GLACIAL GEOLOGY OF BLACKWOOD CANYON, LAKE TAHOE, CALIFORNIA: IMPLICATIONS FOR LANDSLIDES AND TSUNAMIS

Blackwood Canyon (BC), a 600-m-deep U-shaped glacial valley, provides key evidence for Pleistocene glacial advances, a megalandslide, and associated tsunami erosion in the Lake Tahoe basin. BC and Ward Canyon (WC) to the north are the only two large glacial valleys in the eastern Sierra lacking lateral moraines at their mouths, possibly a result of removal by the McKinney Bay megalandslide during Holocene(?) time. New field studies have provided several new lines of evidence that BC was fully glaciated, confirming earlier deductions of Lindgren (1896). New evidence includes: 1) Tahoe and Tioga moraines within cirques on both forks of Blackwood Creek at 2,300-2,400 m asl; 2) Tioga-age glacial polish and striations on bedrock within cirques; 3) abundant glacial erratics along canyon walls in lower parts of the canyon, including granitic erratics of extrabasin origin; 4) small remnants of Tahoe(?) till at the mouth of BC near Eagle Rock; and 5) Tioga(?)-age erratics of both intrabasin and extrabasin origins on top of Eagle Rock at the mouth of BC, over 70 m above the canyon bottom. These observations clearly indicate that Tahoe and Tioga glaciers occupied the entirety of BC and that ice depth exceeded 70 m at the mouth of the canyon. Lindgren’s (1896) inference that granitic erratics near Eagle Rock were carried by ice spill-over from an ice cap southwest of BC is supported by identification of the Rockbound Valley granodiorite (Loomis, 1981), exposed 6-10 km south of the head of BC, as the source of some of the large erratics. Comparison of BC with other glacial valleys in the basin suggests that high-standing (ca 200-m-high) Tahoe and Tioga moraines once extended 4-5 kms east of the mouth of BC into what is now McKinney Bay. However, the mouth of BC near Eagle Rock is instead a low lying, smoothly beveled surface underlain by < 1 m of sand and pebble gravel. This surface is developed on much of the western shoreline of the lake. Furthermore, USGS bathymetry shows no evidence of morainal features along the sides or bottom of McKinney Bay. We conclude that an extensive morainal complex at the mouth of BC and probably WC was removed by one or more post-Tioga collapse events that formed McKinney Bay, and that morainal remnants near the mouths of BC and WC were strongly beveled by backfill tsunami waves. Studies of possible tsunami deposits are in progress.