Quantifying Sedimentation Patterns of Small Watersheds in the Central Oregon Coast Range Using Landslide-Dammed Lakes

Up to 250 years of sedimentation patterns in headwater streams are preserved with detail in landslide-dammed lakes of the central Oregon Coast Range. I hypothesize that both anthropogenic and natural perturbations should increase linear and mass sediment accumulation rates and be discernible spatially and temporally in the sediment record with use of 137Cs, high resolution charcoal stratigraphy, and aerial photography. Klickitat Lake and Wasson Lake are landslide-dammed lakes in small watersheds (\u3c10 \u3ekm2) that contain drowned Douglas-fir stumps that are used for accurate dendrochronology and precise timing of the lake formation. An age-depth relationship was developed using 137Cs and identifiable fire events, which demonstrates that each lake has high linear sedimentation accumulation rates (0.2 – 4.5 cm y-1) and variable mass accumulation rates. Both lakes exhibit similar changes in mass and sediment accumulation, with rates remaining low after formation of each landslide-dammed lake and stabilizing after the initial lake filling. Stand-replacing wildfires of the mid-19th century increased short- and long-term sedimentation and mass accumulation rates, but they are not a primary driver in sediment mobilization and deposition. Sedimentation to the lakes systematically and gradually increased after the 1920s. Rates peaked in the mid-20th century associated with above-average peak discharge events with wetter and cooler conditions across the Pacific Northwest. Sediment deposition to the repositories has since decreased, but remains elevated compared to pre-industrial logging and road development. Comparisons to other landslide-dammed sediment repositories in the region suggest higher sedimentation rates in the steeper southern Tyee Formation, and lower sediment mobilization potential in the deep-seated dominated northern Tyee Formation. Results of this study indicate that small landslide-dammed lakes are viable tools for assessing sediment mobility patterns in headwater channels and can be used to further understand sedimentation and erosional patterns in the central Oregon Coast Range

Geochronology of the middle Eocene Purple Bench locality (Devil’s Graveyard Formation), Trans-Pecos Texas, USA

Purple Bench is a middle Eocene fossil locality in the Devil’s Graveyard Formation of the Trans-Pecos region of West Texas. In addition to yielding a range of taxa characteristic of the Uintan North American Land Mammal Age, the Purple Bench locality is noteworthy in documenting a number of endemic species that are known only from the site. Despite the Uintan character of the mammalian fauna, the absolute age of Purple Bench is a matter of debate. This uncertainty stems from the wide interval of time encompassed by current radiometric dates bracketing the Purple Bench locality and from conflicting magnetostratigraphic correlations in the Devil’s Graveyard Formation. This study constrains the absolute age of the Purple Bench locality through detrital zircon U-Pb geochronological analyses. For these analyses, 147 new detrital zircon U-Pb ages were collected from five tuffaceous sandstones and reworked tuff horizons and analyzed via Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS). These new detrital zircon U-Pb geochronological analyses suggest a maximum depositional age of 43.7 +0.8 / -0.2 Ma for the Purple Bench tuff, a significant marker horizon immediately below the Purple Bench locality. These new maximum depositional age dates presented here provide constraints on the true depositional age of the lower and middle members of the Devil’s Graveyard Formation, bringing clarity to the previously ambiguous age of the fossil-bearing Purple Bench locality. The age constraints presented here also aid the characterization of the temporally and spatially variable Uintan North American Land Mammal Age

Dendrochronological dating of landslides in western Oregon: Searching for signals of the Cascadia A.D. 1700 earthquake

Large-magnitude earthquakes and hydrologic events in mountainous settings commonly trigger thousands of landslides, and slope failures typically constitute a significant proportion of the damage associated with these events. Large, dormant deep-seated landslides are ubiquitous in the Oregon Coast Range, western United States, yet a method for calculating landslide ages with the precision required to diagnose a specific triggering event, including the A.D. 1700 Cascadia earthquake, has remained elusive. Establishing a compelling connection between prehistoric slope instability and specific triggers requires landslide ages with precision greater than that provided by 14C dating of detrital materials. Tree-ring analysis is the only known method capable of determining landslide age with this precision. Dozens of landslide-dammed lakes in western Oregon present an opportunity to use tree rings from drowned snags, or “ghost forests,” to establish the year of death, and thus landsliding. We cross-dated tree-ring indices from drowned Douglas fir trees with live tree-ring records from the Oregon Coast Range that exhibit synchronous, time-specific patterns due to regional climate variations. Our analyses determined that the landslides responsible for creating Wasson and Klickitat Lakes occurred in A.D. 1819 and 1751, respectively. The 14C dates from selected tree rings and landslide deposit detritus are consistent with our tree-ring analysis, although the ages exhibit high variability, revealing the limitations of using 14C dating alone. Because dendrochronology provides annual precision for landsliding, sampling of tree rings at additional landslide-dammed lakes throughout the Oregon Coast Range can be used to constrain the potential effects of ground motion and major storms on Cascadia landscapes