Ichnological signatures of shallow freshwater lakes in the glacial Itararé Group (Mafra Formation, Upper Carboniferous–Lower Permian of Paraná Basin, S Brazil)

Abstract

The Upper Carboniferous–Lower Permian glacigenic deposits of the Itararé Group (Paraná Basin) are widely known and cover an extensive area in the southern Brazil. These deposits consist of different arrangements of massive shales, diamictites, rhythmites, and fine- to medium-grained sandstones with trough cross-stratification. Five genetically distinct thickening-, locally coarsening-upward successions of regular rhythmites can be distinguished in the Mafra/Rio Negro region (Santa Catarina and Paraná states, southern Brazil), representing the landward portion of an incised valley filled during deglaciation. A low diversity invertebrate ichnofauna composed of a freshwater, shallow burrow-dominated suite, and trackway-dominated suite occurs in these rhythmites. The freshwater shallow burrow-dominated suite is composed of Cochlichnus anguineus, Cruziana cf. problematica, Gordia arcuata, Gordia marina, Hormosiroidea meandrica, Rusophycus cf. carbonarius, and Treptichnus pollardi, as well as Undichnia consulca and intermittent rusophiciform traces. It represents a Mermia ichnocoenosis and occurs in the rhythmites of the upper sets, commonly overlapped by D. gouldi and D. biformis, in palimpsest preservation. D. gouldi and D. biformis compose the trackway-dominated suite, which occur in all ichnofossiliferous paired mudstone–siltstone of the basal sets of the rhythmite succession, and in almost all of the upper sets. It represents an atypical Scoyenia ichnocoenosis and is generally preserved over microbially induced wrinkle structures. The rhythmic deposition suggests distal underwater gravity-flow deposits, filling shallow lakes or ponds in depressed areas, the siltstones deposited by melting currents and the mudstones by sediment fall-out, after the flux ceased. The abundant wrinkle structures suggest quiet waters. The overlying deposits bearing a brackish-water ichnofauna and marine fossils suggest a coastal setting, connected with the sea. Although atypical, the occurrence of a Scoyenia ichnocoenosis in almost all ichnofossiliferous rhythmite couplets, preserved over wrinkle structures or overlapping Mermia ichnocoenosis, suggests that these shallow lakes were episodically dried up, at least partially. The present data allow interpreting the regular ichnofossiliferous rhythmites of the Mafra Formation as deposited in shallow lakes or ponds developed in outwash plains at the landward side of a fjord embayment during deglaciation.