UNIQUE ORGANIC PETROGRAPHY OF HIGH-RANK ANTARCTIC COAL AND BLACK SHALE

The geochemical and petrographic uniqueness of Antarctic organic-rich sediments have been noted by previous studies (Schapiro and Gray, 1966; Shopf and Long, 1966), and in more recent years sediments from the Permian-Triassic boundary have been the focus of paleoenvironmental isotope studies. Most of the organic matter from Permian coal seams and black shale deposits in Antarctica has been extensively altered after burial by localized high heat flow and, in some cases, contact metamorphism associated with dikes and sills. The rank of the samples prior to intrusion is estimated to be medium to high volatile bituminous. The majority of the samples analyzed petrographically (96%) have been altered to above low volatile bituminous rank based on vitrinite reflectance, most (83%) are semi- to meta-anthracites, and a few have been altered to anisotropic and isotropic cokes. In the highest rank samples, the reflectance of the liptinites has surpassed that of the vitrinite and the inertinites exhibit “zoned” alteration around their edges; in many high rank and coked samples the vitrinite reflectance surpasses that of the inertinite. The liptinites also exhibit remarkable anisotropy. Pyrolitic carbon has also been observed in several samples. The samples do not appear to follow the typical burial maturation geochemical track, as they are higher in volatile matter (%, daf) and O (%, dmmf), and lower in H (%, dmmf) than coals of the same rank that have undergone normal burial maturation. Carbon stable isotopic data indicate a complicated relationship with coal rank as well as with the amount of pyrolitic carbon. Although the isotopically lightest samples measured in this study are located within what is interpreted to be the Permian-Triassic boundary, the effects of thermal alteration of organic matter on δ¹³C values are still unclear.