PALEOGENE RECORD OF ANACONDA METAMORPHIC CORE COMPLEX UNROOFING AND SEDIMENT DISPERSAL IN THE DEER LODGE BASIN OF SOUTHWEST MONTANA

The Anaconda Metamorphic Core Complex of southwestern Montana makes up the western footwall-margin of the Deer Lodge Basin. Paleogene coarse-grained alluvial, colluvial, and fluvial bodies on the hanging wall margin demonstrate the link between unroofing of the core complex and sediment delivery to the eastward basin.

Alluvial deposits upon the hanging-wall margin include debris flow, grain flow, sheet flow, mud flow, and fluvial facies. Clast composition commonly consists of metamorphosed and unmetamorphosed Proterozoic calcsilicate, Proterozoic subarkose, Paleozoic carbonate, and Eocene tuff and rhyolite. Subordinate clasts may include granite, marble, garnet schist, brittle-fractured clasts, and hornfels. In one case, debris flows containing abundant angular, cobble- and boulder-sized limestone clasts indicate a proximal, high-relief source area of Paleozoic strata in the adjacent Flint Creek Range. Although rare, a debris flow containing meter-scale granite boulders records exhumation of the Flint Creek pluton. In all cases, alluvial paleoflow data indicate eastward transport of detritus. In addition, paleotalus deposits of angular metaquartzite clasts in unconformable contact with folded and faulted metaquartzite strata serve as evidence of colluvial processes along a high-relief tectonic landscape. Small and large-scale fluvial bodies in the southern Deer Lodge Basin document SE and SSE transport. In contrast, fluvial bodies in the northern Deer Lodge Basin record NW transport of detritus derived from two-mica granites in the Flint Creek Range.

The volcanic detritus marks nearly contemporaneous erosion of nearby Eocene Lowland Creek Volcanics that are reportedly associated with initial extension. The Paleozoic and Proterozoic sedimentary clasts document erosion of strata above the detachment whereas metamorphic detritus reflects dissection of the underlying shear zone. Granitic debris documents incision into the footwall. The presence of transported fractured clasts, post-depositional folding of Paleogene strata, and fault-related Paleogene talus are indicative of syndepositonal tectonism. Divergent fluvial paleoflow within the Deer Lodge Basin suggests that a paleo-drainage divide was present in the central part of the paleobasin as opposed to the modern setting.