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| Titel |
Kosi megafan: Sub-surface stratigraphy, sediment provenance and paleoclimate proxies |
| VerfasserIn |
Rajiv Sinha, Manish Yadav, Debajyoti Paul, François Chabaux, Mathieu Granet |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250104606
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| Publikation (Nr.) |
 EGU/EGU2015-4035.pdf |
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| Zusammenfassung |
| The Kosi megafan is a well-known Himalayan megafan formed due to high sediment delivery
of Kosi River and frequent migration of the trunk river. The topography of conical Kosi
megafan is convex-upward and the numerous paleochannels on its surface are the remnants of
westward movements of the Kosi during last 200 years. The sub-surface stratigraphy
of Kosi megafan has been reconstructed after detailed facies analysis of the drill
core sediments. Near-surface (~20 m) strata have pockets of mud and silt within
large amalgamated sand bodies whereas shallow sub-surface (~50 m) deposits are
sandy and devoid of mud and silt pockets. Luminescence dating of core sediments
suggests that the Kosi megafan has been evolving at least since MIS 4. Depositional
phases characterized by aggradation and lithological discontinuity preserved in
sedimentary succession has signatures of multiple aggradational events and autocyclic
movements of channels. Variable net deposition rates (0.3 – 2.4 mm/yr) since MIS 3,
calculated in different cores, relate to channel residence times as well as major
climatic shifts. Stratigraphic reconstruction suggests that the Kosi River has been a
multi-channel system for the last 65 ka due to high sediment flux from the Himalaya. We
have recorded five cycles of channel activity: (i) > 65ka (MIS4), (ii) 32 to 50ka
(MIS3), (iii) 12 to 28ka (MIS2), (iv) 9 to 11ka (Early Holocene), and (v) 5ka to
Modern (Late Holocene) in the studied cores. Multiple aggradational events and
abandonment due to migration of the river show some correlation with the major
climatic shifts, which in turn influenced the sediment flux from the Higher/Lesser
Himalaya. Proximal to distal progradation of the megafan in response to climate
and energy conditions reflects gradual decrease in flow energy through time or
change in sediment supply from the hinterland. After confirming that the radiogenic
isotopic ratios (87Sr/86Sr and 143Nd/144Nd) in Kosi core sediments are not affected by
sediment leaching/sorting, these are used for provenance determination. Isotopic
composition (87Sr/86Sr: 0.7785 to 0.8592; ÉNd: -21.2 to -17.8) of Kosi core sediments are
the most radiogenic reported so far in the Gangetic basin, and also suggest binary
mixing of sediments contributed from both Higher Himalayan (HH) and Lesser
Himalayan (LH) sources. Down core isotopic variability shows variation in sediment
provenance due to major climatic shifts - high 87Sr/86Sr and low ÉNdcharacterize the
interglacial period whereas low 87Sr/86Sr and high ÉNddominate the glacial period.
This is attributed to the litho-tectonic setting of the Kosi basin wherein deformed
sequence of Himalayan zone exposes the HH rocks at a lower topographic level. In
summary, stratigraphic reconstruction of one of the largest megafans in the world
is unraveled and the controls of provenance and climate change during the Late
Quaternary period are inferred from geochemical composition of the sediment cores. |
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