Abstract: Active Growth Faulting on the Washington Continental Margin

Kenneth A. Piper, Lisa C. Mcneill, Chris Goldfinger

An abundance of large-scale growth faults were identified on the Washington Outer Continental Shell (OCS) during Minerals Management Service analysis for the National Assessment of undiscovered Oil and Gas Resources. Fault offset relationships and age determinations from federal OCS wells strongly suggest that these faults had their inception during late Miocene to early Pliocene with continued activity into Quaternary time. Seafloor offsets associated with faults near the shelf-slope boundary suggest that they are still active.

In September 1994, a research team from Oregon State University conducted a seafloor investigation in the vicinity of Grays Canyon, offshore Washington. Investigation included remote observation of surface expressions of three growth faults using side-scan sonar, and direct observation of one using a two-man submarine. The direct observation confirms that the seafloor offset is an active fault scarp. Other growth faults are inferred to be active.

Some growth faults appear to be associated with submarine canyons off Washington. This initially suggested that they represent slumping into the canyons; however, the regularity of the fault growth strata argues against simple slumping. Local Neogene sedimentation rates off southern Washington and northern Oregon, indicated by seismic and well data, suggest that in early Pliocene, Columbia River deposition was north of its present location. It is possible that the canyon and growth fault system began at that time, and the canyons persist because of continued mass movement from faulting.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California