Delta initiation under low sediment discharge: Insights from Holocene incised valley fills north of Tokyo Bay, central Japan

Abstract

Modern delta formation initiated as a result of the deceleration in the rate of sea-level rise during the middle Holocene. In Asia, many deltas initiated at the mouths of rivers with large sediment discharge at 8 ka, early in the middle Holocene, when the volume of sediment supply exceeded that of the accommodation space created by sea-level rise. However, in the Japanese Islands, even some deltas formed by rivers with quite small sediment discharges initiated at 8 ka. In the area north of Tokyo Bay, a narrow, 70-km-long bay was formed during the last deglacial sea-level rise. The Tone and Watarase rivers flowed into this bay from the west and east, respectively. We carried out facies analysis and radiocarbon dating of two sediment cores obtained in the vicinity of the Watarase River mouth. The delta that formed at the tide-dominated Watarase River mouth was characterized by bay mud sediments with a narrow sand body along the axis. This deltaic sand body, which is 5–15 m thick and coarsens upward, filled a tidal channel that was 1–3 km wide and 30 km long. At the mouth of the sediment-starved Watarase River, the delta initiated at the onset of the sea-level stillstand at 7 ka. In contrast, at the mouth of the sediment-rich Tone River, a delta initiated at 8 ka, when the rate of sea-level rise decreased from 14 to 7 m/kyr. By reviewing delta initiations in the area north of Tokyo Bay and in 17 coastal lowlands of the Japanese Islands, we demonstrate that the sediment discharge threshold governing whether delta initiation occurred at 8 or 7 ka was equivalent to a mean annual water discharge of 100 m3/s.