SEDIMENTARY RECORD AND CENOZOIC EVOLUTION OF THE TRANSTENSIONAL FLINT CREEK BASIN, WEST CENTRAL MONTANA

Sedimentary and volcanic rocks of the Flint Creek basin (FCB) in west central Montana record the region's tectonic history from middle Eocene through late Miocene time. Dextral transtensional stress along the Lewis and Clark Lineament to the north and rapid extension within the Anaconda metamorphic core complex to the southeast were synchronous with initiation of FCB subsidence and local volcanism during the middle to late Eocene. 3D and 2D gravity models suggest <1000m of FCB sedimentary fill and indicate the presence of a major northwest-dipping normal fault. The fault locally accommodated much of the basin subsidence and may be a reactivated Laramide-style thrust fault.

Paleocurrent indicators in late Oligocene (Arikareean) sandstones of the upper Renova Formation record a northwesterly paleoflow of arkosic detritus. Abundant muscovite and foliated metamorphic rock fragments in these sandstones suggest erosional unroofing of the Anaconda metamorphic core complex. A 2-meter thick boulder bed in the lowermost Sixmile Creek Formation locally overlies Upper Renova strata. Smectitic clay and barite nodules immediately below the boulder bed are consistent with missing Hemingfordian stage fauna at this stratigraphic level, indicating the presence of a mid-Miocene unconformity (~17-15 Ma). This unconformity is widespread across the northern Basin and Range and represents a fundamental tectonic reorganization associated in part with initiation of the Yellowstone hot spot. Paleocurrent indicators measured from the Sixmile Creek Formation boulder bed record an easterly paleoflow towards the adjacent Deer Lodge basin. Groundwater calcretes in lower Sixmile Creek strata overlying the boulder bed correlate with lacustrine strata down depositional dip in the Deer Lodge lake basin and suggest the development of an internally-drained basin. A final period of uplift and faulting prior to deposition of the upper Sixmile Creek Formation (late Barstovian) eroded paleovalleys into the underlying lower Sixmile Creek Formation and established the west-flowing external drainage system that exists today.