LATE MIOCENE-EARLIEST PLIOCENE RIVER BASIN REORGANIZATION IN THE NORTHERN ANDES

Deciphering the evolution of Earth’s topography and drainage networks through geologic time has important consequences for understanding the fluxes of sediment and nutrients to adjacent sedimentary basins and the distribution of Earth’s biodiversity. In this study, we constrain the timing of incision of the Cauca River Canyon in the Northern Andes (South America) using bedrock thermochronology and a sedimentary provenance analysis in a sedimentary basin located downstream of the canyon. The Cauca River is the major tributary of the Magdalena River and they constitute the largest source of freshwater and sediment to the Caribbean Sea. To constrain the rate and timing of canyon incision we used apatite (AHe) thermochronology on 10 bedrock samples from the eastern wall of the Cauca Canyon. The mean AHe age of the samples ranges from 5–60 Ma and the data shows a positive age vs. elevation relationship. Samples from lower elevations (400–1,500m) show a steeper age–elevation relationship compared to the samples from 1,500–2,500m. This relationship indicates a change in the exhumation rate at about 5–10 Ma, which we interpret to be the result of the onset of canyon incision. The late Miocene – earliest Pliocene incision of the Cauca River Canyon coincides with a change in the depositional environments of the adjacent sedimentary basin, which transitions from shallow marine (~15 Ma) to coarse-grained fluvial environments (5–8 Ma). The provenance analysis shows a large input of volcanic minerals and clasts and suggests that the sources of sediments were the Central and Western Cordilleras of the Northern Andes. The analysis suggests that by 5–8 Ma the Cauca River was incising the bedrock to form the canyon and the modern drainage basin had been established. The establishment of the Cauca River caused an increase in sediment supply to the basin. The sediment supply was higher than the subsidence rate, causing the basin to overfill and transition from marine to continental environments. Our results, together with previous provenance and paleontological data, show that the landscape of northern South America has been very dynamic in the past 10 Ma. Since the Miocene, there was a large-scale drainage reorganization of rivers such as the Cauca, Magdalena, and Amazon which directly affected the dispersal pathways of aquatic organisms.