Clay-mineral transformations in Silurian sediments from the Southern Uplands of Scotland and North-West England

Pearce, Richard Brian (1992) Clay-mineral transformations in Silurian sediments from the Southern Uplands of Scotland and North-West England. University of Southampton, Doctoral Thesis.

Abstract

Discrepancies between illite/smectite (I-S) expandability and organic maturity indicators have been observed from several localities in the Southern Uplands of Scotland and north-west England. In the Southern Belt, I-S expandability ranges from 18 to 5% and chitinozoan reflectance from 1.0 to 2.1% R_{o max}; a clear correlation exists between I-S expandability and organic maturity. At Dob's Linn in the Central Belt, I-S expandabilities range from 16 to 9% and chitinozoan reflectance from 3.6 to 4.8% R_{o max}. Elsewhere, near Brighouse Bay in the Hawick Group, and in several Ordovician/Silurian inliers in north-west England, the mean I-S expandability range is from 5 to 8%. This is associated with maturity values of 3.4% R_{o max} and 3.4 to 4.4% R_{o max} respectively. The I-S expandabilities from Dob's Linn Brighouse Bay and NW England are all anomalous with respect to the maturity, both in comparison with the Southern Belt and with previous work from other regions. It is believed that low K+ activity, retrograde metamorphism and residence time at temperature respectively, were the reaction-controlling factors in these regions. The I-S in the K-bentonites from Dob's Linn is slightly less phengitic in composition than that from the Southern Belt K-bentonites. These differences are believed to be due to temperature driven substitution of Al for Mg + Si. This substitution appears to have taken place during the later stages of the illitization reaction. A positive relationship between temperature and chlorite `crystallinity' exists within the Kirkudbright area. The results from this study confirm previous observations that the use of illite/smectites as palaeogeothermometers should be restricted to regions where the kinetic factors influencing illitization are reasonably well-constrained. The combination of organic and clay mineral maturity indicators is clearly beneficial in highlighting anomalous situations and through the use of chitinozoan reflectance can be extended into terranes of Lower Palaeozoic age.

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