1st Symposium on Neoproterozoic-Early Paleozoic Events in sw-gondwana”




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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 1

The vergence of the Paraguay Belt in the Context of the collage of the West Gondwana 3

Distribution, Genesis and Paleoecological Significance of Iron Formation Through Time 6

Stratigraphy, paleontology and age of Las Ventanas Formation (Neoproterozoic, Uruguay) 8

New level of diamictites in the Corumbá Group (Ediacaran), Paraguay Belt, South America 10

Cloudina from the Itapucumí Group (Vendian, Paraguay): age and correlations 13

Micropaleontological aspects of Lowermost Cambrian black shales and cherts on the Yangtze Platform, China 16

Diamictites overlying Marinoan-age Carbonates of Araras Formation, Paraguay Belt, Brazil: evidence of a new glaciation? 18

CRUSTAL STRUCTURES OF PARAGUAY BELT FROM GRAVITY AND MT DATA: A CAMBRIAN SUTURE IN WESTERN GONDWANALAND? 20

Impact of a late Vendian, non-global glacial event on a carbonate platform, Polanco Formation, Uruguay 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 24

Lithostratigraphy, biostratigraphy and correlations of Neoproterozoic to early Paleozoic sedimentary basins on the Kalahari Craton and its margins (Southern Africa) 27

Chemostratigraphy and diagenetic constraints on Neoproterozoic carbonate successions from the Sierras Bayas Group, Tandilia System, Argentina 30

Unravelling chemostratigraphic signatures of sedimentation and diagenesis in Paleoproterozoic iron and manganese formations 33

The Cajamar basin (SP-Brazil) and the microfossil Titanotheca coimbrae of the Ediacaran period 35

TECTONIC EPISODES RELATED TO WEST GONDWANA AMALGAMATION IN THE RIBEIRA OROGEN (SE BRAZIL) 36

Neoproterozoic fossils in Canada - an overview 39

Neoproterozoic to Early-Cambrian volcano-sedimentary successions of the Camaquã Basin, Rio Grande do Sul State, Brazil 40

GEOTECTONIC EVOLUTION OF URUGUAY DURING THE NEOPROTEROZOIC-CAMBRIAN 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? 46

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 49

Microbial reefs of the Nama Group, Namibia: Composition and growth dynamics in relation to accommodation space and sediment input 52

THE STEPTOEAN POSITIVE C-ISOTOPE EXCURSION (SPICE) RECORDED IN LATE CAMBRIAN CARBONATES OF THE ARGENTINE PRECORDILLERA 53

CONFLICTING CORRELATIONS OF NEOPROTEROZOIC RECORDS 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 58



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 Sr-isotope 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 sand-dolostone 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.


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.







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 (Dn) 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 Sn 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 (Dn+2) is orthogonal to the other phases and occurs as crenulations and kink bands with NW axial planes (Sn+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 Sn+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.


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

III

IV

V



D1

D2




D3









D1




D3

D4



D1

D2




D3

D4






D1

D2

D3

D4









D1

D2

D3



Sn-1 (?)

Sn

Sn+1

Sn+2

Sn+3









Sn*

Sn+1**

Sn+2***

 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 Dn 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 Sn (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 Sn. 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 Sn+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 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.


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 iron-precipitating 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. 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 iron-formation. 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, 1079-1082.


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 km2. 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 fan-dominated 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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