Um sistema deposicional de planície de maré aberta, dominado por ondas: a Formação Lagarto, Ordoviciano (?), Domínio Estância, Sergipe

Abstract

The Lagarto Formation was interpreted as a depositional system of tidal flat where storm waves and tides controlled the sedimentation. This type of depositional system, which is presently relatively common, but uncommon in the geological record, contains hummocky cross-stratifications (HCS) formed in shallow water. Detailed facies analysis was used to define the physical characteristics of the sediments, a depositional architectural organisation, a depositional model and the controlling factors that preserved HCS in shallow-water. Seven facies, formed by oscillatory, unidirectional and combined flows, were recognised. They compose three architectural elements that represent the intertidal zone of a wave-dominated, open-coast tidal flat. The most distal element is characterised by thick and spaced HCS and small amount of mudstone. On the contrary, the most proximal elements display small wave or combined ripples and a high amount of mudstone. This organisation is due to the progressive attenuation of wave and tide energy approaching to the coast. Physical, biological, and geological factors influenced the preservation of HCS in shallow-waters. Fair-weather waves and tidal currents were not able to rework the storm deposited sandstone in the intertidal area. Environmental conditions did not allow the development of a biological community that could disrupt the sedimentary structure of the HCS. High sedimentation rate has allowed the rapid burial and preservation of the HCS beds.