Maryville coal measures, North Wanganui Basin, New Zealand: coal facies and depositional environments

Abstract

Deposition of the late Early Miocene Maryville Coal Measures occurred during shallowing of the North Wanganui Basin at a time of maximum east -west compression. Five coalfields are recognised viz. Mokau, Waitewhena, Ohura, Tangarakau and Aria. The Aria Coalfield is not discussed. The coal measures are divided into the Maryville Lower Coal Member, Maryville Sandstone Member and Maryville Upper Coal Member. Sediment facies boundaries and and coal seam splitting have been strongly influenced by contemporaneous N-S and ENE -WSW faulting. Coal petrologic studies indicate deposition in ombrotrophic mires forming by paludification. The coals are high in huminite (88%) and liptinite (10%) and low in inertinite (2%). Principal components analysis of macerals and maceral ratios did not assist in palreoenvironmental interpretation. Generalised techniques for mire type determination by maceral analysis are shown to give erroneous results, not only for the Maryville Coal Measures but for many coals. Interpretation of depositional environment by traditional microlithotype analysis also gives erroneous results. Plant parts, coupled with maceral analysis, of a pillar sample gives a better picture of mire development. Coal lithotype analysis indicates dulling-up sequences, defined by decreasing bright coal. The very high vitrinite content results in dull coal having a field appearance O.\+l,.,ov5\.., similar to bright coal/\ they are distinguishable on polished surfaces and microscopically. Mean random reflectance of humotelinite varies from 0.28% to 0.45% and is greatest in the central part of the basin. Maximum burial depth of 1.5km is inferred. Rank ranges from sub-bituminous A to high volatile C. Trends in volatile matter are related to petrographic composition, reflectance and sulphur content. Sulphur is medium to high (0.44 -10.50%). It is highest in areas of greatest marine influence and mostly organically bound. Microprobe studies show sulphur is greatest in liptinite and lowest in inertinite. The Maryville Lower Coal Member represents deposition in barrier/back-barrier environments including tidal deltas and channels, sand and mud flats and peat accumulation in raised mires. Widespread deposition of the Maryville Sandstone Member permits its use for inter-coalfield correlations. New coal seam correlations proposed for the Ohura and Tangarakau Coalfields are based in part on the distribution of this member. During Maryville Upper Coal Member time, uplift adjacent to the Ohura Fault resulted in erosion and non-deposition, especially in the Waitewhena Coalfield. Conglomeratic alluvial fans and braid-plains, with associated coal deposition, entered the basin from the east. Barrier/back-barrier environments in the Mokau Coalfield migrated westward. Rank ranges from sub-bituminous A to high volatile C. Trends in volatile matter are related to petrographic composition, reflectance and sulphur content. Sulphur is medium to high (0.44 -10.50%). It is highest in areas of greatest marine influence and mostly organically bound. Microprobe studies show sulphur is greatest in liptinite and lowest in inertinite. The Maryville Lower Coal Member represents deposition in barrier/back-barrier environments including tidal deltas and channels, sand and mud flats and peat accumulation in raised mires. Widespread deposition of the Maryville Sandstone Member permits its use for inter-coalfield correlations. New coal seam correlations proposed for the Ohura and Tangarakau Coalfields are based in part on the distribution of this member. During Maryville Upper Coal Member time, uplift adjacent to the Ohura Fault resulted in erosion and non-deposition, especially in the Waitewhena Coalfield. Conglomeratic alluvial fans and braid-plains, with associated coal deposition, entered the basin from the east. Barrier/back-barrier environments in the Mokau Coalfield migrated westward.