The geology of the Rotokohu coal measures, Inangahua Valley, Buller.
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
The late Altonian to Waiauan Rotokohu Coal Measures formed as a fault controlled, rapidly subsiding fan-delta/ delta sequence. The coal measures gradationally overlie rapidly shallowing-upward Waitakian to late Altonian sediments of the marine Inangahua Formation, and are unconformably overlain by Opoitian to Waipipian shallow marine to marginal marine sediments of the Giles Formation. Three members are recognised within the coal measures: a) the late Altonian to Clifdenian (?) Thomson Member occurs north of Inangahua Landing, and is interpreted as a marine influenced lower delta plain environment. b) the late Altonian to Waiauan (?) Camp Member occurs north of McMurray Creek, and is interpreted as a humid alluvial fan characterised by gravel-dominated "scott-type" channel sediments, and locally thick sequences of fine grained overbank sediments interbedded with thick conglomeratic crevassesplay deposits. c) the late Altonian to Waiauan (?) Donkey Member occurs in the southern Inangahua Valley, and is interpreted as a parallic alluvial plain characterised by low sinuosity, predominatly sandy rivers, and thick low sulphur coal. Microlithotype and maceral analyses suggest that Rotokohu coals were predominantly formed from "woody" vegetation under telmatic (i.e. periodically inundated) conditions. Five reactives-rich, inerts-poor coal types are recognised, and are related to possible paleoenvironmental settings. Coal type variation in exinite content and vitrinite chemistry appear to be major influences on volatile matter yields and specific energy, while variations in vitrinite chemistry are interpreted as a major influence on vitrinite reflectance. Vertical and lateral rank variations, weathering, and a possible variable marine influence are also probable influences on the analytical properties of the outcrop coal samples used in this investigation. Syngenetic mineral matter in Rotokohu coals consists predominantly of detrital quartz, kaolinite and locally muscovite, and dominates the ash mineralogy of most samples. organically bound elements, Ca, Mg, Na and Al are a major influence on the ash mineralogy of some low ash samples. Early diagenetic, low temperature silicification is relatively common, and is locally a major influence on the mineral matter content of some samples.