Superposed deformed beds produced by single earthquakes (Tecopa Basin, California): Insights into paleoseismology

Well-exposed soft-sediment deformation (SSD) structures occur in Tecopa paleolake beds largely composed of ash from the Bishop (760 kyr) and the Lava Creek (640 kyr) ultra-plinian eruptions. In both cases the SSD structures affected volcaniclastic deposits concentrated by overland flow from the surrounding topography into the lower slope and lake environments. Re-sedimentation of ash produced alternations of volcaniclastic layers with different grain sizes and ash content that allowed for reverse density gradients, which favoured liquefaction and deformation. In the Tecopa Basin, extensive outcrops show these deformed strata continuing laterally for hundreds of metres, along with three-dimensional exposures. This study illustrates the 3-D complexity of soft-sediment deformation including some novel morphologic features. Of particular interest were examples of superposed deformed beds that laterally change into one single deformed horizon. Heterogeneities in ash concentration, grain-size, and water content produced contrasting permeabilities that were barriers to liquefaction and soft-sediment deformation. The probable causes for liquefaction were Middle Pleistocene paleo-earthquakes. There are several active faults in the region with enough seismic potential to produce moderate to large earthquakes. The fact that a single deformed deposit laterally merges into multiple superposed deformed beds indicates that multilayer liquefaction could be produced by a single shaking event. In these cases, SSD layers are insufficient and unreliable indicators of paleo-seismic recurrence intervals.