The stratigraphy of the Kimmeridge Clay Formation (Jurassic) of the Isle of Portland, Dorset, UK

Abstract

A proposed gas-storage site at the former Upper Osprey Ministry of Defence site on the north east coast of the Isle of Portland is underlain by landslide debris and man-made deposits that wholly conceal an outcrop of Kimmeridge Clay Formation. Historical accounts of major landslides in the 17th to 19th centuries have been interpreted as evidence of deep-seated failure surfaces in the Kimmeridge Clay. A pattern of continuously cored boreholes, mostly 25 m to 100 m deep, was drilled to determine the stratigraphy and structure of the area beneath and adjacent to the proposed installations. Comparison of the succession proved in the boreholes with that exposed at the type succession of the formation in the cliffs at and adjacent to Kimmeridge Bay enabled numerous laterally persistent lithological marker beds to be identified. These were used in combination with total-gamma-ray logs to show that the successions proved in the borehole cores could be correlated with one another with a stratigraphical accuracy of ± 0.2 m or better. Taken together, the borehole and geophysical data showed that the Kimmeridge Clay dips steadily SSW at c. 02° beneath the site and the adjacent areas. No tectonic folding or faulting was detected, nor any significant disturbance within the rock mass that could be attributed to a deep-seated landslide. A subsequent multibeam sonar survey of Portland Bay, made for reasons unrelated to the gas-storage site, confirmed the uniform low SSW dip in the Kimmeridge Clay in the offshore area adjacent to the Upper Osprey site