Holocene evolution of the Kumozu River delta, Mie Prefecture, central Japan

Abstract

The Kumozu River delta, in central Japan, is a fluvial-wave dominated delta facing Ise Bay in the Pacific Ocean. We reconstructed the Holocene evolution of the Kumozu River delta area in response to sea-level changes using sedimentary facies analyses, grain-size analyses, and radiocarbon dating of two new cores, KM and KM , obtained from the delta plain as well as previously published data. The core sediments, in ascending order, consisted of fluvial gravelly sand, tidemud and upward-coarsening delta-front muddy sand, and modern fluvial sand and mud. The maximum flooding surface (MxFS) was identified between the estuarine and deltaic sediments, and was dated at about calibrated radiocarbon years ago (cal kyr BP). At that time the shoreline was located near the site of core KM , about km inland from the present shoreline. Before the MxFS, the present delta area was a fluvial environment until cal kyr BP, and then changed to an estuarine environment during cal kyr BP. After the MxFS, the Kumozu River delta formed and prograded eastward in the incised valley between Pleistocene terraces during cal kyr BP. After deltaic sediments filled the incised valley, the delta advanced into Ise Bay and became lobate. Delta-front migration shows that the delta prograded northeastward, mainly during and cal kyr BP, and southeastward during cal kyr BP.

Evolution of Depositional Environments in Response to the Holocene Sea-Level Change in the Lower Delta Plain of Nakdong River Delta, Korea

“…This layer consists of sandy sediments with a fining upward trend from a lowermost gravel layer. These facies are interpreted as a channel fill sediment [11,12,34]. The abundant organic matter and absence of shell fragments in sediments also support this interpretation.…”

“…The large number of shell fragment layers and lenticular bedding with sand-mud couplets in the lower and middle part of the Unit C (32.20~28.00 m) are interpreted as a submarine intertidal and subtidal flat environment (Figure 4) [11,12]. The upper part of Unit C (28.00~23.50 m), which shows a fining upward trend and contains shell fragment, is interpreted as an upward deepening inner-shelf environment caused by rapid sea-level rise [34]. Thus, this lower part of Unit C is interpreted as a transition zone from the estuarine zone to the shallow marine rather than the totally marine environment (Figure 4).…”

“…The abundant organic matter and absence of shell fragments in sediments also support this interpretation. A number of charcoal and sand layers in the upper part of Unit A (45.50~43.16 m) indicate a terrestrial environment because the charcoals and sand layers usually occur in salt marsh and flood plain environments (Figure 4) [12,34]. The reddish soil indicates that the sediments were exposed and oxidized in the subaerial conditions [35].…”