EOCENE VOLCANIC STRATIGRAPHY AND TERTIARY FAULT GEOMETRY OF THE SOUTHERN DEER LODGE VALLEY, SW MONTANA

The Eocene Lowland Creek Volcanic field (LCVF) is a calc-alkaline, silicic pyroclastic suite deposited during regional Challis magmatism. The LCVF is ~800 m thick at the southern margin of the Deer Lodge valley and analytical data support field discrimination of five 1:24,000 scale map units on the basis of variations in mineralogy, texture, and degree of welding in ash-flow tuffs. In stratigraphic order the units are: (1) ~300 m of white to gray, biotite-rich, nonwelded to partially welded, lithic-bearing rhyolite (70.4 - 75.1 wt. % SiO₂) base surge and ash-flow tuff deposits. An ⁴⁰Ar/³⁹Ar age of 52.50 +/- 0.32 Ma obtained from the upper welded part is a minimum age for the unit. (2) ~20 m of black, plagioclase-phyric, vitrophyre. (3) ~350 m of maroon, plagioclase-phyric, partially divitrified rhyodacite (67.0 - 69.8 wt. % SiO₂) ash-flow tuff with an ⁴⁰Ar/³⁹Ar age of 51.79 +/- 0.51 Ma. (4) ~130 m of dark gray, aphanitic, andesite to dacite (62.6 - 65.2 wt. % SiO₂) lavas and shallow intrusions with an ⁴⁰Ar/³⁹Ar age of 49.33 +/- 0.34 Ma (Dudas and others, 2010). (5) Silicified breccia and block and ash deposits that exhibit wide variability in clast composition, size, form and stratigraphic position. The contact between the LCVF and the Boulder Batholith is exposed north and south of the southern Deer Lodge valley where it is offset by as much as 3 km in a right-lateral sense. Faulting in part predates emplacement of the ~49 m.y. old aphanitic andesite/dacite complex, which intruded and filled an 8 km² water-saturated depression. Core from deep wells located immediately north of the surface exposure indicate that the andesite/dacite complex is abruptly dropped ~30 m and covered by Tertiary valley fill. We believe geologic relationships and fault geometries at the southern margin of the Deer Lodge valley record Tertiary deformation in a WNW-striking extensional transfer zone. In a regional sense, the Deer Lodge valley may be a pull-apart basin whereby en echelon dextral shear zones that consist of concentrations of oblique-slip normal faults form the N and S physiographic limits to the valley.