Effects of igneous intrusions on thermal maturity of carbonaceous fluvial sediments: a case study of the Early Cretaceous Strzelecki Group in west Gippsland, Victoria, Australia

This study has investigated the effect of small igneous intrusions on Early Cretaceous carbonaceous fluvial sediments of the Gippsland Basin of south-eastern Australia, based on outcrops in the onshore western part of the basin. Vitrinite reflectance measurements, quantitative mineral phases and clay mineralogy were carried out on twenty two samples of organic-rich mudstone up to 25. m east and west of a 1. m wide dyke. Samples of the dyke allowed classification as an alkali-basalt, based on its optical characteristics, and major- and trace-element properties.Background vitrinite reflectance of 0.75%-0.80% [Rmax] in the sediments is elevated up to 6.67% [Rmax] close to the dyke margin, with slightly different patterns on each side of the intrusion. Analysis of seven selected organic-rich mudstone samples shows that they all have very similar XRD patterns, with the main minerals in all cases being quartz, feldspar (mainly plagioclase), chlorite, illite and interstratified illite/smectite. These observations suggest that the thermal maturity of the organic matter in the sediments was locally elevated by heating due to emplacement of the dyke, but no significant changes were observed in host-rock mineralogy that could be related to the heat generated by the dyke and the associated vitrinite reflectance variations.