Graduation Date

Fall 2021

Document Type

Thesis

Program

Master of Science degree with a major in Environmental Systems, option Geology

Committee Chair Name

Dr. Melanie Michalak

Committee Chair Affiliation

HSU Faculty or Staff

Second Committee Member Name

Dr. Susan Cashman

Second Committee Member Affiliation

HSU Faculty or Staff

Third Committee Member Name

Dr. Laura Levy

Third Committee Member Affiliation

HSU Faculty or Staff

Abstract

The Klamath Mountains Province (KMP) records a robust history of active North American margin tectonics and crustal growth through the subduction-accretion of oceanic and island-arc terranes spanning Devonian through Cretaceous time. However, preservation of Cenozoic rocks is minimal, rendering the geologic record, topography, and tectonics during this timeframe difficult to reconstruct. The only Miocene terrestrial sedimentary unit that exists in the KMP is the fluvially deposited Tertiary Weaverville Formation (Tw) that is preserved in five fault-bounded basins in the southern part of the province. This study presents new uranium-lead (U-Pb) age data and epsilon hafnium (εHf) isotopic ratios from detrital zircons from the Tw from four of the five basins (Weaverville, Lowden Ranch, Hayfork, and Hyampom basins) to determine provenance source, reconstruct Miocene topography, and constrain depositional age of the Tw. The majority of Tw detrital zircons (n=700) are Mesozoic in age (88%) with the bulk of ages between ~130-224 Ma. Precambrian and Paleozoic ages range between ~275 Ma to 2.79 Ga (~11.7%). These ages are likely sourced from KMP bedrock terranes. A unique detrital zircon age signature is ~400 Ma, which can be explained by exhumation of the Trinity ultramafic sheet via activity on the La Grange detachment fault. Select εHf values across a range of ages in the Tw also match local KMP sources. This study also presents new U-Pb detrital zircon age data and εHf isotopic ratios from two additional units: 1) a Great Valley Group sandstone from the Reading Creek basin, and 2) a northern Klamath Mountains fluvial terrace deposit that is hypothesized to be Pliocene in age. Overall, evidence presented in this study suggests three new findings: 1) A new mid-Miocene maximum depositional age for the upper fluvial section of the Tw based on two U-Pb detrital zircon ages of 10.6 ± 0.2 Ma and 16.7 ± 0.1 Ma in the Lowden Ranch basin that are presumably ash fall from the Cascade volcanic arc, 2) The southern KMP was topographically isolated, preventing fluvial deposition of common regional sources from outside of the KMP during the mid-Miocene (e.g., Idaho or Sierra Nevada batholiths), and 3) The southern KMP experienced post-mid-Miocene watershed reorganization, possibly due to changes in topographic development influenced by the northward migration of the southern plate boundary of Cascadia. These new data also support interpretations of previous studies: 1) Basins containing Tw sediments formed syn-post deposition from local KMP sources and Tw basins show variability of source contribution and suggest each basin accumulated independently from multiple localized sources and fluvial directions, and 2) Deposition of the Tw was coeval with La Grange fault activity.

Citation Style

GSA

Included in

Geology Commons

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