Abstract

Modern coarse-grained deltas are common in tectonically active coastal zones. However, the stratigraphic architecture and evolution of these deltas in relation to allogenic forcing are poorly understood compared with fine-grained deltas, mainly because of short vertical exposure in outcrop and lack of detailed ages. This paper investigates the coarse-grained delta in the lower reaches of the Tenryu River developed at an island-arc collision coast through analyses of radiocarbon-dated core sediments and borehole data. The vertical successions of the proximal and distal part of the delta constitute basal, coarse-grained braided-river deposits overlain by fine-grained sand and mud accumulated under temporary marine influence. The fine-grained deposits in turn are covered mainly with coarse-grained braided-river deposits in the proximal part and upper shoreface to beach sand and gravel in the distal part. At the apex of the delta, in contrast, the fine-grained deposits are almost absent and thick gravels dominate.

Allogenic forcing, especially the rate of glacioeustatic sea-level change, has influenced the accumulation rates and stacking pattern of the delta. Most of the sediment may have been transported to the deep sea via a canyon, and deposition has been minimal under the present subaerial delta before 14 ka. Although aggradation almost equivalent to rapid sea-level rise (∼ 15 mm/yr) is found at the proximal part between ca. 11.5 and 8 ka, even with significant basin sediment yield and supply, transgression occurred and gravel supply to the coast waned. Notably, only 30% of the sediment yield from the upstream catchment might have been trapped under the present delta plain during that period. This means that large amounts of sediment were transported farther offshore or alongshore. In contrast, slow aggradation in the proximal part and apex of the delta accompanied by progradation in the distal part might become dominant during the past 8 ka, when the rate of sea-level rise decreased to ∼ 10 mm/yr. A similar response was also observed at other Holocene fluvio-deltaic systems, although they are very different in their geological setting such as sediment supply, water discharge, tectonic movement, and gradient of river and shelf. The results imply that the rate of sea-level rise and its change have a greater significance on coastal depositional systems than other factors such as fluvial sediment supply. This study contributes to an improved understanding of evolution of modern and ancient coarse-grained deltas and reconstruction of sea-level change from geological data.