Paleogeographic Reconstruction of the Renova Formation in Southwestern Montana
The University of Montana - Department of Geosciences Missoula, Montana; [email protected]
Analysis of paleocurrent indicators and clast lithologies from multiple outcrops of the Eocene to Miocene Renova Formation in southwestern Montana suggests that Paleogene paleogeography was dominated by relict topography inherited from Cretaceous orogeny and local topography formed by contemporary volcanic eruptions. In the Sage Creek Basin north of Dillon, paleocurrents and clast compositions measured from conglomerates and sandstones in the Eocene Dell Beds (mean paleocurrent direction from N82W) and the late Eocene to Oligocene Cook Ranch Member (mean paleocurrent direction from N89W) of the Renova Formation indicate primary derivation from local sources in the Cretaceous Beaverhead Formation and the Eocene Hall Springs Basalt. In the Ruby River Valley, paleocurrents and clast lithologies indicate that the primary source terrains were the Archean metamorphics of the Ruby Range and local Paleogene volcanics. Further to the east, the modern Gravelly Range appears to represent a topographically inverted, north-south oriented Paleogene paleovalley. In the northern Gravelly Range, the informally recognized Butcher Creek conglomerate represents the basal unit of this paleovalley sequence with paleocurrent indicators (mean paleocurrent direction from N60E) and felsic metamorphic clast compositions which suggest deposition along the southern flank of a Paleogene analog of the modern Tobacco Root Mountains. Thick deposits of Paleogene sandstones and mudstones, overlying several enigmatic conglomerate units, in the central Gravelly Range probably represent a southern continuation of this paleovalley.
AAPG Search and Discovery Article #90083 © 2008 AAPG Foundation Grants in Aid