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| Titel |
Brackish to hypersaline lake dolostones of the Mississippian |
| VerfasserIn |
Carys Bennett, Timothy Kearsey, Sarah Davies, David Millward, John Marshall |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250135832
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| Publikation (Nr.) |
 EGU/EGU2016-16743.pdf |
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| Zusammenfassung |
| Flat-lying ferroan dolomite beds are common in the Mississippian and occur along the
southern margin of Laurassia, from Kentucky USA to Poland. These rocks are important as
they record shallow marine to coastal plain environments that may have acted as a pathway or
refugia for animals that were radiating into freshwaters such as tetrapods, fish, molluscs and
arthropods.
This study is a contribution to the TW:eed Project (Tetrapod World: early evolution and
diversification), that 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, which contains rare fish and tetrapod fossils. The formation is
characterised by an overbank facies association of siltstone, sandstone and palaeosols,
interbedded with dolostone and evaporite units, and cut by fluvial sandstone facies
associations of fining-upwards conglomerate lags, cross-bedded sandstone and rippled
siltstone.
Two sites are used as a case study to describe the sedimentological, faunal and
ichnofaunal diversity of these dolostones. More than 270 dolostone beds are recorded in each
of the 500 metre depth Norham Core (near Berwick-upon-Tweed) and from a 520 metre thick
field section at Burnmouth. The beds are laterally extensive, over ∼1 km, although
individual units do not appear to correlate between the core and the field site. In
the Norham Core dolostones comprise up to 14% of the succession. 17% of the
beds contain marginal marine fossils: Spirorbis, rare orthocones, brachiopods and
putative marine sharks. More common fauna include ostracods, bivalves, plants,
eurypterids, gastropods and sarcopterygian fish, which are interpreted as brackish to
freshwater tolerant. Bioturbation is fairly common with Serpula colonies within
dolostone beds and Chondrites burrowing down from the base of dolostone beds.
Some rare units in the field section have a bulbous bed surface and preserve tree
root traces, and 9% of all dolostone beds in the Norham Core are pedogenically
altered. The isotopic composition of dolomite beds is δ18O –3.6‰ to –1.7‰ and
δ13C –2.6‰ to 1.6‰ which is consistent with a brackish as opposed to marine
origin.
The dolostones are categorised by their sedimentary composition: Facies 1:
Cemented siltstone and sandstone; Facies 2: Homogeneous micrite to micro-crystaline
dolomite, within a clay matrix; Facies 3: Bedded dolomite and siltstone; Facies
4: Mixed calcite and dolomite; Facies 5: Dolomite with gypsum and anhydrite.
Formation processes are diverse, and include diagenetic cementation (Facies 1),
deposition in saline (brackish) lakes (Facies 2), deposition in saline lakes with clastic
sediment input (Facies 3), lagoonal to shallow-marine carbonate deposition (Facies
4), and hypersaline lake to sabkha environments (Facies 5). 60% of the beds are
facies 2 or 3 and their sedimentology, fauna, ichnofauna and isotopic composition
indicate a brackish-water origin. Other Mississippian dolostones from around the
world also contain a fairly restricted fauna and have been interpreted as brackish
water deposits. The mechanism of dolomite formation under these conditions is
discussed. These dolostones provided extensive coastal lakes that may have been an
important habitat for tetrapods and other transitional groups during the Mississippian. |
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