Assessing the mechanical state of sediment and rock during deformation: the example of the Bude Formation, South-west England

The general thesis aim was to establish geometric criteria to distinguish between folds formed in unlithified sediment and lithified rock using dip isogon analyses on folds in sediment, lithified rock, migmatites and model materials. The methods involved connecting points of equal dip and measuring layer thicknesses at these points to calculate fold classes and to understand class changes around the folded layer. The criteria were applied to the Bude Formation folds and confirmed that the slump folds developed in sediment, but that also the Variscan tectonically-generated ‘upright’ chevron folds developed in sediment. ‘Inclined-to-recumbent’ chevron folds exhibit characteristics of folding in interbedded sediment and rock, suggesting that Variscan deformation affected the formation during lithification. The methods can be used to determine compressional structural evolutions with respect to lithification in other basins. The specific thesis aim was to study the sedimentary and structural evolution of the Late Carboniferous Bude Formation, Culm Basin, SW England. New sedimentary structures identified in the formation include: centimetric-scale muddraped ripple laminations; decimetric-scale mud-draped and non mud-draped troughs; and metric-scale tabular cross-stratification. The sedimentary structures, plus analyses of palaeoflow indicators, ichnofabrics and geochemistry, suggest that the formation records a mixed depositional environment, with fluctuating base levels and mainly fresh-to-brackish water conditions in a lacustrine or possibly marginal-marine setting. The map work revealed decametric-scale local structures in the Black Rock foreshore that formed prior to chevron folding. The basin-scale Widemouth South Fault (WSF) juxtaposes the Bude and Crackington formations that are separated stratigraphically by 300 m. The movement along the WSF is considered using three structural models from the deformation accommodated to its north and its south. Although no model holds fully, the Freshney et al (1972) ‘late’ normal-faulted Widemouth South Fault model is the most plausible, suggesting that the Culm Basin accommodated progressive compressional deformation prior to extension.