Paleogeography, sedimentology and basin development of the Eocene Rapahoe Group in the Punakaiki-Westport area
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
The Rapahoe Group as presently defined consists of marine and marginal marine sediments deposited during a regional transgressive cycle, which occur between Brunner Coal Measures and Nile Group Limestones. It is proposed that the Rapahoe Group be expanded to include all transgressive deposits that underlie the Nile Group Limestones, including the Brunner Coal Measures. The limestone in and around the quarry at Cape Foul wind should also be included as the Cape Foul wind limestone rather than part of the Oligocene Waitakere Limestone, as it is formed during a fluctuating transgression in the Eocene rather than high-stand conditions in the Oligocene, even though the two are similar in composition.
The basin between Punakaiki and Westport is a narrow N to NE trending basin controlled on the west by the Cape Foulwind Fault and on the east by the complex Paparoa Tectonic Zone. Maximum subsidence occurred on the SE margin of the basin, but due to post-Oligocene uplift and erosion most of the evidence for the nature of this margin has been removed. The Cape Foul wind Fault is an enigmatic structure, its orientation, exact location and history is unknown. Slide deposits md the occurrences of Miocene Welsh Formation limestone onlapping basement on the eastern side of the basin suggest a land mass to the east, that was not transgressed until the Miocene. A similar land mass to the southeast was probably transgressed in the latest Eocene, however, all direct evidence of landmasses to the east of the basin has been removed by erosion caused by post-Oligocene uplift.
The Little Totara Sand can be subdivided at most outcrops and was deposited in a number of different shoreline environments, including beach, dune and tidal bar and channels. At Gibsons Beach and the Cape Foul wind Quarry it is underlain by a laminated silt and clay sequence up to 50 cm thick, that contains lenses of coarser sand and is very carbon rich. This is inferred to be a lagoon deposit. The Little Totara Sand is time transgressive as it occurs both over and underlying the limestone at the Cape Foulwind Quarry.
A basin wide transgression-regression-transgression sequence has been recognised in all closely examined sections, the regression culminating in the Lower Whaingaroan with a surface of erosion or sub-aerial exposure recognised in most sections. This unconformity is correlated with an unconformity in the Inangahua and Buller Gorge regions, and possibly with breccia deposits around the mouths of the Little Wanganui and Mokihinui Rivers, but similar unconformities are not recorded for Greymouth, Buller Coalfields or Murchison areas. The regression and unconformity are therefore thought to be due to local tectonic events, rather than regional or global changes in sea-level.