Integrated stratigraphy of the Waitakian‐Otaian Stage boundary stratotype, Early Miocene, New Zealand

Abstract

The base of the type section of the Otaian Stage at Bluecliffs, South Canterbury, is recognised as the stratotype for the boundary between the Waitakian and Otaian Stages. Principal problems with the boundary are the restriction 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 depths (200-600 m) in a marginal basin which was partially sheltered from fully oceanic circulation by a submarine high and islands. The site was covered by cool-temperate water and was probably adjacent to the Subtropical Convergence. 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 indicated. A cyclic pattern of abundance spikes in benthic and planktonic foraminifera commences 9 m above base and extends to 73 m in the banded lithofacies. Oxygen isotope excursions (up to 2.08%o) in Euuvigerina miozea and Cibicides novozelandicus are greatest within the interval containing the abundance spikes. The stage boundary occurs in the banded lithofacies 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 as a biostratigraphic reference.Spores, pollens, dinoflagellates, calcareous nannofossils, foraminifera, bryozoans, and ostracods are preserved near the boundary, but molluscs principally occur higher, in the shallower upper siltstone lithofacies. Siliceous microfossils are rare. There is considerable scope for further biostratigraphic research.The primary event marking the boundary at 73 m is the appearance of the benthic foraminifer Ehrenbergina marwicki. This is a distinctive and widely distributed event but is restricted to shelf and upper bathyal environments. Supplementary events in planktonic foraminifera and calcareous nannofossils were researched. Highest occurrences of Globigerina brazieri and G. euapertura are recorded at 47 and 58 m. There is a marked decline in relative abundance 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. They occur in the interval of prominent spikes in foraminiferal abundance.The Waitakian-Otaian boundary is dated at 21.7 Ma by strontium isotopes. Stable primary remanence could not be determined in a pilot paleomagnetic study of Bluecliffs specimens. However, specimens trended towards reversed polarity, and remagnetisation great circle analysis will allow directions to be calculated in future collections.

A coherent middle Pliocene magnetostratigraphy, Wanganui Basin, New Zealand

“…In all three sections the HO of Cibicides molestus occurs above R3 in the long normal polarity interval N3 (Hayton 1998;. Finally, the LO of Z. delicatula, along with that of M. convexum, lies within the Hautawa Shellbed, in the uppermost reversed polarity interval R4, and corresponds with the base of the Nukumaruan Stage (Beu 1990;Morgans et al 1996).…”

“…We are concerned here with the middle to late Pliocene part of the succession, which includes the uppermost part of the Opoitian, and the Waipipian and Mangapanian Stages (Morgans et al 1996). The studied sections in the Turakina and Wanganui River valleys in the central parts of the basin (Fig.…”

“…The highest occurrence (HO), using the boundary defining event notation of Cooper (2004) of G. puncticulata occurs in reversed polarity interval R1, near the base of each of the Turakina and Wanganui sections (Hayton 1998). The lowest occurrence (LO) (Cooper 2004) of dextral G. crassaformis, the datum used by Hornibrook (1981) and Morgans et al ( 1996) to locate the base of the Mangapanian in deep water sediments, occurs within the Utiku Subgroup, near the base of reversed polarity interval R3 in the Rangitikei and Turakina sections, and in the Koroniti Sandstone, just below R3 in the Wanganui section (McIntyre 2002). In all three sections the HO of Cibicides molestus occurs above R3 in the long normal polarity interval N3 (Hayton 1998;.…”

“…It is when the local (New Zealand) framework needs to be aligned with the international timescale that careful, reliable magnetostratigraphy provides the crucial link to the numerical ages of the GPTS. Morgans et al (1996) presented a Cenozoic timescale for New Zealand, in which they assigned ages from the GPTS to the local biostratigraphically defined framework. They employed reliability criteria, based on earlier work by Edwards et al ( 1988) that included a letter based on the integrity of the bio-event and a numeral based on whether the chron boundary was identified directly or by extrapolation, in New Zealand or elsewhere.…”

Olistostromes marking tectonic events, East Coast, New Zealand

“…Several samples collected within underformed layers of the upper part of the sequence, as close as possible to the lowest exotic lenses, raised problems as the foraminiferal fauna commonly appeared to be winnowed, lacking reliable age-diagnostic taxa, and with assemblages commonly contaminated by older microfossils. An Oligocene age was proposed by Moore (1988b) but additional sampling of the Owahanga olistostrome showed it to be Otaian (late Aquitanianto lower Burdigalian; Morgans et al 1999Morgans et al ,2002; Table 1, samples U25/f205 and f325).…”