Cretaceous and Eocene geology of South Westland

• Main Author: Adams, D. P. M
• Format: Other/Unknown Material
• Language: English
• Published: University of Canterbury. Geology 1987
• Subjects: Greenland
• Online Access: http://hdl.handle.net/10092/7117; https://doi.org/10.26021/5696

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 ...