Abstract
In Late Cretaceous times, regional subsidence resulted in deposition of a transgressive marine continental shelf sequence across the east of the South Island, which reportedly reached its peak in the Late Oligocene, followed by a Neogene regression. The Waiareka Volcanic-Ototara Limestone sequences of this study formed in the later phases of the Late Cretaceous-Mid Cenozoic transgression when Zealandia was substantially submerged. Previous studies interpreted the volcanics as formed by short-lived localized marine Surtseyan volcanoes which erupted in a continental shelf setting probably tens of km from land, remote from a source of terrigenous components.
This thesis reports on the stratigraphy, sedimentology, and especially the foraminiferal micropaleontology for the basaltic Waiareka Volcanics and the immediately overlying Ototara Limestone at 3 localities in North Otago, New Zealand. These strata have been reported widely in studies on regional geology, stratigraphy, paleontology, volcanology and igneous petrology, but few details have been published on the physical stratigraphy and biostratigraphy of limestone and associated sedimentary rocks above the Waiareka-Deborah volcanics.
This study uses physical sedimentology and foraminiferal micropaleontology to deduce details of the Late Eocene marine environment associated with the Waiareka-Deborah volcanics. Composite lithostratigraphic sections of 18-20 m meters were made for the limestone-dominated strata above the Waiareka volcanics at the 3 study localities of Reidston, Maheno and Bridge Point, North Otago. Samples from those sections indicated lower to middle Runangan Stage, Late Eocene (later Priabonian) for all 3 localities, bounded by the first occurrence of Bolivina pontis at the base of Runangan, about 36.4 Ma and the last occurrence of Globigerinatheka index at 34.6 Ma (end of Runangan). The absence of G. brevis suggests that the uppermost Runangan was not represented. Thus, sequences span from the base of the Runangan stage to at least the end of the Subbotina linaperta zone, uncertainly a little older than 34.6 Ma.
Paleodepths were equivalent to inner to mid shelf, as deduced from micropaleontological assessments using benthic and planktic foraminifera, taking also physical sedimentology in account. Warm to subtropical paleotemperatures were deduced by the presence of warm stenothermal fossil foraminifera including species in large benthic genera such as Asterocyclina, Asterigerina, Wadella, and Cribrorotalia, as well as by planktic species such as Globigerinatheka index, and Hantkenina alabamensis.