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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 • • • • • 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: • • • • 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 • • • • • 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 • • • • • 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 • 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
