doi:10.1144/SP294.18 2008; v. 294; p. 343-364 Geological Society, London, Special Publications

 P. J. Pazos, L. S. Bettucci and J. Loureiro  

critical reappraisalThe Neoproterozoic glacial record in the Río de la Plata Craton: a 

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The Neoproterozoic glacial record in the Rıo de la Plata Craton:

a critical reappraisal

P. J. PAZOS1, L. S. BETTUCCI2 & J. LOUREIRO2

1CONICET-UBA, Dpto. de Ciencias Geologicas, Fac. Cs. Ex. y Nat., Ciudad Universitaria

Pabellon II (1428), Ciudad Autonoma de Buenos Aires, Argentina

(e-mail: pazos@gl.fcen.uba.ar)2Dpto. de Geologıa, Facultad de Ciencias, Universidad de la Republica, Igua 4225 (11400),

Montevideo, Uruguay

Abstract: Neoproterozoic glacial successions have been described in South America, but theglacial deposits of the Rıo de la Plata Craton have been neglected in previous studies addressingthe global distribution of glacially influenced successions. The Rıo de la Plata Craton containsNeoproterozoic glacial deposits in the Sierra del Volcan Formation (Tandilia System, Argentina),glacial influenced deposits in the Playa Hermosa and Zanja del Tigre formations (Dom FelicianoBelt, Uruguay) and suspected glacially influenced deposits in Passo da Areia (Sao Gabriel block,Brazil). The Tandilia System glacial record includes diamictites, dropstones and rhythmites depos-ited in glaciomarine conditions in a tectonically stable depositional setting. The Dom FelicianoBelt includes a thin section with ice-rafted clasts in carbonates and a thicker section containingdiamictites, rhythmites, outsized clasts and deformed beds in a volcano-sedimentary succession.The Sao Gabriel block occurrence deserves more attention to confirm any glacial influencein the fine-grained part of the succession. Glaciation is considered to be contemporaneous withthe Gaskiers glaciation (580 Ma), with the exception of the carbonates with dropstones thatmay represent a previous event correlative with one of the glaciations described in the KalahariCraton, prior to Kalahari–Rıo de la Plata assembly in the proto-western Gondwana margin.

The Precambrian is one of the most enigmatic timesin Earth history, involving drastic changes in thebiosphere, mostly documented in the last part ofthe Neoproterozoic. This was a time when dramaticfluctuations in climate, sea-water composition, con-tinent assembly, interaction between tectonism andclimate, and life diversification took place (e.g.,Brasier & Shields 2000; Butterfield 2000; Evans2000; Hurtgen et al. 2002; Bowring et al. 2003;Eyles & Januszczak 2004; Allen & Hoffman2005; Halverson et al. 2005). Sedimentary succes-sions archive crucial information for the evaluationof the magnitude and pattern of such transform-ations. For instance, the Neoproterozoic rockrecord contains significant diamictite intervals thathave traditionally been interpreted as evidence fortwo severe global glaciations, informally knownas Sturtian and Marinoan (Hambrey & Harland1985; Hoffman et al. 1998), but a younger and prob-ably more geographically restricted glaciationtermed Gaskiers has also been well established(see Halverson et al. 2005 for references).However, the nature, timing and correlation ofthese ‘global’ glaciations are still the subject ofintense debate, and other Neoproterozoic glacia-tions cannot be completely ruled out. Someinterpretations suggest general synchronicity and

glaciation extending into low latitudes, offering apicture of the Earth’s surface that was totallyfrozen with complete shut-down of the hydrologicalcycle and life crisis (the Snowball Earth hypoth-esis); the aftermath was a rapid change to green-house conditions (Hoffman et al. 1998). Criticismof the hypothesis suggests that global correlationof glaciations requires more detailed age constraintsto corroborate the fundamental idea of a quasi-synchronous record. For instance, new Re–Osages from Australia indicate a long-lived Sturtianglaciation (711–643 Ma), or diachronism notmuch different from that of the Pleistocene glacia-tion (Kendall et al. 2006). Other papers questionthe true nature of the assumed glacial deposits.Some of the best known glacial diamictites havebeen interpreted as meteoritic impact eject (e.g.,Rampino 1994). Alternatively, subaqueous debrisflow, sometimes not related to glacial influence atall but triggered by tectonic instability in synchro-nous mega-rift ‘zipper-rift’ basins, has beensuggested for deposits previously interpreted astillites (e.g., Eyles & Januszczak 2004), althoughsuch synchronous rift evolution has been provenmistaken by Fanning & Link (2004). Icebergcirculation (e.g., Condon et al. 2002) and multipleglacial advances and retreats indicate incomplete

From: PANKHURST, R. J., TROUW, R. A. J., BRITO NEVES, B. B. & DE WIT, M. J. (eds) West Gondwana:Pre-Cenozoic Correlations Across the South Atlantic Region. Geological Society, London, Special Publications,294, 343–364. DOI: 10.1144/SP294.18 0305-8719/08/$15.00 # The Geological Society of London 2008.

shut-down in the hydrological cycle (e.g., Leatheret al. 2002), contrary to the Snowball Earth hypoth-esis. Interestingly, a low-latitude glacial record israrely questioned (see Evans 2000) and has been con-firmed for Dobrzinski et al. (2005) for the Sturtian andMarinoan deposits in the Yangtze platform of SouthChina. Such uncertainty about climate evolutionduring the Neoproterozoic explains the re-invigoratedinvestigation of sedimentological aspects of diamic-tites with a supposed glacial origin (e.g., Arnaud &Eyles 2002, 2006; Arnaud 2004), the palaeomagnet-ism of glaciated intervals (e.g., Li et al. 2004;Macouin et al. 2004), U–Pb dating (e.g., Brasieret al. 2000; Lund et al. 2003; Fanning & Link 2004;Hoffmann et al. 2004; Condon et al. 2005), Re–Osdating (Kendall et al. 2006), geochemistry andchemostratigraphy (e.g., Kennedy et al. 1998;Gorjan et al. 2000; Alvarenga et al. 2004; Gaucheret al. 2005a, b; Halverson et al. 2005), tectonic andpalaeogeographic correlations (Veevers 2004, 2005)and palaeobiological approaches (Corsetti et al.2006), among many others.

South America contributes to the database of theNeoproterozoic glacial record with undisputedglacial or glacially influenced successions, mainlyin Brazil. The best known and studied glacial suc-cessions are located in the Sao Francisco Craton(Fig. 1). Their sedimentary record includes diamic-tites and striated pavements assigned to the Sturtianglacial event (Martins Neto et al. 2001; Pedrosa-Soares et al. 2001), but other glacial deposits arealso distributed in tectonically deformed beltsbetween cratonic blocks (e.g., Cukrov et al. 2005).The younger glacial deposits were assigned to theMarinoan glaciation and restricted to the southernpart of the Amazon Craton and Paraguay Belt(Alvarenga & Trompette 1992), but have beenrecently re-interpreted and assigned to theyounger (580 Ma) Gaskiers glaciation by Nogueiraet al. (2003). The Rıo de la Plata Craton is tradition-ally considered to be devoid of equivalent glacialdeposits, even though glaciomarine deposits aredocumented at its northern border (CorumbaBasin, Alvarenga & Trompette 1992). But theglacial deposits of the Corumba Basin, those docu-mented in the Tandilia System by Spalletti & delValle (1984) and in southern Brazil by Eerola(1995) were not considered by Eyles & Januszczak(2004) and the glacially influenced origin of someNeoproterozoic units in the Rıo de la Plata Cratonhas been not mentioned in discussions of thetectono-sedimentary and climatic evolution of theeastern margin of this craton during the Neoproter-ozoic (e.g., Gaucher et al. 2003, 2005a). In conse-quence a ‘climatic window’ between Namibianand Brazilian records has been suggested toexplain the absence of glacial record in the Rıo dela Plata Craton. However, in the last decade

further glacially influenced deposits have beendocumented (e.g., Pazos et al. 2003), confirmingthat this craton, like the neighbouring ones, was atleast partially glaciated during the Neoproterozoic.Different palaeomagnetic reconstructions for theNeoproterozoic locate the Rıo de la Plata Cratonin either mid-high or lower latitudes (see Meert &Torsvik 2004). Sanchez Bettucci & Rapalini(2002) suggested a low to intermediate palaeolati-tude and a congruent polar wander path at 600 Mabased on preliminary results from the Neoprotero-zoic Playa Hermosa Formation in southeasternUruguay. Uncertainties in the stratigraphic positionand sedimentary features of glacially related depos-its within the sedimentary cover of the Rıo de laPlata Craton, as well as their correlation withother documented glacial deposits in West Gond-wana, are the focus of this paper.

Rıo de la Plata Craton

The assembly of the western margin of Gondwanawas a progressive process that included several col-lisions between large blocks (cratons), but alsosmall and complex micro-blocks, during theclosure of ancient oceanic basins. However, theexact extent of each craton is not well established,e.g., the Rıo de la Plata Craton includes igneousand metamorphic rocks with Palaeoproterozoicages (2.2–1.6 Ga), with no Grenvillian ages(Cordani et al. 2000), distributed in outcrops inUruguay and Argentina. Grenvillian ages between1.0 and 0.9 Ga were obtained in the Punta delEste terrane by U–Pb (ID-TIMS, Preciozzi et al.1999, 2003), in rocks that were strongly reworkedduring the Brasiliano orogeny. According toRapela et al. (2007), the western boundary of theRıo de la Plata Craton is against the Pampean (Cam-brian) rocks of the Eastern Sierras Pampeanas.Suggested results of collision of the Pampean Beltwith the Rıo de la Plata Craton include magmatismand metamorphism of latest Neoproterozoic toEarly Cambrian age (Escayola et al. 2007). Thenorthern extent of the Rıo de la Plata Craton isspeculative: Alkmim et al. (2001) interpreted theParaguay fold belt as an ancient basin closedduring collision of the Rıo de la Plata Craton withAmazonia in the late Precambrian–Early Cam-brian. However, a recent proposal by Trindadeet al. (2006) is that the main collisional intervalwas Cambrian (530–525 Ma). The eastern marginis also a matter for debate, particularly given thecomplex tectonic interpretations for the geologicalevolution of southeastern Brazil and Uruguay.A complex nomenclature of tectono-stratigraphicdomains divides the Precambrian geologyof Uruguay and southern Brazil into a puzzle of

P. J. PAZOS ET AL.344

blocks, termed ‘terranes’ that collided diachro-nously. In Uruguay, at least four terranes are recog-nized (Fig. 2). One of them, the Dom Feliciano Beltlocated to the east of Nico Perez terrane, containsthe glacial sections discussed in this paper. Bossi& Gaucher (2004) speculated that the Nico Perezterrane was a block attached to a northern extensionof the Rıo de la Plata Craton (the Piedra Altaterrane) in the Mesoproterozoic. Another exoticblock (Punta del Este terrane) is situated to theeast of Sierra Ballena shear zone (Fig. 2) and, infact, is part of the Kalahari Craton (Preciozzi

et al. 2003), which collided with the Rıo de laPlata Craton during the latest Precambrian–EarlyCambrian (Frimmel & Basei 2006). The Alferez-Cordillera shear zone, with a NE-SW to east–west trend, is interpreted as the expression of thesuture of that collision (Basei et al. 2005). Thesinistral Sierra Ballena mega-shear zone (Fig. 2)has a NE–SW orientation and was correlated withthe Purros shear zone of the Kaoko Belt ofNamibia by Oyhantcabal (2005). In southernBrazil other exotic blocks (e.g., Sao Gabrielblock, Luis Alves) were accreted during the

1

Congo (Zaire)São Francisco

Craton

Rio de la PlataCraton Kalahari

Craton

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ICE SHEET

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3

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Fig. 1. Distribution of Neoproterozoic glacial deposits, cratons and orogenic belts of West Gondwana (modifiedfrom Alvarenga & Trompette 1992 and Pazos et al. 2003): 1, tillites (s.s) of the northern part of the Taudeni Basinincluding those of the Adrar and Mauritania; 2, glacial rocks of Mali and Senegal boundary (Kayes area) and theMali and Guinea boundary; 3, glacio-marine deposits of southwestern Mali; 4, tillites (s.s) of the northern Volta Basin;5, glacial deposits of the Pan-African Dahomeyides; 6–7, glacio-marine deposits and diamictites of the Brasiliano–Paraguay Belt and southern border of the Amazon Craton; 8, Ghaub Formation (diamictites and dolomite) and NamaGroup; 9, diamictites of (a) Sierra del Volcan Formation, (b) Playa Hermosa Formation, and (c) Passo da Areiasequence; 10, Jequitaı Formation, glacio-marine deposits (Sao Francisco Craton); 11, Congo Craton, Otavi Group.

NEOPROTEROZOIC GLACIATION IN ARGENTINA 345

Cryogenian and Ediacaran (Chemale 2000;Campanha et al. 2005), but the complete tectonicevolution of the craton is still far from beingcompletely understood.

Magmatism and deformation involved in thedifferent stages of amalgamation correspond tothe Brasiliano/Pan-African cycle (650–530 Ma).According to Campos Neto & Figueredo (1995),this comprises an older or classical Brasiliano mag-matic event (650–600 Ma) and a younger, or ‘RıoDoce’ event (590–530). The tectonic evolutionand assembly envisaged by Alkmim et al. (2001)implies an earlier collision between the Rıo de laPlata and Sao Francisco cratons at 0.75 Ga. Later,movement between these cratons generated theDom Feliciano Belt, with subsequent collision and

incorporation of minor blocks. The accretion ofPampia marks the last pre-Ordovician collisionevent in the western margin; other terranes, suchas Cuyania and Chilenia, were annexed duringOrdovician and Devonian–Carboniferous, repre-senting the final accretionary phase of the westernmargin of Gondwana (Astini 2003).

The glacial record

Tandilia System

A synthesis of the sedimentology and palaeontol-ogy of the Tandilia System (Fig. 3a) has recentlybeen presented by Poire et al. (2003). The

Fig. 2. Geological sketch of the eastern margin of the Rıo de la Plata Craton and location of the study areas.SYSZ, Sarandı del Yı Shear Zone; SBSZ, Sierra Ballena Shear Zone. Studied units indicated: a, Sierra del VolcanFm; b, Playa Hermosa Fm; c, Zanja del Tigre Fm; d, Paso de Areia sequence.

P. J. PAZOS ET AL.346

sedimentary succession includes limestones,quartzites, shales, abundant stromatolites andbanded-iron levels that comprise the lithostrati-graphic units of the Sierras Bayas Group. Thisgroup is 175 m thick and contains at the base a dolo-mitic succession (Villa Monica Formation) attribu-ted to the Cryogenian (700–900 Ma) on the basis ofdiagenetic evolution and stromatolites (see GomezPeral et al. 2003; Poire & Spalletti 2005). It isunconformably overlain by other units that haveless diagenetic overprint, suggesting a considerabletime lapse in the unconformity. The Sierras BayasGroup is unconformably overlain by the CerroNegro Formation, which includes tide-dominateddeposits containing acritarchs and possible Clou-dina shells that confirm an Ediacaran age (Poire &Spalletti 2005). However, according to Gaucheret al. (2005b), the Sierras Bayas Group is Ediacaranbased on palynomorphs, presence of Cloudina and

C and Sr-chemostratigraphy. These authorssuggest correlation of the Cerro Largo, LomaNegra and Cerro Negro formations (Fig. 4) withthe lower part of the Arroyo del Soldado Group,but also suggest an Ediacaran age for the VillaMonica Formation based only on the low diversityof acritarchs. Although low diversity could be anartefact of preservation or a response to palaeoen-vironmental controls in tidal and marginal marinefacies. This new interpretation contradicts previousisotopic ages of 800 Ma for pelites of the VillaMonica Formation (e.g., Rb–Sr, Cingolani & Bon-homme 1982) and is difficult to reconcile with theobserved diagenetic differences and the stromato-lite stratigraphy that suggest 800–900 Ma (Poire2002). The Cambro-Ordovician Balcarce For-mation (Poire et al. 2003; Poire & Spalletti 2005)(pre-Late Ordovician according to the age of thedykes that cut the unit, Rapela et al. 1974)

Fig. 3. (a) Location of the studied area. (b) Logged sections at the Del Volcan and La Vigilancia hills. Taken fromSpalletti & del Valle (1984).

NEOPROTEROZOIC GLACIATION IN ARGENTINA 347

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unconformably overlies the Sierras Bayas Groupand other units which do not form part of thatgroup, such as the diamictitic Sierra del VolcanFormation and the subsurface Punta Mogotes For-mation. The glacial-related Sierra del VolcanFormation (Fig. 3b) itself unconformably overliesthe Palaeoproterozoic to Mesoproterozoicigneous–metamorphic Buenos Aires Complex(Spalletti & del Valle 1984), but by lithostrati-graphic correlation it has been considered younger(Ediacaran–Cambrian) than the Sierras BayasGroup; unfortunately, these units are neverexposed in the same section, making interpretationuncertain. One of the elements taken into accountby Spalletti & del Valle (1984) for suggesting apost-Sierras Bayas Group age included the differentgrade and type of alteration of the basement com-pared with the saprolitization that preceded depo-sition of the Sierras Bayas Group, implying atleast two stages of palaeo-weathering prior to depo-sition of the Sierra del Volcan diamictites. Thesefeatures developed under different climate con-ditions preclude any correlation with the basalunit of the group. The other feature mentioned bySpalletti & del Valle (1984) is the presence ofunmetamorphosed quartzitic, outsized blocks inthe diamictites, seemingly precluding provenancefrom the Sierras Bayas Group. However, GomezPeral et al. (2007) concluded that the SierrasBayas Group is unmetamorphosed, so that theexotic blocks could indeed be sourced from thequartzites, as in the new interpretation of Poire &Spalletti (2005). The Sierras Bayas Group includestidal and wave deposits, karstic surfaces andinternal unconformities, evidencing shallowmarine environmental deposition through severalsedimentary cycles (see Andreis et al. 1992). TheArroyo del Soldado Group is a succession morethan 3000 m thick that, according to Gaucheret al. (2003), spanned 580–530 Ma based on paly-nomorphs, skeletal fossils and chemostratigraphy.The passive margin basin suggested by Gaucheret al. (2003) seems incompatible with the rate ofaccommodation space typical of passive marginsand with the immature 1500 m thick conglomerates(Barriga Negra Formation) interpreted by theseauthors as deposits related to a drop in sea level.

Generation of accommodation space and depositionof thick coarse-grained rocks is more compatiblewith a tectonic control during sedimentation.

Nico Perez Terrane – Dom Feliciano Belt

The Nico Perez terrane (see Bossi & Campal 1992)is located between the Sarandı del Yı–Solıs andMarıa Albina shear zone (Fig. 2) and forms partof the Rıo de la Plata Craton (Brito Neves &Alkmim 1993); traditionally, the Sierra Ballenashear zone was its the eastern limit (Bossi &Campal 1992, among others). Hartmann et al.(2001) have separated the Nico Perez terrane intolithotectonic units (Arroyo del Soldado and Laval-leja groups, Valentines, La China and Las Tetascomplexes). The age of the units is not accuratelyconstrained (see Bossi & Gaucher 2004) but theArroyo del Soldado Group is stratigraphicallyassigned to 580–530 Ma (Gaucher et al. 2003;2005b) and correlated with the postglacial(post-580 Ma) units of the Corumba Basin(Nogueira et al. 2003).

The Dom Feliciano Belt (Fragoso Cesar 1980),named by Preciozzi et al. (1999) the Cuchilla deDionisio Belt, is represented in Uruguay by sedi-mentary and volcanic rocks metamorphosed anddeformed during the Brasiliano orogenic cycle(sensu Almeida et al. 1973) and also by coevaligneous rocks (Figs 2 & 4). The Campanero Unit(Sanchez Bettucci 1998; Sanchez Bettucci et al.2003a) is the Palaeoproterozoic basement to Neo-proterozoic supracrustal rocks (Lavalleja Group).This unit is represented by granitoids with variabledeformation grade. The U–Pb ages yield a 1735þ32/217 Ma upper intercept and a 723þ240/2210 Ma lower intercept (Sanchez Bettucciet al. 2003b); the latter suggesting deformation athigh temperature. Mallman et al. (2003) obtaineda similar U–Pb age to the upper intercept(1754 + 6.8 Ma) using the Sensitive High Resol-ution Ion Micro-Probe (SHRIMP) method. TheBrasiliano magmatism is recorded in the CarapeComplex as three major tectono-magmatic eventsat 850–750 Ma, c. 600 and 540–500 Ma (SanchezBettucci et al. 2003a). This granitic complex is

Fig. 4. Distribution of the main lithostratigraphic units. References: a, Hartman et al. (2001); b, Bossi et al. (1998); c,Heaman in Campal & Schipilov (1995); d, Mallmann et al. (2003); e, Preciozzi et al. (2005); f, Sanchez Bettucci et al.(2003b); g, Sanchez Bettucci et al. (2003a); h, Gaucher et al. (2005b); i, Sanchez Bettucci & Linares (1996); j,Oyhantcabal et al. (2005a); k, Oyhantcabal et al. (2007); l, Sanchez Bettucci & Ramos (1999); m, Pazos et al. (2003); n,Sommer et al. (2006); n, Eerola 1995; o, Cingolani & Bonhomme (1982); p, Oyhantcabal et al. (2005b); q,Philipp et al. (2002); r, Hartman et al. (2000); s, Chemale (2000); t, Sanchez Bettucci (1998); u, Fragoso Cesar (1991);v, Basei et al. (2006); w, Borba & Mizusaki (2002); x, Gomez Peral et al. (2003); y, Cingolani et al. (2002); o,Colombo et al. (1999); £, Alvarenga & Trompette (1992); ¥, Wood et al. (2002); #, Hoffmann et al. (2004); w,Hoffmann et al. (1996); ¢, Boggiani et al. (2005). SYSZ, Sarandı del Yı shear zone; CTSZ, Cueva del Tigre shear zone;MASZ, Marıa Albina shear zone; SBSZ, Sierra Ballena shear zone.

NEOPROTEROZOIC GLACIATION IN ARGENTINA 349

correlated with the Pelotas batholith of southernBrazil (Fig. 4).

The Lavalleja Group displays a complex strati-graphic organization (Bossi & Navarro 1991;Sanchez Bettucci 1998; Sanchez Bettucci &Ramos 1999; Sanchez Bettucci et al. 2001). Thisgroup is represented by a metamorphic associationof very low to medium grade, occurring in narrowbands of volcanic and sedimentary rocks (Ryoke–Abukuma type) and it was separated (Sanchez Bet-tucci 1998; Sanchez-Bettucci & Ramos 1999) intothree formations: Zanja del Tigre, Fuente delPuma and Minas. This group is affected byfolding, thrusting, and transcurrent faults. Regard-less of the tectonic setting, the Lavalleja Groupwas affected by metamorphism and deformationduring the Brasiliano orogenic event. The twolower formations attained middle amphibolitefacies to greenschist tacies conditions, whereas theupper formation only reached greenschist facies toanchimetamorphism. The Lavalleja Group wascorrelated with other tectono-stratigraphic units ofsouthern Brazil (Fig. 4), such as the PorongosGroup (Fragoso Cesar et al. 1982a, b, 1987, 1994;Fragoso Cesar & Machado 1997; Trompette 1994;Sanchez Bettucci et al. 2001; Saalmann et al.2006; among others). The Lavalleja Group wouldbe equivalent to the Brusque Complex in Santa Cat-arina (Jost & Bitencourt 1980; Almeida et al. 2000).The Porongos Group and Brusque Complextogether form the Tijucas Belt.

The Zanja del Tigre Formation (Fig. 5) is in tec-tonic contact with the Campanero Unit (basement)and the Fuente del Puma Formation. It is constitutedby quartzites with muscovite and/or fuchsite,andalusite, biotite and muscovite schists, marbles,meta-gabbros, carbonates, banded iron formationand amphibolites. Oyhantcabal et al. (2005b)present U–Pb ages (both ID-TIMS and SHRIMP)ranging from 1.42 to 1.49 Ga for re-workedmeta-ignimbrites and metasedimentary rocks,which is indicative of a Mesoproterozoicsource area.

The Fuente del Puma Formation (Fig. 5)appears in tectonic contact with the underlyingunit and is overlain by the Minas Formation. It isaffected by lower (chlorite zone) to upper (garnetzone) greenschist facies metamorphism. This for-mation is represented by a volcano-sedimentarysequence and was separated by Sanchez Bettucciet al. (2001) into three members: volcanic (basicand acidic), sedimentary (carbonates, siliciclasticand volcaniclastic rocks), and igneous (gabbros).U–Pb data for rutile from metabasalts of this for-mation yield ages of c. 670 Ma (crystallization)and 643 Ma (metamorphism; Sanchez Bettucciet al. 2003b, 2004). U–Pb SHRIMP analyses ofdetrital zircons from metasediments has yielded

ages between 3197 and 702 Ma (Preciozzi et al.2005), the youngest confirming a Neoproterozoicsource.

The Minas Formation is exposed in the neigh-bourhood of the city of Minas (Fig. 5) and com-prises metasedimentary rocks with very low grademetamorphism and deformation. The most repre-sentative lithologies correspond to grey calcareoussiltstones and dolomites with pelites, quartzitesand brown-coloured carbonates with psammiticintercalations (Sanchez Bettucci et al. 2001).

The Brasiliano magmatism in this area is rep-resented by intrusive rocks emplaced in the Campa-nero basement and the Lavalleja Group. It ischaracterized by several late- to post-orogenicmetaluminous to peraluminous granitic suitesnamed the Carape Granitic Complex by SanchezBettucci et al. (2003), and can be correlated withthe Dom Feliciano granitic suite (Fragoso Cesar1980, Figueiredo et al. 1990; Gastal et al. 2005,among others).

The Sierra de Las Animas Complex is constitutedby bimodal magmatism representing two events(Sanchez Bettucci & Rapalini 2002), the first onewith Neoproterozoic ages of c. 615–570 Ma(Sanchez Bettucci & Linares 1996; Oyhantcabalet al. 2007), and the second one ranging from 520to 500 Ma (Bossi et al. 1993; Sanchez Bettucci &Linares 1996; Sanchez Bettucci 1998). This post-orogenic magmatism was a consequence of anextensional relaxation episode related to the Brasi-liano and Rio Doce orogenic cycles. The volcanic,sub-volcanic and plutonic Sierra de Las AnimasComplex is composed of basalts, trachytes, syenites,rhyolites and volcanic breccias with alkaline andsubalkaline affinities, interstratified with sedimen-tary deposits (Sanchez Bettucci 1997).

A sedimentary succession with a major volcano-sedimentary composition at the top constitutes thePlaya Hermosa Formation, exposed in the south-western extreme of the Nico Perez terrane, on thecoast of the Rıo de la Plata estuary (SanchezBettucci & Pazos 1996; Pazos et al. 1998). Thisunit is affected by tectonic tilt, sometimes relatedto magmatic intrusions, but internal deformationor metamorphism is generally absent. Althoughprecise dates are lacking, the lower member ofPlaya Hermosa Formation is coetaneous with thefirst bimodal volcanic effusions of the Sierra deLas Animas Complex (Loureiro et al. 2006),suggesting a Neoproterozoic age. The presence ofpeperites and vesicular basalts suggests shallowwater deposition. The absence of regional meta-morphism in Playa Hermosa Formation is consist-ent with a younger age than the Lavalleja Group(Pazos et al. 2003).

Sanchez Bettucci & Rapalini (2002) obtained apreliminary virtual geomagnetic pole (VGP) for

P. J. PAZOS ET AL.350

c. 600 Ma from the lower member of PlayaHermosa Formation that agrees with the CampoAlegre (Tohver et al. 2006) pole of similar age.The result suggests that the glacially influenceddeposits could have been produced at low tointermediate latitudes.

Sao Gabriel block

The Sao Gabriel block is a terrane located insouthern Brazil, close to the Brazil–Uruguayboundary (Fig. 6), immediately to the west of theDom Feliciano Belt and to the east of the Rıo de

Fig. 5. Geology of the southern area of Uruguay with the location of the glacial sections.

NEOPROTEROZOIC GLACIATION IN ARGENTINA 351

la Plata Craton. The geology of the Sao Gabrielblock has been analysed by Chemale (2000), butnew Sm–Nd isotopic data from volcano-sedimentary units evidence juvenile Neoprotero-zoic oceanic crust to the east of the Rıo de laPlata Craton (Saalmann et al. 2005) and givemore information about the NE border of thecraton. The basement is composed of gneisses ofCryogenian age (750–700 Ma, U–Pb), covered bya metamorphosed volcano-sedimentary succession.The undeformed succession that unconformablyoverlies the volcano-sedimentary sequence com-prises the units of the Camaqua Basin, intruded bylate tectonic granites of 595 Ma (see Saalmannet al. 2005). The suspected glacial deposits in theSao Gabriel block were mentioned for the firsttime by Carvalho & Pinto (1938). In the area ofLavras do Sul (Fig. 6), Eerola (1995, 2001)suggested glacial influence in deposits belongingto the Camaqua Basin. The stratigraphy of thisbasin was extensively described by Paim et al.(1995, 2000) who defined five allogroups: IMarica (620–600 Ma), II Bom Jardim(592–580 Ma) III Cerro do Bugio (573–560 Ma),IV Santa Barbara (559–540 Ma) and V Guaritas(470 Ma). Eerola (1995) named a succession withglacial influence as the Passo da Areia sequenceand mentioned that it is covered by the ‘SantaBarbara units’. For this reason, the glacially influ-enced facies should have been deposited contem-poraneously with alkaline volcanism of the Cerrodo Bugio allogroup. Sommer et al. (2006) suggestthat the shoshonitic volcanism (Hilario Formation,

Bom Jardim Group) represents the oldest (608–592 Ma), whereas the magmatic unit in theCamaqua Basin and the bimodal magmatism ofAcampamento Velho (Cerro do Bugio Group) andCampo Alegre yield ages between 602 and549 Ma. It is possible to correlate the younger(Acampamento Velho) volcanism with the Sierrade Las Animas Complex (II event).

Palaeoclimatic evidence

Tandilia System

The Sierra del Volcan Formation is a very thin unit(Fig. 3b), less than 8 m thick (Spalletti & del Valle1984). The succession overlies a weathered(kaolinite-rich) basement and begins with rhythmic,fine-grained sandstones and pelites containing drop-stones of varied size and composition (unit 1) exhi-biting roughing and bending structures (e.g.,Thomas & Connell 1985) and syn-sedimentaryfolding (Spalletti & del Valle 1984). Outsizedclasts (Fig. 3b) present different sizes (mainlypebbles) and roundness; they are usually facetedwith glacially related bullet shapes. Composition-ally they include quartz, quartzites, migmatitesand other igneous–metamorphic lithologies,sometimes with similar weathering grade to thebasement, providing good evidence of pre-depositional alteration of the basement (Spalletti& del Valle 1984). Sandstones and carbonates areabsent. The host rocks show undulatory lamination

Fig. 6. Geology of the Lavras do Sul area in the Sao Gabriel block. Taken from Eerola (2001).

P. J. PAZOS ET AL.352

indicative of incipient ripple lamination. Pelites arethe result of settling from a suspension in a shallowmarine environment (Spalletti & del Valle 1984).The succession continues with 1.20 m of fine-grained siltstones and claystones containing drop-stones (unit 2) with a dominance of quartzite frag-ments. This level passes upward to pebblysandstone (unit 3), and presents normal gradingand faint parallel stratification to the top. The Bal-carce Formation is deposited over the Sierra delVolcan Formation with sharp angularunconformity.

The diamictites were revisited by van Stadenet al. (2005) to analyse the provenance and deposi-tional framework of the ‘tillites’. More than 15samples distributed in the intervals previouslydescribed by Spalletti & del Valle (1984) were ana-lysed geochemically. They also studied grain sur-faces using electron microscope techniques,yielding evidence of transport under glacial con-ditions (e.g., striations, conchoidal breakage onlarger grains, flat cleavage fractures and facetedgrains). The surface studies indicate subaqueoustransport and the large number of impact cratersindicates a highly energetic beach environment(van Staden et al. 2005). Geochemically, majorand trace elements analysis did not help to decipherthe grade of alteration of samples, Nb/Y and Zr/Tiratios being homogeneous throughout the entire‘tillite’. Rare earth element data indicate an uppercrustal composition and some samples revealeddifferences in the light rare earth element (LREE)concentrations. Overall the results allowed vanStaden et al. (2005) to suggest an evolution fromoxic to anoxic conditions from base to top. Spalletti& del Valle (1984) pointed out variable domains ofkaolinite or illite in the analysed samples, but theauthors emphasized the dominance of kaolinite inthe altered basement. The geochemical compositionsuggests a source area composed of both igneous-metamorphic basement and sedimentary units (vanStaden et al. 2005). More recently Zimmermann(pers. comm.), having in mind that kaolinite is themost abundant clay present in the diamictitematrix (see Spalletti & del Valle 1984) and is alsoextremely abundant in the overlying Balcarce For-mation (Zimmermann & Spalletti 2005), speculatedthat glacial deposits of the Sierra del Volcan For-mation could be Ordovician, like the overlying Bal-carce Formation. This would imply that the glacialrecord is late Ordovician (Ashgillian?). This specu-lation is not followed in this paper since the angularunconformity necessarily places the Balcarce For-mation in the Silurian, although radiometric agessupport an Ordovician age (e.g., Rapela et al.1974, 1998, 2007). The kaolinitization of the base-ment preceded deposition of the Sierra del VolcanFormation and explains the abundance of kaolinite

in the diamictites. Interestingly, van Staden et al.(2005) did not discuss the angular unconformityor comment about the geometrical disposition,which could be the result of pinching out of faciesor local tilting. However, a Precambrian age forthe glacial deposits was suggested. The contrastingchange from anoxic conditions (upper diamictite) totidal dominated deposits, well oxygenated sub-strates, compositionally mature sandstones, and aCruziana-type ichnofauna that characterizes theBalcarce Formation (see Poire & Spalletti 2005)confirm the contrasting depositional settingbetween these units and confirm a sequence bound-ary between them. In our view, geological evidence(angular unconformity) supports the traditionaldifferentiation of two independent stratigraphicunits and favours the previously suggested Precam-brian age for the Sierra del Volcan Formation (e.g.,Spalletti & del Valle 1984; Poire & Spalletti 2005;van Staden et al. 2005). The stratigraphic relation-ship of the unit with respect to the Sierras BayasGroup is still dubious, but a latest Neoprotoerozoicage seems probable. The glacial origin is not ques-tionable and the succession represents the first con-firmed glacial deposits in southern South Americaduring the Precambrian. Sedimentologically, it isnecessary to point out that the term ‘tillite’ usedby van Staden et al. (2005) is inappropriate, forthe following reasons: the section includes threeunits rather than a single ‘tillite’; the lower andmiddle ones contains evidence of rain-out and canbe termed rain-out diamictites rather than tillite;the host deposits are heterolithic rhythmites depos-ited by weak currents (ripples) and suspension andfall-out, and were not directly deposited from aglacier. The upper unit is regarded as a ‘diamictite’rather than a ‘tillite’ following Spalletti & del Valle(1984) because it does not contain evidence ofglacio-tectonic deformation or erosive featuresindicative of glacial erosion and lodgement depo-sition, but also because normal grading and faintlamination to the top suggest a cohesion-lessdebris flow deposited in a probably unstable deposi-tional setting below wave base level.

Dom Feliciano Belt

(a) Playa Hermosa Formation. A complete analysisof the sedimentary facies of the Playa Hermosa For-mation is still lacking. However, the lower sectionhas been particularly targeted in investigationsregarding the possible glacial origin of the succes-sion. The lower section is approximately 50 mthick (Fig. 7) and comprises two facies associationsaccording to Pazos et al. (2003). Facies AssociationI (Fig. 7) includes breccias, conglomerates, sand-stones and mudstones, and records subaqueousdeposition on unstable slopes adjacent to glacial

NEOPROTEROZOIC GLACIATION IN ARGENTINA 353

Fig. 7. Logged section in the lower part of the Playa Hermosa Formation, modified from Pazos et al. (2003).

P. J. PAZOS ET AL.354

centres and probable active faults (Pazos et al.2003). The combined features of interbedded brec-cias, conglomerates, sandstones and minor mud-stones indicate a wide availability of rock detritusbut the intense soft-sediment deformation andcomplex geometry of psefitic deposits suggest syn-sedimentary remobilization, probably due to highsedimentation rate or seismically induced slumping.Facies Association II includes diamictites, sand-stones and mudstones. This facies association pre-sents clear climate indicators of at least seasonalfreezing of the body water and melting of icebergs.The diamictites represent tabular deposits contain-ing angular clasts of different sizes, composed ofgranites, quartz, quartzites, feldspar and sometimesinclude deformed sedimentary clasts of rhythmite-type (Fig. 8a). Clasts may be randomly distributedor form patches. These deposits do not represent‘tillites’ according to Pazos et al. (2003, p. 69)but should be regarded as good examples ofresedimented till, deposited like gravity flows witha variable grade of homogenization in a pro-glacialand unstable depositional setting. Sandstones aremassive, graded or, more commonly, contain driftripple-cross lamination, show strongly asymmetri-cal sections, were affected by down-slope defor-mation prior to lithification, and indicatepalaeo-flow and palaeo-slopes to the NE (Figs 7& 8b). Some ripples present high climbing anglesindicative of rapid aggradation. These rippleswere interpreted as ‘micro-hummocky’ by Fambriniet al. (2003) and used as strong evidence of amarine depositional setting. The storm origin isincorrect and any inference about the marine orlacustrine depositional setting may be based ondrift-cross lamination. However, the fine-grainedsection of the Las Ventanas Formation, whichmay be correlated with the fine-grained section ofthe Playa Hermosa Formation (Pazos et al. 2005),contains palynomorphs and indicates marineinfluence at least for the fine-grained intervals (seeBlanco & Gaucher 2004).

The clearest evidence of seasonal freezing orrain-out processes from icebergs is a large quartzitedropstone (block size), with bullet shape andpolished surface, that disrupts the underlying depos-its (Fig. 8c). No clast in the host deposits is largerthan a small pebble. Moreover, the host depositsconstitute crudely stratified rhythmites with a thick-ness of less than 5 cm. The impact structures arecoincident with the examples typified by Thomas& Connell (1995). Rhythmites, resembling varves,(Fig. 8d) also occur stratigraphically higher andcontain diamictite intervals with isolated blocks,fine-grained sandstones with granite clasts and soft-sediment deformation structures. These deposits areintercalated with pelites, starved rippled depositsand fine-grained turbidites (Fig. 5d in Pazos et al.

2003). The association indicates a more distal andpossible deeper depositional setting than FaciesAssociation I.

Evidence from the analysed section is not con-clusive about a direct glacial origin but the combi-nation of diamictites, rhythmites with varve-likedeposits and dropstones is a strong evidence tosupport glacially influenced deposition (Pazoset al. 2003). The palaeocurrent pattern indicates apalaeo-high area situated to the south-west, possiblyin the vicinity of the Tandilia System.

(b) Zanja del Tigre Formation. The metamorphosedand deformed limestones of this unit include a stra-tigraphically thin (2 m) interval containing outsizedclasts (Pazos et al. 2005) that vary from pebbles toblocks and include gabbros, quartz-quartzites(Fig. 8e) and granites (Fig. 8f ). They are isolated,disrupting the lamination in different form to thatresulting from tectonic deformation (rotationaldeformation). Some are very similar to theexamples illustrated by Condon et al. (2002) inthe compilation of dropstones intervals fromNeoproterozoic successions around the world.

Sao Gabriel Block

Passo da Areia sequence. Eerola (1995) describedand illustrated a sedimentary succession that pre-sents features that may be ascribed as ‘glaciallyinfluenced’ deposits: outsized clasts isolated in fine-grained, rhythmically laminated layers (Figs 9and 10 in Eerola 1995,), diamictites with clasts(pebbly sandstones and mudstones), and rhythmi-cally laminated shales. Eerola (1995) also mentionsother areas of suspect glacial deposits but, unfortu-nately, did not mention the composition of theoutsized blocks. The local abundance of volcanicand pyroclastic deposits introduces more uncer-tainty about the origin of this sequence.

Discussion

The differentiation between tillites and diamictitesof non-glacial origin was emphasized by Eyles &Januszczak (2004). However, in tectonicallyactive basins or in topographically irregular deposi-tional scenarios the most abundant deposits are theresult of gravity flows and reworking that convertedthem into diamictites. This is not to negate aprimary glacial origin. Tectonism was activeduring deposition of some of the units discussedin this paper, related to the Brasiliano cycle, andindicates that extension, probably related to strike-slip basins, was prominent in the eastern part ofthe Rıo de la Plata Craton. Volcanism is documen-ted in the tectonically active areas of the craton(Eerola 2001; Pazos et al. 2003; Saalmann et al.

NEOPROTEROZOIC GLACIATION IN ARGENTINA 355

Fig. 8. Sedimentary features of Playa Hermosa Formation: (a) diamictites with mudstone intra-clasts, (b)rippled-sandstones with soft-deformation overprinted (taken from Pazos et al. 2003), (c) dropstone deforming coarserhythmites and detail of the top view, (d) varve-like rhythmites (taken from Pazos et al. 2003). Sedimentary features ofthe Zanja del Tigre Formation: (e) quartzite clasts of IRD (ice rafted diamictites) origin, (f) granite clast immersedin carbonates.

P. J. PAZOS ET AL.356

2005) and is also coetaneous (Fig. 9a, b) with thePlaya Hermosa Formation (Loureiro et al. 2006),suggesting that seismic shock during high-ratedebris production, a common feature in glacialdepositional settings, may be the trigger of slump-ing and folding in the Playa Hermosa Formation(see Pazos et al. 2003). However re-sedimentationand deformation controlled by high sedimentationrates in a retreating glacial environment is heresuggested, taking into account that the fine-graineddeposits contain intra-basin clasts and that soft-deformation and scarce entombed angular granitepebbles suggest rain-out processes. The palaeogeo-graphic and palaeotopographic framework is notwell known, but in an active tectonic basin, rela-tively high topography and glaciers advancingfrom mountains into the coast (fjords?), as insouthern Chile, are not ruled out. Such casespermit explanation of glacial deposits under con-ditions that are not severe and also not in high-latitude locations. Recently, Gaucher et al.

(2005b) reported palynomorphs from the fine-grained intervals of the Las Ventanas Formationthat forms part of the same depositional cycle asthe Playa Hermosa Formation (Blanco & Gaucher2004; Pazos et al. 2005), evidencing marine influ-ence. The combination of psefites, diamictites,rhythmites, slumping and dropstone-levels issimilar to glacial–postglacial transitions in Carbon-iferous successions of Argentina where a fjordenvironment has been suggested (e.g., Knelleret al. 2004; Pazos et al. 2007). Such environmentsare characterized by high rates of sedimentations,rapid facies shifting and high discharge from melt-water (e.g., Hansen 2004). This could equallyexplain the absence of marine palynomorphs in thefine-grained intervals of the type locality of thePlaya Hermosa Formation compared with thefertile Las Ventanas Formation. Thus the absenceof fossils may be explained by palaeo-ecologicaland taphonomical controls and is a weak criterionto support the conclusion of Gaucher et al. (2005b)of different ages for the two units. Gaucher et al.(2003, 2005b) suggested than the relatively diverseassociation of palynomorphs present in the platformlimestones of the Arroyo del Soldado Group and nowin the siliciclastic rocks of the Las Ventanas For-mation, is strong evidence of the inconsistencywith the Snowball Earth hypothesis, whichassumes cut-off of marine life. Interestingly, Pazoset al. (2003) pointed out that the Playa HermosaFormation marks an earlier stage of the basin infill,previous to the carbonate platform depositionsuperbly recorded in the Arroyo del Soldado Groupafter glaciation disappeared (Pazos et al. 2003).Corsetti et al. (2006) compared the biota containedin Neoproterozoic glacial deposits with frozen seaareas of Antarctica and concluded that most of theforms that appeared previous to the glaciation arenot affected by climate deterioration, and theterminal Proterozoic diversification in the biota isnot connected with the glacial aftermath because itoccurred million of years after the last glacialvestige disappeared.

The dropstone levels documented in themetamorphosed Zanja del Tigre Formation maybe regarded as distal glacially influenced depositsor the result of a Heinrich-type event. The Heinrichlayers occur at times of relative cold within theglacial period, related to increased icebergproduction (Heinrich 1988). There are numerousglacially related successions that comprise distalglaciomarine deposits where only rain-out debrismarks the glacial event (see Condon et al. 2002).The distribution of icebergs is not random andmarine currents and winds control their trajectory,and the size and quantity of the released debrisduring melting is dependant on such control (Prinset al. 2002).

Fig. 9. Interaction between magma and wetunconsolidated sediments (peperites) at Playa Hermosabeach. (a) Outcrop view of magma–sedimentinteraction; the bedding has been partially destroyed(Tch, trachyte; P, pelite; G, conglomerate). (b) Fluidalpeperitic texture related to trachyte. The host sediment ispelite. a, Irregular (amoeboid) contact; b, juvenile clastis trachyte.

NEOPROTEROZOIC GLACIATION IN ARGENTINA 357

West Gondwana correlations

The glacially influenced deposits in the centre of theRıo de la Plata Craton (Tandilia System) are con-firmed by macroscopic and microscopic evidence.The stratigraphic age is not well constrained andabsolute dating is lacking. However, differencesin the grade of diagenesis, higher in the oldestunit of the Sierras Bayas Group (the Villa MonicaFormation) and lower in the glacially relatedSierra del Volcan Formation and the rest of theSierras Bayas Group, suggest a younger age forthe latter. As mentioned, the age of the VillaMonica Formation is matter of debate but it wastraditionally considered to be pre-Ediacaran(700–800 Ma) until the proposal of an Ediacaranage from palynomorph content (Gaucher et al.2005a). The gap involved in the unconformitybetween Villa Monica and Cerro Largo formationsmay be of more than 200 Ma in the traditionalstratigraphic interpretation, or less, if the Gaucheret al. (2005a) proposal is accepted. Thus theglacial deposits of the Sierra del Volcan Formationcould be correlated with any of the widely acceptedglacial events. However, clasts composition in thediamictites suggest provenance from unmetamor-phosed quartzites (Spalletti & del Valle 1984),possibly a quartzite interval in the Cerro LargoFormation (see Poire & Spalletti 2005), and in con-sequence the age of the glacial deposits would needto be Ediacaran. The absence of carbonates clasts inthe diamictites may suggest that they are youngerthan the glacial deposits. This scenario is congruentwith the stratigraphic scheme of the CorumbaBasin, and strongly suggests a correlation betweenboth glacial sections. Poire & Spalletti (2005,Fig. 2) suggested that the diamictites of the Sierradel Volcan are Cambrian like the Cerro NegroFormation, but without any explanation for thisstratigraphic position. Similarly, the glacially influ-enced deposits of the Playa Hermosa Formationwere originally correlated with the Varanger glacia-tion by Pazos et al. (2003) based on preliminary andnot very robust palaeomagnetic data that exhibit apolar wonder path congruent with the c. 600 Mapalaeo-pole (see Sanchez Bettucci & Rapalini2002). Interestingly, volcanic intrusions withmicro-syenites confirm syn-sedimentary volcanismin the lower part of the succession. This effusiveevent was related to the early stages of the Sierrade Animas Complex (Sanchez Bettucci et al.2006). The syenites in the area were dated at579 + 1 Ma (Ar–Ar) by Oyhantcabal et al.(2007). In consequence, the glacially related depos-its are definitively not Marinoan, but more probablyGaskiers or even younger. In the Paraguay fold belt,glacial deposits of the Puga Formation have beencorrelated with the Marinoan glacial event taking

into account a 627 Ma (Pb–Pb) age for postglacialcarbonates (Babinsky et al. 2006), but other diamic-tite intervals interpreted as glacial in origin weredescribed in the Serra Azul Formation (Figueiredoet al. 2006) and correlated with the Gaskiers glacia-tion. The absence of cap-carbonates seems to be acommon feature in Gaskiers-age deposits, whileMarinoan or Sturtian deposits usually have them(see Halverson et al. 2005). The glacial influencesuggested for the Zanja del Tigre Formation byPazos et al. (2005), where banded iron formationdeposits are common, may be connected with theearlier postglacial stages of the Marinoan glaciationand can be partially correlated with the Puga For-mation in Brazil and the Ghaub Formation inNamibia. In this context, and accepting the newideas that suggest that Punta del Este terrane is apart of the Kalahari Craton, the dropstone-richinterval could be connected distally with glacialcentres located in Namibia. The Marinoan glacialrecord in Namibia exhibits sedimentation in aslope and outer platform environment (Domack &Hoffman 2003). Thus in Uruguay this eventwould be a very distal glaciomarine ice-rafted inter-val, during periods of extensive iceberg production,similar to the Heinrich events in the Quaternary (seeHesse et al. 2004).

Surprisingly, the Gaskiers glaciation is probablymore widespread in southern South America thanpreviously imagined. This event has not beendescribed in Africa and is almost absent in WestGondwana with the exception of Tasmania, whereCalver et al. (2004) suggested a Gaskiers recordfor diamictites. Assumed isochronism and lithostra-tigraphic correlations for the Sturtian glaciationhave been challenged recently in Australia byKendall et al. (2006), suggesting a long term glacia-tion encompassing the classical Sturtian andMarinoan records, with multistage advances andretreats as in Phanerozoic counterparts. Theseauthors also questioned a glacial origin for theGaskiers-age glacial deposits analysed by Calveret al. (2004). The idea of diachronism and multipleadvances and retreats is a very important and indi-cates that Neoproterozoic glaciations were severebut comparable with other well-understood Palaeo-zoic and Cenozoic glaciations. Diachronism iswidely accepted in the Late Palaeozoic glaciationwhere at least three stages between Lower Carboni-ferous and Lower Permian have been defined (seeIsbell et al. 2003). For instance, glacial centreswere located in western South America in theLower Carboniferous but migrated to the eastand Africa in the Late Carboniferous (see Pazos2002). This scenario may explain the absence ofGaskiers-age glacial deposits in Africa, takinginto account that the glacial centres were insouthern South America. However, sea level drops

P. J. PAZOS ET AL.358

described by Saylor (2003) in Late Ediacaransuccessions in Namibia may be connected with aglacio-eustatic control.

Conclusions

The salient conclusions of this paper includestratigraphical, sedimentological and palaeogeogra-phical aspects.

(1) Glacially influenced deposits of Precambrianage are confirmed in the Rıo de la Plata Craton andmost probably represent a latest Neoproterozoicrecord. It suggests that Gaskiers-age glacial depos-its were more widespread in the craton thanpreviously envisaged.

(2) A tectono-sedimentary framework indicatestectonic activity (mainly extensional) and magma-tism during coetaneous glacial activity in theeastern border of the craton and stability withlimited accommodation space around the centre(Tandilia System).

(3) An older (Marinoan-age?) distal glaciomar-ine record is documented in the Zanja del Tigre For-mation and may be correlative with the glacialdeposits of the Ghaub Formation and equivalentsin Africa. These predate the collision and amalga-mation of the Kalahari (Punta del Este terrane)and Rıo de la Plata cratons.

(4) In the Rıo de la Plata Craton there is no evi-dence of direct or primary glacial deposition andmost of the record is composed of diamictitesdeposited in an unstable setting with rain-out inter-vals. However, a glacially related origin is undis-puted in the Sierra del Volcan diamictites. ThePlaya Hermosa Formation exhibits glacial influenceunder active tectonism, while glacial influence inthe Passo da Areia section must be a matter ofdebate until new detailed sedimentological infor-mation provides information on the compositionof the outsized clasts and the nature of thefacies deposition.

This work received financial support by FCE (8255),Comision Sectorial de Investigacion Cientıfica, Universi-dad de la Republica, Uruguay. We thank Santiago Starec-zek and Gonzalo Sanchez for providing polished rockslides. This paper is a contribution to the IGCP 512.Finally, we express our gratitude to Bob Pankhurst forthe invitation to contribute to this special volume.

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