396
Journal of Paleontology 91(3):393–406 A glacial influence upon sedimentation of the Jacadigo
Group in the study area near Corumbá has been proposed on the basis of lonestones, diamictites, and recently discovered distinctive δ13C and δ57Fe signatures (Angerer et al., 2016) associated with the iron deposits in the Santa Cruz Formation in the upper part of the group (Barbosa, 1949; Dorr, 1973; Leeuwen and Graf, 1987; Urban et al., 1992). At Morro do Puga (Fig. 3), ~70km south of the fossiliferous unit studied here, diamictites and a pink cap-carbonate, within the type-section of the Puga Formation (Maciel, 1959), have likewise been inter- preted in light of one of the late Neoproterozoic ‘Snowball Earth’ glacial scenarios (Hoffman et al., 1998; Boggiani and Coimbra, 2002; Hoffman and Schrag, 2002; Babinski et al., 2013). Stratigraphic relationships between this formation and the Jacadigo Group are not clear, however. Freitas et al. (2011) interpreted sedimentation in the Group to reflect continental rifting rather than global glaciation. Moreover, they identified a succession of alluvial fan, siliciclastic lacustrine, fan-delta, and bedload-dominated fluvial depositional systems within the Urucum Formation, conformably underlying the Santa Cruz Formation, that are difficult to relate directly to a glacial paleoenvironment. Based on a proposal by Leeuwen and Graf (1987), these apparently opposing views may be reconciled if deposition of the Urucum-Santa Cruz sequence began just prior to or penecontemporaneously with one of the three major Neoproterozoic glaciations—the Sturtian (~716 Ma), Marinoan (~635 Ma), or Gaskiers (~580 Ma) (ages according to Cohen et al., 2013). Despite the broad constraints on age of the Jacadigo Group,
available data does allow some insight into possible temporal relationships with the Neoproterozoic glaciations. The Jacadigo Group is demonstrably younger than its underlying granitic basement, dated at ~889±44 Ma by K-Ar (K-feldspar from granite; Hasui and Almeida, 1970), and is evidently older than
~587±7 Ma, which is the minimum depositional age based on 40Ar/39Ar dating of the late diagenetic or early metamorphic crystallization of cryptomelane in the manganese ore near the top of the formation (Piacentini et al., 2013). The Jacadigo Group may be younger than ~623±15 Ma (O’Connor and Walde, 1985), based on the K-Ar age of a single sample of poorly exposed quartz porphyry of the La Pimienta Formation that cuts the basement underlying the Bolivian equivalent of the Jacadigo Group (Litherland and Bloomfield, 1981; Litherland et al., 1986). This value and the stratigraphic relationships of the Bolivian quartz porphyry require corroboration. If it could be proved that glacial activity during deposition of the Santa Cruz Formation overlying the Urucum Formation was penecon- temporaneous with sedimentation of the Puga Formation at Morro do Puga and in the Serra da Bodoquena, 200 km to the southeast from the site here studied (Walde and Oliveira, 1980; Boggiani, 2010), then the age of ~706±9Ma (U-Pb, SHRIMP) obtained by Babinski et al. (2013) for the youngest detrital zircon grain in the Puga Formation would also be relevant to that of the Jacadigo Group. Corroboration of this suggestion would require additional data supporting both the correlation between the Puga and Santa Cruz formations and the interpretation that the glaciation affecting the two formations was related to a global rather than a local event. Were such strictures to be substantiated, the age of deposition of the Jacadigo Group
would be constrained to the interval from ~590 Ma to ~706 Ma, suggesting the Marinoan and Sturtian events rather than the Gaskiers as candidates for the Puga-Santa Cruz glaciation. The VSMs described here are not necessarily coeval with
any of these glaciations. They occur in clasts of unknown provenance—like the youngest detrital grains of zircon in the Puga Formation (Babinski et al., 2013)—and may come from an eroded pre-Urucum source that is appreciably older than the Puga Formation 706 Ma-old detrital zircon. Thus, they are possibly older than the 740–750 Ma-oldVSMs of the Chuar and Yukon groups of North America (Porter et al., 2003; Strauss et al., 2014) and the ~770 Ma-old VSMs of the Chichkan Formation of Kazakhstan (Sergeev and Schopf, 2010).
Materials and methods
VSMs in the Urucum Formation were originally reported from dolostone clasts from diamictite near the base of the formation at the northern end of the Morraria do Rabicho collected by Zaine (1991) and from an arkosic breccia near the top of the formation at Morro do Urucum (Fig. 3) collected by Barbour (Fairchild et al., 1978, p. 77, pl. 1, figs. 7–9). The latter outcrop, however, has not been re-located and VSMs have not been discovered in any other arkosic or carbonate beds within the formation. Similarly, nowhere else in the Corumbá region have we found VSMs in other dolostone-bearing units, such as the Bocaina Formation, nor in cap dolostones at the type locality of the Puga Formation at Morro do Puga (Fig. 3; Boggiani et al., 2003; Babinski et al., 2013). Rare, morphologically markedly different VSMs, currently under study by L.M., occur in the Serra da Bodoquena region, preserved in a younger phosphorite of the Corumbá Group. In summary, the source of the Urucum VSM-bearing
clasts is presently unknown and apparently located outside the depositional basin of the Jacadigo Group, which is an interpretation that justifies their description as extraclasts. The VSM-bearing dolostone extraclasts were collected at
three localities, including that of Zaine (1991), within a single extensive outcrop of diamictite near the base of the Urucum Formationatthe northern endofMorrariadoRabicho,inthe channel leading from the Paraguai River to the Lagoa Negra (locality 1: 19°1'49.98''S, 57°28'11.94''W; locality 2: 19°1'52.61''S, 57°28'15.16''W; locality 3: 19°1'53.20''S, 57°28'15.64''W). VSMs were also studied in four dolostone clasts reportedly collected near the top of the formation at Morro do Urucum, all localities occurring near Corumbá,Mato Grosso do Sul, west-central Brazil (Fig. 3; Fairchild et al., 1978). The following morphological characters regarded as
significant by Porter et al. (2003) are used here to describe the Urucum VSMs: body shape, total length (L) and width (W), aperture diameter (AD), neck length (NL), wall thickness (WT) and uniformity, and test composition. Aspect ratios (L/W) provide an estimate of the sphericity of the tests and are helpful in defining body shape. For tests that exhibit long straight or flared necks (Limeta lageniformis n. gen. n. sp. and Palaeoamphora urucumense n. gen. n. sp., respectively), the aspect ratio was calculated as BL/W, in which BL is “body length” defined as the total test length (L) minus the neck length (NL). In the taxonomic
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