The lignite resources of the Southland coal region comprise a reported 71% of New Zealand’s recoverable coal resources. The various coalfields within this region are multi-seam with up to 15 minable horizons.
Most of the Southland coal region basement rocks are Early to Middle Mesozoic vocanigenic greywackes and argillites. These are unconformably overlain by Late Cretaceous – Tertiary sediments from the East Southland group, including the upper sandstone-dominated unit with minor lignite deposits, a middle unit consisting of thick, multiple, laterally extensive seams and an upper unit of conglomerates, sandstones and mudstones. Lignite seams within the middle unit may be up to 20 m thick.
Structurally, deformation is only significant on the buried basement ridges, with moderates to gentle folding within the lignite deposits themselves.
The Ohai coalfield is located about 80km northwest of Invercargill and is contained within the north-east limb of a major syncline preserved within a fault-controlled east trending depression. Sediment in this depression is reported to be up to 2,000 m thick.
The Late Cretaceous Ohai group contains two coal measure sequences, being the basal Wairio coal measures and the economically important upper Morley coal measures. The thick New Brighton conglomerate separates these two formations.
Coal seams within the Morley coal measures are up to 23 m thick, but are characterised by splits, washouts and lenticular geometry. Overburden downwardly comprises a thin Quaternary gravel underlain by a thick marine mudstone interbedded with siltstones and rare muddy sandstones.