CRETACEOUS ZIRCONS IN EOCENE TO OLIGOCENE SANDSTONES RECORD SOUTHWARD PALEODRAINAGE IN THE HANGING WALL OF THE PALEOGENE MUDDY-GRASSHOPPER DETACHMENT, SOUTHWEST MONTANA AND EASTWARD FLOW OFF THE EASTERN RIFT FLANK

Contrasting models have been proposed for paleodrainage within the north-striking Paleogene rift system in southwest Montana, in the western hanging wall of the Muddy-Grasshopper detachment. Models most notably disagree over whether northern or southern drainage within the rift basins persisted from the Eocene to Oligocene.

Detrital zircon populations from sandstones in and adjacent to the Paleogene Grasshopper, Anaconda and Nicholia supradetachment basins in the hanging wall of the Muddy-Grasshopper detachment fault have diverse provenance. Different sands contain a) Proterozoic (1400-1750 Ma) zircons recycled through the Belt Supergroup, b) mixed cratonally derived Archean, Paleoproterozoic, and Grenville zircons from nearby ranges, and c) Cretaceous zircons of two distinct populations (65-85 and 90-110 Ma).

Oligocene two-mica sandstones from axial part of the Grasshopper basin contain a 65-85 Ma grain population, likely derived from local granites to the north and northwest. Paleocurrents suggest derivation from the north-northwest. A distinct 90-110 Ma population is also found in sandstones of the Medicine Lodge beds in the intra-rift Grasshopper and Nicholia Creek basins, and in older middle-Eocene “Renova” sandstone from Mantle Ranch, MT, east of the rift zone. The younger (90-100 Ma) grains of these populations were most likely recycled from 93-97 Ma porcellanites of the Vaughn Member of the Blackleaf Formation. The source of 100-110 Ma zircons is currently unknown but may be older undated volcaniclastics of the Blackleaf Formation.

These data require that fluvial systems within Paleogene supradetachment basins drained not only plutons from the north and west, but also Cretaceous foreland basin sediments which crop out in a north trending belt on the eastern flanks of the Montana Pioneer Mountains from just north of Dillon to Melrose, MT. Our data are consistent with a model of southward drainage within the rift system west of the Muddy-Grasshopper master detachment and eastward drainage off the eastern flanks of the rift shoulder.

Oligocene two-mica sandstones in the type Renova Formation near Whitehall, MT contain abundant 75-85 Ma Boulder batholith grains and confirm eastward drainage into the main Renova basin off the rift shoulder.