Abstract

LOPEZ DE AZAREVICH, Vanina L; OMARINI, Ricardo H; SUREDA, Ricardo J  and  AZAREVICH, Miguel B. Tidal rhythmites in the Puncoviscana Formation (s.l.) at Rancagua locality, Northwestern Argentina: tidal dynamics and paleoastronomic considerations. Rev. Asoc. Geol. Argent. [online]. 2010, vol.66, n.1-2, pp.104-118. ISSN 0004-4822.

Tidal rhythmites are clear evidence of sedimentation affected by tides and could preserve tidal periods astronomically induced during the time of deposition. Investigations of tidal periodic sequences lateral or vertically accreted on modern and ancient units had improved notable advances during the last 25 years, but, there are no data on the Neoproterozoic-Early Cambrian transition. The Puncoviscana Formation (NW Argentina) at Rancagua locality is composed by shoreface to offshore lithofacies, which includes tidal rhythmites of Neoproterozoic-Early Cambrian age. Lunar periods involved in sedimentation by tides can be approach by harmonic analysis carried out with fast Fourier transforms from raw data on lamina thickness. A semidiurnal tidal system with tropical periods of 26.64 days was obtained, from which Earth-Moon distance was calculated as 3.78 x1010 cm. These results provide relevant information about the evolution of the lunar orbit through geological time. Results suggest an astrophysical event of singular characteristics during the Paleo-Mesoproterozoic transition with an increment in retreat velocity up to Early Cambrian, with minor discreet events in Early Cambrian and Upper Paleozoic with decrease in retreat velocity in a 50%.

Keywords : Tidal rhythmites; Fourier transforms; Earth-Moon distance; Puncoviscana Formation; Northwest Argentina.

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