Siliceous Petrifactions in the Permian of the Parnaíba Basin, Central-North Brazil: Sedimentary Environment and Fossilization Pathways

Siliceous petrifactions belong to the most spectacular and widely distributed terrestrial plant macrofossils. Recorded from different environments and times during the geological history, they permit deep insights into the composition and former function of tissues, organs, organisms, and even whole habitats. Regardless of sharing the main constituent, silica, in various modifications, the way to be formed cannot only vary considerably, but it can also favorably reveal a site-specific pattern. In this contribution, we review progress made in our understanding of both depositional environments and fossilization pathways as variously offered by silicified rocks. Based on the analysis of accompanying sediments, lithofacies types and architectures, the Permian Motuca Formation and its fossil record are studied in the Parnaíba Basin of Northern Tocantins. In the context of their enclosing rocks, anatomically preserved trunks, silcretes, and coquinas are investigated by light and cathodoluminescence microscopy to reveal taphonomic indications. As a result, we provide evidence of similar silicification patterns exhibited by quite different fossils, lithotypes, and host strata, which were most likely elicited by silica precipitation in seasonally fluctuating groundwater. Lithofacies is regarded as typical for deposits referred to as wet red beds. The sediments show high compositional maturity and point to deposition in sand-bed braided to anastomosing rivers in a vast alluvial plain. A subtropical climate characterized by marked seasonality promoted hygrophilous gallery forests along channels, and plant remains entered the fluvial system by repeated floods and avulsion. Our results provide crucial evidence not only for the significance of silicifications in disclosing plant fossilization but also for elucidating paleoenvironments in deep time.