Tidal rhythmites: geochronometers for the ancient Earth-Moon system

Abstract

Inquiry into the history of both the Earth’s，rotation and the lunar orbit through the analysis of growth increments in marine invertebrate fossils has stalemated in recent years because of uncertainties in the interpretation of various fossil structures. Cyclically laminated and thin-bedded rhythmites of tidal or igin, recently, recognized in Precambrian and Paleozoic deposits, provide the timely prospect that sedimentary rocks independently may record paleotidal and paleorotational data that support and extend the paleontological fi ndings Late Proter ozoic clastic ebb-tidal rhythmites in South Australia indicate about 400 days and尸 I lunar months per year and about 21.9 hours per day at about 650 mi llion year s (Ma少，as well as other paleotidal per iods . Tidal rhythmites of likely estuar ine or igin that occur widely in the Carbonif - er ous of the east-central United States are a poten- tial source of such information, for the late Paleo- zoic. Early Proterozoic banded iron一rmation in Western Australia may provide paleotidal data for about 2,500 Ma. The Proter ozoic data appar en吻 rule out a close approach of the Moon during the past 3,000 Ma. Overall, the study of tidal rhyth- mites promises to illuminate greatly the history of the Earth 's r otation and the Moon 's revolution.