RI-67 Lunisolar Tidal Signatures in the ~1.9 GA Pokegama Formation, Northeastern Minnesota: Implications for the Lunar Orbit

Abstract

Specimens of siltstone beds from a roadcut in the lower member of the ~1.9 Ga Pokegama Formation at the village of Midway near Virginia, Minnesota contain laminae that record possible neap-spring cycles and a strong diurnal inequality. These laminae provide support for the tidal origin of the formation proposed by R. Ojakangas (1983) on the basis of other sedimentary structures. In order to enhance observed cyclicities in the presence of significant diagenetic alteration, a new algorithm was developed that transforms digital images of lamina sequences to horizontal uniformity, and then averages them horizontally to reduce noise and enhance lamina boundaries. Although too short and noisy to provide results of appreciable confidence, spectral analysis of a lamina sequence from the Pokegama Formation suggests minimum values for the lunar orbit radius (greater than 30 earth radii) and length of day (greater than 10 hours) at the time of deposition, consistent with theoretical expectations and extrapolation of other cyclic tidal data. The analysis presented here illustrates the potential value of identifying well-preserved tidal sequences in Proterozoic sedimentary rocks of the Lake Superior region.