| Abstrakt: |
Vertically accreted, cyclic tidal rhythmites—laminated, fine-grained sandstones, siltstones and mudstones—occur in the late Cryogenian glaciogenic Elatina Formation (ca 635 Ma) and early Cryogenian Sturt Formation (ca 660 Ma) in the South Flinders Ranges region of the Adelaide Rift Complex within the Adelaide Superbasin, and in Cryogenian strata from Nicholson 2 drill hole in the eastern Officer Basin, South Australia. The rhythmites provide insights into Cryogenian littoral environments. The Elatina rhythmite was deposited during an interstadial and the Sturt rhythmite during early glacial advance, whereas the Nicholson 2 rhythmite is assigned to interglacial beds. The resultant raised sea levels and drowned valleys provided the tidal inlets, ebb-tidal deltas, estuaries and fjords favouring rhythmite deposition. The rhythmites display semidiurnal and diurnal (lunar day) tidal laminae grouped in fortnightly neap–spring cycles and record the semiannual tide. Periods of 26.2 ± 0.9 neap–spring cycles displayed by the Elatina rhythmite and ∼27 neap–spring cycles by the Nicholson 2 rhythmite mark the non-tidal annual variation of sea level, which results from seasonal surface winds and changes in atmospheric pressure and temperature acting on the waters of marine shelves and marginal seas. The strong annual signals and absence of dropstones in the Elatina and Nicholson 2 rhythmites indicate that respective marine shelves and marginal seas were ice-free during rhythmite deposition. The Sturt rhythmite, by contrast, shows a weak annual period of ∼27 neap–spring cycles and contains dropstones and till pellets, indicating that the adjacent marine shelf and sea were largely ice-covered. Paleotidal data for the Elatina rhythmite have illuminated Earth's late Cryogenian paleorotation and the Moon's orbit, and paleomagnetic studies of the Elatina rhythmite indicated a low paleolatitude for late Cryogenian glaciation. Contrary to recent modelling by others, strong tides existed at shallow-water continental margins during Cryogenian glaciations. Cyclic tidal rhythmites are associated with early and late Cryogenian (ca 660 and ca 635 Ma) glaciogenic successions and interglacial beds in South Australia. Rhythmite deposition occurred when raised sea levels provided the coastal environments favouring rhythmite deposition. The rhythmites display semidiurnal and diurnal tidal laminae grouped in fortnightly cycles, and record the semiannual tide and the non-tidal annual variation of sea level. Strong tides existed at shallow-water continental margins during early and late Cryogenian low-latitude glaciations and the interglacial interval. [ABSTRACT FROM AUTHOR] |
