STRATIGRAPHIC CONTEXT REVITALIZES SOLAR INTERPRETATION FOR THE NEOPROTEROZOIC DEGLACIAL ELATINA RHYTHMITES

South Australia's "Snowball” deglacial sedimentary record includes remarkable rhythmites in Elatina Formation equivalents. Meters of alternating sand and silt couplets are organized cyclically into cm-scale bundles of 10-16 couplets each. This continuous bimodal sedimentation is interpreted to record 60+ years of semi-diurnal tides, of which time series analysis quantifies various ancient orbital and rotational parameters of the Earth-Moon-Sun system.

The current tidal understanding contrasts with early interpretations of Elatina rhythmites, which supposed that bundles of ~12 couplets at the best-exposed locality reflected climatic influence of the ancient solar magnetic cycle. Solar interpretation was replaced by tides mostly because a new occurrence of rhythmites near Hallett Cove, South Australia, was recognized frequently to include ~16 couplets per bundle rather than ~12, and because at Hallett Cove, consecutive rhythmite bundle thicknesses often were asymmetric in a character easily modeled by modern tidal dynamics. Other considerations, including an appreciation for the challenge of linking Earth surface climate to subtle solar variability, also mitigated for tides and against solar cycles, but these latter ultimately were model- rather than data-driven arguments.

Despite the crucial character of the Hallett Cove exposure relative to other rhythmite outcrops, the tidal interpretation was adopted absent full stratigraphic context for rhythmites at Hallett Cove. That full ~100 m section is described here. Bimodal alternation of carbonates and siliciclastics also describes cycles that require very fast-modeled (tidal) deposition of rhythmites in the middle of the section to be far more nonuniformitarian than previously appreciated. If, however, rhythmites are interpreted as annual varves, and bundles as ancient solar cycles, then millenial-scale oceanographic variability can describe lithofacies throughout the section in a relatively conventional way. Magnetostratigraphy of several rhythmite-bearing sections appears to demand this latter, sedimentologically less-exceptional scenario. Consequently, continuous solar cycle variability should be reconsidered as a plausible, perhaps even favored, interpretation for Elatina rhythmites.