The sedimentary geochemistry of the Moffat Shales : a carbonaceous sequence in the Southern Uplands of Scotland

Abstract

The Moffat Shales form a 100-metre condensed politic succession of middle Ordovician to Silurian age. 210 samples were collected from several inliers and were dated by contained graptolites. Mineralogy indicates low greenschist facies regional metamorphism. Correlation and multivariate statistical techniques assess the interdependence of the range of major and trace elements analysed, and demonstrate progressive change related to an increasing input of clay. Physiography is deduced as the main factor controlling sedimentation with physico-chemical conditions and the decay of organic matter having important influences. Amino acids were not detected. Minor amounts of alkanes occur, with the highest average yield extracted from sediments at the locality of Hartfell, and lesser amounts from Dobb's Linn and Clanyard Bay. Their abundance and distribution is related to thermal effects of metamorphism and to biodegradation. Metamorphism of kerogen has produced a graphite-d1a structure. Structural ordering is highest in Hartfell kerogens and can be related to the proximity of reverse faults. Optical properties and textural relationships of carbonized graptolite fragments are related to distance from a porphyrite dyke. Samples show increasing reflectivity, refractive and absorptive indices approaching the intrusion from the south. To the north, three discontinuities in the reflectivity trend are attributed to reactivation of reverse faults following intrusion. Dispersion of reflectivity with wavelength shows a rise from blue to red corresponding to that of anthracitic vitrinites. The relationship between optical parameters and temperature indicates that carbonization has occurred at lower temperatures than for the coal-carbon transformation. Contrasts are attributed to initial chemistry, and rank, also to the unknown effects of time and tectonically induced pressure and temperature. Reflectivity of dispersed graptolite fragments is used as a rank index for Moffat Shales cropping out in Luce Bay and near Moffat. At Luce Bay values correspond to those of coals in the low volatile bituminous to anthracitic range with high values occurring near faults. Coalification is related mainly to regional metamorphism; secondary processes, e.g. migration of hot fluids along fractures, may also have operated. At Moffat, comparable rank ranges are distributed along the southeast flank of a Caledonoid trending dome, low values occurring adjacent to the Ettrick Valley Thrust, high at Hartfell. Secondary thermal effects from an unexposed pluton are deduced. Further evidence of enhanced thermal activity in the Hartfell sediments is presented from a study of the electrical conduction of powdered samples. The low resistance of Hartfell samples is attributed to the graphitic nature of the relatively abundant carbonaceous material.