Abstract

The new method relies on the assumption that foraminiferal accumulation rates are uniform, or are at least steadier than bulk sediment accumulation rates, over the time scale of intervals between calibrated control points. Where this condition is fulfilled, interpolated ages can be assigned to sampled horizons (n) in each age-bracketed interval by apportioning the difference in ages between control points at the bottom and top of each age-bracketed interval (Δt), to samples in each interval. This is done on the basis of the proportional subdivision of the sum total of weight-standardized counts of foraminifera (Cn) in each interval, which can be simply expressed as Δt*(Cn/∑Cn).

The new method is described here in relation to well (cuttings) samples, but it can be adapted for use with whole-rock samples from cores and outcrops. A case study of the Pukearuhe-1 well demonstrates the method, its application, and outputs in relation to deep-water Miocene-age strata, in Taranaki Basin, New Zealand. The study of this well shows that foraminiferal accumulation rates may not be a precise guide for elapsed time, but in tectonically active basins, and in settings on the margins of basins where foraminiferal concentrations can be shown to be regulated by influxes of siliciclastic sediment, it is better to interpolate ages between calibrated control points on the basis of a uniform foraminiferal-accumulation scale than a uniform bulk sediment-accumulation scale.