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The Kimmeridge Clay in Dorset

The Kimmeridge Clay in Dorset

Introduction

The Kimmeridge Clay Formation is the penultimate formation of the onshore British Jurrasic Succession. William Smith was the first to document this distinct formation on his map of 1815, and to name as the Oaktree Soil. In 1817, he gave the name Oaktree Clay for the layers of clay between the “Portland Rock” and “Coral Rag and Pisolite” but in 1816 Webster was the first to describe in details the formation and changed to the name now known as kimmeridge Clay Formation after the English village of Kimmeridge on Dorset’s “Jurassic Coast” a place frequently visited by fossil hunters. (Cox and Gallois, 1981).

The sediments of the Kimmeridge Clay Formation were deposited during Late Jurassic between around 160 and 145 million years ago. The outcrop and subcrop from Kimmeridge Clay in England extends from Dorset to North Yorkshire but due to the lack of exposure inland, all stratigraphic studies of the formation has occurred in the costal sections between Weymouth and Swanage in Dorset ( Figure 1) (Cox and Gallios, 1981)

Figure 1: Kimmeridge Clay Formation outcrops in the Dorset area

Is largely, but not completely exposed at the surface at Kimmeridge. Thus the thickness can not be directly measured. Seismic reflection profiles by British Petroleum Ltd have shown that the full thickness is between 535m and 585m in the area (Gallois, 2000)

Kimmeridge Clay is arguably the most economically important unit of rocks in the whole of Europe since constitutes one of the main North Sea source rock, however over onshore England and Wales, it has log responses and distinctive physical properties. Such rocks are of prime interest to oil industry while...

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...d, just above the limit of the range of Aulacostephanus ( Arkell, 1947) and at the lower limit of the range of Pectinatities( Cope, 1967).

The total thickness of the Kimmeridge area is therefore about 508 metres.

Lithology

Lithologically the Kimmeridge Clay Formation can be subdivided into four units (Figure 2), despite of beds at the base of the succession are poorly exposed. However, in contrast, the units from mid-eudoxus Zone to the top of the formation that are well seen in the Formation. At the base of the formation until the eudoxus Zone, the strata are made up almost entirely of bioturbated shelly clays and in between beds, horizons of oil and bituminous are present. The upper part of the Eudoxus Zone until the upper Pectinatus Zone the composition is kerogen-rich mudstones and coccolith limestone.

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