TOPOGRAPHIC REJUVENATION OF TABLELAND LANDSCAPES DRIVEN BY DRAINAGE REORGANIZATION IN THE NORTHERN ANDES

Drainage reorganization is a fundamental process with major repercussions for landscape evolution and biodiversity. We integrate new quantitative data with published geological and sedimentological information to study how the Mesas-Cuestas tableland landscape of Colombia’s Eastern Cordillera became a transient landscape due to stream piracy. The Mesas-Cuestas landscape has been subjected to significant late Cenozoic drainage reorganization between the transverse Sogamoso and the longitudinal Chicamocha and Suárez rivers. In these basins, we analyzed longitudinal river profiles, determined the normalized channel steepness index (ksn), chi anomalies, and the Gilbert metrics to examine drainage reorganization and the (in)stability of the main divide. We interpret the ksn pattern, along with several slope-break knickpoints above 1400 m in elevation as related to an erosional wave traversing the Mesas-Cuestas as a transient landscape. The combination of observations, including pronounced across-divide differences in erosion rates evidenced by heterogeneous values of ksn, chi and the Gilbert metrics, along with qualitative geomorphic evidence (e.g., beheaded alluvial fans, depressed divides, anomalous drainage network) all strongly suggest that the main divide between the Suárez and Chicamocha drainage basins is unstable. To further constraint the timing of capture and subsequent events, we used published cosmogenic nuclide dating of fluvial sediments deposited in wind gaps to model the knickpoint retreat rate for the Mesa de Xerira (one of the Mesas-Cuestas landforms), and find an erosional wave retreat rate that might represent the capture of the Proto-Chicamocha River by the Suárez River. Knickpoints retreat rates are hypothesized to be modulated by changes in bedrock erodibility and local-scale area-loss mechanisms. Ultimately, this feedback between autogenic processes and fluvial erosion is a potential first-order driver of drainage reorganization and river dynamics in tableland landscapes.