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.
Ojakangas, Gregory W.; Ojakangas, Richard W..
RI-67 Lunisolar Tidal Signatures in the ~1.9 GA Pokegama Formation, Northeastern Minnesota: Implications for the Lunar Orbit.
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