Late Pleistocene lithostratigraphy and chronology of alluvial fan successions in the Tacutu graben (Boa Vista FM., Roraima, Brazil

The Boa Vista Formation covers much of the Tacutu Basin in the central portion of Roraima state, northern Brazil. The genesis of this formation has been linked to geological and paleoclimatic events in the Amazon, and its stratigraphic positioning has been controversial, mainly due to the scarcity of geochronological data. In this context, the present study focuses on the geomorphological and distributional aspects of this formation in the outcrop area, integrated with facies, stratigraphic, and geochronological data (Optically Stimulated Luminescence Dating - OSL) obtained from nine outcrops in the northeastern and northwestern portions of the Tacutu Basin. The facies association allowed relating the deposition of this formation to the proximal and distal portions of a Pleistocene alluvial fan environment, based on the gravel facies (AFC) deposited by gravitational flows approximately 82.1 ± 6.0 ka and the sandy facies (AFA) deposited by current flows between 66.6 ± 2.2 and 39.4 ± 2.7 ka, respectively. Locally, the fan deposits are overlain by aeolian facies, deposited by wind reworking approximately 2.13 ± 0.3 ka ago. The data obtained are consistent with a geographic and temporal continuum of the Quaternary coverage of the Tacutu Basin, in an alluvial setting in which gravitational flow processes evolve into areas dominated by streamflows and other pertinent processes correlatable to Late Quaternary alluvial fan systems identified in the northern Amazon. In particular, the unprecedented LOE data allow us to define the sedimentation stages of the fans, allowing them to be placed within the context of recent geological evolution of Western Amazonia.