RITMOS DA DINÂMICA FLUVIAL NEO-CENOZÓICA CONTROLADOS POR SOERGUIMENTO REGIONAL E FALHAMENTO: O VALE DO RIO DAS VELHAS NA REGIÃO DE BELO HORIZONTE, MINAS GERAIS, BRASIL

Résumé

The study of fluvial dynamics of the Rio das Velhas in the section connecting the Quadrilátero Ferríferoand the Bambuí basin, relying on a compilation of geomorphological, sedimentological and structuraldata indicates a Cenozoic morphogenesis controlled by uplift of regional scale with interference of faultslocated at the contacts of morphostructural compartments. These compartments are strongly adapted tothe Precambrian litostructural domains controlling the geomorphological configuration of the differentparts of the river valley.The initial chronological datum is marked by remainders of the Sulamericana Surface (Neogene).During the Plio-Pleistocene period three levels of stepped terraces were formed. During the Holocene ainner floodplain developed in the Lower Terrace.The level of the Upper Terrace (Pliocene) corresponds to the end of dissection phase which reaches adepth of 150 m and shows a equilibrated longitudinal profile over the whole area. The sediments of theMiddle and Lower Terraces have been deposited after new dissection phases with minimum depths of 60and 10 m, respectively. These data of dissections depths and the stamped position of the floodplain confirmsthe gradual decreasing of the regional uplift intensity during the Pleistocene times.The last three events of the valley formation indicate a superimposition of the Rio das Velhas onto thestructures of the Quadrilátero Ferrífero with formation of a water-gap at the passage of the Serra do curral,named “Fecho de Sabará”. In consequence occurs a strong local retention of alluvial sediments causing aimportant alteration of river dynamics.The decreasing uplift during Holocene times is accompanied by the transition from braided channelssystems, prevailing during the Pleistocene, to a meandering channel type. The effects of historic miningactivity induce the return to the braided system.Tectonic activity by faulting is demonstrated by dislocations in the alluvial sequences of the LowerTerrace and in the longitudinal profile of the Middle and Upper Terraces with a total off-set of 50 m aswell as by river capture younger than the Middle Terrace.