Fluvial Evolution of the Eastern Andean Piedmont: Late Quaternary Sedimentary Records from the Caquetá and Guaviare Rivers, Colombia

The sedimentary record of fluvial systems is crucial for understanding the impact of climate change on landscapes over time. While fluvial dynamics in temperate and arid/semi-arid regions have been well-studied, research on Quaternary fluvial processes in tropical mountainous areas is still limited. This study proposes an evolutionary model for the piedmont regions of the Eastern Andean Cordillera, on Caquetá (1°N) and Guaviare (4°N) rivers basin in Colombia, which are characterized by increased tectonic activity in the northern part of the region during the late Cenozoic. We employ a combination of geomorphological mapping and luminescence dating (OSL and IRSL) to investigate sediments forming alluvial fans and river terraces. Our results reveal three levels of river terraces and three alluvial fan units in the Caquetá River basin, as well as eight river terrace levels and six alluvial fan units in the Guaviare River basin. The sedimentary deposits of the alluvial fans and river terraces in both areas are primarily coarse-grained, dominated by conglomerates and sandy-conglomeratic units (Gm, Gt, Sgm). The floodplains of the Caquetá River are composed mainly of fine-grained sediments (silt and clay). In contrast, the Guaviare River floodplains are predominantly coarse-grained and conglomeratic. This difference is attributed to increased tectonic activity in the northern region, which has intensified erosion and sediment transport. The OSL dating in the Caquetá River basin has allowed the identification of three evolutionary phases: (i) 120 to 65 ka, marked by active alluvial fans with braided distributary channels; (ii) 65 to 15 ka, characterized by valley incision and drainage reorganization, leading to the formation of tributary networks like modern systems; and (iii) the last 15 ka, dominated by low terraces and meandering floodplains. In the Guaviare River basin, OSL and IRSL data suggest that the distributary system remained active from at least 300 ka to 50 ka, with floodplains forming during the early Holocene (10 ka). Tectonic activity, including faulting, has also been recorded on river terraces (4°N) dating to at least 110 ka. Paleoenvironmental and palynological data indicate that the shift from distributary to tributary systems is linked to changes in precipitation patterns in the Northern Tropical Andes, driven by the shifting position of the Intertropical Convergence Zone (ITCZ) due to insolation cycles. Decreased rainfall, associated with northern ITCZ positions, supported alluvial deposition in both distributary and tributary systems. A phase of valley incision during MIS 3, followed by a reduced precession and obliquity signal amplitude, promoted the transition to a more stable landscape with a dominant tributary fluvial pattern. The integration of cosmogenic nuclides (¹⁰Be and ²⁶Al; in preparation), alongside OSL and IRSL dating, will further enhance our understanding of paleo-erosion and erosion rates, improving the robustness of the proposed paleoenvironmental model for the Colombian Eastern Andean Cordillera. (FAPESP grant #2021/14947-6)