QUATERNARY ACTIVITY ON THE GALES CREEK FAULT, NORTHWEST OREGON

We use LiDAR imagery, ground penetrating radar (GPR), and fault scarp excavations to evaluate the recency and style of faulting at a site ~40 km west of Portland, OR. We interpret multiple Quaternary surface rupturing earthquakes on the Gales Creek fault, the youngest of which is likely Holocene in age. The Gales Creek fault, ~35 km west of Portland, is NW Oregon’s largest fault, extending at least 80 km NW from Newberg, OR along the boundary between the Coast Range and Tualatin Valley. Recent geologic mapping shows the fault’s long history, with 12 km of right lateral and 5 km of vertical displacement of the Eocene basalt basement.

One trench was excavated across a topographic scarp along a previously mapped bedrock fault on a hilltop saddle. This trench exposed a repeated pattern of bedrock faulted against or thrust over colluvium that is likely mid to late Pleistocene in age based on well-developed soils (numerical analyses are pending). Underlying the Quaternary deposits, the same faults juxtapose tuffaceous mudstone of late Eocene Keasey Formation rocks against basaltic sandstone of the late Eocene-Oligocene Pittsburg Bluff Formation rocks and, along another splay, placed Pittsburg Bluff Formation rocks against a sandstone unit not previously recognized in the area. Buried soils and both faulted and unfaulted colluvium suggest the topographic scarp is the result of multiple surface rupturing events. The relatively weak soil developed into the youngest eolian unit that drapes the scarp is suggestive of a late Pleistocene or younger minimum age of the most recent event.

Two trenches excavated across a subtle lineament that crosses the floodplain of Scoggins Creek exposed inferred Holocene overbank and marsh deposits cut by fractures and faults. GPR surveys across the lineament showed possible offset reflectors. Along one transect the reflectors step down to the east ~ 1 m and, along the other, the reflectors form a synform or channel and are also down to the east in the far field. At the scarp, the deposits are heaved up about 0.5 m and the overbank sand is folded and broken into rotated blocks with vertical shears in a disturbed zone about 4 m wide in both trenches. Numerical ages are pending, but deposits are presumed to be Holocene based on geomorphic position and weak soil development.