Rolling back the ‘mudstone blanket’: complex geometric and facies responses to basin architecture in the epicontinental Oxford Clay Formation (Jurassic, UK)

Abstract

Facies variability of mudstones is likely greater than generally perceived, with important implications for their behaviour in major civil engineering, energy and waste disposal applications. Here, we explore this variability for the UK Oxford Clay, a widely studied Middle/Upper Jurassic mudstone. Evidence from wire-line logs, geochemistry, sequence stratigraphy and biofacies analyses are combined to reveal heterogeneity within the Peterborough Member (Lower Oxford Clay) and to explore the extent to which it blanketed basin features or responded dynamically to them. Thickness modelling suggests that the Mid North Sea High, formed by Mid Jurassic thermal doming, likely influenced sediment pathways, favouring thick sediment accumulation in the Wessex Basin, thinner successions across the East Midlands Shelf, and sediment starvation in the Weald Basin. Biofacies patterns, determined using a novel combination of detrended correspondence and cluster analysis, vary significantly and suggest a complex patchwork of environments related to local basin setting. The Type Section of the Peterborough Member seems to represent only a narrow range of conditions that influenced its deposition, and cautions against developing basin-scale models based on a few well exposed and heavily researched outcrop successions.