80,000 years of geophysical stratigraphic record at the Serra da Capivara National Park, in northeastern Brazil: Uncovering hidden deposits and landforms at a canyon's floor

Abstract

Acquisition of high-resolution geophysical spatial data at the Serra Branca canyon, in the Serra da Capivara National park, in the semiarid of the Northeastern Brazil, enables depicting deposits and relict landforms that have never been reported before for the area. Ground Penetrating Radar (GPR) and GPS real-time kinematics (RTK) enable uncovering wide cut-and-fill structures, a relict perched river channel, relatively well-stratified alluvial deposits and two relict pediment surfaces that border the current canyon's thalweg. Optically Stimulated Luminescence (OSL) geochronology shows that the stratigraphic archive tends to fit important global climatic markers of the last glacial cycle. The relict river channel was active at least from marine isotopic stage (MIS) 5a to MIS 2, showing low sedimentation rates during about 60,000 years. Between the last glacial maximum (LGM) and the Holocene, the paleochannel was abandoned. The canyon's floor was incised, and the current thalweg was carved out, probably following a local baselevel drop. Well-structured alluvial deposits have accumulated near the outlet of the canyon since at least the Early Holocene. During the Middle Holocene, colluvial and alluvial deposits accumulated in sites near the cliffs that border the canyon. The study's geochronology coincides with the pace of independent paleoclimatic proxies from Northeastern Brazil, suggesting that a humid millennial drift was associated with the incision of local bedrocks and accumulation of alluvial and colluvial deposits during the Early and the Middle Holocene. Abandonment of the Pleistocene river channel, incision of the local baselevel, and creation of the canyon's current thalweg were probably caused by a Late Glacial humid spell that may coincide with Heinrich event 1 (H1), about 16,000 years ago. However, complementary geochronologic and sedimentologic data are crucial for improving knowledge about this major drainage system change at the study site.