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A Contribution to the Calibration of the Ediacaran Carbon-Isotope Curve

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A Contribution to the Calibration of the Ediacaran Carbon-Isotope Curve Precambrian Research 182 (2010) 382–401 Contents lists available at ScienceDirect Precambrian Research journal homepage: www.elsevier.com/locate/precamres Chemostratigraphy of the Tamengo Formation (Corumbá Group, Brazil): A contribution to the calibration of the Ediacaran carbon-isotope curve Paulo C. Boggiani a,∗, Claudio Gaucher b,c, Alcides N. Sial d, Marly Babinski a, Cynthia M. Simon a, Claudio Riccomini a, Valderez P. Ferreira d, Thomas R. Fairchild a a Instituto de Geociências, Universidade de São Paulo, Rua do lago 562, São Paulo, SP, Brazil b Departamento de Geología, Facultad de Ciencias, Iguá 4225,11400 Montevideo, Uruguay c Institute of Geography and Geology, University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen, Denmark d NEG-LABISE, Department of Geology, Federal University of Pernambuco, Recife, PE, Brazil article info abstract Article history: The Corumbá Group, cropping out in the southern Paraguay Belt in Brazil, is one of the most complete Received 31 July 2009 Ediacaran sedimentary archives of palaeogeographic, climatic, biogeochemical and biotic evolution in Received in revised form 19 April 2010 southwestern Gondwana. The unit hosts a rich fossil record, including acritarchs, vendotaenids (Vendo- Accepted 2 June 2010 taenia, Eoholynia), soft-bodied metazoans (Corumbella) and skeletal fossils (Cloudina, Titanotheca). The Tamengo Formation, made up mainly of limestones and marls, provides a rich bio- and chemostrati- graphic record. Several outcrops, formerly assigned to the Cuiabá Group, are here included in the Tamengo Keywords: Formation on the basis of lithological and chemostratigraphical criteria. High-resolution carbon isotopic Neoproterozoic ı13 Cloudina analyses are reported for the Tamengo Formation, showing (from base to top): (1) a positive C excur- − Chemostratigraphy sion to +4‰ PDB above post-glacial negative values, (2) a negative excursion to 3.5‰ associated with Paraguay Belt a marked regression and subsequent transgression, (3) a positive excursion to +5.5‰, and (4) a plateau Brazil characterized by ı13C around +3‰. A U-Pb SHRIMP zircon age of an ash bed interbedded in the upper part of the ı13C positive plateau yielded 543 ± 3 Ma, which is considered as the depositional age (Babinski et al., 2008a). The positive plateau in the upper Tamengo Formation and the preceding positive excur- sion are ubiquitous features in several successions worldwide, including the Nama Group (Namibia), the Dengying Formation (South China) and the Nafun and Ara groups (Oman). This plateau is constrained between 542 and 551 Ma, thus consistent with the age of the upper Tamengo Formation. The negative excursion of the lower Tamengo Formation may be correlated to the Shuram–Wonoka negative anomaly, although ı13C values do not fall beyond −3.5‰ in the Brazilian sections. Sedimentary breccias occur just beneath this negative excursion in the lower Tamengo Formation. One possible interpretation of the ori- gin of these breccias is a glacioeustatic sea-level fall, but a tectonic interpretation cannot be completely ruled out. Published by Elsevier B.V. 1. Introduction Asia (Chumakov, 2009). A severe perturbation of the carbon cycle during both glacial events is evidenced by negative ı13C excur- The Neoproterozoic Era, and the Ediacaran Period (Knoll et al., sions, much like in the older Cryogenian glaciations (Halverson et 2004) in particular, were characterized by severe biogeochemical al., 2005, 2009; Kaufman et al., 2009). One of the most dramatic and climatic oscillations (e.g. Gaucher et al., 2009a). Several ice negative ı13C anomalies (Shuram–Wonoka anomaly) is recorded ages occurred in the Neoproterozoic (e.g. Hoffman, 2009), which in middle Ediacaran carbonates of Oman, Australia, southern China, were characterized by coeval positive to negative ı13C excursions Namibia, Norway and Siberia (Brasier et al., 2000; Walter et al., in marine carbonates. 2000; Amthor et al., 2003; Halverson et al., 2005; Fike et al., 2006; In the Ediacaran, the glacial record comprises the ca. 582 Ma Le Guerroué et al., 2006; McFadden et al., 2008; Melezhik et al., Gaskiers glaciation (Bowring et al., 2003) and the ca. 547 Ma Vinger- 2008). The Shuram–Wonoka anomaly ended just before 551 ± 1Ma breek glacial event (Germs, 1995; Kaufman et al., 2009), the latter and reached its nadir before 555 ± 6 Ma in south China (Condon et being possibly correlative to the Baykonurian glaciation in Central al., 2005; Zhang et al., 2005). The anomaly may be associated to the Gaskiers glaciation (Halverson et al., 2005, 2009), or post-date it (Sawaki et al., 2010), and is still a matter of debate regarding ∗ ı13 − Corresponding author. Tel.: +55 11 3091 4202; fax: +55 11 3091 4207. the meaning of extremely negative C values down to 12‰ E-mail address: [email protected] (P.C. Boggiani). VPDB (e.g. Le Guerroué et al., 2006; Bristow and Kennedy, 2008; 0301-9268/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.precamres.2010.06.003 P.C. Boggiani et al. / Precambrian Research 182 (2010) 382–401 383 Le Guerroué and Cozzi, 2010). Precise documentation of the secu- Group, is especially rich and thus central to our understanding lar variations of marine ı13C in the Ediacaran is thus important for of the overall environmental evolution in the Paraguay Belt and understanding the causes of paleoclimatic perturbations, evolution associated cratonic cover. In this work, we revisit the geographi- and extinction of organisms in general and the advent of the Meta- cal distribution of the Corumbá Group and associated units in the zoa in particular. Especially interesting are carbonate-dominated, Paraguay Belt, and report new carbon-isotope analyses of carbon- fossiliferous sedimentary successions with radiochronometric age ates. Emphasis is given to the correlation of the isotopic excursions constraints, such as the Nama Group in Namibia (e.g. Germs et al., recorded in the Corumbá Group, as well as their environmental 2009, and references therein). significance. In South America, one of the units providing the most complete record of Ediacaran climatic, biogeochemical and biotic evolution 2. Geological setting and age: Paraguay Belt and Corumbá is the Corumbá Group and the older Jacadigo Group and Puga For- Group mation, which crop out in the southern Paraguay Belt (Boggiani, 1998; Gaucher et al., 2003; Fig. 1). The most salient features of 2.1. Geological setting these units are the occurrence of probable glacial deposits asso- ciated to Rapitan-type banded iron formation (Puga Formation and The Paraguay Belt, a late Pan-African-Brasiliano age fold belt in Jacadigo Group, e.g. Urban et al., 1992; Alvarenga et al., 2009), thick southwestern Gondwana, is characterized by its convex, arc shape carbonates (Bocaina and Tamengo formations), abundant micro- (Fig. 1). It comprises a southern and a northern branch with distinc- and megafossils and phosphorites. The bio- and chemostratigraphic tively different lithostratigraphy. The main lithological similarity record of the Tamengo Formation, in the middle-upper Corumbá between the southern and northern Paraguay Belt is the presence Fig. 1. Simplified geological map of the Paraguay Belt showing the location of the studied sections. 384 P.C. Boggiani et al. / Precambrian Research 182 (2010) 382–401 of extensive diamictite exposures at the base, generally assigned to the Tamengo Formation and acritarch assemblages similar to other the Puga Formation and interpreted as of glacial origin (Alvarenga late Ediacaran occurrences (Boggiani, 1998; Gaucher et al., 2003). and Trompette, 1992). Overlying carbonate units in the southern During the rift to drift evolution, widespread carbonate deposition (Corumbá Group) and northern (Araras Group) Paraguay Belt are of the Tamengo Formation represents an important transgression lithologically and biostratigraphically different, and probably of over the Apa River Block, which is also observed in Paraguayan ter- different age, as will be explained below. ritory (Itapucumí Group), where Cloudina fossils were also found The most expressive diamictite exposures have been described (Boggiani and Gaucher, 2004). No foreland basin deposits overlie in the northern Paraguay Belt, and initially defined as Jangada For- the Corumbá Group, highlighting the different basin evolution com- mation (Almeida, 1964a,b; Rocha Campos and Hasui, 1981) and pared to the northern Paraguay Belt, as suggested by Alvarenga et al. later correlated to the Puga Formation in the southern part of (in press). Gaucher et al. (2003, 2009b) suggested that this may be the belt (Corrêa et al., 1979; Araújo et al., 1982; Alvarenga and explained if the southern Paraguay Belt marks the eastern boundary Trompette, 1992). In the northern part, the Puga Formation exhibits of the Río de la Plata Craton, as opposed to the northern Paraguay evidence of glacially influenced sedimentation, such as faceted and Belt, which is clearly associated with the Amazonian Craton. striated pebbles and blocks. The unit is interpreted as deposited in a glaciomarine environment, with turbiditic flows occurring in 2.2. Age of the Corumbá Group and associated units eastern, more distal portions (Alvarenga and Trompette, 1992). Diamictites in the northern Paraguay Belt are overlain by well- Despite the absence of more accurate geochronological data, characterized cap carbonates belonging to the Araras Group, which diamictites of the Puga Formation in both the northern and south- present typical sedimentary features observed in other cap
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