Aspectos da sedimentação e estruturas sedimentares dos depósitos da represa billings, são paulo

Several aspects of sedimentation and sedimentary structures occurring on the Billings dam, situated near the city of Sao Paulo, Brazil, are described. The dam was built up in 1934. A great drought exposed the sediments in the end of the year 1963 and begining of 1964, the time we realized the study of its deposits. The average thickness of the unconsolidated and water rich sediments is 30 cm so the rate of sedimentation is 1 cm per year. This is a high rate for the fine grained sediments that cover most of the bottom. On the talweg of some streams that existed in the area before the dam, the thickness of sediments exceeds 2 m. Clay predominates in spite of the small width (see fig. 1) and depth (around 12 m) of the dam. The reasons are mainly the geological conditions of the area: most of the rocks around are precambrian micaschits poor in quartz and a highly weathered terrain, originally covered by rain forests with very scarse outrops. A great part of the sediments are characterized by fine rithmic lamination, dark and light, the dark ones richer in organic matter. Both dark and light laminae are clay, the dark ones with about 1% of silt and the light ones with about 25%. Most of the silty materials are mica, quartz beeing very scarse. Colored biotites are rare. The cause of lamination is probably alternations of periods of stagnation and circulation of the water of the dam. A great number of laminae was deposited during one single year so it is hazardous to infere annual rithmic lamination in sediments of ancient lakes.