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| Titel |
A multi-proxy approach to identifying short-lived marine incursions in the Early Carboniferous |
| VerfasserIn |
Carys Bennett, Sarah Davies, Melanie Leng, Andrea Snelling, David Millward, Timothy Kearsey, John Marshall, Emma Reves |
| 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 |
250112973
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| Publikation (Nr.) |
 EGU/EGU2015-13164.pdf |
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| Zusammenfassung |
| This study is a contribution to the TW:eed Project (Tetrapod World: early evolution and
diversification), which examines the rebuilding of Carboniferous ecosystems following a
mass extinction at the end of the Devonian. The project focuses on the Tournaisian Ballagan
Formation of Scotland and the Borders, which contains rare fish and tetrapod material. The
Ballagan Formation is characterised by sandstones, dolomitic cementstones, paleosols,
siltstones and gypsum deposits. The depositional environment ranges from fluvial,
alluvial-plain to marginal-marine environments, with fluvial, floodplain and lacustrine
deposition dominant.
A multi-proxy approach combining sedimentology, palaeontology, micropalaeontology,
palynology and geochemistry is used to identify short-lived marine transgressions onto the
floodplain environment. Rare marginal marine fossils are: Chondrites-Phycosiphon,
Spirorbis, Serpula, certain ostracod species, rare orthocones, brachiopods and putative marine
sharks. More common non-marine fauna include Leiocopida and Podocopida ostracods,
Mytilida and Myalinida bivalves, plants, eurypterids, gastropods and fish. Thin
carbonate-bearing dolomitic cementstones and siltstone contain are the sedimentary deposits
of marine incursions and occur throughout the formation.
Over 600 bulk carbon isotope samples were taken from the 500 metre thick Norham Core
(located near Berwick-Upon-Tweed), encompassing a time interval of around 13 million
years. The results range from –26oÂto –19 δ13Corg, with an average of –19o much
lighter than the average value for Early Carboniferous marine bulk organic matter (δ13C of
–28 to –30). The isotope results correspond to broad-scale changes in the depositional setting,
with more positive δ13C in pedogenic sediments and more negative δ13C in un-altered grey
siltstones. They may also relate to cryptic (short-lived) marine incursions. A comparison of
δ13C values from specific plant/wood fragments, palynology and bulk sedimentary
organic matter from the core is used to identify further changes in environment and
vegetation.
From the base to the top of the formation, there is a gradual increase in relatively drier
conditions, with more developed palaeosols and deep desiccation cracks. However, the
main character of the formation is that of rapidly changing deposition between
silts, sands and carbonates with many periods of pedogenesis and/or desiccation
suggesting frequent switching from alluvial-plain to coastal environments. Marine
incursions were short-lived, but important and caused a significant increase in the macro
and microfaunal diversity. This temporal variability in the environments may have
been an important factor in the evolution of tetrapods in the Early Carboniferous. |
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