Controls on deposition of coal and clastic sediment in the Waikato coal measures
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Coal seams in the Waikato Coal Measures can vary significantly in thickness over distances of hundreds of meters to kilometers. Previously, the primary depositional controls on these variations have been inferred to be syn-depositional normal faulting and pre-depositional paleotopography. The data presented in support of these models are typically equivocal and which, if any, of these processes provide the principal control on the geometry and spatial distribution of coal seams in the Waikato Coal Region is uncertain. This study utilizes a large database of drill-logs, seismic-reflection lines and mine exposures in four areas (Huntly, Maramarua, North HuntlylWaikare and Rotowaro Coalfields) to test whether syn-depositional faulting and/or paleotopography influence coal seam architecture. These data were used to construct cross sections across faults and basement topography, which in turn, offer information on the relative timing of faulting and coal measure deposition, together with information on the spatial relations between seam thicknesses, faulting and paleotopography. Cross sections and isopach maps together with examination of spatial and temporal variations in fault displacements reveal that syn-depositional normal faulting had little or no impact on the deposition of the Waikato Coal Measures. Only in the Maramarua study area was any evidence found of fault control on coal measure deposition, with the Landing Fault accruing displacement between deposition of the Kupalrupa Seam and the end of coal measure sedimentation. The vast majority of faults in the Waikato Coalfield were, however, active following coal measure deposition. For example, the Foote, Kimihia and Pukekapia faults show evidence of displacement accrual, which commenced during deposition of the Mangakotuku Formation (37-35 Ma BP). The duration of this episode of faulting is difficult to determine, but may have ceased about 30 Ma ago. In addition, a number of faults (e.g. Foote Fault) display evidence oflate stage extension during the last 5 Ma. Given the lack of stratigraphic evidence for fault displacements during deposition of coal measures, it is suggested that the Mangakotuku and Waipuna basement scarps are erosional rather than tectonic features. Cross sections, together with structure contour and isopach maps in each of the four study areas examined, indicate that basement topography was the dominant control on the spatially variable accumulation of peat. These data show coal seams both thinning into, and away from, topographic lows. To account for this observation a model is proposed, in which peat accumulation is controlled by basement relief and sediment supply to parts of the depositional system. In the model it is postulated that the Waikato Coal Measures depositional system was a continuum between two end members. In one end member, with a high sediment supply, sediment is channeled into the lowest topographic areas and peat accumulates mainly on topographic highs. In the other end member, with little or no sediment supply, peat accumulates to its greatest thickness in areas of relatively low topography, in addition to on basement ridges. In the Rotowaro and North Huntly/Waikare study areas, the thickest peat developed on basement highs and the lows acted as a conduit for sedimentation. On basement highs, peat mires were largely sheltered from clastic sediment influx. In the Huntly East and Maramarua study areas, the thickest peat accumulated in basement lows, with comparable clastic sedimentation in highs and lows. The proposed model has application to other coalfields where peat accumulated on an undulating topographic surface and sediment supply was channelised. Prediction of coal seam thickness, as well as lithological types, is crucial in coal exploration and development. The methodology developed and employed in this study can be applied to other basins to access and model coal and clastic sediment distribution.