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

摘要

Several kinds of data from the Cretaceous Morley and Eocene Beaumont CoalMeasures at Ohai Coalfield were integrated in order to develop models for sedimentaryelastic deposition and mire formation. These data comprise lithostratigraphic relationships atoutcrop and basin scale and information on coal chemistry, petrography and palynology.Results indicate that although deposition of both Morley and Beaumont Coal Measures weretectonically controlled by the development of sedimentary sub-basins, Cretaceous mires andsedimentary regimes differ from those of the Eocene.Although the data available at Ohai Coalfield are insufficient for interpretation of fluvialchannel planform, other characteristics of the sedimentary environments can be deduced.Accumulation of the Morley Coal Measures occurred in two types of non-contemporaneousenvironment, 'S'-environments, in which widespread sand was deposited by fluvialchannels and few mires developed, and 'C'-environments in which only fine-grained elasticdeposition occurred and mires were extensive and persistent. Three environments, whichwere sometimes contemporaneous, have been identified in the Beaumont Coal Measures. InBeaumont 'S'-environments, sandy elastic sediment was deposited widely by fluvialchannels whereas in the 'C'- and 'C-S'-environments, mires developed and sedimentation inshallow lakes and streams was widespread. In the 'C'-environments channels carried mudwith 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 rarelyflooded and may have been domed. In contrast, Beaumont mires were frequently floodedand probably not domed. Most Morley mires developed in environments with widespreadmires drained by low energy streams. In contrast high energy fluvial activity was morecommon in Beaumont mire-forming environments; Beaumont mires are inferred to havefrequently developed on lake margins. Palynological evidence indicates that the Morleyfloral assemblage was dominated by gymnosperms whereas both angiosperms andgymnosperms formed significant proportions of the Beaumont flora.The information available on Morley coal allows development of a model for peataccumulation. Peat accumulation was influenced by a number of interdependent parametersincluding water table level, nutrient supply and acidity. In response to environmentalconditions two different peat types formed. At the base, top and margins of mires peat wasgenerally woodier, less degraded and less oxidised. In contrast, peat in the mire centressuffered both more non-oxidative degradation as well as oxidation and contained less woodymaterial. As mires developed, the initially diverse gymnosperm flora became dominated bythe podocarp Phyllocladidites mawsonii however this change in vegetation did not affectthe character of the peat.