The power of the physical fabric of mudstone in creating trace fossil like redoximorphic features, case study from the late Eocene, Sirt Basin

Abstract

This article presents complex paleo-pedoturbation features, preserved in a massive mudstone stratum belonging to a siliciclastic sequence in the upper Eocene. The outcrop of these turbations presents an intricate network of iron oxide in semi tubular forms. The mudstone to which these structures are restricted is lithologically similar to other mudstone beds, below and above it in the sequence. In contrast, it presents more paleo-pedogenic features.Attributing these tubular structures to a specific burrow is difficult for two reasons: the apparent similarities in morphology to numerous trace fossils coupled with the absence of diagnostic excavation-locomotion proxies, as well as the lack of any faunal or floral body fossils in the parent mudstone. Moreover, these structures do not convincingly conform to the simplified image documented in numerous publications for fossil roots.Taking into account the exclusive, physicochemical properties of the clay rich rocks (shrink-swell behaviour, erratic permeability and cation-exchange capacity), this article provides potential evidence that such trace fossil-like features could, alternatively, be produced by abiotic redox processes without pre-existing faunal or floral predecessors. Regardless of the extent to which the provided justifications are acceptable, they might increase awareness as to just how treacherous trace fossil identification can be. The studied case draws attention to the fact that, in mudstones, the dividing line between valid and spurious trace fossils can be very subtle. It also assists in better recognition of paleosol features in mudstones.