Traços fósseis dos depósitos marinhos rasos da Formação Pitinga, Siluriano superior da Bacia do Amazonas, Rio Tapajós, PA, Brasil

Abstract

The Silurian-Devonian interval of the Amazonas Basin exhibits low trace fossils diversity. Transgressive post-glacial deposits (upper Silurian) in the Tapajos River edge shows a trace fossils assemblage composed by Arthrophycus, Beaconites, Bifungites, Diplichnites, Lockeia, Monocraterion, Planolites and Phycodes, also indeterminate tubes and indeterminate arthropod scratches. This assemblage illustrates the Cruziana Ichnofacies preserved in laminated siltstone, wavy bedding sandstone with tabular and sigmoidal cross-stratification. The integration of ichnologic and sedimentologic datasets suggests a wave-dominated shallow marine palaeoenvironment. The predominance of epifaunal trace fossils indicates relatively calm water and deposition just below of the fair-weather wave base. The tracemakers were mostly worms and arthropods, but also rare bivalves. The gradual retreat of glaciers in the Silurian–Devonian transition led to the eustatic rise and the emergence of news ecospace in transgressive post-glacial sea. Thereby, the low icnofaunistic diversity could be related to cold seas that dominated the Gondwana coast enabling infaunal occupation by organisms adapted to the environment stress during the Silurian. Oscillations in the sedimentation rates and erosion of the shallow-tiers traces fossils may also be responsible by the low diversity recorded in the studied deposits.