Ichnology in storm and fair-weather beds of an epeiric sea (Pimenteira Formation, Devonian, Brazil)

Abstract

Trace fossils in storm-generated beds of shelf siliciclastic settings have been investigated mostly to understand the effect of waves in the infauna distribution. It is possible to identify a post-storm ichnofauna (with features attributable to Skolithos ichnofacies) and a resident ichnofauna (attributable to Cruziana ichnofacies) and this ichnologic signature can bring data about the frequency of storms (weak, moderate, or strong) in the lower shoreface zone. Thus, the ichnologic signature can be a key feature to understand the depositional setting in storm-dominated environments. An excellent unit to apply these concepts is the Pimenteira Formation (Eifelian-Frasnian, Parnaíba Basin), interpreted as result of deposition in a storm-dominated platform. In ichnological terms, this unit has a great ichnodiversity and abundance of ichnofossils, but few studies have integrated the trace fossil suites with a facies analysis to understand the storm influence during deposition. Thus, this study aims to (i) characterize storm-related ichnofaunas from the Pimenteira Formation, ParnaíbaParnaíba Basin, Brazil; and (ii) understand the paleocological controls in the colonization of shoreface beds in an epeiric sea. The ichnotaxons were grouped in four suites: Skolithos, Palaeophycus, Bifungiites, and Lophoctenium suites. The Skolithos suite characterizes the post-storm ichnofauna, while Palaeophycus, Bifungiites, and Lophoctenium suites characterizes the resident ichnofauna. The predominance of proximal Cruziana ichnofacies, associated with layers generated by storm events, attests colonization in shallow marine environment. However, the general absence of deposit-feeders indicates short term in the colonization window for the resident ichnofauna, implying in a shoreface setting moderately impacted by storms.