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Initial core data of 2018 sediment cores from Lake Powell, Utah

November 12, 2021

The Lake Powell Coring Project was a USGS research effort, in cooperation with the Utah Department of Environmental Quality, U.S. Bureau of Reclamation, and U.S. National Park Service. In the fall of 2018, hydraulic piston cores targeted sediment that had been deposited in Lake Powell. This large reservoir on the Colorado River in Utah and Arizona was created after the completion of Glen Canyon Dam in 1963. Retrieval and analysis of cores was undertaken in response to the Gold King mine release from the Bonita Peak Mining District in Colorado on August 5, 2015. This event resulted in the containment loss of three-million gallons of mine-impacted waters which flowed from the Animas River into the San Juan River, and ultimately into Lake Powell. Cores were retrieved from 40 holes, totaling nearly 500 m of core, between November 9 and November 30, 2018. At least 17 holes penetrated into the pre-Glen Canyon Dam land surface, typically in the antecedent river channel. Coring was primarily focused on the San Juan River delta, as it is the most likely location to detect Gold King-derived material, though cores were distributed across the reservoir. Full-depth holes, from the sediment top to the pre-Glen Canyon Dam surface, exceed 30 m at the thickest. Shallow holes, particularly in the San Juan River delta region, targeted only the upper 6 m of sediment. At the water-sediment interface of most cores, particularly those nearest the river mouths, an unconsolidated flocculent layer or "slurry" was typically present. Herein it is treated as the uppermost stratigraphic unit of the sediment column and stratigraphy. The area immediately behind the dam was not targeted; the nearest site to Glen Canyon Dam is located approximately 40 miles upriver. The reservoir sediment is primarily composed of thinly laminated muds with varying amounts of sand or silt. Rotary coring tools were used at the bottoms of holes that penetrated the pre-Glen Canyon Dam surface. The material encountered at the bottom of these holes (commonly bedrock, boulders, and coarse sand) could not be penetrated by hydraulic piston core. Recovery percentages of pre-Glen Canyon Dam sediment were not high, but sufficient material exists for characterization of this surface. Upon completion of the coring campaign, all cores were protected from freezing and freighted to the National Lacustrine Core Facility (LacCore) at the University of Minnesota in Minneapolis. There, each core was logged through its casing with a Geotek Multi-Sensor Core Logger-S (MSCL-S) that measured gamma density, volume-normalized magnetic susceptibility, p-wave amplitude and velocity, and electrical resistivity with a loop sensor. The cores were then split and cleaned, imaged with a Geotek Core Imaging System (MSCL-CIS) linescan camera at 10 pixels/mm (254 dpi) and scanned at a 5-cm interval on a Geotek Core Workstation (MSCL-XYZ) with a Konica Minolta CM2600D spectrophotometer for point-sensor magnetic susceptibility and color, relative gloss, and UV characteristics. The spectrophotometer uses a d/8 measuring geometry to collect light from the ultraviolet to infrared range (360-740 nm wavelengths) and provide CIE (Commission Internationale de l'Eclairage) XYZ wavelengths and L*a*b* color indices for each measurement. As the MSCL-S and MSCL-XYZ are separate logs, the respective data do not usually have corresponding analysis depths. All cores, including archive and working halves, are held in curation at the LacCore repository.