Strong Rhythmicity in the ~2.46-2.50 Ga Banded Iron Formation of the Hamersley Group (W. Australia): Evidence for Sub-Orbital to Milankovitch Scale Cycles

Banded iron formations (BIF's) are some of the world's most controversial geological deposits, found on most of the Precambrian shields. In addition to their economic significance, BIF's are of great importance for understanding Earth's early development over an extended interval of geologic time (~3.8-0.6 Ga). Currently, research is focused on understanding the genesis and evolution of these deposits, and the characteristics and differences between Paleoproterozoic and Neoproterozoic BIF's. Research has centered on tectonic settings of BIF occurrence, and the physico-chemical conditions associated with BIF deposition. The characteristic banding of BIF's has also on occasion been described in great detail, mainly in terms of petrology. Missing from all BIF investigations has been the analysis of the evolution of the compound banding that so strongly dominates BIF stratigraphy. Here, we use time-frequency analysis to investigate banding along a ~142 meter core collected from the Dales Gorge Member (Brockman Iron Formation) of the well-preserved Archean-Proterozoic Hamersley Basin, Western Australia (1). Geochronology indicates that this BIF was deposited between 2461±6 Ma and 2495±16 Ma (2); this allows us to assign periodicity to characteristic rhythms that are observed in the core stratigraphy. We assembled high-resolution greyscale series from core photographs; this series was examined using a variety of time-frequency methods. Conversion from stratigraphic to time domain results in the identification of remarkable and persistent harmonic features along the entire length of the core. These harmonics occur with sub-orbital to Milankovitch timescales; preliminary comparison of the latter to frequencies predicted for ~2.5 Ga (3) suggest that revisions in assumptions for tidal dissipation through geologic time may be necessary. References: 1. Trendall & Blockley, 1968, Geol. Surv. W. Austral. Bull. 119: 48; 2. Trendall et al., 2004, Austral. J. Earth Sci., 51: 621; 3. Berger & Loutre, 1994, Spec. Publ. Int. Assoc. Sediment., 19:15.