Paleoenvironments, coal properties and their interrelationship in Paparoa and selected Brunner coal measures on the West Coast of the South Island (1985)
AuthorsNewman, J.show all
An extensive study of Paparoa Coal Measures (Upper Cretaceous - Paleocene) and Brunner Coal Measures (Paleocene - Eocene) in 3 major West Coast coalfields has been undertaken, incorporating data from field exposures and drillholes, coal petrology, and interpretation of coal analyses. Earlier paleogeographic interpretations for Paparoa Coal Measures at Greymouth Coalfield are amplified and links established with Pike River Coalfield, where Paparoa Coal Measures are described and inferred to represent more proximal sedimentation in a narrow fault-bounded basin common to both coalfields. It is proposed that syndepositional differential subsidence acted in both areas to localise major fluvial channels to zones of most rapid subsidence. The complex paleogeography caused by sedimentation in a tectonically active sedimentary basin was locally more favourable for accumulation of thick clean peat than would be expected if the basin had subsided uniformly. Petrological studies demonstrate that, despite an overwhelming predominance of vitrinite, both Paparoa and Brunner coals exhibit significant variability in maceral characteristics and assemblages, which can be related principally to swamp drainage. Type variation is manifest not only by maceral characteristics but also by substantial variability in volatile matter yield and vitrinite reflectance, which exhibit a linear inverse relationship in isorank (equal thermal maturity) samples, attributed to variability in peat oxygenation. Relationships between variability in (a) lithostratigraphic characteristics, such as member thickness, texture and composition, and abundance of coal, (b) seam thickness and coal quality, and (c) coal type, including any analytical property which varies as a consequence of type variation, are used in detailed paleogeographic and paleoenvironmental reconstruction, with emphasis on coal swamps and their interaction with adjacent sedimentary environments. The resulting models are proposed as a basis for seam correlation in situations where paleoenvironmental complexity causes rapid variations in seam thickness and quality. Several case studies including both intermontane Paparoa and marginal marine Brunner Coal Measures are demonstrated.