TERTIARY-QUATERNARY DEBRIS FLOW AND FLUVIAL GRAVEL DEPOSITS THAT FORM THE MELROSE/JEFFERSON RIVER DIVIDE, SW MONTANA

Archean schist and gneiss, folded and thrust-faulted Paleozoic and Mesozoic sedimentary, Tertiary basalt, rhyolite, ash and tuffs, and faulted debris flow and fluvial gravels rocks are exposed in SW Montana, between Butte and Dillon. Of particular interest to this study are the Tertiary materials associated with collapse of the Laramide orogin and Basin and Range formation because mapping, dating, and characterizing the changing lithology of the alluvial deposits through the Cenozoic section will help elucidate the progressive evolution of the basins that controlled the stream system development.

There are four main gravel units mapped: a debris flow (Eocene), Six Mile Creek Formation (Neogene?), Big Hole terrace (Neogene?), and modern channel gravel (Quaternary). At the present divide, 400-500 m above the modern Melrose and Jefferson River basins respectively, volcanic basalt underlies and intrudes the basal debris flow (Renova Fm.?) which is Ar³⁹/Ar⁴⁰ dated as approximately 50 my (Eocene). Large angular to subangular clasts, up to 4 meters in diameter, include quartzite, sandstone (Quadrant and Kootenai Fms.), conglomerate (Kootenai Fm.), and basalt. The overlying fluvial unit (Six Mile Creek Fm.?) extends in a fan shape 16 km southeast from the divide. It includes mainly subrounded to rounded duriclasts of white, gray, red, pink, and purple quartzites (Bonner Supergroup exposed to the west) and few decomposed granite clasts with a mean diameter of 22 cm. This fan gravel has been reworked and includes local bedrock clasts of Archean metamorphics and Cretaceous Black Leaf Fm. to form a Neogene terrace along the Big Hole River and modern floodplain/channel deposits of streams that incise the fan gravel. Locally, a poorly sorted deposit of silt, with sand and rare cobbles from underlying and surrounding units occurs in low-lying areas.