Sedimentary Record and Tectonic Evolution of the Flint Creek Basin, West Central Montana
In this study we combined 1:24,000 or larger scale geologic surface mapping, sedimentary facies analysis from outcrop and core, sandstone compositional data, geochronologic information, and 2-D gravity data analysis and modeling to document the record of Middle Eocene through Upper Miocene sedimentary and volcanic fill in the Flint Creek basin (FCB) in west central Montana. This basin lies in the northern Basin and Range and in the footwall of the Anaconda Metamorphic Core Complex (AMCC). Middle to Late Eocene initiation of subsidence and volcanism was synchronous with dextral transtensional stress along the Lewis and Clark Lineament to the north and rapid extension within the AMCC to the south. Gravity results suggest the presence of a major southwest-dipping normal fault that accommodated up to 800m of sedimentary fill within the FCB. Combined with borehole data, bedrock relationships along the basin margin suggest that this fault is a reactivated Sevier-style thrust. Arikareean sandstones of the upper Renova Formation contain abundant foliated metamorphic rock fragments, ~64 Ma muscovite (40Ar/39Ar), 77-63 Ma K-feldspar (40Ar/39Ar) and ~76 Ma zircon (SHRIMP U-Pb) suggesting erosional unroofing of 2-mica bearing footwall rocks in the AMCC. These sandstones directly overlie an air fall tephra yielding an 40Ar/39Ar age date of ~27 Ma. The top of the Renova Formation is locally truncated by erosion and overlain by a 2-meter thick boulder-bearing conglomerate bed forming the basal Sixmile Creek Formation. Smectitic clay, pedogenic slickensides and barite nodules immediately below the boulder bed are consistent with missing Hemingfordian stage fauna at this stratigraphic level and indicate the presence of a mid-Miocene unconformity (~17-15 Ma). Paleocurrent indicators measured from the basal Flint Creek boulder bed record an easterly paleoflow towards the adjacent Deer Lodge basin. Flint Creek strata overlying the boulder bed contain abundant groundwater calcretes and correlate temporally with lacustrine strata down depositional dip in the Deer Lodge lake basin, suggesting the development of an internally-drained basin. By ~6Ma, a series of westward-sloping paleovalleys filled with conglomeratic strata of upper Six Mile Creek Formation incised into the lower parts of the formation. These paleovalleys breached the Deer Lodge lake basin and initiated the westerly-flowing external drainage system that exists today.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009