Palaeobotany of the Bristol coalfield : a critical period

of global changeJanine Pendleton

Overview

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

Carboniferous timescale

Age of study area

?

Carboniferous palaeogeography

http:/jan.nau.edu/~rcb7/300Marect.jpg

300Ma

•Two main continents, Euramerica and Gondwana•Southern ice sheet•Central Pangean Mountains began to uplift ~4Ma before coal deposition began, and glaciation intensified

Adapted from Waters et al. (2007)

British Westphalian basins

•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

Early Westphalian Latest Westphalian

B

A

COALPIT HEATH

WINTERBOURNE

PARKFIELD

SHORTWOOD

WESTERLEIGH

KEYNSHAM

Local geology

5km

A B

• 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 MeasuresPennant Measures

History of research

Moore & Trueman (1937)Ramsbottom et al. (1984)

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)

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

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

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

?

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

T. obscura

?Disaccites striatiti

Medullosan pteridosperm pollen(Schopfipollenities ellipsoides)

• >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

?Laevigatisporeites(?Sigillaria)

Scale bars=200um

Megaspores

Megaspores?

Scale bars=200um

Megaspore sp

?Laevigatisporeites(?Sigillaria)

Megaspores

• Megaspore rich coal halfway up the Downend Formation

?Tuberculatisporites brevispiculus(Sigillaria)

Pennant plant casts

Lycophytes(Lepidodendron and ?Knorria)

Lycophytes(Sigillaria)

Spenophytes(Calamites)

Spenophytes(Artisia)

• Coarse Pennant Measures sandstone yields sandstone casts of plants

• Megafloral specimens from roof shale assemblages provide a better tool for biostratigraphic work

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

Alethopteris serlii

Lobatopteris miltoni

Coalpit Heath palaeobotany

Senftenbergia plumosa

•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

Cyathocarpus cf. arborescens

Parkfield palaeobotany

Annularia spinulosa

Lobatopteris miltoni Laveineopteris tenuifolia

Alloiopteris radstockensis

•Flora at Parkfield Colliery contains Radstockia sphenopteroides and Alloiopteris radstockensis

•These taxon may indicate Stephanian aged rocks are present in the youngest coal measures

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