Sedimentology, coal chemistry and petrography of the Cretaceous Morley coal measures and the Eocene Beaumont coal measures, Ohai Coalfield, South Island, New Zealand.

Several kinds of data from the Cretaceous Morley and Eocene Beaumont Coal Measures at Ohai Coalfield were integrated in order to develop models for sedimentary elastic deposition and mire formation. These data comprise lithostratigraphic relationships at outcrop and basin scale and information on coal chemistry, petrography and palynology. Results indicate that although deposition of both Morley and Beaumont Coal Measures were tectonically controlled by the development of sedimentary sub-basins, Cretaceous mires and sedimentary regimes differ from those of the Eocene. Although the data available at Ohai Coalfield are insufficient for interpretation of fluvial channel planform, other characteristics of the sedimentary environments can be deduced. Accumulation of the Morley Coal Measures occurred in two types of non-contemporaneous environment, 'S'-environments, in which widespread sand was deposited by fluvial channels and few mires developed, and 'C'-environments in which only fine-grained elastic deposition occurred and mires were extensive and persistent. Three environments, which were sometimes contemporaneous, have been identified in the Beaumont Coal Measures. In Beaumont 'S'-environments, sandy elastic sediment was deposited widely by fluvial channels whereas in the 'C'- and 'C-S'-environments, mires developed and sedimentation in shallow lakes and streams was widespread. In the 'C'-environments channels carried mud with little sand but in the 'C-S'-environments sand deposition was more common. Morley mires, which were larger and longer-lived than Beaumont mires, were rarely flooded and may have been domed. In contrast, Beaumont mires were frequently flooded and probably not domed. Most Morley mires developed in environments with widespread mires drained by low energy streams. In contrast high energy fluvial activity was more common in Beaumont mire-forming environments; Beaumont mires are inferred to have frequently developed on lake margins. Palynological evidence indicates that the Morley floral assemblage was dominated by gymnosperms whereas both angiosperms and gymnosperms formed significant proportions of the Beaumont flora. The information available on Morley coal allows development of a model for peat accumulation. Peat accumulation was influenced by a number of interdependent parameters including water table level, nutrient supply and acidity. In response to environmental conditions two different peat types formed. At the base, top and margins of mires peat was generally woodier, less degraded and less oxidised. In contrast, peat in the mire centres suffered both more non-oxidative degradation as well as oxidation and contained less woody material. As mires developed, the initially diverse gymnosperm flora became dominated by the podocarp Phyllocladidites mawsonii however this change in vegetation did not affect the character of the peat