Planation Surfaces on the Parana Basaltic Plateau, South America

Abstract

SRTM data constituted a good resource for morphometrical analyses of the extensive Paraná Basaltic Plateau (southern Brazil and northeastern Argentina, South America). The plateau is a stepped system of high-level surfaces separated by escarpments and with incised fluvial valleys, mainly belonging to the Upper Paraná and the Upper Uruguay River basins. Palaeosurface remnants of such basins preserve attributes that have been identified in digital elevation models. Generation of hypsometric curves in six representative tributary basins of the Uruguay River basin and also in one tributary basin of the Paraná River in the region permitted to identify, classify and map the main Cenozoic planation surfaces of the basaltic plateau. Other morphometric parameters such as longitudinal profiles and isobase lines were produced also to delimit such surfaces. Field geomorphological analyses were performed, also including the description of toposequences. Three groups of hypothetical hypsometric curves are deduced from proximal value sets for predicted base levels. Each mapped surface was considered between the minimum height for correlative surfaces in all of the sub-basins and the minimum height of the next higher surface. Using that approach, which is based on the clustering from the modelled base level in the river mouth of the sub-basins, three main palaeosurfaces were defined in northeastern Argentina. Complementary, three intermediate or secondary surfaces also were identified based on morphometric analyses, taken account that small flat remnants at the same level suggest that they could be remnants of a formerly extensive plain. Remnants of the Late Cretaceous-Paleogene King´s Sul-American Surface are well-developed in the headwaters of the Uruguay River (on the sub-basins of the Pelotas and Canoas Rivers, Brazil), located immediately westward from the Serra Geral erosion scarp. Sul-Americana 1 (1,277-1,080 m a.s.l) and Sul-Americana 2 (1,080-880 m a.s.l.) Surfaces in this work are equivalent to the King´s Sul-Americana Pediplain, represented by an extended low-relief surface. If the Upper Uruguay River basin corresponded to the lower segment of such surface, a predicted base level value would be estimated around 880 m a.s.l. The higher relict surface of the plateau in Argentina corresponds to a secondary planation surface named Bernardo de Irigoyen Erosion Surface (675-880 m a.s.l.). King´s Velhas Surface (Paleogene) is mainly preserved on water divides as remnants in the Upper Uruguay basin in Brazil and it is represented by a landscape of gentle and well-rounded hills. It is correlated in this study with Aristóbulo del Valle Erosion Surface, which in the Argentine Misiones province comprises the plane-top watershed between the Paraná and Uruguay basins (490- 675 m a.s.l.). Because of the proximity of these large fluvial collectors, pediplanation and pedimentation processes leaved a narrow remnant that locally are restricted to ridges of planned tops. The general landscape of this surface is represented by rounded hills and flat bottomed fluvial valleys. A predicted base level value for this surface could be estimated around 370-425 m a.s.l. The main lower surface identified in this morphometric analysis correspond to the Apóstoles Surface (110-165 m a.s.l), which would be generated by the Plio-Pleistocene King´s Paraguaçú cycle of deep fluvial incision on the Velhas/Aristóbulo Surface. The base level for the lower surface remnant of this erosive cycle is above 100 m a.s.l. The parallel retreat of slopes along the major valleys to their coalescence generated an extended erosion surface at the southeastern area of the Upper Uruguay basin, named Apóstoles Pediplain. It is formed by convex, low and long hills with very gentle and simple slopes, alternating with wide and very shallow fluvial valleys that have consumed much of the interfluves bearing remnants of the Velhas cycle. The formation of the Late Cretaceous-Cenozoic regional erosion surfaces mapped in a relatively distant area of the passive margin is mainly related to fluvial erosion and slope retreat process. Most fluvial erosion was concentrated in tributary valleys of the Paraná and Uruguay Rivers destroying the flat basaltic surface and generating a new erosion surface below. Field data also indicate that geomorphic processes like fluvial, surface wash and mass movement (rock fall, debris flow) were important. Uplift and rejuvenation of valleys before the new surface can be graded to the lower level was required. For the planation surfaces identified in the study region, the base level to which the erosion process developed was the Uruguay River or the Paraná River. The steeper longitudinal profile observed in a tributary of the Paraná, compared to the closest Uruguay tributary in northeastern Argentina, points out that erosion is more active on the Paraná system. Planation surfaces exert control on the active headward erosion by valley development and rock landslides.