MAPPING THE TERTIARY MEGAFAN GRAVELS OF MADISON COUNTY, MONTANA

Madison County, SW Montana, has a rich and diverse geologic history. Surface lithologies in the study area include: Archean metamorphics, Paleozoic sedimentary rocks, Tertiary intrusives, extrusives, and fan gravels as well as Quaternary fluvial deposits along channels that have incised the fan.

Five different Tertiary/Quaternary sedimentary units of differing lithologies, rounding, and sorting were mapped in the study area: The oldest unit, Paleogene ash and relatively fine-grained tuffaceous sediment of the Renova Formation has scattered outcrops, mainly in the southern and eastern portions of the study area. The overlying extrabasinal Neogene Six Mile Creek Formation (?) of quartzite and quartz sandstone duriclast cobbles forms a megafan that unconformably overlies both Archean and Paleozoic rocks. The basal portion of the fan is either penecontemporaneous with or younger than the Eocene extrusives. This fan is now detached from its western source area and the distal margin terminated at the Jefferson Valley to the east or the distal fan sediment is buried within the Jefferson Valley fill. One of two younger gravels is Neogene colluvium north of the megafan. It is derived from Archean metamorphic rocks of the Highland Mountains to the west and forms an alluvial apron east of the mountain front. A second Neogene colluvium is along the southwest margin of the megafan. It is derived from Cretaceous sedimentary rocks and forms an alluvial apron along the southeastern and southern flanks of McCartney mountain. Both of these locally derived deposits include clasts of the extrabasinal Six Mile Creek gravel at their respective boundaries with the fan. The youngest gravel, found along the present channels that incise the megafan, is reworked older fan gravel mixed with local bedrock exposed in the stream channel.