Integrated stratigraphy of the Waitakian-Otaian stage boundary stratotype,Early Miocene, New Zealand

The base of the type section of the Otaian Stage at Bluecliffs, South Cante rbury, is recognised as the stratotype for the boundary between the Waitaki an and Otaian Stages. Principal problems with the boundary are the restrict ion of existing bioevent proxies to shelf and upper slope environments and its uncertain age. These topics are addressed by a multidisplinary study of a 125 m section about the boundary, which examines its lithostratigraphy, depositional setting, biostratigraphy, correlation, and geochronology. The lower siltstone lithofacies (0-38.5 m) was deposited at upper bathyal d epths (200-600 m) in a marginal basin which was partially sheltered from fu lly oceanic circulation by a submarine high and islands. The site was cover ed by cool-temperate water and was probably adjacent to the Subtropical Con vergence. This unit is succeeded by the banded lithofacies (38.5-106 m) and the upper siltstone lithofacies (basal 19 m studied). Paleodepth probably declined up-sequence, but deposition at shelf depths is not definitely indi cated. A cyclic pattern of abundance spikes in benthic and planktonic foram inifera commences 9 m above base and extends to 73 m in the banded lithofac ies. Oxygen isotope excursions (up to 2.08 parts per thousand) in Euuvigeri na miozea and Cibicides novozelandicus are greatest within the interval con taining the abundance spikes. The stage boundary occurs in the banded litho facies at the highest abundance spike (73 m). Although condensed intervals might affect the completeness of the section, they are not associated with sedimentary discontinuities, and we consider that the section is suitable a s a biostratigraphic reference. Spores, pollens, dinoflagellates, calcareous nannofossils, foraminifera, br yozoans, and ostracods are preserved near the boundary, but molluscs princi pally occur higher, in the shallower upper siltstone lithofacies. Siliceous microfossils are rare. There is considerable scope for further biostratigr aphic research. The primary event marking the boundary at 73 m is the appearance of the ben thic foraminifer Ehrebergina marwicki. This is a distinctive and widely dis tributed event but is restricted to shelf and upper bathyal environments. S upplementary events in planktonic foraminifera and calcareous nannofossils were researched. Highest occurrences of Globigerina brazieri and G. euapert ura are recorded at 47 and 58 m. There is a marked decline in relative abun dance of Paragloborotalia spp. at 62 m. Helicosphaera carteri becomes more abundant than H. euphratis between 56 and 87 m. These events are not exact proxies for the boundary but they may usefully indicate proximity to it. Th ey occur in the interval of prominent spikes in foraminiferal abundance. The Waitakian-Otaian boundary is dated at 21.7 h/Ia by strontium isotopes. Stable primary remanence could not be determined in a pilot paleomagnetic s tudy of Bluecliffs specimens. However, specimens trended towards reversed p olarity, and remagnetisation great circle analysis will allow directions to be calculated in future collections.