Abstract

Lake sediments commonly contain detrital layers that record events such as floods or earthquakes, but these may be disturbed or partially destroyed by bioturbation. Here we use a novel combination of techniques to relate microscopic sediment fabric features to processes at the lake basin scale. X-radiography and micro-X-ray fluorescence analysis of cores are complemented by back-scattered electron imagery and energy-dispersive X-ray microanalysis of resin-embedded sediment. Together, the microfabric and geochemical methods enable the identification of clay layer mass transport deposits despite bioturbational mixing of the original end-members. Two cores with robust radionuclide chronologies contain correlative clay layers dated to 1979 (1974–1982) and 1979–1980 (1973–1986), respectively. These clay layers probably represent the distal turbidite generated by a major mass flow deposit identified from multi-beam swath bathymetry and sediment grab sampling. A probable trigger for the mass flow and associated turbidity current is the 4.7 ML 1979 Carlisle earthquake. The lake basin slope was probably preconditioned for failure by increased sedimentary biogenic gas production and sediment in-wash as a result of anthropogenic activities, coupled with sediment disruption and dredging. This study highlights the effectiveness of microstratigraphic techniques in the recognition and characterization of event layers in sediments where bioturbative disruption has occurred.

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