Abstract
Analyses of a fluvial sedimentary sequence from the lower reaches of the Narmada River establish a record of rhythmic cycles of sediment facies that represent floods during the late Holocene. The south-west Indian monsoon strongly influences the study area, and heavy rainfall or cyclones which originate from either the Bay of Bengal or the Arabian Sea, also affect the region. Optically stimulated luminescence dating places the 8 m thick sediment sequence in the climate transition phase which ranges from the Medieval Warm Period to the Little Ice Age. Multi-proxy analyses including high-resolution granulometry, magnetic susceptibility, ferromagnetic mineral concentration, facies major oxide geochemistry, and micro-fossil records (from two sedimentary units) are used to study these late Holocene flood events. The latter are characterised by multiple sediment facies, depositional events, changes in channel morphology, and distinctive flood signatures. Integration of these records enables to identify two distinct aggradations viz. phase I and phase II, as well as a relative change in channel morphology. The study describes 11 flooding events and their imprints over multi-proxy records. Historic documents and instrumental records from the town of Bharuch referring to floods, movement of channel sand, channel shallowing, and the dysfunction of the ancient port of Bharuch further validate the inferences drawn from the sedimentary sequence. The study exemplifies the need to use high resolution and multi-proxy studies to interpret paleoflood records and climate signatures in order to build archives of monsoonal rivers.
Highlights
River terrace sequences are widely recognised as sources for understanding Quaternary paleo-environments over the continent (Gao et al, 2016; Kolb et al, 2016; Stanford et al, 2016; Sun et al, 2016; Vázquez et al, 2016)
The present study examines neo-bank sequences that preserve historic flood imprints and help to understand changes in the southwest Indian monsoon (SwIM) dynamics
We discuss the interrelationship of the proxies as outlined in the above paragraphs for the 8 m thick sequence at Uchediya
Summary
River terrace sequences are widely recognised as sources for understanding Quaternary paleo-environments over the continent (Gao et al, 2016; Kolb et al, 2016; Stanford et al, 2016; Sun et al, 2016; Vázquez et al, 2016). The LrNR draws attention to three prominent landforms, namely, relic alluvial fans, paleo-banks, and neo-banks (Figure 1-IV). A high-resolution, multi-proxy fluvial record from the site of Uchediya (21◦43 2.22 N, 73◦ 6 26.22 E) located 10 m a.s.l in the core portion of “the neo-bank” is the focus of this study. This landform has laterally aggraded for about 45 km (from the village of Rajpardi in the east to the village of Hansot in the west) within the southern Narmada paleo-bank (Figure 1-IV). Our findings lead us to propose an aggradation model with the sequence of change in the channel morphology corresponding to late Holocene climate and flood events
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