Hothouse Hydrology: Evolving River Dynamics in the Eocene Montllobat and Castissent Formations, Southern Pyrenees

ABSTRACT Rivers are highly sensitive to climate and tectonic change, and understanding how fluvial systems respond to greenhouse climates in dynamic tectono‐geomorphic settings is vital to projecting imminent landscape change in the face of global warming. We look to the southern Pyrenean Tremp‐Graus basin during the Early Eocene Climatic Optimum (EECO), analogous to future anthropogenic climate scenarios. We focus on the fluvial deposits of the Montllobat and Castissent Formations, deposited during the early Pyrenean orogeny. This succession records a significant shift in geomorphology involving a 20 km progradation of the shoreline and its feeder rivers in < 0.8 Myrs. Using field‐based quantitative palaeohydrology, we reconstruct the evolving morphometry and hydrodynamics of ancient river systems in a foreland basin. The transition from the Montllobat Formation into the Castissent Formation at c. 50.5 Ma is associated with a sharp change in palaeohydraulics: a statistically significant reduction in cross‐set height, a 40% increase in water discharge, and a 15% increase in total sediment flux. This intensification in hydrological regime implies a clear climate driver and is compounded with a switch in interpreted fluvial planform morphology from anastomosing to a dominantly braided planform at the onset of the Castissent interval and a 1.4‐fold increase in channel slope. We suggest the transient hydrological signature of the Castissent Formation was driven by Ypresian hyperthermal events superimposed on a levelling‐off in the global cooling trend at the end of the EECO and an increase in tectonic uplift rates at c. 50 Ma. This analysis holistically reconstructs the dynamics of ancient rivers in the Eocene Hothouse, and in conjunction with isotope and exhumation records, reveals the potential to extract complex tectono‐climatic signals from fluvial stratigraphy.