PRELIMINARY INVESTIGATION OF PALEOGENE FLUVIAL DISPERSAL AND PROVENANCE RELATIONSHIPS NEAR THE FRONTAL MARGIN OF THE ANACONDA METAMORPHIC CORE COMPLEX, SOUTHWEST MONTANA

The metamorphic core complex of the Anaconda and Flint Creek Ranges in western Montana marks a western footwall-margin of the Deer Lodge and northern Big Hole intermontane basins. Paleogene fluvial conglomerate bodies demonstrate the link between unroofing of the core complex and delivery of detritus to adjacent basins.

In the Deer Lodge Basin, sheets of crudely bedded, upward-fining, framework-supported fluvial conglomerates make up one of several syntectonic lithofacies deposited upon the western hanging-wall margin. Depending upon location, clast composition is dominated by metamorphosed and unmetamorphosed Proterozoic calcsilicate, Proterozoic subarkose, or Paleozoic carbonate. Subordinate clasts include quartz sandstone, marble, garnet schist, muscovite-quartz gneiss, quartz-feldspar gneiss, brittle-fractured clasts, hornfels, and Eocene tuff and rhyolite. Paleoflow was to the SSE.

About 28 km SE of the central Anaconda Range, a fluvial, biotite- and muscovite-rich conglomerate of interpreted Paleogene age is exposed along a paleovalley that extends southeastward, away from the range and from the northern Big Hole Basin, across the Pioneer Mountains. The succession consists of normally graded, amalgamated sheets and lenses of conglomerate and a capping paleovalley-side debris flow. Clasts are dominantly Proterozoic subarkose. Subordinate clasts include Paleozoic quartz arenite, tuff, muscovite-biotite schist, muscovite-biotite-rich gneiss, and two-mica granite. Paleoflow was ESE.

Overall, a longitudinal trunk-fluvial tract flowed SSE along the western margin of the Deer Lodge Basin, transporting detritus derived from the Flint Creek orogen toward, and perhaps into, the northernmost Big Hole Basin. Farther south, transport was transverse through the Pioneer Mountains and into the Divide Basin. Deformed and metamorphosed clasts at both sites, similar to core-complex shear-zone lithologies, substantiate that the footwall of the detachment had been breached. The presence of two-mica granite clasts in the south document unroofing of a two-mica pluton, most likely the Chief Joseph pluton within the Anaconda Range. Proterozoic clasts were derived from the Anaconda-Flint Creek uplift, local Pioneer Mountain sources, and probably other westward locales.