PRELIMINARY RESULTS OF GEOLOGIC MAPPING AND SEDIMENTOLOGIC ANALYSIS OF THE NORTHEASTERN FLINT CREEK BASIN, WEST CENTRAL MONTANA

Detailed 1:24,000 scale geologic mapping southeast of Drummond, Montana has provided a basis for reconstructing the Cenozoic history of the Flint Creek Basin. This basin is bounded to the north by the Lewis and Clark shear zone and to the south by late Cretaceous plutons and the Anaconda Metamorphic Core Complex of the Flint Creek Range. Bedrock geology consists primarily of folded and thrusted Cretaceous siliciclastics and alkalic intrusives. A red kaolinitic regolith is exposed along the contact between these units and overlying Tertiary rocks. Paleogene sandstones of the Renova Formation contain west-directed paleocurrent indicators and have an arkosic composition suggesting that the adjacent Boulder batholith was the likely sediment source. In the central part of the map area, a locally angular unconformity is well exposed between the Cabbage Patch beds of the Upper Renova Formation (late Oligocene) and the overlying Flint Creek member of the Six Mile Creek Formation (middle Miocene). The uppermost Renova Formation, immediately below the unconformity, predominantly consists of smectite clay with ferruginous mottling, nodules and granular peds containing abundant slickensides. We interpret these features as evidence of pedogenesis. A two meter thick boulder bed at the base of the Flint Creek member occurs locally just above the erosional contact, suggesting a significant reorganization of sediment dispersal systems within the basin. Identification of Arikareean (North American land mammal age) vertebrate fossils below the erosional surface and Barstovian fossils above it supports the interpretation of a depositional hiatus during the late-early Miocene (Hemingfordian). This surface is correlative to a regionally extensive unconformity in the Neogene of southwestern Montana. The presence of abundant calcretes in the Cabbage patch and Flint Creek beds is consistent with a subhumid to arid environment of deposition during the late Oligocene and middle Miocene.