Slope-shelf-nearshore depositional sequences with transgressive conglomerate in a Miocene backarc and arc-junction setting, central Japan

Several third-order slope-shelf-nearshore depositional sequences are found in the Middle to Late Miocene Aoki and Ogawa Formations in the southern part of the Northern Fossa Magna region, central Japan. Each depositional sequence consists of a transgressive systems tract, varying in thickness from several tens of centimetres to about 10 m, a falling stage systems tract several tens of metres in thickness and a highstand systems tract, 20–80 m thick. The highstand systems tract and falling stage systems tract are bounded by the regressive surface of marine erosion. Each depositional sequence mainly consists of coarse-grained sediments. The materials of the transgressive conglomerate beds were derived from the gravelly foreshore and upper shoreface by transgressive shoreface erosion. The thin and conglomeratic transgressive systems tract indicates that these slope-shelf-nearshore sequences were deposited on the steep slope area under a rapid transgression. Therefore, these sequences were formed in response to excess sediment supply and a rapid rise in relative sea-level at an active margin of the backarc side of the arc-trench system.