Lacustrine evidence reveals spatially and temporally distinct Holocene ruptures on the Sawtooth Fault, Central Idaho, USA

Abstract

The Sawtooth Fault in central Idaho, USA, is a range-bounding normal fault with a Late Pleistocene–Holocene scarp near the up-valley ends of several range-front lakes. Cores from Redfish Lake, which spans the fault, exhibit evidence of catastrophic sediment re-mobilization in two sequences consisting variably of intraclastic mud-clast conglomerate, massive homogenite, and graded silt and clay. Event strata of the younger sequence are in unconformable contact with Pleistocene sediments in two cores on the northern lake-basin slope, suggesting transport from the down-valley portion of the lake toward the basin center. A silt cap associated with the event strata is preserved at all coring sites. The event sequence, distributed lake-wide, dates to ca. 4300 cal a bp, while additional event strata in two Redfish Lake cores pre-date the 7627 cal a  bp Mazama ash. We propose seismic triggering of the sediment disruptions. Asynchronous event stratigraphy in Pettit Lake, 15 km south of Redfish Lake, suggests distinct ruptures along two portions of the Sawtooth Fault. The sedimentology and geochemistry of remobilized sediments in both lakes indicates that sources included Late Pleistocene and early Holocene lake-floor sediments and suggests failures that cut deeply into the sedimentary sequence. Cores collected from Stanley Lake (17 km northwest of Redfish Lake) following the nearby 2020 M6.5 Stanley earthquake exhibit a thin, sandy event horizon associated with documented delta collapse. Subaerial slope failures associated with the 2020 event left no widespread disturbance stratigraphy in Redfish Lake. We conclude that the large-magnitude lake-floor sediment remobilization events probably required rupture of fault sections proximal to Redfish and Pettit lakes.