GEOLOGIC AND PALEOMAGNETIC STUDIES OF A PLIOCENE VOLCANIC DEBRIS-AVALANCHE DEPOSIT IN THE NORTHERN SIERRA NEVADA, CALIFORNIA

The Pliocene Barker Pass volcanic debris-avalanche deposit occurs over an area of ~400 km² in the northern Sierra Nevada, California and is inferred to have originated on a stratovolcano located to the east of the Sierran range-front fault zone. This study describes the Forest Hill Divide lobe of the deposit, which is ~380 m thick and occupies part of a southwest-trending paleochannel that is >20 km long. Glaciation has cut an oblique longitudinal section through the deposit that reveals contacts with overlying Pliocene lavas and lahar deposits, and underlying Tertiary volcaniclastic deposits and Paleozoic and Mesozoic basement. Contacts with the paleochannel walls are nearly vertical in places, and are defined by truncated, subhorizontal Miocene lahar deposits and Oligocene ignimbrites. Kilometer-scale megablocks composed of basaltic and andesitic lavas, lahar deposits and hydrothermally altered rock are abundant within the deposit. The megablocks show various stages of disaggregation and in some cases preserve internal stratification. They are inferred to have been derived from the source volcano. The debris-avalanche matrix is a very-poorly sorted mixture of dust- to block-sized particles, and typically shows an ungraded, chaotic fabric; juvenile pyroclastic material is absent. Induration of the matrix has preserved evidence of cataclastic shearing both within the matrix and between blocks. Many of the megablocks are composed of a distinctive, blue-grey andesite/dacite that is found only in the Barker Pass debris-avalanche deposit, and locally shows intrusive relationships with other rock types within composite megablocks. Paleomagnetic studies show that the blue-grey andesite/dacite was emplaced just prior to avalanche initiation and was relocated after it had cooled to 300° C. The data also show that the debris-avalanche matrix was emplaced at the same temperature, suggesting the blue-grey andesite/dacite as the heat source. It is inferred that the andesite/dacite was emplaced as a hypabyssal intrusion, possibly a cryptodome that destabilized the hydrothermally weakened flank of a stratovolcano, leading to the initiation of the debris-avalanche.