Rare preservation of Triassic pedorelicts with biogenic traces from a hot semi-arid upland palaeoenvironment at Portishead, SW England

Mark Howson*, Maurice E Tucker, Fiona F Whitaker

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

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Abstract

Pedorelicts occur sporadically in coastal exposures of Middle to Late Triassic continental sediments, at Portishead, SW England. The clasts include aeolian and bedrock constituents, calcrete textures, vesicles and other pedogenic features reflecting a hot semi-arid palaeoenvironment. Interpreted as being derived from an upland soil, they are classified in palaeosol terms as Calcic Protosol. In the most significant exposure, cryptic tubular trace fossils in branching networks occur within these pedorelict clasts in a localised fluvial deposit. This extremely rare palaeoecological archive offers a glimpse into continental upland life in a Pangaean desert landscape. It is documented for its own merit, to support continuing studies of its palaeobiology and to prompt investigation for comparable deposits.

The pedorelicts occur within the Mercia Mudstone Marginal Facies (MMMF), close to its basal unconformity with underlying Paleozoic strata, at the margin of the Somerset Basin. The soil from which they are derived is interpreted to have developed in an upland regolith over Tournaisian bedrock of interbedded limestone and siltstone. These weathered and, with aeolian dust containing calcite and iron minerals and siliciclastic sand, formed a structured soil. This was aided by intermittent light and moderate rainfall that promoted mainly vadose pedogenic calcretization with displacive calcite crystallisation and siliciclastic grain breakage. In places, the soil was vesicular, as an ‘Av’ horizon, and in others, it was bioturbated with the development of calcite-lined tubules and unlined tunnels. At intervals of perhaps 104 to 105 years, catastrophic deluges eroded the regolith and transported clasts downslope towards the basin to form onlapping coarse clastic beds typical of the MMMF. The soil structure disintegrated but fragments that became pedorelicts were segregated in a fault-controlled palaeo-valley, possibly partly as a debris-flow, to be deposited as localised conglomeratic lenses with fluvial sand. After burial by further sedimentation, this deposit underwent diagenetic calcite cementation and baryte mineralisation before its present exposure by coastal erosion.
Original languageEnglish
Pages (from-to)572-588
Number of pages17
JournalProceedings of the Geologists' Association
Volume133
Issue number6
Early online date26 Jul 2022
DOIs
Publication statusPublished - 1 Dec 2022

Bibliographical note

Funding Information:
Thanks are due to staff members at the University of Bristol, School of Earth Sciences, including Thomas Davies and Dr. Liz Martin-Silverstone regarding CT-scanning, Dr. Stuart Kearns for SEM tuition, Claudia Hildebrandt and Adam Parker for microscopy assistance, Dr. Samuel Mitchell for sample preparation, Professor Sir Stephen Sparks for his field observation and Professor Susan Marriott and other reviewers. Outside the university, thanks go to local landowners Dr. Lindsay Smith and Paul Smith for access, local friends Andrew Curtain, Graham Hunter and Joe Morris for field assistance, Dr. Chris Blake and Richard Ashley for early petrography support, Robert Nuttall and Robin Kear for software help regarding CT-scan data, Peter Speight for guidance around Backwell, Anthony Oldroyd at Cardiff University for excellent thin-section preparation, The National Trust for permission and assistance to visit quarries on the Tyntesfield Estate, North Somerset Council and The Crown Estates for permission to collect specimens on their property, and Natural England for permission to collect within Sites of Special Scientific Interest (SSSIs). The manuscript benefitted from thoughtful comments from Dr. V. Paul Wright and other reviewers.

Publisher Copyright:
© 2022 The Geologists' Association

Keywords

  • pedorelict
  • Triassic
  • Calcrete
  • Regolith
  • Continental

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