Varves versus Snowball: Seasonal rhythmite in the glacial deposits of the Nichatka Formation (South Siberia).

The paleomagnetic data obtained from the Neoproterozoic rocks stratigraphically related to glacial deposits suggest the ice sheets' near-equatorial occurrence. Based on these data, the Snowball Earth hypothesis proposing the Cryogenian period's total glaciations has been developed and became almost a paradigm. Quaternary glacial successions usually contain varves (seasonally laminated deposits) as they were formed in high latitudes. Therefore, we suggest that varves provide an independent sedimentological test of the paleolatitude position of Precambrian glacial deposits.

We carried out a sedimentological study on thinly laminated rhythmites in the Neoproterozoic glacial deposits in Southern Siberia, and found that they have features characteristic of seasonal varves. The studied rhythmites interstratify the Bolshoi Patom and Nichatka Formations' diamictites at the base of the Dal'nyaya Taiga Group. The seasonality is clearly manifested in the rhythmites of the Nichatka Formation. The rhythmites are represented by interbedding of millimeter-scale siltstones and mudstones with sandy and gravelly admixture. The coarse-sandy and gravelly component is interpreted as ice-rafted clasts, as it has characteristic features of dropstones and contains unconfined till pellets. Ice rafted clasts saturate siltstone laminas and are practically absent in argillite layers.
Thus, argillite laminas can be confidently recognized as deposits of the cold season, during which ice melting and iceberg rafting ceased. On the other hand, siltstone layers with dropstones are deposits of the warmer melting season. The rhythmite's diurnal nature is excluded by its complex structure of the silty layer of the rhythm, which is caused by several sedimentation events separated in time.  The entire set of microfacies of the Nichatka Formation rhythmites reveals similarities with varve microfacies produced by variable flows in ice-contact proglacial lakes. The upper part of the Bolshoi Patom Formation's rhythmites is also formed by varve-like pairs of thin siltstone and mudstone laminas. Dropstones are virtually absent in them, and, therefore, the seasonal nature of the rhythm is less confidently established. The siltstone within the rhythm may have a massive or normally graded texture. The argillite is approximately equal in thickness to the siltstone lamina (about 0.5 mm). The thickness of a pair of siltstone and argillite laminas may remain almost constant when more than 50 pairs are observed. This regularity of laminas thickness in rhythmite is not typical of a tidal setting, but it is difficult to rule out this rhythmite's diurnal nature.  These deposits display high similarity to varves produced by low energy suspension settling during the melt season. The observed seasonal nature of the rhythmites in the glacial deposits of the Dal'nyaya Taiga Group evidence against the validity of the Snowball Earth hypothesis, which assumes the presence of glacial caps near the equator in the Neoproterozoic. The study was supported by RSF Grant No. 20-77-10066.