NEOPROTEROZOIC SEDIMENTATION AND RIFTING EVENTS ON EARTH: POSSIBLE LUNAR INVOLVEMENT

The Neoproterozoic appears to be a critical time in the history of the planet. There are two major horizons of glacial deposits in the more complete sequences and each of the glacial sequences is capped by a significant thickness of laminated carbonates. These features have been interpreted by many investigators as recording alternate "icehouse" and "greenhouse" episodes in a low paleolatitude setting. Many of the glacial-carbonate sequences are several hundred meters thick and are thought to be rift basin fills. Another noteworthy feature of Neoproterozoic sequences is the association of tidally influenced sediments including significant thicknesses of tidal rhythmites.

Peale and Cassen (1978,Icarus,36,p.245) identified an orbital resonance state between Jupiter's orbit and the lunar orbit when the lunar orbit is at 53.4 earth radii. They state that if the resonance is stable, then there could be "profound" effects on both the earth and moon. We have done some 4-body (sun, earth, moon, jupiter) numerical simulations (4th-order Runge-Kutta) on the effects of this resonance. In the short (100 year) runs that we have done on both circular and elliptical lunar orbits with semimajor axes between 50.0 and 53.4 earth radii, we have found a systematic increase in lunar orbital eccentricity. We have also done a series of 3-body (sun, earth, moon) numerical calculations on the orbital and tidal regimes which are characteristic of this lunar-orbit--jupiter-orbit resonance model as well as for the standard circular orbit model. We find that the tidal amplitudes and ranges in the earth's equatorial zone are enhanced by about 3x by a forced eccentricity of 0.25 relative to the tidal amplitudes and ranges of a near circular orbit with an eccentricity of 0.055.

In general, a geologically short-lived excursion in lunar orbital eccentricity could explain the abundance of tidally influenced sediments of that time and the enhanced rock tides could have an ancillary influence on the development of continental rift zones.