Land-Marine Paleoseismic Integration for the Northern Cascadia Margin, USA

Abstract:

New and archive cores (N=70), bathymetric, backscatter and sub-bottom data from the Washington margin reveal patterns of Holocene sediment transport and deposition. Barkley, Nitinat, Juan de Fuca (JDF), Quillayute, Grays, Guide, and Willapa Canyons each have different post-glacial mechanisms of loading and dispersal of sediment via turbidity currents. In high-stand conditions, the northern canyons, Barkley, Nitinat, JDF, and Quillayute are mostly relict systems. The remaining canyons, Quinault, Grays, Guide, and Willapa, are recharged to varying degrees by northward transport of Columbia River derived sediment. All systems are nonetheless active conduits for turbidity currents during the Holocene. Sedimentologic and CT analyses, supported by radiocarbon ages, micropaleontology, and the Mazama Ash show that the Holocene sedimentary sequence consists of a series of sand to mud turbidites in the active portions of all systems, interbedded with hemipelagic sediment. The relict systems are finer grained, commonly not visually detected, with Holocene turbidite counts the same as recharged systems. Use of 1960’s core sketches (Atwater) fails to capture the full record, as noted by Barnard (1973). Hydrodynamic models and heavy mineral distributions show that the northern canyon systems (Barkley, Nitinat, JDF, Quillayute) are independent of the southern systems, (Quinault, Guide, Grays, Willapa) during the Holocene. Best fitting flow inversions suggest turbidity currents range in height from 80-170 m, consistent with earlier work and backscatter observations. Mass balancing suggests sediment supplied to the slope canyons and abyssal channels is 4-6 times greater than supplied by recharge to the canyon heads by external sources, strongly indicating autogenic sourcing by earthquakes. Turbidite deposition off Washington is not very sensitive to either sediment supply or slope angle. Lithostratigraphic correlation and age models of Holocene turbidite sequences suggests deposition of ~ 20 Holocene turbidites in most parts of the Canyon systems, with little variation. The explanation most consistent with the data is triggering by a series of 18-20 Holocene earthquakes, in agreement temporal and lithostratigraphic linkages to new marine sites in Canada, as well and land paleoseismic data.