CENOZOIC EVOLUTION OF THE GOLD CREEK BASIN, WESTERN MONTANA: MAPPING AND DOCUMENTING THE GEOLOGY OF THE ROCK CREEK LAKE 7.5' USGS QUADRANGLE

This study presents results from 1:24,000 scale surface mapping and related analysis of geology within the Rock Creek Lake 7.5’ USGS Quadrangle in western Montana. The purpose of this study is to 1) document the Cenozoic structural evolution and sedimentary fill record associated with tectonic development of the northern Flint Creek Range and in particular, the Anaconda metamorphic core complex and 2) determine the existence and extent of Pleistocene glacial deposition in the study area. Gneissic mylonitic granodiorite along the southeastern contact of the Royal Stock intrusion indicates the Anaconda metamorphic core complex detachment zone trends through the center of the map area but diminishes to the north. Synthetic top-to-the-east normal faults throughout the study area are interpreted as joining the main detachment at depth. Granitic and metamorphic rock fragments observed during provenance studies of sandstones and conglomerates indicate the initial opening of the Gold Creek basin was contemporaneous with the unroofing of the Anaconda metamorphic core complex. The upper Renova Formation observed in the study area is equivalent to the upper Renova Cabbage Patch beds best developed in the Flint Creek basin to the west and indicate primarily lacustrine, fluviatile and palustrine depositional environments. Quaternary glacial deposits attributed to the Pinedale Glaciation consist of narrow, sharp-crested moraines (mean crest width of 5m) with high distal slope angles (average 30°) and primarily consist of relatively unweathered granitic boulders. Deposits interpreted as reflecting the Bull Lake Glaciation are characterized by wide, round-crested moraines (mean crest width of 50-100m) and low distal slope angles (average 10-20°), with little exposure of quartzite and weathered granite boulders at the surface. Formation of the Gold Creek basin was influenced by Cenozoic relaxation of Cretaceous thrust faults, extension from the Anaconda metamorphic core complex, and dextral transtension along the Lewis and Clark Lineament.