Abstract

The Kimmeridge Clay Formation of the Wessex Basin, United Kingdom, illustrates the expression of sequence-stratigraphic surfaces and units in a relatively restricted Mesozoic clastic shelf to relatively distal basin setting. This formation is of similar age and depositional setting to the Haynesville Formation (USA), Vaca Muerta Formation (Argentina), and Bazhenov Shale (Russia) but differs significantly in basin configuration and resultant depositional geometries.

This chapter describes in detail the steps of the application of the sequence-stratigraphic method to the mudstone-dominated, organic-matter–rich interval around the Blackstone Band (Tithonian, Wessex Basin, UK) using a subset of the many datasets available for the Kimmeridge Clay Formation. We described 50 m of outcrop section at a 1:10 scale, constructed six detailed cross sections, correlated 21 parasequences across the basin, and mapped systems tracts. The insights gained by this approach allowed us to explain rock-property variations at a local to regional scale.

Meter-scale stratal units and significant stratigraphic surfaces (flooding surfaces, sequence boundaries) could be correlated over more than 25,000 km² in a depositional setting where no single sediment-accumulation process (organic-matter production, destruction, or dilution) obviously dominated across the entire area. In this setting, inherited and syndepositionally evolving bathymetry influenced sediment deposition and thickness trends, and controlled the distribution of rock properties.

At the parasequence scale, the highest enrichment of organic matter occurs not only just above the basal flooding surface but also in some, more distal, areas, a short distance above. At the depositional-sequence scale, lateral changes in thickness occur not just by proportional thickening and thinning (“aggradation”) but also by stratal terminations by truncation, onlap, and downlap.

A comparison of systems-tract–scale maps with ammonite-zone maps illustrates that the higher resolution sequence-stratigraphic framework has the potential to reveal more about mud accumulation and the influences on rock properties and their lateral variation.