SEISMOGENIC SEDIMENT REDISTRIBUTION AND EXPORT AT REDFISH LAKE, IDAHO, DRIVEN BY HOLOCENE FAULTING

The range-front Sawtooth Fault, central Idaho, lies near the upstream ends of several moraine-dammed lakes positioned on the hanging wall of the fault. Recent lidar-assisted fault mapping (Thackray et al 2013) confirms Holocene motion along the Sawtooth Fault, and suggests at least two large (>M6) earthquakes since latest Pleistocene deglaciation.

Sediment cores recovered from Redfish Lake, the lake basin cut near its inlet end by the Sawtooth Fault trace, reveal evidence of a major turbidite-generating subaqueous slope failure dated to 4300 cal yrs BP, as well as less-complete stratigraphic evidence for an earlier Holocene failure event (Johnson 2010). The failure plane from which the slump was generated has not been mapped, but in two cores that fully penetrate the disturbance sequence, translated sediments lie in (erosional) contact with glaciolacustrine varves, showing that the failure incorporated the entire pre-disturbance section of Holocene sediment, along with an unknown thickness of latest Pleistocene glaciolacustrine material. Remobilized glacial flour provided copious <10 micron clastic material, the suspended fraction of which forms a striking 25-cm graded cap on the disturbance sequence. In deep-water sections, the top of this deposit displays 12-15 mm-scale couplets suggesting periodic remobilization of fine disturbance-sequence material from littoral settings.

The occurrence of this suspended deposit at a site nearly 40 m shallower and remote from the main area of turbidite deposition strongly suggests that most or all of the Redfish Lake water column was suffused with remobilized glacial sediment in the wake of this seismically-triggered event. Confirmation of disturbance-triggered sediment export from the Redfish basin is found in a contemporaneous clastic pulse deposit in sediments of down-gradient Little Redfish Lake.

Elsewhere along the Sawtooth Fault, this event registers ambiguously (Hell Roaring Lake, ~10 km south on the fault foot wall) or not at all (Pettit Lake, 15 km south, hanging wall) in sediments recovered by coring. Lack of coeval deformation features in Pettit Lake cores can be attributed to its lower sediment loading and more gently-sloping bathymetry, or perhaps to termination of slip at a possible segment boundary a short distance north of Hell Roaring Lake.