The petrology and diagenesis of Permo-Triassic rocks of the Sellafield area, Cumbria

Abstract

SUMMARY A study of the petrology and diagenesis of the Permo-Triassic rocks of the Sellafield area has been undertaken as part of the Core Characterisation Programme for UK NIREX Ltd. This paper summarizes observations of the petrology and diagenesis from boreholes 1/1A, 2, 3, 7A and 7B of the Brockram, St Bees Evaporites, St Bees Shale and St Bees Sandstone and proposes a series of Diagenetic Episodes (denoted DE1 to DE10) to describe paragenetic relationships inferred from these observations. As might be expected from closely spaced boreholes, overall paragenetic characteristics are laterally similar for each stratigraphic unit, although the degree of development and preservation of authigenic minerals, cements and porosity characteristics varies between boreholes. However, there are more significant differences in diagenetic characteristics between stratigraphic units. In general, the diagenesis of the Brockram breccias is characterized by precipitation of anhydrite, calcite, ferroan calcite, dolomite, ferroan dolomite, ankerite, hematite, barite, kaolinite and illite; the St Bees Shales and Evaporites by sulphates and dolomite; and the St Bees Sandstone by non-ferroan dolomite, quartz, ferroan and non-ferroan calcite and late illite. Total porosity in the Brockram, St Bees Evaporites and St Bees Shales is negligible (<1 %), but there is significant porosity ( c. 15–25%) in the St Bees Sandstone, most of which is considered here to be secondary porosity attributed to removal of carbonates by present-day groundwaters, but ultimately due to early preservation of non-compacted fabrics by an inferred early evaporite cement, probably anhydrite, which was subsequently removed during diagenesis.