Sand waves and early Earth-Moon history

Abstract

The geologic record may provide a test for recent suggestions that the moon became an earth satellite within the last two billion years, at an initial distance one-half to two-thirds of the present distance. Oceanic tidal currents are influenced by the distance between the earth and the moon and by length of day; therefore, tidal current velocities at the time of the origin of the earth-moon system were 4 to 14 times the velocities of today. Sand waves develop on sandy bottoms where current velocities are 1–3 knots and are preserved in sandstones as large-scale cross strata. In today's seas, sand waves occur in geographically restricted areas where tidal currents are increased by topography. At the time of the moon's closest approach, tidal currents of greater than 1 knot were normal for the continental shelves; therefore, a prolific development of sand waves should have occurred in the neritic environment. Cross strata produced by migrating sand waves should be abundant in marine sandstones and calcarenites formed directly after the origin of the earth-moon system and should be less abundant throughout the remainder of geologic time. The presence of abundant large-scale cross strata and other characteristics of the late Precambrian marine shelf sediments of eastern California could be explained by increased tidal currents. Marine shelf sediments of this age throughout the world should show similar evidence of a high-energy environment if the earth-moon system originated in the late Precambrian.