versión On-line ISSN 1678-2690
An. Acad. Bras. Ciênc. v.74 n.3 Rio de Janeiro sep. 2002
LATE PALEOZOIC EXHUMED GLACIAL EROSIVE LANDSCAPE IN SALTO, SP*
JULIANA B. VIVIANI** AND ANTONIO C. ROCHA-CAMPOS
Instituto de Geociências, USP, São Paulo, SP.
Outcrops of the Itu granite (early Paleozoic) in the Tietê river valley, in Salto, SP, expose a variety of glacial erosional landforms of meso-scale extremely well preserved. Structures occur on land or eventually emerge above the water inside the river channel.
The most notable landforms correspond to streamlined bedrock (whalebacks) and stoss and lee features (roches moutonnées). The first category includes the classic roche moutonnée from Salto. Another reported occurrence of glacially abraded granite in the area could not be properly examined. Submerged depressions in the granite that separate landforms inside the river may correspond to rock basins.
Exposed dimensions of landforms vary from 1-15m of length to 1-1.5m of height. Their plan view shape is roughly elliptical. Micro-scale erosional features on the structures include striae, grooves, polishing and quarrying. Orientation of micro-features varies locally, but is in general SE-NW, parallel to elongation of landforms. They indicate a sense of movement of glacier towards NW.
Areal distribution of landforms over a distance of at least 1.2km in the Tietê river valley configures an extensive, exhumed, well preserved late Paleozoic landscape of glacial erosion.
The glacially eroded features of Salto indicate the action of a warm based glacier with subglacial meltwater. The ice mass probably flowed on an irregular, fractured granite floor, generating a complex pattern of effective ice pressure that resulted in different erosional landforms.
Itararé Subgroup rocks overlying the glacially eroded basement in the area vary from lodgement and meltout tillites, and lacustrine (?) rhythmites and sandstone with dropstone on the roche moutonnée from Salto, to cross-bedded and "convoluted'' sandstone beds in the river channel, toward NW. They are interpreted as proximal and distal glacial facies respectively, associated with an advance and retreat of the glacier.
Those rocks are overlain by an extensive and thick shale (pellitic) section that may represent the deposit of a marine transgressive post-glacial episode. Dropstones in the shale denote presence of icebergs and permanence of glacial influence during sedimentation. (December 14, 2001).