Hothouse hydrology: Evolving river dynamics in the Eocene Montllobat and Castissent Formations, Southern Pyrenees

Jonah S. McLeod , Alexander C Whittaker, Gary J Hampson, Rebecca E. Bell, Marine Prieur, Oliver G. Fuller-Field, Luis Valero, Xiang Yan, Jeffery M. Valenza

Environmental forcings have shaped landscapes and basins across geologic history, and Earth’s surface is projected to undergo rapid change in the near future amidst increasing climate extremes. 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 specifically on the fluvial deposits of the Montllobat and Castissent formations, formed during the early Pyrenean orogeny. This succession records a unique shift in geomorphology and 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 Fm. into the Castissent Fm. at c. 50.5 Ma is associated with a sharp change in palaeohydraulics: a statistically significant reduction in cross-set height, a 1.4-fold increase in channel slope, 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 shift in fluvial planform morphology: we interpret a switch from anastomosing to a dominantly braided planform at the onset of the Castissent interval. We suggest the transient hydrological signature of the Castissent Fm. 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 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.