The Geology and Mineralogy of Bentonites and Associated Rocks of the Chuckanut Formation, Mt. Higgins Area, North Cascades, Washington

In the Mt. Higgins area, the Chuckanut Formation is in probable fault contact with pre-Tertiary metamorphlc rocks. The Chuckanut is over- lain by Oso volcanics (zircon fission track age of 43.2 ± 1.9 MY). A diorite body (k/At date = 53 ± 8 MY) crops out in the Granite Lake area and is thought to be intrusive into the Chuckanut. Sedimentary rocks of much of the study area are dominantly thick- bedded arkoses that are generally cross-bedded, and resemble the Chuckanut type section. Sediments cropping out along and near Deer Creek are different; black, bituminous shale is the dominant rock type, and several bentonite beds are also present. Some bentonites are only a few centimeters thick and are not lithified, whereas others are thicker, lithifled and composed of amalgamated beds, some of which are graded. Glass shards and other volcanic detritus of the air-fall tephra have altered to form K-rectorlte upon heating to 275-285°C. A mld-Eocene date of 44.5 MY was obtained by fission track dating of zircons from one of the bentonites. It is proposed that the Deer Creek area sediments were deposited in lakes and other low energy environments on a swampy floodplain. Kalkberg-type mixed-layer illlte/smectite is found elsewhere in the Mt. Higgins area, and is probably indicative of higher temperatures than those which created the K-rectorite. On Mt. Higgins, the presence of andalusite suggests temperatures near 400°C. Cordlerite-bearing sediments indicate temperatures of 500°C. Heating was a result of a large geothermal system initiated by the intrusion of the Granite Lake diorite and possibly other related intrusions, and may be associated with Oso volcanlsm. Magmatic activity may have affected the area for several million years, perhaps sporadically