ILLITIZATION AND ORGANIC MATURITY IN SILURIAN SEDIMENTS FROM THE SOUTHERN UPLANDS OF SCOTLAND

Discrepancies between clay mineral and organic indicators of very low-grade metamorphism have been observed from the Southern Uplands of Scotland. In the Southern Belt, I-S expandability ranges from 18-5%, and chitinozoan reflectance from 0.8-2.1% R0 mean; a clear correlation exists between I-S expandability and organic maturity. At Dob's Linn in the Central Belt, I-S expandabilities range from 18-10%, and chitinozoan reflectance from 3.6-4.8% mean. The results from Dob's Linn show anomalously high I-S expandabilities with respect to maturity, both in comparison with the Southern Belt and with previous work from other regions. K-feldspar is present in variable amounts in the Southern Belt, but is not detectable by XRD at Dob's Linn. Low K+ activity is believed to have resulted in a slower reaction rate at Dob's Linn, and a consequent "lag" in I-S expandability with respect to the Southern Belt.