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Introduction

Southwestern Gondwana comprises Namibia, South Africa, Botswana, Argentina,
Uruguay, Paraguay, Bolivia, the southern half of Brazil, and part of Antarctica. At least six large cratonic blocks form the basement of this region, namely the Kalahari, southern Congo, Río de la Plata, Paraná, southern Amazonian and southern Sao Francisco cratons (Fig. 1).

Figure 1: Research area indicated on a pre-drift reassembly of Gondwana, modified after Porada (1989). PC: Paraná Craton, RPC: Río de la Plata Craton; SFC: Sao Francisco Craton. Red areas: Neoproterozoic to Early Palaeozoic belts and deposits. 1: Sierras Pampeanas Belt, 1b: Cordillera Frontal Metamorphic Belt, 2: Paraguay Belt, 3: Dom Feliciano Belt, 4: Arroyo del Soldado Group, 5: Tandilia System, 6: Saldania Belt, 7: Gariep Belt, 7a: Nama Group, 8: Damara Belt, 9: Kaoko Belt, 10: Ribeira Belt, 11: Ara?uaí Belt, 12: Lufilian Belt, 13: Zambezi Belt, 14: Transantarctic Belt (only the most significant units for the present project have been indicated)

Neoproterozoic to Early Palaeozoic rocks, mostly within 800-400 Ma, are widespread there as cratonic cover and are also integral part of the orogenic belts that mark the sutures between the mentioned cratons. The sedimentary sequences are mostly unmetamophosed, representing sedimentary environments ranging from continental to deep marine. These facts determined that the region has become most important for the dilucidation of some of the most intriguing events in Earth history, such as global Neoproterozoic glaciations, the amalgamation and breakup of the first supercontinent (Rodinia), the so-called Cambrian “explosion” of life and the occurrence of one of the most intense orogenic periods on Earth (Panafrican-Brasiliano Orogeny). Intense debate is currently taking place around these issues, as demonstrated by the innumerable publications specifically based in SW-Gondwana (see below). Therefore, SW-Gondwana is a key region that can greatly improve our understanding of the major paradigms of Earth Sciences: plate tectonics, organismic evolution, global climatic change and -most important- the interaction between these three.

State of the art

The Neoproterozoic-Early Palaeozoic saw the occurrence of some of the most significant events in Earth history, namely:

The advent and diversification of metazoans and skeletons (“Cambrian explosion”)
The occurrence of the most severe glaciations on Earth
Breakup of the (first) supercontinent, Rodinia
Important changes in the chemistry of oceans and atmosphere
Intense orogenesis represented by the Pan African-Brasiliano orogenies

These themes were subject of intense research in recent years. To researchers dealing with any of the issues mentioned, it has become increasingly clear that there was an interplay of different factors that determined such profound changes in the bio- and lithosphere. For example, the advent and early evolution of the Metazoa was largely influenced by palaeoclimatic oscillations and changes in oxigenation of oceans and atmosphere.

Southwestern Gondwana is important for the understanding of these phenomena, because:

It comprises a very complete and largely unmetamorphosed sedimentary record spanning most of the Neoproterozoic and Early Palaeozoic (Poiré 1987; Porada 1989; Bossi & Navarro 1991; Trompette 1994; Germs 1995; Gresse et al. 1996; Boggiani 1998; Bahlburg et al. 2000; Bock et al. 2000; Gaucher 2000; Paim et al. 2000; Teixeira 2000; Alvarenga et al. 2000; Frimmel et al. 2002; Zimmermann & Van Staden 2002).

A remarkable diversity of well-preserved early metazoans, vendobionts and skeletal fossils is recorded in the region (Gürich 1933; Richter 1955; Pflug 1970 a, b; Germs 1972; Yochelson & Stump 1977; Hahn et al. 1982; Hahn & Pflug 1985; Zaine & Fairchild 1985; Grant 1990; Narbonne et al. 1997; Gaucher & Sprechmann 1999; Waggoner 1999; Grotzinger et al. 2000; Gaucher 2000; Hagadorn & Waggoner 2000; Brain et al. 2001; Wood et al. 2002)

It includes the remnants of the Pan African-Brasiliano orogens (Almeida 1969; Porada 1979; 1989; Trompette 1994; Pankhurst & Rapela 1998; Ramos & Keppie 1999; Rozendaal et al. 1999; Campos Neto 2000).

The Neoproterozoic glacial record is present both at the African and South American side (Hambrey & Harland 1985; Durand 1990; Alvarenga & Trompette 1992; Kaufman et al. 1997; Boggiani 1998; Hoffman et al. 1998; Kennedy et al. 1998; Saylor et al. 1998; Alvarenga et al. 2000; Frimmel et al. 2002; Hoffman & Schrag 2002).

Recent years have witnessed great advances in the knowledge of the Neoproterozoic-Early Palaeozoic of SW-Gondwana. The following are just some of the most important contributions:
Detailed U/Pb geochronology (Grotzinger et al. 1995; Soliani et al. 2000; Frimmel 2000; Bossi et al. 2001)
Chemostratigraphy (C, O, Sr, S) of Neoproterozoic-Cambrian successions and its palaeoclimatic implications (Saylor et al. 1998; Hoffman et al. 1998; Kennedy et al. 1998; Sial et al. 2001; Frimmel et al. 2002; Gaucher et al. 2002).
Discussion of the number, extent, timing and causes of Neoproterozoic glaciations in SW-Gondwana (Hoffman et al. 1998; Kennedy et al. 1998; Hyde et al. 2000; Eerola 2001; Hoffman & Schrag 2002)
Definition and description of previously unknown sedimentary sequences of Neoproterozoic-Early Palaeozoic age (Gaucher 2000; Meinhold et al. 2000; Teixeira 2000; Teixeira & Petri 2001; González et al. 2002; Zimmermann & Van Staden 2002)
Discovery of surprisingly diverse and abundant skeletal fossils in Vendian successions (Gaucher & Sprechmann 1999; Grotzinger et al. 2000; Wood et al. 2002).
Recognition of the palaeoecologic, taphonomic and evolutionary importance of microbial mats in Neoproterozoic oceans (Pflüger 1999; Seilacher 1999; Porada & Löffler 2000; Bottjer et al. 2000; Porada 2002).
Preliminary palaeogeographic models for the region (Unrug 1996; Ace?olaza & Tosseli 1999; Rozendaal et al. 1999; Brito Neves et al. 1999; Porada & Berhorst, 2000; Campos Neto 2000; Basei et al. 2000; Chemale 2000; Teixeira 2000; Teixeira & Petri 2001; Gaucher & Germs 2002; Sánchez Bettucci & Rapalini, in press; Gaucher et al., in press).
Preliminary provenance studies in the Neoproterozoic successions in the Sierras Pampeanas (Zimmermann & Van Staden 2002).

The main units where research has been focussed in the Neoproterozoic of S-Africa were the Damara, Katangan, Gariep and Saldania Belts, the Witvlei, Nama and Vanrhynsdorp Groups and associated intrusions in Botswana, DRC, Namibia, South Africa and Zambia. On the South American side, research has been centered mainly in the Ribeira, Dom Feliciano, Paraguai and Cordillera Frontal Belts (Brazil, Uruguay, Paraguay, Argentina and Bolivia), in the Arroyo del Soldado, Bambuí, Camaqua, Corumbá and Sierras Bayas Groups (Argentina, Brazil and Uruguay) and Araras and Puncoviscana Formations (Argentina and Brazil). The abundant late Neoproterozoic to Cambrian magmatism in Brazil and Uruguay has been the subject of intense research as well.

Despite the large amount of data available from certain areas, the main problems of the Neoproterozoic-Early Palaeozoic of SW-Gondwana have not been solved yet, namely:

How SW-Gondwana accreted in the Vendian-Cambrian
Intercontinental correlation of lithostratigraphic units at either sides of the Atlantic
Reconstruction of the palaeoclimatic history of SW-Gondwana
Erection of a bio- and chemostratigraphic scheme valid for the region
Reconstruction of the palaeobiogeography and conditions determining the Cambrian radiation of metazoans

The cause is the great geological complexity and wide extent of the terranes involved, which demand detailed mapping, precise dating and study of poorly-known areas. This will be the main scope of this project, which will allow us to address more satisfactorily the above mentioned problems.