Abstract:
As part of a multidisciplinary effort to obtain annual to decadal paleoenvironmental records in Saanich Inlet, British Columbia, X-radiographs of U-channels of anoxic varved sediments from ODP Holes 1033 B, C and 1034 C, D were digitized, merged, and subjected to detailed varve analysis. High-resolution correlations between the Holocene portions of each pair of holes were achieved by visual inspection of homogeneous, faintly laminated, and well-laminated intervals in x-radiograph prints augmented by comparisons of characteristic features in detailed rock magnetic profiles. Intervals could be correlated between holes and sites with centimeter to decimeter precision. An independent varve chronology established for Site 1033 appears to be complete for at least the last 8000 years and agrees well with the radiocarbon chronology if certain massive units are assumed to be reworked sediments. Sedimentation rates appear to be lower than average during drier conditions of the Mid-Holocene period from 2000 to 5800 years ago. Massive homogeneous units from 0.1 m to 2 m thick can be correlated among holes at a site and between sites. Eleven units occur at the same time in three or more holes. Four other events are coeval in two holes at different sites. Moreover, four simultaneous events are seen in two holes at Site 1033 but not at Site 1034. These units appear to be either resedimented mudflow deposits caused by seismically induced landslides and/or liquefied reworked sediments probably caused by large earthquakes. The estimated ages of events at Saanich Inlet are very similar to ages of turbidites in Lake Washington and buried soils at coastal sites in Washington and British Columbia, suggesting that very large earthquakes (M>7.5) have affected major portions of the Pacific Northwest several times during the Holocene. These events appear to have occurred more frequently in the last 3000 years. The recurrence intervals of these events average between 420 and 730 years for the last 8000 years, but earthquakes occur about every 230 years in the last 3000 years.