Transition from a transgressive to a regressive river-mouth sediment body in Tokyo Bay during the early Holocene: Sedimentary facies, geometry, and stacking pattern

Abstract

Sedimentary facies, geometry, and stacking pattern transitions of river-mouth sediment bodies recording the turnaround from a transgressive estuary to a regressive delta system have not been sufficiently clarified in the Asian sedimentary record. In this study, the temporal and spatial distributions of sedimentary facies of the post-Last Glacial Maximum incised-valley fills (pLGMIVFs), including those recording delta initiation in the Tama River Lowland, on the west coast of Tokyo Bay, are described taking into account 14 sediment cores, 186 radiocarbon dates, and 8745 geotechnical borehole logs. The pLGMIVFs in the Tama River Lowland, which unconformably overlie Plio-Pleistocene deposits, can be divided into braided-river, meandering-river, estuary, and delta systems in an ascending order. The boundaries between the braided-/meandering-river or estuary systems and the estuary/delta systems correspond to a transgressive surface (20 ka) and a maximum flooding surface (8 ka), respectively. In the estuary system, the river-mouth sediment body, accumulated during 10–8 ka, gradually became smaller and coarser-grained landward; in contrast, the delta system's river-mouth sediment body became larger, finer-grained as it prograded seaward during the past 8 ky. With decelerating sea-level rise at 8 ka, the delta initiated as a sediment body, consisting of 5-m-thick, upward-coarsening sand and gravel beds, distributed in an area of 2 km wide and 2 km long in a funnel-shaped river mouth. This initial delta prograded locally in the subaqueous river mouth, with the saltmarsh spreading widely landward at the margins of the delta to the highest sea-level peak at 7 ka. This separation of subaqueous delta progradation and inland saltmarsh expansion occurred because the volume of sediment deposition was almost equal to the increase in accommodation space due to the 7 mm/yr sea-level rise during 8–7 ka.