Drivers of fluvial terrace formation and quantifying their impacts – Application to the Río Santa Cruz in Southern Patagonia

The Rio Santa Cruz in Southern Patagonia preserves a spectacular set of fluvial terraces along its approximately 200-km length across the Patagonian Steppe landscape, stretching from the proglacial Lago Argentino to the Atlantic Ocean. Exposure dating of the terrace levels using 10Be concentrations of surface cobbles has revealed ages ranging mostly from ca. 1 Ma to < 100 ka. Over this long spatial distance and wide temporal range, the terrace record provides a unique opportunity to explore how different mechanisms have affected landscape evolution in the Patagonian Steppe but also river long-profile evolution in a more general setting as they are often used to infer impacts of past environmental drivers. These drivers may include climate forcing in glaciated headwaters (including changes during the Mid-Pleistocene Transition), sea-level changes at the outlet, and geodynamic forcing above a slab window for the Patagonian context.

A first step in our analysis considers how climate forcing may affect aggradation and incision patterns along the river. We utilize GRLP (Wickert and Schildgen, 2019) to model the river long-profile response to (e.g. periodic and pulsed) changes in sediment and water input to specifically explore the magnitude and spatial extent of aggradation-incision cycles. We find that these scenarios, commonly explained as main driver of river terrace formation along whole fluvial systems, only generate aggradation-incision cycles for a limited upstream portion of the system. Further analysis steps include modelling river long-profile evolution due to lithosphere flexure by glacial loading and sea-level changes exposing different offshore slopes to the fluvial system. Results not only show main differences in the spatial and temporal process of terrace formation but overall highlight lag times between environmental forcing and channel evolution with terrace abandonment.

The transfer of these generic results back to the Río Santa Cruz points to different environmental forcings, upstream and downstream. In a last inverse modelling step, we highlight the potential of deriving and quantifying forcing parameters based on the observed terrace sequence along the river.