PROVENANCE AND MAXIMUM DEPOSITIONAL AGES OF THE MONTGOMERY CREEK FORMATION FROM DETRITAL ZIRCON U-PB: AN UNDERGRADUATE COURSEWORK APPROACH

Late Cretaceous-Eocene sedimentary rocks in the Klamath Mountains Province (KMP) are rare, making paleotopographic reconstruction and tectonic interpretation difficult. The Montgomery Creek Formation (MCF) in northern California is primarily composed of meter- to decameter-thick beds of fluvial conglomerate and sandstone. Published constraints on the depositional age of the MCF mostly come from palynology and plant fossils, and indicate Late Cretaceous to Eocene ages. Sedimentologic changes from the lower to upper members represent an overall shift from a higher energy to lower energy fluvial system, and preserve evidence for reworking and multiple disconformities (Aalto, 1988). Previous studies of provenance in the MCF suggested a Klamath Mountains source based on clast compositions and sandstone petrology (Aalto, 1988) or a source in the Idaho Batholith based on Ar-Ar ages of detrital micas (Renne et al., 1990).

We present new detrital zircon (DZ) U-Pb age data (n=1473) from five samples collected from the MCF, including three samples from the upper sandstone member and two samples from sand lenses within the lower conglomerate member. Our approach to integrating this project into a semester-long upper division course is to divide samples among student co-authors to be analyzed individually, then to interpret the data collaboratively. This allows us to determine maximum depositional ages (MDA) and test competing provenance models.

Preliminary DZ results confirm a Late Cretaceous MDA for the lower MCF, and a late Eocene MDA for the upper MCF, suggesting a major unconformity between them. Multidimensional scaling (MDS) models of DZ age spectra show that the lower MCF plots close to the Jurassic-Cretaceous Dothan Formation, suggesting a similar provenance for these two units. In MDS space, the upper MCF plots close to the Late Cretaceous Hornbrook and late Eocene Tyee Formations, implying that they share a similar source. Alternatively, the upper MCF could be recycled from the Hornbrook Formation. In summary, this study introduces undergraduate students to the research community via a project integrated into an upper-division course, and a group of students will present new MDA and provenance constraints for the Late Cretaceous to Eocene Montgomery Creek Formation.