EVIDENCE FOR A LATE PLEISTOCENE GLACIALLY INDUCED LAHAR ON THE SOUTH SIDE OF MOUNT SHASTA, NORTHERN CALIFORNIA

During historic times, rapid melting of glaciers has been a common cause for lahars on Mount Shasta in the Cascade Range of northern California. These recent lahars only occur in drainages currently headed by glaciers, but they almost certainly occurred in drainages that were occupied by glaciers during late the Pleistocene. A large lahar in the Panther Creek drainage on the south flank of Mount Shasta exhibits evidence of originating from the rapid melting of a glacier. The proximal portion of the lahar is confined between a set of moraines, suggesting a localized triggering mechanism. Immediately south of the moraines, Everitt Hill diverted some of the lahar to the west, while the rest of the flow would have continued down Panther Creek toward the town of McCloud, some 10 km to the southeast. The lahar contains andesitic clasts up to a meter in diameter; these clasts are also similar in lithology to the clasts in the moraines. Poor sorting and lack of stratification support the idea that the deposit is a lahar instead of the result of a fluvial process such as outwash. Though the moraines were emplaced before the lahar occurred, the degree of weathering in both deposits suggests they are close in age. Weathering rinds on clasts from the lahar and from the left lateral moraine have average thicknesses of 0.11 mm and 0.16 mm, respectively. Colman and Pierce (1981) found rinds of similar thicknesses on andesitic clasts from glacial deposits on Mount Lassen, ~120 km south of Mount Shasta, and correlated the deposits containing the clasts to the last glacial maximum. Excavations a kilometer apart in the lahar and moraine reveal ~ 60 cm of tephra overlying moderately weathered diamictons. The physical properties of the weathering profiles suggest at most a slight age difference. Younger lahars have not been found in or around Panther Creek, implying that despite eruptive activity in the Holocene, large lahars are infrequent without a glacier heading the drainage.