“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Summary
ISOTOPE STRATIGRAPHY OF THE ARARAS GROUP (NEOPROTEROZOIC) IN THE SOUTHEAST
BORDER OF THE AMAZON CRATON Alvarenga, C.J.S.; Dardenne, M.A.; Santos, R.V.; Dantas, E.L.;
Brod, E.R.; Gioia, S.M.C.L.; Sial, A.N. ...................... ......................................................................................... 1
THE VERGENCE OF THE PARAGUAY BELT IN THE CONTEXT OF THE COLLAGE OF THE WEST
GONDWANA Barboza, E.S.; Geraldes, M.C.; Pinho, F.E.C. ............................................................................. 3
DISTRIBUTION, GENESIS AND PALEOECOLOGICAL SIGNIFICANCE OF IRON FORMATION
THROUGH TIME Beukes, N.J. & Gutzmer, J. ..................................................................................................... 6
STRATIGRAPHY,
PALEONTOLOGY
AND
AGE
OF
LAS
VENTANAS
FORMATION
(NEOPROTEROZOIC, URUGUAY) Blanco, G. & Gaucher, C. ......................................................................... 8
NEW LEVEL OF DIAMICTITES IN THE CORUMBÁ GROUP (EDIACARAN), SW BRAZIL, SOUTH
AMERICA Boggiani, P.C.; Fairchild, T.R.; Riccomini, C. .................................................................................. 10
CLOUDINA FROM THE ITAPUCUMÍ GROUP (VENDIAN, PARAGUAY): AGE AND CORRELATIONS
Boggiani, P.C. & Gaucher, C. ................................................................................................................................. 13
MICROPALEONTOLOGICAL ASPECTS OF LOWERMOST CAMBRIAN BLACK SHALES AND
CHERTS ON THE YANGTZE PLATFORM, CHINA Braun, A.; Chen, J-Y; Waloszek, D.; Maas, A. ........ 16
DIAMICTITES OVERLYING VARANGER-AGE CARBONATES OF ARARAS FORMATION –
PARAGUAY BELT, BRAZIL – EVIDENCE OF A NEW GLACIATION? Figueiredo, M.F.; Babinski, M.;
Alvarenga, C.J.S.; Pinho, F.E.C. ............................................................................................................................ 18
CRUSTAL STRUCTURES OF PARAGUAY BELT FROM GRAVITY AND MT DATA: A CAMBRIAN
SUTURE IN WESTERN GONDWANALAND? Fisseha, S.; Ussami, N.; Padilha, A.L.; Vitorello, I.;
Trindade, R.I.F. ......................................................................................................................................................... 20
IMPACT OF A LATE VENDIAN, NON-GLOBAL GLACIAL EVENT ON A CARBONATE PLATFORM,
POLANCO FORMATION, URUGUAY Gaucher, C.; Sial, A.N.; Pimentel, M.M.; Ferreira, V.P.................. 21
RECORD OF 483-429 MA U-PB AND 40AR-39AR AGES IN SW RONDONIA AND SUNSÁS
PROVINCES: THE ROLE OF NEOPROTEROZOIC MOBILE BELTS OVERPRINT WITHIN THE
AMAZON CRATON Geraldes, M.C.; De Paulo, V.G.; Barboza, E.S.; Vasconcelos, P.; Teixeira, W. ....... 24
LITHOSTRATIGRAPHY, BIOSTRATIGRAPHY AND CORRELATIONS OF NEOPROTEROZOIC TO
EARLY PALEOZOIC SEDIMENTARY BASINS ON THE KALAHARI CRATON AND ITS MARGINS
(SOUTHERN AFRICA) Germs, G.J.B. & Gaucher, C. ...................................................................................... 27
CHEMOSTRATIGRAPHY AND DIAGENETIC CONSTRAINTS ON NEOPROTEROZOIC CARBONATE
SUCCESSIONS FROM THE SIERRAS BAYAS GROUP, TANDILIA SYSTEM, ARGENTINA
Gómez Peral, L.E.; Poiré, D.G.; Zimmermann, U.; Strauss, H. ................................................................. 30
UNRAVELLING CHEMOSTRATIGRAPHIC SIGNATURES OF SEDIMENTATION AND DIAGENESIS IN
PALEOPROTEROZOIC IRON AND MANGANESE FORMATIONS Gutzmer, J.; Scheiderhan, E.;
Beukes, N.J. .............................................................................................................................................................. 33
THE CAJAMAR BASIN (SP-BRAZIL) AND THE MICROFOSSIL TITANOTHECA COIMBRAE OF THE
EDIACARAN PERIOD Hachiro, J.; Teixeira, A.L.; Gaucher, C.; Sprechmann, P. ....................................... 35
TECTONIC EPISODES RELATED TO WEST GONDWANA AMALGAMATION IN THE RIBEIRA
OROGEN (SE BRAZIL) Heilbron, M.; Valeriano, C.; Tupinambá, M.; Almeida, J.; Duarte, B.; Valladares,
C.; Schmitt, R.; Geraldes, M.; Ragatky, C.D.; Palermo, N.; Gontijo, A. ........................................................... 36
NEOPROTEROZOIC FOSSILS IN CANADA - AN OVERVIEW Hofmann, H.J. .......................................... 39
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“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
NEOPROTEROZOIC TO EARLY-CAMBRIAN VOLCANO-SEDIMENTARY SUCCESSIONS OF THE
CAMAQUÃ BASIN, RIO GRANDE DO SUL STATE, BRAZIL Janikian, L.; Almeida, R.P.; FragosoCesar, A.R.S.; Martins, V.T.S.; D'agrella Filho, M.S.; Mcreath, I.; Dantas, E.L. ............................................. 40
GEOTECTONIC EVOLUTION OF URUGUAY DURING THE NEOPROTEROZOIC-CAMBRIAN Pecoits,
E. & Oyhantçabal, P. ................................................................................................................................................ 43
SEDIMENTARY HISTORY OF THE NEOPROTEROZOIC OF OLAVARRÍA, TANDILIA SYSTEM,
ARGENTINA: NEW EVIDENCE FROM THEIR SEDIMENTARY SEQUENCES AND UNCONFORMITIES
- A “SNOWBALL EARTH” OR A “PHANTOM” GLACIAL? Poiré, D.G. ..................................................... 46
THE CHARACTERIZATION OF A CAMBRIAN COLISIONAL OROGENY IN THE RIBEIRA BELT (SEBRAZIL) AND THE TECTONIC EVENTS IN THE KAOKO BELT (NW-NAMIBIA)- NEW
GEOCHRONOLOGICAL DATA AND INSIGHTS ON THE WEST GONDWANA EVOLUTION Schmitt,
R.S.; Trouw, R.A.J.; Passchier, C.W.; Van Schmus, W.R.; Pimentel, M.M.; Todt, W.; Poller, U.; Kroner, A.
..................................................................................................................................................................................... 49
MICROBIAL REEFS OF THE NAMA GROUP, NAMIBIA: COMPOSITION AND GROWTH DYNAMICS
IN RELATION TO ACCOMMODATION SPACE AND SEDIMENT INPUT Schröder, S.; Adams, E.W.;
Grotzinger, J.P. .......................................................................................................................................................... 52
THE STEPTOEAN POSITIVE C-ISOTOPE EXCURSION (SPICE) RECORDED IN LATE CAMBRIAN
CARBONATES OF THE ARGENTINE PRECORDILLERA Sial, A.N.; Peralta, S.; Ferreira, V.P.;
Gaucher, C.; Toselli, A.J.; Aceñolaza, F.G.; Parada, M.A.; Pimentel, M.M. .................................................... 53
CONFLICTING CORRELATIONS OF NEOPROTEROZOIC RECORDS Soares, P.C. .............................. 55
TERRANE TRANSFER DURING THE GRENVILLIAN ASSEMBLY OF RODINIA: IMPLICATIONS OF
AMAZONIAN CRUST IN THE SOUTHEASTERN APPALACHIANS FOR THE NEOPROTEROZOIC
PALEOGEOGRAPHY OF THE IAPETUS OCEAN Tohver, E. & Bettencourt, J.S. ..................................... 58
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“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Isotope stratigraphy of the Araras Group (Neoproterozoic) in
the southeast border of the Amazon Craton
Carlos J. S. de Alvarenga*, Marcel A. Dardenne*, Roberto V. Santos*, Elton L. Dantas*,
Emanuela R. Brod*, Simone M.C.L. Gioia*, Alcides N. Sial**
* Instituto de Geociências, Universidade de Brasília (UnB), Brasília, DF, 70910-900, Brazil, alva1@unb.br
**NEG-LABISE, Departamento de Geologia, Universidade Federal de Pernambuco (UFPE), C.P. 7852,
Recife, PE, 50670-000, Brazil
The neoproterozoic carbonate sequence on the southeastern border of Amazon Craton may be divided
into two lithostratigraphic units: a basal unit characterized by limestones and mudstones of the Guia
Formation; and an upper unit characterized by dolarenites, dolorudites and sand-dolostone of the Nobres
Formation (Hennies 1965, Boggiani and Alvarenga 2004). Within the first 25 m of the basal unit occurs a
cap dolomite that overlies Marinoan-age glacial diamictites. In this paper we compare C-isotope and Srisotope data of these carbonate sequence within two different domains: on border of the basin (cratonic
domais), in which it has a reduced thickness; and in the inner- to outer-shelf domain (fold belt domain), in
which it has up to 1300 m thick.
The cap dolomites on the border of the basin have approximately 22 m thick and are placed between
diamictites of the Puga Formation at the base, and laminated limestone-mudstone at the top (Alvarenga
and Trompette, 1992, Nogueira et al., 2003, Alvarenga et al., 2004). The upper part of this sequence are
characterized by intercalations of dolarenites and sand-dolostone that are followed abruptly by mudstone
with trace fossil and centimetric layers of anhydrite-pseudomorph.
In the inner- to outer-shelf domain, the 1300 m of continuous carbonates sequence is folded and present
weak signs of very-low metamorphic grade. Overlying the diamictites there are 18 m of cap dolomite that
are found only in borehole and which are followed by a thick mud-limestone sequence (200-300 meters).
Dolarenite and dolorudite are found in the upper part of the carbonate sequence and present sanddolostone intercalations near the upper contact with sandstones of the Raizama Formation.
Among the three studied sections on the border of the basin, carbon isotope varies between -10.5 to 4.1‰ in cap dolomite, and between -5.4 and -2.7‰ in above laminated limestone and mud-limestone
(Figure1). The intercalations of dolarenites and sand-dolostone placed in the upper part of the sequence
present 13CPDB values ranging between -6.1 and -3.7‰. Samples of limestones with high Sr content (>
400 ppm) exhibited 87Sr/86Sr ratios ranging from 0.70740 to 0.70803.
Cap-dolomites from the inner- to outer-shelf present 13CPDB values ranging between -4.8 and -1.7‰
(Figure 2). The limestone and mud-limestone unit above these rocks (>200 m thick) present carbon
isotope that increase smoothly from -3.8 at the base to 0.1‰ at the top (Figure 2). These limestones
grade upwards within 3 meters to dolostones that are characterized by homogeneous and positive
13CPDB values (+1.9 to +2.7‰). The dolarenites and sand-dolostone from the upper Nobres Formation
present 13CPDB values up to +9.6‰ that decrease down to -1.0‰ just below the contact with the upper
siliciclastic sequence (Raizama Formation). Limestones and mud-limestones from the deeper portion of
the basin exhibit 87Sr/86Sr ratios ranging from 0.70763 to 0.70780.
A comparative isotope stratigraphy between the border and the inner part of the basin shows significant
difference in 13CPDB values. The cap dolomite, limestones, dolostone and sand-dolostone of the Araras
Group has more negative carbon isotope on the border of the basin (Mirassol d’Oeste and Tangará) than
the same correlative unit in the outer shelf of the basin (Nobres). These lower values can be related to a
shallower environment conditions and to a stronger influence of the continental border, as suggest the
presence of evaporates and restrictive environmental conditions. The 87Sr/86Sr ratios are the same in the
both areas, thus suggesting that they have the same age of sedimentation.
References
Alvarenga, C.J.S.de and Trompette, R. 1992 Glacial influenced turbidite sedimentation in the uppermost Proterozoic
and Lower Cambrian of the Paraguay Belt (Mato Grosso, Brazil). Palaeogeogr. Palaeoclimatol.
Palaeoclimatol. 92: 85-105.
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“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Alvarenga, C.J.S., Santos, R.V. and Dantas, E.L., 2004. C-O-Sr isotopic stratigraphy of cap carbonates overlying
Marinoan-age glacial diamictites in the Paraguay Belt, Brazil. Precambrian Research 131:1-21.
Boggiani, P.C. and Alvarenga, C.J.S. de, 2004. A Faixa Paraguai. In.: V. Mantesso Neto, A. Bartorelli, C. D. R.
Carneiro, B. B. B. N. (coords) O Desvendar de um continente: a moderna geologia da América do Sul e o
legado da obra de Fernando Flávio Marques de Almeida. (prelo).
Hennies, W.T. 1966. Geologia do centro-norte mato-grossense. Tese, Esc. Politécnica Univ. São Paulo, São Paulo
65p.
Nogueira, A.C.R., Riccomini, C., Sial, A.N., Moura, C.A.V. and Fairchild, T.R., 2003. Soft-sediment deformation at the
base of the Neoproterozoic Puga cap carbonate (southwestern Amazon craton, Brazil): confirmation of rapid
icehouse to greenhouse transition in snowball Earth. Geology, 31: 613-616.
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“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
The vergence of the Paraguay Belt in the Context of the
collage of the West Gondwana
Elzio da S. Barboza* **, Mauro C. Geraldes**, Francisco E. C. Pinho*
*Universidade Federal do Mato Grosso (UFMT), Grupo de Pesquisas Recursos Minerais de Mato
Grosso, Brazil, elziosb@yahoo.com.br, aguapei@yahoo.com
**TEKTOS Research Group, Universidade do Estado do Rio de Janeiro (UERJ), Rua São Francisco
Xavier 524, Rio de Janeiro, RJ, 20550-013, Brazil, geraldesl@uerj.br
The Paraguay Belt occupies the western portion of the Tocantins Province, forming the southeastern
border of the Amazon Craton and eastern border of the Rio Apa Block (Alvarenga et al., 2004). It is
partially covered by the Parana Basin (southeast), and by the Bananal and Pantanal Basins (east and
south). Over the past decades, several aspects of the Paraguay Belt have been investigated by different
workers: geochemical and stratigraphical (Alvarenga & Saes 1992; Boggiani et al. 1999); structural (Silva,
1999; Pires et al. 1986 ca.). Few works address the tectonic significance of the Paraguai Belt, with the
possible exception of the contribution of Alvarenga & Trompette (1993).
Geochronological data of the Paraguay Belt suggest a Brasiliano age for collisional processes. Notable
among these data is the Rb/Sr age of 484 ± 19 Ma for schist, interpreted as the final stage of the orogenic
evolution (Barros et al., 1982). The crystallization ages of the late orogenic igneous rocks (São Vicente
Granite) is given by the Rb/Sr age of 504±12 Ma (Almeida & Montalvani, 1975). The Coxim and Taboco
Granites yield a K/Ar cooling age of 453 Ma (Beuriem, 1956, in Luz et al. 1980). The existence of Ar/Ar
ages between 541 and 531 Ma has been suggested as a constraint on the cooling from metamorphism
for the Cuiabá Group in the of Nova Xavantina region (Geraldes and Tassinari, 2003). The variation in
presntly available ages makes it difficult to assign a sequence of regional events; thus, ore
geochronological studies are necessary to establish the evolution of tectonic events in the Paraguay Belt.
In addition to this age uncertainty, another question that still remains concerns the vergence of structures
generated during the deformation and the direction of mass transport. In much the same manner as in
other orogneic belts, the number of deformational phases and direction of tectonic vergence imprinted on
rocks are the subject of wide disagreement among different workers. In general, the number of proposed
deformation phases ranges from three to four. The proposal of three coaxial phases and one fourth
orthogonal phases was advanced by Alvarenga & Trompetti (1993) can be summarized as follows: the
first phase does not show clear vergence, the second phase has a SE vergence, and the third with
vergence in direction to the Amazonian craton is visible only in the rocks that are covering the craton.
Other interpretations are summarized in the Table 1.
Our structural studies in the Cuiabá, Poconé and Cangas region, together with an wide review of the
literature, provide additional evidence in support of three continuous deformation phases that affected the
rocks of the Cuiabá Group in the internal portion of the Paraguay Belt. The first deformation phase (D n)
was responsible for the generation of the asymmetrical and inverse regional scale folds showing SE
vergence. The penetrative Sn formed marks the axial plan of these folds, NW dipping. The stretching
lineation is recorded by the alignment of flattened clasts in the S n plane with a shallow northeasterly
inclination. The second phase of deformation (Dn+1) produced open, asymmetrical folds and an axial
plane (Sn+1) that plunges SE with a resultant NW vergence, with NE dipping. The third phase (D n+2) is
orthogonal to the other phases and occurs as crenulations and kink bands with NW axial planes (S n+2)
duping in both SW and NE directions. In the auriferous deposits located at the “Baixada Cuiabana”, the
best gold grades in quartz veins are coincident with S n+2 surface.
The deformation phases that had affected the Paraguay Belt rocks had been generated during the
deformation imposed by the Brasiliano tectonism, responsible for the closing of a molassic basin during
the collision of the Paranapanema and Rio Apa blocks with the Amazonian Craton.
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“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Table 1 - Deformation phases that had affected the Paraguay Belt during the Brasilian cycle, with our
proposal for the internal portion of the belt in boldface. Modified after Silva (1999).
Deformation Phases

I
II
D1
D2
D3
D1
D3
D4
D2
D3
D4
D2
D3
D4
D1
D2
D3
Sn+1
Sn+2
Sn+3
Sn*
Sn+1**
Sn+2***


D1

D1


Sn-1 (?)

Sn
III
IV
V
 Luz et. al. (1980);  Pires et al. (1986);  Silva (1990);  Alvarenga & Trompette (1993);  Gheler
(1997);  Silva (1999);  This study: Casa de Pedra, Cangas and Poconé gold deposits. Obs.:
Vergence: SE (*); NW (**) e SW ? (***).
The existence of the Paranapanema Block (Almeida et al., 2000), also referred to as the Paraná Block
(Campos Neto et al., 2000) seems to entrenched in the literature. This block is a geotectonic unit that
represents part of the basement of the Parana Basin. It is a cratonic nucleus of igneous and metamorphic
Precambrian rocks surrounded by mobile belts of Brasiliano age. Some authors had proven the existence
of this craton nucleus through geophysical studies, however these proposals lack geometric and
quantitative aspects. The evidences of the existence of this geotectonic unit, as well as the agreed upon
necessity of studies to define its boundaries, appears to be widespread in the literature.
Discussion and Conclusions
Most of the penetrative deformation recorded in the rocks of the Cuiabá Group occurs during the D n
phase, followed by the Dn+1. In addition, the occurrence of Dn+2 seems to be conditioned by lithology.
Table 1 shows the proposed tectonic sequencec (lines) presented as deformation phases in the rocks of
the internal portion of the Paraguay Belt. The numbered columns from 1 to 5 indicate continuous
deformational phases. This contribution seeks to summarize the findings of different studies with respect
to the generation of axial planes during these deformation phases in light of our own field observations. In
column 1, we have described the development of bedding parallel cleavage, considered by some authors
to be generated by flexural slip. However, in some outcrops in Cangas and Poconé it is possible to
distinguish the hinge of the folds related to the S n (column 3), from the S0 layering in these hinge, which
are are parallel in the limbs due the great amplitude of the folds. Column 4 describes Dn+1, which gently
folds S0 and Sn and shows vergence toward the Amazonian Craton, in contrast to the S n. The presence of
an open fold axial plane define Sn+1, which can give the impression that Sn has vergence toward the
Amazonian Craton. The Dn+2 phase is represented in column 5, and defines a S n+2 orthogonal to Sn and
Sn+1, i.e. a NW direction that is almost always SW dipping and more rarely NE dipping. The deformation
represented in column 2 was not observed in the visited areas. Alvarenga & Trompette (1993) described
this phase of deformation only in the external zone of the Paraguay Belt, however Silva (1999) identified
this phase in some localities of the internal zone.
The structural geology studies in the Paraguay Belt demonstrate that many deformation phases affected
the rocks of the region. Discussions tend to related only to localized areas, avoiding problems of more
regional scope. We attribute the origin of these deformations and the variation in vergence to be the result
of a collision of large cratonic masses, involving the Amazonian Craton and the Rio Apa and
Paranapanema blocks. The irregular boundary of these cratonic masses and the rheology is responsible
for the local intensity of the deformation in each deformational phase during the collision of the blocks. In
a general way, most authors attribute the variations in the described vergence as the result of bulkheads
(cratons). Thus, the Paraguai belt presents vergence to the NW or N near the Amazon craton, with the
vergence changing to the SE or S adjacent to the Paranapanema Block. The existence of other rigid
blocks identified by geophysic data under of the sediments of the Parana Basin, might have resulted in an
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“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
even more complex variation in tectonic vergence. In this way, the structural analysis in the Paraguay Belt
must take in account the variations of the cratonic masses and the geometry of the structures can be
resulted of the number of blocks involved in the collision.
References
Almeida, F.F.M.; Brito Neves, B.B.; Carneiro, C.D.R., 2000. The origin and evolution of the South
American Platform. Earth-Science Reviews 50: 77–111.
Almeida, F.F.M. & Mantovani, M.S.M., 1975. Geologia e Geocronologia do Granito São Vicente, Mato
Grosso. Anais da Academia Brasileira de Geociências, Rio de Janeiro, Brasil, 47: 451-458.
Alvarenga, C.J.S. & Saes, G.S., 1992. Estratigrafia e Sedimentologia do Proterozóico Médio e Superior
da Região Sudeste do Craton Amazônico. Revista Brasileira de Geociencias, 22(4): 493-499.
Alvarenga, C.J.S.; Santos, R.V.; Dantas, E.L., 2004. C-O-Sr isotopic stratigraphy in cap carbonate
overlying Marionan-age glacial diamictites in Paraguay Belt, Brazil. Precambrian Research,
Amsterdam, 131(1-2): 1-21.
Alvarenga C.J.S. & Trompetti, R., 1993. Evolução Tectônica Brasiliana da Faixa Paraguai: A
Estruturação da Região de Cuiabá. Revista Brasileira de Geociências, 18: 323-327.
Barros, A.M.; Silva, R.M.; Cardoso, O.R.F.A.; Freire, F. A.; Souza, J. J. Jr.; Rivetti, M.; Luz, D.S.;
Palmeira, R.C.B.; Tassinari, C.C.G., 1982. Geologia, In: Radambrasil, Folha SD-21 Cuiabá, Rio
de Janeiro, MME – SG, (Levantamento de Recursos Naturais), 26: 25–192.
Boggiani, P.C.; Coimbra, A.M.; Gesicki, A.L.; Sial, A.N.; Ferreira, V.P.; Ribeiro, F.B.; Flexor, J.M., 1999.
Tufas Calcárias da Serra da Bodoquena. In: Schobbenhaus, C.; Campos, D.A.; Queiroz, E.T.;
Winge, M.; Berbert-Born, M. (Edit.) Sítios Geológicos e Paleontológicos do Brasil. Publicado na
Internet no endereço: http://www.unb.br/ig/sigep/sitio034/sitio034.htm
Campos Neto, M.C.; Caby, R.; Janasi, V.A.; Garcia, M.G.M.; Perrota, M., 2000. Continental subduction
and inverted metamorphic pattern: South of São Francisco Craton, SE Brazil. 31st International
Geol. Congress, Rio de Janeiro. Abstracts, CD-ROM.
Geraldes, M.C. and Tassinari, C.C.G., 2003 40Ar/39Ar metamorphic record of a collision related to the
western gondwana collage: the (541-531 Ma) Paraguay belt in the Nova Xavantina (MT) region.
Simpósio Nacional de Estudos Tectônicos. Buzios-RJ, 54-57 pp.
Gheler, W. L. 1997. Contribuição à geologia do Grupo Cuiabá, na região do Rio Jatobá –
Campinápolis/MT. Trabalho de conclusão do curso de Geologia – UFMT (inédito). Cuiabá, 62 pp.
Luz, J.S.; Oliveira, A.M.; Souza, J.O.; Motta, J.F.M.; Tanno, L.C.; Carmo, L.S.; Souza, N.B. 1980. Projeto
Coxipó, Goiânia, Dnpm/Cprm, Rel., 1: 136 pp.
Pires, F.R.M.; Gonçalvez F.T.T.; Siqueira A.J.B. 1986. Controle da Mineralizações Auriferas do Grupo
Cuiabá, Mato Grosso. In: Congresso Brasileiro de Geologia, 34. Goiânia, S B G, Anais, 5: 2383 2396.
Silva, C.H., 1999. Caracterização Estrutural de Mineralizações Auriferas do Grupo Cuiabá, Baixada
Cuiabana (MT). Dissertação de Mestrado. Rio Claro, Unesp, 134 pp. (Dissertação de Mestrado).
Silva, L. J. H. D. 1990. Ouro no Grupo Cuiabá, Mato Grosso: controles estruturais e implicações
tectônicas. Anais do XXXVI Congresso Brasileiro de Geologia. Natal, Anais SBG, 6:2520-2534.
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“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Distribution, Genesis and Paleoecological Significance of
Iron Formation Through Time
Nicolas J. Beukes and Jens Gutzmer
Department of Geology, Rand Afrikaans University, P.O. Box 524, 2006 Auckland Park, South Africa,
njb@rau.ac.za
Iron formation is an enigmatic rock type, abundant in Precambrian sedimentary successions and scarce
to absent in the Phanerozoic. Certain iron formations, especially in the early Paleoproterozoic and
Neoproterozoic, contain interbeds of sedimentary manganese ores. There is very little consensus about
the origin of these rocks, largely because of a lack of modern analogues, except perhaps for iron and
manganese-bearing sediments associated with present day deep sea hydrothermal vents. In spite of
this, detailed sedimentological studies of iron formations may provide us clues about depositional and
diagenetic environments in ancient oceans.
In this contribution we address four pertinent questions regarding the origin and environmental
significance of iron formation through time, namely
a) What controlled secular variations in the abundance of iron formation.
b) Which mechanisms were responsible for the precipitation of iron minerals.
c) What can we learn about microbial metabolic pathways, for both primary production and
degradation of organic matter, from the study of iron formation.
d) What is the relationship between iron formations and glacial deposits especially in the
Neoproterozoic.
Reconstruction of depositional settings and basin analyses of iron formations, provide the following
answers to these questions.
Iron formations typically represent starved shelf deposits formed during major transgressions. Their
abundance as a rock type through time is thus perhaps best defined by the frequency with which they
occur along transgressive surfaces in rock successions of different ages. Defined as such they appear
perhaps more abundant in Archean than in Paleoproterozoic successions. The so-called peak of iron
formation deposition at ~2,45 Ga, based on size (Gole and Klein, 1981), may merely be an artifact of
preservation and of deposition of iron formation of the Transvaal and Hamersley Provinces in one large
unique sedimentary basin. Contrary to popular belief it is thus unlikely to represent a global event of iron
formation deposition and to be related to the rise of oxygen in the atmosphere (Cloud, 1973). In both the
Transvaal and Hamersley successions, a carbonate depositional basin was replaced by an ironprecipitating basin, with retention of the whole spectrum of depositional environments from deep basin to
shallow shelf. This is best explained by increased hydrothermal plume activity bringing iron and silica into
a basin on a shallow shelf and replacing the water from which carbonates were initially deposited.
Hydrothermal plume activity may thus have been a major controlling factor on the distribution of iron
formations through time (Isley and Abbott, 1999). Scarcity of iron formations in rock successions younger
than 1,9 Ga, could thus also be an artifact of preservation as a result of decreased plume activity and
restriction of hydrothermal plumes to off-shelf, deep oceanic environments.
Facies relationships between iron formation and associated rock types, especially stromatolitic carbonate
and black carbonaceaous chert, indicate that iron formations were most commonly deposited from a
stratified ocean in which the shallow (< 200 m depth) surface layer, including the entire photic zone, was
depleted in dissolved iron. It is thus highly unlikely that photochemical oxidation of ferrous to ferric iron
(Cairns-Smith, 1978) and/or anaerobic photosynthetic iron-oxidizing bacteria (Konhauser et al., 2002)
could have been responsible for the precipitation of oxide-facies iron formations in the absence of free
oxygen in the Archean. In turn, this implies that free oxygen must have been available in the upper layer
of a stratified ocean for the deposition of oxide-facies iron formations in the Archean. Such iron
formations occur as far back as 3,8 Ga at Isua in Greenland (Dymek and Klein, 1988).
The distribution of organic matter in iron formation successions strongly suggest that the precipitation of
iron oxides was decoupled from primary production of biomass (Klein and Beukes, 1989). Oxide facies
iron formation apparently accumulated in areas of very low organic carbon supply or primary productivity.
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In areas of higher organic carbon supply, iron oxides were transformed to siderite most probably through
microbial ferric iron respiration. Certain Archean iron formations occur in close association with
manganese carbonates and it is quite possible that these carbonates were derived from microbial
respiration of earlier Mn4+ - oxyhydroxides. The presence of Mn4+ - oxyhydroxides requires free oxygen,
which would support the notion that at least some oxygen was available in shallow ocean water in the
early Precambrian. Oxygenic photosynthesis is the most likely source of free oxygen, which implies that
this metabolic pathway was present very early on in earth history.
Iron formations are known to be directly associated with glacial deposits in the Mesoarchean,
Paleoproterozoic and Neoproterozoic. All of these iron formations appear to have been deposited either
during interglacial periods, or at the end of glacial episodes associated with post-glacial flooding events.
It is most unlikely that a permanently stratified ocean could have been maintained in the presence of polar
ice caps even as far back in time as the Mesoarchean. Stratification of oceans in the Precambrian may
thus have been episodic and could have been controlled by interplay of global climatic change and
hydrothermal (tectonic) activity.
Iron formations and associated manganese deposits in the
Neoproterozoic appear to be directly related to episodes of ice-house or Snowball Earth conditions in that
period (Klein and Beukes, 1993). It is possible that similar conditions were present at specific intervals in
the Paleoproterozoic and the Mesoarchean.
References
Cairns-Smith, A.G., 1978. Precambrian solution photochemistry, inverse segregation, and banded iron
formations. Nature, 276, 807-808.
Cloud, P., 1973. Paleoecological significance of the banded iron-formation. Econ. Geol., 68, 1135-1143.
Dymek, R.F. and Klein, C., 1988. Chamistry, petrology and origin of banded iron- formation lithologies
from the 3 800 Ma Isua supracrustal belt, West Greenland. Precambr. Res., 39, 247-302.
Gole, M.J. and Klein, C., 1981. Banded iron-formations through much of Precambrian time. J. Geol., 89,
169-183.
Isley, A.E. and Abbott, D.H., 1999. Plume-related mafic volcanism and the depositon of banded ironformation. J. Geoph. Res., 104, 15461-15477.
Klein, C. and Beukes, N.J., 1989. Geochemistry and sedimentology of a facies transition from limestone
to iron-formation deposition in the early Proterozoic Transvaal Supergroup, South Africa. Econ.
Geol., 84, 1733-1774.
Konhauser, K.O., Hamade, T., Raiswell, R., Morris, R.C., Ferris, F.G., Southam, G. and Canfield, D.E.,
2002. Could bacteria have formed the Precambrian banded iron formations. Geology, 30, 10791082.
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Stratigraphy, paleontology and age of Las Ventanas
Formation (Neoproterozoic, Uruguay)
Gonzalo Blanco* & Claudio Gaucher**
*Department of Geology, Rand Afrikaans University, Auckland Park 2006, Johannesburg, South Africa,
blancogonzalo2@yahoo.com.ar
**Departamento de Geología, Facultad de Ciencias, Iguá 4225, 11400 Montevideo, Uruguay.
gaucher@chasque.apc.org
Las Ventanas Formation was defined and mapped by Midot (1984). It is composed by a thick
conglomeratic succession, which crops out in the vicinity of the homonymous hill. The outcrops are
located in the south of the Nico Perez Terrane, to the north and southwest of the town of Pan de Azúcar,
covering an area of more than 120 km 2. Clasts composition shows a provenance from a mixed volcanic
(basic and acid) and granitic source area. Both rhyolitic and basic volcanic flows occur in the succession,
with an evolution from basic to acid volcanism towards the top. Pelites and sandstones occur mainly at
the top, but are always subordinate (Fig. 1).
The Las Ventanas Formation is intruded by syenites of the Sierra de Animas Formation with an age by
Rb-Sr of 520 5 Ma (Bossi et al., 1993) and the Pan de Azcar Granite, which yielded a Rb-Sr age of
55928 (Preciozzi et al., 1993). K-Ar datings of 572 ± 7 Ma obtained for synkinematic muscovites
(Cingolani, in Bossi and Campal, 1992) crystallized along the Puntas del Pan de Azúcar Thrust, suggest
a minimum Vendian age for the unit. However Snchez-Betucci and Pazos (1996) proposed an
Ordovician age for Las Ventanas Formation, and describe syenitic clasts derived from Sierra de Animas
Formation in the conglomerate of the Las Ventanas Formation. Its type area represents a large syncline
(Cerro Las Ventanas Syncline) with an axis plounging 35° to the S20W (Fig 1.). The lithostratigraphy of
the Las Ventanas Formation is described, and separated into the following informal units: basic volcanics
and breccias, polymictic conglomerates, sandstones and conglomerates, and laminated pelites. The
Formation represents a thinning- and fining-upward sequence, recording evolution from an alluvial fandominated environment to shallow marine conditions with occasional storms. Sedimentary structures and
petrography of conglomerates and sandstones point to a steep palaeorelief. A number of organic-walled
microfossils is described for the first time, namely: Leiosphaeridia tenuissima, L. minutissima,
Lophosphaeridium sp., Soldadophycus bossii, S. major, Soldadophycus sp., Vendotaenia sp. and psilate,
branched filaments. The assemblage is characterized by its low diversity, abundance and large size (up
to 400 µm) of Leiosphaeridia. Wrinkle structures occur in the laminated pelite unit. Based on the
microfossils and its stratigraphic relationships to the overlying Arroyo del Soldado Group (Gaucher,
2000), we assign the Las Ventanas Formation to the lower Vendian (Varangerian, ca. 600 Ma). The Playa
Hermosa Formation can be interpreted as a lateral facies of the Las Ventanas Formation, or be –
alternatively- younger than the latter unit. On the basis of microfossil assemblages, we envisage that the
Las Ventanas Formation immediately predates the Arroyo del Soldado Group which represents a thick
successions of siliciclastic, carbonates and BIF devoid of volcanic and volcaniclastic rocks. This unit is
interpreted as a passive margin deposit with an upper Vendian age (Gaucher, 2000). An extensional
geotectonic setting, possibly a rift, is postulated for Las Ventanas Formation on the basis of:
1) evolution from continental to marine environments fom base to top;
2) steep palaeorelief, shown by coarse and immature siliciclastic deposits;
3) synsedimentary, bimodal volcanism, evolving from basalts to rhyolites up section, and
4) age of the Las Ventanas Formation, immediately pre-dating Arroyo del Soldado Group, which is
here interpreted as the drift stage of the platform.
These results confirm that the rifting event that affected the Río de la Plata Craton is significantly younger
than its counterpart in the Kalahari Craton, supporting the stepwise rifting model of Gaucher & Germs
(2002).
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References
Bossi, J. & Campal, N., 1992. Magmatismo y tectónica transcurrente durante el Paleozoico Inferior en
Uruguay. In: Gutierrez- Marco, J. G., Saavedra, J. & Rabano, I. (Ed.), Paleozoico Inferior de
Iberoamérica, Mérida, pp. 343- 356.
Bossi, J.;Cingolani, C.;Llambias, E.;Varela, R.;Campal, N. 1993. Características del magmatismo postorogenico finibrasiliano en el Uruguay: Formación Sierra de Ríos y Sierra de Animas. Revista
Brasileira de Geociencias 23: 282-288.
Gaucher, C. 2000. Sedimentology, paleontology, and estratigraphy of Arroyo del Soldado Group
(Vendian to Cambrian, Uruguay). Beringeria 26: 1-120.
Gaucher, C.; Germs, G.J.B. 2002. Stepwise rifting of Rodinia as the prelude to the amalgamation of WGondwana ? New insights from Uruguay and Brazil.-16th International Sedimentological
Congress, Abstracts Volume: 111-112; Johannesburg.
Midot, D. 1984. Etude Geologique et diagnostic Metallogenique l’exploration du Secteur de Minas
(Uruguay). These presentee pour l’obtention du Diplome de Docteur de 3e Cycle a l’Universite
Pierre et Marie Curie (París), 175 pp.
Preciozzi, F.; Masquelin, H; Sanchez Bettucci, L. 1993. Geología de la Porción Sur del Cinturón Cuchilla
de Dionisio, La Paloma, Uruguay. Primer simposio internacional del Neoproterozoico-Cámbrico
de la Cuenca del Plata, Minas-La Paloma, DINAMIGE.
Sánchez Bettucci, L; Pazos, P. 1996 Análisis Paleoambiental y Marcotectónico en la Cuenca Playa
Verde, Piriápolis, Uruguay. In: XIII Congreso Geológico Argentino y III Congreso de Exploración
de Hidrocarburos, I: 405-412.
Figure1. Geological sections of Cerro Las Ventanas Syncline.
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
New level of diamictites in the Corumbá Group (Ediacaran),
Paraguay Belt, South America
Paulo C. Boggiani, Thomas R. Fairchild, Claudio Riccomini,
Instituto de Geociências, Universidade de São Paulo (USP), Rua do Lago, 562, São Paulo, SP, 05508080, Brazil, boggiani@usp.br
Supported by FAPESP (proc. 04/01233-0)
Introduction
The Puga Formation is an important Neoproterozoic glaciogenic unit in central South America, first
described in the southern part of the Paraguai Mobile Belt (Maciel, 1959), but shortly thereafter identified
in the northern part of the belt as well (Almeida 1964, 1965). In both regions, the Puga Formation is
overlain by predominantly carbonate successions, the Araras Group in the north and the Corumbá Group
in the south. Cap carbonates have been described in both groups (Boggiani et al. 1996, Nogueira et al.
2003, Boggiani et al. 2003). This unit has been interpreted in the northern part of the belt as glaciomarine, and metassediments of the Cuiabá Group have been considered as coeval distal turbidites
(Alvarenga 1988, 1990, Alvarenga & Trompette 1992).
Recent work on the upper Corumbá Group in the Mina Laginha, off highway BR-262, 16 km south of
Corumbá, Mato Grosso do Sul, revealed a new occurrence of diamictite, here named the Laginha
Diamictite, above the Cloudina-bearing Tamengo Formation in the pelitic Guaicurus Formation at the top
of the Corumbá Group.
The new diamictite level in the Corumbá Group – the Laginha Diamictite
The new level of diamictite occurs in a recently opened portion of the Mina Laginha as a lens in the
pelites of the Guaicurus Formation near the contact with the underlying black limestones of the Tamengo
Formation (Figure 1). The lens is 15 m long and up to 1.5 m thick with a predominantly silty-clayey matrix
and concentrations of sand and granules. The clasts comprise no more than 10% of the lens and reach
25 cm in diameter. They are angular and all are black limestone derived from the Tamengo Formation.
Many clearly deform the underlying sediments. Slumped, convoluted beds were observed at the same
level as the lens. The lens occurs 5 m above the oolitic calcarenites of the Tamengo Formation at one
point and 15 m above them 150 m away along strike. Pyrite nodules occur at the contact.
Among the smaller clasts within the pelitic matrix of the Laginha Diamictites are jumbled, cylindrical
bodies of millimetric dimensions with thin calcareous walls, which may represent Cloudina-like fossils.
Fossils already identified in the Guaicurus Formation include the alga Eoholynia corumbensis (Gaucher,
2000, Gaucher et al. 2003).
Discussion – glacial or slump?
We are working with two hypotheses for the origin of the Laginha Diamictite, one glacial and the other
related to slumping. The strongest evidence for a glacial origin is the deformation observed at the base of
the clasts, as is common beneath glacial dropstones. The homogeneous lithology of the clasts, all of
black limestone from the underlying Tamengo Formation, indicates a local source and is not in keeping
with a strictly glacial interpretation, and the presence of slump structures within the pelite at the same
stratigraphic level close to the clustered clasts suggests gravitational movement.
If the glacial origin were to be confirmed, then which of the Neoproterozoic glacial epochs (Kendall et al.
2004, ) would be represented, Older Sturtian (~740-720 Ma), Younger Sturtian (~685 Ma), Marinoan
(600-620 Ma), Gaskier (~ 580 Ma), or an even younger local glaciation? The fossils Corumbella and
Cloudina in the underlying Tamengo Formation would support the last of these alternatives.
The discovery of diamictites above the Araras Group in the northern Paraguai Belt (Figueiredo et al – this
volume) opens up the possibility of other glacial events in the Paraguay Belt besides that represented by
the Puga Formation.
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“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Conclusions
A new diamictite level is described near the base of Guaicurus Formation, above the Corumbella- and
Cloudina–bearing Tamengo Formation. It is not associated with the widespread Puga Formation, which
underlies the Corumbá Group. Whether the new diamictite is glaciogenic or has been deposited by
gravity flow in a slope setting requires more detailed study. If it does prove to be glaciogenic, it would
have to represent a Late Edicaran glacial event.
Pe lite
Mud sto ne
0
Slumps
Lime stone cla st
may be from
Tamengo Formation
m
2
Coarser matrix
Details of Laginha Diamic tite
Guaicurus
Formation
Laginha Diam ic tite level
Tamengo
Formation
Bocaina
Formation
Cerradinho
Formation
Cadiueus
Formation
Puga
Formation
(p ossible Ma rinoan)
Figure 1 – Detail of Laginha Diamictite and its stratigraphic position.
References
Almeida, F.F. M. de 1964. Glaciação Eocambriana em Mato Grosso. Boletim da Divisão de Geologia e
Mineralogia, DNPM, Notas Preliminares e Estudos, 117: 1-11.
Almeida, F.F.M. de 1965. Geologia da Serra da Bodoquena (Mato Grosso), Brasil. Boletim da Divisão de
Geologia e Mineralogia, DNPM, 219:1-96.
Alvarenga, C.J.S de. 1988. Turbiditos e a glaciação do final do Proterozóico superior no Cinturão
Paraguai. Revista Brasileira de Geociências. 18:323-327.
Alvarenga, C.J.S. de & Trompette, R. 1992. Glacially influenced sedimentation in the later Proterozoic of
the Paraguay Belt (Mato Grosso, Brazil). Palaeogeography, Palaeoclimatology, Palaeoecology,
92:85-105.
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Alvarenga, C.J.S. de 1990. Phénomènes sédimentaires, structuraux et circulation de fluides développés
à la transition chaîne-craton. Exemple de chaîne Paraguai d’âge protérozoïque supérieur, Mato
Grosso, Brésil. Thèse doct. Univ. Aix-Marseille III, France, 177 p.
Alvarenga, C.J.S. de; Santos, R.V.; Dantas, E.L. 2004. C-O-Sr isotopic stratigraphy of cap carbonates
overlying Marinoan-age glacial diamictites in the Paraguay Belt, Brazil. Precambrian Research
131:1-21.
Boggiani, P.C.; Coimbra, A.M.; Fairchild, T.R 1996. Stromatolitic reefs of the Bocaina Formation
(Corumbá Group - Neoproterozoic-Cambrian) Mato Grosso do Sul, Brazil. Anais da Academia
Brasileira de Ciências, Resumo das Comunicações, 68(4):596-597.
Boggiani, P.C.; Sial, A. N.; Babinski, M.; Ferreira, V.P. 2003. New carbon isotopic data from the Corumbá
Group as a contribution to a composite section for the Neoproterozoic III in South America. In: III
International Colloquium Vendian-Cambrian of W-Gondwana. Cape Town, October 2003. IGCP478 meeting. Programme and Extended Abstracts, p. 13-16.
Figueiredo, M.F.; Babinski, M.; Alvrenga, C.J.S.; Pinho, F.E.C. 2004. Diamictites overlying Varanger-age
carbonates of Araras Formation, Paraguay Belt, Brazil: Evidence of a new glaciation?. In: 1st
Symposium on Neoproterozoic-Early Palaeozoic Events in SW-Gondwana, São Paulo, Brazil, 2nd
Meeting IGCP-478 Neoproterozoic Events in SW Gondwana.Extended Abstract, p….
Gaucher, C. 2000. Sedimentology, palaeontology and stratigraphy of the Arroyo del Soldado Group
(Vendian to Cambrian, Uruguay). Beringeria, 26:1-120.
Gaucher, C.; Boggiani, P.C.; Sprechmann, P.; Sial, A. N.; Fairchild, T.R. 2003. Integrated correlation of
Vendian to Cambrian Arroyo del Soldado and Corumbá Groups (Uruguay and Brazil):
palaeogeographic, palaeoclimatic and palaeobiologic implications. Precambrian Research, 120(34):241-278.
Kendall, B.S., Creaser, R.A.; Ross, G.M.; Selby, D. 2004. Constraints on the timing of Marinoan
“Snowball Earth” glaciation by 187Re – 187Os dating of shale in Western Canada. Earth and
Planetary Science Letters, 222:729-740.
Maciel, P. 1959. Tilito Cambriano (?) no Estado de Mato Grosso. Boletim da Sociedade Brasileira de
Geologia, São Paulo, 81:31-39.
Nogueira, A. C.R.N.; Riccomini, C.; Sial, A. N.; Moura, C.; Fairchild, T. 2003. Soft-sediment deformation
at the base of the Neoproterozoic Puga cap carbonate (southwestern Amazon Craton, Brazil):
Confirmation of rapid icehouse-greenhouse transition in snowball earth. Geology., 31(7):613-616.
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Cloudina from the Itapucumí Group (Vendian, Paraguay): age
and correlations
Paulo C. Boggiani* & Claudio Gaucher**
* Instituto de Geociências, Universidade de São Paulo (USP), Rua do Lago, 562, São Paulo, SP, 05508080, Brazil, boggiani@usp.br
** Departamento de Geología, Facultad de Ciencias, Iguá 4225, 11400, Montevideo, Uruguay,
gaucher@chasque.apc.org
Geological setting
The Itapucumí Group is a predominantly carbonatic succession 300-400 m in thickness cropping out in
northern-central Paraguay. The equivalence between the Itapucumí Group and the Corumbá Group in
Brazil was postulated by Harrington (1950) and Almeida (1965, 1984), although a detailed stratigraphic
column of the Vallemí Mine presented by Boggiani (1998) showed that thes units are lithologically
different. At the Vallemí Mine the following units occur from base to top (Boggiani 1998):
1) Medium-grained red arkoses, which according to Hutchinson (1979) occur at the base of the
Itapucumí Group.
2) Marls and marl/mudstone rhythmites 50 m in thickness.
3) 70 m of ooid grainstones formed in high-energy environments such as barrier islands and oolitic
shoals. Partial dolomitization of grainstones occurred under evaporitic conditions, as indicated by
gypsum pseudomorphs.
4) Marls.
Outcrops of the Itapucumí Group near Colonia San Alfredo (22° 54’ S, 57° 25’ W) are lithologically similar
to the Corumbá Group and different from the succession occurring at Vallemí Mine, described above.
This raises the possibility that the Itapucumí Group might be separated into two different units, one that
can be correlated with the Corumbá Group and another one that cannot.
We report here for the first time the occurrence of Cloudina Germs (1972) at Estancia Belo Horizonte (to
the north of Colonia San Alfredo), which provides biostratigraphic support for the correlation of part of the
Itapucumí Group with the Corumbá Group.
Palaeontology
Tubular fossils occur in two samples of massive, impure, light brown calcisiltites. The rock is composed of
silt-sized, rather angular clasts of calcite, quartz, phyllosilicates and opaque minerals.
Five longitudinal, six oblique and fifteen tranverse cross- sections of fossils show a structure of
eccentrically nested cones (Fig. 1A). Maximum diameter of longitudinal sections vary between 0.3 and
1.3 mm. Cross sections range between 0.4 and 0.9 mm in diameter. Shells are composed of fine-grained
calcite crystals, and are rather thin (20-50 µm, Fig. 1). Broken specimens and loose fragments occur,
indicating that the organism possessed a rigid skeleton. Axes of most specimens are subparallel to one
another. However, due to the absence of bedding it is not clear if this orientation is due to the fossils
being in life position or if they were current-oriented. The fact that size-frequency distribution of
longitudinal sections is rather wide makes orientation by currents rather unlikely.
The fossils described above can be confidently assigned to the genus Cloudina (Germs 1972), since all
diagnostic characteristics are present in the studied material, namely: (1) calcitic shell, (2) cone-in-cone
structure, (3) eccentrically nested cones giving bilateral simmetry (Fig. 1A), and (4) size similar to
Cloudina riemkeae Germs, 1972 or Cloudina lucianoi (Beurlen & Sommer) Zaine & Fairchild 1985.
Discussion
The species identification of Cloudina in the Itapucumí Group is not obvious, since size-frequency
distribution of fossils fall within the ranges known for Cloudina riemkeae and Cloudina lucianoi (Germs
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
1972, Hahn & Pflug 1985, Gaucher et al. 2003). Specimens larger than 2 mm in diameter, known for C.
lucianoi but not for C. riemkeae, were not observed. However, since the available material is limited and
considering that most specimens appear as cross sections, we cannot rule out the occurrence of larger
individuals. On the other hand, nested cones in the studied material are quite long and thin-walled,
bearing a strong resemblance to Cloudina lucianoi from the Corumbá Group (Gaucher et al. 2003).
Comparison with Cloudina riemkeae from the type area in Driedoornvlakte farm, Namibia (Germs 1972),
also showed considerable differences between this species and the fossils from Paraguay. Cloudina
riemkeae shows more robust walls and mostly short cones, although specimens with long cones also
occur (forma ß, Germs 1972). Therefore we assign the material from the Itapucumí Group to Cloudina
lucianoi (Beurlen & Sommer) Zaine & Fairchild, 1985.
Age and correlations
Cloudina occurs exclusively in late Vendian (Ediacaran) successions worldwide (Grant 1990, Gaucher &
Sprechmann 1999, Hofmann & Mountjoy 2001, Gaucher et al. 2003, Amthor et al. 2003). Whereas the
extinction of Cloudina coincides with the Neoproterozoic-Cambrian boundary at 542 Ma (Grotzinger et al.
1995, Amthor et al. 2003), the lower age boundary for the taxon is less clear. The oldest occurrences so
far reported are probably those of the lower Arroyo del Soldado Group, Uruguay (Gaucher & Sprechmann
1999, Gaucher 2000, Gaucher et al., 2003). On the basis of C- and Sr-chemostratigraphy combined with
fossil evidence, an age of ca. 580-575 Ma is assumed for Cloudina riemkeae in Uruguay (Gaucher et al.
2004). Therefore, we can be confident of an late Vendian age for the Itapucumí Group near Colonia San
Alfredo. No fossils have been described for the section at Vallemí Mine (see above), but isotopic data
were reported by Boggiani (1998). Carbonates yielded least altered 87Sr/86Sr values of 0.7081, 13C PDB
varying between 0.3 to 1,8 0/00. These values are common in Vendian successions worldwide (Walter et
al. 2000, Melezhik et al. 2001), suggesting that the Vallemí Mine succession, though different from the
Colonia San Alfredo section, could also be Vendian in age.
From a regional perspective, our findings strengthen the hypothesis put forward by Gaucher et al. (2003),
that an extensive carbonate platform developed in the Vendian on the eastern margin of the Río de la
Plata and Amazonia cratons between southern Uruguay and south-western Brazil. The studied outcrops
occupy an intermediate position in the Corumbá-Arroyo del Soldado shelf. Thus, the shelf deposits are
represented by at least three lithostratigraphic units, which are from north to south the Corumbá,
Itapucumí and Arroyo del Soldado groups. More work is needed to determine if units located farther to the
south (Sierras Bayas Group, Argentina) or to the north (Araras Group, Brazil) of this area are also related
to this palaeoshelf.
Fig. 1: Cloudina cf. C. lucianoi (Beurlen & Sommer) Zaine & Fairchild, 1985 from the Itapucumí Group,
Estancia Belo Horizonte, Paraguay. A: Cross section 0.49 mm in maximum diameter, showing three
eccentrically-nested cones (1 to 3) and sparitic infill. B: Cross section showing only one cone 0.65 mm in
diameter with variable wall thickness. Shell infill is made up of carbonate and hematite.
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References
Almeida, F.F.M. de 1965. Geologia da Serra da Bodoquena (Mato Grosso), Brasil. Boletim da Divisão de
Geologia e Mineralogia, DNPM, 219:1-96.
Almeida, F.F.M. de 1984. Província Tocantins, setor Sudoeste. In: O Pré-Cambriano do Brasil (Almeida,
F.F.M. de & Hasui, Y., Eds.). São Paulo, Edgard Blücher, p. 265-281.
Amthor, J.E.; Grotzinger, J.P.; Schröder, S.; Bowring, S.A.; Ramezani, J.; Martin, M.W. and Matter, A.
2003 Extinction of Cloudina and Namacalathus at the Precambrian-Cambrian boundary in Oman.
Geology 31, 431-434.
Bogggiani, P.C. 1998. Análise Estratigráfica da Bacia Corumbá (Neoproterozóico) – Mato Grosso do Sul.
Tese de Doutorado, Instituto de Geociência – USP, São Paulo, 181 p.
Gaucher, C., 2000. Sedimentology, palaeontology and stratigraphy of the Arroyo del Soldado Group
(Vendian to Cambrian, Uruguay). Beringeria 26, 1-120.
Gaucher, C., Sprechmann, P., 1999. Upper Vendian skeletal fauna of the Arroyo del Soldado Group,
Uruguay. Beringeria 23, 55-91.
Gaucher, C., Boggiani, P.C., Sprechmann, P., Sial, A.N., Fairchild, T.R., 2003. Integrated correlation of
the Vendian to Cambrian Arroyo del Soldado and Corumbá Groups (Uruguay and Brazil):
palaeogeographic, palaeoclimatic and palaeobiologic implications. Precambrian Res. 120, 241278.
Gaucher, C.; Sial, A.N.; Blanco, G., Sprechmann, P., 2004. Chemostratigraphy of the lower Arroyo del
Soldado Group (Vendian, Uruguay) and palaeoclimatic implications. Gondwana Res. 7: 715-730.
Germs, G. J. B., 1972. New shelly fossils from Nama Group, South West Africa.- American Journal of
Science 272, 752- 761.
Grant, S.W.F., 1990. Shell structure and distribution of Cloudina, a potential index fossil for the terminal
Proterozoic.-American Journal of Science 290-A, 261-294.
Grotzinger, J. P., Bowring, S. A., Saylor, B. Z., Kaufman, A. J., 1995. Biostratigraphic and Geochronologic
Constraints on Early Animal Evolution.- Science 270, 598- 604.
Hahn, G., Pflug, H. D., 1985. Die Cloudinidae n. fam., Kalk- Röhren aus dem Vendium und UnterKambrium.- Senckenbergiana lethaea 65, 413- 431.
Harrington, H.J. 1950. Geología del Paraguay Oriental. Univ. Buenos Aires, Fac. Cienc. Exact. Fis. Nat.,
Contribution Cientifica Série E Geol. 1:1-82.
Hofmann, H.J., Mountjoy, E.W. 2001. Namacalathus-Cloudina assemblage in Neoproterozoic Miette
Group (Byng Formation), British Columbia: Canada’s oldest shelly fossils. Geology 29, 10911094.
Hutchinson, D.S. 1979. Geology of the Apa High. DRM-MODC, T.A.C., unpublished internal repport,
Asunción, 24 pp.
Melezhik, V.A.; Gorokov, I.M.; Kuznetsov, A.B., Fallick, A.E., 2001. Chemostratigraphy of Neoproterozoic
carbonates: implications for “blind dating”. Terra Nova 13, 1-11.
Walter, M.R., Veevers, J.J., Calver, C.R., Gorjan, P., Hill, A.C., 2000. Dating the 840-544 Ma
Neoproterozoic interval by isotopes of strontium, carbon and sulfur in seawater, and some
interpretative models. Precambrian Res. 100, 371-433.
Zaine, M.F., Fairchild, T.R., 1985. Comparison of Aulophycus lucianoi Beurlen & Sommer from Ladário
(MS) and the genus Cloudina Germs, Ediacaran of Namibia.-Anais Academia Brasileira de
Ciências 57 (1), 130.
15
“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Micropaleontological aspects of Lowermost Cambrian black
shales and cherts on the Yangtze Platform, China
A. Braun*, J.-Y. Chen**, D. Waloszek***, A. Maas***
*Institute of Paleontology, University of Bonn, Nussallee 8, D-53115 Bonn, Germany,
Braun@uni-bonn.de
**Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjing 210008, PR,
China
***Section Biosystematic Documentation, University of Ulm, D-89069 Ulm, Germany
Black cherts, phosphorites and black shales on the Yangtze Platform, China yield evidence for a very
intense biological input to sedimentation during the Lowermost Cambrian (Anabarites trisulcatus –
Protohertzina anabarica assemblage zone). Biological input is preserved a) as "organic" (coaly and
bituminous) remains and b) as very abundant siliceous hardparts.
Coaly and bituminous remains derive from phytoplankton. Spiny acritarchs (Micrhystrium) are present in
rock forming quantities in early diagenetic cherts of the Hetang black shale formation. Abundance of
phytoplankton in the sedimentary rocks clearly indicates that plankton blooms of extraordinary scale have
had a strong influence on the lithological composition of the sediments. Besides, they presumably caused
the deeper water body as well as the sea bottom to be anoxic (see also Goldberg et al. 2003), largely
preventing a deeper water benthos from settlement during this time. The only benthic organisms most
probably inhabiting the anoxic sea floors and leaving abundant remains were sponges.
Besides organic microfossils, clay-rich and siliceous sedimentary rocks contain a large amount of
siliceous hard parts. Based on their high content of siliceous fossils these rocks are in fact largely
biosiliceous sediments. Siliceous microfossils are almost exclusively sponge spicules. Among the sponge
spicules are megascleres as well as microscleres. Both spicule types allow to state, which particular
sponge groups were present in the respective depositional environment. Remarkably, some of the
solution residues consist exclusively of spicules derived from lithistid demosponges, whereas others (e. g.
most „black“ shale and black chert samples) contain exclusively spicules of hexactinellid sponges.
Relating sedimentary facies to the systematics of sponges, biofacial and ecologic differences seem to
have been present already by the earliest Cambrian. Rocks rich in siliceous microfossils from the
mentioned occurrences all derive from a shelf or slope („transitional“) environment. In the deeper oceanic
basin of the Yangtze platform (depositional environment of the „Hetang Shales“) siliceous sponge
remains are not common in solution residues. Eventually, the deeper water sea bottoms, being strongly
anoxic following extensive phytoplankton blooms have not been colonized by sponge faunas by the
earliest Cambrian, or only temporarily during time periods of more favourable ecological conditions.
Besides sponges, the oldest known remains of radiolarians are present in the solution residues, but they
are extremely rare. Two occurrences (cf. BRAUN & CHEN 2003; Braun et al., submitted) yielded a few
specimens. These display strong similarities to "advanced" spherical radiolarian morphologies found in
Paleozoic rocks younger than Ordovician.
The significant contribution of biosiliceous particles to early Cambrian sedimentation on the Yangtze
platform as well as to other occurrences in Kazakhstan and Europe implies, that silica-biomineralizing
organisms have played a significant role in the geochemical cycle of silica in the oceans by the beginning
of the Phanerozoic. The high abundance of sponge spicules in the sediments indicate that sponges
(Porifera) played a major ecological role in the early Cambrian environment of the investigated areas.
This is supported by finds of complete sponges and spicule clusters on bedding planes of clay rich
sediments and black shales (REITNER 1994, RIGBY 1995).
Phosphatic microfossils and phosphatic particles from black shales and bedded black cherts contribute to
the ecological picture of the oceanic basins as well as to the environments of the anoxic sea bottoms
during earliest Cambrian times. The common occurrence of Protohertzina (interpreted as Chaetognath
hooks by SZANIAWSKI 2002) and microcoprolite-shaped phosphatic particles could possibly indicate that
the surface water masses in shelf and deep-water areas containing rich phytoplankton could have served
as a food source not only for a rich benthic sponge fauna, but eventually also for pelagic metazoans in
the earliest Cambrian.
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“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Field work and sampling within the framework of IGCP 478 in SW Gondwana aims at finding and
investigating correlative lithologies and biologic remains in S-Africa and S-America, especially with
respect to cherts and phosphorites.
References
BRAUN, A. & CHEN, J.-Y., 2003. Plankton from early Cambrian black shale series on the Yangtze Platform,
and its influences on lithologies. – Progress in Natural Science, 13 (10): 777-782.
BRAUN, A., CHEN, J., W ALOSZEK, D. & MAAS, A. (submitted): First Early Cambrian Radiolaria.
CHEN, J.-Y., BRAUN, A., W ALOSZEK, D., PENG, Q. & MAAS, A., 2004. Lower Cambrian yolk-pyramid
embryos from Southern Shaanxi, China. – Progress in Natural Science, 14 (2):167-172.
GOLDBERG, T., STRAUSS, H., GUO, Q.-J. & LIU, C.-Q., 2003. Late Neoproterozoic to Early Cambrian
sulphur cycle – An isotopic investigation of sedimentary rocks from the Yangtze platform. –
Progress in Natural Science, 13 (12): 946-950.
REITNER, J. L., 1974 New phylogenetic and palaeoecological aspects of late Precambrian and early
Cambrian sponge communities. In: Ecological aspects of the Cambrian radiation.(IGCP 366). –
Terra abstracts, 6 (suppl. 3): 6-7.
RIGBY, J. K., 1995 Lower Cambrian demosponges and hexactinellid sponges from Yunnan, China. – J.
Paleont., 69 (6): 1009-1019.
SZANIAWSKI, H., 2002 New evidence for the protoconodont origin of chaetognaths. – Acta Palaeontologica
Polonica, 47 (3): 405-419.
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Diamictites overlying Marinoan-age Carbonates of Araras
Formation, Paraguay Belt, Brazil: evidence of a new
glaciation?
Milene F. Figueiredo*, Marli Babinski*, Carlos J.S. Alvarenga**, Francisco E.C. Pinho***
*Instituto de Geociências, Universidade de São Paulo (USP), Rua do Lago, 562, São Paulo, SP, 05508080, Brazil, mimff@ig.com.br, babinski@usp.br
**Instituto de Geociências, Universidade de Brasília (UnB), Brasília, DF, 70910-900, Brazil, alva1@unb.br
***Depto. de Geologia, Universidade Federal de Mato Grosso (UFMT), Av. Fernando Correa s/n, Cuiabá,
MT, 78060-900, Brazil, aguapei@yahoo.com
The Neoproterozoic Era was marked by important glacial events, which have increasingly attracted the
attention of scientists. However, there is no consensus about how many glaciations took place during the
Neoproterozoic or their ages. Recent work done by Halverson et al. (2004) suggests three glaciations: the
older named Sturtian-Raptian (ca. 740 Ma), Marinoan (ca. 600 Ma) the second, and the younger named
Varanger (ca. 580 Ma).
The deposition of Paraguay Belt sediments started during the second glaciation with: (i) a Neoproterozoic
basal glaciomarine and glacial sequence corresponding to Cuiabá Group and Bauxi and Puga formations
(Alvarenga, 1988); overlying the (ii) thick succession of marine carbonates of Varanger to post-Varanger
age from the Araras Formation (Fairchild, 1978; Nogueira et al., 2003; Pinho et al., 2003); and (iii) a top
Cambrian siliciclastic sequence of Raizama and Diamantino formations (Ribeiro Filho et al., 1975). The
rocks of the Paraguai Belt were deformed during the Early to Middle Cambrian by collision of Paraná
Craton with Amazonian Cráton and Rio Apa Block (see figure 01).
Recent field work carried out in the northern portion of the Paraguay Belt, where a dominant east-west
trend is observed, it was possible to observe the presence of diamictites overlying dolomites of the Araras
Formation and overlain by Raizama Formation sandstones, along the Azul and Morro Selado Mountain
Ranges, in Mato Grosso state. This diamictite layer is approximately 60m thick and is composed of a pink
muddy matrix with sandstone, arkose, chert, carbonate and crystalline clasts, varying in size from grains
to boulders, and in shape (rounded to angular). Conformable to this level, there is a thin but ubiquitous
bed (~ 60 cm) of yellow laminated siltstone with variegated granules. Overlying this is a very thick layer of
bronwish clay-rich shale, which grades to gray towards the top of the section, with over 100m in
thickness. A graded contact is observed with the overlying sandstones of the Raizama Formation. The
basal contact of the diamictite with the carbonates from the Araras Formation was not observed.
The described diamictites and clay-rich shales are generally not well exposed. They are covered by talus
deposits or strongly weathered. In relief, these rocks occupy longer and lower areas between mountain
ranges sustained by the Raizama Formation sandstones and hills formed by the Araras Formation
carbonates. The best exposures are found only in drainages, quarries and road cuts.
These diamictites suggest the presence of a new Neoproterozoic glaciation in the southern portion of the
Gondwana supercontinent, confirming the negative 13C incursion observed by Alvarenga et al. (2004) on
carbonates next to the top of the Araras Formation, in the Paraguay Belt, with the third glacial period
proposed by Halverson et al. (2004) to African Neoproterozoic basins. These data, however, are
preliminary and a detailed field work as well as a chemostratigraphy study has to be done in order to
confirm the evidence of this new glaciation in the northern part of the Paraguay Belt.
References
Alvarenga, C.J.S & Trompette, R. 1993. Evolução Tectônica Brasiliana da Faixa Paraguai na região de
Cuiabá. Revista Brasileira de Geociências, 23(1):18-30.
Alvarenga, C.J.S., 1988. Turbiditos e a Glaciação do Final do Proterozóico Superior no Cinturão
Paraguai, Mato Grosso. Revista Brasileira de Geociências, 18:323-327.
18
“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Alvarenga, C.J.S., Santos, R.V., Dantas, E.L., 2004. C-O-Sr isotopic stratigraphy of cap carbonates
overlying Marinoan-age glacial diamictites in the Paraguay Belt, Brazil. Precambrian Research
131:1-21.
Fairchild, T.R., 1978. Evidências paleontológicas de uma possível idade Ediacariana ou Cambriano
Inferior, para parte do Grupo Corumbá, Mato Grosso do Sul. In: Congresso Brasileiro de
Geologia, 30, Recife, PE, Brazil. Boletim Especial, p. 38-39.
Halverson, G.P.; Maloof, A.C., Hoffman P.F.2004. The Marionan glaciation (Neoproterozoic) in northeast
Svalbard. Basin Research (in press).
Nogueira, A.C.R., Riccomini, C., Sial, A.N., Moura, C.A.V., Trindade, R.I.F., 2003. C and Sr isotope
variations and paleoceanographic changes as recorded in the Late Neoproterozoic Araras
Carbonate Plataform, Amazon Craton, Brazil. In: IV South American Symposium on Isotope
Geology, Salvador, BA, Brazil. Short papers, p. 380-381.
Pinho, F.E.C., Sial, A.N. and Figueiredo, M.F., 2003. Contribution to the Neoproterozoic C- and Oisotopic record, carbonate rocks from the Paraguay Belt, Mato Grosso, Brazil. In: IV South
American Symposium
Figure 1 - Regional geological map of Paraguay Belt in Mato Grosso State (modified from Alvarenga and
Trompette, 1993), showing the diamictite occurrences.
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“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
CRUSTAL STRUCTURES OF PARAGUAY BELT FROM
GRAVITY AND MT DATA: A CAMBRIAN SUTURE IN WESTERN
GONDWANALAND?
Shimeles Fisseha* **, Naomi Ussami*, Antonio L. Padilha**, Ícaro Vitorello**, Ricardo I. F.
Trindade*
*Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo (USP), Rua do
Matão, 1226, São Paulo, SP, 05508-080, Brazil
**Divisão de Geofísica Espacial, Instituto Nacional de Pesquisas Espaciais (INPE), Av. dos Astronautas,
1758, São José dos Campos, SP,12227-010, Brazil, shimeles@dge.inpe.br
The Paraguay fold and thrust belt and the actual positions of the Amazonian/Paraná cratonic
borders, under the vast Cenozoic sedimentary cover of the Pantanal Wetland, are still
unresolved and constitute a major problem in understanding the geological and tectonic
evolution of the central and southern South American Platform. In order to provide geophysical
constraints to help solving these geological uncertainties, we have begun to carry out high
resolution magnetotelluric (MT) studies along an ENE – WSW profile between the cities of
Corumbá and Coxim. The analyses of the MT data, including dimensionality studies and 2D
inversion indicate the presence of a prominent electrical resistivity anomaly (< 150 Ωּm) and
strong electrical anisotropy in central Pantanal basin at depths greater than 6 km. Variations in
the local geomagnetic transfer functions, shown as induction vectors also support the presence
of these mid-crustal conductors and show the complexity of the deep geoelectric structures. In
conjunction with gravity data, these crustal conductors are attributed to reflect the presence of a
suture zone related to graphitized metasediments and/or circulation of mineralized fluids in mid
crustal fractures whereas the gravity anomaly is thought to be caused either by remnants of
deformed magmatic-arc terranes and/or by an overthrusted lower crust, along a collisional
boundary between two petrophysically distinct lithospheric blocks. Within the framework of the
regional geology, the western and eastern blocks are attributed to the Rio Apa Craton (or
Amazon plate) and the Paraná block, respectively
Project sponsored by FAPESP grants 1999/12690-2 and 2000/00806-5
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Impact of a late Vendian, non-global glacial event on a
carbonate platform, Polanco Formation, Uruguay
Claudio Gaucher*, Alcides N. Sial**, Marcio M. Pimentel***, Valderez P. Ferreira**
*Departamento de Geologia, Facultad de Ciências, Iguá 4225, 11400, Montevideo, Uruguay
gaucher@chasque.apc.org
**NEG-LABISE, Departamento de Geología, Universidade Federal de Pernambuco (UFPE), C.P. 7852,
Recife-PE, 50670-000 Brazil
***Instituto de Geociencias, Universidade de Brasilia, Brasília-DF, 70910-900 Brazil
Introduction
The Polanco Formation of the Arroyo del Soldado Group (ASG) is a thick (up to 900 m) carbonate
succession composed by clastic limestones, limestone-dolostone rhythmites and dolostones (Gaucher,
2000). Carbonates are rich in organic matter, which gives them a bluish grey to black colour.
Age of the unit is constrained by: (1) Cloudina riemkeae occurring at the top of the underlying Yerbal
Formation (Gaucher & Sprechmann, 1999; Gaucher et al., 2003; Fig. 1); (2) a low-diversity assemblage
of acritarchs reported by Gaucher (2000), (3) U-Pb SHRIMP datings of granitoids of the basement of the
ASG (Hartmann et al., 2002) and Rb-Sr datings of intrusive granites (Kawashita et al., 1999), which give
maximum and minimum age constraints of 633 ± 12 Ma and 532 ± 11 Ma respectively, and (4) detailed C
and Sr isotopic data reported by Gaucher et al. (2004). Altogether, these lines of evidence suggest that
the Polanco Formation was deposited between 575 and 550 Ma.
Lithostratigraphy
Facies of the Polanco Formation vary according to the location of the section within the basin. Whereas
proximal sections in the west show an alternation of calcarenite-dolosiltite rhythmites with calcareous
storm deposits, distal sections are made up of a fine intercalation of dolsiltites and calcareous turbidites
(Gaucher, 2000; Gaucher et al., 2004). The whole unit was deposited in a remarkably stable carbonate
shelf under tropical conditions. The Polanco Formation represents a shallowing-upward sequence, a
trend that can be seen in both proximal and distal sections (Gaucher, 2000).
Gaucher et al. (2004) studied in detail the lithostratigraphy of the unit at Calera de Recalde, one of the
most proximal sections. The authors subdivided the formation into six informal units, which were named
unit A to F (Fig. 1). These units are characterized by different grain size, sedimentary structures, organic
matter content, colour, organic-walled microfossils and carbon-isotopic signatures. Gaucher et al. (2004)
13C -peaks were
established a correlation of 13C and other attributes with palaeobathymetry. Positive
associated with high sea-level stand, high organic-carbon burial and relatively higher microfossil diversity
13C -excursions occur in carbonates with less organic matter, less microfossil
(Fig. 1), while negative
diversity and are always associated to regressions. We show here that, as suggested by Gaucher et al.
(2004), the regressive intervals represented by units B, and F of the Polanco Formation actually record
glacial events.
Evidences of a glacial event
The near-primary nature of C and Sr isotopic signatures recorded in the Polanco Formation at Calera de
Recalde has been shown by Gaucher et al. (2004). We carried out 9 87Sr/86Sr determinations on
limestones (calcarenites and calcisiltites) containing more than 500 ppm Sr, which were combined with 4
analyses reported by Kawashita et al. (1999). Unit A and the upper part of unit C (Fig. 1) yielded relatively
high 87Sr/86Sr values (0.7083-0.7085). A significant drop to values as low as 0.7073 occurs in Unit B and
the upper Unit E-Unit F. In both cases the following changes in measured parameters occur (Fig. 1):
-Positive to negative
-Low
87Sr/86Sr
13C
–excursions
values
-Remarkable sea-level drop
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
-Lower organic matter contents
-Lower diversity of organic-walled microfossils.
We argue that the most plausible explanation for these combined phenomena is a severe, but non-global
glacial event. Glaciation would have caused a lowering of sea-level, a decrease in plankton
bioproductivity and the deposition of organic matter, environmental stress leading to lower acritarch
13C–excursion and a lowering of 87Sr/86Sr values due to diminished chemical
diversity, a negative
weathering and continental runoff. Absence of glacial deposits indicates that the glacial event did not
reach tropical latitudes. According to palaeomagnetic data reported by Sánchez Bettucci & Rapalini
(2002), the Río de la Plata Craton, onto which the Polanco Formation was deposited, was located at
17.5° latitude by 550 Ma.
Discussion and conclusions
A decrease of 87Sr/86Sr values of the magnitude observed (0.0010) was predicted by Jacobsen &
Kaufman (1999) for a global glacial event. Assuming a diagenetic flux of Sr similar to present values, they
postulate that a decrease of this magnitude in 87Sr/86Sr would imply a length of 1 Ma for the snowball
event. Since the Polanco Unit B glacial event was not global in extent, we envisage a longer duration for
this glaciation. A thickness of 200 m of storm-dominated shelf carbonates analogue to Unit B is deposited
in 2 to 5 Ma in modern environments (Einsele, 2000), consistent with the previous estimates.
An unexpected feature of the obtained 87Sr/86Sr curve is that isotopic ratios begin to decrease before
13C shifts to negative values (Fig. 1). The same trend is recorded in pre-glacial Neoproterozoic
carbonates of Australia (Walter et al., 2000), Namibia (Fölling & Frimmel, 2002) and elsewhere (Melezhik
13C excursion,
et al., 2001). This might imply either that ice sheets started to grow before the negative
or that other factors (e.g. changing hydrothermal inputs) were involved.
The glacial events represented by units B and E-F of the Polanco Formation are significantly younger
than the age currently accepted for the youngest Neoproterozoic glaciation, named Gaskiers event (580
Ma, Bowring et al., 2003). This raises the possibility that further, non-global glacial events took place in
the Vendian, which are potentially important for event stratigraphy, as suggested by Germs (1995) for the
Nama Group.
References
Bowring, S.; Myrow, P.; Landing, E.; Ramezani, J.; Grotzinger, J.P. 2003. Geochronological constraints
on terminal Neoproterozoic events and the rise of metazoans. Geophysical Research Abstracts 5,
219.
Fölling, P. G.; Frimmel, H. E. 2002. Chemo-stratigraphic correlation of carbonate successions in the
Gariep and Saldania Belts, Namibia and South Africa. Basin Research 14, 69-88.
Gaucher, C., 2000. Sedimentology, palaeontology and stratigraphy of the Arroyo del Soldado Group
(Vendian to Cambrian, Uruguay).-Beringeria 26, 1-120.
Gaucher, C., Sprechmann, P., 1999. Upper Vendian skeletal fauna of the Arroyo del Soldado Group,
Uruguay. Beringeria 23, 55-91.
Gaucher, C., Boggiani, P.C., Sprechmann, P., Sial, A.N., Fairchild, T.R., 2003. Integrated correlation of
the Vendian to Cambrian Arroyo del Soldado and Corumbá Groups (Uruguay and Brazil):
palaeogeographic, palaeoclimatic and palaeobiologic implications. Precambrian Res. 120, 241278.
Gaucher, C.; Sial, A.N.; Blanco, G., Sprechmann, P., 2004. Chemostratigraphy of the lower Arroyo del
Soldado Group (Vendian, Uruguay) and palaeoclimatic implications. Gondwana Res. 7: 715-730.
Germs, G.J.B. 1995. The Neoproterozoic of southwestern Africa, with emphasis on platform stratigraphy
and paleontology. Prec. Res. 73, 137-151.
Hartmann, L.A. , Santos, J.O. Bossi, J., Campal, N., Schipilov, A., Mac Naughton, N. J., 2002. Zircon and
titanite U-Pb SHRIMP geochronology of Neoproterozoic felsic magmatism on the eastern border
of the Rio de la Plata Craton, Uruguay. J. South Am. Earth Sci. 15, 229-236.
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Jacobsen, S.B., Kaufman, A.J., 1999. The Sr, C and O isotopic evolution of Neoproterozoic seawater.Chem. Geol. 161, 37-57.
Kawashita, K., Gaucher, C., Sprechmann, P., Teixeira, W., Victória, R., 1999. Preliminary
chemostratigraphic insights on carbonate rocks from Nico Pérez Terrane (Uruguay).In: Actas II
South American Symposium on Isotope Geology. Córdoba (Argentina), pp. 399-402.
Melezhik, V.A.; Gorokov, I.M.; Kuznetsov, A.B., Fallick, A.E., 2001. Chemostratigraphy of Neoproterozoic
carbonates: implications for “blind dating”. Terra Nova 13, 1-11.
Sánchez Bettucci, L.; Rapalini, A.E. 2002. Paleomagnetism of the Sierra de Las Animas Complex,
southern Uruguay: its implications in the assembly of western Gondwana. Precambrian Res. 118:
243-265.
Walter, M.R., Veevers, J.J., Calver, C.R., Gorjan, P., Hill, A.C., 2000. Dating the 840-544 Ma
Neoproterozoic interval by isotopes of strontium, carbon and sulfur in seawater, and some
interpretative models. Precambrian Res. 100, 371-433.
Figure 1: Litho-, bio- and chemostratigraphy of the Polanco Formation at Calera de Recalde (modified
from Gaucher et al., 2004). Shaded areas represent glacial events.
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“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Record of 483-429 Ma U-Pb and 40Ar-39Ar ages in SW
Rondonia and Sunsás provinces: the role of Neoproterozoic
mobile belts overprint within the Amazon craton
Mauro C. Geraldes*, Valéria G. de Paulo*, Elzio da S. Barboza* , Paulo Vasconcelos**,
Wilson Teixeira***
* Universidade do Estado do Rio de Janeiro (UERJ), Rua São Francisco Xavier 524, Rio de Janeiro, RJ,
20550-013, Brazil, geraldes@uerj.br, tzara@pop.com.br, elziosb@yahoo.com.br
**University of Queensland, Earth Sciences, Steele Building-Brisbane, Qld 4072, Australia,
paulo@earth.uq.edu.au
***Instituto de Geociências, Universidade de São Paulo (USP), Rua do Lago, 562, São Paulo-SP, 05508080, Brazil, wteixeir@usp.br
The Amazon craton began a period of tectonic quiescencee at the Meso-Neoproterozoic time boundary,
after the collision between this craton and Laurentia and Baltica that led to the formation of the Rodinia
supercontinent (Hoffman, 1991; Dalziel, 1992). The latest tectonic events in the SW part of Amazonia
comprise the Sunsás orogen in Bolivia (1200-1000 Ma), the Nova Brazilândia orogen in Rondônia state
(1100-1000 Ma) and the Aguapeí thrust belt in Mato Grosso state (920-910 Ma) - Fernandes et al, (2003),
Rizzotto (1999), Litherland et al., (1986), respectively.
After the break-up of Rodinia (ca. 900-700 Ma), the Amazon craton became part of the Gondwana
supercontinent (Figure 1; Almeida et al., 2001), the westernmost portion of which resulted from a collage
of various, previously-dispersed crustal fragments, such as West Africa (northeast), the Rio de La Plata
(southern) and Paranapanema blocks (southwest); the São Francisco-West Congo craton (east), and the
Kalahari craton (southeast). The collision belts between these various blocks developed from ca. 800 Ma
to 503 Ma (Geraldes et al., 2003).
The Tucavaca Neoproterozoic belt localized in the Bolivia marks the eastern boundary of the craton and
presents a broad synclinal structure. According to K-Ar data , deformation of this belt is Cambrian in age
(Litherland et al., 1986). The Alto Paraguay fold belt partly forms the eastern boundary of the Guaporé
block. The Brasiliano metassediments of this belt grade westward into a subhorizontal cover sequence
that forms a narrow ribbon overlapping the eastern margin of the Amazon craton. Rb/Sr data collected
from the rocks of the Alto Paraguay Group provide ages of 569 ± 20 Ma in shales of the Sepotuba
Formation, 484 ± 19 Ma on a schist of Cuiaba Group, and 500 ± 4 Ma and 504 ± 12 Ma (K-Ar on biotites)
on the São Vicente granite (Trompette, 1994).
Recent mapping in SW Amazonia and newly geochronological data (e.g., Tassinari et al., 2000; Geraldes
et al., 2001) provide the basis for better understanding of geologic evolution and tectonic settings. U-Pb
studies in SW region of Mato Grosso state, Rondonia-Sunsás belt, defined four major events: (i) The Alto
Jauru orogen (1.79 to 1.74 Ga); (ii) The Cachoeirinha orogen (1.58 to 1.54 Ga); (iii) The Santa Helena
orogen (1.45 Ma to 1.42 Ga); and (iv) Rio Alegre orogen (1.51-1.49 Ga).
Here we report U-Pb and 40Ar-39Ar radiometric studies carried out on pegmatitic and granitoids rocks near
Pontes e Lacerda city (Mato Grosso state). The results have important implications for the tectonic
evolution of the SW Amazon craton, taking into account the surrounding Neoproterozoic framework.
The pegmatite studied is intrusive into sedimentary rocks of the Late-Mesoproterozoic Aguapeí Group.
Seven of nine analyzed zircon grains (Figure 2) yielded a U-Pb discordia plot with upper intercept at ca.
1260 Ma. The two remaining grains yielded upper intercepts at about 500-400 Ma, suggesting that the
sample contains a mixture of zircons grains (7) from the country rocks and two zircons (2) formed during
Cambrian-Ordovician times. From the results above presented we may assume that U-Pb age may be
interpreted as the age of the pegmatite.
24
“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
WAF
AMZ
AA
PP
RAP
SF-CNG
RDP
KHR
Figure 1. Western Gondwana according to Almeida et al. (2000). Cratonic areas: Amazônia (AMZ); West
África (WAF); Arequipa-Antofala (AA); Rio Apa (RAP); Rio de La Plata (RDP); Paranapanema (PP); São
Francisco-Congo (SF-CNG); Kalahari (KHR).
Pegmatite
97-114
(9 points)
Upper Interc epts
T1 = 1260 Ma
T2 = 500-400 Ma
0
1
2
3
Figure 2. Concordia diagram of the pegmatite sample (97-114).
Within the border of the Santa Helena and Rio Alegre orogen rocks the Sararé S-type granite occurs. The
intrusion emplaced at 917 Ma ago (U-Pb age). A sample of this granite collected in an outcrop located in
the Cuiabá-Porto Velho road close of the river Sararé yielded 40Ar-39Ar integrated biotite age of 483.3 
2.8 Ma, may be interpreted as cooling age of the biotite (Figure 3). 40Ar-39Ar ages in biotite and muscovite
has been reported by Ruiz (2003) and the results define ages in the range from 906 Ma and 903 Ma,
interpreted as cooling ages of the Sararé Granite.
From the above, the roughly contemporary, youngest U-Pb and 40Ar-39Ar ages suggest that thermal
overprinting and igneous activity took place during the Cambrian-Ordovician period. We speculate that
such episodes may have a genetic relationship with the Neoproterozoic mobile belts that surround the
SW border of the Amazon craton. The results here presented comprise the first U-Pb and Ar-Ar postProterozoic ages reported within Amazon Craton and are of potential importance for mineralization in
pegmatites.
25
Age (Ma)
“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
box heights are 2
600
590
580
570
560
550
540
530
520
510
500
490
480
470
460
450
440
430
420
410
400
Biotite
Sararé Granite
(97-101)
Integrated age = 483.3 ± 2.8 Ma
0,0
0,2
0,4
0,6
0,8
1,0
Cumulative 39Ar Fraction
Figure 3. 40Ar-39Ar ages diagram of the Sararé Granite (sample 97-101).
References
Almeida, F.F.M.; Brito Neves, B.B.; Carneiro, C.D.R., 2000. The origin and evolution of the South
American Platform. Earth-Science Reviews 50: 77–111
Dalziel,I.W.D., 1992. On the organization of American plates in the Neoproterozoic and the breakoup of
Laurentia. GSA Today 2,1-2.
Fernandes, C.J.; Geraldes, M.C.; Tassinari, C.C.G.; Kuyumjian, R.M. 2003b. Idades 40Ar/39Ar para a
Faixa Móvel Aguapeí, sudoeste do estado de Mato Grosso, fronteira Brasil-Bolívia. In: SBG/
Núcleo Centro Oeste, 9o Simpósio de Geologia do Centro Oeste, Anais.
Geraldes, M.C.; Van Schmus, W.R.; Condie, K.C.; Bell, S.; Teixeira, W e Babinski, M., 2001. Proterozoic
geologic evolution of the SW part of the Amazon Craton in Mato Grosso state, Brazil.
Precambrian Research 111, 91-128p.
Geraldes, M.C.; Castro, N.; Paulo, V.G e Teixeira, W., 2003. A aplicação de imagens LANDSAT como
suporte a estudos tectônicos e geocronológicos dos terrenos Paleo e Mesoproterozóicos do SW
do craton Amazônico. SNET. Búzios, Rio de Janeiro.
Hoffman, P.F., 1991. Did the breakout of Laurentia turn Gondwana inside out? Science, 252, 1409-1412.
Litherland M., Annells, R.N. Applenton, J.D., Berrangé, J.P., Boomfield, K., Burton, C.C.J., Darbyshire,
D.P.F., Fletcher, C.J.N., Hawkins, M.P., Klinck, B.A., Llanos, A., Mitchell, W.I., O’Connor, E.A.,
Pitfield, P.E.J., Power, G. and Webb, B.G., 1986. The geology and mineral resources of the
Bolivian Precambrian shield. Overseas Mem. Br. Geol. Surv., n9.
Ruiz, L.M.B.A., 2003. Caracterização petrológica, geoquímica e geocronológica (U/Pb e Ar/Ar) do Maciço
Sararé – Nova Lacerda – MT. Dissertação de Mestrado. UNESP. Rio Claro-SP.89 p.
Rizzotto, G.J., 1999. Petrologia e Ambiente Tectônico do Grupo Nova Brasilândia – RO. Universidade
Federal do Rio Grande do Sul, Porto Alegre- RS, Brazil, 136p.
Tassinari, C.C.G.; Bettencourt, J.S.; Geraldes, M.C.; Macambira, M.J.B. e Lafon, J.M., 2000. The
Amazon Craton. Tectonic Evolution of Southamerica. p.41-95. Rio de Janeiro.
Trompette, R., 1994. Geology of Wertern Gondwana (2000-500Ma) Pan-African-Brasiliano Aggregation
of South America and Africa. University of Marseille, France. p.350.
26
“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Lithostratigraphy, biostratigraphy and correlations of
Neoproterozoic to early Paleozoic sedimentary basins on the
Kalahari Craton and its margins (Southern Africa)
Gerard J.B. Germs* & Claudio Gaucher**
* Department of Geology, Rand Afrikaans University, Auckland Park 2006, Johannesburg, South Africa,
gagerms@global.co.za
** Departamento de Geología, Facultad de Ciencias, Iguá 4225, 11400, Montevideo, Uruguay,
gaucher@chasque.apc.org
The Neoproterozoic to early Paleozoic was an eventful time-period in the geological history of the Earth.
During this period major glaciations took place, the supercontinent Rodinia broke up, the Gondwana
supercontinent assembled and the first true animals originated. All these events are related to each other.
The main aims of IGCP Project 478 are to determine which events took place during the Neoproterozoic
to early Paleozoic in SW-Gondwana, their number (e.g. glaciations) and timing. These aims can be
achieved by dating and correlating the rocks which formed as a result of these Neoproterozoic to early
Paleozoic events. The dating can be accomplished by radiometric, chemo- and biostratigraphical tools.
We have undertaken the task to date biostratigraphically (predominantly by studying organic-walled
microfossils) the sedimentary successions, which are related to the Kalahari Craton, using radiometric
and chemostratigraphical data for calibration. We present here new data and discuss previous reports
(Germs, 1983; Le Roux and Gresse, 1983; Gresse, 1986; Germs and Gresse, 1991; Hegenberger, 1993;
Frimmel and Frank, 1998; Saylor et al., 1998; Le Roux, 2000; Frimmel et al., 2002; Belcher and Kisters,
2003; Gaucher and Germs, 2003) aiming at correlation of the Kalahari sedimentary basins.
27
“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
The Neoproterozoic to early Paleozoic sedimentary basins, which are related to the Kalahari Craton,
occur as localised remnants of larger basins. These remnants are called basins but do not represent true
sedimentary basins. They form part of various orogenic belts occurring in southern Africa (Fig. 1). The
basins are related to the inland branch of the Damara Orogen (called the Khomas Belt), to the Gariep
(and equivalent Vanrhynsdorp) Belt and the western and southern branches of the Saldania and Zambezi
Belts. The “Kalahari” basins formed as the result of the break-up of the Rodinia Supercontinent and the
subsequent collisions of the Congo, Kalahari, Rio de la Plata, Antarctica (?) and Australia (?) plates.
These and other collisions formed the supercontinent Gondwana. The Nosop-Witvlei-northern Nama
basins occur in the inland branch of the Damara Belt which is called the Khomas Belt. The Gariep–
southern Nama basin and Gariep (Gifberg)–Vanrhynsdorp basin are related to the Gariep Belt. The
Gariep Belt in the Vanrhynsdorp area is also called the Vanrhynsdorp Belt. The Malmesbury basin is
related to the western and southern branch of the Saldania Belt. The Cango Caves, Gamtoos and
probably also the Natal –Port Shepstone basins are related to the southern branch of the Saldania Belt.
The Neoproterozoic to early Paleozoic sediments of the Tengwe River and Sjirarira Groups occur in
Zimbabwe and are probably related to the northern Zambezi Belt.
Table 1: Neoproterozoic-Cambrian fossiliferous sedimentary units of the Gariep and Saldania belts.
Sources of data: (1) Germs (1995); (2) Gresse (1986); (3) Germs et al. (1986), Germs (1995) and
references therein; (4) Gaucher & Germs (2003); (5) This work; (6) Gaucher, Frimmel & Germs
(submitted)
Age
Basin
Nama (Fish River)
Fossils of biostratigraphic significance
Treptichnus pedum(1)
Cambrian
Vanrhynsdorp Group
(Brandkop
and Knersvlakte)
Nama (Schwarzrand
and Kuibis)
Cango Caves Group
Ediacaran
Gamtoos Group
Port Nolloth Group
(Holgat-Numees)
Treptichnus pedum, Oldhamia(2)
Cloudina, Namacalathus, Ediacaran fossils, Leiosphaeridia,
Bavlinella, Chuaria, Vendotaenia(3)
Bavlinella, Soldadophycus, Leiosphaeridia, Micrhystridium,
Chuaria(4)
Bavlinella, Soldadophycus, Leiosphaeridia(5)
Soldadophycus, Leiosphaeridia(6)
Previous data (including radiometric data) and our biostratigraphical studies (Table 1) enable us to
subdivide the Neoproterozoic to early Paleozoic “Kalahari” sediments into intervals of long duration. The
sediments are predominantly of Cryogenian (850 – 630 Ma), Ediacaran (630 – 542 Ma) and early
Paleozoic (younger than 542 Ma) age. Sediments of the Tonian period (1000 – 850 Ma) appear to be
generally absent. A database composed of 105 samples analyzed for organic-walled microfossils (mainly
acritarchs) enabled the recognition of Ediacaran deposits in the Gariep and Saldania belts. They are
characterized by depauperate acritarch assemblages containing the genera Leiosphaeridia, Bavlinella
and Soldadophycus as the most prominent components of the microbiota (Table 1). They can be easily
distinguished from Early Paleozoic deposits on the basis of their acritarch assemblages. So far, no fossils
have been reported from older, Cryogenian units.
With our present knowledge it is possible to correlate stratigraphic units of the various “Kalahari” basins.
The Blaubeker (Witvlei Group), Kaigas (Gariep Supergroup) and Karoetjes Kop (Gifberg Group)
diamictites are most probably correlates and are of Sturtian (Cryogenian, ca. 720 Ma) age. The Numees
(Gariep Supergroup) and Kobe (Gifberg Group) diamictites are probably correlates of the Gaskiers (ca
580 Ma) glaciation. The diamictite at the base of the Wallekraal Formation (Gariep Supergroup) may be
related to the (ca 630 Ma) Marinoan glaciation. The Swartland and Boland Groups of the Malmesbury
Supergroup of the western branch of the Saldania Belt are Cryogenian and Ediacaran in age respectively,
whereas the Cango Caves and Gamtoos Groups of the southern branch of the Saldania Belt are
Ediacaran in age. The proposed correlations display how incomplete the Neoproterozoic to early
Paleozoic stratigraphic record in southern Africa is, indicating how difficcult chemo-stratigraphic
correlations can be.
28
“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Research in course aims both at the biostratigraphic subdivision of the Neoproterozoic (especially the
Ediacaran Period) and also at studying Cryogenian sedimentary units using micro-palaeontologic tools.
References
Belcher, R.W. and Kisters, A.F.M., 2003. Lithostratigraphic correlations in the western branch of the PanAfrican Saldania belt, South Africa: the Malmesbury Group revisited. South African Journal of
Geology, 106: 327-342.
Frimmel, H.E. and Frank, W., 1998. Neoproterozoic tectono-thermal evolution of the Gariep Belt and its
basement, Namibia/South Africa. Precambrian Research, 90: 1-28.
Frimmel, H.E., Fölling, P.G. and Eriksson, P.G., 2002. Neoproterozoic tectonic and climatic evolution
recorded in the Gariep Belt, Namibia and South Africa. Basin Research, 14: 1-18.
Gaucher, C. & Germs, G.J.B. 2003 Preliminary biostratigraphic correlation of the Arroyo del Soldado
Group (Vendian to Cambrian, Uruguay) with the Cango Caves and Nama groups (South Africa
and Namibia). Revista Soc. Urug. Geol. 3, Publ. Especial, 1: 141-160.
Germs, G.J.B., 1983. Implications of a sedimentary facies and depositional envirornmental analysis of
the Nama Group in Southwest Africa/Namibia. In: R. McG. Miller (Editor), Evolution of the
Damara Orogen. Geological Society of South Africa. Special Publication, 11, pp. 89-114.
Germs, G.J.B., 1995 The Neoproterozoic of south-western Africa, with emphasis on platform stratigraphy
and paleontology. Precambrian Research, 73: 137-151.
Germs, G.J.B. and Gresse, P.G., 1991. The foreland basin of the Damara and Gariep Orogens in
Namaqualand and southern Namibia: stratigraphic correlations and basin dinamics. South
African Journal of Geology, 94:159-169.
Gresse, P.G., 1986. The tectonosedimentary history of the Vanrhynsdorp Group. Unpublished Ph.D.
thesis, University of Stellenbosch, South Africa, 183 pp.
Hegenberger, W., 1993. Stratigraphy and sedimentology of the latest Precambrian Witvlei and Nama
Groups, East of Windhoek. Geological Survey of Namibia. Memoir 17, 82pp.
Le Roux, F.G., 2000. The geology of the Port Elizabeth-Uitenhage area, Explanation of Sheets 3325 DC
& DD, 3425 BA Port Elizabeth, 3325 CD & 3425 AB Uitenhage, 3325 CB Uitenhage Noord and
3325 DA Addo. 55pp.
Le Roux, J.P. and Gresse, P.G., 1983. The sedimentary-tectonic realm of the Kango Group. In: A.P.G.
Söhnge and I.W. Hälbich (Editors), Geodynamics of the Cape Fold Belt. Special Publication
Geological Society of South Africa, 12: 149-164.
Saylor, B.Z., Kaufman, A.J. and Grotzinger, J.P., 1998. The partitioning of Neoproterozoic time:
constraints from Namibia. Journal of Sedimentary Research, 68: 1223-1235.
29
“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Chemostratigraphy and diagenetic constraints on
Neoproterozoic carbonate successions from the Sierras
Bayas Group, Tandilia System, Argentina
Lucía E. Gómez Peral*, **, Daniel G. Poiré*, Udo Zimmermann**, Harald Strauss***
*Centro de Investigaciones Geológicas, CONICET-UNLP, 1 Nº 644, 1900 La Plata, Argentina.
lperal@cig.museo.unlp.edu.ar, poire@cig.museo.unlp.edu.ar
**Department of Geology, RAU University, Auckland Park 2006, Johannesburg, South Africa
***Geologisch-Paläontologisches Institut, Westfälische-Wilhelms-Universität Münster, Corrensstrasse 24,
48149 Münster, Germany
Carbon-oxygen isotopic data, combined with a detailed diagenetic study, obtained from undeformed,
unmetamorphosed dolomites and limestones from the Sierras Bayas Group (Neoproterozoic), Argentina,
provide a new record of isotopic stratigraphic variation.
The Sierras Bayas Group is composed of the Villa Mónica (sandstones and dolomites), Cerro Largo
(siliciclastic rocks) and Loma Negra (limestones) Formations, unconformably followed of the Cerro Negro
Formation (siliciclastic rocks with basal marls), and grouped into four depositional sequences bounded by
unconformities.
The Villa Mónica Formation can be placed between 800 and 900 Ma, based on stromatolites. The
geochronological data from the Sierras Bayas Group are sparse and sometimes non-consistent with the
geological framework. In this sense, a Rb/Sr age from interbedded fine-grained sedimentary rocks of 793
±32 Ma was reported by Cingolani and Bonhomme (1988) for the Villa Mónica Formation.
The Cerro Largo Formation age constraints are poor, a whole rock Rb/Sr age of 725 ±36 Ma was
presented by Kawashita et al., 1999a. Bonhomme and Cingolani (1980) reported a Rb/Sr age of 769 ±12
Ma on the same unit.
A Rb/Sr age of 723 ±21 Ma was performed on fine fractions from Cerro Negro Formation by Cingolani and
Bonhomme (1982). Later on, a K/Ar whole rock analysis reported by Cingolani et al. (1991) point to a
minimum depositional age of 680 Ma.
In contrast with geochronological data, micropaleontological studies of the Cerro Negro Formation,
displayed an association of acritarchs which were assigned to a Vendian age (Cingolani et al., 1991).
Diagenesis
The diagenetic trend for Villa Mónica Formation begins with the development of a dolomitic mosaic,
where crystals recrystallised uniformly or in two recrystallization phases from a micritic calcite base. This
mosaic has probably grown from primitive low-Mg-calcite (protonuclei) placing them in the field of
secondary dolomites. This stage corresponds to an earlier dolomitization process (limestone replacement
by dolomite). A later dolomitization process is associated with burial diagenesis conditions.
Later on, three stages of cementation could be distinguished: Stage 1: dolosparitic cement (100 to 750
m); Stage 2: quartz cement, (500 m to 7 cm); and Stage 3: high-Mg-calcite (HMC) cement (~ 650m).
All these cements are filling voids and fractures up to 15 cm in width respectively diameter and their origin
is related to the interaction of rock and meteoric fluids at low temperatures.
In the micritic limestones facies of the Loma Negra Formation the following diagenetic processes are
interpreted:
Stage 1: Recrytallization of the calcitic base to non-planar micrite, microsparite or sparite preserving the
existing microstructure.
Stage 2: Chemical dissolution generates porosity in form of veins and voids.
Stage 3: Precipitation of calcite cements filling porosity generated in Stage 2.
Stage 4: Pressure–solution produces irregular stylolites and their mineralogical components.
30
“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Stage 5: Silicification caused by precipitation of silica gels along veins and as partial replacement of
carbonate crystals.
Geochemistry and stable isotopes
Abundance of total, carbonate and organic carbon identify most samples from the Villa Mónica Formation
as carbonates, containing on average 0.8 % organic carbon. Lower values for TIC in the uppermost
samples reflect a higher content of siliciclastic material. 13C values vary between -0.65 and +2.20 ‰.
18O values range from -6.67 to -2.11 ‰.
Dolostones from the Villa Mónica must be regarded as diagenetically altered, as discernible from very low
Sr concentrations and strongly elevated Mn and Fe concentrations. In particular, the top part of this unit,
comprising a higher proportion of siliciclastic material as evident from low TC values, shows the highest
Mn and Fe concentrations. Microprobe analyses show replacement of rhombohedral dolomite nuclei by
Fe-oxides. Samples with extremely high Fe concentrations are iron-rich dolostones. All features point to a
rather pervasive alteration of the Villa Mónica dolostone with an iron-rich fluid.
Interestingly, the oxygen isotopic composition for all Villa Mónica samples is more positive than -7 ‰. In
fact, towards the top, the 18O values rise to -2 ‰. However, these samples contain a higher proportion of
siliciclastic material. The higher 18O values could reflect partial equilibrium with 18O enriched siliciclastics.
Trace element data are distributed rather uniformly across the entire Loma Negra Formation with an
average Sr concentration of 360 ppm, an average Mn concentration of 390 ppm and an average Fe
concentration of 5900 ppm. Most Mn/Sr values are <1.4, suggesting a low degree of diagenetic alteration.
Interestingly, the oxygen isotopic composition for the Loma Negra Formation displays a clear stratigraphic
variation tracing the differences in petrography. 18O values between -13.5 to -11.2 ‰ characterise the
reddish micritic limestones. The black micritic limestones in display substantially more positive values,
ranging from -7.9 to -7.1 ‰ in the middle section of the formation. Towards the top of the succession, the
18O values become as negative as -14.1 ‰. It is unclear whether this unusual distribution in 18O is
related to the late diagenetic silicification (diagenetic Stage 5) or whether this could even reflect some
preserved near-primary feature.
Based largely on the even stratigraphic distribution of trace element compositions throughout the Loma
18O data and
any trace element composition or ratio, we regard the carbon isotope data for the Loma Negra Formation
as near-primary seawater signal.
13C values are all positive, ranging from +2.8 to +4.5 ‰. Such data are consistent with carbon isotope
values for other Neoproterozoic carbonate successions.
Finally, the difference between the carbonate and organic carbon isotopic compositions () recorded for
the Loma Negra Formation varies between 30.5 and 32.3 ‰, with no discernible stratigraphic variation.
The latter suggests that the organic matter has not been altered post-depositionally to any great extent.
This range in  indicates photosynthetic carbon fixation. Neither enhanced thermal maturation nor a
substantial contribution of 13C depleted bacterial biomass is indicated.
Chemostratigraphy
Positive carbonate carbon isotope values around +3.5 ‰ and strontium isotope data between 0.7068 and
0.7075 published by Kawashita et al. (1999a) support a Vendian age for the Loma Negra Formation
(Jacobsen and Kaufman 1999; Melezhik et al., 2001). More specifically, an age of ~580-590 Ma appear
plausible. This is in contradiction to previously advocated views based on Rb/Sr and K/Ar data
(summarised by Kawashita et al., 1999a).
Similar isotope data reported here were described for limestones of the Polanco Formation (Arroyo del
Soldado Group) in Uruguay (Gaucher et al., 2003; 2004) and Corumbá Group in Brazil (Boggiani, 1998;
Boggiani et al., 2003). The age of the Polanco Formation has been determined as Vendian (555-580 Ma)
on the basis of acritarch biostratigraphy, Cloudina and C and Sr chemostratigraphy (Gaucher et al.,
2004). The age of the Corumbá Group was determined to be between 600 and 535 Ma based on Sr
chemostratigraphy and biostratigraphy (Boggiani et al., 2003). Furthermore, the regional geological
context shows similarities as well with glacial deposits and cap carbonates being absent from the Sierra
Bayas Group.
31
“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
This is a contribution to the IGCP 478 “Neoproterozoic-Early Paleozoic events in SW-Gondwana”.
References
Boggiani, P.C., 1998. Análise estratigráfica da Bacia Corumbá (Neoproterozoico) – Mato Grosso do Sul.Unpublished Ph. D. Thesis, Universidades de Sao Paulo, Brazil, 181 pp.
Boggiani, P.C., Sial, A.N., Babinski, M., Ferreira, V.P., 2003. New carbon isotopic data from the Corumbá
Group as a contribution to a composite section for the Neoproterozoic III in South America. III
International Coloquium Vendian-Cambrian of W-Gondwana, Ed H.E. Frimmel. Ext Abst. Cape
Down, South Africa, pp. 13 –16.
Bonhomme, M.G., Cingolani, C., 1980. Mineralogía y geocronología Rb-Sr y K-Ar de fracciones finas de la
“Formación La Tinta”, provincia de Buenos Aires. Revista Asociación Geológica Argentina 35 (4),
519-538.
Cingolani, C., Bonhomme, M.G., 1982. Geocronology of La Tinta Upper Proterozoic Sedimentary Rocks,
Argentine. Precambrian Research 18, 119-132.
Cingolani, C., Bonhomme, M.G., 1988. Resultados geocronológicos en niveles pelíticos intercalados en las
dolomías de Sierras Bayas (Grupo La Tinta), provincia de Buenos Aires. Segundas Jornadas
Geológicas Bonaerences. Buenos Aires, Argentina, pp. 283-289.
Cingolani, C., Rauscher, R., Bonhomme, M.G., 1991. Grupo La Tinta (Precámbrico y Paleozoico inferior),
Provincia de Buenos Aires, República Argentina: Nuevos datos geocronológicos y
micropaleontológicos en las sedimentitas de Villa Cacique, Partido de Juárez. Revista YPF B.
Gaucher, C., Boggiani, P.C., Sprechmann, P., Sial, N.A., Fairchild, T., 2003. Integrated correlation of the
Vendian to Cambrian Arroyo del Soldado and Corumbá Groups (Uruguay and Brazil):
palaeogeographic, palaeclimatic and palaeobiologic implications. Precambrian Research 120,
241-278.
Gaucher, C., Sial, A.N., Blanco, G., Sprechmann, P., 2004. Chemostratigraphy of the lower Arroyo del
Soldado Group (Vendian, Uruguay) and paleoclimatic implications. Gondwana Res. 7 (3): 715730.
Jacobsen, S.B., Kaufman, A.J., 1999. The Sr, C and O isotopic evolution of Neoproterozoic seawater.
Chemical Geology 161, 37-57.
Kawashita, K., Varela, R., Cingolani, C., Soliani, Jr. E., Linares, E., Valencio, S.A., Ramos, A.V., Do
Campo, M., 1999a. Geochronology and Chemostratigraphy of “La Tinta” Neoproterozoic
Sedimentary rocks, Buenos Aires Province, Argentina. II South American Symposium on isotope
Geology. Brazil, pp 403-407.
Melezhik, V.A.; Gorokhov, I.M.; Kuznetsov, A.B., Fallick, A.E., 2001. Chemostratigraphy of
Neoproterozoic carbonates: implications for “blind dating”. Terra Nova 13, 1-11.
Poiré, D.G., 1993. Estratigrafía del Precámbrico sedimentario de Olavaría, Sierras Bayas, Provincia de
Buenos Aires, Argentina. XII Congreso Geológico Argentino y II Congreso de Exploración de
Hidrocarburos Act. II, 1-11.
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Unravelling chemostratigraphic signatures of sedimentation
and diagenesis in Paleoproterozoic iron and manganese
formations
Jens Gutzmer, Eva Scheiderhan, Nicolas J. Beukes
Paleoproterozoic Mineralization Research Group, Department of Geology, Rand Afrikaans University,
P.O. Box 524, 2006 Auckland Park, South Africa, jg@rau.ac.za
The origin of banded iron formations (BIF) remains one of the most enigmatic problems in Precambrian
geology. Various authors have controversially discussed the origin of BIF’s (among others, Beukes &
Klein, 1992; Canfield, 1998). The existence of manganese-rich beds, which occur occasionally
intercalated with BIF as so called manganese formations (MnF), is commonly neglected. However,
understanding the reasons for the formation of alternating iron- and manganese-rich precipitates may
yield important clues for constraining the origin of Precambrian BIF, and may furthermore contribute to
our understanding of the evolution of System Earth.
The Voëlwater Subgroup of the Paleoproterozoic Transvaal Supergroup in Griqualand West, South Africa
is one of the best preserved examples of intercalated BIF and MnF, closely associated with shallow
marine carbonate rocks. Excellent exposures of the Voëlwater Subgroup are available through miningrelated outcrops and numerous diamond drill cores in the Kalahari Manganese-Field. This study provides
the first comprehensive geochemical data set for the different sedimentary lithologies of the Voëlwater
Subgroup.
The 2.22Ga old Voëlwater Subgroup is constituted by a conformable succession of three major
lithological units. The Ongeluk Formation is the lowermost of these units; it is composed of basaltic
andesites that extruded onto the submerged western portion of the Kaapvaal Craton (Cornell et al.,
1996). An increasing abundance of hyaloclastites, jasper beds and chert mark the transition to the
overlying Hotazel Formation, which consists of three conspicuous symmetrical cycles with highly
manganiferous braunite lutite (MnF) at their centers, surrounded by BIF. The transition between BIF and
MnF is gradational, and marked by carbonate-rich jacobsite lutite and hematite lutite. The Hotazel
Formation is in gradational contact with dolostones and less common limestones of the Mooidraai
Formation that marks the top of the Voëlwater Subgroup.
For this study, samples were taken from a diamond drill core that intersected the entire Voëlwater
Subgroup in the southern part of the Kalahari Manganese-Field. Detailed petrographic studies of this
sample set did not yield any evidence for the presence of siliciclasticic detritus, and mineral assemblages
developed point to a diagenetic to very low metamorphic overprint of the succession.
The most abundant chemical components in the Hotazel and Mooidraai Formation are CO 2, MnO, Fe2O3,
CaO, MgO and SiO2. Their concentrations change systematically between the different lithological units.
Furthermore, total inorganic carbon and CaO increase steadily towards the top on the expense of Fe 2O3
and SiO2. Major and trace elements indicating detrital input (Al2O3, TiO2, Zr, Rb) and trace elements in
general are negligible, with the exception of Ba and Sr, which are enriched in carbonate-rich MnF.
Analyses of 13CPDB and 18OPDB (total carbonate) yield signatures typical for Fe-bearing carbonates in
Precambrian BIF and diagenetic Mn-carbonates. 13CPDB signatures of Fe- and Mn-rich carbonates of the
Hotazel Formation define a first order trend to more positive values towards the top of the stratigraphy.
This overall trend is modulated by obvious second order fluctuations. No such first-order trend is
displayed by 18OPDB values, but with respect to second order variations d 18OPDB co-varies negatively
with 13CPDB. Furthermore, 18OPDB signatures vary between two distinct end-member values, -15‰ and 7‰. The carbonates of the Mooidraai Formation have a normal marine 13CPDB signature; 13CPDB and
18OPDB do not show any apparent co-variation or first order trends. Trace amounts of organic carbon
were detected in only few samples from the Hotazel and Mooidraai Formations. This organic matter is
isotopically light (13Corg ranges between -41.1‰ and -33.4‰), typical for well-preserved organic matter in
the Paleoproterozoic Era. In the Hotazel Formation 13Corg values show an evolution up-section to more
positive values. In contrast, the Mooidraai Formation presents values that remain constantly around 36‰.
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
values show similar fluctuations as observed for 18OPDB. Analyses of 87Sr/86Sr permit the
differentiation of all samples into two groups. The first group displays a non-radiogenic signature of 0.705,
and contains hematite lutite, braunite lutite and samples from the carbonate-rich uppermost IF unit. The
second group shows a radiogenic signature with values around 0.715, and consists of jaspilites
intercalated with volcanic rocks of the Ongeluk Formation, carbonate-poor silicate-facies IF, thickly
laminated oxide-carbonate, microlaminated oxide-facies IF in the lower part of the Hotazel Formation, and
dolostones of the Mooidraai Formation.
87Sr/86Sr
The results suggest that the alternation of BIF and MnF in the Hotazel Formation modulates a first-order
transition from chert-rich to carbonate-rich lithologies. This is reflected in systematic major element
geochemical changes as well as the gradual transition into shallow marine carbonate rocks of the
Mooidraai Formation. Both of these trends are a reflection of sedimentary processes and may be best
explained by episodic upwelling of hydrothermally-dominated Fe-Mn-rich deep marine water into a
depositional basin that evolved through time from an outer, shelf environment to a shallow marine
carbonate platform.
The transition from negligible sedimentary or early diagenetic carbonate formation at the base of the
Hotazel Formation to abundant carbonate deposition in the Mooidraai Formation is of particular
importance. Every sample is affected to some degree by diagenetic or epigenetic fluid flow that resulted
in recrystallization and/ or neoformation of carbonates. Postdepositional fluid flow also affected the
isotopic composition of carbonates; the effect of the diagenetic overprint is especially profound on
carbonate-poor lithologies (jasper, chert-rich BIF), as indicated by elevated 87Sr/86Sr ratios. The effect of
late diagenesis on the carbon isotopic composition of whole rock carbonates is similarly profound.
Unusually light δ13CPDB values are linked to the most carbonate-poor samples. This effect of late
diagenetic carbonate formation must be distinguished from early diagenetic formation of Mn-rich
carbonates. In both cases, decreasing δ13CPDB signatures are thought to reflect an increasing proportion
of isotopically light CO2 derived from oxidized organic matter.
References
Beukes, N.J. & Klein, C. (1992). Models for Iron-Formation Deposition. In: Schopf, J.W. & Klein, C., eds.,
The Proterozoic Biosphere. Cambridge University Press, 147-151.
Canfield, D.E. (1998). A new model for Proterozoic ocean chemistry. Nature, 396, 450-453.
Cornell, D.H., Schütte, S.S., Eglington, B.L. (1996). The Ongeluk basaltic andesite formation in
Griqualand West, South Africa: submarine alteration in a 2222 Ma Proterozoic sea. Precambrian
Research, 79, 101-123.
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
The Cajamar basin (SP-Brazil) and the microfossil
Titanotheca coimbrae of the Ediacaran period
Jorge Hachiro*; Antonio L. Teixeira**; Claudio Gaucher***; Peter Sprechmann***
* Instituto de Geociências, Universidade de São Paulo (USP), Rua do Lago, 562, São Paulo-SP, 05508080, Brazil, jhachiro@usp.br
**Instituto Geológico, Secretaria do Meio Ambiente do Estado de São Paulo (IG-SMA), Av. Miguel
Stefano, 3900, São Paulo, SP, 04301-903, Brazil
*** Departamento de Geología, Facultad de Ciencias, Iguá 4225, 11400, Montevideo, Uruguay
The Cajamar basin is situated at about 25 km (NE) of the municipal district of São Paulo. It is an
individualized unit, overlying the São Roque Group rocks (Neoproterozoic), that occupied a
graben which subsidence occurred partly limited for failed blocks. It was filled out by terrigenous
and carbonate sediments with ± 300m of thickness presenting evidences of a metamorphism
that attained the very low grade. Considered as relict remainder of a widespread basin, now
their deposits occupy only a small area no longer than 1km2. It was recognized through drilling
cores, once a thicken alteration mantle and the covering of Cenozoic sediments don't allow the
blooming of these rocks.
Among the terrigenous rocks of the Cajamar basin it was found remains of a primitive
foraminifer, Titanotheca coimbrae Gaucher & Sprechmann (1999), with agglutinated test
ornamented by microscopic and surprising rutile needles. The rutile crystals were selected and
attached on the fine and flexible walls of the tests to mold a sophisticated “armour”.
Gaucher & Sprechmann (1999) were the pioneers who identified this protozoan in the Late
Ediacaran sediments of the Arroyo del Soldado Group (Uruguay). They believed that this
organism had a defensive purpose to build resistant tests. The hardness and the insolubility
front to the acid attack choose the rutile as the ideal material against predators, mainly the shellboring predators which existed since remote times. Based in the vase form and robustness of
the walls of the Titanotheca’s tests the authors inferred a epibenthic habit of life for these
organisms, probably on the marine substratum of continental platforms in waters of shallow to
medium depth, similar to the suitable environment for the terrigenous and carbonate sediments
of the Cajamar basin.
Reference
Gaucher, C. & Sprechmann, P. 1999. Upper Vendian skeletal fauna of the Arroyo del Soldado
Group, Uruguay. Beringeria 23:55-91.
35
“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
TECTONIC EPISODES RELATED TO WEST GONDWANA
AMALGAMATION IN THE RIBEIRA OROGEN (SE BRAZIL)
Monica Heilbron; Claudio Valeriano; Miguel Tupinambá; Júlio Almeida; Beatriz Duarte;
Claudia Valladares; Renata Schmitt; Mauro Geraldes; Célia D. Ragatky; Nely Palermo;
Ambrosina Gontijo
TEKTOS Research Group, Universidade do Estado do Rio de Janeiro (UERJ), Rua São Francisco Xavier
524, Rio de Janeiro, RJ, 20550-013, Brazil, heilbron@uerj.br
Tectonic setting snd objectives
The Ribeira belt occupies a central position in West Gondwana and is one of the key units for
reconstructing the history of this supercontinent. Recent geological data reveal that, from the Criogenian
to the Cambrian/Ordovician transition, a complex history of orogenic construction involved the accretion
of a cordilleran arc and the collision of at least two terranes onto the eastern margin of the São Francisco
plate, while basin formation was taking place elsewhere in West Gondwana
The Ribeira belt extends for 1400 km along the Atlantic coast of Brazil (Almeida, et al., 2000). Together
with its northern extension, the Araçuaí belt, it forms an orogenic belt developed at the eastern and
southeastern borders of the São Francisco Craton. It formed in response to the convergence between the
São Francisco plate and another plate or microplate located to the east (Campos Neto & Figueiredo,
1995; Trouw et al., 2000; Heilbron et al., 2003, 2000, 2003, 2004, Brito Neves et al., 1999;).
This contribution presents a synyhesis of the tectonic evolution of the belt based on the combination of
detailed geological and U-Pb isotopic data (Heilbron et al. 1995, 2000, Machado et al. 1996, Schmitt
2000, and Tupinambá et al. 2000 Schmitt et al., 2004).
Tectonic organization of the belt
The present crustal structure of the central segment of the Ribeira Belt is defined by four different
tectonostratigraphic Terranes thrusted to NW onto the eastern margin of the São Francisco craton. From
NW to SE, these are: a) the Occidental Terrane, interpreted as the reworked margin of the São Francisco
Continent; b) The Paraíba do Sul Klippe; (c) the Oriental (Costeiro, Serra do Mar, Vitória) Terrane, which
probably includes another cratonic block or microplate (Campos Neto and Figueiredo, 1995; Heilbron et
al., 1998; Ebert et al., 199-); and d) the Cabo Frio Terrane (Fonseca et al., 1984; Fonseca, 1993).
The Oriental terrane can be subdivided into three tectonic domains: the Cambuci domain, the Costeiro
domain and the Italva klippe (Figure 1). The Cambuci domain represents the basal thrust sheet of the
Oriental terrane at the Northern portion of the Rio de Janeiro State. The Costeiro domain overrides the
Cambuci domain and the Occidental terrane. The Italva klippe represents the uppermost thrust slice of
the Oriental terrane and overrides the Costeiro domain
Tectonic episodes
The central segment of the Ribeira belt may have evolved through the following stages (Figure 1):
1.000-790 Ma: passive margin
The Neoproterozoic passive margin at the São Francisco side is represented by the siliciclastic
Andrelândia Megasequence, wich also shows indication for glacial deposists. Tholeiitic amphibolites
evolve from WPB to E-Morb indicatind crustal thinning (Paciullo et al., 2000). Another passive margin
sequence is developed at the border of the eastern continent (Oriental terrane or Serra do Mar
microplate). It is chacterized by pelitc gneisses with plataformal marbles interleaved with amphibolies,
and minor calcsilicatic and quartzitic intercalations.
Although no ocean floor has been recognized in the central sector of the belt, the occurrence of MORBtype magmatism as the oldest magmatic event is compatible with the existence of an ocean between the
São Francisco margin and the Oriental Terrane at ca. 848 Ma. This age is similar to the 816±72 Ma Sm36
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Nd isochron age for a unit interpreted to represent ocean floor in the Araçuaí Belt (Pedrosa-Soares et al.,
1998). Thus, break-up of a former supercontinent, generally admitted to have occurred at ca. 900 Ma
based in part on the age of mafic dikes of the São Francisco craton, was followed by the generation of
ocean floor between the craton and a continental landmass to the east between ca. 848 Ma and ca. 790
Ma, the age of the oldest arc rocks.
790-590 Ma: subduction of São Francisco paleoplate
Subduction of oceanic lithosphere from the São Francisco paleoplate beneath the eastern continent
(Oriental terrane) led to the formation of one or more arc complexes starting at ca. 790 Ma. The main Rio
Negro arc complex was generated in the 640-620 Ma interval and possibly lasted until ca. 590 Ma, when
collision with the São Francisco continental margin initiated. New Nd and Sr isotopic data indicate juvenile
contribution.
590-550 Ma: collision 1 São Francisco X Oriental Terrane
Collision of the Oriental terrane onto the São Francisco margin led to the reactivation of the basement
associated with north-westward thrusting of foreland crustal blocks, and to the generation of granitoids
and of the main structures (the main foliation and large-scale D2 recumbent folds) in the Oriental terrane.
535-510 Ma: collision 1 São Francisco+ Oriental Terrane X Cabo Frio Terrane
Continued convergence led to the first Cambrian event represented in the central Ribeira belt: the
development of major dextral transcurrent faults and associated metamorphism in the foreland thrust
complexes at 540-520 Ma. Partially overlapping in time, the collision of the Cabo Frio terrane with the
already accreted Oriental terrane occurred during the Cambrian-Ordovician at 530-490 Ma (Schmitt et al.,
2004).
510-480 Ma: collapse of the orogen
Late-stage deformation comprises east-vergent folds associated to sub-horizontal extensional shear
zones, and two sets of transtensional sub-vertical shear zones. They are associated to the intrusion of
post-collisional granitoid rocks. These features are regarded as related to the collapse of the orogenic
building.
References
Almeida, F.F.M. et al. 2000. The origin and evolution of the South American Platform Earth-Science
Reviews 50, 77-111.
Brito-Neves, B.B., Campos-Neto, M.D., Fuck, R.A., 1999. From Rodinia to Western Gondwana: An
approach to the Brasiliano-Pan African Cycle and orogenic collage. Episodes 22(3): 155-166.
Heilbron, M. et al., 2000 From Collision to Extension: The Roots of the Southeastern Continental Margin
of Brazil. IN: Atlantic Rifts and Continental Margins, Talwani & Mohriak eds. , 354ps. Geophysical
Monograph Series, V 115, p:
Heilbron, M. & Machado, N. 2003 Timing of terrane accretion in the Neoproterozoic-Eopaleozoic Ribeira
orogen (SE Brazil). Precambrian Research 125:87-112.
Machado, N., Valladares, C.S.; Heilbron, M. & Valeriano, C.M., 1996. U-Pb Geochronology of Central
Ribeira belt, Prec. Res., 79, 347-361.
Pedrosa Soares, A.C.; Vidal, P.; Leonardos, O.H.; Brito-Neves, B.B., 1998. Neoproterozoic oceanic
remnants in eastern Brazil: further evidence and refutation of an exclusively ensialic evolution for
the Araçuaí-West Congo orogen. Geology 26(6):519-522
Schmitt, R.S. et al. 2004. Late amalgamation in the central part of West Gondwana:new geochronological
data and the characterization of a Cambrian collisional orogeny in the Ribeira belt (SE Brazil)
Precambrian research 133:26-61
Trouw, R.A. J. et al. 2000. The central segment of the Ribeira belt, in: U.G. Cordani, E.J. et al. (Eds),
Tectonic Evolution of South America, 854 p. 31st International Geological Congress, Rio de
Janeiro, 287-310.
Tupinambá, M. et al. 2000. Neoproterozoic Western Gondwana assembly and subduction-related
plutonism: the role of the Rio Negro Complex in the Ribeira Belt, Southeastern Brazil. Revista
Brasileira de Geociências 30, 7-11.
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Figure 1- Cartoon with envisaged tectonic evolution of the central Ribeira belt, modified from Heilbron et
al. (2000, 2003). Legend: 1- São Francisco plate, 2- São João del Rei rift sucessions, 3- Intraplate
Carandaí sucessions, 4- Andrelândia passive margin, 5- Oceanic crust, 6- Oriental paleoplate, 7Costeiro passive margin sucessions, 8- Rio Negro plutonic rocks, 9- Rio Negro volcanics, 10- Paraíba
nad Cambuci fore arc sucessions, 11- Búzios back arc sucessions, 12-Cabo Frio paleoplate, 13- syncollisional I granites, 14- late to post-collision I granites, 15- tholeiitic magmatism, 16-tectonic vergence,
17- sense of lateral shear zones, 18- normal faults
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Neoproterozoic fossils in Canada - an overview
Hans J. Hofmann
Dept. of Earth & Planetary Sciences, and Redpath Museum, McGill University,
Montreal, QC H3A 2A7, Canadá, hofmann@eps.mcgill.ca
Outcrop areas of Neoproterozoic sequences in Canada are confined to the peripheral areas of
the continent, occurring mainly in the Appalachian and Cordilleran fold belts, and in platform
sequences in the western Canadian Arctic. In addition to bona fide fossils, a number of
dubiofossils and pseudofossils have been described. The first discoveries of Neoproterozoic
fossils were made in the latter half of the 19th century, in the area around St. John's,
Newfoundland. The first named taxon, Aspidella terranovica, was described and illustrated by
Elkanah Billings in 1872, and represents the most common form now recognized in the Avalon
biota and elsewhere in the world. This biota remained controversial for a long time, until
additional taxa of more complex macroscopic fossils were found in the Avalon sequence in the
1960s, and on other continents. The now well known Mistaken Point assemblage, has received
the most study in Canada. Microfossils are common, but are poorly preserved. Stromatolites
occur sporadically, but only in conglomerate clastss. A second area of Neoproterozoic fossils in
the Appalachian belt is centered around St. John, New Brunswick, and another occurs in Nova
Scotia, both characterized by stromatolitic carbonates. The first named Precambrian
stromatolite, Archaeozoon acadiense, was described in 1990 from the Green Head Group of St.
John by George F. Matthew, who also reported what are now considered to be pseudofossils. In
several areas of the Cordilleran belt of western Canada, various assemblages of microfossils,
macrofossils (carbonaceous compressions as well as Ediacaran soft-bodied and shelly fossils),
trace fossils, stromatolites, and dubiofossils are known in both Ediacaran and pre-Ediacaran
units. In the western Arctic, microfossils, carbonaceous compressions, and abundant
stromatolites occur in pre-Ediacaran sequences; all await more detailed study. The biotas are
inventoried in Hofmann (1998).
Reference
Hofmann, H.J. 1998: Synopsis of Precambrian fossil occurrences in North America. Chapter 4
in Geology of the Precambrian Superior and Grenville provinces and Precambrian fossils
in North America, S.B. Lucas and M.R. St-Onge (co-ordinators), Geological Survey of
Canada, Geology of Canada, no.7, p.271-376 (also Geological Society of America, The
Geology of North America, v. C-1).
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“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Neoproterozoic to Early-Cambrian volcano-sedimentary
successions of the Camaquã Basin, Rio Grande do Sul State,
Brazil
Liliane Janikian*, Renato P. de Almeida*, A. Romalino S. Fragoso-Cesar*, Veridiana T. de
S. Martins*, Manoel S. D'Agrella Filho**, Ian McReath*, Elton L. Dantas***
*Instituto de Geociências, Universidade de São Paulo (USP), Rua do Lago, 562, São Paulo-SP, 05508080, Brazil, lijanikian@yahoo.com.br
; **Instituto de Astronomia, Geofísica e Ciências; Atmosféricas, Universidade de São Paulo (USP); Rua
do Matão, 1226, São Paulo, SP, 05508-080, Brazil
***Instituto de Geociências, Universidade Federal de Brasília (UnB), Brasília, DF, 70910-900, Brazil
Supported by FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo)
The Camaquã Supergroup is constituted by siliclastic and volcano-sedimentary successions of
Neoproterozoic to Early Paleozoic age that crop out in the south-central region of Rio Grande do Sul
State (southern Brazil). It comprises five units, from base to top: Maricá Group (marine and fluvial clastic
deposits), Bom Jardim Group (basic to acid volcanic rocks and lacustrine clastic successions),
Acampamento Velho Formation (mainly acid volcanic and volcaniclastic rocks), Santa Bárbara Group
(post-volcanic continental siliclastic successions) and Guaritas Group (aluvial and aeolian successions),
intruded by the Rodeio Velho Intrusive Suite. These units are exposed in three sub-basins separated by
basement highlands, each one representing a distinct episode of tectonic subsidence with different
depocentre and basin configuration.
This paper presents new geochronological data, a revision of the stratigraphy and an interpretation of the
paleoenvironmental evolution of the volcano-sedimentary successions of the Camaquã Supergroup: the
Bom Jardim Group, the Acampamento Velho Formation and the Rodeio Velho Intrusive Suite.
Bom Jardim Group
Because of the rapid lateral variation of depositional environments typical of small fault-bounded basins,
the Bom Jardim Group is characterized by lithologic variation among different regions. As the depocentre
and basin area have changed trough time, there is a marked pattern of onlap towards the west in the
period of deposition of the Bom Jardim Group and the Acampamento Velho Formation. Thus, the older
successions of the Bom Jardim Group occur only in the Central Camaquã Sub-basin, and the younger
ones overlay directly the Maricá Group or the basement of the Camaquã Basin in the Western Camaquã
Sub-basin. The Bom Jardim Group is divided into three formations, following the stratigraphic column
proposed by Janikian et al. (2003):
Cerro da Angélica Formation
Having its occurrences restricted to the Central Camaquã Sub-basin, the Cerro da Angélica Formation is
approximately 1500m thick at its northernmost exposure (Bom Jardim region) and 1700m thick at the
southernmost (Casa de Pedra region). In the Bom Jardim region, the unit is composed of rhythmic
centimeter-scale cycles of sandstones, siltstones and mustones locally cut by channel-shaped bodies of
pebbly sandstones, interpreted as lobe and channel deposits of sub-lacustrine fans which overlay deltaic
fans of the base of the unit. Volcaniclastic rocks formed by the intrusion of basic magma in unlithified
sediment (peperite) are also found. In the Casa the Pedra region, the correlated succession is
represented by proximal facies, mostly conglomerates and pebbly sandstones of alluvial fans and fandeltas.
Provenance analysis of the Cerro da Angélica Formation revel source-areas dominated by granitic
lithologies and low-grade metassedimentary rocks of the Dom Feliciano Belt and the Rio Vacacaí terrane.
Metacarbonate clasts are found in alluvial fan deposits in the Casa de Pedra region and are relatable to
small occurrences of carbonates in the basement located west of the Central Camaquã Sub-basin. As the
area of occurrence of carbonate clast bearing alluvial fans is restricted to the vicinity of the basement
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
carbonate exposures, the basin border faults are thought to have been mainly normal, leading to few
lateral displacement between the source areas and the corresponding deposits.
The correlation between the two areas is based on the recognition of bounding surfaces, depositional
sequences and systems tracts, and is corroborated by a U-Pb age of 593,7±2,8 Ma, obtained in a granitic
apophysis that cuts the top of the succession of the Casa de Pedra region, which is close to the
crystallization age of the Hilário Formation that overlays the succession of the Bom Jardim region.
Hilário Formation
The Hilário Formation (Ribeiro & Fantinel 1978) is approximately 1000m thick in the Bom Jardim region
(northern portion of the Central Camaquã Sub-basin) and over 2500m thick in the Lavras do Sul region
(southern portion of the Western Camaquã Sub-basin), at the type-area of the unit. It is constituted by
volcanic rocks of basic, intermediate and acid composition (basalts, latite-basalts, latites, andesites and
rhyolites) placed in subaquous environments in the Bom Jardim region and subaereous environments in
the Lavras do Sul region (Nardi & Lima 2000). Related pyroclastic rocks are also present (lapilli-tuffs and
coarse-grained lithic and vitric tuffs), formed by primary pyroclastic flow processes, secondary gravity
flows or water settling.
In the Bom Jardim region, those volcanigenic rocks are interbedded with rhythmic successions of pelites
and fine-grained sandstones deposited in a lacustrine pro-delta environment.
The great difference in thickness between the volcanic and pyroclastic successions of the two regions
suggest that the Lavras do Sul region was closer to the volcanic centers and the Bom Jardim region was
nearer to the basin depocentre, with less contribution of volcanic flows in a lacutrine pro-delta
environment.
In the Lavras do Sul region, Ar-Ar plagioclase analysis of the volcanic rocks revel a crystallization age of
590±6 Ma for a sample below the main pyroclastic deposits and ages of 588±7 Ma and 586±8 Ma for two
samples of the upper portion of the unit. U-Pb ages of pyroclastic deposits of the Hilário Formation in the
Bom Jardim region show compatible results, with crystallization ages of 589±5,3 Ma and 590,5±5,7 Ma.
Picada das Graças Formation
The Picada das Graças Formation overlays the Hilário Formation in the Bom Jardim region, where it is
approximately 1800m thick, and was deposited directly over the Maricá Group in the northern portion of
the Western Camaquã sub-basin (Serra do Espinilho region), where it is approximately 700m thick.
In both sub-basins, the lower portion of the unit is characterized by rhythmic layers of fine-grained
sandstones, siltstones and mudstones, deposited in lacustrine pro-delta environments, overlaid by thick
conglomeratic and sandy successions of delta front environment.
The upper portion of the unit shows latteral variation between the sub-basins. In the Central Camaquã
Sub-basin, it is constituted by pebbly sandstones and sandstone-siltstone successions of riverdominated deltaic environment. In the Western Camaquã Sub-basin, the same stratigraphic level is
characterized by fluvial conglomerates that gradually pass to fluvio-deltaic sandstones and siltstones.
This upper portion of the Picada das Graças Formation marks the progradation of river-dominated deltas
and the cessation of border-fault induced proximal deposits, suggesting a post-rift stage.
Acampamento Velho Formation
With occurrences restricted to the Western Camaquã Sub-basin, the Acampamento Velho Formation
(sensu Ribeiro & Fantinel 1978) is composed mainly of volcaniclastic and volcanic rocks of acid
composition, formed in subaereous environments, and minor andesitic rocks. The unit is approximately
600m thick and is in angular unconformity with the Bom Jardim and Maricá groups.
The lower succession of the Acampamento Velho Formation is composed of approximately 100m thick
coarse-grained vitric, lithic and crystal moderately to strongly welded tuffs, formed by pyroclastic flows.
Those coarse-grained tuffs are gradually overlaid by 15m of lapilli tuffs in massive tabular beds, some
levels being moderately to strongly welded. Tuff-breccias occur above the lapilli tuffs, constituting a
succession of approximately 200m dominated by coarse grained pyroclastic deposits composed of lithic
fragments (mainly tuffs and acid volcanic rocks).
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
The upper portion of the Acampamento Velho Formation is characterized by acid volcanic rocks, mainly
rhyolites, that reach 150m in the Cerro do Bugio area. Above the main acid volcanic succession, new
reworked lapilli tuffs (approximately 50m thick) and andesites occur.
A crystallization age of 574±7 Ma for the Acampamento Velho Formation was obtained by U-Pb analysis
of rhyolites.
Rodeio Velho Intrusive Suite
Sub-volcanic rocks of intermediate to basic composition intrude all stratigraphic levels of the
Camaquã Supergroup, even the Guaritas Group. These rocks have been mistaken for the Hilário
Formation in early works (Robertson 1966) but they register a younger magmatic event related to the
subsidence cycle of the Guaritas Group. Ar-Ar whole rock step heating ages of the Rodeio Velho Intrusive
Suite at a location where it intrudes the Guaritas Group point a crystallization age of 535±8 Ma
(unpublished data of Almeida, R.P), positioning the Guaritas Group in the Early Cambrian.
Conclusions
The volcano-sedimentary successions of the Camaquã Supergroup, included in the Bom Jardim
Group and the Acampamento Velho Formation, are one of the most important elements for the
understanding of the geological evolution of south-east South America in the Neoproterozoic as they
register tectonic and paleogeographical conditions in their various stratigraphic levels and present the
possibility of direct dating of volcanic rocks interbedded with the sedimentary successions.
The first unit of the Bom Jardim Group, the Cerro da Angélica Formation, registers the prevolcanic stage of the basin, when the basin was restricted to the area of the Central Camaquã Sub-basin
and limited by normal faults. The southern deposits are dominated by alluvial fans, revealing the
presence of nearby basin margin faults, and the northeast deposits are characterized by deep-water
lacustrine deposits, related to the basin's depocentre.
The onset of the major volcanic activity was around 590 Ma, represented by the Hilário
Formation. At this time the basin expanded to the west, where the volcanic centers probably developed,
but the depocentre was still in the Central Camaquã Sub-basin. After the volcanic events, the basin
expands even more, with no evidence of proximal border faults in the Picada das Graças Formation,
probably related to post-rift thermal subsidence. A younger volcanic event occurred at approximately 575
Ma, characterized by the dominance of acid rocks (Acampamento Velho Formation).
Those volcanic successions have developed in a lacustrine basin with no evidence of glacial
deposits. The last magmatic event registered in the Camaquã Basin is the Rodeio Velho Intrusive Suite,
dated of 535 Ma, related to the last subsidence cycle of the Camaquã Basin in the Early Cambrian.
References
Janikian L., Almeida R.P., Fragoso Cesar A.R.S., Fambrini G.L. 2003. Redefinição do Grupo Bom Jardim
(Neoproterozóico III) em sua área-tipo: litoestratigrafia, paleogeografia e significado tectônico das
sucessões vulcano-sedimentares do Supergrupo Camaquã, RS. Revista Brasileira de
Geociências, 33(4):349-362.
Nardi L.V.S. & Lima E.F. 2000. O magmatismo shoshonítico e alcalino da Bacia do Camaquã – RS. In:
M. Holz & L.F. De Ros (eds). Geologia do Rio Grande do Sul. p. 119-131.
Ribeiro M. & Fantinel L.M. 1978. Associações petrotectônicas do Escudo Sul-Riograndense: I Tabulação
e distribuição das associações petrotectônicas do Escudo do Rio Grande do Sul. Ihneríngia,
Série Geologia, 5:19-54.
Robertson J.F. 1966. Revision of Stratigraphy and nomenclature of rock units in Caçapava-Lavras
Region. Notas e Estudos, IG-UFRGS, Porto Alegre, 1(2): 41-54.
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
GEOTECTONIC EVOLUTION OF URUGUAY DURING THE
NEOPROTEROZOIC-CAMBRIAN
Ernesto Pecoits & Pedro Oyhantçabal
Departamento de Geología y Paleontología, Facultad de Ciencias, Iguá 4225, CP 11400, Montevideo,
Uruguay, epecoits@adinet.com.uy
Introduction
The tectonic model presented here is a review and a reinterpretation of the eastern edge of the Río de la
Plata craton evolution from rifting of Rodinia to Gondwana amalgamation. Unlike previous interpretations
this geotectonic view does not consider units of Brazil and it is based on long term field work carried out
in Precambrian units of Uruguay, South Africa and Namibia (Gariep Belt and Nama Basin). Even though
this evolutionary model accomodates the available geological evidence from Uruguay and southwestern
Africa, it represents an attempt to constrain tectonic interpretations in the future.
Proposed Tectonic Model
The opening of the Adamastor Ocean, as a consequence of the Rodinia fragmentation, would have
occured ca. 800 Ma ago (Fig. 1A). During the rift and early drift phases the deposition of the lower
Gariepian volcano-sedimentary rock sequence (Gariep Supergroup) took place. The onset of crustal
thinning is indicated on the African side by the emplacement of an alkaline granitic to syenitic suite
(Richtersveld Igneous Complex) between 833 ±2 and 771 ±6 Ma (Frimmel et al., 2001). Similar age data
were obtained by Robb et al. (1999) and Raith et al. (2003) from syn-metamorphic granites in adjacent
parts of the Bushmanland Terrane. These data were interpreted to reflect an elevated heat flow from the
mantle during crustal extension and thinning of the Mesoproterozoic crust prior to the opening of the
Adamastor Ocean (Frimmel & Germs 2003).
In Uruguay, similar ages (753 ±14 Ma) have been obtained from syn-metamorphic granites of the
Mesoproterozoic crust (Preciozzi et al., 1999). This data can be interpreted in the same way as for the
related rocks of Africa.
Subsequently, on the western side of the ocean, the development of a back-arc basin recorded by
deposition of the Lavalleja Group (Fig. 1B). The geology and geochemical signature of the igneous rocks,
mainly metagabros, and basic and acidic metavolcanic rocks, indicates a back-arc basin tectonic setting.
The best age constraint for deposition of the Lavalleja volcanosedimentary unit is provided by a U-Pb age
of 667 ±4 Ma (Sánchez Bettucci et al., 2004). In addition, a metasedimentary sample analyzed by
SHRIMP methodology yields ages between 3197 and 702 Ma, in confirmation of the proposed age.
Between 650 and 600 Ma this basin was progresively closed as the arc collided with the Río de la Plata
craton (Fig. 1C). This event produced deformation and metamorphism of the volcanosedimentary
sequence. The peak of the metamorphism was reached at 624 ±14 Ma as is indicated by U-Pb age in
zircons from a metabasalt (Sánchez Bettucci et al., 2004).
Voluminous granites were generated at the same time, as is shown by very similar ages, in both the
craton and in the arc. In the latter, this event is represented by various syn-tectonic granites assocciated
to the Sierra Ballena Shear Zone, which could be generated during the closure of the basin or as a
consequence of the reactivation of a palaeosuture. An important sequence of Vendian (ca. 600-570 Ma)
magmatism to extensional events have been documented in Uruguay, as demonstrated by Pecoits
(2003). Most of this magmatism is assocciated to the generation of transtensional and transpressional
basins, as recorded by conspicuous volcanosedimentary sequences (Pecoits et al., 2004). Likewise,
shear zone and thrust development and reactivation during this period of time was accompanied by the
intrusion of large granitic bodies (Fig. 1D).
The deposition of the Arroyo del Soldado Group in a foreland type basin occurred at around 570 Ma (Fig.
1E). This resaonably well constrained unit consists of siliciclastic to shelf carbonate deposits with a rich
fauna of upper Vendian age (Gaucher, 2000).
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Finally, the collision of the Kalahari craton closed that basin, generating deformation and low grade
metamorphism. During this closure the Sierra Ballena Shear Zone was reactivated and followed by an
important phase of post-tectonic magmatism (Fig. 1F). The largest igneous province representative of this
magmatism in Uruguay is the Sierra de Ánimas Complex which is developed near the Sarandí del YíPiriápolis Lineament (Oyhantçabal et al., 1993). Based on age and preliminary geochemical and
petrological data, this unit and the Kuboos pluton from Africa are proposed to be correlated.
Figure 1. Schematised tectonic model of the eastern edge of the Río de la Plata craton (Uruguay) and
southwestern Africa (~800-520 Ma).
Acknowledgements
We thank geologists Gerard J. B. Germs and Hartwig E. Frimmel who introduced E. P. to the geology of
the external Gariep Belt and Nama Basin (South Africa-Namibia). We discussed many aspects of
geochronology and geotectonica of Uruguay with Leda Sánchez and Fernando Preciozzi. All the abovementioned colleagues have our deep gratitude. Funding by research project “Lithostratigraphy of the
Fuente del Puma Group and their correlation with other units of Uruguay and southwest of Africa” of the
Comisión Sectorial de Investigación Científica” (CSIC, Uruguay) is gratefully acknowledged. This paper is
a contribution to project IGCP 478 (Neoproterozoic-Early Palaeozoic events in SW-Gondwana).
References
Frimmel, H.E. & Germs, G.J.B., 2003. Geology of the external Gariep Belt and Nama Basin, South
Africa/Namibia. IGCP478 Field Workshop, 25-31 October, University of Cape Town, Cape Town,
55 pp.
Frimmel, H.E., Zartman, R.E., & Späth, A., 2001. Dating Neoproterozoic continental break-up in the
Richtersveld Igneous Complex, South Africa. The Journal of Geology, 109; 493-508.
Gaucher, C. 2000. Sedimentology, Paleontology and tratigraphy of the Arroyo del Soldado Group
(Vendian to Cambrian, Uruguay). Beringeria. Würzburg.120 pp.
Oyhantçabal, P., Derregibus, M.T. & De Souza, S. 1993., Geología do extremo sul da Formaçao Sierra
de Asnimas (Uruguay). Simpósio Sul-Brasileiro de Geología I: 4-5, Curitiba.
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Pecoits, E., 2003. Age and preliminary correlation of the Las Ventanas Formation and Bom Jardim-Cerro
do Bugio allogroups (Vendian, Uruguay and Brazil). III International colloquium VendianCambrian of W-Gondwana. University of Cape Town, Cape Town, p. 32-34.
Pecoits, E., Aubet, N., Oyhantçabal, P. & Sánchez Bettucci, L., 2004. Estratigrafía de sucesiones
sedimentarias y volcanosedimentarias neoproterozoicas del Uruguay. Revista de la Sociedad
Uruguaya de Geología. (in press).
Preciozzi, F., Masquelin, H. & Basei, M.A.S., 1999. The Namaqua - Grenville Terrane of Eastern
Uruguay. In: South American Symp. on Isotope Geology, 2, Asoc. Geol. Argentina, Carlos Paz,
Actas.
Raith, J.G., Cornell, D.H., Frimmel, H.E. & de Beer, C.H., 2003. New insights into the geology of the
Namaqua Tectonic Province, South Africa, from ion probe dating of detrital and metamorphic
zircon. The Journal of Geology, 111; 347-366.
Robb, L.J., Armstrong, R.A. & Waters, D.J., 1999. The history of granulite-facies metamorphism and
crustal growth from single zircon U-Pb geochronology: Namaqualand, South Africa. Journal of
Petrology, 40; 1747-1770.
Sánchez-Bettucci, L., Oyhantçabal, P., Peçoits, E., Aubet, N., Peel, E., Preciozzi, F. & Basei, M.A.S. (in
press). Stratigraphy of the supracrustal successions of the Dom Feliciano Belt (Proterozoic,
Uruguay). Journal of South American Earth Sciences.
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Sedimentary history of the Neoproterozoic of Olavarría,
Tandilia System, Argentina: new evidence from their
sedimentary sequences and unconformities - A “snowball
Earth” or a “phantom” glacial?
Daniel G. Poiré
Centro de Investigaciones Geológicas, UNLP-CONICET, Calle 1 n° 644, 1900, La Plata, Argentina,
poire@cig.museo.unlp.edu.ar
The Precambrian sedimentary succession of Olavarría crops out in the north-western part of the Tandilia
System, Argentina. These deposits overlie a crystalline basement (Buenos Aires Complex, Marchese and
Di Paola, 1975), which is composed of granitoids, migmatites, ectinites, milonites, anphibolites and basic
dykes, yielding Sm-Nd model ages averaging 2620 ±80 Ma (Pankhurst et al., 2003).
This succession consist of: a) the Villa Mónica Formation (Poiré, 1993), b) the Cerro Largo Formation
(Poiré, 1993), and c) the Loma Negra Formation (Borrello, 1966), all of these being constituents of the
Sierras Bayas Group (Dalla Salda and Iñiguez, 1979; Poiré, 1993), and d) the Cerro Negro Formation
(Iñiguez and Zalba, 1974). These lithostratigraphic units were grouped by Spalletti et al. (1996) into four
depositional sequences: the Tofoletti (I), Malegni (II) and Villa Fortabat (III) sequences (Neoproterozoic),
and the La Providencia sequence (IV) (Vendian).
The Sierras Bayas Group being unfossiliferous, it carry biogenic sedimentary structures (trace fossils and
stromatolites) as the only evidence of biocoenosis in the Neoproterozoic sea of this region. Stromatolites
are located in the Villa Mónica Formation, where they are arranged in biostromes and bioherms dated
between 800 and 900 Ma (Poiré, 1987; 1993). Trace fossils are scarce and show a poor ichnodiversity.
Palaeophycus isp. and Didymaulichnus isp. have been described in the Cerro Largo Formation (Poiré et
al., 1984), while Helminthopsis isp. and probable medusa resting traces have been found in the Loma
Negra Formation. Skolithos isp. has recently been registered in the lower part of the Cerro Negro
Formation (Poiré et al., 2003).
By the other hand, acritarchs were reported by Cingolani et al. (1991), consisting of simple forms
Sphaeromorphis such as Synsphaeridium sp., Trachysphaeridium sp. and Leiosphaeridia sp., which are
probably Vendian in age.
Sedimentary Sequences
The four sedimentary sequences of the Sierras Bayas Group (167-195 m) and Cerro Negro Formation
(100-400 m) are divided by regional unconformities (Poiré, 1987; 1993).
The oldest depositional sequence (Tofoletti, 52-80 m) shows two sedimentary facies associations: a)
quartz-arkosic arenites to the base and b) dolostones and shales to the top. The former is composed of
shallow marine siliciclastic rocks (conglomerates, quartz and arkosic sandstones, diamictites and shales),
and the latter is characterised by shallow marine stromatolitic dolostones and shales. This sequence has
been dated by stromatolites and Rb/Sr ages in 800-900 Ma.
The second depositional sequence (Malegni, 75 m) consists of a basal succession composed of chert
breccia, fine-stratified glauconitic shales and fine-grained sandstones, followed by cross-bedded quartz
arenites which are in turn covered by siltstones and claystones. This sequence represents a shallowing
upward succession from subtidal nearshore to intertidal flat deposits. An age of 700-800 Ma has been
defined from Rb/Sr dating (Bonhomme and Cingolani, 1980).
The third depositional sequence (Villa Fortabat) is a 40 m thick unit composed almost exclusively of red
and black micritic limestones, originated by suspension fall-out in open marine ramp and lagoonal
environments.
The Vendian Cerro Negro Formation (La Providencia Depositional Sequence) appears on top. It is a
more than 100 m thick unit characterised by claystones and heterolithic fine-grained sandstone –
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
claystone interbeds, mainly formed in upper to lower intertidal flats. Through the presence of acritarchs
the underlying Cerro Negro Formation has been considered as Vendian (Cingolani et al., 1991).
Unconformities
Between the crystalline basement and the sedimentary cover the oldest regional unconformity (UA) is
developed. Below the later an arkosic saprolite is suggesting a palaeoweathering surface (Poiré, 1987;
Zalba et al., 1992). This alterated level (4 m) is composed of yellow, red, light green, and grey,
“pseudostratificated”, unconsolidated saprock, consisting of quartz, microcline, plagioclase, biotite,
muscovite (like the basement rocks, but without mafic minerals, carbonate veins or chlorite), and very
abundant Fe-oxides and illite. Marine sediments are covering this saprock in a very pasively way based
on the little evidence of erosion on the top and in some places sandstones with ripples (low regime
current) occur in the lowersmost bed of the sedimentary cover. This suggests a very gentle sea
transgression on the paleoweathered landscape (Poiré, 1993).
The unconformity (UB) appears between the Tofoletti and Malegni depositional sequences. The top of the
dolostones is very irregular making its thickness range from 19 to 58 m. Above the unconformity a wide
range of lithologies are developed, where chert breccias and massive mudstones bearing “big clasts”
and/or “slumped beds” are dominated. Wether this diamictite deposits are glacial or not, is being studied.
The contact between the Malegni and the Villa Fortabat depositional sequences (UC) has been
interpreted as a low grade unconformity to allow the developing of a carbonate ramp above a siliciclastic
subtidal sequence, with a regional diferential subsidence (Poiré, 1987; 1993).
On top of the Sierras Bayas Group a regional unconformity (UD) is recognised (Barrio et al., 1991). This
surface has been related with a sea-level drop. Meteoric dissolution of the carbonate sediments is
interpreted as a karstic surface on which residual clays and brecciated chert accumulated.
Discussion
Diagenetic, geochemical and C-O-Sr-isotope studies show a very different diagenetic degree for the
dolostones of the Villa Mónica Formation in relation with the limestones of the Loma Negra Formation.
Dolostones show a much higher diagenetic degree than the limestones, although only separated by the
75 m thick second depositional sequence (Malegni). In this sense, a diagenetic temperture around 123°C
has been measured from fluid inclusions in quartz crystals formed in siliciclstic sediments of the Villa
Mónica Formation.
As regards the lower unconformity (UA) and the unconformity (UC) below the limestones of the Loma
Negra Formation, are very gently and show non erosive processes. On the other side, the unconformity
above the dolostones of the Villa Mónica Formation (UB) and the above the limestones of the Loma
Negra Formation (UD), are very erosive with karst process signals.
This data suggests new ideas about the sedimentary history of this Neoproterozoic units. First of all, why
is there a disrup in the diagenetic trend? It is very clear that the dolostones were buried much deeper
than the limestones. Then, has there ever been any sedimenentary record between dolostones and the
Cerro Largo Formation now lost?
Secondly, there is a lack of glaciogenic sediment record below the limestones of the Loma Negra
Formation, and there is no sign of erosive unconformity, as well. Therefore, no evidence to suggest any
relationship between these carbonates and the Neoproterozoic glacial events is present. Cozzi et al.
(2003) have discussed the Snowball Earth hypotesis through sedimentary research in the Late
Neoproterozoic Shuram Formation of Oman. The Sierras Bayas Group gives new support to point
towards the fact that the Earth was not entirely covered like a snowball, at least during the period of the
pre-Loma Negra Formation deposition. May Cozzi´s idea of the “phanton” glacial be drifting the “snowball
Earth” apart?
References
Barrio, C., Poiré, D.G. and Iñiguez, A.M., 1991. El contacto entre la Formación Loma Negra (Grupo
Sierras Bayas) y la Formación Cerro Negro, un ejemplo de paleokarst, Olavarría, provincia de
Buenos Aires. Asociación Geológica Argentina Revista 46: 69-76.
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Bonhomme, M. and Cingolani, C., 1980. Mineralogía y geocronología Rb-Sr y K-Ar de fracciones finas de
la “Formación La Tinta”, provincia de Buenos Aires. Asociación Geológica Argentina Revista 35:
519-538.
Borrello, A.V., 1966. Paleontografía Bonaerense. Fascículo V. Trazas, restos tubiformes y cuerpos fósiles
problemáticos de la Formación La Tinta, Sierras Septentrionales, provincia de Buenos Aires.
Comisión de Investigaciones Científicas, Provincia de Buenos Aires 42 pp.
Cozzi, A., Brasier, M.D., Allen, P.A., Mccarron and Amthor, J.J., 2002. Last gasp of “snowball Earth”? – A
“phantom” glacial from the Late Neoroterozoic Shuram Formation of Oman. 16 International
Sedimentological Congress, Abstract :68-69.
Cingolani, C.A., Rauscher, R. and Bonhomme, M., 1991. Grupo La Tinta (Precámbrico y Paleozoico
inferior) provincia de Buenos Aires, República Argentina. Nuevos datos geocronológicos y
micropaleontológicos en las sedimentitas de Villa Cacique, Juarez. Revista Técnica de YPFB 12(2):
177-191.
Dalla Salda, L. and Iñiguez, A.M., 1979. La Tinta, Precámbrico y Paleozoico de Buenos Aires. VII
Congreso Geológico Argentino Actas 1: 539-550.
Di Paola, E.C. and Marchese, H.G. 1974. Relación entre la tectosedimentación, litología y mineralogía de
arcillas del Complejo Buenos Aires y la Formación La Tinta (provincia de Buenos Aires). Revista de
la Asociación Argentina de Mineralogía, Petrología y Sedimentología 5(3-4):45-58.
Iñiguez, A.M. and Zalba, P.E., 1974. Nuevo nivel de arcilitas en la zona de Cerro Negro, partido de
Olavarría, provincia de Buenos Aires. LEMIT, Provincia de Buenos Aires Serie II(264):93-100.
Pankhurst, R.J., Ramos, A., and Linares, E. 2003. Antiquity of the Río de la Plata craton in Tandilia,
southern Buenos Aires province, Argentina. Journal of Southamerican Earth Sciences, 16, 5-13.
Poiré, D.G., 1987. Mineralogía y sedimentología de la Formación Sierras Bayas en el núcleo
septentrional de las sierras homónimas, Partido de Olavarría, Provincia de Buenos Aires.
Unpublished PhD Thesis, Fac. Ciencias Naturales y Museo, Universidad Nacional de La Plata,
La Plata, 271 pp.
Poiré, D.G., 1993. Estratigrafía del Precámbrico sedimentario de Olavarría, Sierras Bayas, provincia de
Buenos Aires, Argentina. XII Congreso Geológico Argentino and II Congreso Exploración de
Hidrocarburos Actas II: 1-11.
Poiré, D.G., Del Valle, A. and Regalía, G.M. 1984. Trazas fósiles en cuarcitas de la Formación Sierras
Bayas (Precámbrico) y su comparación con las de la Formación Balcarce (Cambro-Ordovícico),
Sierras Septentrionales de la provincia de Buenos Aires. IX Congreso Geológico Argentino, Actas
4:249-266.
Spalletti, L.A., Poiré, D.G., Isla, F. and Zárate, M., 1996. Litoral atlántico bonaerense y Sistema de Tandilia.
VI Reunión Argentina de Sedimentología, Guía de Excursión Geológica, 15 pp.
Zalba, P.E., Poiré, D.G., Andreis, R. and Iñiguez, A.M., 1992. Precambrian paleoweathering records and
paleosurfaces of Tandilia System, Buenos Aires Province, Argentina. In Schmitt, J and Gall, Q.
(Eds.) Mineralogical and Geochemical Records of Paleoweathering. ENSMP Memoires des
Sciences de la Terre 18: 153-161.
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
The characterization of a Cambrian colisional orogeny in the
Ribeira Belt (SE-Brazil) and the tectonic events in the Kaoko
Belt (NW-Namibia) - new geochronological data and insights
on the West Gondwana evolution
R.S. Schmitt 1,6, R.A.J.Trouw2, C.W.Passchier3, W.R. Van Schmus4, M.M.Pimentel5,
W.Todt6, U.Poller6, A. Kroner 3,6
1 DGRG,
Faculdade de Geologia, Universidade do Estado do Rio de Janeiro (UERJ), Rio de Janeiro, RJ,
Brazil, renataschmitt@uol.com.br
2 Departamento de Geologia, Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, RJ, Brazil
3 Johannes Gutenberg Universität, Mainz, Germany
4 Departament of Geology, University of Kansas, 120 Lindley Hall, Lawrence, KS 66045, USA.
5 Instituto de Geociências – Universidade de Brasília (UnB), Brasília, DF, Brazil
6 Max Planck Institüt für Chemie, Ab. Geochemie, Mainz, Germany
During the mid-Cambrian the central part of West Gondwana was still undergoing a high-grade
tectonometamorphic event corresponding to collision (Schmitt et al., 2004). This event, named Búzios
Orogeny, from 520 to 490 Ma, is so far the youngest identified in this belt and probably represents one of
the final stages of the Gondwana amalgamation. Recently more data on Cambrian thermotectonic events
in Pan-African-Brasiliano belts have been shown (e.g. Dürr & Dingeldey, 1996; Jung et al., 2000; Seth et
al., 2000; Ring et al., 2002). We present here an overview of the Búzios Orogeny and new data regarding
coeval events in the Kaoko belt, NW Namibia, in order to discuss the Neoproterozoic-Cambrian evolution
of West Gondwana.
The Búzios orogeny was identified in the southeastern part of the pan-african-brasiliano Ribeira Belt (SE
Brazil), so called the Cabo Frio Tectonic Domain (CFTD). It is limited to the NW by a major NE-SW
striking thrust zone which separates it from the Neoproterozoic “Oriental terrane”, whereas to the SE it is
covered by the Atlantic Ocean. The domain comprises a Paleoproterozoic orthogneissic basement
tectonically interleaved with younger supracrustal rocks, folded and metamorphosed at upper amphibolite
to granulite facies during the mid-Cambrian. The supracrustal rocks are subdivided in two successions:
Búzios (Al-metapelites, calcsilicates and amphibolites) and Palmital (quartz-feldspathic metasediments
with minor metapelites). These successions were deposited in a deep oceanic environment between ca.
620 and 525 Ma as indicated by SHRIMP U-Pb data for detrital zircons and by T DM model ages (Schmitt
et al., 2004). The metamorphic peak, defined by the mineral associations Ky+Kfs in metapelites and
Cpx+Grt+Qz in amphibolites, occurred at minimum pressure of 9 kbar and temperature above 780 o C. At
this stage migmatites were generated by partial melting in all lithostratigraphic units, including the
amphibolites. The metamorphic peak was also contemporaneous with top to the NW thrusting, testified by
mineral and stretching lineations related to progressive deformation phases D1 and D2. The metamorphic
peak was dated between 525 and 520 Ma, as determined by U-Pb analyses of zircons of leucosomes.
During deformation phase D3, large recumbent folds developed with NW-SE axes, parallel to the main
direction of movement. The CFTD was juxtaposed at this stage to the “Oriental terrane” by a major NESW striking thrust fault. U-Pb dating of monazites from metapelites and of sphenes from amphibolites
revealed ages of about 510 Ma for the mineral growth. The sillimanite, aligned as L 3, partially replaced
kyanite, indicating a clockwise P-T-t-path for the central and eastern areas of the CFTD.
After docking into the Ribeira belt, during the late Cambrian, the western limit of CFTD was affected by a
transcurrent dextral shear zone that developed a NE-SW stretching lineation related to D4, under
amphibolite facies conditions. This is recorded in monazites and zircons within this shear zone with U-Pb
ages ranging from 505 to 490 Ma. At this stage, the central and eastern parts of CFTD were already
cooling at a rate of 10oC/Ma. After 480 Ma, the cooling rate diminished to 5 oC/Ma. A 207Pb/206Pb age in
rutile (4805 Ma) and a U/Pb zircon age in a post-tectonic pegmatite (44011Ma) mark the stabilization of
the area during the Ordovician-Silurian transition.
In the African counterpart, the Kaoko Belt consists of NS-trending strips of Archaean and
Palaeoproterozoic basement and Neoproterozoic cover, separated by a steep NS trending sinistral
transcurrent shear zone, tightly folded on a km-scale by folds with subhorizontal NS-axes. This central
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
and western part has low to medium pressure and increasing metamorphic grade from east to west,
culminating in migmatites along the coast. This part of the belt apparently operated in sinistral
transpression which obliterated earlier structures. Towards the east these transpressional structures
grade into a thrust belt associated with transport of imbricates towards the Congo Craton in the east. The
high-grade metamorphic peak was dated in syntectonic granites with ages between 567 and 552 Ma
(Seth et al., 1998).
Our study is focused in syn-D2 plutonic bodies located in the southern Kaoko Belt, Lower Ugab and
Goantagab domains (Voetspoor intrusion, Doros intrusion, Brandberg West intrusion, Drie Krone
Granite). These bodies generate a contact aureole in a turbiditic sequence (Zerrissene system – Miller et
al., 1983) consisting of deep-water siliciclastic and carbonatic sediments, with a depositional age between
750-540 Ma. The metamorphic peak is recorded at lower greenschist facies related to D1-D2 phases
(Passchier et al., 2002). The plutons are mainly syenitic to granitic with more mafic facies (amphibole
monzonite, quartz-monzonite) and are cross cut by several leucocratic veins. There are no deformational
structures within the bodies, though they present folded and boudinated veins by D2 phase at the
contacts with the supracrustals. The plutons cross cut the D1 folds. Regionally, D3 axial planes clearly
deflect around the intrusions (Passchier et al., 2002).
The Voetspoor intrusion was dated as 530 Ma by the evaporation Pb-Pb method (Seth et al., 2000). Our
preliminary data on zircons, U-Pb conventional analysis and evaporation technique, show an age range
between 550 and 500 Ma with large errors. These are due to the complex morphology of the zircons.
Cathodoluminescense and SEM images revealed inherited cores and several apatite inclusions. The
igneous overgrowths are fine rims comparing to the cores. Some crystals present metamorphic internal
structures. These inherited features could be either inherited from the igneous source or from the wall
rocks (xenoliths are numerous).
Although the southern Kaoko Belt is geographically connected to the Dom Feliciano Belt (southern
Brazil), its Cambrian deformational and metamorphic ages are coeval to the central segment of the
Ribeira belt. The present distribution of the tectonic domains could have been well disturbed by late
transcurrent faults (eg. Purros shear zone – NW Namibia).
The Cambrian-Ordovician tectonic activity described in this study is also coeval with well-known
orogenies along the margins of Gondwana. The Pampean Orogeny (~525 Ma) and the Fammatian
Orogeny (~490Ma) are responses to subduction in the ancient Andean region in Argentina (Rapela et al.,
1998). The first is the result of the subduction of an oceanic plate to the west, generating 530 Ma
granitoids in a passive margin sequence. The collision between the margin with the Pampean terrane
occurred at 525 Ma and was accompanied by crustal thickening, ophiolite obduction and metamorphic
conditions of 8.6 kbar and 810oC (Rapela et al.,1998). In Antarctica and Australia, the Ross and the
Delamerian orogenies are coeval and related to the subduction of Pacific Ocean floor after rifting of
Laurentia (Dalziel, 1991).
The newly formed Gondwana supercontinent was not only surrounded by marginal orogenies but was
also still amalgamating in its interior part. The Neoproterozoic continents that had not yet collided were
forced to amalgamate by the marginal orogenies “trap”, leading to the closure of young seas. This could
well be the cause of the Búzios Orogeny, since its time span coincides with most of the “marginal”
orogenies. It is important to emphasize that the eastern limit of the CFTD is unknown. Rough estimates
indicate that at least 250 km of continental crust is covered in Brazilian and African coastal platforms. This
would correspond to almost the total width onshore of the Ribeira Belt in its Central Segment. The
occurrence of Cambrian high grade belts along present African (Kaoko and West Congo) and Brazilian
coast lines (Ribeira, Araçuaí and Dom Feliciano) shows that the Mesozoic South Atlantic rifting closely
follows Paleozoic sutures of West Gondwana.
Acknowledgments
This project is partially financed by CAPES-DAAD exchange program – PROBRAL (n. 148-02).
References
Dalziel, I.W.D., 1991. Pacific margins of Laurentia and East Antartica-Australia as a conjugate rift pair:
evidence and implications for an Eocambrian supercontinent. Geology, 19:598-601.
Dürr, S.B., Dingeldey, D.P., 1996. The Kaoko belt (Namibia): Part of a late Neoproterozoic continentalscale strike-slip system. Geology, 24(6): 503-506.
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Heilbron, M.; Mohriak, W.U.; Valeriano, C.M.; Milani, E.J.; Almeida, J.; Tupinambá, M. 2000. From
collision to extension: The roots of the southeastern continental margin of Brazil. In: W.U.Mohriak
& M.Talwani (Eds), Atlantic Rifts and continental margins – Geophysical Monograph 115.
American Geophysical Union. p. 1-32.
Jung, S., Hoernes, S., Mezger, K., 2000. Geochronology and petrogenesis of Pan-African, syn-tectonic,
S-type and post-tectonic A-type granite (Namibia): products of melting of crustal sources,
fractional crystallization and wall rock entrainment. Lithos, 50: 259-287.
Miller, R. McG., Freyer, E.E., Hälbich, I.W.1983. A turbidite succession equivalent to the entire Swakop
Group. In: Miller, R. McG. (Ed), Evolution of the Damara Orogen. Special Publication of the
Geological Society of South Africa, vol.11, pp. 65-71.
Passchier, C.W., Trouw, R.A.J., Ribeiro, A., Paciullo, F.V.P. 2002. Tectonic evolution of southern Kaoko
Belt, Namibia. Journal of African Earth Sciences, 35: 61-75.
Rapela, C.W., Pankhurst, R.J., Casquet, C., Baldo, E., Saavedra, J., Galindo, C., 1998. Early evolution of
the proto-Andean margin of South America. Geology, 26 (8): 707-710.
Ring, U., Kröner, A., Buchwald, R., Toulkeridis, T., Layer, P.W., 2002. Shear-zone patterns and eclogitefacies metamorphism in the Mozambique belt of northern Malawi, east-central Africa: implications
for the assembly of Gondwana. Precambrian Research, 116 (1,2): 19-56.
Schmitt, R.S.; Trouw, R.A.J.; Van Schmus, W.R.; Pimentel, M.M., 2004. Late amalgamation in the central
part of West Gindwana: new geochronological data and the characterization of a Cambrian
collisional orogeny in the Ribeira Belt (SE Brazil). Precambrian Research, 133: 29-61.
Seth, B., Okrusch, M., Wilde, M., Hoffmann, K.H. 2000. The Voetspoor intrusion, southern Kaoko Zone,
Namibia: mineralogical, geochemical and isotopic constraints for the origin of a syenitic magma.
Communs geol. Surv. Namibia, 12: 125-137.
Seth, B., Kröner, A., Mezger, K., Nemchin, A.A., Pidgeon, R.T., Okrusch, M., 1998. Archean to
neoproterozoic magmatic events in the Kaoko belt of NW Namibia and their geodynamic
significance. Precambrian Research, 92:341-363.
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Microbial reefs of the Nama Group, Namibia: Composition
and growth dynamics in relation to accommodation space
and sediment input
S. Schröder*, E. W. Adams, J. P. Grotzinger
Massachusetts Institute of Technology, Earth, Atmospheric & Planetary Sciences, Cambridge, USA
*presently at Rand Afrikaans University, Johannesburg, South Africa, email: sts@rau.ac.za
The Neoproterozoic Kuibis Subgroup (Nama Group, ca. 550 Ma) was deposited on a storm-dominated
ramp in the Nama foreland basin of Namibia. Microbial reefs occur at several stratigraphic levels on this
ramp and excellent exposures allow recognition of internal fabrics, reef geometry and the relationship
between reef growth, accommodation space and sediment input.
Internal reef fabrics comprise stromatolites and thrombolites that form individual columns, domes and
composite mounds with sizes ranging from a few centimeters to 20 m. Within the microbialite structures,
massive thrombolites are commonly surrounded by a crudely laminated, stromatolitic thrombolite margin.
These observations seem to correspond to increased current influence around the column margin, rather
than to a change in the reef community. Thrombolites contain major concentrations of the skeletal fossils
Cloudina and Namacalathus, as well as the recently discovered colonial organism Namapoikia. Fossils
occur embedded in the microbial framework and as reworked components in bioclastic grainstones. The
organisms apparently preferred the stable substrate provided by the thrombolite structure, but did not
actively contribute to framework construction.
Reef growth on the Kuibis ramp can be related to variations in accommodation space and sediment input.
Low sediment input and increased accommodation space, which characterize transgressive depositional
systems, favored reef development. This relationship is illustrated by the growth dynamics of the
Driedoornvlakte isolated carbonate platform, which developed in a down dip position on the ramp.
Driedoornvlakte contains microbial reefs in a variety of environments such as the interior of the platform,
the platform margin, toe-of-slope, and basin.
A relative progressive increase in accommodation space occurred over the lifetime of the platform. Three
accommodation cycles are recognized, and each subsequent cycle experienced progressively greater
influence of the long-term accommodation increase. Aggradation and progradation during the first cycle
produced a flat-topped, sheet-like platform. Facies developed as a coarsening- and shallowing-upward
sequence; they include only small subtidal columnar microbialites because microbial growth competed
with elevated flux of clastic carbonates on the platform. In addition, growth of toe-of-slope mounds was
suppressed by sediment shedding into the basin, while basinal mounds were restricted to the
transgressive phase.
During the second cycle, higher accretion of the platform edges relative to the platform interior created a
distinct margin and a bucket-shaped geometry. The margin contained microbialite mounds, whereas
columnar microbialites, arranged in well-bedded units, dominated the platform interior. The development
of a bucket-shaped geometry with distinct margins indicates reduced sediment production on the platform
and therefore reduced sediment shedding into the basin, which consequently allowed growth of toe-ofslope mounds.
While the interior drowned during the last cycle, the platform margins kept up with rising sea level and a
pinnacle reef with fused large thrombolite mounds formed. The reef mounds nucleated preferentially on
the antecedent topography generated during the second cycle. Mounds are less common in the platform
interior, suggesting that either the water depth was too deep or the platform-derived sediment flux too
high. Breakup of the pinnacle reef into isolated large microbial mounds flanked by shales indicates a final
give-up stage before drowning of the entire platform.
The Driedoornvlakte case study illustrates that accommodation variation and sediment flux exerted a
dominant control on late Neoproterozoic platform development and microbial reef growth, analogous to
many well-documented Phanerozoic examples.
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
THE STEPTOEAN POSITIVE C-ISOTOPE EXCURSION (SPICE)
RECORDED IN LATE CAMBRIAN CARBONATES OF THE
ARGENTINE PRECORDILLERA
Alcides N. Sial1, Silvio Peralta2, Valderez P. Ferreira1, Claudio Gaucher3, Alejandro J.
Toselli4, Florencio G. Aceñolaza4, Miguel A. Parada5, Marcio M. Pimentel6
1NEG-LABISE,
Departamento de Geologia, Universidade Federal de Pernambuco (UFPE), C.P. 7852,
Recife, PE, 50670-000, Brazil; ans@ufpe.br
2Instituto de Geología, Universidad Nacional de San Juan-CONICET, Argentina, 5400;
3Departamento de Paleontologia, Facultad de Ciencias, Iguá, 4225, 11400, Montevideo, Uruguay;
4INSUGEO, Miguel Lillo 205, S.M. Tucuman, Argentina, 4000
5Departamento de Geología, Universidad de Chile, Santiago, Chile
6Instituto de Geociências, Universidade de Brasília (UnB), Brasília, DF, 70910-900, Brazil
The highly oscillatory C-isotope record of the Late Neoproterozoic coincides with major glacial to
interglacial fluctuations. Except for a glaciation at the Vendian/Cambrian boundary coincident with a large
13C negative excursion (Bertrand-Sarfati et al., 1995), in the Cambrian period “greenhouse” conditions
seem to have predominated (Tucker, 1992). In addition, the widespread occurrence of phosphorites in the
Early Cambrian suggest vigorous oceanic circulation and upwelling (Cook and Shergold, 1984). The
falling amplitude of C-isotopic values between 750 and 500Ma may be related, somehow, to the change
from glacial “icehouse” conditions in the Cryogenian to “greenhouse” conditions in the Middle Cambrian.
C-isotope oscillations in the Early Cambrian result, probably, from climatic oscillations either than from
glaciations that have not been proved to have occurred (Brasier and Sukhov, 1998).
A large and global Steptoean positive carbon isotope excursion (SPICE; +5‰ PDB; Saltzman et al., 1998,
2000) has been reported from North America, Kazakstan, South China and Australia and is one of the
largest Phanerozoic C-isotope excursions. It represents a major perturbation of the carbon cycle ~500Ma
and, as a peculiarity, a worldwide mass extinction (trilobites) coincides with the onset of the positive shift
(base of the Pterocephalyd biomere). SPICE is a valuable tool for precise regional and intercratonic
correlations and can be used globally to locate primary subdivisions of the Cambrian system in
unfossiliferous carbonate sequences.
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Platformal carbonate successions of the Precordillera, western Argentina, are one of the best proxies of
the Cambrian evolution in South America (Keller, 1999; Finney et al., 2003). We established C-isotope
profiles in Late Cambrian carbonate sequences at: (a) Chica de Zonda Range, eastern Precordillera
(Quebrada de la Flecha), (b) Cerro la Silla, central Precordillera and (c) Quebrada de Angostura, northern
part of the Precordillera, searching for the SPICE.
At Quebrada de La Angostura, near Guandacol, 200km north of San Juan, Early Cambrian Cerro Totora
Fm. limestones overthrust La Flecha or La Silla Formation limestones that, by their turn, overthrust
Tertiary pelites. 13C in carbonates of La Flecha Fm. and lowermost portion of La Silla Fm. varies from -2
to +5.6‰ and seem to have recorded the SPICE, reported here by the first time South America. The peak
of the anomaly corresponds to the transition of La Flecha to La Silla Fm., characterized by intercalation of
up to 2m-thick layers of black shales (sea-level change).
At Quebrada de La Flecha, 13C (from -2.7 to +0.6‰) shows one discrete positive excursion in the Zonda
Form. dolomites (SPICE?). The cyclicity of La Flecha Fm. is well reflected in the C-isotope profile. At
Cerro la Silla, Zonda dolomitic carbonates exhibit 13C values ~ -1‰, increasing slightly at the transition
to La Flecha Fm. (mainly composed of microbialitic cycles; 13C from 0 to -1‰). The transition of La
Flecha to La Silla Formations is characterized by intercalation of black shales and a very discrete positive
excursion (SPICE?).
The peak of SPICE coincides with a time of maximum regression in Laurentia, in North America
(intercalation of terrigenous facies). At Quebrada de La Angostura, however, the peak of SPICE coincides
with the onset of a transgression/subsidence event revealed by presence of black shales interlayered with
limestones at the transition of La Flecha to La Silla Formations.
The location of SPICE in the sections at Quebrada de La Flecha, Cerro La Silla and La Angostura allows
precise stratigraphic correlations between them and confirms the diachronic deposition of Zonda and
Flecha Formations.
Acknowledgements. This study has been supported by the CNPq/PROSUL project n. 490180/2003-5.
References
Bertrand-Sarfati, J., Moussine-Pouchkine, A., Amard, B. And Ait Kaci Ahmed, A. 1995 First Ediacaran
fauna found in western Africa and evidence for an Early Cambrian glaciation. Geology, 23 (2):
133-136.
Brasier, M.D. and Sukhov, S.S., 19978. The falling amplitude of carbon isotopic oscillations through the
Lower to Middle Cambrian: northern Siberia data. Can. J. Earth Sci., 35: 353-373.
Cook, P.J. and Shergold, J.H., 1984. Phosphorus, phosphorites and skeletal evolution at the
Precambrian-Cambrian boundary. Nature, 308: 231-136.
Finney, S. C., Gleason, J. D., Gehrels, G. G., Peralta, S. H., and Aceñolaza, G. F., 2003. Early
Gondwanan Connection for the Argentine Precordillera Terrane. Earth and Planetary Science,
Letters, 205: 349-359. Elsevier.
Keller, M., 1999, Argentine Precordillera. GSA Special Publication 341: 140.
Saltzman, M.R., Ripperdan, R.L., Brasier, M.D., Lohmann, K.G., Robison, R.A., Chang W.T., Pemf. S.,
Ergaliev, E.K. and Runnegar, B., 2000. A global carbon isotope excursion (SPICE) during the
Late Cambrian: relation to trilobite extinctions, organic-matter burial and sea level.
Palaeogeography, Palaeoclimatology, Palaeoecology 162.: 211-223.
Saltzman, M.R., Runnegar, B. and Lohmann, K.C., 1998. Carbon isotope stratigraphy of the Upper
Cambrian (Steptoean Stage) sequence of the eastern Gretab basin: record of a global
oceanographic event. GSA Bulletin, 110: 285-297.
Tucker, M.E., 1992. The Precambrian-Cambrian boundary: seawater chemistry, ocean circulation and
nutrient supply in metazoan evolution, extinction and biomineralization. Journal of the Geological
Society, 149: 655-668.
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
CONFLICTING CORRELATIONS OF NEOPROTEROZOIC
RECORDS
Paulo César Soares
Universidade Federal do Paraná (UFPR), C.P. 19001, Curitiba, PR, 81531-990, Brazil, soares@ufpr.br
Introduction
The Neoproterozoic collision belts around Paraná and Rio de la Plata continental blocks comprise many
separate allochthonous units with different stratigraphic organizations. These belts are bounded by two
different and non-contemporaneous Neoproterozoic fault systems: (1) the syn-metamorphic piling up by
overthrust system and (2) the syn- to post sedimentary cover disrupted by frontal shortening to lateral
escape transcurrent fault system. Overthrust bounded terranes present gaps in the facies sequence,
metamorphic facies, and ages. Two groups of correlated terranes are bounded by large overthrusta: (a)
the lower one, Mesoproterozoic, made up mainly of quartzites, muscovite biotite amphibole schists,
marbles, wacke, volcanics previously metamorphosed in upper greenschist to amphibolite facies; and (b)
the upper one, Neoproterozoic, made mainly of weakly metamorphosed pelites, sandstones, lime and
dolostones with mafic volcanics and intrusives. The latter preserves a sedimentary and volcanic texture
and structure, except in the sheared metamorphic bands, and includes glacial and proglacial deposits
(Vazantes, Itaú de Minas, Araxá, Corumbá-Cuiabá, Açungui, São Roque, Brusque, Vacacaí, MinasLavalleja). Autochthonous units with preserved stratigraphy and little deformation (Bambuí, Bocaina,
Cerro San Francisco etc) are possibly correlative with cratonic sequences.
Early-Mid Neoproterozoic
Neoproterozoic metamorphic complexes comprise two lower order tectonosedimentay sequences. The
first one is a transgressive-hemi sequence with near shore quartzarenites grading to offshore and
platformal pelites and ramp carbonates, followed by pelagic sediments and abundant mafic volcanics.
The regressive hemi sequence is made of a thousand meter thick section of prograding turbidites
culminating in conglomerates and abundant cannibalism in Iberia belt. The second sequence begins with
a T-hemi sequence made of matrix supported conglomerates that are glacially related, covered by two
extensive siliciclastic to carbonate cycles. It includes limestones, dolomites, phosphorites, iron formation
and basalts. The R-hemi sequence represents the final overfilling of margin basins with coarsening
deltaic deposits, ending in thick quartzarenites and arcosic arenites, sometimes conglomeratic. A 1000 to
650 Ma age is attributed for this complex. The lithologic correlation between allochthonous and
autochthonous zones is surprisingly good, as exemplified by Vazantes and Bambui. Syn-collision granite
intrusions (630-670 Ma) in allochthonous terranes provide a minimum age for deposition.
Late Neoproterozoic
After collision and erosion of deformed complexes, several sedimentary basins developed, partially
preserved today as a folded and faulted package made of red to gray erogenous conglomerates,
sandstones, marine pelites with ichnofossils, in basins either within the belt (Eleutério, Camarinha,
Camaquã etc.) or at the continental margin (Itajaí, Barriga Negra etc.). A T-hemi sequence, from fanconglomerate to marine turbidites, with volcanics, is followed by a R-hemi sequence reaching thick
conglomerate with rhyolite clasts. The coarsening upward indicates active tectonism, compression and
basin fill at the end of the cycle, as well exemplified by Itajaí Basin. A strong escape tectonic event has
deformed and reorganized the previous blocks and terranes, in the southeastern and eastern borders of
Paraná and Rio de la Plata blocks, contemporaneous with mainly frontal tectonism in the western and
northern margins. Calc-alkaline volcanism is observed, with andesites, rhyolites and pyroclastic rocks
emplaced contemporaneously. Profuse elongated granite intrusions along faults and antiforms indicate an
age range of 570 to 550 Ma for the deformational event that followed this immature orogenic sequence.
These movements represent the last convergent event in the formation of Gondwana, an event that
resulted in a widespread thermal event and geometric reorganization of blocks, metamorphic terranes
and sedimentary covers.
Early Paleozoic Volcanism
Many transcurrent faults developed early in the cycle show indicators of kinematic reversal, mainly in
brittle regime, with left-lateral movements in the NE system. Profuse granite and thick volcanic
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Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
successions, mainly rhyolites, and continental conglomerates, immature sandstones and pelite present
ages around 550-490 Ma.
Discussion
The pelite-carbonate continental margin units overlap a thick terrigenous unit in the deep margin and
extends to cover old cratonic basement; the sequence is common for both geotectonic sites, including
basal glacial deposits, although its age remains uncertain. The sequence hosts a rich fossil assemblage,
including Cloudina, Titanoteca, Soldadophycus, acritarcas and other microfossils and ichnofossils, in
Uruguay and in Mato Grosso do Sul (Brazil), assumed to be Late Vendian (Gaucher et al. 2003) and the
glacial deposits would be Varangerian (Early Vendian, 600-630 Ma). The fossils have been found in both
allochtonous and autochtonous units; other occurrences of the microfossils have been reported from the
Bambuí and São Roque units where isotopic ages constrain deposition to a pre-Vendian age and the
basal Bambuí glaciation to the Cryogenian, 850-650 Ma.
The psamo-pelite molasse units (e.g., Maricá, Itajaí, Camarinha, Eleutério, Barriga Negra),
unconformably overlie and contain clasts of metamorphic precedent pelite-carbonate association and
intruded granites. In addition, these units contain ichnofossils and/or microfossils considered to be
Vendian in age. The Barriga Negra unit was included in the Arroyo del Soldado Group Gaucher et al.
(2003), despite the fundamental differences in facies sequences and basal unconformity, in kinematic
records, and even in the fossil and ichnofossil assemblage, alike other regions. The hematite iron
formations associated to the pelite-carbonate association are present in the allochthonous complexes like
Açungui (Capiru), Cerro Espuelitas and Bambuí (Itaú de Minas) and autocthonous as in Urucum; in some
cases associated with glacially related deposits.
The Cloudina and Titanoteca bearing pre-orogenic latest meta-pellite-limestones association (Cerro
Yerbal-Polanco, Corumbá) is not overlain by any other unmetamorphosed pelite-carbonate sequence,
but only by a molassic, psamo-pelitic sequence, with an ichnofossil assemblage of late Vendian age,
albeit without trtue fossils. The cratonic margin pellite-carbonate sequence that hosts Cloudina and
Titanoteca fossils does not overly any other metamorphic sequence. Both carbonate associations, either
in the cratonic margin or in the orogenic belts, are covered unconformably by the psamo-pelite molasses
(Barriga Negra over Polanco, Camarinha over Açungui).
Moreover, glaciogenic deposits and subsequent limestones of these complexes, upthrust and ovethrust
the cratonic sequences, like exemplified by Bambuí, Araras, Corumbá and Polanco groups, which
indicates an age of the first Neoproterozoic glaciation (Sturgian) around 780 Ma. The age of these units is
the critical issue in the correlation with many other remnants of pericratonic basins, such Corumbá,
Bambui and Sierras Bayas. Both the Bambui and Corumbá section have been regarded as of Vendian
age, according to paleontological indicators and correlations (Boggiani et al. 1993), but both were
affected by metamorphism and overthrusting in the marginal orogenic zone at around 630 Ma. Pb/Pb
isochronic ages (Babinski, 2001) indicate that the deposition of Bambuí was realized before 686+/-69 Ma,
the probable age of diagenesis. Additionally the presence of Acritarcs above the Macaúba glacial
deposits makes possible the assumption of a pre-Vendian age for the sequences described.
The section where the Cloudina was defined and become the global reference as a Late Vendian index
fossil is the cratonic margin Kuibis unit of the lower Nama Group (Namíbia). This carbonate sequence is
unconformably overlain by the immature terrigenous section of upper Nama. In some places along the
Damara orogenic margin, the equivalent lower unit is strongly deformed. It is well known that the
deformed pellite carbonate association of the Damara sequence is emplaced on top of the autochthonous
basal Nama Group sediments by a major thrust in the southern margin zone. This is a similar
arrangement with the records of Dom Feliciano, Iberia, Brasília and Paraguay belts. Moreover, the fossil
and ichnofossil records of both sections, pre-orogenic pellite carbonate, and late orogenic terrigenous are
wholly different. The facies and the sequence type of Cloudina bearing Kuibis and Schwarzrand pellitecarbonate units are indicative of open shelf basin deposits of the Damara cycle; the molasse type
sequence bearing ichnofossils begins with the unconformable Nomtsas unit. The isotopic age obtained by
small zircon grains in ash beds in the Lower Nama pre-orogenic carbonate section, Cloudina bearing,
was 545-548 Ma (Grotzinger et al. 1996). But post-orogenic granite gives age around 525 Ma (Gresse et
al. 1996). Within a short interval the region would have changed from open platform to Damara orogeny
and then to marine molasse type basin. The main problem with this scenario is the fact that at least two
distinct tectonosedimentary sequences are encompassed by the Namas Group. Similar confusion is
brought by the inclusion of the molasse type Barriga Negra unit in the pelite-carbonate open platform
section. The other is the restriction of Cloudina and Titanoteca range to Late Vendian times.
Here emerges the impasse, with three possible hypothesis:
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“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
(1) Two pellite-carbonate sequences with similar special phosphorites and iron formations exist, being
one of Mid Neoproterozoic (800 the 650Ma) overthrusted and metamorphosed, and another of the
Vendian (Late Neoproterozoic); non-metamorphic and little deformed. (2) The 630-650 Ma ages of the
collision and of the metamorphism are wrong and they would all be from the final of Vendian, having an
overlap between the pelite-carbonate sequence and the psammo-pelite molasse! (3) Cloudina appeared
during the Mid Neoproterozoic in carbonate rich waters like others metazoan and like acritarchs and
overlapped Ediacaran time, until the end of Vendian in open carbonate shelves (like in China).
Furthermore, where psamo-pelite, molasse type, Vendian deposits prevail, even in very expressive
European platform, over the tillite, there is the Ediacara fauna, but not Cloudina.
Against the first hypothesis, the unexpected fact that when one is preserved the other is not! Against the
second, the metamorphic gap and the unconformity between pelite-carbonate platform and molasse type
basal conglomerate with meta-limestone and granite pebbles would have been mistaken! Against the
third, there are the ages given by the small zircon grains in ash beds of Lower Nama Group. It may be
added that the distribution of different fossils and ichnofossils in Meso and Neoproterozoic has been
associated more with the environmental conditions than with evolutionary changes, which could make
them of little chronological value as index fossils.
References
Babinski, M., 2001. Pb Isotopes on carbonate rocks: implications for the evolution of the São Francisco
Basin. IGCP 450 – I Field Workshop. Belo Horizonte, p.38-42.
Boggiani et al. 1993. O Grupo Corumbá (Neoproterozóico-Cambriano) na região… Revista Brasileira de
Geociências, 23(3):301-305
Gaucher et al. 2003. Integrated correlation of the Vendian… Precambrian Research 120:241-278.
Gresse, P. G. et al. 1996. Late- to post-orogenic basins of Pan-African-Brasiliano collision orogen…
Basin Research, 120:241-278.
Grotzinger et al., 1996. Calibrating the Terminal Proterozoic time scale. 30 st International Geological
Congress, Beijing, China, v.2, p.47.
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“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Terrane transfer during the Grenvillian assembly of Rodinia:
implications of Amazonian crust in the southeastern
Appalachians for the Neoproterozoic paleogeography of the
Iapetus Ocean
Eric Tohver & Jorge S. Bettencourt
Instituto de Geociências, Universidade de São Paulo (USP), Rua do Lago, 562, São Paulo-SP, 05508080, Brazil, etohver@usp.br
The opening of the Iapetus Ocean is marked by the development of Neoproterozoic/Cambrian marine
basins that record the rupturing of the Rodinia supercontinent. Although the Amazon craton is widely
considered to form the conjugate margin to Laurentia, the paleogeography of this pre-Atlantic seaway
remains uncertain. Investigations of the Neoproterozoic sequences of western Gondwana and their
potential correlatives in Laurentia can be used to trace the history of this ancient ocean basin. Because
the construction phase of Rodinian history is preserved in the Grenville mobile belt that sutured Amazonia
and Laurentia, the paleogeography of the Iapetan margin directly reflects the final phases of Rodinia’s
assemly. Therefore, reconstructions of the geometry of the developing Iapetan rift margin in the late
Neoproterozoic should reflect the configuration of Rodinia at the end of the Mesoproterozoic.
Whole rock Pb isotope data can be used to determine the provenance of different blocks within the
Rodinia supercontinent, providing a test for paleogeographic reconstructions. Using U-Pb zircon data as
an anchor point, we calculate values for the initial Pb present at the time of rock crystallization using
published whole rock and feldspar data from the Grenville Province of Canada. Calculated Pb isotope
fingerprints are remarkably coherent for the entire Grenville Province, including the Adirondacks of the
northern U.S.A. and Grenvillian rocks of Texas and adjacent New Mexico ca.3000 km away, suggestive
of a homogeneous mantle source for this entire region. A distinctly different source region characterizes
“Grenvillian” basement rocks of the southern Appalachians (Blue Ridge/Mars Hill), suggesting that these
rocks are not of Laurentian origin. The Pb fingerprint of 54 whole rocks samples from the “Grenvillian”
basement rocks of the SW Amazon craton (Rondônia, Brazil), are compared to those from the Grenville
belt of North America and Grenvillian basement inliers in the southern Appalachians, revealing a common
source region for both the SW Amazon basement and the allochthonous Blue Ridge/Mars Hill terrane.
The inferred common source region for the SW Amazon craton and the Blue Ridge/Mars Hill terrane
contrasts with the less radiogenic source region that characterizes the Grenville Province of Laurentia.
The presence of mature continental material as revealed by Nd isotopes and U-Pb zircon geochronology
in the Blue Ridge/Mars Hill terrane is also consistent with the signature of Amazon basement rocks. We
propose that this portion of the S. Appalachian basement is derived from Amazonia and was transferred
to Laurentia during Grenvillian orogenesis at ~1.15 Ga. The presence of these Amazonian rocks in
southeastern Laurentia records the northward passage of the Amazon craton along the Laurentian
margin. Thus, the paleomagnetically defined position of the Amazon craton against southernmost
Laurentia at 1.2 Ga does not define the final geometry at the time of the opening of the Iapetus Ocean.
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“1st Symposium on Neoproterozoic-Early Paleozoic Events in SW-Gondwana”
Extended Abstracts, IGCP Project 478, Second Meeting, Brazil, October 2004
Figure – a) Outline of North America emphasizing the location of the Grenville province (dark grey) on
the eastern margin of the Laurentian craton.b) Position of the Amazon craton (AC) within South America
with major geochronological provinces outlined in white. The dark grey area is the outline of the state of
Rondônia with the Amazon river basin shown in white; dashed black line shows Paragua craton (PC),
accreted at ~1.1 Ga. (main) Schematic illustration of paleogeographic positions previously proposed for
the Amazon craton in late Mesoproterozoic times are shown in light silhouette, from top to bottom. The
southernmost position is the paleomagnetically constrained position of the Amazon at 1.2 Ga, interpreted
as indicating collision with the southern Laurentia. Dark silhouette is the proposed configuration at the
time of the transfer of the Blue Ridge/Mars Hills terrane to the North American craton (ca. 1.15 Ga),. This
model requires net sinistral displacement of ~2.5 cm/yr offset between Laurentia and the Amazon craton
after the initial collision. Numbers are Nd model ages for Laurentia (Labrador; Quebec; reworked portions
of Superior craton; Central Metasedimentary Belt; Adirondacks and Green Mtns.; the mid-continental
region east of the Grenville Front (toothed line); the Llano region and occurrences in W. Texas,
Grenvillian basement inliers in the Appalachian, and the Amazon craton. The bold grey line through the
Appalachians separates Paleozoic accreted terranes from ancestral North America at the end of the
Proterozoic, i.e. at the time of Rodinia breakup. Dashed line separating southeastern midcontinental
region with TDM < 1.55 Ga. Abbreviations for Grenvillian basement occurrences, shown in black outline,
are as follows: CMB – Central Metasedimentary Belt; AM – Adirondack Mts; GMM – Green Mtn. Massif;
BR – Blue Ridge (Virginia, Maryland); BR-MH – Blue Ridge/Mars Hill terrane; EGR – Eastern GraniteRhyolite province; LU; Llano Uplift; Van Horn Mtns.; FM – Franklin Mtns.; HBU – Honeybrook Uplands.
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