Holocene pyroclastic-flow deposits from Shastina and Black Butte, west of Mount Shasta, California

Abstract

A broad apron of pyroclastic-flow deposits derived from dacitic domes of Holocene age at Black Butte and Shastina covers an area of more than 110 km 2 on the west flank of Mt. Shasta volcano. The stratigraphy of the deposits is exposed in readouts along a northwest-southeast line between the cities of Weed and Mount Shasta and includes, from bottom to top, pre-Shastina diamictons, a Shastina pyroclasticflow assemblage, and a Black Butte pyroclastic-flow assemblage. Pyroclastic flows from Shastina, a volcanic cone on the west flank of Mt. Shasta, form a fan of nonvesicular rock debris that overlies part of the Shastina cone and pre-Shastina deposits; the fan deposits thicken northward and underlie the town of Weed. Pyroclastic-flow deposits of both vesicular and nonvesicular rock debris caused by eruptions at the site of Black Butte, a large volcanic dome at the foot of Mt. Shasta, thicken southward and underlie part of the city of Mount Shasta. Soil-profile oxidation is 75-80 cm thick on deposits from both Shastina and Black Butte. As much as 10 m of vertical displacement occurred along east-trending faults 3.5 km northwest of Black Butte after deposition of the youngest two pyroclastic flows from that source. Evidence that faulting and volcanism were nearly simultaneous suggests that the area northwest of Black Butte subsided during a late eruptive phase of the plug dome. Future eruptions similar to those that produced the pyroclastic flows could endanger people and property in any direction downslope from vents, including the communities of Weed and Mount Shasta and possibly other communities in the Shasta Valley and upper Sacramento River area.