Pleistocene-Holocene colluvial facies from the Volcanic Plateau of the Paraná Sedimentary Basin (Rio Grande do Sul, Brazil) – sedimentation processes and paleoenvironmental implications

Colluvial facies regolith have been described in the northern and central sectors of the Volcanic Plateau of the Paraná Sedimentary Basin (VPPSB) to understand the landscape dynamic resulting from environmental changes driven by the climatic variations of the Upper Quaternary. Despite the importance of previous findings, the southern sector of the plateau lacks the necessary stratigraphic information on colluvial facies to establish a regional paleoenvironmental framework for the VPPSB. Thus, the aim of this study was to characterize the colluvial facies in the southern sector of the VPPSB, with emphasis on their formation processes. Spatial distribution of the facies was generated, six sample outcrops were dated using optically stimulated luminescence, and a soil micromorphology/microsedimentology tool was applied to understand the sedimentation processes and verify their paleoenvironmental significance. The colluvial facies are found on paleolandforms (smooth paleohillslope/pediment, paleovalley head, paleovalley base 2nd-order stream) that are currently situated on top due to relief inversion. It was found that the ages of the colluvial facies suggest that morphogenesis acted with differing intensities and has been recurrent during the last 70 ky BP (Upper Pleistocene to Holocene – MIS 4 to MIS 1), leading to the generation of spatially discontinuous deposits. Locally, the colluvial facies have gaps caused by episodic erosion (gully formation) and a pedogenesis phase (44.86–28.35 ky BP) similar to that verified in the northern and central sectors of the VPPSB. The colluvial facies are composed of mud, sandy mud, sand, and muddy gravel, varying from massive to parallel-laminated, with the former interpreted as the result of climatic fluctuations (to the order of ≤3 ky) from drier to more humid within a dry climate regime, and the latter as resulting from a drier climate. Based on the meso- and microscale lithological properties, and applying rheological behavior criteria and the grain-support mechanism, the sedimentation processes of the colluvial facies were classified in relation to sediment concentration and groundmass organization into: i) high-density flow with a predominance of mud for the massive mud (mineral or organo-mineral) lithofacies; massive sandy mud (mineral or organo-mineral) lithofacies; and massive muddy gravel lithofacies; and layers of parallel-laminated, organo-mineral, sandy mud lithofacies; and parallel-laminated organo-mineral, g