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The debris-avalanche deposits underlie the western two-thirds of Shasta Valley,
which is a broad depression between the Klamath Mountains on the west and the
Cascade Range on the east. The valley is drained by the Shasta River, which
flows northward across the avalanche deposits and younger basaltic lava flows.
The river enters a bedrock gorge in the Klamath Mountains northwest of Montague
and joins the Klamath River about 10 kilometers farther downstream. The floor
of the southern part of Shasta Valley slopes northward from an altitude of a
little over 900 meters near Weed to about 760 meters near Montague, and part of
the valley that lies north of Montague slopes gently southward. Mount Shasta
volcano, which has a summit altitude of 4,316 meters and an estimated volume of
about 335 cubic kilometers (Williams, 1932),
lies at the south end of the valley.
Shasta Valley is flanked on the west and north by complexly folded and faulted metamorphic, sedimentary, volcanic, and ultramafic rocks of Paleozoic age and by eastward-dipping marine sandstone and conglomerate of the Upper Cretaceous Hornbrook Formation (Peck and others, 1956; Hotz, 1977; Nilsen, 1984). A recent geologic map (Nilsen and others, 1983), which includes the northern part of Shasta Valley, shows the Hornbrook to dip 5-17 degrees generally easterly in the area east of Yreka. Volcanic rocks of Tertiary age border the north and east sides of Shasta Valley and also form a few hills that rise above the central part of the valley floor, such as Steamboat Mountain and Owls Head. Gregory Mountain at Montague is a cylindrical neck or plug of hornblende andesite (Hotz, 1977). Steamboat Mountain is shown on the geologic map of Nilsen and others (1983) to be underlain by volcanic rocks of Tertiary (?) age. However, sedimentary rocks of the Hornbrook Formation crop out on the eastern slopes of the mountain topographically lower than the volcanic rocks, which cap the mountain. At one point on the eastern side of Steamboat Mountain these sedimentary rocks strike nearly due north and dip about 15 degrees east. The presence of the Hornbrook near the center of Shasta Valley suggests that the valley south of Montague is largely underlain by that formation. Modern Mount Shasta is made up of at least four overlapping cones, the oldest of which is more than 100,000 years old and froms the south flank of the Volcano (Christiansen and others, 1977). Even this oldest cone, however, probably postdates the debris-avalanche deposits in Shasta Valley and may itself represent more than one "evolutionary cycle" in the growth of Mount Shasta. Volcanic rocks that evidently are older than this cone, and that predate the debris avalanche, crop out at the west base of the volcano (Crandell and others, 1984). A sample of these rocks collected by R.L. Christiansen and potassium-argon dated by G.B. Dalrymple of the U.S. Geological Survey had and age of 593,000 +/- 41,000 years. The age of the avalanche is roughly bracketed by radiometric dates on rocks within the avalanche deposits and on a postavalanche lava flow. Charles Chesterman of the California Division of Mines and Geology collected samples for potassium-argon dating from two andesite blocks within the avalanche deposit. These rocks were analyzed by James Aronson of Case Western Reserve University, who reported ages of 360,000 +/- 40,000 and 380,000 +/- 60,000 years (Crandell and others, 1984). A basalt lava flow that locally overlies the avalanche deposits was dated by Aronson as 300,000 +/- 100,000 years old. Thus, the debris avalanche seems to be between roughly 300,000 and 380,000 years old. This age is consistent with the presence, within the avalanche deposits, of clasts of a volcanic ash, described subsequently, that is about 400,000 years old. |
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