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U-Pb zircon data for Cenozoic clastic and volcaniclastic units deformed along the Gales Creek Fault zone, northwestern Oregon

April 5, 2023

This data release includes laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb zircon geochronology data from four samples collected from fault-bounded bedrock units in northwestern Oregon. Samples were collected from a paleoseismic trench excavated across the Scoggins Valley strand of the Gales Creek Fault and nearby quarry. The trench is located on a ridge top saddle on the Scoggins Creek-Tualatin River divide, where it crosses a lidar scarp trending 300°, up to the east, with suggestion of dextral slip. Trench trends 060° across scarp, is 35 m long in bedrock. Loess, paleosols, and colluvium overlie bedrock. The trench exposes tuffaceous sediments of the Keasey Formation, which records initial deposition of Cascade-derived sediment into the forearc basins from the newly formed Cascade arc. Three samples were collected in the trench from the tuffaceous mudstone of the Keasey Formation; the basaltic sandstone in the Stimson Mill member of the Keasey Formation, and a lithic sandstone bed in the Keasey Formation, respectively. One sample was collected from a quarry near the paleoseismic trench, in a 13m-thick, indurated silicic tuff bed of the Keasey Formation, which may correlate with one of the large, dated ash flow tuffs recognized in the Cascade arc and backarc between ~41 and 25 Ma. The sequence is capped with modern gray loess-derived soil of variable thickness. Ages of samples WN17-1, -2, and -7 overlap, and all have a large tuffaceous component from the initiation of Cascade arc volcanism. Sample WN17-6 has a slightly older detrital age spectra and is a dark brown weathered lithic (basaltic) sandstone, distinct from the three other light colored tuffaceous rocks. The tuff bed (sample WN17-7) is a local marker bed in the basaltic sandstone. The older age for the basaltic sandstone may reflect a separate Coast Range source. The data supports the geologic map of Scoggins Dam, Henry Hagg Lake, and the Scoggins Valley area, Washington County, Oregon.

Publication Year 2023
Title U-Pb zircon data for Cenozoic clastic and volcaniclastic units deformed along the Gales Creek Fault zone, northwestern Oregon
DOI 10.5066/P91ZSKV3
Authors Laura S Pianowski, Christopher S Holm-Denoma, Lydia M Staisch, Ray E Wells
Product Type Data Release
Record Source USGS Digital Object Identifier Catalog
USGS Organization Geology, Geophysics, and Geochemistry Science Center