60-26 Sediment Composition and Texture of Exposed Reservoir Deposits in Lake Powell Tributary Canyons, Utah

Session: 2YC and 4YCU Geoscience Student Research Poster Showcase

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Abstract:

Since the construction of Glen Canyon Dam in the 1960s, sediment carried by the Colorado River and its tributaries has been trapped in Lake Powell Reservoir. As water levels have dropped in recent years, large areas of these stored sediments are now exposed. This project focuses on describing and comparing the composition and grain texture of exposed reservoir sediment in three tributary canyons: Clearwater, Gypsum and Dark in southeastern Utah. We collected 63 sediment samples across the three canyons. Laser Grain Size Analysis (LGSA) was used to measure grain size distribution, with results grouped into clay (<4 µm), silt (4-62.5 µm) and sand (>62.5 µm). Based on LGSA results, 20 fine grained samples interpreted to be reservoir deposits were selected for further analysis. These samples were analyzed using X-ray Fluorescence (XRF) to identify major elements and X-ray Diffraction (XRD) to determine mineral composition. Scanning Electron Microscopy (SEM) images were taken at 5, 20 and 100 µm to observe grain shape, angularity and surface texture. The grain size results showed four distinct sediment types based on grain size distribution patterns, including clay-rich, silt-rich, sand-rich and mixed type samples. These textural differences were observed both within and across canyons. XRF results showed similar overall chemical composition across sites, with high levels of Si, Al and Ca. XRD confirmed that common minerals including quartz, calcite and illite were present in most samples. SEM images revealed clear differences in grain shape and texture across sites. While overall chemistry is similar across canyons, the textures of the sediments are not, suggesting localized differences in depositional conditions across the reservoir. Differences in composition and texture may also influence styles and mechanisms of failure and erosion as the sediments are exposed and reworked. Future work will focus on adding stratigraphic context, completing additional SEM and XRD analyses and using these findings to better understand how exposed reservoir sediments might behave in the future.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7991

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