A palynological investigation of two samples of the Thrussington Till from the Leicester district

Abstract

Sample 1 proved extremely organically-productive; the palynoflora comprises Carboniferous, Triassic, Jurassic and Quaternary forms. Upper Carboniferous (Westphalian) miosopores are the most prominent element (40.9%), thereby indicating reworking of significant levels of Coal Measures into the Thrussington Till south of Leicester city centre. The most likely source of these Carboniferous spores is the Pennines to the north. Late Triassic (Rhaetian) palynomorphs are also present in moderate proportions (5.9%). The most likely lithostratigraphical units which provided this Rhaetian input are the Westbury and/or Lilstock formations from the local area. Jurassic palynomorphs are relatively prominent (27.1%). These are dominated by miospores that comprise 26.4% of the entire flora. The presence of key markers indicates input from Toarcian and Mid-Late Jurassic strata. It is possible that many of these miospores were derived from the several paralic deposits of the Middle Jurassic succession of the East Midlands Shelf to the north-east. Jurassic dinoflagellate cysts are, by contrast, rare (0.7%) and indicate input from the late Callovian to early Oxfordian Stewartby and/or Weymouth members of the Oxford Clay Formation from the outcrop north-east of Leicester. Low levels (1.3%) of Quaternary pollen grains of aspect were observed and these are assumed to have been derived locally. Forms that are stratigraphically non-diagnostic are present in significant proportions (24.8%), and no evidence of input from the Lower Palaeozoic, Devonian, Permian or Cretaceous-Neogene was observed. Sample 2 also yielded an abundant and well-preserved palynoflora that is entirely comprised of Westphalian (Upper Carboniferous) miospores. This means that large levels of Coal Measures were incorporated into the Thrussington Till at Braunstone. It is highly unusual to encounter a till which is characterised by a non-heterogenous assemblage. The source of these spores is most likely to be the Pennines. The absence of Late Triassic forms in this sample is interesting because other Thrussington Till samples have yielded Rhaetian palynomorphs. The Westphalian miospores represent the far-travelled component; whereas the Rhaetian palynomorphs are locally-derived. It is possible, for example, that the local component diluted the far-travelled component dependent on the availability of Rhaetian strata in the path of the glacier. A detailed study of a complete section of the Thrussington Till would be required in order to fully assess the organic heterogeneity of this unit