TECTONIC CONTROLS ON STYLES OF SEDIMENT ACCUMULATION IN THE LATE CRETACEOUS MORLEY COAL MEASURES OF OHAI COALFIELD, NEW-ZEALAND

The tectonic environment of a sedimentary basin can have a very signif icant effect on the style of sediment deposition in that basin. Effect s of tectonic environment can be hard to separate from those of purely sedimentary processes. However, in the Morley Coal Measures of the Oh ai Coalfield, tectonic effects are very marked. The development of the basin through two differing tectonic regimes, first extensional, then oblique slip, resulted in the character of sediment deposited varying considerably within the coalfield. In particular, the sediments diffe r between two sub-basins, the Ohai and Mossbank, of which the Ohai Sub -basin was oriented more favourably for extension in the tectonic sett ing. The other major effect of tectonism on the Morley Coal Measures w as in creating alternating environments of elastic deposition with pea r deposition during the Late Cretaceous, seen now as vertically altern ating units of sandstone/mudstone with coal/carbonaceous mudstone. It is conjectured that these alternating units were the result of periods of active tectonism and erosion with transport of elastic sediments f ollowed by more quiescent times when pear deposition occurred, relativ ely unaffected by elastic sediments. (C) 1995 Academic Press Limited.