Thickness variations in the Kimmeridge Clay Formation in southern and eastern Britain: implications for Late Jurassic basin evolution

The Kimmeridge Clay Formation is a mudrock-dominated succession deposited during the Upper Jurassic –Kimmeridgian and Lower Tithonian stages– in a shallow shelf environment, below fair- weather wave base. With a maximum thickness of 712m onshore, and 1400m in the northern North Sea, the formation has several intervals of bituminous shales, rich in organic matter, that make it a major oil source rock in the North Sea.

In the Late Jurassic, north-western Europe was part of the Laurasian Seaway, a shallow marine area underlain by a system of interconnected extensional basins. In Britain, sedimentation of the Kimmeridge Clay Formation took place in two main depo-centres; the onshore Wessex and Weald basins and adjacent Channel Basin in southern Britain, and the East Midlands Shelf and adjacent Cleveland Basin in north-eastern Britain, extending offshore into the North Sea. These depo-centres were bounded by normal fault systems. with significant syn-depositional activity associated with  Late Jurassic crustal extension allowing  development of thick sedimentary successions in the hangingwalls of these structures.

The sedimentary patterns of the Kimmeridge Clay formation are rhythmic, with intervals of mudstones, organic-rich mudstones and carbonate-rich stone bands, that give characteristic inflection patterns to wireline logs. These patterns are very similar across the Wessex and Weald basins but differ slightly from those of the East Midlands Shelf and Cleveland Basin; perhaps a consequence of compartmentalisation of accommodation space by the Anglo-Brabant Massif.

We present a new correlation of borehole geophysical logs (gamma & sonic) and associated thickness maps and structural maps for the Kimmeridge Clay Formation. They reveal lateral changes in thickness that reflect the influence of underlying basin structure on patterns of deposition and post-depositional erosion.