Peixe Basin Research Articles

Heavy metals in six Brazilian watersheds: Levels of contamination in the state of São Paulo

The levels of Cd, Pb, and Hg were determined in bed sediments and suspended particulate material collected in six Brazilian watersheds (Alto Paranapanema, Peixe, Aguapei, Sao Josh dos Dourados, Mogi-Gaucu, and Piracicaba) in the State of Sao Paulo (Southeast, Brazil), bimonthly from January to December, 2001. The levels of Cd and Pb in total and exchangeable fraction of the sediment were determined in the fraction lower than 63 μm by GFAAS. Cd and Pb in suspended particulate material were determined in the fraction lower than 0.45 pm by GFAAS. The levels of Hg were determined by AFS. The levels of metals did not show temporal variability. Pb concentrations were low in all basins. Exchangeable Pb did not represent a significant fraction of the sediment. Cd concentrations were higher in the Piracicaba River basin, as a result from anthropogenic activities in the basin, such as industrial and domestic effluents damped in the river. Exchangeable Cd was up to 60% of the total Cd in the sediments. Hg concentrations were higher in the Peixe basin. All metals analyzed did not show high values, but the sources of Cd to the Piracicaba River and Hg to the Peixe River have to be evaluated.

Strike-slip relay ramps

Areas of reorientated bedding at contractional oversteps between strike-slip faults are here called strike-slip relay ramps. Metre-scale examples are described from the Jurassic sediments at East Quantoxhead, Somerset U.K. Larger strike-slip relay ramps occur in the Rio de Peixe Basin, NE Brazil, along the Newport-Inglewood Trend, California, and in the Bovey Basin, SW England. Although the geometry and development of strike-slip relay ramps are similar to those of relay ramps in normal fault systems, there are differences in the structures which accommodate the transfer of displacement between the overstepping faults. Whereas strike-slip relay ramps are typically transpressional, with pressure solution often occurring, relay ramps in normal fault systems are dominated by extension or transtension. Care needs to be taken when interpreting areas of reorientated bedding between overstepping faults, particularly when displacement directions are unknown, for example when using seismic data. This is because relay ramps can occur in both strike-slip and normal fault zones.

Faulting in the Early Cretaceous Rio do Peixe basin (NE Brazil) and its significance for the opening of the Atlantic

The intracratonic Rio do Peixe basin (NE Brazil) developed during the Early Cretaceous. Ductile shear zones in the Precambrian basement became reactivated as brittle master faults, bounding the Brejo das Freiras, Sousa and Pombal sub-basins. Sediments are well-dated as Berriasian to Barremian. Large growth folds are distributed en échelon along the master faults, which are dominantly strike-slip. We have measured the attitudes of 300 minor faults and striations at 11 localities in basin sediments. For 160 faults, we found the sense of slip to be unequivocal. Most common are right-lateral strike-slip faults, striking NNE; then normal faults, striking ENE; then reverse faults, striking ESE; and finally, left-lateral strike-slip faults, striking ENE. We have used a kinematic contouring method to estimate regional strain and the method of Etchecopar to estimate stresses. At eight localities, the principal direction of compression is nearly horizontal and strikes ENE; at three localities it is nearly vertical. At eight localities, the principal direction of extension is nearly horizontal and strikes SSE; at three localities, it is nearly vertical. Thus the regional pattern is one of wrenching, with local deviations towards either transpression or transtension. Stratigraphic constraints on the timing of fault motions are sparse. Master faults were active throughout the entire history of sedimentation. In the Pombal sub-basin, transtensional conditions prevailed. In the Brejo das Freiras sub-basin, transpression replaced transtension some time after the Berriasian. From stress trajectories and other data, we infer that left-lateral wrenching along the E-W Patos shear zone dominated over right-lateral wrenching along smaller faults striking NE-SW. This is confirmed by palaeomagnetic results, indicating small block rotations about vertical axes, counterclockwise in the northwest and clockwise in the southeast. Early Cretaccous wrenching along the reactivated Patos shear zone appears to correlate with that reported for the Benue trough in Africa. Intracontinental deformation was probably associated with a northward-propagating rift zone, prior to opening of the South Atlantic.

MICROFÁCIES DE SEDIMENTOS CLÁSTICOS

The concept of microfacies as proposed hy Carozzi has been a pplied for non-fossiliferous, clastic sediments, taking as an example the Piranhas Formation of the Rio do Peixe basin. A great number of detailed microfacies types has been distinguished by determination in thin section of petrography, diagenesis, medium size, morphometry, micro-structures and maturity of sandstones, and as far as possible also of siltstones, claystones and shales. These detailed microfacies have been grouped into five composed microfacies. Interpreting the occurrences of samples in the studied section and the composed microfacies to which they belong, permitted the conclusion of a deposition in a piedmont - alluvial fan environment, in five successive rhythms.