Summary

The Keuper strata in the Severn Estuary region can be divided into two fades, a scree/rock fan deposit known as the Dolomitic Conglomerate and a basin facies referred to as the Keuper Marl. Both facies contain gypsum or evidence indicating that gypsum was once present. As a result of diagenetic changes, such as the release of strontium when aragonite changes to calcite, and also calcite to dolomite, in some areas the initial gypsum, anhydrite accumulation has been replaced by celestine.

Recent construction works have indicated that in parts of the area cavities occur in the Keuper rocks. Some of these cavities have caused engineering problems. Those near the top of the Keuper Marl in the Airport Road tunnel caused machine stress due to the Dosco boom attempting to deflect to the areas of nil resistance. In addition, there was some overbreak and increased grout take.

Data obtained on samples from Llandough Quarry, Cardiff are presented to support the hypothesis that where dolomitic strata combine with gypsum, dedolomitization may occur resulting in degraded, very weak mudstone. Pronounced dolomitised ‘veins’ occur in one area of the quarry; these are described and X-ray diffraction traces are presented to support dolomitization from a joint outwards.

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