Shelf sequence and characteristic bounding surfaces in a wave-dominated setting: Latest Pleistocene-Holocene examples from Northeast Japan

Abstract

This paper presents modern models for coastal to shelf sequences in a microtidal and wave-dominated setting, based on detailed analyses of latest Pleistocene-Holocene sequences on the Pacific side of Northeast Japan. The latest Pleistocene-Holocene sequences on the Pacific side of Northeast Japan, created in response to sea-level changes for the last 30 kyr, can be assigned to lowstand, transgressive and highstand systems tracts related to sea-level changes, and show characteristic bounding surfaces and depositional systems. These basically support the sequence architecture model of Vail. However, transgression and regression occurred almost simultaneously with rise and fall of sea level in the study areas, and so inflection points of sea-level curves, where the absolute slope or the rate of change is greatest, are less significant for these high-frequency sequences. The characteristic bounding surfaces are a sequence boundary, a transgressive surface which is the boundary between the lowstand and the transgressive systems tracts, a ravinement surface which is found mainly in deposits of the transgressive systems tract, and a maximum flooding surface which is the boundary between the transgressive and the highstand systems tracts. The sequence boundary was created in response to falling sea level. Fluvial entrenchment shifted seaward with falling sea level. Therefore, the boundary is a time-transgressive unconformity and is not an isochronous boundary. The transgressive surface is a nearby isochronous boundary that occurred when the shoreline had started to move landward from its maximum seaward excursion. The ravinement surface is a diachronous surface created by shoreface erosion during transgression. The shoreface erosion shifted landward with rising sea level, and so the erosion surface created is time-transgressive. This surface is the most distinct bounding surface in the shelf sequence. These characteristics of the bounding surfaces support and clarify the model of Thorne and Swift (1991). The transgressive systems tract on the shelf can be divided into two backstepping wedges separated by the ravinement surface. The lower wedge consists of coastal plain and fluvial systems, and the upper wedge is composed of transgressive sand and/or sand ridges. Both the wedges show a backstepping depositional pattern, which can be recognized in seismic records and cores. These wedges correspond to the backstep backbarrier wedge and backstep shelf wedge of Thorne and Swift (1991), respectively.