Tidal bar cyclicity record in the Lower Permian: The Rio Bonito Formation, Paraná Basin, southern Brazil

Abstract

Tidal bars register gravitational interactions between the Earth, Moon, and Sun and allow inferences about astronomical positioning through the cyclic variation of sandstone layer thickness throughout depositional events. Measurements of sandstone layers thickness in a tidal bar of the Rio Bonito Formation (Paraná Basin, southern Brazil) and further spectral analysis allowed the definition of the tide patterns imprinted on rock layers. The study was based on a remarkable tidal bar outcrop located in the Candiota paleovalley in the southern basin, Candiota town, Brazil, with sedimentary characteristics that suggest to a tidally dominated estuary depositional environment. The tidal bar shows a medium- to large-scale cross-stratification, reactivation surfaces, bidirectional structures, and tidal bundles. The spectral analysis resulted in a significant frequency peak (0.0329) which corresponds to a period of 30.45 layers per cycle. Six of these cycles are recognizable in the tidal bar by its cyclic thickening-thinning of layers, that range from 1.5 to 11 cm. This cyclic pattern is interpreted as being produced by the neap-spring tide cycle. Our study also allows us to infer that the south Paraná Basin Permian tide had a semi-diurnal pattern, with a neap-spring tide period of 14.9 days. Comparison with previous studies points to small changes in the duration of the neap-spring tide cycle in the last 900 Ma (15.2 days in the Cambrian, 14.9 days in the early Permian and 14.7 days today), explained by the distance and acceleration of the movement of the Moon's revolution and decrease in the speed of rotation of the Earth.