Cyclic sedimentation in Brazilian caves: Mechanisms and palaeoenvironmental significance

Caves associated with doline slopes in the tectonically stable area of eastern Brazil display remarkable sequences of clastic sediment intercalated with calcite layers. Sediment erosion has also occurred allowing access to formerly sediment-filled passages. The palaeoenvironmental meaning and chronology of these three processes (i.e. clastic sediment input, clastic sediment erosion, and speleothem precipitation) were studied in both semi-arid Campo Formoso and sub-humid Lagoa Santa areas through ²³⁰Th dating and stratigraphical analyses. The dry climate of the Campo Formoso area prevents speleothem deposition at present, but soil erosion results in valley aggradation and cave infilling. Growth periods of speleothems and travertines in this area have allowed the recognition of recurrent past phases of increased humidity correlated with wet conditions recorded in southeastern Brazil speleothem calcite. At the Lagoa Santa area there is limited speleothem precipitation and sediment input at present. However, sediment entrainment is actively exhuming speleothems and exposing cave passages. Sediment erosion inside caves in the area is interpreted as being due to intermediate climatic conditions, not wet enough to favour speleothem deposition and not too dry to allow doline slope erosion and sediment transport into caves. Due to the low rates of denudation and isostatic rebound inherent to tectonically stable areas, cave passages will remain within the range of sediment infill and erosion for a much longer time than in tectonically active areas spanning, in average, at least three full glacial-interglacial cycles. As uplift proceeds, cave passages will be decoupled from the doline bottom and no longer will be affected by erosion or infilling episodes. Sediment filled passages in many caves in the Lagoa Santa region are relict features that display ancient clastic and chemical precipitation. The three processes described above have occurred throughout the life history of the caves, resulting in complex sediment assemblages that can, however, show significant intra- and inter-site variations.