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Early Mississippian sandy siltstones preserve rare vertebrate fossils in seasonal flooding episodes

Early Mississippian sandy siltstones preserve rare vertebrate fossils in seasonal flooding episodes
Early Mississippian sandy siltstones preserve rare vertebrate fossils in seasonal flooding episodes
Flood-generated sandy siltstones are under-recognised deposits that preserve key vertebrate (actinopterygians, rhizodonts, and rarer lungfish, chondrichthyans and tetrapods), invertebrate and plant fossils. Recorded for the first time from the lower Mississippian Ballagan Formation of Scotland, more than 140 beds occur throughout a 490 m thick core succession characterised by fluvial sandstones, palaeosols, siltstones, dolostone ‘cementstones’ and gypsum from a coastal–alluvial plain setting. Sandy siltstones are described as a unique taphofacies of the Ballagan Formation (Scotland, UK); they are matrix-supported siltstones with millimetre-sized siltstone and very fine sandstone lithic clasts. Common bioclasts include plants and megaspores, fish, ostracods, eurypterids and bivalves. Fossils have a high degree of articulation compared with those found in other fossil-bearing deposits, such as conglomerate lags at the base of fluvial channel sandstones. Bed thickness and distribution varies throughout the formation, with no stratigraphic trend. The matrix sediment and clasts are sourced from the reworking of floodplain sediments including desiccated surfaces and palaeosols. Secondary pedogenic modification affects 30% of the sandy siltstone beds and most (71%) overlie palaeosols or desiccation cracks. Sandy siltstones are interpreted as cohesive debris flow deposits that originated by the overbank flooding of rivers and due to localised floodplain sediment transport at times of high rainfall; their association with palaeosols and desiccation cracks indicates seasonally wet to dry cycles throughout the Tournaisian. Tetrapod and fish fossils derived from floodplain lakes and land surfaces are concentrated by local erosion and reworking, and are preserved by deposition into temporary lakes on the floodplain; their distribution indicates a local origin, with sediment transported across the floodplain in seasonal rainfall episodes. These deposits are significant new sites that can be explored for the preservation of rare non-marine fossil material and provide unique insights into the evolution of early terrestrial ecosystems.
Carboniferous, desiccation, overbank, palaeosol, sandy siltstone, taphofacies, tetrapod, vertebrate
0037-0746
1677-1700
Bennett, Carys E.
e1684f72-df39-4d08-8321-f5d512f66727
Kearsey, Timothy I.
d81e2518-1290-4c61-87ed-e6f0d4b8e7c1
Davies, Sarah J.
9c08e102-7013-4ee4-9076-565ad12235cc
Millward, David
c438e99d-66d1-46c9-8caa-00967348c49a
Clack, Jennifer A.
0210709c-82fb-46f9-934e-58c31bc644a1
Smithson, Timothy R.
1f34d9ca-d0ee-44d7-876e-b167fef8e811
Marshall, John E.A.
cba178e3-91aa-49a2-b2ce-4b8d9d870b06
Fielding, Chris
926a7d00-2d94-417b-b56a-d74b5b20c873
Bennett, Carys E.
e1684f72-df39-4d08-8321-f5d512f66727
Kearsey, Timothy I.
d81e2518-1290-4c61-87ed-e6f0d4b8e7c1
Davies, Sarah J.
9c08e102-7013-4ee4-9076-565ad12235cc
Millward, David
c438e99d-66d1-46c9-8caa-00967348c49a
Clack, Jennifer A.
0210709c-82fb-46f9-934e-58c31bc644a1
Smithson, Timothy R.
1f34d9ca-d0ee-44d7-876e-b167fef8e811
Marshall, John E.A.
cba178e3-91aa-49a2-b2ce-4b8d9d870b06
Fielding, Chris
926a7d00-2d94-417b-b56a-d74b5b20c873

Bennett, Carys E., Kearsey, Timothy I., Davies, Sarah J., Millward, David, Clack, Jennifer A., Smithson, Timothy R., Marshall, John E.A. and Fielding, Chris (2016) Early Mississippian sandy siltstones preserve rare vertebrate fossils in seasonal flooding episodes. Sedimentology, 63 (6), 1677-1700. (doi:10.1111/sed.12280).

Record type: Article

Abstract

Flood-generated sandy siltstones are under-recognised deposits that preserve key vertebrate (actinopterygians, rhizodonts, and rarer lungfish, chondrichthyans and tetrapods), invertebrate and plant fossils. Recorded for the first time from the lower Mississippian Ballagan Formation of Scotland, more than 140 beds occur throughout a 490 m thick core succession characterised by fluvial sandstones, palaeosols, siltstones, dolostone ‘cementstones’ and gypsum from a coastal–alluvial plain setting. Sandy siltstones are described as a unique taphofacies of the Ballagan Formation (Scotland, UK); they are matrix-supported siltstones with millimetre-sized siltstone and very fine sandstone lithic clasts. Common bioclasts include plants and megaspores, fish, ostracods, eurypterids and bivalves. Fossils have a high degree of articulation compared with those found in other fossil-bearing deposits, such as conglomerate lags at the base of fluvial channel sandstones. Bed thickness and distribution varies throughout the formation, with no stratigraphic trend. The matrix sediment and clasts are sourced from the reworking of floodplain sediments including desiccated surfaces and palaeosols. Secondary pedogenic modification affects 30% of the sandy siltstone beds and most (71%) overlie palaeosols or desiccation cracks. Sandy siltstones are interpreted as cohesive debris flow deposits that originated by the overbank flooding of rivers and due to localised floodplain sediment transport at times of high rainfall; their association with palaeosols and desiccation cracks indicates seasonally wet to dry cycles throughout the Tournaisian. Tetrapod and fish fossils derived from floodplain lakes and land surfaces are concentrated by local erosion and reworking, and are preserved by deposition into temporary lakes on the floodplain; their distribution indicates a local origin, with sediment transported across the floodplain in seasonal rainfall episodes. These deposits are significant new sites that can be explored for the preservation of rare non-marine fossil material and provide unique insights into the evolution of early terrestrial ecosystems.

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Bennett_et_al-2016-Sedimentology_accepted_manuscript.pdf - Accepted Manuscript
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More information

Accepted/In Press date: 20 February 2016
e-pub ahead of print date: 19 September 2016
Published date: October 2016
Keywords: Carboniferous, desiccation, overbank, palaeosol, sandy siltstone, taphofacies, tetrapod, vertebrate
Organisations: Paleooceanography & Palaeoclimate

Identifiers

Local EPrints ID: 401451
URI: https://eprints.soton.ac.uk/id/eprint/401451
ISSN: 0037-0746
PURE UUID: 07330a03-7dce-4d2f-a700-b0af81c70d4b
ORCID for John E.A. Marshall: ORCID iD orcid.org/0000-0002-9242-3646

Catalogue record

Date deposited: 12 Oct 2016 08:44
Last modified: 06 Jun 2018 13:17

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