Petrography and depositional environment of coal seams in the Carboniferous Morien Group, Sydney Coalfield, Nova Scotia

Abstract

Listen

Translate article

The Morien Group (Westphalian A – Stephanian) of the Sydney Basin, Nova Scotia, contains numerous coal seams, many of which can be traced across the onshore part of the basin. Depositional environments during coal formation range from proximal, fluvially dominated to intercalated distal fluvial, to restricted marine. Most of the coal samples analyzed in this study come from the Bras d'Or section, a near-continuous coastal exposure of the Sydney Mines Formation. Petrographic compositions of the samples are discussed in the context of the depositional setting of the enclosing strata. The coal seams have a high vitrinite content (70–90 vol. %), with moderate inertinite and low liptinite. Petrographic indices suggest that most coals of the Sydney Mines Formation are similar to seams deposited in interdistributary or lower delta plain settings in other basins and provide support for the interpreted sedimentological setting of this sequence. The coal seams of the Bras d'Or section show little variation in gross petrographic composition relative to their stratigraphic position, although a slight trend of increasing vitrinite content towards the upper part of the sequence may indicate peat accumulation in a more distal setting. Coal seams that developed in the lower part of the section have relatively high amounts of detrital macerals, indicative of higher water levels and a higher degree of water circulation within the mires. The common occurrence of red beds in the upper part of the section and the increase of structured macerals, including fusinite, suggest somewhat drier conditions during peat accumulation. Petrographic compositions of underlying coals of the South Bar Formation (Gardiner and Tracy seams) indicate deposition in a somewhat more proximal setting than most coals from the Sydney Mines Formation, reflecting the overall retrogradational development of the Morien Group.