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Abstract
The biostratigraphy of a rapidly deposited, deep bathyal, lower Altonian (Early Miocene) sequence through Whakai Formation, East Cape, is presented. We interpret the depositional milieu, identify and describe foraminiferal and nannofossil bioevents potentially useful in biostratigraphy, and relate these to the magnetostratigraphy, 40Ar/39Ar ages, and ages using the Sr isotope sea‐water curve.
Although lower and upper contacts of the 623 m section through Whakai Formation are obscured, a high (c. 0.9 m/ka) flux of muddy sediment has produced an amplified record of most of the lower Altonian Globorotalia incognita Zone and the base of the overlying Globorotalia zealandica Zone. No significant breaks were identified. Benthic foraminiferal paleodepth proxies suggest that the site was on the lower slope (>1000 m) throughout, but the moderate abundance of planktonic taxa shows that it was not in fully oceanic water. Planktonic assemblages identify warm‐temperate water masses. Abundance data for both nannofossils and planktonic foraminifera are cyclic, and temperature oscillations in the Milankovitch band are inferred. Nannofossil proxies indicate that sea temperatures were warmest in the upper Globorotalia incognita Zone. There is a prominent “crash” in populations of warm‐water taxa at 457 m, at the base of the middle Altonian zone.
Among the benthic foraminifera, Bolivina punctato‐striata occurs at the base of the section, and its first appearance datum (FAD) may be a useful proxy in the vicinity of the base of Altonian; the FAD of Sigmoilopsis compressa occurs near the base of the middle Altonian Globorotalia zealandica Zone. Globorotalia praescitula (planktonic foraminifera) is persistent in very low numbers through the sequence; individuals are small and easily overlooked, but they are distinctive and considered to identify strata above the base of Altonian reliably. The FAD of Paragloborotalia bella is near the base of the section in the Globorotalia incognita Zone and may be a useful supplementary criterion for basal Altonian. Qualitative criteria for recognition of the FAD of Globorotalia zealandica are cited. The species arises by relatively rapid phyletic transformation from Globorotalia incognita and defines the boundary between the lower and middle Altonian zones. Transitional populations occur over 50 m. The lower Altonian Globorotalia incognita Zone appears to be missing at the Altonian stratotype.
Apaleomagnetic, 40Ar/ 39Ar, and 87Sr/86Sr composite age model for the Tangakaka Stream section indicates that much of the lower Altonian Globorotalia incognita Zone lies in Chrons C5En and C5Er. The lower Altonian/middle Altonian (Globorotalia zealandica Zone) boundary occurs within Chron C5En. The Otaian/Altonian boundary occurs below the base of the Tangakaka Stream section, but above the top of the nearby Hoia section, in the vicinity of the Chron C6n/ C5Er boundary (c. 19.0 Ma).
