2013 Volume 122 Issue 2 Pages 291-307
There have been fewer studies on strata formation in tectonically subsiding areas than in tectonically stable and uplifting areas. In this study, (1) 1000 year scale sediment stacking patterns, (2) seismic subsidence and meltwater pulse (MWP) 1B, and (3) event stratigraphy derived from seismic subsidence are revealed on the basis of sedimentary facies and 162 radiocarbon dating of three sediment cores obtained from the latest Pleistocene to Holocene Shinano River incised-valley fill (Alluvium) in the Echigo Plain. Aggradation is dominant in sediment stacking patterns of the incised-valley fill due to tectonic subsidence from the Nagaoka Plain Western Margin Fault Zone. Two continuous layers with abundant marine diatoms and rapid sediment accumulation rates are identified in transgressive salt marsh sediments. These layers may be due to seismic subsidence, but the latter layer (11.2-11.6 cal kyr BP) coincides with MWP 1B. If MWP 1B exists, the thickness of the latter layer indicates that the magnitude of the relative sea-level jump is less than 4 m. The event stratigraphy derived from seismic subsidence in salt marsh sediments is characterized by grading, dominant marine diatoms, and intense sediment accumulation rates. The presence of this event stratigraphy in the Echigo Plain suggests that seismic events strongly affect strata formation in tectonically subsiding areas in Japan.
