Timing of normal faulting in the Waikato Coal Measures, New Zealand, and its implications for coal‐seam geometry

Coal seams within the Waikato Coal Measures can vary significantly (>50%) in thickness over short lateral distances (<500 m). Normal faulting during coal‐measure deposition could produce the observed variations in seam thicknesses, but the timing of faulting has not been determined precisely. To constrain the timing of faulting, and its potential impact on coal seam geometry, we have analysed displacements for nine normal faults striking both NNW to N and NNE to NE. Displacements were derived from drillholes, mine workings, and seismic reflection lines. The data indicate that normal faulting occurred after peat accumulation in the coal measures, in some cases perhaps commencing in the latest Eocene to early Oligocene (i.e., c. 33–36 Ma). Normal faulting had little or no impact on the accumulation of peat in the Waikato Coal Measures. Variations in coal‐seam thickness were controlled by basement paleotopography and fluctuations in sediment supply.