Enhanced channel mobility assessed from the preservation of floodplain elements in fluvial to coastal sandstones: An example from the Paleocene Eocene Thermal Maximum (PETM) in the Southern Pyrenees (Spain)

Climate is a primary driver of sedimentary processes from source to sink. A key challenge for stratigraphic studies in deep time is to constrain how perturbations of hydroclimate in the past have affected surface processes and source-to-sink dynamics. In modern fluvial systems, precipitation and the global hydrological cycles are fundamental determinants of sediment production, transport and deposition, and have also long been shown to exert major influence on river channel dynamics. For instance, recent works suggest that increased seasonality in precipitation could enhance lateral river dynamics, through less-frequent but higher-energy flood events and decreased bank stability associated to droughts and vegetation decline. In such settings, channel mobility has more impact than water discharge on the export of fine sediments downsystem, hence predicting specific stratigraphic patterns potentially diagnostic of hydroclimate perturbations in deep time.

This hypothesis has been tested by paleohydraulic reconstructions and estimates of avulsion occurrences based on facies analyses, but lacks direct assessment of the intensity of floodplain reworking. Therefore, to further test this hypothesis, we propose to quantify the degree of floodplain reworking by looking at the amount and spatial distribution of floodplain elements contained in fluvial to coastal sandstones within a source-to-sink system during an important climate perturbation.

Our study focuses on the Paleocene-Eocene Thermal Maximum (PETM, 56Myr) in the Southern Pyrenees (Tremp-Graus Basin, Spain). Several authors described the PETM in many localities over the area, showing a widespread deposition of an anomalously thick conglomeratic interval suggesting a perturbation of the fluvial channel dynamics at that time. We point-counted the content of 45 sandstone samples that span the pre-, syn- and post-PETM intervals in continental channels to deltas and carbonated platform depositional environments. Reworked floodplain was identified by the presence of microcodiums and carbonate nodules. To assess whether these elements were reworked from floodplain significantly older (erosion of underlying layers) or contemporaneous (lateral erosion by channel migration), we attempted LA-MC-ICPMS U-Pb dating on carbonates. Preliminary results show an increase of the amount of reworked floodplain in distal environments during the PETM climatic event. This observation is in adequation with other studies predicting a link between enhanced seasonality during global warming and channel migration rate, and provides a new way of assessing and testing fluvial dynamics in deep time as a proxy for perturbations of ancient hydroclimates.