Tectonic controls on basin development, fluvial architecture and coal occurrence in the Paparoa coal measures at the Pike River coalfield, West Coast of the South Island

Abstract

The Paparoa Coal Measures at the Pike River Coalfield are an outlier of Late Cretaceous to Early Paleocene non-marine sediments. The coalfield is located in the southern Paparoa Range at an altitude of around 800m in rugged, bush covered hill country. Excellent exposure on an escarpment 2.5km long and up to 250m high allows recognition of a rapid west to east facies change. Fluvial sediments on the west part of the escarpment are characterized by coarse sandstone bodies up to 15m thick, with only a few metres of fine overbank sediments between the channel deposits. In contrast, 200-300m to the east sandstone bodies are less than 5m thick, laterally discontinuous, and are separated by up to 5m or more of overbank sediments, which include coal. Channel deposits and overbank fines occupy their respective positions throughout the sequence and for the entire length of the escarpment. Coal properties indicate that peat accumulated in both low lying and raised mires. Raised mire floral assemblages were dominated by gymnosperms. The palynomorph P. mawsonii is particularly abundant, as is woody tissue, suggesting a relationship between plant type and mire conditions. Floral assemblages were more diverse in low mires, with angiosperms palynomorphs being more abundant, possibly because of an enhanced nutrient supply. Clean, thick coals occur in close proximity to major fluvial channels. A half-graben, with subsidence controlled by a bounding fault on the eastern side, produced an elongate locus of deposition where channel sediments consistently accumulated. Another basin, with a depocentre 2km east of the escarpment, is separated from the western basin by a basement high. The tectonic style is attributed to transtensional rifting related to the separation of New Zealand from Australia in the Latest Cretaceous and Early Paleocene.