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Palaeoenvironments of the Bristol
coalfield : a critical period of global
change
Janine Pendleton
Overview
• Timing, geography and climate of the
Carboniferous : An introduction
• Overview of the main tropical plant groups
• Description of field area
• Aims and objectives of the study
• Conclusion and questions
The Carboniferous Period
•359-299 million years ago (Ma)
•This study will look at rocks from the late Carboniferous but
range is debated
?
Age of
study
area
Lower Carboniferous
•359-318 million years ago (Ma)
•Britain was part of Euramerica and lay at tropical latitudes
•Limestones deposited on shallow continental shelves
•Southern continent glaciated
•Atmospheric CO2 levels comparable to modern levels
http://www.scotese.com/early.htm
Upper Carboniferous
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In the middle of the Carboniferous, Euramerica and Gondwana collided
Central Pangaean Mountains began to uplift and the continents pushed together
Glaciation of Gondwana intensified, perhaps triggered by altering circulation cells
Widespread peat mires and deltas formed over Euramerica with lush vegetation
Extinction event of tropical plants at the Westphalian-Stephanian boundary
(~307Ma) led to the rise of a more drought tolerant plant assemblages
Westphalian
tropical forests
(deltas and
mires)
Pangaea
http://www.scotese.com/late.htm
Late Carboniferous
There are several factors that may have caused/contributed to the floral turnover:
• Pulse of uplift in Euramerica occurred at the boundary
• Rising Central Pangean Mountains may have caused a rain shadow and shed
sediment
• Negative δC13 excursion suggest pulse of increased atmospheric CO2
• Gondwana ice retreated may have changes atmospheric circulation patterns
-5‰ shift in δC13
http://www.scotese.com/late.htm
Plants in the Carboniferous
LYCOPSIDS (e.g Lepidodendron)
40m tall spore producing trees. Needed wet substrates to reproduce and were
common in the wetter parts of the deltas. Main coal producers in the Westphalian.
Terminal branch
Lepidostrobus
(cone)
www.ucmp.berkeley.edu/carboniferous
/carblife.html
Stigmaria
(roots)
Lepidodendron
(bark)
Cleal & Thomas (1999) Fossil Plants
Plants in the Carboniferous
SPHENOPSIDS (e.g Calamites)
River margins and levees were colonised by ancient horsetails around 20m tall.
Calamites
(internal cast of stem)
Annularia
(leaf whorl)
Cleal & Thomas (1999) Fossil Plants
Plants in the Carboniferous
FILICOPHYTA (Tree ferns) and PTERIDOSPERMS (Seed ferns)
Tree ferns (e.g Psaronius) lived in the drier regions of the mires and in the uplands.
Seed ferns (e.g Neuropteris) lived in flood disturbed regions (e.g levees and river
margins)
Pecopteris
(frond)
Neuropteris
(frond)
http://www.emc.aricopa.edu/facult
/farabee/BIOBK/BioBookPaleo4.html
http://www.emc.aricopa.edu/facult
/farabee/BIOBK/BioBookPaleo4.html
Cleal & Thomas (1994) 994 Plant fossils of
the British Coal Measures
Cleal & Thomas (1994) 994 Plant fossils of
the British Coal Measures
Plants in the Carboniferous
GYMNOSPERMS (e.g Cordaites)
These early gymnosperms lived in the drier parts of the swamp, and in upland
areas.
Cordaites
(leaf)
http://www.turbosquid.com/FullPreview/In
dex.cfm/ID/273678
Bristol coalfield
•Northern most basin of the Bristol and Somerset coalfield
•Exposure of youngest Carboniferous coal measures
•Limited exposure
•Three main areas will be studied in conjunction with British Geological Society
boreholes
Aims
The two main aims of this study are:
1.
2.
Reconstruct the palaeoenvironment of the Upper Coal Measures of
Bristol
Date the Upper Coal Measures in the area to aid correlation to other
coalfields
http://www.naturenorth.com/dragonfly/DOM/Page03_Palaeobiology.html
1. Palaeoenvironment
A multidisciplinary approach will be taken to aid reconstruction of the
palaeoenvironment
SEDIMENTOLOGY
• Detailed field data recorded at sites (field sketches, outcrop measurements etc)
will provide and initial depositional setting
• Current indicators and rock composition may highlight river paths and sources
• Can be used as a proxy for
some climate factors (e.g red beds
form in semi-arid conditions)
1. Palaeoenvironment
Delta systems are characterised by several distinct depositional settings, these are
usually discernable from rock outcrops:
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Coals
Massive coarse sandstone with erosive base, unidirectional/concordant current
indicators, coal/clay rip up clasts
Sandy wedge shaped draped with shales or mudstone
Levee
drapes
River channel
Peat
mire
Floodplain
Laminated mudstone with
occasional thin sandstone beds
which do not persist laterally
(crevasse splay).
Divergent current indicators
1. Palaeoenvironment
PLANT MEGAFOSSILS:
• Macroscopic plant fossils (e.g trunks, leaves, seeds, cones etc)
• Catalogue Bristol University, Bristol Museum, Natural History Museum (London)
and field samples
• Megafossils will show species which are living
nearby in the deltas and mires
1. Palaeoenvironment
PALYNOLOGY
• A palynomorph is an organic microfossil liberated when
a sedimentary rock is macerated with acid
• Spores and pollen will be used to illustrate floral diversity
• Field and borehole samples will be prepared in the palynology lab at Sheffield
University
• Slides will be made of samples to record species abundances (200 random
palynomorphs from 30g samples of rock)
1. Palaeoenvironment
PALYNOLOGY
Photomicrographs will be taken to illustrate diversity. Shallow depth-of-field will
be overcome using z-stacking where appropriate.
Raistrickia saetosa
(Loose)
1. Palaeoenvironment
PALYNOLOGY
• When correlated with the megafloral record, extra-basinal species may be seen
due to their lack of megafloral representation
Upland vegetation
2. Age of the Coal Measures
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A biostratigraphy will be produced from the palynological and megafloral data
Several zonation schemes will be used to determine the age of the coal measures
(stage or sub-stage)
Previously the Supra-Pennant Measures have been classified as Stephanian by
Moore & Trueman (1937) (this is seen in other southern Britain coalfields)
Megaflora examined Ramsbottom et al. (1984) suggests that there are no
Stephanian rocks in Bristol
A palynological and megafloral biostratigraphy would therefore be a logical step
towards a more accurate date
Supra-Pennant Measures
Pennant Measures
?
Moore
& Trueman
Ramsbottom
et al. (1937)
(1984)
2. Age of the Coal Measures
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A widespread extinction of lycopsids occurred at the Westphalian-Stephanian
boundary across tropical Euramerica
Extinction likely linked to drier conditions, allowing tree ferns to take over
Westphalian-Stephanian boundary will be represented by a dramatic turnover
event (if present)
δC13 isotope data will be collected from long ranging species
δC13 isotope will aid identification of the boundary and correlation of the units
Stephanian
Westphalian
δC13
Bar thickness
represents
307Ma Group
abundance
Conclusion
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This study will focus on the late Carboniferous rocks of the Bristol coalfield
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Palynology, palaeobotany,and sedimentolgy will be used as indicators of the
palaeoenvironment of the Upper Coal Measurse
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A biostratigraphy will be constructed from palynological and megafloral data to aid
dating of this sequence. δC13 will also be used if feasible.
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The nature of the floral turnover at the Westphalian-Stephanian boundary will be
studied, if present
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The late Carboniferous is an important analogue of contemporary climate change
and illustrates possible effects on coastal habitats