POTENTIAL SEISMIC DISTURBANCE OF HOLOCENE LACUSTRINE SEDIMENTS IN THE REDFISH LAKE DRAINAGE, SAWTOOTH MOUNTAINS, CENTRAL IDAHO

Holocene activity along the recently located Sawtooth Fault is shown by a fault scarp cutting latest Pinedale age glacial landforms in the Sawtooth Valley. Scarp characteristics indicate that motion occurred along this fault at least twice within the last 14,000 years. Seismic activity on the fault should be recorded in sediments from lakes and marshes within the Sawtooth Valley. Holocene seismic activity may be recorded in the numerous lakes lying on or adjacent to the fault as lake bottom slump deposits, distortion and faulting of laminated sediments, and seiche deposits in peripheral marshes. 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 breccia, 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 some cores from Redfish Lake but is not present in the down-lake cores, suggesting that the deposit originated down-lake and flowed toward the center of the basin. We propose that this sequence represents a seismically generated slump deposit that is capped by an intra-lake slump-generated turbidite deposit (homogenite), which formed due to motion along the adjacent Sawtooth Fault. 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) suggest the possibility of lake-wide effects on biota. Analyses of mineral composition, grain-size distribution, organic geochemistry, and ¹⁴C-based depositional chronology from Redfish Lake and down-drainage Little Redfish Lake are underway to evaluate the presence of more subtle disturbance sequences and the possible importance of seismic triggering to intra-lake and inter-lake redistribution of sediment.