Evidence for mountain glaciation events in the

Brasiliano Fold Belts surrounding the

São Francisco Craton

 

Walde, D. H.G¹; Gonzaga, G.M.²

 

1. Instituto de Geociências, Universidade de Brasília, Brasília - DF, Brazil - detlef@unb.br

2. Gonzaga Geologia S/C Ltda, Brasília - DF, Brazil - gmg@terra.com.br

 

 

 

Abstract

Based on new field work and detailed literature reviews the occurrences of a large number of mountain glaciogenic deposits related to the Brasiliano Orogeny are now interpreted from a new perspective and described in the following areas: Brasília Belt: Samburá Fm., base of the Vazante Gr., diamictites of Cristalina, diamictites of Formosa, etc. Araçuaí Belt: upper portion of the Macaúbas Gr., Jequitinhonha Complex, Salobro Fm. Sergipano Belt: Palestina Fm. Rio Preto Belt: Canabravinha Fm.  Ribeira Belt: upper portion of the Andrelândia Basin.

Key words: Neoproterozoic, Glaciations, Brasiliano Belts, Tectonics 

Introduction and selected examples 

     A continental Neoproterozoic glacial event (São Francisco Glaciation, ~750 Ma) related to the São Francisco Craton that occurred prior to the Brasiliano Orogeny (~630-540 Ma) is well documented in the literature. Its cap carbonate (Sete Lagoas Fm.) is dated 740±22 Ma (Babinski & Kaufmann, 2003). Even though a second (later) glacial event during the Neoproterozoic is known for the Paraguay Belt (W-Brazil) it has rarely been viewed in correlation to certain glacial deposits throughout Brazil. This paper argues that a considerable number of these deposits, which were formerly either related to the São Francisco Glaciation, or not considered glaciogenic, or simply not known at all, have to be associated with glacial events related to the Brasiliano Orogeny. Gonzaga (2001) suggested that mountain glaciogenic deposits were directly related to the tectonic evolution of different Brasiliano Belts. This is because gravitational reworking of glaciogenic deposits were observed, and locally, fosforite and/or iron formation, typical for the Ediacarian, can be found. This second Neoproterozoic glacial event for the State of Minas Gerais  was called the 'Samburá Glaciation' (Gonzaga, 2001).

     Support for the above suggested interpretation is provided by the following stratigraphic,  sedimentological, and radiometric new facts and findings. Brasília Belt: Samburá Fm. (Bambuí Gr.), Gonzaga & Tompkins (1991) defined a glaciogenic unit in the Serra da Canastra region with detritic zircon of maximum age of 670 Ma (Dardenne et al., 2003) and W-provenance of the foreland basin sediments (Castro & Dardenne, 1996). Region of Coromandel, MG: phosphorite (Vazante Gr.) with minimum age of deposition at 690±76 Ma, and 87Sr/86Sr ratio of 0,70777, which are coherent with Neoproterozoic (730-600 Ma) values for seawater (Babinski et al., 2002). Correlation of  the Pamplona Fm. (Vazante Gr) with the Neoproterozoic Corumbá Gr., MS, based on  d¹³C values (Misi et al., 2003). Region of Cristalina, GO: the NW-provenance of the clastics of  diamictites (Pimentel et al., 2002) and their sedimentological and petrographical characteristics (Lima et al., 2003b) are not compatible with the São Francisco glaciation. Araçuaí Belt: Macaúbas Gr: in the western section, the basal formation Serra do Catuni is product of the São Francisco Glaciation. But in the eastern section, Nova Aurora Fm. and Chapada Acauã Fm. there are BIF and phosphate intercalations, and stratigraphic findings indicate a mountain glaciation during the development of the Araçuai Belt (~580 Ma), cf. Walde & Gonzaga (2005c). The Salinas Fm. presents zircon age of 568 Ma (Lima et al., 2003a). The Jequitinhonha Complex as the highest metamorphic equivalent of the Macaúbas Gr, has zircon age of 630±19 Ma (Noce et al., 2004). In the State of Bahia, the Salobro Fm. was mainly deposited during the Brasiliano Event (Pedreira, 1999) with glaciogenic features (Trompette, 1994), already interpreted by Gonzaga (2001). Sergipano Belt: Vaza Barris Gr. (Palestina Fm.): diamictites, positioned below the metacarbonatic unit, which is contemporary with the Brasiliano tectonic event (D'el-Rey Silva, 1999). Rio Preto Belt: Canabravinha Fm., base of Bambuí Gr.: diamictites with dropstones (oral comm., G. Gonzaga), intercalations of  BIF, turbidites (Andrade Filho et al., 1994), W-provenance of the sediments (Uhlein et al., 2004), and the absence of a cap carbonate (i.g. Sete Lagoas Fm) are not compatible with the São Francisco Glaciation. Ribeira Belt: top of Andrelândia Basin, MG: diamictites, pelitic sequences with dropstones (Trouw et al., 2000), locally high proportions of apatite (Ribeiro et al., 2003), maximum zircon age of 670 Ma (Valeriano et al., 2004). Heilbron et al. (2004) suggest correlation with the Brasiliano Orogeny.

 

Discussion

     Based on the above presented evidence, the only plausible interpretation seems to be the one suggested: discontinuous and diachronous glaciogenic deposits, which are not restricted to a single correlatable horizon, often glacio-marine with strong gravitational reworking, turbidities, with BIF and phosphorite intercalations are product of  mountain related glaciations, and not coherent with the continental São Francisco Glaciation. According to Schermerhorn (1983) tectonic evolution, i.g. uplifted sourcelands and subsiding basins created favorable conditions for glaciation where till formation were deposited (mountain glaciation). From these highlands, glaciclastic debris was transported into the basins. Locally, glacier tongues descended to sea level, depositing glacial intercalations. These glacial incursions are superposed on normal sedimentation, i.g. unsorted tilloid flows, sand, silt or dolomite, which offer strong support for our interpretation of mountain glaciaton with tectonic influence during the Neoproterozoic tectonic event. Furthermore new radiometric data clearly indicate a more recent depositon during the end of the Neoproterozoic and Ediacarian age. As an important consequence, this new model, proposed initially by Gonzaga (2001) and further developed by Walde & Gonzaga (2005 a,b) argues that the deposition of the Bambuí Gr. and the Vazante Gr. began with glaciogenic deposits during the orogeny of the Brasília Belt (630-580 Ma), and is not related to the earlier São Francisco Glaciation (ca. 750 Ma). This approach answers also the question raised by Kaufman (1998) concerning an expected second Neoproterozoic glacial event with regard to the State of Minas Gerais. In addition, the suggested scenario is compatible with that one generally discribed for W-Africa (Caby, 1987 in: Trompette, 1994; Bertrand-Sarfati et al, 1987). This is supported by Evens (2000a), who proposed that a minimum of two glacial events were determined in the Neoproterozoic of W-Africa with the following ages: ~740 Ma and ~600 Ma. Frimmel & Fölling (2000) using radiometric analyses supported these ages for glaciation and proposed the correlation with Sturtian and Ediacarian ice periods. Ice persisted (or reappeared) locally because Cambrian glaciogenic deposits are known from West Africa (Evans, 2000 b).

 

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