PRECAMBRIAN TIDAL AND GLACIAL ELASTIC DEPOSITS - IMPLICATIONS FOR PRECAMBRIAN EARTH-MOON DYNAMICS AND PALEOCLIMATE

Over the past decade the study of Precambrian elastic tidal rhythmites - stacked laminae of sandstone, siltstone and mudstone that display p eriodic variations in thickness reflecting a strong tidal influence on sedimentation - has provided accurate palaeotidal and palaeorotationa l data. Palaeotidal records obtained from tidal rhythmites may be syst ematically abbreviated, however, and derived periods and frequencies c an be misleading. The validity of such values, including past length o f day, can be assessed by testing for internal self-consistency throug h application of the laws of celestial mechanics. Such a test supports the estimated length of day of 21.9 +/- 0.4 h derived from the late N eoproterozoic (similar to 620 Ma) Elatina-Reynella rhythmites in South Australia, and the indicated mean rate of lunar retreat of 2.16 +/- 0 .31 cm/year since similar to 620 Ma. The validity of estimated lengths of day obtained from other Precambrian tidal rhythmites remain unveri fied because the data sets contain only one primary value directly det ermined from the rhythmites. The Elatina-Reynella data militate agains t significant Earth expansion at least since similar to 620 Ma, and su ggest that the free nutation or 'tipping' of the Earth's fluid core ha s undergone a resonance with the Earth's annual forced nutation since the Neoproterozoic. Glaciogenic deposits are readily distinguishable f rom ejecta resulting from impacts with Earth-crossing bodies. Palaeoma gnetic data, based on the geocentric axial dipole model for the geomag netic field, indicate that Neoproterozoic and Palaeoproterozoic glacia tion and cold climate near sea level occurred in low palaeolatitudes. This enigmatic finding may imply global glaciation or an increased obl iquity of the ecliptic, and is relevant to modelling the effect of ice sheet formation on the Earth's obliquity history by obliquity-oblaten ess feedback mechanisms. Through multidisciplinary studies, elastic se dimentology and geophysics together can make substantial contributions to understanding Precambrian Earth-Moon dynamics and global palaeoenv ironments. (C) 1998 Elsevier Science B.V. All rights reserved.