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

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.