Quartz silt in mudrocks as a key to sequence stratigraphy (Kimmeridge Clay Formation, Late Jurassic, Wessex Basin, UK)

Sequence-stratigraphic interpretation of mudrocks is often carried out using proxy indicators for grain size or by consideration of other aspects of lithology thought to relate to sea-level change, such as organic-matter content. An alternative stratigraphic analysis, based on direct estimation of quartz-silt content, was carried out on a major Late Jurassic mudrock (and oil source rock), the Kimmeridge Clay Formation of the Wessex Basin, Dorset, UK. The new data, generated by backscatter SEM, X-ray and image analysis, show decametre-scale stratigraphic patterns that are incompatible with many previous sequence-stratigraphic interpretations based on gamma-ray logs or visual lithofacies and biofacies description. Correlation with a basin-margin section in the Boulonnais, northern France, indicates that silt-rich intervals in basinal facies are coeval, within the limits of biostratigraphic resolution, with shallow-water sand-rich packages on the margin. Variation in silt content in the Kimmeridge Clay therefore appears to be a record of relative sea-level change of at least regional extent. It is suggested that analysis of silt content offers the most reliable basis for generation of a regional sequence stratigraphy in basinal mudrocks. A revised relative sea-level curve fair the Wessex Basin Kimmeridgian and early Tithonian is presented based on this premise.