LATE PLEISTOCENE GLACIAL HISTORY AND EVALUATION OF THE SAN JUAN FAULT, SOUTHWESTERN VANCOUVER ISLAND, BRITISH COLUMBIA

The Cordilleran ice sheet that existed throughout the late Pleistocene covered an area ranging from Alaska and the Yukon Territory, all the way down into Washington, Idaho, and Montana. The San Juan River Valley in Southwestern Vancouver Island, B.C., fell within the bounds of the maximum extent of the ice sheet experienced approximately 14,000 years ago. Melting of the ice sheet occurred rapidly however. By approximately 12,000 years ago, the ice sheet retreated from this region of the island. This quick downwasting of the ice sheet lead to the ubiquitous deposition of glacio-fluvial, deltaic, and lacustrine sediment in this region.

Large deposits of these facies fueled by the melting of the cordilleran ice sheet is evident in available outcrops surrounding the densely forested San Juan River Valley and surrounding mountains. Within Smaller valleys in which tributaries to the San Juan River are located, sediment interpreted to have been deposited in paleo-fluvial systems are found. Further down these valleys, and at the edges of the San Juan River Valley itself, topset and forset beds are found. Within the valley, lacustrine clays indicative of bottomset beds are seen. This sequence of deposits leading to the San Juan River Valley have provided strong evidence for the existence of a paleo-lake: Lake Pacheedaht.

Two distinct sets of faults were identified and measured within the Pacific Marine Road Quarry located a few miles north of the town of Port Renfrew. One set of faults is considered to be growth faults associated with kettles. At the time of deposition, some ice blocks that were incorporated into fluvial systems were eventually dropped in place, covered with sediment, and insulated for a period of time. As the ice blocks melted, sediment dropped to fill the vacated space, forming a bowl-shaped deposit. The second group of faults are considered to be of a different origin. Possibilities include either tectonic or gravitational slumping derived faulting. The latter would be caused by erosion of the valley side by the tributary. The former would be caused by the San Juan Fault. This fault is considered to be last active during the Eocene, however, some evidence suggests activity as recent as the Quaternary. Further analysis of these observed faults aims to pin down their origin.