Chapter 49 The Neoproterozoic Macaúbas Group, Araçuaí orogen, SE Brazil

Abstract

The Neoproterozoic Macaúbas Group records accumulation in the precursor basin of the Araçuaí orogen, located on the eastern margin of the São Francisco craton (SE Brazil). The Macaúbas basin evolved from a late Tonian continental rift to a passive margin that lasted at least untilc.660 Ma. It was orogenically inverted during the late Neoproterozoic Brasiliano event. The Macaúbas Group includes the pre-glacial Matão, Duas Barras and Rio Peixe Bravo formations, the glaciogenic Serra do Catuni, Nova Aurora and Lower Chapada Acauã formations, and the post-glacial Upper Chapada Acauã and Ribeirão da Folha formations. In the central sector of the Araçuaí orogen, the oldest glaciogenic unit of the group, the Serra do Catuni Formation, overlies the sandstone-conglomerate package of the Duas Barras Formation. The Serra do Catuni Formation consists of massive diamictite with minor sandstone and rare pelite, deposited mostly in a proximal glaciomarine environment. This unit passes upward and eastward into the Lower Chapada Acauã Formation, a thick succession of stratified diamictite, graded sandstone, pelite, transitional basalt and rare carbonate. This distal glaciomarine unit is covered by the diamictite-free Upper Chapada Acauã Formation, which passes eastward into the Ribeirão da Folha Formation, which includes fine-grained turbidite, pelite and ocean-floor rocks. In the northern sector of the Araçuaí orogen, the sandstone-pelite succession of the pre-glacial Rio Peixe Bravo Formation is covered by the Nova Aurora Formation, the glaciomarine unit rich in diamictite and Fe-rich diamictite, with minor graded sandstone and rare pelite. The Nova Aurora Formation is covered by the sandstone-pelite package of the Upper Chapada Acauã Formation. The pre-glacial and glaciogenic successions record the continental rift to transitional stages of the Macaúbas basin. The post-glacial succession represents proximal and distal passive margin to ocean floor environments. The Serra do Catuni Formation seems to be a proximal glaciomarine equivalent of the Jequitaí glacio-terrestrial deposits located on the São Francisco craton.

New evidence of an Ediacaran age for the Bambuí Group in southern São Francisco craton (eastern Brazil) from zircon U–Pb data and isotope chemostratigraphy

“…Our new data provide better temporal constraints to these events, since it places a maximum depositional age for the base of the Bambuí Group at around 560 Ma, suggesting that foreland sedimentation in the craton has started later than previously thought. In fact, our data shows that the Sete Lagoas Formation sedimentation is coeval with the construction of the Araçuaí Belt and may have been strongly influenced by it (Pedrosa-Soares et al, 2011a, 2011b. Nevertheless, the time span between the supposed flexural subsidence (~620 Ma; Pimentel et al, 1999;Pimentel et al, 2011) and the start of the sedimentation (~560 Ma) is too long and problematic.…”

“…The source of the most important zircon population (~625 Ma -almost 70% of the dated grains; Fig. 6) is probably the G1 supersuite of the Araçuaí orogen (Pedrosa-Soares et al, 2011a, 2011b. The other important peak is~557 Ma (about 27% of the grains) and its source may be the rocks from syn-collisional G2 supersuite.…”

“…Despite tectonic discontinuities (e.g., thrust faults along craton limits), the Neoproterozoic glaciogenic cover has equivalents in both the Araçuaí and Brasília belts, represented by units of the Macaúbas and Ibiá groups, respectively (Uhlein et al, 1999;Martins-Neto et al, 2001;Pedrosa-Soares et al, 2011a;Babinski et al, 2012).…”

“…1). At least in the Araçuaí belt, the carbonatepelite succession typical of the Bambuí Group has not been found , 2011aBabinski et al, 2012).…”

“…Around the EdiacaranCambrian boundary, the São Francisco craton and related orogenic belts occupied a central place in the newborn western Gondwana (Trindade et al, 2006;Li et al, 2008). By that time, both the Araçuaí and Brasília belts have already been uplifted against the São Francisco craton, showing Neoproterozoic stratigraphic unit thrust onto the cratonic covers included in the Bambuí Group Alkmim et al, 2006;Valeriano et al, 2008;Pedrosa-Soares et al, 2011a).…”