2020 Volume 74 Issue 2 Pages 65-76
The Pleistocene Katata Formation, Kobiwako Group distributed in the western littoral area of Lake Biwa is constructed by lacustrine delta system. By sedimentary facies analysis of the Katata formation, we identified several depositional environments including delta plane, shore slope, delta front, prodelta, and lake floor with the occurrence of subaqueous sediment-gravity flow deposits, such as hyperpycnites. The succession of the facies indicates that the paleo-lake levels changed asymmetrically: the depositional environment shallowed gradually and deepened rapidly. The asymmetric changes formed thick accommodations and thick deposition of stratum. Notably, every horizon of rapid rise of lake level is accompanied with the deformation structures, such as convolute and slump structures. This suggests that the increase of the lake level was caused by tectonic subsidence of the lake basin. The formative mechanism of the Katata Formation in the Kobiwako Group was firstly reported in this study: repetitions of lake-level rising by seismogenic faulting accumulated a thick sediment succession.
