ANOMALOUS LONGITUDINAL DISPERSAL OF SEDIMENT FROM OLDER, MORE EXTENDED TO YOUNGER, LESS EXTENDED SEGMENTS OF AN EOCENE-OLIGOCENE FOLD-PRONE RIFT ZONE, SW MONTANA

The Grasshopper basin, SW Montana is a major graben within a N-trending Eocene-Oligocene rift zone that includes the Medicine Lodge and Horse Prairie half graben to the south and the Big Hole basin in the north. Multiple phases of extension produced depocenters for meandering stream, lacustrine, alluvial fan, and megabreccia deposits in Grasshopper basin, starting after 45.7±0.14 Ma and continuing after deposition of an ash (27.57±0.64 Ma) and coeval basalt flows (27.76±0.20 Ma). The bulk of the basin fill, the Medicine Lodge beds, was deposited during an early phase of simple translation on the west-dipping Muddy-Grasshopper detachment fault. The relatively small volume of sediment derived from the active eastern and hinged western margin of the basin suggests that this basin was starved for sediment during most of its history, and that this supradetachment basin has facies typical of rifts. A SSE-flowing meandering stream drained through Grasshopper basin into a persistent lacustrine depocenter within the Medicine Lodge subbasin throughout the basin’s long history. This river system filled most of the basin floor and tapped a distinctive granitoid source with large biotite and muscovite. Argon and zircon geochronology on detrital grains shows that the plutonic source was emplaced between 70 and 83 Ma and cooled below mica and K-spar closure temperatures in the early Tertiary (K-spar plateau ages 47-74 Ma). The two-mica bearing Chief Joseph pluton in the footwall of the southern Anaconda core complex, > 50 km to the NNW, is suspected to have been the source for the arkosic fluvial system. The Anaconda complex was active in Middle Eocene time (O’Neill et al., 2004) yet continued to supply arkosic sediment to Grasshopper basin for at least another 20 m.y. This pattern is unexpected because the Anaconda complex is clearly more extended than Grasshopper basin and should have been a paleolow, not a paleohigh. The Georgetown thrust in the footwall of the Anaconda detachment fault appears to be an offset portion of the Johnson thrust fault in its hanging wall. If so, there was about 25-30 km of extension in the Anaconda complex whereas roughly 10 km produced Grasshopper basin. Extensional folds are plentiful, large and intersecting in the Grasshopper basin, and most formed late during the basin’s evolution.