Petrology of the A and B Seams, Ermelo Coalfield (South Africa): Indications for changing palaeoenvironmental and sedimentary conditions

Abstract

Petrological and mineralogical features are key for inferring palaeoenvironmental and sedimentary conditions of coal-bearing sediments. Following the logging of a borehole core, this study reports on the organic petrology and mineralogy of the A and B Seams, Ermelo Coalfield, Main Karoo Basin (MKB), South Africa. Seventeen coal samples collected from the core, differentiated based on changes in lithotypes, were assessed using detailed petrography (macerals, microlithotypes, and mean random vitrinite reflectance), quality analyses (proximate, elemental, and gross calorific value), and mineralogy in order to deduce depositional environments for the Ermelo coals. The lithologies present in the core were typical of the MKB, and the coals were inertinite- and mineral matter-rich, apart from samples occurring at the base of the seams. The latter samples also had a higher proportion of structured inertinite macerals (fusinite and semifusinite) and lower ash yields. The rest of the samples had higher inertodetrinite contents, occurring in association with detrital quartz and kaolinite, resulting in higher ash yields. The formation of each coal seam thus commenced with the prevailing conditions conducive to the preservation of structured macerals and concluded with the accumulation of predominantly inertodetrinite. Select samples from the B Seam also had durite, an association of inertodetrinite and liptinite (including alginite). The co-occurrence of inertodetrinite and the detrital minerals alongside alginite suggests two depositional episodes. The first episode relates to the formation of primary inertinite macerals through charring, followed by fragmentation and redeposition as inertodetrinite. This latter process likely also reflects seasonal flooding, resulting in fluctuating water levels within the palaeomire. The second episode was quieter, occurring in deeper waters, and conducive for the growth of algae. An allochthonous origin is thus proposed for inertodetrinite and the associated detrital minerals, whereas alginite deposition was autochthonous. The cyclical upward-fining and upward-coarsening facies of the MKB were observed in the core, indicative of fluvial and deltaic palaeoenvironments, respectively. The fluvial facies were generally associated with the occurrence of structured vitrinite and inertinite macerals, suggesting in-situ formation, and mostly occur at the base of the coal seams. In contrast, the inertodetrinite-rich samples appear to be related to the overlying deltaic deposits.