The Pleistocene-Holocene history of a fluviokarst landscape in Central Brazil: An analysis of the river sediments of the Extrema, das Pedras and Ventura dry valleys

The present study aims to understand the evolutionary and depositional dynamics of the karst system of the Nascentes do Rio Vermelho Environmental Protection Area – APANRV, Goiás, Central Brazil, by analyzing three dry valleys in the region, formed as a result of fluvial/karst transitions. Their sediments were subjected to stratigraphic, mineralogy, granulometry, geochemical and stable isotope (δ¹³C) analyses, as well as radiocarbon dating and OSL geochronology. The three selected dry valleys – Rio Ventura, Rio Extrema and Rio das Pedras – were proven to have once belonged to rivers that were captured by sinkholes and carried underground, since their channels present interspersed sedimentary packages typical of fluvial environments. Depositional phases were defined for each case according to the characterization of the profiles in the field and to stratigraphic analysis, which identified the last deposition stage before stream capture and valley abandonment. In the Rio Pedras dry valley, surface sediments are composed of colluvial material from intermediate layers; there, deposition began around 3.0 kyr, with a change in flow around 1.5 kyr, and a final stage at 0.5 kyr, suggesting a fluvial transition. The different depositional phases found in the two profiles in the Ventura River dry valley suggest a significant change in flow, starting with sandy deposits around 2 kyr and 1.2 kyr, and later deposition of calcareous tufa, with fossils occurring only in the middle of the valley. This scenario was interpreted as representing a partial stream capture with subsequent completion. Finally, in the Extrema River dry valley, the depositional change around 13.0 kyr with the fluvial/karst transition, while the colluvial material deposited later was dated to 1.0 kyr. According to the evolution of the valleys and to the depositional phases, the ages of both the deposits and captures are related to the distance between the valleys and the erosive escarpment of the Serra Geral de Goiás.