The West Coast coal region is situated on the West Coast of the South Island. It comprises coalfields formed during the Late Cretaceous and Early Tertiary period within a narrow, elongate, fault-bound basin (the Paparoa trough), which extends from Ross in the south to Seddonville in the north.
A marine incursion from the Eocene to Oligocene period resulted in up to 2,500 m of marine sediments being deposited into the trough over the coal-bearing sediments, which were later, uplifted by a major change of tectonic activity, reducing cover across the region.
The Buller History is located adjacent to the western coast of the South Island, approximately parallel to the coastline and centred close to the coastal town of Westport.
The coalfield is on a tilted basement block, bounded on the west by about 2,000 m of vertical uplift along the Kongahu fault, which resulted in the formation of the Papahaua Range and erosion of the tertiary marine sediments overlying the coal measures. This tectonic activity also caused the formation (through basin eversion) of a pattern of northeast trending normal and reverse faults, which are characteristic of the coalfield.
The Brunner coal measures, formed in the Buller, have a lithological sequence of basal pebble-conglomerate, quartzose sandstone, mudstones and coal seams. Shallow marine sediments overlay the measures.
Most of the coal resources within the Buller History occur within a single, thick base seam traditionally named the Mangatini seam. A notable characteristic of this seam is the relatively high sulphur levels in the roof and floor plies, with very low ash and sulphur levels within the middle plies.
Stockton Opencast Mine
Within the Stockton coal mining licence there is generally a single seam present, the Mangatini seam, which ranges in thickness from 5 m to 10 m. The seam dips 5 to 10 degrees to the northeast with local variations around faults. Depth-of-cover ranges from 10m to 30m. Quartzose sandstone units form erosion-resistant caps, aiding high wall stability.
The Greymouth History is located near the town of Runanga, about 100 km south of Westport. Most of the coal resource is contained within the Paparoa coal measures, a terrestrial unit of Late Mesozoic to Early Cainozoic, overlain on a basement of Palaeozoic metasediment. These coal measures comprise four coal-bearing units each separated by a lacustrine mudstone. Unconformably overlying these coal measures are the Brunner coal measures.
The coal-bearing units have been upwardly named the Jay, Morgan, Rewanui and Dunollie formations. Late Tertiary tectonism produced eversion of the Paparoa trough and subsequent erosion has removed large areas of the overlying Tertiary marine sediments.