Cretaceous and Eocene geology of South Westland

The Cretaceous and Eocene sequence of South Westland crops out within a 6km wide coastal area from Ship Creek to the Mahitahi River. The oldest unit, the Otumotu Formation (Motuan-Arowhanan), lies with angular unconformity on the Paleozoic Greenland Group. It is divided into two members, an older Tauweritiki Member (new) overlain by the Topsy Member. Both are entirely clastic but the lower unit is significantly coarser ranging from boulder conglomerate to mudstone, while granule conglomerate, sandstone analysis of sedimentary features to pebble breccia and the upper member comprises and mudstone. Detailed suggests that the lower member represents alluvial fan and plain sedimentation in a tectonically active setting changing to a more stable semi-arid fluvial and lacustrine depositional regime in the younger deposits. The Butler Formation (new) (Piripauan), which lies unconformably on the Otumotu Formation, consists of conglomerate, sandstone and mudstone, with high and medium volatile bituminous coal seams. The sediments represent an environment of rivers and coal forming swamps and lakes which produced thick (up to 3m) coal seams. The Tauperikaka Formation (new), previously the Tauperikaka Coal Measures, (Haumurian) overlies the Butler Formation, with a disconformity marked by a low relief scour surface, and is divided into the Moeraki (lower), Paringa (middle) and Rasselas (upper) Members. The Moeraki Member consists of pebble conglomerate, cross-bedded and horizontally bedded pebbly and granular sandstone and carbonaceous massive silty mudstone. The sequence is thought to represent a coastal fluvial environment. The Paringa Member includes large scale planar tabular cross-bedding with mud drapes (“tidal bundles”), bi-directional flaser bedded, trough and planar cross-bedded sandstone, siltstone and mudstone. The depositional environment is interpreted as a tide dominated coastline. The Rasselas Member, which consists of interbedded burrowed and structureless glauconitic sandstone in which both the density and diversity of burrows and the sediment grain size decrease upwards, was probably deposited in a large open marine bay. The sediments of the Otumotu, Butler and Tauperikaka Formations are derived from a Greenland Group and Tuhua Group source which probably lay to the west of the basin. The change in depositional environment within the Tauperikaka Formation, from a marginal marine to an off shore marine environment is responsible constituents in the sediment composition, and the rock fragment component has been greatly depleted. The eruption of the Arnott Basalt towards the end of the Haumurian is possibly related to extension which led to thinning of the crust. The Eocene Law Coal Measures (new) (Kaiatan) are composed of clast supported very well rounded cobble to pebble conglomerate, well sorted medium sandstones, carbonaceous siltstone and mudstone and thick (up to 4m) high volatile bituminous coal seams. The sequence is interpreted as marginal marine, with coal forming reed swamps developing between fluvial clastic fans. A marine transgression from the east resulted in the end of coal measure sedimentation. The Tititira Formation (Miocene) lies unconformably on the Law Coal Measures. Differences in coal type and coal geochemistry distinguish the coal in the Butler Formation from coal in the Law Coal Measures. The pH of the Law Coal Measure swamps was elevated by a marine influence which has produced a distinctive coal type characterised by a low Tissue Preservation Index. The coal also contains very little inertinite compared with coal from the Butler Formation. The Coal in the Law Coal Measures can be distinguished using the relatively high Na₂O content which is totally organically associated and is present in a constant amount within different seams. The Butler Formation coal contains a high proportion of clay compared to the coal in the Law Coal Measures and has negligible Na₂O. A thrust system involving both Paleozoic basement and cretaceous and Tertiary cover rocks has developed in post, Miocene time and accounts for a substantial amount of shorting (in the range of 40km and possibly more). The Mistake fault, a splay off the Alpine Fault, is the sole thrust of the Mistake Thrust Sheet which is part of a duplex thrust system which has subsequently been buckled into an antiformal stack. The anti formal stack includes at least two other thrust sheets, one below and one above the Mistake Thrust Sheet. The thrust complex appears to extend south to Milford Sound and up to 100km north of the area mapped and it is likely similar thrust systems are developed along the entire length of the Alpine Fault