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

?

•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

Lower Carboniferous

http://www.scotese.com/early.htm

Upper Carboniferous• 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

http://www.scotese.com/late.htm

Pangaea

Westphalian tropical forests

(deltas and mires)

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

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.

Cleal & Thomas (1999) Fossil Plants

Stigmaria(roots)

Terminal branchwww.ucmp.berkeley.edu/

carboniferous/carblife.html

Lepidodendron(bark)

Lepidostrobus(cone)

Plants in the Carboniferous

SPHENOPSIDS (e.g Calamites)River margins and levees were colonised by ancient horsetails around 20m tall.

Cleal & Thomas (1999) Fossil Plants

Annularia(leaf whorl)

Calamites (internal cast of stem)

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)

Cleal & Thomas (1994) 994 Plant fossils of the British Coal Measures

Pecopteris (frond)

http://www.emc.aricopa.edu/facult/farabee/BIOBK/BioBookPaleo4.html

Cleal & Thomas (1994) 994 Plant fossils of the British Coal Measures

Plants in the Carboniferous

Neuropteris (frond)

http://www.emc.aricopa.edu/facult/farabee/BIOBK/BioBookPaleo4.html

GYMNOSPERMS (e.g Cordaites)These early gymnosperms lived in the drier parts of the swamp, and in upland

areas.

http://www.turbosquid.com/FullPreview/Index.cfm/ID/273678

Cordaites(leaf)

Plants in the Carboniferous

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. Reconstruct the palaeoenvironment of the Upper Coal Measures of Bristol2. 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 bedsform in semi-arid conditions)

1. Palaeoenvironment

Delta systems are characterised by several distinct depositional settings, these are usually discernable from rock outcrops:

• Coals• Massive coarse sandstone with erosive base, unidirectional/concordant current

indicators, coal/clay rip up clasts • Sandy wedge shaped draped with shales or mudstone

drapes• Laminated mudstone with

occasional thin sandstone beds which do not persist laterally (crevasse splay). Divergent current indicators

Peat mireRiver channel

Levee

Floodplain

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

PALYNOLOGYPhotomicrographs 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

• 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

?Moore & Trueman (1937)

Supra-Pennant MeasuresPennant Measures Ramsbottom et al. (1984)

2. Age of the Coal Measures• 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

Bar thickness representsGroup abundance

307MaStephanianWestphalian

δC13

• This study will focus on the late Carboniferous rocks of the Bristol coalfield

• Palynology, palaeobotany,and sedimentolgy will be used as indicators of the palaeoenvironment of the Upper Coal Measurse

• A biostratigraphy will be constructed from palynological and megafloral data to aid dating of this sequence. δC13 will also be used if feasible.

• The nature of the floral turnover at the Westphalian-Stephanian boundary will be studied, if present

• The late Carboniferous is an important analogue of contemporary climate change and illustrates possible effects on coastal habitats

Conclusion