ABSTRACT

The Kakegawa Group is exposed in Shizuoka, central Japan. It is the fill of a Plio-Pleistocene fore-arc basin and constitutes a third-order sequence (2.6-1.0 m.y.). The upper Kakegawa Group consists of a transgressive systems tract (TST) and a highstand systems tract (HST). The TST comprises parasequence sets that are retrogradational on the shelf and progradational on the slope. The shelf parasequences consist of upward-coarsening facies of tempestites, bounded by ravinement surfaces. The shelf parasequences can be traced downslope, where they comprise upward-thinning turbidite successions (fourth- or fifth-order transgression) containing backstepping sand-dominated intervals, and silt-dominated, upward-thickening successions (regression).

The HST consists of progradational parasequence sets. The shelf parasequences are similar to those of the TST except for greater basinward distribution of shelf sand. Outer-shelf to upper-slope parasequences incorporate silt-dominated, upward-thinning successions (transgression) and overlying upward-thickening turbidite successions (regression) including sand-dominated intervals at their tops. The upward-thinning successions are much thinner than those in the TST. A condensed interval dividing these systems tracts consists of bioturbated silt and might include the maximum flooding surface.

In the TST parasequences, slope sand packets are thicker, are present on the lower slope, and were formed during fourth- or fifth-order transgression. In contrast, the HST slope turbidite sands are thinner, are present on the outer shelf to upper slope, and were deposited during fourth- or fifth-order regression. During transgression, the large volumes of sediment derived from intense shoreface erosion bypassed the narrow shelf (ca. 10 km wide), and contributed to channeling on the upper slope and accumulation of larger packets on the lower slope. In the HST, deposition of slope turbidites was induced by progradation of shelf sand during regression. Smaller rate supply of sandy sediment to the slope resulted in deposition of smaller packets. The sand might have been deposited at the o ter edge of a wider shelf (15-20 km) of the HST.