Depositional timing of lobate landforms in the Park and Leon Creek drainages with comparisons to West Salt Creek landslide, Grand Mesa area, Mesa County, Colorado, The

Abstract

The purpose of this research is to characterize two lobate deposits in the valleys adjacent to the West Salt Creek Landslide (WSCL) to assess whether these deposits can be used in magnitude/frequency hazard assessments of WSCL type failures. Present-day hazards cannot be assessed by looking at the deposits of the region without understand the process and timing that led to their deposition due to the many agents that shaped the Grand Mesa. The origin of the two lobate deposits is in question, as various authors have mapped them as debris flow, glacial, or bedrock deposits. The two lobate deposits were dated and mapped using a variety of absolute and relative dating techniques, including radiocarbon and 3He cosmogenic nuclide dating, Schmidt Hammer, pit-depths, lichenometry, and rind-thicknesses. Dating and mapping the lobes in each deposit allows for an understanding of the magnitude, frequency, and origin of each deposit. The combination of absolute dating methods shows that there is a lag time between deposition and accumulation of datable organic matter, suggestive of a glacial origin. The Park and Leon Deposits are found to be of Pinedale age, with the Park Deposit likely covering its entire valley in one event, and the Leon Deposit covering its valley in two events. The deposits are thought to be moraines, and therefore, should not be used in assessing WSCL type hazards. The Schmidt Hammer is found to be a low cost and reliable method to estimate relative ages of the glacial deposits in this region, whereas the other relative dating methods did not reflect the same conclusions reached by absolute dating. Pit-depths did not reflect deposit age due to the vesicularity of the basalts creating different starting points for pit-development depending on the original vesicle sizes. Rind-thicknesses have not appreciably developed because not enough time has elapsed since deposition. Lastly, lichen species present at the elevations investigated were not ideal for the age of the deposits.