Chemostratigraphy of the Lower Arroyo del Soldado Group (Vendian, Uruguay) and Palaeoclimatic...

Gondwana Research, V. 7, No. 3, pp. 715-730.© 2004 International Association for Gondwana Research, Japan.ISSN: 1342-937X

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Chemostratigraphy of the Lower Arroyo del Soldado Group(Vendian, Uruguay) and Palaeoclimatic Implications

Claudio Gaucher1*, Alcides Nóbrega Sial2, Gonzalo Blanco1 and Peter Sprechmann1

1 Departamento de Paleontología, Facultad de Ciencias, Iguá 4225, 11400 Montevideo, Uruguay, E-mail: [email protected] Departamento de Geología, Centro de Tecnologia e Geociências, Universidade Federal de Pernambuco (UFPE), Caixa Postal

7852, Recife PE-50732-970, Brasil* Corresponding author

(Manuscript received November 30, 2002; accepted October 8, 2003)

Abstract

New C- and O-isotopic determinations from the Vendian lower Arroyo del Soldado Group are reported and combinedwith sedimentologic and biostratigraphic data. On the basis of different geochemical and petrographic criteria, theprimary nature of the C- and Sr-isotopic signature is shown. Positive d 13C-values characterize the mainly siliciclasticupper Yerbal Formation, which contains oxide-facies BIF and a diverse assemblage of skeletal fossils, including Cloudinariemkeae. A series of positive and negative δ13C-excursions occur up-section in the overlying Polanco Formation, mainlycomposed of limestones and limestone-dolostone rhythmites. The transition to the overlying Barriga Negra and CerroEspuelitas Formation, which consists of conglomerates and shales/cherts/BIF respectively, is marked by a furthernegative excursion. On the basis of sedimentary structures, a correlation of δ13C with palaeobathymetry is established.Positive δ13C -peaks are associated with high sea-level stand, high organic-carbon burial and relatively higher microfossildiversity, while negative δ13C -excursions occur in carbonates with less organic matter, less microfossil diversity and arealways associated to regressions. These observations can be readily explained by palaeoclimatic models which postulatethat enhanced bioproductivity due to higher availability of nutrients (P, N, Fe) was the key factor controlling Neoproterozoicglaciations. The mentioned models are discussed in view of the new data. The occurrence of at least four cold periodsin the upper Vendian is envisaged. These cold periods led to sea-level fall and possibly glaciation at higher latitudes.The absence of glacigenic rocks associated to negative δ13C-excursions in the Arroyo del Soldado Group is probably dueto the tropical setting of the basin. Finally, the upper Vendian age of the lower Arroyo del Soldado Group is confirmed,on the basis of C- and Sr-isotopes. The onset of carbonate deposition at the base of the Polanco Formation is estimatedat 580 Ma by comparison with existing global isotopic curves.

Key words: Vendian, Uruguay, isotopes, palaeoclimate, glaciation.

Introduction

Arroyo del Soldado Group (ASG) is a 5 km-thickplatform succession unconformably overlying a mainlyArchean to Mesoproterozoic basement (Nico PérezTerrane), that occurs in central-eastern Uruguay (Gaucheret al., 1996, 1998; Gaucher, 2000; Fig. 1). The group ischaracterized by an alternation of siliciclastic (Yerbal,Cerro Espuelitas, Barriga Negra and Cerros San FranciscoFormations) and carbonate units (Polanco and CerroVictoria Formations, Fig. 2). Volcanic, volcanoclastic andpyroclastic rocks are absent. Carbonate units representpericontinental, storm-dominated carbonate ramps(Einsele, 2000). Siliciclastic units were also deposited ina basin with a gentle palaeoslope, with the only andlocalized exception of the Barriga Negra Formation. Inboth cases, facies are laterally very persistent over

hundreds of kilometers. Sandstones are texturally andmineralogically mature quartzarenites or subarkoses(Gaucher, 2000). All together, these facts strongly indicatea stable, Atlantic-type continental platform as thegeotectonic setting for the Arroyo del Soldado basin(Gaucher, 2000).

ASG is notable for its extensive and very thick carbonatedeposits, namely (a) the Polanco Formation, withmaximum thickness of more than 900 m, (b) the CerroVictoria Formation, composed of up to 400 m ofstromatolitic and oolitic limestones (Gaucher et al., 2003;Sprechmann et al., 2004), and (c) various isolateddolomite and limestone strata in the lower Cerro EspuelitasFormation and upper Yerbal Formation (Fig. 2). Theseare excellent prerequisites for chemostratigraphicstudies in the late Neoproterozoic, a time of extreme

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biogeochemical oscillations, environmental instability andbiological innovations. Carbon and oxygen isotopic datafrom the ASG have been previously reported by Boggiani(1998), Kawashita et al. (1999), Gaucher (2000) andGaucher et al. (2002). Kawashita el al. (1999) present fourSr-isotopic determinations from the lowermost PolancoFormation (see below). We report here the results of a seriesof 50 C- and O-isotopic determinations from the Yerbal,Polanco, lower Barriga Negra and lower Cerro Espuelitasformations (Table 1), and combine them with availabledata, as well as with reported Sr-isotopic data. Newgeochemical and biostratigraphic data are also discussed.

Age of the Arroyo del Soldado Group

Age of the ASG is geochronologically constrained by:(a) a maximum U-Pb SHRIMP zircon age of 633±12 Mafor the Puntas del Santa Lucía pluton (Hartmann et al.,2002), which is overlain with erosional unconformity bythe ASG; and (b) a minimum Rb-Sr isochron age for the

Guazunambí Granite of 532±11 Ma (Ro = 0.70624:Kawashita et al., 1999), which intrudes into the ASG inthe Isla Patrulla Block (Fig. 1), causing contact-metamorphism. Additional data are provided by K-Ar agesranging between 532±16 and 492±14 Ma for therecrystallization of pelites belonging to the group(Cingolani et al., 1990; Gaucher, 2000). Basal rhythmitesof the Polanco Formation in the Calera de Recalde Synclineyielded 87Sr/86Sr values of 0.7078 according to fourdeterminations carried out by Kawashita et al. (1999, seebelow). This isotopic ratio corresponds to an age of 580Ma for the basal Polanco Formation, according to theglobal Sr-isotopic curves presented by Jacobsen andKaufman (1999) and Walter et al. (2000).

Biostratigraphic data also point to an upper Vendianage for the lower ASG, and a lowermost Cambrian agefor the top of the unit (Cerro Victoria Formation). Anassemblage of skeletal fossils containing Cloudina riemkeaeGerms (1972), Titanotheca coimbrae Gaucher andSprechmann (1999) and at least five other genera and

Fig. 1. Geographic distribution of the Arroyo del SoldadoGroup and other Neoproterozoic-Cambrian(meta)sedimentary units in Uruguay, and someimportant Cambrian intrusive granites, modifiedfrom Gaucher (2000). SYPSZ�Sarandí del Yí-Piriápolis Shear Zone; SBSZ�Sierra Ballena ShearZone; CPT�Cerro Partido Thrust; IPSZ�IslaPatrulla Shear Zone; IPB�Isla Patrulla Block;ICR�Isla Cristalina de Rivera; ICA�Isla Cristalinade Aceguá. Numbers 1-8 indicate sites refered toin the text. Rectangle around point 3 correspondsto the geological map shown in figure 3. Detailedgeological information on the Arroyo del SoldadoGroup south of Minas is reported by Gaucheret al. (2004).

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species occur in the uppermost Yerbal Formation of theASG (Gaucher and Sprechmann, 1999; Gaucher, 2000;Gaucher, 2002). Cloudina is recognized as an index-fossilof the upper Vendian (Grant, 1990), and has a globaldistribution in rocks of that age. On the other hand, thereare increasing data from other South American unitssuggesting that Titanotheca coimbrae is also restricted tothe upper Vendian (Valdaian, Ediacaran), at least in aregional scale (Teixeira and Gaucher, 2001; Gaucher etal., 2003a). A low-diversity, high-abundance assemblage

of organic-walled microfossils is preserved in the ASG aswell (Gaucher et al., 1996, 1998; Gaucher, 2000). Thegenera Bavlinella and Soldadophycus are dominant in thesiliciclastic units, while in the Polanco Formation a slightlymore diverse assemblage (Leiosphaeridia Lophosphaeridiumassemblage) occurs (Gaucher, 2000). These data are inaccordance with an upper Vendian (Valdaian) age for thelower-middle ASG, especially considering the absence oflarge sphaeromorphs and acanthomorphs (Vidal andMoczydlowska, 1997; Knoll, 2000).

Fig. 2. Generalized stratigraphiccolumn of the Arroyo delSoldado Group and C-isotopic composition ofcarbonates, modified afterGaucher (2000). Sources ofδ13C-data: Boggiani (1998,13 analyses), Kawashita etal. (1999, 4 analyses),Gaucher et al. (2003 a, b; 13and 30 analyses respectively)and this work (Table 1).

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Table 1. Carbon and oxygen isotopic data for carbonates of the Arroyo del Soldado Group. Samples marked with a single asterisk were preparedusing standard palynological techniques for the isolation of organic-walled microfossils. Double asterisks indicate data previously reportedby Gaucher et al. (2003). CRSyncline: Calera de Recalde Syncline (Figs. 5�6). AA�, BB� and CC� represent different sections, indicated infigure 5. BNFm.: Barriga Negra Formation (Fig. 7).

Sample Unit, locality Lithology δ18OSMOW� δ18OPDB � δ13CPDB �

001116/1* Polanco Fm., Unit A, CRSyncline (Fig. 5: AA�) Calcisiltite from rhythmite +22.00 �8.59 +2.96001116/2 Polanco Fm., Unit A, CRSyncline (Fig. 5: AA�) Calcisiltite from rhythmite +21.45 �9.13 +3.22001117/1* Polanco Fm., Unit C, CRSyncline (Fig. 5: AA�) Calcareous dolosiltite +22.24 �8.36 +0.93001117/2 Polanco Fm., Unit A, CRSyncline (Fig. 5: AA�) Calcisiltite from rhythmite +24.11 �6.55 +3.40001117/3* Polanco Fm., Unit A, CRSyncline (Fig. 5: AA�) Calcisiltite +20.85 �9.72 +5.26001117/4 Polanco Fm., Unit A, CRSyncline (Fig. 5: AA�) Calcisiltite +22.13 �8.47 +2.97001117/5 Polanco Fm., Unit A, CRSyncline (Fig. 5: AA�) Fine, tempestitic calcarenite +23.25 �7.38 +4.35001117/6* Polanco Fm., Unit B, CRSyncline (Fig. 5: AA�) Dolomitic calcisiltite +20.26 �10.28 �2.53001117/7* Polanco Fm., Unit B, CRSyncline (Fig. 5: AA�) Very fine, temp. Calcarenite +21.48 �9.10 �3.06001117/8* Polanco Fm., Unit B, CRSyncline (Fig. 5: AA�) Calcisiltite from rhythmite +21.09 �9.48 �3.26001117/9* Polanco Fm., Unit B, CRSyncline (Fig. 5: AA�) V.f.calcarenite from rhythmite +24.04 �6.62 �2.79001117/10* Polanco Fm., Unit C, CRSyncline (Fig. 5: AA�) Dolosiltite from rhytmite +23.95 �6.70 +2.06001117/11 Polanco Fm., Unit B, CRSyncline (Fig. 5: AA�) Tempestitic, dol. calcarenite +20.02 �10.51 �1.63001117/12 Polanco Fm., Unit B, CRSyncline (Fig. 5: BB�) Dolosiltite-calcisiltite +23.93 �6.73 �2.71001117/13* Polanco Fm., Unit B, CRSyncline (Fig. 5: BB�) Tempestitic, fine calcarenite +24.58 �6.10 �1.03001117/14 Polanco Fm., Unit C, CRSyncline (Fig. 5: BB�) Calcisiltite +23.75 �6.90 +1.26001117/15* Polanco Fm., Unit C, CRSyncline (Fig. 5: BB�) Dolomitic calcisiltite +24.05 �6.61 +1.48001117/16 Polanco Fm., Unit C, CRSyncline (Fig. 5: BB�) Very fine calcarenite +21.45 �9.13 +2.07001117/17a* Polanco Fm., Unit D, CRSyncline (Fig. 5: BB�) Calcisiltite from rhythmite +22.73 �7.89 �0.70001117/17b* Polanco Fm., Unit D, CRSyncline (Fig. 5: BB�) Dolosiltite from rhythmite +23.43 �7.21 +0.04001015/2 Polanco Fm., Unit E, CRSyncline (Fig. 5: CC�) Dolomitic calcisiltite +19.76 �10.77 +1.85001015/3 Polanco Fm., Unit E, CRSyncline (Fig. 5: CC�) Calcarenite from rhythmite +24.65 �6.02 +2.79001015/5* Polanco Fm., Unit E, CRSyncline (Fig. 5: CC�) Dolomitic calcisiltite +23.08 �7.54 +0.43001118/6 Upper Polanco Fm., parastratotype BNFm. (Fig. 7) Calcisiltite from rhythmite +20.91 �9.65 �1.43001118/7 Upper Polanco Fm., parastratotype BNFm. (Fig. 7) Calcilutite +18.96 �11.54 �0.66001118/8 Upper Polanco Fm., parastratotype BNFm. (Fig. 7) Dolosiltite from rhythmite +17.51 �12.95 �1.35001118/9 Upper Polanco Fm., parastratotype BNFm. (Fig. 7) Calcisiltite from rhythmite +20.31 �10.23 �1.46001118/10 Upper Polanco Fm., parastratotype BNFm. (Fig. 7) Calcisiltite from rhythmite +19.57 �10.96 �1.86001118/11 Upper Polanco Fm., parastratotype BNFm. (Fig. 7) Calcisiltite-dolosiltite +21.63 �8.95 �1.11001118/12 Upper Polanco Fm., parastratotype BNFm. (Fig. 7) Calcisiltite from rhythmite +20.77 �9.79 �1.32980317/7a** Boundary Polanco-BNFm., parastratotype (Fig. 7) Calcarenite from rhythmite +20.51 �10.04 �1.74980317/7b** Boundary Polanco-BNFm., parastratotype (Fig. 7) Dolosiltite from rhythmite +21.46 �9.12 �1.44980317/5** Lowermost BNFm., parastratotype (Fig. 7) Calcarenite +20.25 �10.29 -1.69001119/1 Polanco Fm., stratotype of B.Negra Fm. (Fig. 1, site 5) Dolomitic calcisiltite +15.11 �15.28 +0.61001119/2 Polanco Fm., stratotype of B.Negra Fm. (Fig. 1, site 5) Calcareous dolosiltite +19.27 �11.25 +2.33001119/3 Polanco Fm., stratotype of B.Negra Fm. (Fig. 1, site 5) Calcisiltite +19.53 �10.99 +2.07001119/4 Polanco Fm., stratotype of B.Negra Fm. (Fig. 1, site 5) Dolosiltite from rhythmite +20.36 �10.19 +0.61001119/5 Polanco Fm., stratotype of B.Negra Fm. (Fig. 1, site 5) Calcisiltite +21.98 �8.62 +1.69970317/3 Upper Polanco Fm.,stratotype of Cerro Espuelitas Fm. Dark dolosiltite +26.65 �4.08 +2.13970317/5 Upper Polanco Fm.,stratotype of Cerro Espuelitas Fm. Dark dolosiltite +30.28 �0.56 +1.57970501/2 Upper Polanco Fm.,stratotype of Cerro Espuelitas Fm. Dolosiltite with authig. albite +27.81 �2.95 +1.87010624/3 Upper Polanco Fm.,stratotype of Cerro Espuelitas Fm. Dark dolosiltite +25.09 �5.59 +2.05950123/3 Middle Cerro Esp. Fm. at its stratotype (Fig. 1, site 4) Dark dolosiltite +29.80 �1.02 +2.40010503/2 Uppermost Polanco Fm. at its stratotype (Fig. 1, site 7) Dolosiltite-dololutite +18.97 �11.53 �1.65010503/3 Uppermost Polanco Fm. at its stratotype (Fig. 1, site 7) Dolosiltite-dololutite +19.23 �11.28 �1.59000715/3a** Uppermost Polanco Fm. at its stratotype (Fig. 1, site 7) Dolosiltite-calcarenite +18.14 �12.34 �1.94000715/3b** Uppermost Polanco Fm. at its stratotype (Fig. 1, site 7) Dololutite from rhythmite +20.59 �9.96 �1.55970320/6** Lower C. Espuelitas Fm. at Polanco stratotype Calcisiltite +17.45 �13.01 �2.52970320/7** Basal C. Espuelitas Fm. at Polanco stratotype Calcisiltite-dolosiltite +16.00 �14.42 �1.33990927/1** Upper Yerbal Fm., Arroyo Campanero (Fig. 1, site 1) Pink dolosiltite +26.24 �4.48 +2.15990927/2a** Upper Yerbal Fm., Arroyo Campanero (Fig. 1, site 1) Pink dolosiltite +26.19 �4.54 +1.55980311/6** Upper Yerbal Fm., Arroyo Campanero (Fig. 1, site 1) Dolosiltite clast +26.96 �3.79 +1.96980927/1** Upper Yerbal Fm., Arroyo Campanero (Fig. 1, site 1) Pink dolosiltite +25.17 �5.52 +1.17

Analytical precision for δ13C and δ18O: better than 0.01�

Regarding the age of the upper ASG, trace fossilsassigned by Sprechmann et al. (2004) to Thalassinoidesisp. occur in the Cerro Victoria Formation, indicating a

Cambrian age. On the basis of a high-resolution, C and Oisotopic survey of carbonates of the Cerro VictoriaFormation, Gaucher et al. (2003b) suggest a lowermost

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Cambrian (Nemakyt-Daldyn) age for the unit. A negativeδ13C excursion of �3.5� PDB is recorded at the transitionCerros San Francisco-Cerro Victoria Formation andsuggests according to Gaucher et al. (2003b) that thePrecambrian-Cambrian boundary is located in the upperCerros San Francisco Formation (Fig. 2).

Materials and Methods

Standard thin sections of carbonates were prepared,stained (Alizarin Red-S) and carefully analyzed under apetrographic microscope Leica DM LP. Domains consistingof pure primary carbonates (preferably calcite) andshowing no recrystallization were selected. Approximately1 g of these domains was extracted from the correspondingrock specimen. C and O isotope ratios of the resultingsamples were analyzed at the stable isotope laboratory(LABISE) of the Federal University of Pernambuco. CO2

gas was extracted from powdered carbonate samples byreacting 10�20 mg of sample with 100% orthosphosphoricacid, under high vacuum, at 25ºC for approximately 24hours. The released CO2 gas was analyzed in a SIRA IIdual inlet, triple collector mass spectrometer, using theBSC reference gas (Borborema skarn calcite) calibratedagainst NBS-18 (carbonatite), NBS-19 (toilet seatlimestone) and NBS-20 (Solenhofen limestone), with acomposition of δ18O = �18.3� PDB and δ13C = �8.6 PDB.Whole-rock chemical analyses were carried out on fusedbeads by XRF.

Palynological macerations of carbonates and peliteswere prepared at the Micropalaeontology laboratoriesof the Facultad de Ciencias (Montevideo). Followingcrushing and digestion of samples (ca. 150 g) withconcentrated HCl, 72% HF was applied for 24 hours tothe silicate/organic residues. After neutralization, boiling,concentrated HCl was used to dissolve fluorides.Remaining organic residues were recovered by meansof a 5 µm sieve, stored in glass flasks and mountedwith glycerin-gelatine on standard glass slides. Microfossilswere determined and counted under a Leica DM LPpolarizing microscope, using both transmitted andreflected light (in the latter case with oil immersionobjectives). Organic matter content of samples wasdetermined semi-quantitatively from the amount oforganic residue obtained by means of palynologicalmaceration described above.

Nature of Isotopic Signatures

A geochemical and petrographic survey of the Polancocarbonates was carried out to determine if the isotopicsignatures are reliable indicators of the primary isotopic

composition of seawater. Sampled carbonates includelimestone, dolostone and limestone/dolostone rhythmiteof the Polanco Formation in the Isla Patrulla Block(Fig. 1). Limestone/dolostone rhythmite is composed ofa millimetric to decimetric alternation of calcarenite/calcisiltite with dolosiltite/ dololutite (Gaucher, 2000).The limestone in the rhythmite is of clastic origin, whereasthe dolostone is of primary origin and probably related tothe activity of sulphate-reducing bacteria (Vasconcelos andMcKenzie, 1997; Warthmann et al., 2000). According toGaucher (2000), the evidences supporting this origin fordolomite in the Polanco Formation are: (1) dolostone isvery fine-grained (dololutite to fine dolosiltite) andconsiderably finer than interbedded calcarenite/calcisiltite(Fig. 4H); (2) dolomite forms pure dolostone layers, oftenshowing millimetric alternations with limestone layers(Fig. 4D, H); (3) they are almost terrigenous-free (Fig.4H); (4) they do not show displacive textures; (5) theiriron carbonate content of up to 5.5 mol %, which is typicalof other Neoproterozoic primary dolomites (Fairchild,1980); (6) dolostone layers are organic-rich and oftenpyritic, supporting anoxic conditions required by thesulphate-reduction model (Vasconcelos and McKenzie,1997; Warthmann et al., 2000); and (7) dolosiltite alsooccurs as rip-up intraclasts in overlying calcarenites,indicating a syngenetic or penecontemporaneous originfor dolomite.

Carbonates in the studied sections meet the criteriaproposed by Jacobsen and Kaufman (1999) to distinguishbetween primary and diagenetic signatures, namely:(1) the sampled micro-domains in the carbonates werechosen after careful petrographic examination, only fromprimary mineral phases; (2) δ18O values >-10� PDB inca. 90% of the considered samples (Table 1); (3) Mn/Srranges between 0.03 and 0.62 (50 analyses), and is thusclearly below the boundary of 2 accepted for unalteredcarbonates (Jacobsen and Kaufman, 1999); and (4) highSr concentrations always in excess of 500 ppm, andreaching 2000 ppm. A near primary nature of the C- andSr-isotopic data is, therefore inferred.

Chemostratigraphy of the Lower ASG

The sections studied were (Figs. 3, 6�7): ArroyoCampanero (Fig. 1, point 1) for the Yerbal Formation;Arroyo Yerbalito Syncline (Fig. 1, point 2), Calera deRecalde Syncline (Fig. 1, point 3) and Cerro EspuelitasSyncline (Fig. 1, point 4) for the Polanco Formation; thestratotype and parastratotype (Gaucher, 2000) of theBarriga Negra Formation (Fig. 1, points 5�6) and the TapesGrande Syncline (Gaucher, 2000; Fig. 1, point 7) for boththe Polanco and Cerro Espuelitas Formations.

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Yerbal Formation

Four analyses of dolostones of the upper YerbalFormation in the Arroyo Campanero section near Minas(Fig. 3) have been reported by Gaucher et al. (2003a;Table 1). These pink dolosiltites underly Cloudina,Waltheria, Titanotheca, Bavlinella and Soldadophycus-bearing siltstones. The dolomite yielded δ13C valuesbetween +1.17 and +2.15� PDB, increasing up-section(Fig. 3). This trend continues into limestone/dolostonerhythmites of the lowermost Polanco Formation (seebelow). Corresponding δ18O values range between �3.79and �5.52� PDB, not showing any co-variance with δ13C.It is worth noting that some 150 m above the dolomites,

oxide-facies BIF with up to 24% magnetite/hematiteoccurs, reaching a thickness of 50 m (Fig. 3, Fig. 4A�C).No glacial features, such as dropstones, have beenobserved in these BIF thus far. The BIF conformably overlybanded siltstones bearing Cloudina riemkeae (Fig. 3). Allfacies are rich in organic matter and show goodpreservation of organic-walled microfossils. Theassemblage is strongly dominated by Soldadophycus bossiiand Bavlinella faveolata (Gaucher et al., 2004).

Polanco Formation

The best studied section of this unit is located inthe northern flank of the Calera de Recalde Syncline(Figs. 5�6, Table 1). There, the contact between the Yerbal

Fig. 3. Correlation chart between the different sections of the lower ASG dealt with in this paper, showing available bio- and chemostratigraphicdata. Symbology is the same as figure 2, except for skeletal fossils (see Fig. 6). Location of sections in figure 1: La Salvaje Anticline: point8 (Gaucher et al., 2004); Arroyo Campanero: point 1; Arroyo Yerbalito Syncline: point 2; Calera de Recalde Syncline: point 3 (see alsoFig. 5). 87Sr/86Sr data shown for sections C and D (lowermost Polanco Formation) taken from Kawashita et al. (1999). Southern sections(A-B) represent deeper shelf deposits, shallowest conditions occurring in the northernmost section (D).

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Fig. 4. Particular lithologies and sedimentary structures of the lower ASG. (A) BIF of the upper Yerbal Formation, as exposed in a shallow explorationtrench. Note fine banding. Length of knife: 9 cm. (B) Close up a polished slab showing a pure chert band (2.5 cm in thickness) in BIF of theupper Yerbal Formation. (C) Polished slab of BIF of the upper Yerbal Formation containing ca. 30% iron oxides. Field of view is 9 cm wide.(D) Limestone-dolostone rhythmites, Polanco Formation. Note dolostone layers (marked �Dol�). Length of scale: 8 cm. (E) Convolutebedding in limestone-dolostone rhythmites, lower Polanco Formation. Knife is 9 cm long. (F) Calcareous tempestite showing well-developed,hummocky and swaley cross stratification, Polanco Formation. Scale is 8 cm long. (G) Basal carbonatic breccias of the Barriga NegraFormation, interbedded with subordinate limestone/dolostone rhythmites and calcarenites (see Fig. 7). Length of hammer: 40 cm. Thebreccias are the main infill of an U-shaped structure cut into the Polanco Formation, that may be interpreted as a paleovalley. (H) Alizarinred-stained thin section of limestone-dolostone rhythmite. Note intercalation of fine-grained dolosiltite layers with calcarenite layers (arrowed)including medium to coarse-grained, clear terrigenous clasts (mainly quartz). Field of view is 2.5 mm wide.

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and Polanco Formations is well exposed, and thecarbonates of the Polanco Formation form excellentoutcrops. The formation begins with a 300 m-thick, basalunit of dark limestone/dolostone rhythmites (Fig. 4D) withsporadic intercalations of thin tempestitic calcarenitesshowing hummocky cross-stratification 15�20 cm inwavelength (unit A, Figs. 5�6). Up section, storm deposits(tempestites sensu Einsele, 2000) become predominantin the next 250 m. The beds are thicker, coarser-grainedand often show amalgamation, specially at the top ofthe package (unit B, Figs. 5�6). Hummocky- and swaley-cross stratification is widespread (Fig. 4F), reachingmaximum wavelengths of 70 cm. A distinct level of black,carbonaceous, laminated calcisiltites follows, ca. 110 min thickness (unit C, Figs. 5�6). Storm deposits arerare and very thin in this level but become predominantagain in the next 50 m. There, distinct sandstone bedsindicate enhanced input from the continent (unit D,Fig. 6). Finally, the top 200 m exposed in the sectioncorrespond to a rapid alternation of rhythmites,tempestites and laminated calcisiltites (unit E, Figs. 5�6).Hummocky cross-stratification with wavelengths of 60 cmoccurs in these storm deposits (Fig. 4F). At the very top,coarse-grained calcarenite showing low-angle cross-stratification is interpreted as beach foreshore deposits(unit F, Fig. 6).

Basal rhythmites of the Polanco Formation in the Calerade Recalde and Arroyo Yerbalito Synclines yielded87Sr/86Sr values of 0.7078, 0.7079, 0.7080 and 0.7082(Fig. 3) according to determinations carried out byKawashita et al. (1999). Since the first two values showhigher Sr concentrations (1380 and 1460 ppmrespectively) than the latter values (580 ppm for both),Kawashita et al. (1999) accept the ratio of 0.7078 as thebest estimate of seawater composition. The sampleyielding this ratio showed also the lowest Rb/Sr ratio(0.0029), which according to Jacobsen and Kaufman(1999) reflects least-altered conditions. The basalcarbonatic rhythmites show δ13C consistently between+2.56 and +2.96� PDB (5 analyses), graduallyincreasing up-section, thus continuing the trend recordedin the upper Yerbal Formation (Figs. 3, 6, Table 1). Peakvalues (P1) of +5.3� PDB are reached 180 m above thecontact with the Yerbal Formation (Fig. 6). An impressivenegative δ13C excursion to �3.3� PDB follows (N2), andcorresponds to unit B, dominated by calcareoustempestites (Fig. 6). Up-section, a positive δ13C-peak of+2.1� PDB (P2) is recorded by black, carbonaceous andlaminated calcisiltite (Fig. 6, Table 1).The return to slightlynegative values (-0.70� PDB) occurs in calcareoustempestites 650 m above the base of the PolancoFormation in the Calera de Recalde Syncline (N3).

Uppermost tempestitic calcarenites and interbeddedrhythmites of unit E show positive δ13C-values of up to+2.8� PDB (P3, Fig. 6). There is a clear trend to negativevalues at the top of the section exposed in the Calera deRecalde Syncline (unit F, Fig. 6).

We carried out seven C- and O-isotopic analyses(Table 1) at the stratotype of the Polanco Formationin the Tapes Grande Syncline (Gaucher, 2000; Fig. 1:point 7), which were combined with previously reporteddeterminations by Boggiani (1998) and Gaucher et al.(2003a). The isotopic signal there shows approximatelythe same peaks described above, but with less amplitudeand shifted towards negative values. This can beinterpreted as a deep-water signal (Calver, 2000), as alsosuggested by sedimentary structures and facies. Gaucher(2000) shows the occurrence of frequent turbidites in theunderlying Yerbal Formation in the same section,confirming the deeper sedimentary environment for thestratotype of the Polanco Formation. The same trendtowards more negative values in deeper-water facies hasbeen shown recently in carbonates of the VendianBloeddrif and Dreigratberg Members of the Gariep Belt,southern Namibia (Frimmel and Fölling, 2004). More Cisotopic determinations are required at the stratotype ofthe Polanco Formation to determine the shape of the curve.Therefore, it is proposed here to locate the chemostratotypeof the Polanco Formation in the Calera de Recalde Syncline(Figs. 5�6), because of the unequivocal preservation andinterpretation of primary isotopic signatures there. Thischemostratotype might be used in the future as a referencesection for the Polanco Formation.

Lower Barriga Negra Formation

In the parastratotype of the Barriga Negra Formation(Fig. 1, point 6; Fig. 7) the contact between the Polancoand Barriga Negra formations is characterized by anegative δ13C-excursion to �1.9� PDB (Fig. 7, Table 1),which continues into the lowermost Barriga NegraFormation (N4). Limestone/dolostone rhythmites andcalcarenites intercalated with the basal breccias of theBarriga Negra Formation (Gaucher, 2000; Fig. 4G) yieldedδ13C-values between �1.44 and �1.74� PDB (Gaucher etal., 2003a; Table 1). In the case of the stratotype of theBarriga Negra Formation (Fig. 1, point 5), a U-shapedstructure cut into the the Polanco limestones and filledwith Barriga Negra breccias and conglomerates (Fig. 4G)is here interpreted as a paleovalley. Moreover, the δ13Ccurve of the upper Polanco Formation is truncated,suggesting a depth of several hundreds of meters for thepalaeovalley, in accordance with preliminary geologicalmaps by the authors. Ongoing research aims to reveal thegeometry of the structure, thus allowing inferences on its

723



genesis. Carbonates become rarer up-section, disappearingcompletely 150 m above the contact with the PolancoFormation. Therefore, no δ13C data are available for themiddle-upper Barriga Negra Formation. An impressiveregression is recorded in that part of the section, leadingto cannibalization of the ASG-deposits and redepositionin stream-dominated alluvial fans (Gaucher, 2000).

Lower Cerro Espuelitas Formation

Only a few isotopic analyses are available for the lower/middle Cerro Espuelitas Formation (Table 1), where onlysporadic carbonate beds are intercalated. Thesepreliminary studies were carried out in the CerroEspuelitas Syncline (Fig. 1, point 4) and the Tapes GrandeSyncline (Fig. 1, point 7). Limestones and rhythmites in

Fig. 5. Geological map of the Calera de Recalde Syncline (see Fig. 1,point 3 for location). Sections AA� and CC� are dessignated aschemostratotype of the Polanco Formation. The correspondingstratigraphic column is shown in figure 4.

the lower Cerro Espuelitas Formation yielded negativeδ13C-values of �2.5� PDB (Table 1), thus confirming theexistence of a further negative excursion (N4) at thetransition from the Polanco to the Cerro Espuelitas/BarrigaNegra Formation (its lateral equivalent). Black carbonatesinterbedded in the lower-middle Cerro EspuelitasFormation at its stratotype yielded δ13C values of +2.4�PDB, probably indicating a further positive excursion (Fig.2, Table 1).

Biostratigraphy

In the Calera de Recalde Syncline (Figs. 5�6), 15samples of organic-rich carbonates of the PolancoFormation, whose C- and O- isotopic compositons weredetermined, were prepared using standard palynologicaltechniques to recover organic-walled microfossils(Table 1). This procedure allowed to directly compare theisotopic curves with biostratigraphic data, such asmicrofossil diversity (Fig. 6). Of the analyzed samples, 10were found to contain palynomorphs. Facies sampled weremostly fine, organic-rich rhythmites and laminated, darkcalcisiltites for all intervals, so that preservational biasescan be ruled out.

Organic-walled microfossil species occurring in thedifferent units of the Polanco Formation in the studiedsection (Fig. 6) are indicated in table 2.

Most of the mentioned species are illustrated byGaucher (2000) and Gaucher et al. (2004). As for theupper Yerbal Formation in the studied region, the speciesBavlinella faveolata Schepeleva (Vidal, 1976),Soldadophycus bossii Gaucher et al. (1996) andSoldadophycus major Gaucher (2000) are reported byGaucher (2000).

Considering the determined taxa for each of the units,a palynomorph diversity-curve was prepared for thesection exposed in the Calera de Recalde Syncline(Fig. 6). Samples containing no fossils are indicated, butwere not used to draw the curve, because absence offossils more likely indicates nonpreservation ratherthan nonexistence. Comparing the diversity curve withthe δ13C-curve, the following conclusions can be drawn:(1) the assemblages preserved in the upper Yerbal andPolanco Formations in the studied section are allcharacterized by low diversity (maximum 5 species);(2) diversity peaks of 3 to 5 species occur concomitantlywith positive δ13C peaks, as in the case of P2, P3 and possiblyP1; and (3) negative δ13C-intervals yield very low-diversityassemblages.

Finally, an assemblage containing at least 5 species ofskeletal fossils occurs in the uppermost Yerbal Formation,within δ13C-peak P1, (Gaucher and Sprechmann, 1999;

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C. GAUCHER ET AL.

Fig. 6. Detailed stratigraphic column of the uppermost Yerbal and Polanco Formations along sections AA� to DD� of the Calera de Recalde Syncline(Fig. 3), and corresponding δ13C-data (Table 1). Relative organic matter content (low, medium, medium-high, high and very high) andorganic-walled microfossil diversity is shown. For the latter, open squares represent afossiliferous samples. Criteria for the determination ofpalaeobathymetry are explained in the text.

Gaucher, 2000). The most important components of thisassemblage are Cloudina riemkeae Germs (1972),Waltheria marburgensis Gaucher and Sprechmann (1999),and Titanotheca riemkeae Gaucher and Sprechmann(1999). The carbon-isotopic composition of Cloudina shellscannot be determined, because the fossils are largelyreplaced by hematite (Gaucher and Sprechmann, 1999)

Discussion

Geochronologic aspects

The least-altered 87Sr/86Sr-value of 0.7078 reported byKawashita et al. (1999) corresponds, according toJacobsen and Kaufman (1999) and Walter et al. (2000),to an age of 580 Ma, which is taken here as a good estimate

725



A, C and E, Fig. 6; Fig. 4D�E) show predominantly positivevalues and yield higher diversity microfossil-assemblages,whereas intervals dominated by storm deposits (units B,D and F; Fig. 4F) are characterized by 13C-depletedcarbonates, less organic matter and very low diversity(Fig. 6). Sedimentary structures allow a rough

Table 2. Distribution and relative abundance of organic-walledmicrofossils in the different units of the Polanco Formation inthe Calera de Recalde Syncline (see Fig. 6).

Organic-walled UNIT UNIT UNIT UNIT UNITmicrofossil species A B C D E

Soldadophycus bossiiGaucher et al. (1996) o o o - -Soldadophycus majorGaucher (2000) x o o - oGlenobotrydion aenigmatisSchopf (1968) - o o - oMyxococcoides sp. - - o - -Lophosphaeridium montañaeGaucher (2000) - - x - -Branched, cyanobacterial (?)sheaths (Gaucher, 2000:pl. 10.4) - - o - oSpherical, dense microfossils(bacterial colonies ?) o - o - -Vendotaenid fragments(undetermined) - - - - x

-: Absent x: Rare o: Common

Fig. 7. Stratigraphic column of the parastratotype of the Barriga NegraFormation (Fig. 1, point 6), showing C isotopic data. Symbologyis the same as for figure 2. Note negative δ13C values at thePolanco-Barriga Negra transition. Substantially modified fromGaucher (2000).

of the absolute age of the basal Polanco Formation. Thiscorroborates the biostratigraphic data (Cloudina andBavlinella-Soldadophycus assemblage), which indicate apost-Varangerian/Marinoan (600�590 Ma) age for thewhole lower ASG.

As for the C and O isotopic data, the C-isotopic profilefor the upper Yerbal and Polanco Formations is reasonablywell-determined and consistent. Despite poor absolute agecontrol of the available global δ13C-curves (Jacobsen andKaufman, 1999; Walter et al., 2000), the upper Yerbal-lower Polanco positive excursion (+5.3� PDB) matchesa global positive excursion around 580 Ma, corroboratingthe Sr-isotopic data mentioned above. The N2 peak of themiddle Polanco Formation (-3.3� PDB) matches anegative excursion at 573 Ma in the global curve (Jacobsenand Kaufman 1999, Walter et al., 2000). Peaks P2 to N4 ofthe upper Polanco Formation and lowermost BarrigaNegra-Cerro Espuelitas formations, probably correspondto the 570�550 Ma interval in the curve of Walter et al.(2000), although the upper date is still uncertain. It isnoteworthy that the curve presented here for the PolancoFormation, despite rather coarse sampling, shows morepeaks than most of the curves reported for the period,and could be due to a more detailed record thanks to thegreat thickness and almost pure carbonates typical of theunit.

Palaeobathymetry and C-isotopes

The relationship between C-isotopic composition,microfossil diversity and facies of the Polanco Formationin the Calera de Recalde Syncline is evident. The finer,more dolomitic and organic-carbon rich lithologies (units

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C. GAUCHER ET AL.

palaeobathymetric estimation, within the following depthranges: (a) from 0 (beach deposits) to fair weather wave-base (ca. 0�20 m), (b) between fair weather and stormwave-base (ca. 20�50 m), and (c) deeper than storm wavebase (>50 m; Fig. 6). The palaeobathymetric criteria usedwere mainly the presence/absence of tempestites (Fig. 4F),their thickness, hummocky cross-stratification wavelength(Ito et al., 2001), occurrence of amalgamated tempestites(Einsele, 2000), grain-size and other sedimentarystructures. The Polanco Formation as a whole is ashallowing-upward sequence, as first stated by Gaucher(2000), but higher-frequency sea-level oscillations areoverprinted in the general trend. A close comparison ofthe resulting palaeobathymetric curve with the δ13C profileshows that periods of high sea-level correspond to positiveδ13C peaks (units A, C and E, Fig. 6), while regressions areclosely associated with negative peaks (units B, D and F,Fig. 6). Due to the general trend towards shallowerenvironments up-section, unit E is composed at its top ofstorm deposits but shows positive δ13C-values, in contrastto even shallower overlying calcarenites (unit F), whichare 13C-depleted.

The described relationships between palaeobathymetryand δ13C could be ascribed to carbon fractionation in thewater column, as already reported for Neoproterozoiccarbonates by Calver (2000) and Frimmel and Fölling(2004). Nevertheless, the amplitude of the δ13C-oscillationsin the ASG (up to 9� PDB) is far greater than bathymetricfractionation reported for modern oceans. An enrichmentin shallow marine waters of 1� PDB relative to the O2-minimum layer, at ca. 1 km depth, has been measured inmodern oceans (Hoefs, 1997). Furthermore, in the PolancoFormation the negative values are found in the shallowerdeposits, while positive values correspond to deepersettings (Fig. 6), just opposite to the expected water-column C-fractionation (Hoefs, 1997; Calver, 2000).Therefore, an explanation in terms of climatically-induced,eustatic sea-level oscillations coupled with δ13C-excursionsis prefered here (see below).

Palaeoclimatology

It is well known that carbonates associated withNeoproterozoic glacigenic rocks are strongly 13C-depleted(Kaufman et al., 1991; Kaufman and Knoll, 1995; Kennedy,1996; Kaufman et al., 1997; Hoffman et al., 1998;Jacobsen and Kaufman, 1999; Walter et al., 2000; Frimmelet al., 2002; Hoffman and Schrag, 2002). Theseobservations are in line with evidence presented here, thatthe palaeobathymetry-oscillations are the result ofclimatic-driven, eustatic sea-level oscillations, whichoccurred concomitantly with δ13C-excursions. The higherabundance of tempestites in the negative δ13C-intervals

cannot be ascribed to a higher storm frequency or intensityin the basin during these periods. Highest tempestitefrequency, hummocky cross-stratification wavelength andtherefore intensity of storm waves have been shown tooccur in periods of enhanced greenhouse effect and highsea-level stand (Ito et al., 2001). This is not the case ofunits B and D of the Polanco Formation, allowing aninterpretation of these tempestite-dominated levels asregressive periods, in which palaeobathymetry wasshallower than storm wave-base.

No glacial rocks occur in the δ13C-negative intervals ofthe Polanco Formation or elsewhere in the ASG, probablydue to a tropical setting (Gaucher et al., 2003a). An openquestion is whether the above-mentioned palaeovalleys,cut into the Polanco Formation and filled with BarrigaNegra breccias and conglomerates (Fig. 4G), representan erosive glacial feature or not. Still, the co-variation ofδ13C and palaeobathymetry in the manner described beforestrongly supports the existence of at least three coldperiods that caused moderate to strong sea-level fall, allof them recorded in the Polanco to lower Barriga NegraFormations. Carbonate deposition continued in thePolanco carbonate ramp during cold periods, but thismight be the result of a tropical setting as well. Sayloret al. (1998) suggested that high-frequency, high-amplitude sea-level oscillations recorded in theSchwarzrand Subgroup of the Nama Group in Namibiacould be related to glacioeustasy. Important climatically-induced sea-level changes have also been reported for theupper Gariep Supergroup in southwestern Africa (Frimmelet al., 2002).

Microfossil-diversity and organic-matter concentrationshed some light on the causes of these oscillations.Horizons characterized by positive δ13C values contain,without exception, more organic matter and relativelymore diverse microfossil-assemblages (Fig. 6). Therefore,it follows that enhanced bioproductivity was the cause ofCO2 drawdown through enhanced 12C-burial, followed byglobal cooling and sea-level fall (Fig. 8), as first postulatedby Kaufman et al. (1997). Bidigare et al. (1999) showthat induced iron-fertilization of the equatorial Pacific ledto a sevenfold increase in the export of particulate organiccarbon to the sediments, and a δ13C positive shift of7� PDB. Upwelling of iron-rich waters was indeedimportant in the positive peaks P1 and P4 of the ASG(Figs. 2�3, 6), as recorded by BIF occurring within theseintervals in the Yerbal and Cerro Espuelitas formations(Figs. 2�3, 4A�C).

Kaufman et al. (1997) first suggested that enhancedorganic carbon burial might have facilitated glacial growthby reducing greenhouse capacity. Gaucher (2000) andKaufman (2000) independently postulated a model basedon CO2 drawdown caused by enhanced bioproductivity

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driven by high nutrient availability to explainNeoproterozoic glaciations. This model also helps toexplain the alternation of high-bioproductivity, positiveδ13C-periods, with lower bioproductivity, negative δ13Cstages recording eustatic lowering of sea-level (Gaucheret al., 2003a). This greenhouse-icehouse alternation canbe summarized in the following stages (Fig. 8):

(1) Enhanced upwelling of nutrient-rich waters (Fe, P, N,trace elements) stored in the deep-ocean reservoir triggermassive phytoplankton blooms (Palacios, 1989; Vidal andNystuen, 1990; Gaucher, 2000), deposition of BIF andphosphorites (the latter not in the ASG). Upwelling zonesrepresent only 0.1% of the present ocean surface (Baturin,1982), but this need not be constant through geologictime. The ocean becomes largely eutrophic, and largeamounts of organic matter (13C-depleted) accumulate onthe shelves. Carbonates deposited in this stage are,therefore, 13C-enriched (Fig. 8a). Sulfate-reducing bacteriafind optimal conditions in the anoxic, lower water-layer,leading to anomalous sulfate consumption, primarydolomite deposition (Vasconcelos and McKenzie, 1997;Warthmann et al., 2000) and shift in δ34S towardsextremely positive values (Walter et al., 2000; Kaufman,2000). During this stage, progressive sequestering ofatmospheric CO2 by blooming phytoplankton populationstakes place, reducing greenhouse effect (Fig. 8a).

(2) After reaching atmospheric CO2 thresholdconcentrations of pCO2~10-4 to 10-5 bar (Hoffman andSchrag, 2002), glaciation to latitudes of 45�30° andresultant sea-level fall takes place (Fig. 8b). Except for acircum-equatorial zone, the ocean and most continentsare covered with thick pack-ice, greatly reducing primarybioproductivity, silicate weathering and carbonatedeposition. δ13C of carbonates shift to negative values dueto collapse of primary productivity and continuing inputof light carbon from volcanic outgassing. The absence ofglacials in the Polanco Formation associated with thenegative peaks indicates that no runaway ice-albedofeedback took place, as postulated for the more severeSturtian and Varangerian/ Marinoan glaciations (Hoffmanand Schrag, 2002). The observed events in the PolancoFormation closely resemble computer simulations of near-snowball conditions by Hyde et al. (2000).

(3) After hundreds of thousands to millions of years oficehouse-conditions, the accumulation of atmospheric CO2

from volcanic outgassing restores greenhouse effect andleads to global warming, ice melting and sea-level rise(Fig. 8c). Phytoplankton populations re-colonize theoceans in an opportunistic way. Plankton blooms occur,fueled by abundant nutrients delivered by upwelling ofdeep waters and continental runoff after a period ofintense chemical weathering (Fig. 8c), starting the wholeprocess from the beginning.

Fig. 8. Palaeoclimatic-palaeoceanographic model for the observedrelationships between δ13C, sea-level, organic matter contentand plankton diversity in the ASG. A: upwelling of deep,nutrient-saturated waters cause phytoplankton blooms, CO2drawdown and deposition of heavy (δ13C-positive) carbonates.Note concomitant BIF deposition. B: low CO2 levels causeglaciation at mid to high latitudes, sea-level drop, and reductionof phytoplankton productivity. Due to diminishedbioproductivity, silicate weathering and carbonate deposition,CO2 from volcanic outgassing accumulates. Carbonatesdeposited at this stage are δ13C-negative. C: CO2-accumulationand subsequent return to greenhouse conditions leads to icemelting, sea-level rise and deposition of heavy carbonates. Highnutrient input through upwelling and enhanced chemicalweathering triggers phytoplankton blooms, starting the wholeprocess from the beginning.

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We envisage that the system would oscillate betweengreenhouse and icehouse conditions provided thecontinuous availability of a deep-ocean nutrient (P, Fe, N)reservoir. Enhanced hydrothermal activity related to therifting of Rodinia mantained this reservoir until the EarlyCambrian, as shown by 87Sr/86Sr-relationships ofNeoproterozoic carbonates (Walter et al., 2000).

Judging from an estimated sea-level fall of 50 to 100 mduring the greatest regressive (δ13C-negative) stages inthe Polanco Formation, we conclude that ice volume musthave been comparable to that of Pleistocene glaciations(Van Andel, 2000), although a very differentpalaeogeography and even seawater-composition preventthe use of these glaciations as a �modern� analogue. It isworth noting that upper Vendian (ca. 590�543 Ma) tolowermost Cambrian glaciations have been proposed fora number of basins in W-Gondwana, such as the TaoudenniBasin in Algeria (Bertrand-Sarfati et al., 1995) and theNama Basin in Namibia (Vingerbreek glaciation: Germs,1995; Saylor et al., 1998).

Conclusions

Biostratigraphy, Sr and C isotopes indicate an upperVendian (Valdaian) age for the lower ASG. The onset ofcarbonate deposition in the Polanco Formation isestimated at 580 Ma by comparison with existing globalisotopic curves.

Positive δ13C excursions are associated with high sea-level stand, high organic-carbon content and highermicrofossil diversity in the Polanco Formation. BIFdeposition also occurred during positive δ13C intervals inthe ASG. Negative δ13C excursions are recorded incarbonates with less organic matter, less microfossildiversity and are always associated to regressive events.These results strongly support palaeoclimatic modelswhich postulate that enhanced bioproductivity due tohigher availability of nutrients (P, N, Fe) was the key factorcontrolling Neoproterozoic glaciations.

It is proposed that many glacial events took place inthe upper Vendian (Valdaian), which were not so severeas the Sturtian and Varangerian/Marinoan events. Theupper Vendian glacial events were not global, but restrictedto higher latitudes. At low latitudes, their effects weresea-level fall and less bioproductivity. The amplitude ofobserved sea-level oscillations suggest that ice volume athigh latitudes must have been comparable to Pleistoceneglaciations. A correlation of the upper Vendian-lowermostCambrian negative δ13C excursions with proposed glacialhorizons of the same age in the Nama Group (Vingerbreekmixtite), Taoudenni Basin and elsewhere seems probable.These post-Varangerian glaciations have profoundimplications for the stratigraphy, palaeoclimatology and

palaeobiology of the terminal Neoproterozoic, suggestingthat palaeoclimatological factors played a determinant rolein the advent and diversification of the Metazoa.

Acknowledgments

The authors are indebted to Gerard J.B. Germs, whoactively participated in field work and contributed withstimulating ideas. Constructive and helpful commentswere provided by the reviewers, Hartwig Frimmel andAlan Jay Kaufman. Field work and publication expenseswere financed by research project 6007 of the ConsejoNacional de Investigación Científica y Tecnológica(CONICYT), Uruguay, and research projects C-32 and C-39 of the Comisión Sectorial de Investigación Científica(CSIC, Uruguay).

This paper is a contribution to projects IGCP 478(Neoproterozoic-Early Palaeozoic events in SW-Gondwana), IGCP 493 (The rise and fall of the Vendianbiota) and IGCP 450 (Proterozoic sediment-hosted basemetal deposits of western Gondwana).

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Chemostratigraphy of the Lower Arroyo del Soldado Group (Vendian, Uruguay) and Palaeoclimatic...

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