther east in Idaho and the volcanic cover of the Idaho batholith supplied detritus for the first time for the Montana foreland basin. Transitional marine/non-marine environments of deposition dominated as a result of interplay of marine transgression and eastward progradation of an alluvial system outward from the orogenic souce areas. Within the foreland basin positive stuctures were influencing sediment dispersal patterns as early as Albian time. However, coarse-clastic facies derived from these intra-foreland structures were not deposited extensively until the Late Cretaceous. Extensional tectonics brought about a breaking up of the foreland basin in Early Cenozoic time. Immature arkosic sandstone and conglomerate were derived from nearby basement-cored block uplifts as well as from the unroofed Boulder batholith. Deposition was in intermontane basins. Cyclical variations in both the compositional maturity of sandstone and nature of the sandstone depositional environments characterize the evolution of the southwest Montana foreland basin.