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

Carys E Bennett,Sarah J Davies,David Millward,Timothy R Smithson,John E A Marshall,Timothy I Kearsey,Jennifer A Clack,Chris Fielding

doi:10.1111/sed.12280

Abstract

AbstractFlood‐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.

Highlights

The 25 million years that followed the end of the Devonian (360 Ma) has been regarded as fossil-poor (‘Romer’s Gap’), attributed in part to the end Devonian mass extinction (Kaiser et al, 2015)
Tetrapod fossils commonly occur in fluvial conglomerate lags and overbank deposits: a review of Euramerican Pennsylvanian tetrapod sites identified swampy pools, oxbow lakes and abandoned fluvial channels filled with organic material as the most common environments where fossils are preserved (Milner, 1987)
Late Devonian tetrapods from Red Hill, Pennsylvania, Upper Cretaceous Judith River Formation (USA), were deposited on an alluvial floodplain in an inter-channel deposit consisting of basal lags and overbank deposits (Cressler et al, 2010)

Summary

Introduction

The 25 million years that followed the end of the Devonian (360 Ma) has been regarded as fossil-poor (‘Romer’s Gap’), attributed in part to the end Devonian mass extinction (Kaiser et al, 2015). Smithson et al, 2012) and associated fishes (Smithson et al, 2016) have been recovered from this time interval from localities in Scotland, northern England and Nova Scotia (Anderson et al, 2015). Why do these particular sedimentary successions preserve such abundant fossil evidence of the early terrestrial ecosystems? Fine-grained mudstone or siltstone overbank facies from floodplain successions preserved in the sedimentary record commonly contain concentrations of vertebrate macrofossils and microfossils (Wilson, 2008; Buck et al, 2004). Late Devonian tetrapods from Red Hill, Pennsylvania, USA, were deposited on an alluvial floodplain in an inter-channel deposit consisting of basal lags and overbank deposits (Cressler et al, 2010)

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Conclusion