INITIAL DESCRIPTION OF A NEW ROCK CORE FROM THE PALEOGENE RENOVA FORMATION, NINEMILE VALLEY, WESTERN MONTANA: A WELL-PRESERVED ARCHIVE OF PALEOCLIMATE AND TECTONICS

We report the initial description of a 93 meter core cut from the Paleogene Renova Formation in the Ninemile Valley of western Montana (SE1/4 NW1/4 SE1/4 SE1/4, sec. 8, T15N, R22W). Well-bedded, commonly laminated silt and clay-rich sediments deposited in glacial Lake Missoula (GLM) are exposed at the core location, and the uppermost 9 meters of the core consist of calcareous pink-to-brown laminated silt and clay intepreted as sublacustrine GLM deposits. Abundant carbonized woody debris and leaf impressions in a dark gray-brown claystone at 9.1 meters depth mark the first occurrence of Paleogene Renova Formation (late Eocene to middle Miocene) in the core. The uppermost ~4.5 meters of Renova Formation are dominated by light-gray to brown, non-calcareous siltstone and claystone with abundant carbonaceous debris and several dm-scale sandstone interbeds. Carbonaceous claystone and cm-scale seams of coal first appear at 14.3 meters depth, and carbonaceous shale consistently is a common lithology in the core to a depth of 69.1 meters. Claystone and siltstone dominate the coal-bearing portion of the core. A ~2 meter thick section of massive, gray-white tuffaceous sandstone between 32 and 34 meters depth is interpreted as a volcanic ash. Below ~71 meters depth, a broad transition is inferred from well-wetted coal-forming depositional environments to better drained terrestrial environments. Below this depth, the core is progressively more characterize by green, purple, yellow-brown, and red mottled sandy mudstones. Local carbonate nodules in this facies are interpreted as pedogenic, and yellow- to red-colored clay-rich units are interpreted to reflect laterization of fine-grained clastic sediments comprising the lower Renova Formation. The core ends in a mottled red-brown claystone interpreted to be a laterite within the Renova. The abundance of claystone in the core is interpreted to reflect the importance of devitrification of abundant volcanic tephra and the local ponding of sediment in small pull-apart basins associated with the adjacent Ninemile Fault System. We posit that Paleogene sediments in the core provide a unique opportunity to investigate the transition from the Eocene Thermal Optimum to the establishment of subsequent Oligocene 'Ice House' paleoclimate conditions.