Paleoenvironment and age constraints of Paleoproterozoic alluvial fans in the Sao Francisco Craton, Brazil

摘要

The Ouricuri do Ouro Formation represents sedimentary deposits of the Statherian period in Sao Francisco Craton. Previous works have focused just on the Formation mapping, interpreting the depositional system only in general aspects. However, this study focuses on detailed fades analysis and geochronological investigations of the Ouricuri do Ouro Formation in the central area of Sao Francisco Craton, Brazil. Four facies associations are recognized: i) Proximal deposits dominated by non-cohesive debris flow; Proximal sheetfloods; Intermediate sheetfloods; and iv) Sandy plains of distal floods. Paleocurrent directions derived from cross-bedding indicate a radial pattern to NE-N. The vertical stacking of distal deposits overlying the proximal facies association suggests a retrogradation of the alluvial fan system, indicating a change in the dynamic of the depositional system. Retrogradation might have been caused by a decrease in the regional gradient declining of the tectonic activity or the retreat of the basin's margin faults system. Furthermore, the presence of higher frequency fining-upward cycles also suggests a climatic control in those deposits. These cycles could be results of the progressive decrease in flow capacity and competence, associated with a decrease of superficial runoff over time. Allied with the dominant paleocurrent, the geochronological data of four tuffaceous sandstones reveal the contribution of lthyaclan to Mesoarchean zircons, suggesting that they came with sediments transported from SW and S of the studied area during the Statherian. The transported sediments came from the southern portion of Gaviao Paleoplate. The contribution of volcanic zircons recognized in the tuffaceous sandstones is related to syndepositional magmatism of Statherian Rift of Espinhaco Supergroup. The tuff layer at the base of drill hole (Sample FLV-61 A) contains a zircon age with dominant fraction represented by Statherian zircons defining the syn-sedimentary age of 1722 +/- 13 Ma at the basal section of the Ouricuri do Ouro Formation. The present results and available geochronological data constraints the depositional interval of Lower Espinhaco Basin or Megasequence (in sense of Chemale et al., 2012; Guadagnin et al., 2015a, b) between 1.80 and 1.68 Ga in the Chapada Diamantina Region.