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In Brazil drainage patterns and slopes were studied in the Valley of Tres Forquilhas near Terra de Areia on the seaward edge of the volcanic plateau of northeastern Rio Grande do Sul. We studied this area to explain why the escarpment of the plateau is closest to the sea here and to establish the erosional history of this small part of the escarpment. Here two small rivers, Tres Forquilhas and Maquine, follow the Torres Syncline to the Atlantic Ocean. These rivers probably started to significantly erode the syncline and the volcanic plateau starting in the Middle to Late Miocene in response to far field Andean tectonics and epirogenic uplift across most of South America. Subsequent erosion probably occurred in pulses rather than at an uniform rate in both watersheds much as short bursts of intense heavy rainfall “pulse” local erosion of the escarpment every few years today. Study of the offshore sequence stratigraphy of the Tertiary fill of the adjacent Pelotas Basin helped us establish this local erosional history (seven unconformity bound sequences, thickness variations between these unconformities, and large scale slumping). Study of an onshore subsurface cross section also identified two paleovalleys in Mesozoic rocks beneath the present valleys of Tres Forquilhas and Maquine Rivers. There is a strong structural control on drainage at all scales. Consideration of slope process contributed significantly to our understanding of drainage evolution and the origin of the escarpment of the volcanic plateau.

Três Forquilhas Valley in Southern Brazil - evidence for the uplift of the volcanic plateau