Sedimentary records in dry karstic valleys: The case of Mambaí, central Brazil

Abstract

Karstic areas in Central Brazil show significant depositional and erosive facies placed within abandoned dry valley-systems and associated with fluviokarstic dynamics. These valleys have distinct depositional characteristics that offer sedimentary records of both past climatic events in the region and its current fluviokarstic system. The purpose of this paper is to point out and characterize such sedimentary packages, protected by dry valleys that were deposited up until the fluviokarstic transition, thus enabling the correlation of such sediments to past climatic events. The study is based on a combination of geomorphic and sedimentological analyses through the employment of multiple techniques, such as mineralogy, geochronology, stable isotopes, and granulometry, to the analysis of our case studies. These are the three dry valleys around the Ventura, Pedras and Extrema rivers, which in turn comprise the River Vermelho Basin, northeastern Goiás State. Our results suggest that the valleys contain fluvial deposits remnant from the transition of superficial fluvial flow to the underground karstic system. Records provide a general view of the depositional stages placed before and up until the period of fluvial capture in terms of their morphology, dynamics, and sediment architecture. OSL ages indicate that fluviokarstic processes had been in operation since 23 Ka, when the oldest river capture was found to have occurred. Depositions occurred in a reasonably continuous manner, mainly during the last 3.5 Ka, and concentrated around relatively dry and wet events, exhibiting typical depositional gaps in fluvial environments. Consequently, no relationship between the depositions and paleoclimatic events were observed for the analyzed fluviokarstic system.