Decrypting Milanković-driven sedimentary rhythms in nearshore strata of the Archean Moodies Group, South Africa

To advance the next generation of astronomical solutions, there is a need to establish constraints on the Earth-Moon distance and the related precession and obliquity parameters throughout Earth's history. These constraints can be derived by extracting precise precession and/or obliquity signals from geological records. The recently drilled ICDP BASE cores from the Moodies Group in the Barberton Greenstone Belt (South Africa) provide a unique opportunity to determine an Earth-Moon distance datapoint at 3.2 Ga using cyclostratigraphy. In this study, we present initial cyclostratigraphic results from BASE Site 5A, which represents a relatively deeper and quieter depositional environment with finer-grained sediments compared to other ICDP BASE drill sites. To detect a potential Milanković signal, we performed time-series analyses on a suite of elemental proxies obtained via XRF core scanning, tracing temporal changes in redox conditions and siliciclastic input.

BASE Site 5A reveals superimposed cycles of 4–6 meters and 30–50 meters, visible in both redox-sensitive elements and siliciclastic elemental proxies. However, interpreting this sedimentary cyclicity is challenging due to the absence of radio-isotopic age constraints at this site. Existing U-Pb ages from the Barberton Supergroup suggest extremely high sedimentation rates of approximately 25 to 1000 cm/kyr for the Moodies Group as a whole (Heubeck et al., 2013). Given that Site 5A was selected for its finer-grained sediments, its sedimentation rates may be on the lower end of this range. Additionally, variations in lithology, ranging from sandstones of varying grain sizes to jaspilites and siltstones, complicate sedimentation rate estimates and duration calculations for this interval. Nevertheless, preliminary evolutive time-series analyses (evolutive harmonic analysis, evolutive TimeOpt and eASM) suggest no significant sedimentation rate changes, except near the stratigraphic top of the record. Sedimentation rates estimated by these evolutive analyses range from 35 to 55 cm/kyr, corresponding to a total duration of 820–1300 kyr for the 450-meter-long Site 5A core. Based on these derived sedimentation rates, the 4–6-meter cycles could potentially correspond to precession, while the 30–50-meter cycles may reflect short ~100-kyr eccentricity cycles. However, we emphasize that these interpretations are preliminary and remain inconclusive. At this stage, these results provide only an indication of potential astronomical Milanković forcing, which will require thorough scrutiny against additional sedimentological and statistical analyses before an inference on Earth-Moon distance can be made.

Heubeck, C., Engelhardt, J., Byerly, G. R., Zeh, A., Sell, B., Luber, T., and Lowe, D. R.: Timing of deposition and deformation of the Moodies Group (Barberton Greenstone Belt, South Africa): Very-high-resolution of Archaean surface processes, Precambrian Research, 231, 236–262, https://doi.org/10.1016/j.precamres.2013.03.021, 2013.