Small-scale (< 5.0 m) vertical heterogeneity in mudstones: implications for high-resolution stratigraphy in siliciclastic mudstone successions

48 mudstone samples, from the Peterborough Member, Oxford Clay Formation, h ave been analysed to determine the small-scale Vertical and lateral lithofa cies variability in this succession. The samples were collected every 0.15 m from Saxon and King's Dyke brickpits (Whittlesey, Peterborough, UK) over a short vertical interval (15 m). The sampled interval spans the obductum t o the grossouvrei Subzones of the Callovian (Upper Jurassic). Lithofacies p resent were described using combined field logging, optical and backscatter ed electron optical techniques. The succession predominantly comprises sift-rich mudstones, with subordinat e clay-rich mudstones, shell-beds and shell-pavements. The individual units are variously composed of detrital clay, quartz, pyrite, calcite with mino r apatite, kaolinite and glauconite. In some units millimetre-scale upward- fining couplets are present; elsewhere, the original bedding structures hav e been destroyed by bioturbation. Comparison of successive thin sections re veals that upward-coarsening is present on a metre-scale and that shell-enr iched units are located close to the tops of these upward-shoaling units. C omparison of samples collected laterally on a 0.01-1.0 m scale from the sam e level (3.0 m below the top of Bed 14) indicates that there is lateral lit hofacies variability. The small-scale (0.001 m) couplets are interpreted to be beds deposited by waning flow currents. Significant breaks between these depositional events allowed a dysaerobic infauna to colonize the sediment. The metre-scale upwa rd-coarsening intervals which are capped by shell-enriched units are interp reted to be parasequences. The observed small-scare lateral lithofacies variability is interpreted to be a product of small, but significant vertical miscorrelation errors rathe r that systematic lateral facies changes. These data suggest that producing reliable, bed-scale lateral correlations over distances of more than 1 km in this, and successions like it, will be very difficult.