Sedimentary features of the seismo-turbidites, Lake Biwa, Japan

Seismo-turbidite layers are deposited at the foot of the steep slope of the half graben in which Lake Biwa has accumulated. Sedimentary studies, especially of grain-size distribution of selected turbidites provide excellent information for clarifying the distinctive features of general seismo-turbidites. Development of the two constituent sublayers (lower sandy sublayer and upper silt sublayer), lack or limited appearances of traction-indicating structures and characteristic grain-size distribution in the two constituent sublayers are most remarkable and significant. The characteristic grain-size distribution features are coarse tail grading in a relatively thick sandy sublayer, exceptionally rapid distribution grading through the thin boundary interval between the two sublayers, height increase and fluctuation of the silt mode, and high clay content. Brief discussion was made on the causes of these sedimentary features. The need and importance of further comparative studies of various turbidites known triggering forces are emphasized.