Improved UK Zechstein palynology
The UK Zechstein (Lopingian, Upper Permian) is a thick pile of carbonate/evaporite sediments that accumulated over a period of several million years. In the past rather sparse palynology and palaeobotanical data hinted at minimal evolutionary change over this period suggesting that palynological subdivision (biozonation) would be difficult. New work on the extraction of palynomorphs directly from UK Zechstein salt, rather than from argillaceous sediments associated with salt - and new data from individual mudstone units like those in Cadeby Quarry suggest that there is potential for more and better palynology in the UK Zechstein. This would support the succession’s increased importance in decarbonisation technologies such as subsurface hydrogen storage.
The Zechstein Sea was located in Pangaea north of the equatorial Pangaean mountains. It extended over present day eastern England, the central North Sea, Denmark and northern Germany, as well as to the north into the proto-Atlantic. The stratigraphy of the Zechstein Sea is dominated by carbonate/evaporite cycles caused by the sea increasing and decreasing in size mainly through evaporation and replenishment. At the margin of the sea, clastic units of marl, mudstone and siltstone can be locally common. There are seven evaporation and replenishment cycles at the centre of the basin, and five cycles towards the margins, known as English Zechstein (EZ) 1–5. The UK Zechstein outcrop extends from Country Durham south to Nottinghamshire. To the west upland separated the Zechstein Sea from the smaller Bakevellia Sea.
Gibson & Wellman (2021) described palynomorph assemblages from EZ1 to EZ5 dominated by pollen, with rare trilete spores, and even rarer acritarchs and microforaminiferal test linings. According to Gibson & Wellman (2021) the composition of the assemblages changed very little through EZ1 to EZ5. This lack of evolutionary development is also suggested by a palynomorph study from the long ranging section of the Hilton Borehole described by Warrington (2008) from the Bakevellia succession. Plant macrofossils are extremely rare through EZ1 to EZ5.
However recent work may be able to improve the potential for correlation using palynomorphs. Gibson and Bodman (2021) described a new method to recover palynomorphs direct from salt, as opposed to argillaceous sediments associated with salt, the first published work on purely salt palynology for the UK Zechstein. The new technique which involved dissolution in boiling water and treatment with hydrofluoric acid and zinc chloride solution produced excellent results. 49 samples of rock salt were processed and full counts were possible for 16 samples, many of which contained very well preserved bisaccate pollen grains, some spores, and plant tissue fragments, such as tracheids and cuticle (see fig. 5 Gibson and Bodman, 2021). Although these palynomorph yields are not as good as those of many argillaceous rocks, they are still good enough for biostratigraphy and suggest that geologists and palynologists approaching the Zechstein should now think of salt as well as argillaceous sediments as targets for palynological analysis.
Other grounds for UK Zechstein palynology optimism is recent work on mudstone units from the Cadeby Formation (Fig. 1). Seventeen samples from thin mudstone units at the quarry face at Cadeby yielded organic residues with some superbly preserved palynomorphs (see Plate 1, and plates 1 to 5, Stephenson and McLean 2023).
Fig. 1. Field photograph of the measured section at Cadeby looking NNE, showing relationship of the bedded dolostones and palyniferous mudstones to the patch-reefs on either side. B) Annotated photograph. Vertical black bar indicates position of measured section. From Stephenson and McLean (2023)
Although the short stratigraphic span of the sequence (less than 5m) did not yield big differences that could lead to zonation within the short span, the variety of palynomorphs (as well as materials such as cuticle) suggest that - as for other Upper Permian sequences –palynological fingerprinting of individual units– rather than classical zonation - might be possible, improving chances of correlation.
Plate. 1. Well preserved pollen from the Cadeby Formation. Taeniaesporites noviaulensis, E35, 03, focus on taeniae; 2) Taeniaesporites noviaulensis, E35, 03, focus on saccus attachment; 3) Alisporites nuthallensis, D41, 03; 4) Taeniaesporites noviaulensis, D35, 03, focus on taeniae; 5) Taeniaesporites noviaulensis, D35, 03, focus on distal folds associated with saccus; 6) Taeniaesporites noviaulensis, P40, 03; 7) Taeniaesporites noviaulensis, P41/1, 03, focus on saccus detachment; 8) Taeniaesporites noviaulensis, P41/1, 03, focus on taeniae; 9) Taeniaesporites noviaulensis, M39/3, 03; 10) Lueckisporites virkkiae (variant A), P28/3, 03, corpus intexine fo-cus; 11) Lueckisporites virkkiae (variant A), P28/3, 03, saccus detachment focus; 12) Lueckisporites virkkiae (variant A), P28/3, 03, distal focus; 13) Limitisporites rectus, H41/1, 03; 14) Limitisporites sp., D27/4, 03; 15) Lueckisporites virkkiae (variant A), V28/1, 04, focus on taeniae. From Stephenson and McLean (2023)
An example of the potential for fingerprinting of argillaceous units is reported by Stephenson and Powell (2013) and Stephenson (2022). Around 30 individual mudstone units within the Lopingian Umm Irna Formation, were sampled for their spores and pollen. Overall the argillaceous units revealed pollen and spore assemblage that were very varied, being influenced mainly by their position on the floodplain and their isolation in relation to the main river flow (Table 1). No evolutionary trends were identified through the Umm Irna Formation succession, however in many cases individual mudstone layers often had close to unique characters distinguished by percentages of monolete spores (such as Torispora), trisulcate pollen (e.g. Pretricolpipollenites) and small taeniate bisaccate pollen (Protohapolxypinus uttingii) (Table 1). Although much more work needs to be done, it seems possible that as in the Lopingian of Jordan, it may be possible to show that individual Zechstein halite or mudstone units have unique characters if not in overall composition, then in percentages and ratios of taxa. This could allow fingerprinting of units which could aid correlation at local or sub-regional level.
Table 1. Selected mudstone units and their differing palynological identity from the Lopingian Umm Irna Formation, Jordan
It’s clear that we need more work on Zechstein palynostratigraphy: high resolution sampling with an aim of distinguishing small units, and more routine sampling of Zechstein salt. This would help to support the succession’s increased importance in decarbonisation technologies such as subsurface hydrogen storage.
References
Gibson M.E. & Wellman C.H. (2021) - The use of spore-pollen assemblages to reconstruct vegetation changes in the Permian (Lopingian) Zechstein deposits of northeast England. Review of Palaeobotany and Palynology, 288: 104399. doi.org/10.1016/j.revpalbo.2021.104399
Gibson, Martha E. and David J. Bodman. “Evaporite palynology: a case study of the Permian (Lopingian) Zechstein Sea.” Journal of the Geological Society 178 (2021): jgs2020-174.
Stephenson M H 2022. Palynology and outcrop analogue studies for reducing uncertainty of reservoir heterogeneity in carbon capture and storage. Abstracts Madrid 83rd EAGE Annual Conference & Exhibition, Jun 2022, Volume 2022, p.1 - 5
Stephenson M.H. & McLean D. 2023. Lopingian (Late Permian) palynomorphs from the Cadeby Formation, Cadeby Quarry, Yorkshire, UK. Riv. It. Paleontol. Strat., 129(3): 25-46.
Stephenson, M H, and Powell, J H. 2013. Palynology and alluvial architecture in the Permian Umm Irna Formation, Dead Sea, Jordan. GeoArabia, 18, 3, 17-60.
Warrington, G. 2008. Palynology of the Permian succession in the Hilton Borehole, Vale of Eden, Cumbria, U.K. Proceedings of the Yorkshire Geological Society, 57, 123–130.
Mike Stephenson is available for consulting