This paper discussed the characteristics of delta front deposits, especially lower part of them, based on the analysis of three borehole core sediments (NB, SW, and AP1 cores) obtained from the Nobi Plain, central Japan. Lower delta front deposits have the following features: (1) upward-coarsening succession and upward-decreasing mud content showing convex-upward trend, (2) small luminosity compared with the underlying prodelta deposits under the wet condition, and (3) increase in accumulation rates from the prodelta deposits with concave-upward trend, and large accumulation rates (more than 10 mm/yr). When we determine the timing of passage of delta front through a site with sufficient accuracy, it is possibly effective using the horizon with mud content of fifty percent (almost equivalent to the boundary of sand and mud) because the horizon is identified easily, found commonly in the almost same depth among borehole sites, and can be formed near the river mouth with a relatively short period. It is expected that research on Holocene coastal and fluvial depositional systems will further progress by applying the detailed analysis result of borehole core sediments to reexamination of a huge number of existing boring logs.
