Integrated correlation of the Vendian to Cambrian Arroyo ... fileindicated by fossils for both units. Previously reported strontium isotopic data are also consistent with this age

Integrated correlation of the Vendian to Cambrian Arroyo delSoldado and Corumba Groups (Uruguay and Brazil):palaeogeographic, palaeoclimatic and palaeobiologic

implications

Claudio Gaucher a,*, Paulo Cesar Boggiani b, Peter Sprechmann a, AlcidesNobrega Sial c, Thomas Fairchild d

a Facultad de Ciencias, Departamento de Paleontologıa, Igua 4225, 11400 Montevideo, Uruguayb Universidade Federal de Mato Grosso do Sul (UFMS), Caixa Postal 549, Cidade Universitaria S/N, 79070-900 Campo Grande, MS,

Brazilc Departamento de Geologıa, Centro de Tecnologia e Geociencias, Universidade Federal de Pernambuco (UFPE), Caixa Postal 7852,

Recife PE-50732-970, Brazild Instituto de Geociencias, Universidade de Sao Paulo, Caixa Postal 11348, Sao Paulo SP 05422-970, Brazil

Received 30 May 2001; accepted 6 September 2002

Abstract

The Corumba Group of SW Brazil and the Arroyo del Soldado Group (ASG) of Uruguay are correlated on the basis

of litho-, bio- and chemostratigraphy. Both units represent marine sedimentation with alternating siliciclastics and

carbonates developed on a stable continental shelf. In the Corumba basin, sedimentation began in the Varangerian,

represented by the glaciomarine Puga Formation. A series of sea-level fluctuations coupled with climatic changes are

recorded up section. While uppermost deposits of the ASG are of lowermost Cambrian age, sedimentation ceased in the

latest Vendian in the Corumba basin. An assemblage of six species of organic-walled microfossils dominated by

Bavlinella faveolata and Soldadophycus bossii , three species of vendotaenids and two species of skeletal fossils (Cloudina

and Titanotheca ) is described from the Corumba Group. The vendotaenid Eoholynia corumbensis sp. nov is described

from siltstones of the Guaicurus Formation. An important diversity of skeletal fossils in the Corumba, Arroyo del

Soldado and Nama groups points to favourable Vendian palaeoclimatic conditions in SW-Gondwana. Preliminary

carbon isotopic data show a series of alternating positive and negative excursions, corroborating the upper Vendian age

indicated by fossils for both units. Previously reported strontium isotopic data are also consistent with this age. It is

postulated that the Corumba and ASGs were deposited onto the same shelf, which opened to the east. The Rio de la

Plata Superterrane (Craton) extends farther to the north than previously expected, or it was already amalgamated with

the Amazonian Craton by Vendian times. Collision of the platform with the Parana Block caused closure of the basin

during the Cambrian-Early Ordovician. Finally, models of Neoproterozoic glaciations based on enhanced

bioproductivity driven by high nutrient availability are discussed.

# 2002 Elsevier Science B.V. All rights reserved.

* Corresponding author.

Precambrian Research 120 (2003) 241�/278

www.elsevier.com/locate/precamres

0301-9268/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 3 0 1 - 9 2 6 8 ( 0 2 ) 0 0 1 4 0 - 7

Keywords: Vendian; Neoproterozoic; Brazil; Uruguay; Palaeogeography

1. Introduction

The correlation of the Arroyo del Soldado

Group (ASG) of Uruguay with the Corumba

and Jacadigo Groups of southwestern Brazil

(Fig. 1) was first postulated by Gaucher et al.

(1996: 360�/361), on the basis of similar age,

similar depositional environments, probable con-

tinuity in subsurface, and existence of Bavlinella -

dominated acritarch-assemblages in these groups.

This working hypothesis proved to be a powerful

predictive tool, because it could explain many

observations made thereafter. Gaucher and

Sprechmann (1998, 1999) described a skeletal

assemblage from the Yerbal Formation of the

ASG containing Cloudina , which is a common

fossil in the limestones of the Tamengo Formation

of the Corumba Group (Hahn and Pflug, 1985;

Zaine and Fairchild, 1985; Boggiani et al., 1993;

Boggiani, 1998; Gaucher 1999). Boggiani (1998)

presented sedimentologic, carbon and strontium

isotopic data for the Corumba and ASG, con-

sidering that a correlation of both units is possible.

Gaucher (1999, 2000) compares the fossil content,

lithostratigraphy, chemostratigraphy and sedimen-

tary environments of the Arroyo del Soldado,

Jacadigo and Corumba Groups. The author con-

cludes that while the Arroyo del Soldado and

Corumba Groups are correlative and of Red-

kinian�/Kotlinian (upper Vendian) age, the Jaca-

digo Group is composed of glacial deposits and

BIF’s of the Rapitan-type, probably generated

during the Varangerian glaciation, as also postu-

lated by Almeida (1984), Walde (1987: 103),

Alvarenga (1990) and Boggiani (1998). In this

paper, we present in some detail the evidence that

supports the correlation of these important Ven-

dian to Cambrian units of South America. Newly

discovered microfossils from the Corumba Group

and carbon isotopic profiles of the ASG are

described as well. Finally, the paper deals with

the consequences of this new data for the poorly-

known palaeogeography of the Rıo de la Plata

Fig. 1. Map of SE-South America showing outcrop areas of the

Corumba and ASGs and major tectonostratigraphic units.

Boundary of post-Cambrian sedimentary cover is indicated as

well. PAT, Piedra Alta Terrane; NPT, Nico Perez Terrane;

CDT, Cuchilla Dionisio Terrane; RAB, Rıo Apa Block.

C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278242

Superterrane (Gaucher, 1999, 2000) during theVendian�/Cambrian (Unrug, 1996).

2. Geological setting

2.1. The Arroyo del Soldado Group

This lithostratigraphic unit was defined by

Gaucher et al. (1996), to include thick (�/5000m) marine shelf deposits, which occur in the Nico

Perez Terrane of Uruguay. The boundaries of the

Terrane also represent the boundaries of the

occurrences of the ASG (Fig. 2), namely: the

Sarandı del Yı-Piriapolis Shear Zone to the west,

the Sierra Ballena Shear Zone to the East and the

Cerro Partido Thrust to the south (Gaucher et al.,

1998b; Gaucher, 1999, 2000). To the north, theoccurrences of the ASG are covered by Phaner-

ozoic sedimentary units and volcanics of the

Parana Basin. The northernmost outcrops of the

Group have been recognized by Gaucher et al.

(1998c) and described by Gaucher (1999, 2000) in

the so-called Isla Cristalina de Rivera, near the

boundary with Brazil (Fig. 2). The unit lies with

erosional and angular unconformity on: (1) aMesoproterozoic metasedimentary complex infor-

mally known as Basal Group (Sprechmann et al.,

1994; Gaucher et al., 1996) or Grenvillian Meta-

morphic Complex (Bossi et al., 1998), (2) Archean

(3.4�/2.7 Ga) metasedimentary units recently dis-

covered in the vicinity of Piraraja (Campal and

Schipilov, personal communication, 2000; Fig. 2);

(3) Palaeoproterozoic (17849/5 Ma: Campal andSchipilov, 1995) rapakivi granites and (4) other

granites of undetermined age. The basal Yerbal

Formation (Gaucher et al., 1998a; Gaucher and

Sprechmann, 1999; Gaucher, 2000) represents a

siliciclastic, deepening-upward sequence, and is

characterized by thin conglomerates and arkoses

at the base, passing into green siltstones and

banded siltstones at the top. Oxide-facies BIFhave been recently discovered at the top of this

unit near the city of Minas (Fig. 2). The Yerbal

Formation is concordantly overlain with sharp

contact by the Polanco Formation, which marks

the development of a large carbonate ramp in the

basin. The unit is characterized by bluish gray to

black limestone�/dolostone rhythmites, frequentcarbonatic tempestites, pure calcisiltites and dolo-

siltites, and rare oolitic calcarenites (Gaucher,

1999, 2000). In the shallowest areas of the basin

(to the west), carbonates of the Polanco Forma-

tion are concordantly overlain by conglomerates

and arkoses of the Barriga Negra Formation (Fig.

4), recording a major regression (Gaucher et al.,

1998a; Gaucher, 2000). This unit passes upwardsinto shales and siltstones of the basal Cerro

Espuelitas Formation. In the deepest sections (to

the E), the Polanco Formation shows a direct

transition into the Cerro Espuelitas Formation

(Gaucher, 1999, 2000; Fig. 4). The latter unit is

made up of an alternation of dark shales, thick

chert-deposits and oxide-facies BIF with up to 35

wt.% magnetite and/or hematite (Gaucher andSchipilov, 1994; Gaucher et al., 1996, 1998b;

Gaucher, 1999, 2000). The Cerro Espuelitas For-

mation is truncated by an erosive surface that

probably marks an important regression (Gau-

cher, 2000). The Cerros San Francisco Formation

was deposited above this surface, and is character-

ized by very mature quartz-arenites with wave and

current ripples, hummocky cross-stratification andlow-angle cross bedding (Montana and Sprech-

mann, 1993; Gaucher and Schipilov, 1993; Gau-

cher et al., 1996; Gaucher, 2000). Finally, the

Cerros San Francisco Formation passes upwards

into stromatolitic and oolitic limestones of the

Cerro Victoria Formation, which contains ichno-

fossils of probable Cambrian age (Montana and

Sprechmann, 1993; Gaucher, 1999, 2000).The ASG has been folded and intruded by

granites and syenites in the Cambrian, granitoids

yielding ages around 540�/510 Ma (Bossi and

Navarro, 1991: 342; Kawashita et al., 1999; Fig.

2). K�/Ar datings of illite-rich pelites belonging to

the Group yielded recrystallization ages between

5329/16 and 4929/14 Ma (Cingolani et al., 1990;

Gaucher, 2000), well in accordance with agesobtained for intrusive granites. Considering these

ages, the orogenic event that determined the

deformation of the ASG can be taken as a terminal

event of the Brasiliano Megacycle or, more

probably, represent a separate orogeny (Paraguay

Orogeny of Basei and Brito Neves, 1992). As

stated by Gaucher (2000), the latter interpretation

C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278 243

seems more plausible because the orogeny records

the collision of a typical Brasiliano-block (Cuchilla

Dionisio Terrane) with a cratonized block, onto

which the Arroyo del Soldado-shelf developed.

2.2. The Corumba Group

With minor changes, the lithostratigraphical

scheme proposed by Almeida (1965) has been

Fig. 2. Geographic distribution of the ASG and other Neoproterozoic-Cambrian (meta)sedimentary units in Uruguay, and some

important Cambrian intrusive granites, slightly modified from Gaucher (2000). SYPSZ, Sarandı del Yı-Piriapolis Shear Zone; SBSZ,

Sierra Ballena Shear Zone; CPT, Cerro Partido Thrust; IPSZ, Isla Patrulla Shear Zone; IPB, Isla Patrulla Block; ICR, Isla Cristalina

de Rivera; ICA, Isla Cristalina de Acegua.


adopted by most authors recently dealing with theCorumba Group (Almeida, 1984; Zaine, 1991;

Boggiani et al., 1993; Boggiani, 1998; Alvarenga

et al., 2000). Boggiani (1998) includes the Cadiueus

Formation in the base of the Corumba Group,

after the recognition of a gradational contact

between the Cadiueus and Cerradinho Forma-

tions. The group rests with angular and erosional

unconformity on (1) granites and gneisses of theBasal Complex of the Rıo Apa Block, sensu

Correa et al. (1979), of undetermined age but

probably reworked in the Mesoproterozoic; and

(2) Mesoproterozoic (Uruacuan) biotite gneisses,

micaschists and quartzites of the Alto Terere

Metamorphic Complex (Correa et al., 1979; Al-

meida, 1965, 1984; Schobbenhaus Filho et al.,

1979). The Corumba Group overlies conformablythe Puga Formation at many localities, like at the

Morro do Puga (Correa et al., 1979; Boggiani,

1998; Alvarenga and Saes, 1992; Alvarenga et al.,

2000). In general, the basement of the Corumba

Group/Puga Formation comprises igneous and

metamorphic rocks older than the Brasiliano-Pan

African orogeny, recording a widespread tecto-

nothermal event in the Mesoproterozoic (ca. 1.3�/

1.0 Ga).

The Corumba Group was deposited on a stable

continental margin (Almeida, 1984: 274; Zaine,

1991; Boggiani et al., 1993, 1998; Boggiani, 1998),

and includes alternating siliciclastic and carbonatic

units with a composite thickness of approximately

600 m (Boggiani, 1998). Following glacial and

glaciomarine deposits of the Varangerian/Mar-inoan Puga Formation (Almeida, 1984; Alvar-

enga, 1990; Alvarenga and Trompette, 1992;

Alvarenga et al., 2000), a siliciclastic deepening-

upward sequence was deposited, represented by

the Cerradinho Formation (Almeida, 1965, 1984;

Boggiani, 1998). Boggiani (1998) interpreted the

Cadiueus Formation as alluvial fan, synrift depos-

its generated during opening of the Corumbabasin. While this explanation cannot be ruled

out, it must be born in mind that the underlying

Puga Formation already records (glacio) marine

conditions in the shelf. Furthermore, it is well

known that alluvial fan deposits are very similar to

glacial-outwash fans, also termed sandur deposits

(Ruegg, 1977; Reineck and Singh, 1980: 202;

Miall, 2000). Therefore, the Cadiueus Formationprobably represents glacial-outwash fans depos-

ited during retreat of the Varangerian ice-sheet.

The Cerradinho Formation conformably passes

into carbonates of the Bocaina Formation, or

more rarely, directly into the Tamengo Formation

(Almeida, 1965, 1984; Boggiani, 1998; Fig. 4). The

Bocaina Formation is characterized by thick

deposits of stromatolitic dolostones, that show agreat lateral variation of facies (Boggiani et al.,

1993; Boggiani, 1998), including phosphorites with

up to 34% P2O5. The overlying Tamengo Forma-

tion is mainly composed of dark, organic rich

limestones and marls with occasional Cloudina -

event accumulations (Almeida, 1965, 1984; Zaine,

1991; Boggiani, 1998). Dark limestones and lime-

stone�/dolostone rhythmites included by Almeida(1984) in the Cerradinho Formation were consid-

ered by Gaucher (2000: 107) as part of the basal

Tamengo Formation. Finally, carbonates of the

Tamengo Formation are concordantly overlain by

gray siltstones of the Guaicurus Formation (Al-

meida, 1965; Boggiani, 1998), marking the end of

deposition in the Corumba basin.

The Corumba Group was deformed and meta-morphosed in the Cambrian-lowermost Ordovi-

cian (540�/490 Ma) during the Paraguay Orogeny,

as defined by Basei and Brito Neves (1992). This

orogenic event has been either considered as a

terminal event of the Brasiliano-Pan African

Megacycle (Almeida, 1984; Alvarenga and Tromp-

ette, 1993; Trompette in Alvarenga et al., 2000) or

as a separate orogeny (Basei and Brito Neves,1992: 333; Gaucher, 2000; see above). Almeida

(1984) defined two main structural domains in the

Paraguay Belt, based on observed tectonic style,

metamorphism and lithology. The ‘Metamorphic

Brazilides’ represent the most deformed and me-

tamorphosed domain to the E, in contrast with the

‘Non-metamorphic Brazilides’, which include the

less deformed regions adjacent to the craton(Almeida, 1984; Fig. 3). This scheme proved very

successful in explaining the different tectonic styles

that shows the Corumba Group in the different

domains, and has been adopted by most authors

dealing with the Paraguay Belt (Almeida, 1984;

Boggiani et al., 1993; Alvarenga and Trompette,

1993; Boggiani, 1998; Alvarenga et al., 2000).


Granitogenesis in the Paraguay Belt is mainlyconfined to the ‘Metamorphic Brazilides’. Post-

tectonic granites such as the Sao Vicente Granite

yielded ages of 5049/12 Ma (K/Ar in biotite:

Almeida and Mantovani, in Boggiani, 1998: 24),

giving minimum age-constraints for the Corumba

Group.

3. Lithostratigraphy and sedimentary environments

3.1. Comparative lithostratigraphy

3.1.1. Common features

The comparison of both sedimentary sequences

reveals the following common features of the

Arroyo del Soldado and Corumba groups:

(1) Both successions rest with erosional andangular unconformity on an Archean to Meso-

proterozoic granitic�/metamorphic basement that

is consistently older than the Brasiliano-Pan

African Orogeny (:/750�/600 Ma: Campos Neto

(2000)), a fact of great importance for palaeogeo-

graphic reconstructions of the region.

(2) Both groups begin with a siliciclastic, dee-

pening-upward sequence unconformably overlyingpre-Brasiliano basement, represented by the Yer-

bal and Cerradinho formations. These units con-

tain rather thin, mature, basal conglomerates and

sandstones, with complete absence of volcanics,

volcanoclastics and pyroclastites (Almeida, 1984;

Boggiani et al., 1993; Boggiani, 1998; Gaucher et

al., 1998a,b; Gaucher, 1999, 2000). These facts

indicate that the basin was tectonically stable andhad a low palaeorelief, suggesting that the cause of

the transgression was an eustatic sea-level rise

rather than extensional tectonics. At the top, the

mentioned formations are composed mainly of

green, chloritic and glauconitic siltstones, which

contain an acritarch-assemblage strongly domi-

nated by Bavlinella faveolata (Gaucher, 1999,

2000).(3) The siliciclastic transgressive units of both

groups pass to the top with sharp contact into

thick carbonate deposits (Polanco and Bocaina-

Tamengo Formations; Fig. 4). The Polanco and

Tamengo formations are mainly made up of dark,

organic-rich, clastic limestones and limestone�/

dolostone rhythmites (Boggiani, 1998; Gaucher,

1999, 2000). The carbonatic rhythmites are very

distinctive lithologies, that have been cited only

from few places around the world (Bose, 1979;

Fairchild, 1980; Gaucher, 1999, 2000), and indi-

cates that both the Arroyo del Soldado and

Corumba groups presented the special conditions

necessary for their genesis. Sedimentary structures

and facies indicate that the Tamengo Formation

was deposited in a shallower basin than the

Polanco Formation. Tempestites are frequent in

both units, as documented by occurrence of

graded limestone beds with hummocky cross-

stratification and event-accumulations of Cloudina

in the Corumba Group (Zaine, 1991; Boggiani et

al., 1993; Boggiani, 1998; Gaucher, 2000). These

facts indicate the common occurrence of storms in

both basins, probably due to a tropical setting

(Seilacher and Aigner, 1991).

(4) Carbonate deposits of the Tamengo and

Polanco Formations are overlain by thick silici-

clastic, mainly pelitic units, namely the Guaicurus

and Cerro Espuelitas Formations, respectively

(Boggiani, 1998; Gaucher, 2000). These units are

made up mainly of siltstones and shales, with

minor carbonates at the base. Thick chert and BIF

deposits of the middle Cerro Espuelitas Formation

are absent in the Guaicurus Formation. A possible

explanation could be that only the lower portion

of the Cerro Espuelitas Formation is represented

in the Corumba Group, as also suggested by the

fossils that occur in the Guaicurus Formation. The

chemostratigraphic data obtained so far support

these ideas as well (see below).(5) Palaeocurrents measured in the Corumba

Group consistently indicate that the basin dee-

pened to the east (Boggiani, 1990, 1998; Alvar-

enga, 1990; Boggiani et al., 1993) and point to a

source-area in the Rıo Apa Block, to the west

(Figs. 1 and 3). In the case of the ASG, palaeo-

currents measured also indicate a shelf that

deepened to the SE�/E, with a palaeoshoreline

roughly N�/S and source areas to the west

(Montana and Sprechmann, 1993; Gaucher,

2000). Facies arrangement and thickness in both

basins corroborate palaeocurrent data.


3.1.2. Differences

There are some important differences in the

lithostratigraphy of the Corumba and ASGs which

need to be considered here:

(1) A clear record of glacial episodes in SW-

Brazil is provided by the Jacadigo Group and by

tillites of the Puga Formation (Almeida, 1984;

Alvarenga, 1990; Alvarenga and Trompette, 1992;Alvarenga et al., 2000). Evidence of glaciation is so

far absent in the ASG. The Puga Formation is

composed of massive, polimictic diamictites in-

cluding angular, faceted and striated clasts up to

some decimeters in size (Almeida, 1984: 272�/

273Alvarenga, 1990; Zaine, 1991; Alvarenga and

Trompette, 1992; Boggiani, 1998). Intercalations

of pelites and fine sandstones including decimetricdropstones also occur (Almeida, 1984; Fig. 4) and

corroborate the interpretation of a glacial, most

probably glaciomarine depositional environment

for the unit (Alvarenga, 1990; Alvarenga and

Trompette, 1992; Alvarenga et al., 2000). Taking

into account that the Puga Formation is concor-

dantly overlain by the upper Vendian Corumba

Group (Almeida, 1984; Boggiani, 1998; Alvarengaet al., 2000), these deposits represent the record of

the Varangerian/Marinoan Ice Age (Hambrey and

Harland, 1985; Harland, 1989; Kasting, 1992;

Kaufman et al., 1997; Saylor et al., 1998; Knoll,

2000) in the Corumba basin (Alvarenga, 1990: 34;

Alvarenga and Trompette, 1992). As noted above,

the Cadiueus Formation may represent glacial-

outwash deposits generated during the retreat ofthe ice-sheet. Two main scenarios can be proposed

to explain the absence of Varangerian or older

glacial deposits in the ASG, namely: (a) a tropical

palaeogeographic location, inside the circum-

equatorial ice-free zone (Hyde et al., 2000; Run-

negar, 2000), where glaciation only led to sea-level

drop, leaving no glacial deposits; and (b) glacia-

tion with complete erosion of the sedimentaryrecord of the Varangerian in the basin. Of these

scenarios, the former is prefered because there is

indeed abundant evidence of a tropical setting for

the ASG, but complete erosion of the record

cannot be ruled out (Montana and Sprechmann,

1993; Gaucher et al., 1996; Gaucher, 1999, 2000).

(2) Sedimentary structures indicate that the

Corumba Group was deposited in a shallowerbasin, leading to a greater facies-variation com-

pared to the ASG. While the Tamengo Formation

shows a quite rich facies variability (Boggiani,

1998; Boggiani et al., 1993), the Polanco Forma-

tion presents a great lateral facies-persistence

(Gaucher, 1999, 2000). Composite thickness of

the ASG exceeds 5000 m (Gaucher, 2000) and is

Fig. 3. Generalized geological map of SW-Mato Grosso do

Sul, showing broad distribution of lithologies of the Corumba

Group, adapted from Boggiani (1998). Occurrences of other

Neoproterozoic-Cambrian units is also shown. Numbers in-

dicate sites referred to in the text.


thus roughly 10 times greater than that of the

Corumba Group (600 m: Boggiani, 1998), in

accordance with deposition in a deeper and more

rapidly subsiding basin.(3) The stromatolitic and oolitic dolostones of

the Bocaina Formation (sensu Boggiani et al.,

1993; Boggiani, 1998) do not have a counterpart in

the ASG. Absence of stromatolites at the base of

the Polanco Formation is due to a deeper sedi-

mentary environment. Oolitic calcarenites have

been found, however, in the Polanco Formation

in the Isla Cristalina de Rivera (Gaucher, 2000;

Fig. 2). Moreover, the fact that the Cerradinho

Formation passes directly into non-stromatolitic

carbonates of the Tamengo Formation at many

localities indicates that the Bocaina Formation

could be restricted to the shallowest areas of the

Corumba basin. Boggiani (1998: 64) proposed that

the Bocaina Formation represents the transgres-

sion of the Vendian Sea onto a peneplainized

basement (Pedra Branca surface) generating a

large and shallow epicontinental basin. In deeper

sections like in the Serra da Bodoquena, carbo-

nates of the Tamengo Formation rest directly on

the Cerradinho Formation, closely parallelling the

lithostratigraphy of the ASG (Gaucher, 2000). At

Fazenda Baıa das Garcas (points 3�/4, Fig. 3), for

instance, the upper Cerradinho Formation passes

directly into dark limestones and limestone�/do-

lostone rhythmites, stromatolitic dolostones of the

Bocaina Formation being completely absent at

that locality (Boggiani, 1990, 1998; Zaine, 1991;

Gaucher, 1999, 2000).

(4) Black phosphorites with up to 34% P2O5

have been reported from the Bocaina and basal

Tamengo Formations (Boggiani et al., 1993;

Boggiani et al., 1998). They are made up of

cryptocrystalline fluorapatite and contain large

amounts of organic matter (�/5%). Such litholo-

gies have not been found so far in the ASG, where

the only traces of phosphate are relicts in the shells

of Waltheria marburgensis at the top of the Yerbal

Formation (Gaucher and Sprechmann, 1999;

Gaucher, 2000). As known from recent examples,

phosphorite deposits are mainly the consequence

of upwelling of deep water at the western margins

of continents under favourable wind and current

regime (Baturin, 1982). Evidence of upwelling in

the upper Yerbal Formation has been presented byGaucher (1999, 2000), including acritarch assem-

blages indicative of basin eutrophication, occur-

rence of glauconitic and chamositic siltstones,

banded ferruginous siltstones, and recently dis-

covered BIF. Considering the phosphorite occur-

rences in the Corumba Group restricted to rather

thin beds (Fig. 4) and the mentioned evidences of

upwelling in the ASG, we conclude that the

occurrence of phosphatic rocks in the latter unit

cannot be ruled out. Furthermore, upwelling

played indeed a major role in both groups at the

contact between basal siliciclastics and overlying

carbonates, probably indicating a similar palaeo-

geography for both basins.

(5) The ASG includes thick deposits of chert and

BIF in the Cerro Espuelitas Formation (Gaucher

and Schipilov, 1994; Gaucher et al., 1996, 1998b;

Gaucher, 1999, 2000), which are unknown in the

Corumba Group (Fig. 4). The BIF represent

micro- and meso-banded, oxide-facies BIF

(Beukes, 1973; James, 1983) containing up to 35

wt.% magnetite or hematite and occurring in up to

three horizons with maximum thickness of 200 m.

Thickness of banded, massive and ferruginous

cherts of the middle Cerro Espuelitas Formation

reaches 400 m (Gaucher et al., 1996, 1998b;

Gaucher, 1999, 2000). The Guaicurus Formation

is probably correlative to the lowermost Cerro

Espuelitas Formation, where BIF and chert are

absent (Gaucher 1999, 2000). As already men-

tioned above, BIF of the Jacadigo Group cropping

out in the region of Corumba, are considered older

than both the Corumba and ASG, being probably

of Varangerian or even Sturtian age (Almeida,

1984; Walde, 1987; Zaine, 1991; Gaucher, 2000).

Fig. 4. Composite stratigraphic columns of the Corumba and ASGs, showing correlations between lithostratigraphic units and

stratigraphic distribution of most important fossil-taxa (sources: Boggiani, 1998; Gaucher, 2000 and this work). CV, Cerro Victoria

Formation; Cerros SF, Cerros San Francisco Formation; Cad, Cadiueus Formation. The Cadiueus Formation has not been drawn to

avoid a more complicated representation (see text). Shaded intervals in the ASG represent periods of basin eutrophication.


Fig. 4


(6) Gaucher (2000) recognized an impressiveclimatic deterioration recorded at the boundary

between the Polanco and Barriga Negra forma-

tions, on the basis of: (a) termination of carbonate

deposition at the top of the Polanco Formation;

(b) change of clay-mineral composition of silt-

stones; and (c) destabilization of the shallower

areas of the Arroyo del Soldado shelf due to sea-

level drop, leading to cannibalization of shelfdeposits and local deposition of up to 1500 m of

conglomerates of the Barriga Negra Formation.

These conglomerates do not have a correlative in

the Corumba Group, although the basin was

shallower than the Arroyo del Soldado basin.

Possible explanations could be that: (1) palaeor-

elief was much steeper in some parts of the Arroyo

del Soldado basin than in the Corumba basin; (2)the regression left only an erosive record in the

Corumba Group; and/or (3) influence of isostatic

reactivations of the Sarandı del Yı-Piriapolis

lineament were also responsible for the genesis of

the conglomerates, a factor absent in the Corumba

basin.

3.2. Geotectonic setting and sedimentary

environments

The analysis of the preceding data indicates that

the Corumba and ASGs were deposited in similar

sedimentary environments and geotectonic set-

tings. Both units were deposited on a stable,

Atlantic-type continental margin, in a predomi-

nantly marine environment (Almeida, 1984: 274;

Zaine, 1991; Boggiani et al., 1993; Gaucher et al.,1996, 1998b; Boggiani, 1998; Gaucher, 1999,

2000). Evidence of tectonic stability in both basins

is provided by: (a) the absence of volcanic,

pyroclastic or volcanoclastic rocks; (b) textural

characteristics of sandstones, which are mainly

quartz-arenites and subarkoses (Pettijohn et al.,

1987); (c) provenance of sandstones and conglom-

erates from a deeply eroded, granitic and(para)metamorphic source-area; and (d) develop-

ment of extensive carbonate ramps and stromato-

lite buildups.

The Corumba and ASGs were deposited in a

marine sedimentary environment, as shown by

microfossil assemblages, skeletal fossils, vendotae-

nids (see below), sedimentary structures, petrogra-phy and facies distribution. Both basins record the

transgression that followed the termination of the

Varangerian glaciation, as indicated by biostrati-

graphy and carbon isotopes. This is specially clear

in the Corumba Group, where this glacial episode

is represented by the tillites of the underlying Puga

formation (Almeida, 1984; Alvarenga, 1990). The

glacioeustatic sea-level rise that followed theVarangerian determined the deposition of the

basal units of both groups. In fact, the major

control of sedimentation was provided by (gla-

cio)eustasy and not by tectonic processes, in

accordance with the proposed geotectonic setting.

Evidences of drastic climatic changes coupled with

eustatic sea-level oscillations are widespread in the

Corumba and ASGs. The lower portions of thebasal Cerradinho and Yerbal formations were

deposited under cool and/or arid conditions, as

shown by sandstone composition (Boggiani, 1998:

44; Gaucher, 1999, 2000) and clay-mineral suites

present (Gaucher, 1999, 2000). At the base of the

Bocaina, Tamengo and Polanco Formations, cli-

mate shifted towards warm, tropical conditions,

because: (a) oolites are widespread (Zaine, 1991;Boggiani, 1998; Gaucher, 2000), which are typical

of tropical regions (Fuchtbauer and Richter, 1988:

335); (b) abundant carbonatic tempestites in the

Tamengo and Polanco Formations (Boggiani et

al., 1993; Boggiani, 1998; Gaucher, 2000) indicate

recurrent large storms which are more common at

low latitudes (Seilacher and Aigner, 1991: 252); (c)

clay-mineral and terrigenous are composed mainlyof kaolinite (Gaucher, 1999, 2000) and quartz-

arenite, respectively (Boggiani, 1998: 68�/69; Gau-

cher, 1999, 2000), suggesting intense chemical

weathering in the source-areas (Pettijohn et al.,

1987; Heling, 1988); and finally (d) the huge

amounts of carbonate deposited in both basins

are also indicative of a tropical setting. Up section,

climate deteriorated again, as shown by the inter-ruption of carbonate deposition at the transition

to the Guaicurus and Barriga Negra�/Cerro

Espuelitas formations. Gaucher (1999, 2000)

showed that this climate change caused an im-

portant regression which exposed the shallower

areas of the Arroyo del Soldado shelf. An asso-

ciated negative d13C-excursion in unaltered carbo-


nates (see below), also suggests that this regressioncould be the consequence of global cooling or even

glaciation (Kaufman et al., 1997; Jacobsen and

Kaufman, 1999; Knoll, 2000; Kaufman, 2000).

Upwelling was another important factor in both

shelves, leading to deposition of BIF, chert and

phosphorites, causing phytoplankton blooms and

eutrophication of the basins (Boggiani et al., 1993;

Gaucher et al., 1996: 363; Boggiani, 1998; Gau-cher, 2000; Fig. 4). This fact indicates that the

palaeogeographic setting of both basins was very

similar, with a combination of shoreline orienta-

tion, current and wind regime that promoted

upwelling of deep, cold seawater (Baturin, 1982).

Indications that the basin was stratified and had

episodic periods of eutrophication related to

upwelling are: (a) BIF occurring in the ASG andphosphorites in the Corumba Group; (b) strong

dominance of the low-diversity microfossil assem-

blages by one or two taxa, including Bavlinella

faveolata (Gaucher, 2000; Fig. 4), as also found for

other upper Vendian successions worldwide (Man-

suy and Vidal, 1983; Palacios, 1989; Vidal and

Nystuen, 1990; Steiner, 1994); and (c) occurrence

of organic-rich horizons with up to 11% TOCsuggesting high bioproductivity and enhanced

preservation of organic matter.

A substantial difference observed is, that while

the ASG was deposited in a typical marginal or

pericontinental sea (sensu Einsele, 1992), the

Corumba Group was in part deposited in a

shallower epicontinental sea over a peneplainized

craton. This could explain the greater thickness ofthe ASG, as well as its more complete sedimentary

record (Einsele, 1992: 124). The Arroyo del

Soldado basin was steeper and responds to a

typical marginal sea (Einsele, 1992), with great

lateral facies-persistence over hundreds of kilo-

meters and deposition of a thick sedimentary

wedge.

4. Palaeontology

4.1. Previous work

The first fossils to be reported from the units

considered here were calcareous tubes of the

Tamengo Formation described by Beurlen andSommer (1957) as Aulophycus lucianoi , and later

included in the genus Cloudina by Zaine and

Fairchild (1985) and Hahn and Pflug (1985).

Hahn et al. (1982) reported the occurrence of

possible Scyphozoa in the Tamengo Formation,

erecting the taxon Corumbella werneri . The occur-

rence of acritarchs in the Corumba Group was

first reported by Fairchild and Sundaram (1981)and later confirmed by Zaine and Fairchild (1987),

which assigned them to the species Bavlinella

faveolata and cf. Vandalosphaeridium sp. The first

systematic study of the palaeontology of the

Corumba Group was made by Zaine (1991), who

described acritarchs, filamentous microfossils, ven-

dotaenids, and possible metazoans of the genus

Cloudina and Corumbella . Boggiani (1998) re-ported new occurrences of Cloudina and Corum-

bella , extending the known stratigraphic range of

these taxa in the Corumba Group. Gaucher (2000)

describes Eoholynia sp. from the lower Guaicurus

Formation. Finally, Fairchild et al. (2000) men-

tioned the occurrence of possible ichnofossils,

newly discovered stromatolites, oncolites and

silicified colonies of probably cyanobacterial affi-nities.

Palaeontologic studies began more recently in

the ASG. Montana and Sprechmann (1993) re-

ported the occurrence of stromatolites and ichno-

fossils in the Cerro Victoria Formation. Gaucher

and Schipilov (1994) described the first acritarchs

from BIF of the Cerro Espuelitas Formation. A

number of publications by Gaucher et al. (1996,1998b) and Gaucher and Sprechmann (1998)

followed, describing organic-walled microfossils

from almost the entire ASG. An assemblage of

five genera and species of skeletal fossils including

Cloudina riemkeae Germs (1972) has been re-

ported by Gaucher and Sprechmann (1999). Fi-

nally, Gaucher (2000) presented detailed

palaeontologic and biostratigraphic informationfor the ASG, also describing fossil material from

the Corumba Group.

In the following section, new fossil material

from the Corumba Group is described and com-

pared with the fossil assemblages of the ASG. All

specimens described are deposited in the collection

of the Departamento de Paleontologıa of the


Facultad de Ciencias (FCDP) in Montevideo(Uruguay).

4.2. Organic-walled microfossils

Organic-walled microfossils are abundant and

well preserved in the ASG. Although microfossils

and kerogens are also abundant in the Corumba

Group, preservation is quite poor due to carboni-

zation attributed to a thermal event associatedwith generation of anorogenic basaltic dykes

(Walde, 1987) or unspecified contact metamorph-

ism (Trompette et al., 1998: 595). This thermal

event also affected the Mn-ore paragenesis of the

Jacadigo Group, as well as their oxygen isotopic

composition (Trompette et al., 1998 and references

therein). Therefore, the reported taxa should be

taken only as a fragment of the original micro-biota, because most of the organic remains are

unidentifiable.

Kingdom Eubacteria Woese & Fox, 1977

Phyllum Cyanobacteria Stanier et al., 1978

Classis, Ordo et Fam. indet.

Genus Bavlinella (Schepeleva) Vidal, 1976

Type species: Bavlinella faveolata (Schepeleva)

Vidal, 1976

Bavlinella faveolata (Schepeleva), Vidal, 1976

Fig. 5C�/E, G�/H; Fig. 6F

1974 Sphaerocongregus variabilis Moorman: pls. 1�/3

1976 Bavlinella faveolata Vidal: figs. 7A�/C

1990 Sphaerocongregus variabilis Vidal & Nystuen:

fig. 9A�/B, D�/E, G�/L

1991 Sphaerocongregus variabilis Zaine: fig. 2A.

1992 Bavlinella faveolata Schopf: pl. 54J1�/J3

1996 Bavlinella faveolata Gaucher et al.:

figs. 7.1�/7.2

2000 Bavlinella faveolata Gaucher: pl. 9, pl. 18.1�/18.2

Type specimen. Vidal (1976) adopted the diag-

nosis given by Moorman (1974) for Sphaerocon-

gregus variabilis as the valid diagnosis for

Bavlinella faveolata . Vidal and Nystuen (1990)

find that the type specimen illustrated by Shepe-

leva (1962), in Vidal, 1976) ‘is in fact the organic

residue after maceration of framboidal pyrite’, andrecommend the use of the junior synonim instead

of Bavlinella faveolata for this species. Never-

theless, German et al. (1989) had already desig-

nated a lectotype for the species from the Kotlin

Formation of the former USSR. This lectotype has

been also illustrated by Schopf (1992: pl. 54-J).

Therefore, the valid designation of a lectotype

supersedes any previous restriction of the applica-tion of the name of the genus and species

Bavlinella faveolata . Sphaerocongregus variabilis

Moorman (1974) is thus to be considered as a

junior synonim.

Material. Hundreds of specimens in thin-sec-

tions of siltstones and palynological macerations

of limestones and marls. In the siltstones, the

specimens are commonly permineralized withiron-oxides (Fig. 5G�/H). The species often occurs

in acid macerations, showing advanced carboniza-

tion which gives the vesicles a gray to black colour

(Figs. 5D and 6F).

Description. The observed specimens are single

spheroidal vescicles made up of hundreds of

tightly packed, micron-sized microspheres, thus

corresponding to the endosporangia morphotypeof Moorman (1974).

Dimensions. Specimens preserved in green silt-

stones of the upper Cerradinho Formation range

in diameter between 4.0 and 21.5 mm (mean�/9.9

mm, S.D.�/4.0 mm, N�/40). This population was

measured mainly in thin sections, but specimens in

palynological macerations were also considered.

On the other hand, the population palynologicallyrecovered from marls of the Tamengo Formation

show larger diameters, between 16 and 45 mm

(mean�/27.9 mm, S.D.�/7.8 mm, N�/13). Zaine

(1991: 109) found vesicles with diameters ranging

between 4 and 15 mm (mean�/7 mm, N�/20) in

macerations of the same stratigraphic levels of the

Tamengo Formation. This strongly suggests that

the differences in mean size observed are due eitherto facies dependence of the populations or to

preparation biases (i.e. mesh apertures of sieves

used).

Distribution. The occurrence of this taxon in the

Tamengo Formation was already noted by Zaine

and Fairchild (1987) and Zaine (1991). Bavlinella

faveolata occurs in large masses in green siltstones


of the upper Cerradinho Formation in the Serra da

Bodoquena (point 3, Fig. 3), and also in black,

organic-rich marls of the Tamengo Formation

near Corumba (Pedreira Saladeiro/Itau, point 9

of Fig. 3), as described by the above mentioned

authors.

Remarks. Bavlinella faveolata shows more

variability and better preservation in the ASG,

with occurrence of all morphotypes described by

Moorman (1974), as noted by Gaucher (2000).

Apart from this, the fossils of the Corumba Group

are identical to the corresponding morphotypes of

the ASG. Bavlinella faveolata is closely associated

with green siltstones in the Cerradinho Formation

occurring in large numbers and dominating the

fossil assemblage. In this facies, Bavlinella is

Fig. 5. Organic-walled microfossils from the Corumba and ASGs, as observed in thin sections (C, E�/H) and palynological

macerations (A�/B, D). (A) Vendotaenia antiqua , fragment from green siltstones of the upper Cerradinho Formation (point 3, Fig. 3).

(B) Saucer-shaped colony of Soldadophycus bossii from gray siltstones of the lowermost Guaicurus Formation (Pedreira Laginha,

point 10 of Fig. 3). Note typical transitions between spheroids and filaments. (C) Bavlinella faveolata , vesicle-chain occurring in green

siltstones of the upper Cerradinho Formation (point 3, Fig. 3). (D�/E) Solitary vesicles of Bavlinella faveolata from the same facies and

locality as C. (F) Bavlinella faveolata , specimen FCDP 3189 from the Yerbal Formation of the ASG (near Minas de Corrales). (G�/H)

Solitary, hematized vesicles of Bavlinella faveolata , occurring in green siltstones of the upper Cerradinho Formation (point 3, Fig. 3).

Note loss of details due to ferrification. Scale bars represent 10 mm for all figures.


Fig. 6. Organic-walled microfossils recovered by means of palynological macerations from the Corumba Group. (A) Soldadophycus

bossii , carbonized, saucer-shaped colony from gray siltstones of the lowermost Guaicurus Formation at point 10 (Fig. 3). (B)

Soldadophycus bossii , same procedence as the former. (C�/D) Two filament-balls of Siphonophycus robustum from gray siltstones of the

lowermost Guaicurus Formation at point 10 (Fig. 3). (E) Myxococcoides sp., semicarbonized colony in dark marls of the Tamengo

Formation (Pedreira Saladeiro/Itau, point 9 of Fig. 3). Note psilate, loosely aggregated spheroids (arrowed). (F) Bavlinella faveolata ,

single vesicle from the Tamengo Formation (same procedence as E). (G) Leiosphaeridia sp., carbonized specimen under epi-

illumination. Recovered from gray siltstones of the lowermost Guaicurus Formation (point 10, Fig. 3). Scale bars represent 10 mm for

all specimens.


typically hematized, sometimes obscuring mor-phologic detail (Fig. 5G�/H). The same happens

in the correlative Yerbal Formation and also in the

Cerro Espuelitas Formation of the ASG (Gaucher,

2000). The palynofacies are in fact almost indis-

tinguishable, probably due to eutrophic conditions

fueled by high nutrient (P, Fe, N) availability in

both basins.

Bavlinella faveolata is a long-ranging fossil,

found in successions ranging from the UpperRiphean (Vidal, 1976; Samuelsson, 1997) to the

Ordovician (Reitz, 1991). Well-preserved fossils

clearly belonging to this taxon have been found by

Salamon and Gaucher (in preparation) in Middle

Devonian (Givetian) siliceous shales of the Rhei-

nisches Schiefergebirge of Germany. Nevertheless,

it is clear that the taxon reached its acme in the

upper Vendian, when it was a dominating compo-nent of the biota worldwide (Moorman, 1974;

Mansuy and Vidal, 1983; Knoll and Sweet, 1985;

Germs et al., 1986; Palacios, 1989; Vidal and

Nystuen, 1990; Gaucher, 2000).

Genus Siphonophycus Schopf (1968), emend Knoll

et al. (1991)

Type species: Siphonophycus kestron Schopf, 1968

Siphonophycus robustum Schopf (1968), emend

Knoll et al. (1991)

Fig. 6C�/D

1968 Eomycetopsis robusta Schopf: pls. 82.2�/3; 83.1�/4

1991 Siphonophycus robustum Knoll et al.: figs. 10.3, 10.5

1994 Siphonophycus robustum Butterfield et al.: figs. 26A, G

1994 Siphonophycus robustum Hofmann and Jackson: fig. 11.5

Type specimen: Specimen of plate 83.1 (a, b) of

Schopf (1968)

Material. Three mat fragments and one com-

plete filament ball in macerations of gray siltstones

of the base of the Guaicurus Formation atPedreira Laginha, near Corumba (point 10, Fig.

3).

Description. Psilate, unbranched, nonseptate

filaments 0.8�/3.0 mm in diameter (mean�/1.8

mm, S.D.�/0.5 mm, N�/21). The entangled fila-

ments occur either as irregular mat-fragments or

as filament balls (Fig. 6C�/D).

Remarks. The species of the genus Siphonophy-

cus are distinguished on the basis of trichome-size

(Knoll et al., 1991; Butterfield et al., 1994). The

material from the Guaicurus Formation has a size

distribution intermediate between S. septatum

(Schopf) Knoll et al. (1991) and S. robustum .

The latter is characterized by filament diameters in

the range of 2�/4 mm (Knoll et al., 1991; Butterfield

et al., 1994; Hofmann and Jackson, 1994). Never-theless, the habit of the filament-aggregates de-

scribed here is identical to S. robustum . Butterfield

et al. (1994: fig. 26G) report the occurrence of S.

robustum -balls similar to modern Nostoc-balls in

the Svanbergfjellet Formation of Spitsbergen,

which closely resemble our material. Moreover,

they find that the species is the most common

shale-facies microbial mat-builder, as is the case inthe Guaicurus Formation. Therefore, we assign

the material to S. robustum (Schopf) Knoll et al.

(1991).

Incertae sedis

Group Acritarcha Evitt, 1963

Genus Leiosphaeridia Eisenack (1958), emend

Downie & Sarjeant (1963)

Type species: Leiosphaeridia baltica Eisenack

(1958)

Leiosphaeridia tenuissima Eisenack (1958)

1958 Leiosphaeridia tenuissima Eisenack: pl. 1.2�/1.3

1994 Leiosphaeridia tenuissima Butterfield et al.: fig. 16I

1994 Leiosphaeridia tenuissima Hofmann and Jackson: fig.

12E

1998 Leiosphaeridia tenuissima Gaucher et al.: fig. 4.6

2000 Leiosphaeridia tenuissima Gaucher: pl. 11.5

Material. Two well-preserved specimens in ma-

cerations of siltstones of the lowermost Guaicurus

Formation at Pedreira Laginha (point 10, Fig. 3).

Description. Thin-walled, compressed, psilate

spheroidal vesicles with common folds. Diameter

ranging from 70 to 120 mm (only two specimens).The individuals are partially carbonized, showing

a light gray colour.

Remarks. These are the largest acritarchs of the

Corumba microbiota, as is also the case in the

ASG (Gaucher, 2000). Absence of larger acri-

tarchs is typical of uppermost Vendian successions


(Knoll, 1996b; Vidal and Moczydlowska-Vidal,1997).

Leiosphaeridia sp.Fig. 6G

2000 Leiosphaeridia sp. Gaucher: p. 105.

Material. Two complete specimens and some

fragments recovered by acid maceration from

siltstones of the lowermost Guaicurus Formation

at Pedreira Laginha (point 10, Fig. 3).Description. Compressed ellipsoidal vesicles

with rectilinear or curvilinear folds, 72�/150 mm

in maximum diameter. Wall probably thick, and

opaque due to advanced carbonization. Irregular

sculpture of wall probably due to degradation of

an originally psilate wall.

Remarks. Due to the opacity of the vesicles, no

reliable estimation of wall-thickness could bemade. Nevertheless, the specimens are quite dif-

ferent in their degree of carbonization, type of

folds and superficial features to those described

here under Leiosphaeridia tenuissima from the

same locality. These facts suggest that the species

is thicked-walled, unlike L. minutissima . If this

point is confirmed, then the specimens could be

assigned to Leiosphaeridia jacutica (Timofeev)Mikhaylova and Yankauskas (Hofmann and

Jackson, 1994), but more material is needed.

Incertae sedis

Genus Soldadophycus Gaucher et al. (1996)

Type species: Soldadophycus bossii Gaucher et al.

(1996)

Soldadophycus bossii Gaucher et al. (1996)Fig. 5B, Fig. 6A�/B

1989 Tipo B Palacios, pl. V; figs. 1�/4

1996 Soldadophycus bossii Gaucher et al.: figs. 6.1�/6.5; fig.

6.7

1998b Soldadophycus bossii Gaucher et al.: Figs. 4.7�/4.8

1998 Soldadophycus bossii Gaucher and Sprechmann: p. 184

2000 Soldadophycus bossii Gaucher: pls. 14�/15; 17.4

Type specimen. Holotype: specimen FCDP3188, figured by Gaucher et al. (1996: fig. 6.1).

Paratype of saucer-shaped colonies is the specimen

FCDP 3207b (fig. 6.2 of Gaucher et al., 1996).

Material . Twelve colonies and colony-fragments

in palynological macerations of marls, siltstones

and limestones of the upper Cerradinho, Tamengo

and lower Guaicurus formations. Fossil sites for

each of the mentioned units are the locality of Baıadas Garcas (point 3, Fig. 3), Pedreira Saladeiro/

Itau (point 9, Fig. 3) and Pedreira Laginha (point

10, Fig. 3), respectively. The species occurs more

frequently in the Cerradinho and Guaicurus for-

mations.

Dimensions . Diameter of the spheroidal cells

ranging between 2.6 and 8.2 mm (mean�/4.9 mm,

S.D.�/1.6 mm, N�/41). Maximum width of fila-ments varying between 1.7 and 3.5 mm (mean�/2.3

mm; S.D.�/0.6 mm; N�/7). Diameter of saucer-

shaped colonies varying between 57 and 65 mm. A

single vase or bottle-shaped colony occurs, with a

diameter of 90 and 160 mm long. These values are

completely within the cell and colony sizes typical

for the species (Gaucher et al., 1996; Gaucher,

2000: 78).Description . Soldadophycus bossii is character-

ized by the co-occurrence of psilate, spheroidal

cells and septate, branched filaments (Gaucher et

al., 1996). Nevertheless, some colony-types are

made up only of spheroidal cells. In the Corumba

Group, the saucer-shaped colonies dominate (Figs.

5B and 6A�/B), but subspheroidal and vase-shaped

colonies also occur. The typical transitions fromspheroids into filaments and vice versa were also

observed (Fig. 5B). Unlike in the ASG, the fossils

are always carbonized, showing gray to black

(opaque) colours.

Remarks. As in the ASG, Soldadophycus occurs

in the Corumba Group in relatively large numbers

only in the siliciclastic units bracketing carbonate

deposits (Cerradinho and Guaicurus formations).In the Tamengo Formation they are an accessory

element of the microbiota, as in the case of the

Polanco Formation (Gaucher, 2000).

Genus Myxococcoides Schopf (1968)

Type species: Myxococcoides minor Schopf (1968)


Myxococcoides sp.

Fig. 6.E

2000 Myxococcoides sp. Gaucher: p. 105

Material . Three fairly well preserved colonies in

palynological macerations of marls of the Ta-mengo Formation (Pedreira Saladeiro/Itau, point

9 of Fig. 3) and green siltstones of the Cerradinho

Formation (point 3, Fig. 3).

Description. Spheroidal to ellipsoidal, colonial

microfossils. Walls thin, psilate, and rather hyaline

despite advanced carbonization. Cells loosely

aggregated into irregular colonies of up to 50 mm

in maximum dimension. Maximum diameter ofcells ranging between 4.5 and 8.0 mm (mean�/6.3

mm, S.D.�/1.1 mm; N�/10).

Remarks. The microfossils described here under

Myxococcoides sp. resemble M. siderophila Gau-

cher (2000) in size, loose arrangement of cells in

irregular-shaped colonies and wall thickness.

Moreover, M. siderophila is a common component

of the Arroyo del Soldado microbiota. Never-theless, due to the advanced carbonization of the

material studied, an assignment to that taxon

would be premature.

4.3. Vendotaenids

Class Vendophyceae Gnilovskaya (1986)

Order Eoholyniales Gnilovskaya (1986)

Family Eoholyniaceae Gnilovskaya (1986)

Genus Eoholynia Gnilovskaya (1975)

Type species: Eoholynia mosquensis Gnilovskaya

(1975)

Eoholynia corumbensis sp. nov.

Fig. 7 A�/H

2000 Eoholynia sp. Gaucher: pl. 17.7

Type specimen . Holotype: specimen FC DP

3615 (Fig. 7C�/D). Paratypes of an individual

with multiple basal branchs is the specimen FC

DP 3222 (Fig. 7E, also figured by Gaucher, 2000:

pl. 17.7). As paratype of terminal sporangium, wedesignate the specimen FC DP 3616 (Fig. 7H).

Derivation of name. After the town of Corumba,

near which the fossils were discovered.

Material. Twenty five complete specimens plus

innumerable fragments preserved as carbonaceous

impressions in bedding surfaces of gray siltstones,

lowermost Guaicurus Formation.

Type locality. The material described here wascollected in the Pedreira Laginha, located along

road BR-262, 16 km south of Corumba (point 10,

Fig. 3). The fossils were found in situ in gray

siltstones with frequent slumps, 5�/10 m above the

contact with the Tamengo Formation (see strati-

graphic column in Gaucher, 2000: text*/fig. 39).

Diagnosis. A species of Eoholynia characterized

by a cord- or ribbon-like thallus composed of 3�/5main branches, 0.05�/0.6 mm wide (mean�/0.24

mm, S.D.�/0.15 mm, N�/34; Fig. 9). Main

branches develop secondary branches of several

orders, mostly dichotomously. Spherical bodies

are frequently (but not always) attached to the

surface of the main branch, and also to the ends of

the lateral branches (here interpreted as terminal

sporangia). Size of spherical bodies ranging be-tween 0.3 and 1.2 mm (mean�/0.54 mm, S.D.�/

0.26 mm, N�/11). Height of complete specimens

3�/18 mm, mostly around 8 mm.

Discussion. Eoholynia corumbensis differs from

Eoholynia mosquensis Gnilovskaya (1975) mainly

in its considerably larger size. While E. mosquensis

is characterized by main branches 0.1�/0.15 mm

wide (Gnilovskaya, 1979, 1985), E. corumbensis

reaches 0.6 mm (Fig. 9), thus four times wider than

the largest specimens of the former species. Side

branches of E. corumbensis are also much wider

and not so intensely tapering as that of E.

mosquensis . Finally, in the case of E. corumbensis ,

diameter of spherical bodies interpreted here as

sporangia is up to 10 times larger than that

reported for E. mosquensis (50�/100 mm: Gnilovs-kaya, 1979, 1985; Fig. 11). Considering the above

mentioned differences, we find that the erection of

a new species of the genus Eoholynia is well

justified.

Remarks. As already noted by Gnilovskaya

(1979, 1985) and Hofmann (1994), Eoholynia

represents the remains of eucaryotic algae, prob-


ably Phaeophyta or Rhodophyta. Miaohephyton

bifurcatum Steiner (1994), a morphologically simi-

lar species, has been placed by Xiao et al. (1998)

among the brown algae (Phaeophyta). Neverthe-

less, the demonstration of the palaeobiologic

affinities of the species needs a more detailed

study that would be out of the scope of this paper.

E. corumbensis was probably benthic, as also

suggested for E. mosquensis (Gnilovskaya, 1985;

Burzin, 1996). The accumulations found in silt-

stones of the Guaicurus Formation are parau-

tochthonous, as is the case of most Russian

Fig. 7. Eoholynia corumbensis sp. nov. and Tawuia sp. from bedding surfaces of gray siltstones of the lowermost Guaicurus Formation

at Pedreira Laginha (point 10, Fig. 3). (A�/H) Eoholynia corumbensis sp. nov. (A) Complete and straight thallus (FCDP 3613) with

multiple branchs. (B) Specimen FCDP 3614 with three basal branchs and curious levogire arrangement. (C�/D) Holotype FCDP 3615,

with main branch covered with spherical bodies (sporangia). Terminal sporangia are present at the end of the main branch and at two

secondary branches. (E) Paratype FCDP 3222 with intensely branching thallus. (F) Straight thallus-fragment. (G) Terminal

sporangium. (H) Specimen FCDP 3616, paratype of terminal sporangium. (I�/J) Tawuia sp. (I) Slightly curved specimen FCDP 3617.

(J) Straight specimen with numerous superimposed Eoholynia -thalli. Scale bars represent 1 mm for all figures.


occurrences (Burzin, 1996). Regarding the strati-

graphic distribution and evolution of the genus

Eoholynia , the brazilian material is clearly younger

than E. mosquensis from the Redkino Series

(lower Valdaian) of the East European Platform,

because: (1) the lower Guaicurus Formation con-

cordantly overlies limestones of the uppermost

Tamengo Formation with 87Sr/86Sr isotopic rela-

tions of 0.7086 and d13C of up to �/5� PDB

(Boggiani, 1998; Fig. 12), which can be correlated

with upper Valdaian sections worldwide (Jacobsen

and Kaufman, 1999; Walter et al., 2000); and (2)

Gnilovskaya (1979, 1985) finds that Eoholynia is

typical of lower Valdaian rocks of the EastEuropean Platform, which are overlain by Vendo-

taenia -bearing rocks of the Kotlin Horizon (upper

Valdaian). As will be discussed later, Vendotaenia

occurs immediately below Eoholynia in the Cor-

umba Group (Zaine, 1991). In accordance with

our data, Hofmann (1992, 1994) reports younger

occurrences of Eoholynia , probably reaching the

Lower Cambrian.

It is worth noting that the larger size of E.

corumbensis compared with E. mosquensis could

reflect an evolutionary trend among the Eoholy-

niaceae. Older representatives of the family like the

Riphean genus Ulophyton are even smaller than E.

mosquensis (Gnilovskaya, 1979; Hofmann, 1994),

while younger representatives are mostly larger

(e.g. Steiner, 1994).

Order Vendotaeniales Gnilovskaya (1986)

Family Vendotaeniaceae Gnilovskaya (1986)Genus Vendotaenia Gnilovskaya (1971)

Type species: Vendotaenia antiqua Gnilovskaya

(1971)

Vendotaenia antiqua Gnilovskaya (1971)

Fig. 5A

1971 Vendotaenia antiqua Gnilovskaya: pl. XI.6-8

1979 Tyrasotaenia podolica Gnilovskaya

1985 Vendotaenia antiqua Gnilovskaya

1985 Tyrasotaenia podolica Gnilovskaya

1991 Tyrasotaenia sp. Zaine: fig. 5.6, pl. 10

1994 Vendotaenia antiqua Steiner: fig. 41k, l; fig. 45a�/c; fig. 47

Type specimen . Specimen IGGD AN SSSR, No.

6931/20 from the Kotlin Horizon in St. Petersburg,

figured by Gnilovskaya (1971: pl. XI.8).

Material . Zaine (1991) mentions the occurrence

of 25 measured specimens of Tyrasotaenia sp.

(synonimized by Steiner, 1994 with Vendotaenia )in marls of the Tamengo Formation. Our material

includes five fragments recovered by means of

palynological macerations from siltstones of the

Cerradinho Formation and phosphorites of the

Bocaina Formation (Fig. 5A).

Description . The material recovered from the

Cerradinho and Bocaina formations consists of

Fig. 8. Titanotheca coimbrae from phosphorites of the Bocaina

Formation at Fazenda Ressaca (point 6, Fig. 3). Specimens

shown in A�/C were recovered by means of standard palyno-

logical macerations, while specimen shown in D occurs in a

thin-section of the phosphorites.(A) Vase-shaped individual

with rather long neck. (B) Detail of A, showing broken part of

the test and agglutinated rutile-grains. (C) Vase-shaped speci-

men with large portions of test broken apart. Note septum

dividing initial chamber from neck (arrowed). (D) Large

specimen with broken apical end and coarse-grained wall. Scale

bars represent 25 mm for all specimens.


fragments of ribbonlike thalli, with typical long-

itudinal-fibrous structure due to compaction folds.

No cellular structure is present. Width of the

fragments reaches 0.12 mm. The fossils are carbo-

nized and degraded to some extent, showing gray

to black colour.Remarks . The fossils described by Zaine (1991)

as Tyrasotaenia sp. can be confidently assigned to

Vendotaenia antiqua . Steiner (1994) placed Tyra-

sotaenia in synonimy with Vendotaenia . More-

over, size and morphology of the specimens

illustrated by Zaine (1991) are well in accordance

with the diagnosis of Vendotaenia antiqua (Gni-

lovskaya, 1971, 1979, 1985; Steiner, 1994). The

fragmentary material recovered in this work from

the Cerradinho and Bocaina formations closely

resembles the microstructure of Vendotaenia anti-

qua (Gnilovskaya, 1985: pls. 32.3, 33.5, 34.3�/4;

Vidal, 1989: fig. 1C, E). Therefore, we assume that

they are conspecific with the vendotaenids of the

Tamengo Formation. It is worth noting that the

microstructure of Eoholynia from the Guaicurus

Formation is very different to that of Vendotaenia .

While the former represents eucaryotic, red or

Fig. 9. Size-frequency distribution of selected fossils from the Corumba and ASGs. Data shown for C. riemkeae of the ASG were

taken from Gaucher and Sprechmann (1999). Sources of histogrammes of C. lucianoi are Zaine (1991) (right graph) and this work (left

graph). Note important differences between the size-frequency distributions of this species, due to differential hydrodynamic sorting in

Cloudina -bearing tempestites. Size-frequency distribution of Eoholynia corumbensis n.sp. was measured at a single locality, where the

type material occurs.


brown algae, the latter are probably abandoned

sheaths of sulfide-oxidizing organotrophic bacteria

related to the Beggiatoaceae (Vidal, 1989),

although other interpretations cannot be ruled

out (Gnilovskaya, 1985).

Class Chuariaphyceae Gnilovskaya & Ishchenko,in Hofmann (1994)

Order Chuariales Gnilovskaya (1986)

Family Tawuiaceae Ishchenko, in Hofmann (1994)

Genus Tawuia Gnilovskaya (1975)

Fig. 10. C. lucianoi from thin-sections of carbonatic tempestites of the Tamengo Formation at Pedreira Corcal (point 8, Fig. 3). (A):

Longitudinal section of specimen with two stacked tubes. (B) Single, straight tube with closed apical end. (C) Specimen FCDP 3242

with same structure as C. riemkeae forma b Germs (1972). (D) Long, worm-like specimen (arrowed) with sinuous shell. Note

constriction at the middle of the test. (E) Two transverse sections. Note section with two eccentric tubes and robust spine (arrowed). (F)

Transverse section of two attached specimens. Note deformation of smaller specimen along attachment (arrowed). (G) Small specimen

with clear basal peduncule. (H�/I): Large, chalice-shaped specimen with robust spines (arrowed). Closed apical end of specimen is not

shown in H. (J): Chalice-shaped specimen with basal peduncule and two open ends. (K): Specimen FCDP 3241 with stacked tubes and

short, curved basal-peduncule. All scale bars represent 1 mm.


Type species: Tawuia dalensis Hofmann (1979, in

Steiner, 1994)

Tawuia sp.Fig. 7I�/J

Material. Five carbonaceous compressions in

bedding surfaces of gray siltsones of the Guaicurus

Formation at Pedreira Laginha (point 10, Fig. 3),

co-occuring with Eoholynia corumbensis .Description. Sausage-shaped carbonaceous

compressions, straight or ‘C’-shaped, with more

or less rounded ends. Width of compressions: 1.0�/

2.0 mm and length ranging between 5 and 12 mm.

Remarks. Most of the specimens observed are

either straight or slightly curved. Only one speci-

Fig. 11. C. riemkeae from thin-sections of banded siltstones of the Yerbal Formation (ASG). All specimens are preserved hematized

and filled with quartz and hematite. (A): Specimen FCDP 3001 showing: (1) short basal peduncule (?), and (2) two stacked cones

typical of the species. Compare with Fig. 10A. (B): Specimen FCDP 3236 with same morphology as C. lucianoi of Fig. 10K. (C)

Specimen FCDP 2934, showing bud with cone-in-cone structure (arrowed). (D) Specimen FCDP 2991, with same morphology as C.

riemkeae forma b Germs (1972). Compare also with Fig. 10C. (E) Specimen FCDP 3002, completely filled with hematite. (F)

Pedunculate specimen filled with hematite, closely parallelling the morphology of C. lucianoi shown in Fig. 10G. (G) Poorly preserved,

pedunculate specimen filled with sediment. (H) Long, worm-like specimen with sinuous test, analogous to individual shown in Fig.

10D. All scale bars represent 1 mm.


men shows the more typical C-shape. Steiner(1994: pl. 2) illustrates many specimens from the

Liulaobei Formation and Shilu Group of China

similar to those occuring in the Guaicurus For-

mation. The material described here fits well

within the size-range of Tawuia dalensis from

those units. Nonetheless, we leave these fossils in

open nomenclature until more material is avail-

able.

4.4. Skeletal fossils

Phylum indetClassis et ordo indet

Family Cloudinidae Hahn & Pflug (1985)

Genus Cloudina Germs (1972)

Type species. Cloudina hartmannae Germs (1972)

Cloudina lucianoi (Beurlen & Sommer) Zaine &

Fairchild (1985)Fig. 10A�/K

1957 Aulophycus lucianoi n. sp. Beurlen & Sommer: pls. I�/VI

1985 Cloudina lucianoi Zaine & Fairchild: p. 130

1985 Cloudina waldei Hahn & Pflug: fig. 7, pl. 1, pl. 2.3�/2.4

1987 Cloudina lucianoi Zaine & Fairchild: figs. 1�/7

1990 Cloudina lucianoi Grant: fig. 10E

1991 Cloudina lucianoi Zaine: figs. 5.2�/5.3, pls. 3�/6

1998 Cloudina lucianoi Boggiani: fig. 4.33

1999 Cloudina lucianoi Gaucher & Sprechmann: p. 66

2000 Cloudina lucianoi Gaucher: pl. 21.1�/21.3

Type specimen. Specimen No. 1149 of the

palaeobotanic collection of the Divisao de Geolo-gia e Mineralogia (DNPM, Rio de Janeiro),

figured by Beurlen and Sommer (1957: pl. II.b).

Material. Forty-five well-preserved specimens in

thin-sections of calcareous tempestites of the

Tamengo Formation at Pedreira Corcal (point 8,

Fig. 3), and many others in polished slabs of these

samples.

Dimensions. Maximum diameter of fossils stu-died ranging within 0.5�/3.5 mm (mean�/1.35 mm,

S.D.�/0.64 mm, N�/32), well in accordance with

earlier reports by Zaine (1991, 0.2�/3.8 mm) and

Hahn and Pflug (1985, 2.5�/3.1 mm). However,

size-frequency distribution of studied tafocenosis

(Fig. 9) varies according with competence of

palaeocurrents. Length of tubes reaches 11 mm,but Zaine (1991) describes specimens up to 15 mm

in length.

Description. Calcareous fossils consisting of

straight or curved tubes open at one end. Tubes

are vertically and eccentrically stacked one into

another giving cone-in-cone structures. Axes of

stacked tubes are often not parallel to each other.

A basal peduncule and robust spines are present insome individuals. Pedunculate specimens have

commonly two open ends (Fig. 7G, J�/K). Juxta-

posed specimens also occur (Fig. 7F). Shell

probably calcitic, but an important amount of

organic matter in the shell and incomplete miner-

alization is suggested by the flexibility observed in

some individuals, as noted by Grant (1990) for

Cloudina from the Nama Group.Discussion. Zaine (1991) considers that the

name Cloudina lucianoi (Beurlen and Sommer)

Zaine and Fairchild (1985) has priority over

Cloudina waldei Hahn and Pflug (1985) because

‘according to article 10.f of the International Code

of Zoological Nomenclature (1985), for an organ-

ism not as first classified as an animal but later so

classified, the original name is available if validunder the code that it was published. In this case

the genus Aulophycus must be abandoned, but the

specific name lucianoi is still valid’ (translation

from Zaine, 1991: 80). For these reasons, we

consider here C. lucianoi (Beurlen and Sommer)

Zaine and Fairchild (1985) as the valid name of

this species (see Ride et al., 1985). Therefore, C.

waldei Hahn and Pflug (1985) is to be consideredas a junior synonim of C. lucianoi , contrary to the

opinion of Conway Morris et al. (1990: 248).

The fossils represent benthic organisms which

lived in an upright position. Seilacher (1999)

interpret this organisms as (bio)mat-stickers rather

than sediment stickers, mainly because of their

small size compared with other well-known sedi-

ment stickers such as rugose corals. The inter-pretation of such a lifestyle for Cloudina , allowed

Seilacher (1999) to explain the anatomy of these

fossils in terms of functional morphology. Thus,

stacked cones of varying lengths can be explained

as organisms living in sedimentary environments

with different sedimentation rates, while common

kinks in the shell represent occasional perturba-


tions. Therefore, the two forms (a and b) of C.

riemkeae described by Germs (1972) probably

represent two different ecophenotypes: forma arepresents specimens growing under low sedimen-

tation rate, while forma b grew under faster

accumulating sediment. The same is true for C.

lucianoi , which also shows specimens made up of

stacked cones of different lengths. C. lucianoi

presents other morphological variations, including

the presence of robust spines in 15% of the

observed individuals and 18% of pedunculate

specimens. As already mentioned by Hahn and

Pflug (1985: 425), C. lucianoi is larger than C.

riemkeae (diameter 0.3�/1.5 mm; Fig. 9) but

smaller than Cloudina hartmannae , (diameter

2.5�/6.5 mm: Germs, 1972).

Remarks. There are no substantial differences

between C. lucianoi of the Corumba Group and C.

riemkeae from the ASG other than their size-

frequency distribution (Gaucher and Sprechmann,

1999; Gaucher, 2000; Fig. 9) and preservation.

While specimens of C. riemkeae of the ASG are

completely hematized and preserved in life posi-

tion (autochthonous accumulation), C. lucianoi of

the Corumba Group is preserved in tempestites

(parautochthonous accumulation) with its origi-

nal, slightly recrystallized calcareous shell. There-

fore, it can be expected that C. lucianoi from

autochthonous accumulations would show a wider

size-frequency distribution than observed in our

material.

Order Foraminiferida Eichwald (1830)

Suborder Textulariina Delage & Herouard (1896)

Family Saccamminidae Brady (1884)

Subfamily Sacammininae Brady (1884)

Genus Titanotheca Gaucher & Sprechmann (1999)

Type species. Titanotheca coimbrae Gaucher &Sprechmann (1999)

Titanotheca coimbrae Gaucher & Sprechmann

(1999)

Figs. 8.A�/D

1999 Titanotheca coimbrae Gaucher & Sprechmann:

pls. 5�/6; pl. 7.1-7.2; text*/fig. 9

2000 Titanotheca coimbrae Gaucher: pl. 18.3�/18.7;

text*/fig. 34A�/F

Type specimen. Holotype: specimen figured by

Gaucher and Sprechmann (1999: pl. 5, figs. 1�/2)

with catalogue number FC DP 2947.

Material. Five complete specimens and many

fragments in thin-sections and acid macerates of

black phosphorites of the Bocaina Formation at

Fazenda Ressaca (point 6, Fig. 3).

Description. Spheroidal to vase-shaped fossils

with walls composed of a single layer of aggluti-nated rutile grains. Test is composed of one or two

chambers divided by a septum (Fig. 8C), and has

an open (apertural) end. Agglutinated rutile grains

are well sorted but vary considerably among

different specimens. While in the case of specimens

figured in Fig. 8A�/C long axes of rutile crystals

(3�/4 mm) are approximately 6% of maximum

diameter of the test, specimen shown in Fig. 8D

is composed of relatively larger crystals, with long

axes up to 14% of its maximum diameter (25 mm).

Maximum diameter of specimens ranging within

50�/175 mm. Largest specimen observed is 0.55 mmlong (Fig. 8D).

Remarks. On the basis of the agglutinated

nature of the wall, the simple shape of the fossils,

presence of perforated septa dividing the chambers

and small size, Gaucher and Sprechmann (1999)

assigned this genus to the Foraminiferida (sub-

order Textulariina), representing the oldest for-

aminifers currently known. The mode of life of

Titanotheca is inferred to have been free epi-

benthic, on the basis of the shape and robustness

of the test wall (Gaucher and Sprechmann, 1999;Gaucher, 2000). The specimens recovered from the

Bocaina Formation of the Corumba Group are the

only occurrence known to date other than the type

material from the ASG Group. The species has a

high fossilization potential and is quite abundant

in organic-rich facies such as the phosphorites of

the Bocaina Formation. If the taxon is confirmed

to be restricted to the upper Vendian (Valdaian) as

it seems, it may eventually become a useful index

fossil for that period.


5. Biostratigraphy

The palaeontologic data presented above for the

Corumba Group allows some important biostrati-

graphic correlations. First of all, the six genera of

organic-walled microfossils described from the

Corumba Group are also present in the ASG

(Gaucher, 2000). These genera are represented by

six species, three of which also occur in the ASG.The shared species, Bavlinella faveolata , Soldado-

phycus bossii and Leiosphaeridia tenuissima , are

the most abundant and dominate the microbiota

in both units. While in the ASG 19 species of

organic-walled microfossils have been described to

date (Gaucher, 2000), the diversity in the Corumba

Group is considerably lower. However, there is a

substantial preservational bias due to advancedcarbonization and degradation of fossils in the

latter unit. Most of the organic remains that were

found in macerations of the Corumba Group

could not be identified, for carbonization and

corrosion greatly obscures taxonomic-relevant de-

tails. The assemblages of the Corumba and ASGs

belong to the upper Vendian depauperate assem-

blage found in many successions worldwide(Chauvel and Schopf, 1978; Mansuy and Vidal,

1983; Volkova, 1985; Knoll and Sweet, 1985, 1987;

Germs et al., 1986; Palacios, 1989; Zaine, 1991;

Vidal et al., 1994; Gaucher et al., 1996). This

microbiota would match the Kotlin-Rovno assem-

blage of Vidal and Moczydlowska-Vidal (1997),

which is characterized by low diversity, abundance

of Bavlinella faveolata , rarity or absence ofacanthomorphs and absence of large sphaero-

morphs (Vidal and Knoll, 1983; Volkova, 1985;

Knoll, 1996a; Vidal and Moczydlowska-Vidal,

1997). More recently, another biostratigraphic

scheme has been proposed (Knoll, 2000 and

Walter et al., 2000), which recognizes three in-

formal biozones between the Varangerian/Mar-

inoan glacials and the base of the Cambrian. Theperiod immediately above the Varangerian is

characterized by a simple leiosphere palynoflora

(Walter et al., 2000: 383), followed by a complex

acanthomorph palynoflora described from many

sites worldwide (Knoll, 2000, and references

therein). In the uppermost Vendian (Kotlin-Rovno

assemblage of Vidal and Moczydlowska-Vidal,

1997), plankton diversity decreased dramatically,leading again to a depauperate assemblage domi-

nated by small sphaeromorphs. Although no

acanthomorphic acritarchs at all were found either

in the Corumba or ASG, the latter unit could be

divided into three informal zones according to

organic-walled microfossils. Gaucher (2000) re-

cognized two similar low-diversity assemblages

(Bavlinella -Soldadophycus assemblage) in the Yer-bal and Cerro Espuelitas Formations divided by a

more diverse assemblage occurring in the Polanco

Formation (Leiosphaeridia -Lophosphaeridium as-

semblage). These assemblages correlate chemos-

tratigraphically quite well with those proposed by

Knoll (2000) and Walter et al. (2000), the more

diverse assemblage occurring worldwide at a

positive d13C peak with 87Sr/86Sr values of0.7079, just as in the case of the Polanco Forma-

tion (Fig. 12). Gaucher (2000) proposed that this

alternation could be determined by different

palaeoclimatic conditions. Skeletal fossils strongly

indicate an upper Vendian age for most of both

units studied. Cloudina , for instance, has been

proposed by Grant (1990) as an index fossil of this

period, occurring in many successions worldwide(Germs, 1972; Palacios, 1989; Conway Morris et

al., 1990; Zaine, 1991; Bengtson and Yue Zhao,

1992; Vidal et al., 1994; Boggiani, 1998; Gaucher

et al., 1999; Gaucher, 2000; Hagadorn and Wagg-

oner, 2000; Corsetti and Hagadorn, 2000; Grot-

zinger et al., 2000). Therefore, the Corumba

Group is almost entirely upper Vendian in age,

for Cloudina occurs even at the top of theTamengo Formation (Boggiani, 1998). In the

case of the ASG, Cloudina indicates that the

Yerbal Formation was deposited in the upper

Vendian and not before (i.e. in the Varangerian).

On the other hand, Titanotheca seems to be

restricted to the lower parts of both groups

(Yerbal and Bocaina formations), co-occurring in

the ASG with Cloudina . This might imply that thestratigraphic range of Titanotheca is even more

restricted than Cloudina , but more work is needed

to prove this point.

Vendotaenids from the Guaicurus Formation

also give important biostratigraphic information.

Gnilovskaya (1979, 1985) finds that E. mosquensis

is typical of lower Valdaian rocks of the East


Fig. 12. Chemostratigraphy of the Corumba and ASGs, compared with well studied sections from the Witvlei-Nama groups and the

Windermere Supergroup (Narbonne et al., 1994; Grotzinger et al., 1995; Kaufman et al., 1997; Saylor et al., 1998). Negative (N) and

positive (P) d13C-excursions were numbered starting from the first post-Varangerian/Marinoan deposits. Total thickness represents

approximately 2500 m for the Witvlei�/Nama groups (Saylor et al., 1998) and 5000 m for the Windermere Supergroup (Kaufman et

al., 1997). CSF: Cerros San Francisco Formation, CV: Cerro Victoria Formation, Blaube: Blaubeker Formation, Blass.: Blasskrans

Formation, Buschmannsk.: Buschmannsklippe Formation.


European Platform, which are overlain by Vendo-

taenia-bearing rocks of the Kotlin Horizon (upper

Valdaian). As already noted above, E. corumbensis

of the Guaicurus Formation postdates Vendotae-

nia -bearing marls of the Tamengo Formation, and

is thus younger than E. mosquensis . On the other

hand Vendotaenia antiqua is considered to be

restricted to the upper Vendian and occurs in

many successions worldwide. Some examples arethe East European Platform (Gnilovskaya, 1979,

1985; Burzin, 1996), China (Steiner, 1994 and

references therein) and Namibia (Germs et al.,

1986). The association of Vendotaenia with Ta-

wuia is known from the Dengying Formation of

China (Steiner, 1994), of uppermost Vendian age.

Finally, it has been suggested that Corumbella

werneri Hahn et al. (1982) from the CorumbaGroup (Tamengo Formation) could be a repre-

sentative of the Ediacara fauna (Zaine,

1991Walde, 1987: 103) or even belong to the

sabelleditids (Fairchild et al., 2000). In either

case, these fossils would indicate an upper Vendian

(Ediacaran, Valdaian) age for the Tamengo For-

mation. However, an alternative interpretation of

Corumbella as scyphozoans (Cnidaria) cannot beruled out (Hahn et al., 1982; Hagadorn and

Waggoner, 2000), though it seems rather improb-

able (Zaine, 1991).

Summing up, there is quite a lot of biostrati-

graphic information that points unequivocally to

an upper Vendian (Valdaian) age for both groups.

Furthermore, the striking similarity of the biotas

that lived in both basins suggests that they had anample connection or were even part of the same

shelf. It is worth noting that not only the more

widespread planktonic genera and species are

shared, but also representatives of the benthos

like Cloudina and Titanotheca .

6. Chemostratigraphy

The first stable isotope analyses for the Cor-

umba Group were reported by Zaine (1991), but

the data were unfortunately not ordered against

their stratigraphic position, and are of very limited

value for correlation. Boggiani (1998) reports

detailed carbon, oxygen and strontium isotope

analyses for the Corumba and ASGs, among otherNeoproterozoic-Cambrian units of South Amer-

ica. Gaucher (1999, 2000) presents a curve of d13C

variations in carbonates of the ASG, based on the

data reported by Boggiani (1998). Finally, Kawa-

shita et al. (1999) report d13C, d18O and 87Sr/86Sr

data for four samples from the base of the Polanco

Formation of the ASG. The stratigraphic rele-

vance of these determinations have been discussedby Gaucher et al. (1999). We present here new

d13C and d18O determinations for 13 samples of

the upper Yerbal Formation and lowermost Bar-

riga Negra Formation of the ASG. The database

includes, for the Corumba Group, 37 d13C and

d18O determinations, while in the case of the ASG,

d13C and d18O was determined for 30 samples.

Raw data are given in Table 1, while syntheticd13C curves for the Corumba and ASGs are shown

in Fig. 12. Furthermore, preliminary data of a

high-resolution chemostratigraphic survey of the

Polanco Formation (Gaucher et al., 2002) were

also considered. As for Sr-isotopes, a series of 2087Sr/86Sr determinations for carbonates of the

Corumba Group was carried out by Zaine

(1991). Boggiani (1998) presents 6 87Sr/86Sr ana-lyses for the Tamengo Formation. Finally, Kawa-

shita et al. (1999) report the only 87Sr/86Sr data

available for the ASG, consisting of four determi-

nations in little altered samples (Mn/Sr�/0.1; d18O

PDB�/�/11�) of the lowermost Polanco Forma-

tion.

6.1. Corumba Group

The comparison of the d13C curve of the

Corumba Group with the Vendian global curve

of Kaufman and Knoll (1995) and Kaufman et al.

(1997), Saylor et al. (1998), Jacobsen and Kauf-

man (1999) and Knoll (2000); Walter et al. (2000)

shows many significant coincidences. Dolomites of

the lowermost Bocaina Formation capping glacial

deposits of the Puga Formation yielded d13Cvalues of �/5.5 � PDB (Boggiani, 1998), a feature

typical of cap dolomites worldwide (Aitken, 1991;

Narbonne et al., 1994; Kennedy, 1996; Hoffman et

al., 1998; Knoll, 2000; Walter et al., 2000).

Dolomites of the Bocaina Formation up section

have d13C values near zero (Boggiani, 1998), while


Table 1

Carbon and oxygen isotopic data of carbonates of the ASG

Locality Sample d18OSMOW� d18OPDB� d13CPDB� Precision (d18O) Precision (d13C) Remarks

ILL 2 ILL2A �/24.986 �/5.698 �/3.484 0.028 0.007 Stromatolitic limestone, Cerro Victoria Formation

ILL 4 ILL 4 �/24.989 �/5.695 �/0.174 �/ �/ Stromatolitic limestone, Cerro Victoria Formation

ILL 4 ILL 4B �/24.279 �/6.384 �/1.840 0.013 0.006 Stromatolitic limestone, Cerro Victoria Formation

PRJ 37 980317/5 �/20.254 �/10.288 �/1.692 0.003 0.005 Calcarenite, basal B. Negra Fm

PRJ 37 980317/7A �/20.514 �/10.036 �/1.738 0.005 0.014 Lim/dol rhytmite, calcarenite layer, boundary Polanco-B. Negra Fm

PRJ 37 980317/7B �/21.464 �/9.115 �/1.440 0.006 0.005 Lim/dol rhytmite, dolosiltite layer, boundary Polanco-B. Negra Fm

PRJ 13 PRJ 13C �/16.870 �/13.571 �/2.411 0.011 0.006 Limestone, upper Polanco Fm

PRJ 13 PRJ 13P �/16.122 �/14.297 �/2.597 0.006 0.003 Limestone, upper Polanco Fm

PRJ 14 PRJ 14B �/14.798 �/15.581 �/1.998 0.010 0.004 Limestone, upper Polanco Fm

PRJ 14 PRJ 14a1 �/16.726 �/13.711 �/2.808 0.008 0.003 Limestone, upper Polanco Fm

PRJ 14 PRJ 14 b2 �/17.551 �/12.911 �/2.888 0.005 0.002 Limestone; upper Polanco Fm

CPA 35 CPA 35 �/20.892 �/9.670 �/3.630 0.044 0.011 Lim/dol rhymite, C.Espuelitas Fm

CPA 32 970320/7 �/16.000 �/14.415 �/1.331 0.004 0.005 Lim/dol rhymite, C. Espuelitas Fm

CPA 31 970320/6 �/17.445 �/13.013 �/2.516 0.004 0.012 Limestone, basal C. Espuelitas Fm

CPA 30* 000715/3a �/18.139 �/12.341 �/1.944 0.008 0.011 Lim/dol rhytimite, uppermost Polanco Formation

CPA 30* 000715/3b �/20.589 �/9.963 �/1.547 0.005 0.008 Lim/dol rhytmite, uppermost Polanco Fm

CPA 30* 000715/4 �/21.982 �/8.612 �/1.783 0.003 0.012 Dolosiltite, upper Polanco Fm

CPA 40 970503/6 �/14.248 �/16.115 �/1.397 0.004 0.007 Lim/dol rhytmite, Polanco Fm

CPA 28 CPA28D �/19.328 �/11.187 �/2.596 0.019 0.009 Lim/dol rhytmite, lower PolancoFm

CPA 27 CPA 28C �/21.169 �/9.400 �/1.143 �/ �/ Lim/dol rhytmite, lower PolancoFm

CPA 27 CPA 28B �/15.719 �/14.688 �/1.391 0.007 0.004 Lim/dol rhytmite, lower PolancoFm

CPA 27 CPA 28A �/22.902 �/7.720 �/0.891 0.005 0.006 Lim/dol rhytmite, lower PolancoFm

PYE 31 980305/1 �/ �/12.3 �/2.82 �/ �/ Limestone, lowermost Polanco Fm

PYE 32 980305/2 �/ �/11.0 �/2.86 �/ �/ Limestone, lowermost Polanco Fm

CCU 4 980306/12 �/ �/11.4 �/2.56 �/ �/ Limestone, lowermost Polanco Fm

CCU 4 980306/13 �/ �/11.3 �/2.60 �/ �/ Limestone, lowermost Polanco Fm

MIN 15 99027/1 �/26.237 �/4.484 �/2.154 0.005 0.005 Pink dolosiltite, upper Yerbal Fm

MIN 15 990927/2a �/26.185 �/4.535 �/1.553 0.003 0.003 Pink dolosiltitle, upper Yerbal Fm

MIN 16 980311/6 �/26.957 �/3.786 �/1.957 0.002 0.009 Dolosiltite clast, upper Yerbal Fm

MIN 16 980927/1 �/25.174 �/5.516 �/1.167 0.006 0.009 Pink dolosiltite, upper Yerbal Fm

Data from localities PYE 31-32 and CCU 4 are from Kawashita et al. (1999). Analysis from localities CPA 27-28, PRJ 13-14 and ILL 2, 4 were reported by Boggiani

(1998). Other localities: this work.

C.

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limestones of the lower Tamengo Formationrecord a negative d13C excursion (�/2.5 to �/

3.3� PDB) associated with Corumbella - and

Cloudina -bearing strata (Boggiani, 1998). The

negative peaks of the middle Sheepbed Formation

(Windermere Supergroup) and the upper Witvlei-

lower Nama Group (N2: Fig. 12) resemble values

obtained for the lower Tamengo Formation (Nar-

bonne et al., 1994; Kaufman et al., 1997; Saylor etal., 1998). Moreover, this negative peak is globally

associated with the first occurrence of relatively

diverse Ediacaran fossils and Cloudina (Narbonne

et al., 1994: 1284; Saylor et al., 1998: 1233), as in

the Corumba Group. Up section in the Tamengo

Formation, a positive d13C excursion of up to �/

5.8� PDB is recorded in Cloudina -rich beds,

which has been correlated by Boggiani et al.(1996, 1997), with the younger Ediacaran (upper

Vendian) positive excursion (P2, Fig. 12) recorded

worldwide. Uppermost Tamengo limestones show

also positive d13C values, and are thus older than

the Gametrail-Zaris negative excursion N3 (Fig.

12). This important negative excursion is probably

represented in the lower Guaicurus Formation,

but no isotopic data for this unit are available.Alternatively, it could be correlated to the small

d13C-negative peak in the uppermost Tamengo

Formation, with 87Sr/86Sr values of 0.7086 (Bog-

giani, 1998).

6.2. Arroyo del Soldado Group

Varangerian�/Marinoan deposits are not known

from the ASG. Since the lower Yerbal Formationis devoid of carbonates, the post-Varanger nega-

tive excursion (N1, Fig. 12) is not recorded in the

ASG. Pink dolostones of the upper Yerbal For-

mation show positive d13C values of up to �/2.2�PDB (Table 1, Fig. 12), which continue into the

lowermost Polanco Formation, reaching �/2.9�PDB (Kawashita et al., 1999). This peak corre-

sponds to the positive d13C-peak of the lowerSheepbed Formation (Narbonne et al., 1994;

Kaufman et al., 1997) and middle Witvlei Group

(Saylor et al., 1998), because: (a) it is upper

Vendian (post-Varangerian) in age, as demon-

strated by C. riemkeae occurring in the uppermost

Yerbal Formation; and (b) it includes limestones

with 87Sr/86Sr values of 0.7078 (Kawashita et al.,

1999), similar to those encountered in the lower

Sheepbed Formation (Windermere Supergroup)

and in the Buschmannsklippe Formation of the

Witvlei Group (Narbonne et al., 1994; Kaufman et

al., 1997; Walter et al., 2000: 376). Although the

remarkably thick, pure and little altered carbo-

nates of the overlying Polanco Formation repre-

sent the desideratum for the determination of a

precise d13C curve, reconnaissance studies carried

out by Boggiani (1998) and Gaucher (2000) and

the authors yielded ambiguous results. The stra-

totype of the Formation, located in the Tapes

Grande Syncline (Gaucher, 2000) has shown

exclusively negative values troughout the section

(sites CPA 27 to CPA 30* of Table 1). Never-

theless, other sections in the Isla Patrulla Block

and Piraraja region (sites CCU 4, PYE 31�/32 and

PRJ 13�/14 of Table 1; Gaucher et al., 2002) show

that the lower and upper Polanco Formation

records positive d13C excursions, and that carbo-

nates there experienced little diagenetic overprint

(Mn/Sr mostly within 0.05 and 0.5; d18O between

�/6 and �/11�). Differences in the carbon iso-

topic composition could respond to an enhanced

metamorphic overprint at the stratotype, or more

probably, to a very characteristic, deeper sedimen-

tary environment in which primary dolomite

precipitation (Vasconcelos and McKenzie, 1997;

Burns et al., 2000) due to bacterial sulphate

reduction took place (Gaucher, 2000: 32). In

such disaerobic environments, recycling of organic

matter by bacteria yields isotopically light CO2,

which is then incorporated into carbonates, result-

ing in 13C-depletion (Strauss et al., 1992; Kaufman

and Knoll, 1995: 30; Hoefs, 1997: 122). Calver

(2000) proposed that 13C-depletion of deep-water,

upper Vendian carbonates of the Adelaide Rift

Complex (Australia) responds to water stratifica-

tion, which also played a key role in the ASG

(Gaucher, 2000). The sections in which positive

values have been recorded are characterized by

lower palaeobathymetry, above storm-weather

wave-base, and also by less dolomite precipitation.

In modern marine environments, shallow seawater

is enriched in 13C by approximately 1� PDB

relative to deep water (Hoefs, 1997: 121). Ongoing


research with detailed sampling of sections up to800 m thick is intended to solve this problem.

All things considered, it is reasonably well

established that two positive d13C excursions are

recorded in the lowermost Polanco (P1, Fig. 12)

and upper Polanco Formation (P2), separated by

an abrupt negative excursion to �/3.5� PDB (N2).

The transition from the Polanco Formation into

the Cerro Espuelitas Formation or the lowermostBarriga Negra Formation is marked by a shift to

negative values of �/1.7� PDB (lowermost Bar-

riga Negra Formation) or �/3.6� PDB (lower

Cerro Espuelitas Formation). This negative peak

is tentatively correlated to 13C-depleted carbonates

of the N3 excursion (Fig. 12) recorded in the

Gametrail Formation (Windermere Supergroup;

Kaufman et al., 1997) and uppermost ZarisFormation (Nama Group; Grotzinger et al.,

1995; Saylor et al., 1998). No carbonates are

known from the upper Cerro Espuelitas or Cerros

San Francisco Formation, the youngest record

being stromatolitic and oolitic limestones of the

Cerro Victoria Formation (Montana and Sprech-

mann, 1993; Gaucher, 2000). Three analyses

reported by Boggiani (1998) suggest the presenceof a further negative d13C excursion. Taking into

account the lowermost Cambrian age proposed for

this unit on the basis of trace fossils (Montana and

Sprechmann, 1993; Gaucher, 2000), the mentioned

negative peak could match the negative d13C

excursion of the Proterozoic-Cambrian boundary

N4 (Fig. 12) recorded worldwide (Kaufman et al.,

1997; Saylor et al., 1998; Knoll, 2000; Corsetti andHagadorn, 2000), but more data are needed to

confirm this.

Summing up, there is a reasonably good corre-

lation between the d13C curves obtained for the

Corumba and ASGs with the global curve of the

upper Vendian, thus supporting other lines of

evidence such as biostratigraphy and geochronol-

ogy. The available 87Sr/86Sr data for the Corumbaand ASGs also corroborate an upper Vendian age

for these units (Kaufman et al., 1993; Gorokov et

al., 1995; Kaufman et al., 1997; Knoll, 2000;

Walter et al., 2000). Sedimentation begun already

in the Varangerian in the Corumba basin (Puga

Formation), unlike the ASG, which begins with

post-glacial eustatic sea-level rise. On the other

hand, d13C-data suggest that deposition of theASG probably continued into the lowermost

Cambrian, unlike the Corumba Group. We are

confident that high-resolution carbon chemostra-

tigraphy will allow to refine these correlations

(Gaucher et al., 2002).

7. Discussion

7.1. Palaeogeography

All lines of evidence presented above indicate

that: (1) the Arroyo del Soldado and Corumba

groups are coeval; (2) broadly the same climatic

and eustatic sea-level changes are recorded in both

units; and (3) both basins had either an ample

connection or were part of the same shelf.Furthermore, the units studied here were clearly

deposited in a passive, Atlantic-type continental

margin (Almeida, 1984: 274; Zaine, 1991; Boggiani

et al., 1993; Gaucher et al., 1996, 1998a,b;

Boggiani, 1998; Gaucher 1999, 2000). In both

groups, the platform deepened to the east, with

granitic-metamorphic source areas located to the

west (Boggiani, 1990, 1998; Boggiani et al., 1993;Montana and Sprechmann, 1993; Gaucher, 2000).

Considering the above mentioned facts, the only

plausible explanation is that the Corumba and

ASGs were deposited onto the same shelf. This

hypotesis is supported by the same tectonic

vergence to the W-NW and same structural style

(Almeida, 1984: 269; Boggiani et al., 1993; Gau-

cher et al., 1998b; Gaucher, 2000; Alvarenga et al.,2000: 187), as well as similar ages for the deforma-

tion of these units around 540�/490 Ma (see

chapter 2). Furthermore, there are many outcrops

of probably correlative successions in northern

Uruguay, Paraguay and Bolivia, scattered in a

roughly NS direction between the main outcrop

areas (Fig. 13). Finally, the nature of the basement

is also similar in both cases. This suggests thateither the Rio de la Plata Superterrane (Gaucher,

2000) extends further to the north than previously

thought (Almeida et al., 1973) or that the Rıo de la

Plata Superterrane and the Amazonian Craton

were already amalgamated into a single crustal

block by Vendian times (Fig. 13).


The possibility that the Rıo de la Plata Super-

terrane (Craton) could extend to the north to

latitudes around 208S has been already suggested

by Ramos (1988). While studying a basement

window in southern Paraguay (Fig. 13), Meinhold

(1998) and Meinhold et al. (2000) also postulated

the continuity of the Rio de la Plata Craton to the

north. Ramos (1988) suggested that the collision of

the Rıo de la Plata Craton with the ensialic Alto

Paraguay Terrane, located to the east, resulted in

deformation of the Paraguay belt. The Alto

Paraguay Terrane of Ramos (1988) is roughly

equivalent to the Parana (Continental) Block of

Soares (1988, Fig. 13), and has been considered in

the palaeogeographic reconstructions of Unrug

(1996). The study of this crustal block is hindered

by the sedimentary cover of the Parana Basin

(Figs. 1 and 13), but some data has been obtained

by interpretation of gravimetric and magneto-

metric surveys, as well as datings of basement

Fig. 13. Major cratonic areas in SE-South America, showing extension of the Corumba-Arroyo del Soldado palaeoshelf. Major

lineaments and boundary of post-Cambrian cover were modified from Almeida et al. (1973). Probable boundaries of the Parana Block

according with Soares (1988). Western boundary of the Rıo de la Plata Superterrane taken from Ramos (1988). Position of the

hypothetical Mesoproterozoic suture between the Amazonian Craton and the Rıo de la Plata Superterrane is not known due to

extensive vulcanosedimentary cover of the Parana Basin. DFB, Dom Feliciano Belt; RB, Ribeira Belt.


rocks from oil exploration drilling (Ramos, 1988;Soares, 1988). It follows from this discussion that

the collision of the Rıo de la Plata Superterrane

with the Parana Block (Alto Paraguay Terrane) in

the Cambrian is responsible for the closure and

deformation of the Arroyo del Soldado-Corumba

shelf. The deformed sedimentary cover marks the

suture between this two continental blocks (Fig.

13). A similar palaeogeographic reconstruction hasbeen recently presented by Teixeira (2000), while

studying upper Vendian�/Cambrian (‘Eocam-

brian’) sedimentary successions from SE-Brazil.

The author suggested that closure of the Brazilides

ocean between the Amazonian-Rio de la Plata

cratons and the Parana Block-Sao Francisco

Craton resulted in deformation of the Corumba

shelf in the Cambrian. Brito Neves et al. (1999)and Campos Neto (2000) proposed a slightly

different palaeogeographic scheme for the region.

7.2. Palaeoclimatology

Drastic climatic changes took place during

deposition of the Corumba and ASGs (Section

3.2). Climate shifted quite rapidly between ‘green-

house’ and ‘icehouse’ effects, as known fromsedimentary successions of this age worldwide

(Kasting, 1992; Kirschivink, 1992). Gaucher

(2000) proposed that these changes were driven

mainly by palaeoceanographic factors that

strongly altered bioproductivity. Kaufman (2000)

arrived to a similar model through other lines of

evidence. The model proposed by Gaucher (2000)

includes the following sequence of events:(1) In the Upper Riphean, enhanced hydroter-

mal activity (Knoll, 1994) related to the rifting of

Rodinia (Hoffman, 1991; Dalziel, 1995; Unrug,

1996) pump large quantities of iron, silica, and

other nutrients into the ocean. These accumulate

in the deep ocean, generating an enormous reser-

voir, leading to ocean stratification.

(2) Due to a favourable wind and currentregime, enhanced upwelling takes place. Upwelling

zones represent only 0.1% of the present ocean

surface (Baturin, 1982), but this need not be

constant through geologic time. Enhanced upwel-

ling of nutrient-rich waters stored in the deep-

ocean reservoir trigger massive phytoplankton

blooms (Palacios, 1989; Vidal and Nystuen,1990), deposition of BIF and phosphorites. The

ocean becomes largely eutrophic, and large

amounts of organic matter accumulate on the

shelves. Sulphate-reducing bacteria find optimal

conditions in the anoxic lower water-layer, leading

to anomalous sulphate consumption and shift in

d34S towards extremely positive values (Walter et

al., 2000: 415; Kaufman, 2000).(3) Drawdown of CO2 by blooming phytoplank-

ton-populations reduces greenhouse effect and

triggers glaciation (Kaufman et al., 1997; Kauf-

man, 2000) and eustatic sea-level fall. Concomi-

tantly, large amounts of oxygen would be released

into the atmosphere, accounting for the proposed

Neoproterozoic oxygen-increase (Holland, 1994;

Canfield and Teske, 1996).(4) During snowball earth, silicate weathering,

carbonate deposition and bioproductivity are

greatly reduced, allowing for accumulation of

atmospheric CO2.

(5) Restored greenhouse effect leads to global

warming, ice melting and sea-level rise. Consump-

tion of nutrients stored in deep waters, a reduction

of hydrotermal activity after the Varangerian (asshown by 87Sr/86Sr relations: Knoll, 1994; Jacob-

sen and Kaufman, 1999; Walter et al., 2000) and

better water mixing seem to be the cause for the

final disruption of the system in the Early Cam-

brian.

7.3. Palaeobiology

As already proposed by Grant (1990) andGaucher and Sprechmann (1999), the fact that

shelly fossils were so widespread and relatively

diverse in the upper Vendian suggests that biomi-

neralization and agglutination were not simply a

local phenomenon by latest Proterozoic times.

Grotzinger et al. (2000) describe the calcareous

fossil Namacalathus hermanastes from reefs of the

Nama Group, adding one more species to theknown diversity of Precambrian shelly fossils.

They further report the occurrence of undeter-

mined, tube-shaped, calcareous fossils from the

same reefs. The Nama Group of Namibia and the

ASG of Uruguay are thus the units which contain

the most diverse shelly assemblages reported to


date (Germs, 1972; Gaucher and Sprechmann,1999; Gaucher, 2000; Grotzinger et al., 2000).

This suggests that favourable palaeogeographic

and consequently palaeoclimatic conditions in that

regions (see above) stimulated the diversification

of mineralized fossils. Considering that both

basins were probably located at either sides of

the same ocean (Unrug, 1996; Teixeira, 2000), it is

probable that the ‘oasis’ generated in that wayrepresents in fact the craddle of many different

lineages, which could evolve despite the inhospi-

table conditions elsewhere. Furthermore, the di-

versity of types of biomineralization and

agglutination found in the upper Vendian is

surprising, suggesting that the advent of skeletons

must have occurred much earlier than currently

accepted.

8. Conclusions

The Arroyo del Soldado and Corumba groups

are roughly coeval, and were deposited onto the

same passive continental margin, developed on the

Rıo de la Plata Superterrane. Thus, this continen-

tal block probably extends farther to the norththan previously expected, or, alternatively, it was

already amalgamated to the Amazonian Craton by

Vendian times. Closure of the basin is related to

the collision of the platform with the Parana Block

(Alto Paraguay Terrane) during the Cambrian-

Early Ordovician. Broadly the same climatic and

eustatic sea-level changes are recorded in both

units. Climate shifted quite rapidly between ‘green-house’ and ‘icehouse’ effects, with the consequent

lithological variations. Varangerian/Marinoan gla-

ciogenic deposits are not known from the Arroyo

del Soldado basin, but are represented in the

Corumba basin by the Puga Formation. This

suggests that the ASG was deposited in a lower

palaeolatitude, inside the circum-equatorial (B/

258), ice-free region predicted by recent computersimulations of Neoproterozoic glaciations. New

data obtained further support recent models of

Neoproterozoic climate change and glaciations

proposed independently by Gaucher (2000) and

Kaufman (2000). These models are based on

enhanced bioproductivity driven by high nutrient

availability in the largely stratified Neoproterozoicoceans. Newly discovered fossils are described

from the Corumba Group, with a total of six

species of organic-walled microfossils, three spe-

cies of vendotaenids and two species of skeletal

fossils including Cloudina and Titanotheca . The

vendotaenid Eoholynia corumbensis sp. nov is

described from siltstones of the Guaicurus For-

mation. The biota preserved in the CorumbaGroup correlates well with that represented in

the ASG. An upper Vendian (Valdaian) age is

confirmed for both units, using biostratigraphic,

as well as carbon and strontium isotopic data.

While uppermost deposits of the ASG are of

lowermost Cambrian age, sedimentation ceased

already in the uppermost Vendian in the Corumba

basin. Nevertheless, continuing research in thepoorly known, uppermost units of the Corumba

Group (Guaicurus Formation) might reveal the

Proterozoic/Cambrian transition as well. Finally,

an important diversity of skeletal fossils in the

Corumba, Arroyo del Soldado and Nama groups

points to favourable palaeoclimatic conditions in

the region occupied by the ocean that extended

between the Rıo de la Plata and Kalahari cratons.It is here proposed that this favourable conditions

generated an ‘oasis’, in which skeletal fossils and

metazoans could evolve despite the inhospitable

conditions elsewhere.

Acknowledgements

This study is the product of fruitful cooperationbetween Brazilian and Uruguayan researchers.

The authors are indebted to many individuals

who collaborated in many respects. Deep gratitude

is due to the late Armando Marcio Coimbra

(1949�/1998), who actively participated of field

work and contributed with stimulating ideas, but

could not see the completion of this publication.

Ana Lucia Desenzi Gesicki (Campo Grande) andSilvana Martınez (Montevideo) are thanked for

valuable help during field work in Brazil and

Uruguay, respectively. Antonio Luiz Teixeira

(Sao Paulo) is thanked for providing literature

and for valuable comments. Our sincere apprecia-

tion goes to Prof. Malcom Walter and an anon-


ymous reviewer for their useful comments andsuggestions. Field work in Uruguay has been

financed by research project 1040 of the Consejo

Nacional de Investigacion Cientıfica y Tecnologica

(CONICYT), Uruguay. Analyses and publication

expenses were financed by research projects C-32

and C-39 of the Comision Sectorial de Investiga-

cion Cientıfica (CSIC, Uruguay) and project

CONICYT 6007. C.G. is indebted to the GermanAcademic Exchange Service (DAAD) for a gener-

ous research grant (1995�/1999) and further finan-

cial support in many respects. This paper is a

contribution to projects IGCP 450 (Proterozoic

sediment-hosted base metal deposits of western

Gondwana) and IGCP 419 (Foreland basins of the

Neoproterozoic belts in central to southern Africa

and South America).

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Documents

Integrated correlation of the Vendian to Cambrian Arroyo ... fileindicated by fossils for both units. Previously reported strontium isotopic data are also consistent with this age