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Serra Sul Diamictite of the Carajás Basin (Brazil): a Paleoproterozoic

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Serra Sul Diamictite of the Carajás Basin (Brazil): a Paleoproterozoic https://doi.org/10.1130/G46923.1 Manuscript received 10 June 2019 Revised manuscript received 28 August 2019 Manuscript accepted 3 September 2019 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 15 October 2019 Serra Sul diamictite of the Carajás Basin (Brazil): A Paleoproterozoic glaciation on the Amazonian craton Raphael Araújo1,2* and Afonso Nogueira2 1 Management of Geology and Mineral Resources, Geological Survey of Brazil, Av. Perimetral 3645, Belém 66095-904, Brazil 2 Institute of Geoscience, Federal University of Pará, R. Augusto Corrêa 01, Belém 66075-110, Brazil ABSTRACT we attempted to resolve this issue by producing This paper reports the discovery of glacial deposits of likely Siderian–Rhyacian age (2.58– sedimentological and stratigraphic evidence that 2.06 Ga) in South America (Carajás Basin, Brazil), thereby expanding the potential reach helps to identify the origin of these breccia. of Paleoproterozoic glaciations to the Amazonian craton for the first time. Glacially derived The results provide important evidence of the diamictites are stacked within a hitherto unrecognized ∼600-m-thick siliciclastic succession, paleoenvironmental conditions on the Amazo- here named the Serra Sul Formation. Well-preserved textures, with evidence of glaciotec- nian craton in the early Paleoproterozoic. tonism and ice rafting, indicate deposition in a coastal subglacial to glacial-fed submarine fan system, in which the immediately underlying units (banded iron formation and volcanic MATERIALS AND METHODS rock) were the main source and bedrock. The Serra Sul diamictite may be correlated with A sedimentological and stratigraphic study any of the known Paleoproterozoic glaciations, or with none of them. based on detailed descriptions of drill cores from throughout the Serra Sul Formation was INTRODUCTION mented as a set of ca. 2.75 Ga BIF and mafic undertaken. To cover as much of the lateral Paleoproterozoic glaciations (ca. 2.45–2.22 to felsic volcanic rock (Trendall et al., 1998) stratigraphic information as possible, detailed Ga; Rasmussen et al., 2013) are recorded in al- affected by Cu-Au mineralization at ca. 2.57 Ga measured sections were taken throughout an most every continent, but they have not yet been ( Tallarico et al., 2005). This was partially due to ∼600-m-thick succession from drill cores lo- reported in South America. These glaciations, the lack of an accurate stratigraphic framework cated in three different areas of the Carajás Ba- also called the Huronian Glaciation Event (Tang for the Carajás Basin that has caused several sin (a, b, and c in Fig. 1; see Items DR3, DR4, and Chen, 2013), occurred at the time of the units to be neglected, misinterpreted, or even and DR5). For each of these areas, a simplified Great Oxidation Event (GOE), and it is suggest- overestimated. sedimentary log was created and organized in ed that they were triggered by the superconti- The initial investigation of structures and the north-south section toward the basin dep- nent breakups at the end of the Archean (Aspler textures from drill cores revealed that the strata ocenter (Fig. 2). Small samples (up to 10 cm and Chiarenzelli, 1998; Bleeker, 2003; Strand, of the Serra Sul succession are inverted. This is long) were collected from the drill cores, and 2012). While some studies have argued that they generally observed on the borders of the Carajás polished thin sections were made in order to ver- may represent an early snowball Earth episode Basin that were deformed by regional folds asso- ify the microstructures and compositions of the (e.g., Evans et al., 1997; Hoffman, 2013), oth- ciated with the latest tectonic event that affected diamictite matrices and clasts. Scanning electron ers have suggested that there may have been the area, the Transamazonian cycle at ca. 2.0 microscope (SEM) analyses were performed to a series of more typical glaciations occurring Ga (Machado et al., 1991). In fact, the breccia investigate the microtextures on quartz grain at different times and in different places (e.g., intervals occur within a siliciclastic succession surfaces. For this analysis, small pieces of core Young, 2014). hitherto unrecognized in the basin, hereafter (n = 2) were gradually broken and quartz grains In the Carajás Basin, Amazonian craton formally named the Serra Sul Formation. This (n = 100) were randomly selected, then fixed in (Brazil), intervals of matrix-supported breccia formation unconformably overlies the Neoar- an electrically conductive carbon double-stick were previously reported as a particular rock chean units and unconformably underlies the tape, coated with gold, and imaged using sec- of the Serra Sul succession, and considered a siliciclastic deposits of the Águas Claras For- ondary electrons at the Geological Survey of Neoarchean magnetite-rich breccia of the low- mation of ca. 2.06 Ga (Mougeot et al., 1996), Brazil (Belém). ermost banded iron formation (BIF) unit (Ca- suggesting Paleoproterozoic (2.58–2.06 Ga) age bral et al., 2013). Strangely, these rocks were constraints (Fig. 1). RESULTS never described in the Neoarchean succession In this study, based on a detailed descrip- Foliated to Massive Diamictite Facies (Grão-Pará Group), which is generally docu- tion of well-preserved delicate primary textures Association throughout the Serra Sul Formation (see Items The association of foliated to mas- *E-mail: [email protected] DR1 and DR2 in the GSA Data Repository1), sive diamictite facies comprises diamictite 1GSA Data Repository item 2019397, complete set of core-based sedimentary logs, borehole locations, table of lithofacies, and supplementary figures, is available online at http://www.geosociety.org/datarepository/2019/, or on request from [email protected]. CITATION: Araújo, R., and Nogueira, A., 2019, Serra Sul diamictite of the Carajás Basin (Brazil): A Paleoproterozoic glaciation on the Amazonian craton: Geology, v. 47, p. 1166–1170, https://doi.org/10.1130/G46923.1 1166 www.gsapubs.org | Volume 47 | Number 12 | GEOLOGY | Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/12/1166/4870248/1166.pdf by guest on 27 September 2021 interlayered with massive diamictite and black Amazonian x x 50°W Stratigraphy Ages (Ma) craton x x N shale beds indicates an intrastratal deformation x x Carajás Basin 6 Carajás faulx t Dike swarms 1880.2 ± 1.5 at the time of deposition, and the coeval occur- x Carajás x x x 1 rence of deformed and preserved clasts in the Carajás granite 1880 ± 2 Basin same interval suggests a highly heterogeneous Brazil x Gorotire 5 c ALM-FD037 2011 ± 25 deformational setting (e.g., Menzies, 2012; Bus- ALM-FD083 6°S Formation x ALM-FD099 field and Le Heron, 2013, 2018). The observa- b 4 A43W-DH03 x Águas Claras 2060 ± 13 tion that fractures are restricted to the borders x ALM-FD52 GT41-FD02 Formation Drill hole GT41-F16 of the clasts and rarely cutting them, are prefer- GT41-F13 locations x a x SSD-FD672 GT41-F12 Serra Sul entially oriented perpendicular to foliation, and Synform SSD-FD927 Serra Sul area Formation are filled with quartz and dolomite that replaced Major faults x x 3 x x x 2575 ± 12 Igarapé Bahia v v v v v the original matrix minerals indicates that these Dike swarms x 20 km v v v 2 á Formation v v v v v 2743 ± 11 structures were formed during sedimentation Granite Carajás v v Formation and subsequently infilled by secondary minerals. Volcanic rock Granite Gabbro Group v v v v v Volcano-sedi v v v 2 Parauapebas v v v v v Grão-Par v v v 2757 ± 7 Similar to the interpretation of Cabral et al. mentary units Iron formation Mn-bearing mudrock Formation vv vv v (2013), our observations indicate that the perva- xx Basement and Conglomerate Cu-Au mineralization metamorphic x x x 1 x x Migmatite Mud-rich diamictite Xingu Complex sive deformation fabric present in some diamic- terrains x x 2859 ± 2 Mudstone Iron-rich diamictite and correlates x x x tite intervals is derived from synsedimentary deformation, certainly linked to subglacial Figure 1. Simplified geological map of Carajás Basin (Brazil) highlighting study areas a( , b, mechanisms, rather than tectonic deformations. and c) and location of drill holes (black dots), and stratigraphic framework of basin including The observation in drill cores that the succession exact stratigraphic position of Serra Sul Formation and main geochronological data available (1—Machado et al., 1991; 2—Trendall et al., 1998; 3—Tallarico et al., 2005; 4—Mougeot et al., is only tilted and that no shear bands are present 1996; 5—Pereira et al., 2009; 6—Teixeira et al., 2018). strengthens this hypothesis, as does the fact that no evidence of metamorphism was recorded in interbedded with thin beds of black shale (as Conglomerate-Sandstone-Rhythmite- the thin section. Ultimately, the occurrence of much as 3 m thick). There are uninterrupted in- Diamictite Facies Association well-preserved delicate microtextures such as tervals of diamictite, 150 m thick, in the strati- Conglomerate and sandstone interbedded steps, striations, and fractures on the surfaces of graphic section (Fig. 2, log a; see Item DR1). multiple times with rhythmite and diamictite quartz sand grains confirms that the Serra Sul The diamictite has a foliated matrix that tends compose the final facies association observed diamictite is unmetamorphosed or was meta- toward a massive aspect. The BIF and mafic in this study (Fig. 2, log b; see Item DR2). morphosed to a very low grade (cf. Machado to felsic volcanic-subvolcanic rock clasts vary Normally graded to massive conglomerate and et al., 1991; Pinheiro and Holdsworth, 1997). in size from pebbles to boulders, and vary in sandstone are stacked into 0.30–30-m-thick fin- shape from angular to well rounded.
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