The clay mineral assemblages and microtextures of a suite of mudrocks from the Lias Group of England and Wales indicate important regional differences in burial history.
Samples from the northern Cleveland Basin are characterized by illite-smectite (I-S, 90% illite) and little carbonate whilst samples from the southern Worcester and Wessex basins contain less mature discrete smectite and are often calcite- and dolomite-rich. Lias Group rocks have been buried to 4 km in the Cleveland Basin but to <2 km in the Worcester and Wessex basins. Burial in the Cleveland Basin is deeper than previously estimated and does not need a local heating event. Illite-smectite (80% illite) detected in samples from the East Midlands Shelf suggests burial to 3 km, again deeper than previous estimates for this region.