SIFTING THROUGH THE ASHES: USING K-BENTONITES TO REFINE THE MINERALOGICAL DATA FOR THE DOB’S LINN REGION

The GSSP for the base of the Silurian is located in the Southern Uplands Terrane of Scotland, specifically at Dobb’s Linn. This base was formalized in 1985 with the start of the Silurian being marked by the first appearance of graptolite Parakidograptus acuminatus in the rock sequence. At Dobb’s Linn, 135 K-bentonite beds, 1- 50 cm thick, occur in Ashgill to Llandovery strata, approximately 25My of time being represented. The ashes range from intermediate to felsic composition and are likely tied to volcanic activity associated with the closing of the Iapetus.

The objective of this research was to refine and provide chemical and mineralogical data for the sections above and below this boundary in order to better understand the geologic history of the region. Rock samples from K-bentonite layers above and below the boundary were analyzed with X-ray Diffraction to determine the clay mineralogy. Other methods used included vibrational infrared microscopy and thermogravimetric analysis. Fourier transform infrared spectroscopy(FTIR), Raman spectroscopy, and thermogravimetric analysis (TGA) were implemented to supplement the XRD data, providing a more robust mineralogic data set. X-Ray diffraction analysis revealed that the clay content is composed primarily of R3 mixed-layer illite/smectite with approximately 80-90% smectite, discreet illite, lesser amounts of chlorite, and minor kaolinite. The illite/smectite and discreet smectite make up approximately 90% of the total clay, while the mixed layer smectite fraction does not exceed 10% of total weight. The dominance of 10Å potassium-illite allows for the classification of K-bentonite to be applied for all samples analyzed. Chlorite ranges from 1% to as high as 11% in one sample and is largely iron-rich, while kaolinite is less than 1% of total weight. Microscopic study of the Dob's Linn K-bentonites reveal that they were originally fine-grained, crystal-vitric volcanic ash. The majority of the material consists of clay minerals and primary volcanic grains, such as feldspars, apatite, biotite, quartz, and rare zircon.