Abstract

A. W. Archer, E. P. Kvale, & H. R. Johnson write: in a recent paper Williams (1989a) claims that tidally deposited, rhythmically laminated silstones from the Precambrian (650 Ma) of Australia (Elatina Formation) can be used to extract specific values for parameters of the ancient Earth-Moon-Sun system. However, analysis of modern tidal data indicates that the basic assumptions regarding Precambrian tidal processes may have been oversimplified, with consequent understatement of the uncertainties involved.

We agree that the Elatina siltstones were likely deposited by fluctuating tidal currents. This best explains the pronounced cyclic thickness variability of the vertically accreted laminae documented by Williams. However, other than the details of the laminae thickness variability, little information regarding the sedimentology of these deposits has been published; thus little can be said about the original depositional setting of these strata. Many problems exist regarding the type of environment that could have preserved a continuous series of daily tidal laminations over a period of many years. The following discussion is presented merely to emphasize that, wherever such a setting might have existed, an understanding of modern tidal cycles that could potentially control sedimentation is essential before it will be possible to deduce details of ancient Earth-Moon-Sun geometries.

In order to compare modern tidal data to the ancient tidalities, some assumptions regarding sedimentation must be made. It is assumed by Williams (1989a) that the thickness of a lamina bears a more-or-less linear relationship with tidal height. However, this is clearly an oversimplification of depositional controls on tidal sedimentation

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