GEOLOGY OF THE EOCENE LOWLAND CREEK VOLCANIC FIELD: BUTTE NORTH 30′ X 60′ QUADRANGLE, SOUTHWESTERN MONTANA

A long-range goal of MBMG geologists, as determined by the STATEMAP Advisory Committee, is to complete geologic mapping of all 30′ x 60′ quadrangles in Montana. Our current projects are focused within the geologically complex and mountainous western part of the state, the only part of Montana that lacks complete 1:100,000 scale geologic map coverage. Most of the ~53 - 49 Ma Lowland Creek Volcanic Field (LCV), regionally contemporaneous with Challis magmatism, is located within the Butte North 30′ x 60′ quadrangle, a STATEMAP priority because of interest in the world-class Butte Mining District. Existing geologic maps (scale of 1:50,000 scale or less) take a widely varied approach to mapping the LCV, which are as thick as ~2,000 m and distributed over a ~1550 km² area. We believe that whereas eruption of the LCV was related to E-W extension in the hanging wall of the Anaconda Metamorphic Core Complex, the Great Falls Tectonic Zone, a 30 km wide, NE-trending, ancient Precambrian basement structure, controlled the geometry and distribution of the LCV. The LCV have been mapped at 1:24,000 scale in five quadrangles and two more will be STATEMAP deliverables for FY 2013. Elements of the LCV stratigraphy remain ambiguous, highlighting the need for comprehensive analytical work on the section. The lower cycle of the sequence is well established as a laterally continuous volcaniclastic sedimentary basal unit that grades upward to surge deposits locally, and to massive welded tuff regionally. However, the lateral continuity and distribution of lava flows and tuff in the upper cycle is not clear. We are in the process of adding new petrographic analyses, XRF geochemical data, and ⁴⁰Ar-³⁹Ar age determinations to our field maps in order to update the LCV stratigraphy so that it can be applied to the entire volcanic field. Improvements to the stratigraphy, and new ⁴⁰Ar-³⁹Ar ages, will allow us to better understand the LCV eruption(s) and constrain the timing of NE-striking faults within the volcanic field and along its contact with the Late Cretaceous Elkhorn Mountains Volcanics and the Boulder Batholith.