Organic maturation in the Lower Palaeozoic Welsh Basin

Tricker, Paul Michael (1991) Organic maturation in the Lower Palaeozoic Welsh Basin. University of Southampton, Doctoral Thesis.

Abstract

Chitinozoan reflectance (Rch) is an innovative technique of thermal maturity assessment in sediments of Ordovician to Devonian age. Reflectance measurements are taken from the polished walls of chitinozoa. Chitinozoa are enigmatic marine microfossils which are relatively facies-independent, are generally thick-walled and are easy to identify. Rch can generate maturity data which is reliable, accurate, reproducible and standardised. The technique is ideal for regional thermal maturity studies of Lower Palaeozoic sedimentary basins. A calibration between Rch and vitrinite reflectance (Rv) demonstrates a strong linear relationship (Rch= 1.152Rv-0.08, r= 0.989). This calibration used Late Silurian and Devonian samples from worldwide localities. Rch shows no suppression with Aom and no perceptible species variation. Measurement of the reflectances of other Lowe Palaeozoic organic matter (graptolite, exinite, scolecodonts, cutinite) in relation to chitinozoan reflectance indicates some potential for their use as alternative thermal maturity indicators. Calibration against subjective colour scales (spores TAI, conodonts, CAI) demonstrates the better resolution of the reflected light, quantitative methods. A total of 647 samples were processed for the study of organic maturity in the Welsh Basin. The maturity pattern observed is broadly related to basin form. The thin sedimentary sequences of the palaeo-platform possess low maturities (range 0.5-1% Rch). This is in marked contrast to the thicker, deformed sequences of the palaeo-basin where high thermal maturities are observed (range 3-8% Rch). Transects across the Welsh Borderland Fault System (WBFS) demonstrate progressive increases in reflectance across the major lineaments. Little burial-controlled finite maturity can be observed in either classic palaeo-platform or basin settings. Localised abnormally high maturities are observed in some high strain structural settings. Transects across the Myddfai Steep Felt (within the WBFS) to the south-east of Llandovery demonstrate a systematic increase in maturity with age. This burial metamorphic signature suggests a palaeogeothermal gradient of ≈50^oCkm^-1. The high palaeotemperature estimated from Late Silurian maturities in this area cannot be accounted for by subsequent overburden and may point to a thermal (metamorphic) peak during the Acadian rather than in the Early Ordovician.

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