Sedimentological and ecological control of Westphalian B plant assemblages from West Yorkshire

Abstract

The lowest Middle Coal Measures (Upper Carboniferous, Westphalian B) at five localities in west Yorkshire (Swillington, Thorpe, Lodge Hill, Pugneys and Lowther North) expose strata from the Thornhill Rock to the Two Foot Marine Band. The sections show three major facies associations interpreted as representing lake, delta, and floodplain. These are divided into nine subfacies associations, each characterised by its particular lithology, sedimentary structures, fauna and flora. Facies 1A, medium grey and fissile black shales and non-marine bivalves and Lepidodendron leafy shoots, is interpreted as lacustrine/prodelta clays. Facies 1B, upward-coarsening silty shales to silts containing rare Calamite, represents distributary mouthbar sequences. Facies 2A, erosionally based sheet sands showing small scale trough cross bedding with horizons of comminuted plant debris, is interpreted as a proximal distributary mouthbar. Facies 2B is dominated by laminated silty shales with thin units of silty sandstone, contains abundant comminuted plant debris, and represents interdistributary lake deposits. Rarely, facies 2A and 2B are overlain by a thin coal seam rich in Calamites, facies 2C, which represents marginal swamp deposits. Facies 3A consists of thick coals and seatearths which were formed mainly by lycopods (shown by the palynology) and were dominated by different groups in succession; the facies is broadly interpreted as the "swamp association". Facies 3B, poorly laminated shales with abundant plants (often species rich) and thin siltstones, represents true floodplain/crevasse splay deposits. Facies 3C, thick, variable, upward-fining sandstones showing lateral accretion surfaces, is interpreted as pointbar deposits of meandering rivers which often have shilty shale plugs. Abandoned channels are frequently filled with silty shales (Facies 3D) and have slumped banks, and often contain contorted fronds of fern-like foliage.
The fossil plant assemblages are in part related to the sedimentary environment in which they are found and in part to the communities from which they have come. Preliminary reconstruction of the sedimentary environment allows hypotheses of the original plant communities to be made. The differences between the flora forming the coal, and the flora found in other lithologies are emphasised.