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Palaeobotany of the Bristol coalfield : a critical period of global change

Palaeobotany of the Bristol coalfield : a critical period of global change. Janine Pendleton. Overview. The Carboniferous – timescales, palaeogeography, the British coal basins The field location Preliminary palynological results Preliminary palaeobotanical results Conclusion.

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Palaeobotany of the Bristol coalfield : a critical period of global change

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  1. Palaeobotany of the Bristol coalfield : a critical period of global change Janine Pendleton

  2. Overview • The Carboniferous – timescales, palaeogeography, the British coal basins • The field location • Preliminary palynological results • Preliminary palaeobotanical results • Conclusion

  3. Carboniferous timescale Age of study area ?

  4. Carboniferous palaeogeography 300Ma http:/jan.nau.edu/~rcb7/300Marect.jpg • Two main continents, Euramerica and Gondwana • Southern ice sheet • Central Pangean Mountains began to uplift ~4Ma before coal deposition began, and glaciation intensified

  5. British Westphalian basins Early Westphalian Latest Westphalian • Accommodation space created by earlier rifting a thermal subsidence is increased by flexture of Euramerican plate • Tropical peat mires thrived on the deltas • “Barren Measures” deposited after Coal Measures (and floral turnover event in tropical Euramerica) • Turnover seen in palynological and palaebotanical record Adapted from Waters et al. (2007)

  6. Local geology COALPIT HEATH WINTERBOURNE WESTERLEIGH B A PARKFIELD SHORTWOOD B A KEYNSHAM 5km

  7. History of research • Previously the Supra-Pennant Measures have been classified as Stephanian by Moore & Trueman (1937) using non-marine bivalves (this is seen in other southern Britain coalfields) • Megaflora examined Ramsbottom et al. (1984) suggests that there are no Stephanian rocks in Bristol • A new combined palynological and megafloral biostratigraphy of the coalfield would therefore be a logical step towards a more accurate date ? Supra-Pennant Measures Pennant Measures Moore & Trueman (1937) Ramsbottom et al. (1984)

  8. Palynology • Spores and pollen will be extracted from clastics and coals at Sheffield University Palynological Research Facility • Standard preparation techniques will be used • Hydrochloric and hydrofluoric acid digestion on 20g of siliclastic sample. 20um sieve and centrifuging • Schulze solution (nitric acid super-saturated with potassium chlorate) followed by a potassium hydroxide wash in an ultrasonic bath for coals (20um sieve for miospores and 180um sieve for megaspores) • Counts of 200 palynomorphs will be recorded (Barkley, 1954)

  9. Palynological material • 850m of Upper Coal Measurs • Limited surface exposure • Fine grained clastics rare • Field samples supplemented by British Geological Survery boreholes, sub-samples from museum megafossils and National Coal Board residues

  10. Harry Stoke ‘B’ borehole • Located 1.5 miles south west of Winterbourne • Drilled in 1950 for the National Coal Board • Critical access to lowermost 465ft of the Pennant Measures • 27 mudstone and 2 coal samples from British Geological Survey borehole • 5 coals from National Coal Board residues

  11. Harry Stoke ‘B’ borehole ? • Assemblage from bottom 3m of Pennant Measures suggests SL biozone (Bolsovian) of Clayton et al., 1977 • Assemblage is similar to Ts subzone of Owens et al., 2009 (in press) , suggesting a lower Bolsovian age

  12. The youngest Pennant • Parkfield and Coalpit Heath Colliery samples from museum sub-samples and a road cutting • Verrucate monolete miospore (Thymnospora) inidcate latest Bolsovian-Asturian (OT biozone) age • Brislington boreholes contain poorly preserved palynomorphs • Overwhelmingly dominant monsaccate and bisaccate pollen may indicate latest Asturian-Stephanian, but further study is required ? T. pseudothiesseni ?Disaccites striatiti T. obscura

  13. Megaspores • >180 um coal fraction picked for megaspores • A single 2mm megaspore found in Harry Stoke ‘B’ coals • Second possible megaspore species (alete?) • Large pollen grains most common Megaspores? ?Laevigatisporeites (?Sigillaria) Medullosan pteridosperm pollen (Schopfipollenities ellipsoides) Scale bars=200um

  14. Megaspores • Megaspore rich coal halfway up the Downend Formation ?Laevigatisporeites (?Sigillaria) ?Tuberculatisporites brevispiculus (Sigillaria) Megaspore sp Scale bars=200um

  15. Pennant plant casts • Coarse Pennant Measures sandstone yields sandstone casts of plants • Megafloral specimens from roof shale assemblages provide a better tool for biostratigraphic work Spenophytes (Calamites) Lycophytes (Lepidodendron and ?Knorria) Lycophytes (Sigillaria) Spenophytes (Artisia)

  16. Palaeobotany of the Pennant Neuropteris dussarti • Megafossils from museum collections (from spoil tips), sparse leaf beds and Harry Stoke B • CAVEAT: cataloguing not finished, identifications not checked • Early-mid Bolsovian to early Asturian aged

  17. Coalpit Heath palaeobotany Lobatopteris miltoni Cyathocarpus cf. arborescens • Most plants from museum collections of roof shales from Coalpit Heath and Parkfield Colliery (Supra-Pennant) • Most specimens contain no horizon information, and cataloguing of museum collections isn’t complete yet • At least middle Asturian (Lm biozone) aged rocks present at Coalpit Heath Colliery Senftenbergia plumosa Alethopteris serlii

  18. Parkfield palaeobotany Laveineopteris tenuifolia Lobatopteris miltoni Annularia spinulosa • Flora at Parkfield Colliery contains Radstockia sphenopteroides and Alloiopteris radstockensis • These taxon may indicate Stephanian aged rocks are present in the youngest coal measures Alloiopteris radstockensis

  19. Conclusion • This study aims to produce a palynological biostratigraphy for the scientifically neglected Bristol coalfield • Megafossil collections will be amalgamated and re-examined to bring together a full picture of the megafloral biostratigraphy • The megafloral and palynological records will be united and used to date the age range of the Upper Coal Measures in the area • Preliminary palynological information suggests an early-mid Bolsovian to Asturian age for the Upper Coal Measures, with the southern part of the coalfield showing a possibly younger Stephanian assemblage • Megafloral data also indicates a lower limit of early-mid Bolsovian for the base of the Pennant, and an upper limit of at least middle Asturian • The Supra-Pennant in the north of the basin may extend into the latest Asturian or Stephanian, which will hopefully be resolved with more work

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