Advances in harmonic analysis of tidal rhythmites in the Puncoviscana Formation (Proterozoic - Early Cambrian), northwest Argentina.

LÓPEZ DE AZAREVICH, V.L.

18º International Sedimentological Congress, Abstract Volume

Schwarz, E., Georgieff, S.M., Piovano, E. y Ariztegui, D.

Lugar: La Plata; Año: 2010; p. 549 - 549

The Precambrian-Cambrian sedimentary sequences of northwest Argentina are represented by the Puncoviscana Formation and equivalent units, which are widely distributed in Cordillera Oriental. Depositional environments were recently redefined as coastal systems with tide-, wave- and storm-influenced tidal flats and estuarine facies. Tidal rhythmic sequences register variations in lamina thickness that can be analyzed as harmonic series with Fast Fourier Transforms (FFT), allowing preliminary calculation of Moon-Earth distance for the Precambrian-Cambrian boundary for the first time. This technique is applied in Argentina for the first time and permits characterization of tidal periods astronomically influenced during deposition. Lamina thickness in tidal rhythmites ranges from 0,40 to 36,50 mm. Thickness variation presents an oscillatory pattern that evidences sinusoidal curves with different symmetries. Systematic alternation of thin and thick lamina is common in modern and ancient deposits, and can represent the diurnal inequality of tides. Equal thickness of successive lamina represents equal magnitude in morning and afternoon tides that happens when Moon is directly over the Equator. This is the manifestation of the lunar tropical month. Systematic changes in laminae thicknesses can be related to neap-spring cycles in the synodic month (lunar phase), with higher tides developed when the Earth, Moon and Sun are aligned (syzygy). The harmonic analysis of raw and filtered data with FFT produces a Power Spectral vs Period (events/cycle) diagram of clear spectral density patters with a dominant peak defining two flood events per lunar day (semidiurnal cycle) also recognized in the sedimentary profile. A secondary peak would be related to the tropical month that has also a clear imprint in the sedimentary record. A third peak resemble the interaction of several periods as synodic, tropical or anomalistic (apogee and perigee lunar positions) or to seasonal variations along the year (winter/summer). The last peak represents the conjunction of synodic, tropical and anomalistic periods that occurred twice a year (semiannual period), during which tidal forces reach a maximum due to re-enforcement of each period. The Earth-Moon distance can then be calculated as follows: (Psid/Psid°)² = (a/a°)³, were Psid°: number of solar days in the actual sideral month (27,3186 days), a°: actual Earth-Moon distance (3,844 x10¹º cm), Psid: number of solar days per sideral month in the past, a: Earth-Moon distance in the past. According to FFT analysis in Puncoviscana Formation, Ptrop°=Psid°=26,64 days, and then the Earth-Moon distance in the Precambrian-Cambrian boundary, expressed as lunar semi-major axis, is 3,78 x10¹º cm. Lunar retreat along the geological time was analyzed from Earth-Moon distance obtained for Weeli Wolli (2450 Ma), Big Cottonwood (900 Ma), Elatina (620 Ma), Puncoviscana (540-520 Ma) and Brazil Formations (Carbonifferous), that evolved towards the actual value of 3,844 x10¹º cm. Value for the Puncoviscana Formation is coherent with the evolution curve of lunar retreat between the Paleoproterozoic and today. Data suggests a singular and exceptional astrophysical event during the Paleo-Mesoproterozoic transition (900 Ma) with a significant increase in rate of lunar retreat up to the Cambrian. Minor and discrete events are recognized in the Cambrian and Upper Paleozoic with a reduction of 50% in the lunar retreat rate. Progressive and important stabilization of the lunar retreat from the Cambrian is coincident with species development and specialization on the Earth. Relations between both phenomena have to be analyzed in new detailed studies.