VITRINITE REFLECTANCE ANOMALIES IN THE HIGH-VOLATILE BITUMINOUS COALSOF THE GUNNEDAH BASIN, NEW-SOUTH-WALES, AUSTRALIA

The rank of the Permian coals in the Gunnedah Basin has been analyzed using both petrographic and chemical methods. Apart from the effects o f local igneous intrusions, a number of seams in the sequence have vit rinite reflectance values (R-vmax) that deviate significantly from the trend expected with a steady downward increase in coalification. Corr elation of these anomalies with interpreted depositional environments suggests that abnormally low vitrinite reflectance values in the seque nce occur in seams either overlain by or intimately associated with ma rine strata. The three-dimensional distribution of such low reflectanc e values, in part of the section at least, can be related either to th e lithofacies pattern or post-depositional groundwater flow associated with a major fan-delta system. Coals with anomalously high vitrinite reflectance values appear to contain material described elsewhere as p seudovitrinite, a component not previously reported in Australian Perm ian bituminous coals. Both low-value and high-value anomalies need to be taken into account when interpreting maturation patterns from vitri nite reflectance data. In some cases other rank indicators such as air -dried moisture may be useful to complement vitrinite reflectance in r ank studies of high volatile bituminous coals. Abnormally low vitrinit e reflectance values due to environmental factors such as marine influ ence, on the other hand, may be used to identify flooding-surface sequ ence boundaries in the basin for stratigraphic and sedimentological in vestigations. (C) 1998 Elsevier Science B.V.