Accumulation of thick fluvial sediments in the Shinano River incised-valley fills: implications for sequence stratigraphy and alluvial architecture

Abstract

The relationship between fluvial systems and base-level changes, controlled mainly by sea-level fluctuations and tectonic activity, on time scales of 103–104 years, is important for evaluating and improving nonmarine sequence stratigraphy and numerical and physical alluvial architecture models. However, there are not enough quantitative data available from field studies, especially for periods of rapid base-level rise. The Echigo Plain, Japan, is a fluvial–coastal lowland where the tidal range is very small and where a very deep valley was incised by the Shinano River during the latest Pleistocene. The depth of the incised valley near the shoreline, 160 m below mean sea level, is greater than the amount of sea-level fall during the Last Glacial Maximum (LGM) because of tectonic subsidence. We studied an approximately 100-m-long borehole sediment core obtained from the landward part of the plain, where thick fluvial deposits have accumulated since the LGM. Sediment facies, diatom assemblages, sulfur content, carbon to sulfur ratios, and radiocarbon age dates indicate that most of the core sediments were deposited in a fluvial environment during the past 12 kyr. Sediment accumulation rates were 10–15 m/kyr between 12 and 8 cal kyr BP, and relatively low (less than 5 m/kyr) subsequently, mainly because of glacio-eustatic sea-level rise. The timing of the reduction in the accumulation rate is one possible indicator of the position of the maximum-flooding surface in a sequence composed only of fluvial sediments. The presence of many thin layers of very fine to fine-grained sand (20–80 cm thick), interpreted as crevasse-splay deposits, that formed between 11 and 8 cal kyr BP, when the base level was rising rapidly, implies that crevasse-splay deposits are characteristic of aggrading fluvial sediments and the transgressive systems tract in nonmarine sequence stratigraphy. Tectonic subsidence at approximately 2 m/kyr has contributed markedly to the larger accumulation rate of fluvial sediments of the highstand systems tract on the Echigo Plain, compared with rates in other fluvial–coastal lowlands, since 5–6 cal kyr BP, when the eustatic sea-level rise had mostly ceased.

Reconstructing Late Quaternary Paleovalley Systems of Italy Through mHVSR: A Tool for Seismic Hazard Assessment in Modern Coastal Lowlands

“…Late Quaternary paleovalley systems are found worldwide (Anderson et al., 2016; Hori et al., 2023; T. Ishihara & Sugai, 2017; Li et al., 2000), buried beneath modern coastal lowlands and delta plains, and do not have any geomorphological expression; the progressive drowning of the valley, followed by sedimentation above the interfluves, leads to complete obliteration of the primary erosional feature, making paleovalley identification and reconstruction challenging. In Italy, the stratigraphic architecture of late Quaternary paleovalley systems has been documented from the Tyrrhenian Sea coast (Amorosi et al., 2009, 2012, 2013; Milli et al., 2013, 2016), the Ionian Sea coast (Tropeano et al., 2013) and the Adriatic Sea coast (Amorosi et al., 2016, 2017, 2023; Campo et al., 2022; De Santis et al., 2014; Fontana et al., 2008; Maselli & Trincardi, 2013; Maselli et al., 2014; Morelli et al., 2017; Mozzi et al., 2013; Ronchi et al., 2018, 2021).…”

“…Late Quaternary paleovalley systems are found worldwide (Anderson et al, 2016;Hori et al, 2023;T. Ishihara & Sugai, 2017;Li et al, 2000), buried beneath modern coastal lowlands and delta plains, and do not have any geomorphological expression; the progressive drowning of the valley, followed by sedimentation above the interfluves, leads to complete obliteration of the primary erosional feature, making paleovalley identification and reconstruction challenging.…”