04/02645 A short report of the investigations made on thefacies of German coal deposits: Dehmer, J. International Journal of Coal Geology, 2004, 58, (1–2), 41–51

Abstract

This paper reviews current research in Canada relating to the sedimentology of coal-bearing sequences and the petrographic character of contained seams from selected intermontane and foreland basin settings.The intermontane Cumberland and Stellarton basins of Nova Scotia contain Westphalian coal-bearing strata of different character. Coal seams that formed in fluvial-dominated depositional environments of the Springhill Coalfield, Cumberland Basin, commonly brighten upward and laterally display distinct areal zones: a piedmont zone formed by interfingering of mire and distal fan sheetflow deposits; an inner mire zone largely devoid of partings and a riverine zone with large-scale seam splits about multistorey sandstone bodies. Maceral-based groundwater influence and vegetation indices, combined with miospore analyses, indicate that the peat precursors of the No. 3 seam of the inner mire zone accumulated from a groundwater-influenced (rheotrophic to mesotrophic) forest mire (swamp, fen). Both the gelification of lignin-derived maceral precursors and the domination of mire vegetation by lycopsid trees reflect this groundwater influence.Coal that formed from the frequently flooded paleomires of the Stellarton Basin is generally dull, microbanded and mineral-rich. The location of mires within the basin was largely determined by basin subsidence rates. When subsidence rates were high, mires developed around the margin of a central basin lake. Coal seams that formed from these mires (e.g. McLeod seam) thicken toward basin margins and grade into sapropelic shale near the basin centre. Petrographic analyses of the McLeod seam suggest development from limnic to wet forest swamp conditions and a return to a limnic environment at the top of the seam. Thick coal (e.g. Foord seam) developed near the basin centre during periods of lower subsidence rates and grades into red beds and paleosols toward the basin margin. The Foord seam of the basin centre is characterized by low mineral matter contents and a high proportion of degraded vitrinite macerals in the central parts of the seam, interpreted to reflect prolonged peat accumulation with very limited clastic input. The top and the base of the seam, in contrast, were subjected to significant clastic input, as indicated by microbanding associated with significant amounts of detrital minerals and macerals.Coal seams of the Lower Cretaceous Gates Formation in the Western Canada Basin formed in depositional settings ranging from coastal swamps (strandplains) to the upper delta plain. Strandplain coal is characterized by great lateral continuity and substantial thickness, whereas coal of the upper delta plain is thin and discontinous. The strandplain coal is characterized by variable lithotype sequences (oscillatory, brightening-up, dulling-up). Maceral-based facies parameters indicate a wide range of mire facies for these lithotype successions: open moor, forest moor, herbaceous and/or shrubby marshes and slightly raised mires. Results from megafossil and phyteral analyses suggest that the coastal plain mires were dominated by taxodiaceous conifers, whereas ferns, angiosperms and herbaceous lycopods are thought to have occupied the more marshy and/or marginal (stream/lake side) environments.