IMIS - Marine Onderzoeksgroepen

Recent evidence for astronomical-induced cycles in banded iron formations (BIFs) hints at the intriguing possibility of developing astrochronological, i.e. precise time-stratigraphic, frameworks for the earliest Proterozoic as also reconstructed for parts of the Mesozoic and Paleozoic. The Kuruman Iron Formation (IF) (Griqualand West Basin, South Africa) and Dales Gorge Member of the Brockman IF (Hamersley Basin, Western Australia) are of special interest in this regard, given their inferred temporal overlap at ca. 2.47 Ga and similar long-period orbital eccentricity imprint. This suggests that these two BIFs may be correlated on the basis of their large-scale cycle patterns and using additional radio-isotopic age constraints.

To examine the possibility of establishing such a framework, we generated and analysed several high-resolution proxy records from both drill-core and outcrop, combined with high-precision U-Pb dating of zircon from interbedded shale horizons. Time-series analysis of these records yields a variety of spectral peaks, of which a prominent ~5 m and ~16 m cycle can be linked to the basic stratigraphic alternations and bundling as observed in the field. New and revised ²⁰⁷Pb/²⁰⁶Pb ages calculated from the U-Pb data of the Dales Gorge Member and Kuruman IF, respectively, indicate a comparable average sedimentation rate of 10 to 12 m/Myr for both units. Based on this depositional rate, we attribute the ~5 m cycle to the long (~405 kyr) orbital eccentricity cycle. More tentatively, we interpret the ~16 m cycle as the very long (presently ~2.4-Myr) eccentricity cycle, having a reduced period of ~1.3 Myr due to chaotic behaviour in the solar system. Other identified cycles (~560 kyr, ~700 kyr and ~1.8 Myr) can be explained in terms of weaker orbital eccentricity components and/or as harmonics and combination tones of these cycles resulting from nonlinear responses.