Precambrian Cyclic Rhythmites: Solar-Climatic or Tidal Signatures?
For more than 60 years geologists have sought evidence of solar-climatic cyclicity in rhythmically laminated sedimentary rocks, but claims in general have not been persuasive. Three Precambrian rhythmite sequences in Australia that comprise varve-like laminae recently have received attention, however, as their conspicuous cycles of ca. 10-14 and/or 20-25 laminae have been ascribed a sunspot-cycle origin. They are the 2500 Ma old Weeli Wolli Formation, the 1750 Ma old Wollogorang Formation, and the 650 Ma old Elatina Formation. New observations for the Weeli Wolli Formation, a siliceous banded iron-formation, suggest a cycle period exceeding the 23 microband couplets proposed by Trendall, casting doubt on the solar interpretation. The Weeli Wolli cyclicity may record Earth-tidal rhythms that modulated the discharge and composition of silica- and iron-bearing fumarolic waters. The structure of the cycles of silty dolomite and mudstone in the Wollogorang Formation does not support a sunspot-cycle origin, and a tidal control on sedimentation should be considered. The Elatina sequence of cyclic sandstone and siltstone laminae displays several empirical similarities to the sunspot series. The discovery of thicker, more complex lamina-cycles in the correlative Reynella Siltstone has, however, caused reappraisal of the solar interpretation of the Elatina rhythmites. The Reynella cycles consistently contain 14 or 15 laminae, with many laminae comprising pairs of `semi-laminae'; the overall structure of the cycles is similar to the mixed (semidiurnal and diurnal) fortnightly tidal growth patterns of modern bivalves. Also, recent observations that fine material can be transported in suspension to deeper waters offshore by ebb-tidal currents provides a tidal mechanism that can explain the deposition of thin, laterally extensive, graded laminae whose thickness would be a measure of tidal range and current speed. By this tidal model, the Elatina laminae are regarded as diurnal increments, and the lamina-cycles as commonly abbreviated fortnightly groupings. The Elatina series so interpreted may encode unique information on lunar orbital periods and the Earth's palaeorotation: the data indicate ca. 30.5 days per lunar month, 13.1 lunar months and ca. 400 days per year, and lunar apsides and lunar nodal cycles of 9.7 and ca. 19.5 years respectively some 650 Ma ago. Evidence for significant modulation of terrestrial climate by the solar activity cycle in the geological past may prove as elusive as have convincing indications of solar signals in modern patterns of weather and climate.
Philosophical Transactions of the Royal Society of London Series A
- Pub Date:
- April 1990