POSSIBLE WINDERMERE RIFTS IN THE MESOPROTEROZOIC BELT BASIN OF THE MONTANA ROCKIES

New observations from the Montana Rockies suggest that late Neoproterozoic Windermere rifting may have left a larger structural signature in the Mesoproterozoic Belt basin than previously recognized. The Humbolt anticline of the Pioneer Mountains, SW Montana, represents a tectonically inverted half-graben that may be part of a regionally extensive Windermere-aged rift system. The syntectonic fill of the graben comprises a thick, east-tapering wedge of micaceous mudstone characterized by slump structures, gradationally overlain by thick, vitreous, trough cross-bedded, fine to medium, clean quartz arenite. The Middle Cambrian Wolsey Shale unconformably overlies the graben fill; the Flathead sandstone is missing. The inverted graben trends north for 15 km along strike, whereupon it passes into the Late Cretaceous Pioneer batholith. It emerges from the batholith 25 km to the north and occurs in isolated segments engulfed by granite northward for an additional 40 km. The quartzite has been assigned to various local formations of uncertain age, ranging from Mesoproterozoic to Cambrian. We suggest that the graben fill correlates with the micaceous, slump-folded Garnet Range Formation and clean Pilcher Quartzite of the upper Missoula Group, which may define a similar half-graben in the central Belt basin, cut by mafic sills and dikes dated to 780 Ma. The Garnet Range and overlying Pilcher Quartzite in NW Montana are finer-grained than their southern counterparts. The coarsest grains are very fine sand; most grains are coarse silt. Framework grains include abundant angular quartz and detrital mica and rare feldspar. The Garnet Range has a gradational upper contact with the Pilcher with quartz abundance increasing and detrital mica decreasing. New detrital zircon ages of the Garnet Range feature a Paleoproterozoic peak at 1780 Ma with subordinate Archean peaks and a youngest Mesoproterozoic peak at 1430 Ma. These match reported zircon spectra signatures for the Garnet Range farther south. The proposed graben system resembles Windermere grabens that occur on strike in the Selkirk Mountains of British Columbia, where slump-folded basinal facies are overlain by thick, clean quartz arenites of the late Neoproterozoic Hamill and Gog groups.