LACUSTRINE RECORDS OF SEISMIC EVENTS ON THE SAWTOOTH FAULT, CENTRAL IDAHO

The Sawtooth Fault in central Idaho is an extensional fault with a recently identified Holocene scarp, displaying at least two offsets that cut 14 ka Pinedale age moraines in the Sawtooth Valley Seismic events on this fault should be recorded in sediments from lakes and marshes within the adjacent Sawtooth Valley. Sediment cores from Redfish Lake contain spectacular evidence of sediment re-mobilization and transport in the form of a sequence comprised of 1) intraclastic soft-sediment breccias, 2) an overlying massive to near-massive ‘homogenite’, and 3) a normally graded fine-grained silt cap up to 25 cm thick preserving fine lamellae in its upper 5 cm. The breccia occurs in two cores from the deepest part of Redfish Lake but is not present in the down-lake cores, suggesting that the deposit originated down-lake and flowed towards the center of the basin, toward the fault. We propose that this sequence represents a seismically generated slump deposit capped by a slump-generated turbidite deposit (homogenite). We interpret the graded sequence as a widespread failure-generated suspension with persistent effects on water-column sediment concentrations. Estimates of possible suspended sediment concentrations attributable to the suspended silt (10² to 10³ mg/l range) suggest the possibility of lake-wide effects on biota. These results suggest that two major slump events have occurred during the Holocene, and suggest that major seismic events occur in the heart of central Idaho.