GLACIALLY INFLUENCED SEDIMENTATION IN THE LATER PROTEROZOIC OF THE PARAGUAY BELT (MATO-GROSSO, BRAZIL)

Late Proterozoic (probably early Vendian, 610-590 Ma) glaciogenic and turbiditic sediments and metasediments are present along the transitional zone between the Amazonizan Craton to the northwest and the Paraguay Belt to the southeast, in the Cuiaba region, Mato Grosso, Brazil, A few exposure, of diamictites are associated with conglomerates and graded pebbly sandstones on the cratonic fringe and pass southeastwards into fine-grained sediments deposited in the deeper part of the probably marine basin. In the Paraguay Bell, these rocks have been affected by the Brasiliano Orogeny, possibly around 570 Ma. They show increasing deformation and metamorphism from the craton towards the fold belt. The sedimentation scheme proposed for the western border of the Paraguay Belt and the eastern edge of the Amazonian Craton involves three. probably marine, main depositional systems. To the west,on the craton, the platform domain can be subdivided into a western inner shelf and an eastern outer shelf. The deposits on the inner belt subdomain show an alternation of dominant massive diamictites, sandstones and fine-grained sediments with few dropstones. This diamictite association is tentatively interpreted as glaciomarine in origin. In the outer shelf subdomain, the massive diamictites are progressively replaced southeastwards by an association of massive diamictites, stratified diamictites and fine-grained sediments. This diamictite association is interpreted, as a whole, as resedimented glacial debris deposited by subaqueous debris flows. The slope domain located at the transition between craton and fold belt, is characterized by strong reworking of the glaciomarine sediments by gravity flows responsible for a type of submarine fan deposition. The coarse clastic facies, which result from a reworking by rapid downslope transport of cohesive debris-flows and gravity flows with high viscosity, fill feeder channels in the upper fan. Eastward, inside the belt, a progressive sorting in the deeper portions of the fan is shown by the transition from diaimictites, massive gravels, sandstone intercalations with occasional inverse and for normal grading (debris flow + turbidity current) to normal graded, finer sediments (turbidity current). Sandstone and siltstone intercalations represent interchannel deposits formed by turbidity currents. East of the slope, well-bedded. fine-grained sediments were generated by low-density turbidity currents. Direct glacial influence is reduced to the presence of a few isolated clasts or dropstones. Two distinct Upper Proterozoic glaciogenic units are known in Brazil. The older and more widespread one, dated at about 950 Ma, is located on the Sao Francisco Craton and in the adjacent Brasiliano fold belts. The younger one (almost equal to 600 Ma) is restricted to the Paraguay Belt and the adjacent border of the Amazonian Craton. In southern Western Gondwana, Late Proterozoic glaciation is of mountain-type and developed at latitudes of 60-degrees on highs probably generated by Pan African-Brasilano rifting. At the same time, in it northern part, a widespread ice sheet covered the West African Craton