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Neoproterozoic to Early Paleozoic Deposits Of U N I V E R S I D A D D E C O N C E P C I Ó N DEPARTAMENTO DE CIENCIAS DE LA TIERRA 10° CONGRESO GEOLÓGICO CHILENO 2003 IGCP 478: PROVENANCE IN NEOPROTEROZOIC TO EARLY PALAEOZOIC SUCCESSIONS OF SW GONDWANA: THE STATUS QUO AT THE BEGINNING OF A NEW IGCP INTIATIVE ZIMMERMANN, U.1, FRIMMEL, H.2, GAUCHER, C.3, GERMS, G.1, POIRE, D.4, BLANCO, G.3, GOMEZ PERAL., L.4 and VAN STADEN, A.1 1 Dep. of Geology, RAU University, POB524, Auckland Park 2006, Johannesburg, South Africa, [email protected] 2Univ. of Cape Town, Dep. of Geological Sciences, Rondebusch 7701, south Africa, [email protected] 3INGEPA, Facultad de Ciencias, Iguá 4225, 11400 Montevideo, Uruguay, [email protected] 4CIG, CONICET-UNLP,1 Nº 644, 1900 La Plata, Argentina. [email protected] INTRODUCTION Neoproterozoic to early Paleozoic successions are common on both sides of the southern Atlantic in Brasil, Uruguay and Argentina on the western side and Angola, Namibia and South Africa on the eastern side. These successions form part of southwestern Gondwana. Our “provenance” research project is part of the IGCP-Project 478 (Neoproterozoic-Early Paleozoic Events in SW- Gondwana) and focuses on the qualitative and quantitative petrological and geochemical characterization of clastic deposits in combination with biostratigraphy. The purpose of the project is to characterize source areas and depositional loci of clastic sediments in order to reconstruct the paleotectonic evolution (including the assembly) of southwest Gondwana during the Neoproterozoic to early Paleozoic (Fig. 1). The primary study targets are: the Arroyo del Soldado Group in Uruguay, the Sierras Bayas Group in Argentina, the Damara and Gariep Supergroups in Namibia and the Gariep and Malmesbury Supergroups and Nama, Vanrhynsdorp, Klipheuwel and Gamtoos Groups in South Africa. Associated are studies on Neoproterozoic successions on the western border of the Rio de la Plata Craton, the Puncoviscana Formation, to understand the evolution of the latter craton better. The mentioned successions contain important fossils from a biostratigraphic and palaeobiologic point of view, and a variety of rock types, such as carbonates, sandstone/mudstone intercalations and tillites, locally associated with banded iron formations. GENERAL STRATIGRAPHY AND SAMPLING 1. URUGUAY: ARROYO DEL SOLDADO GROUP (ASG) This lithostratigraphic unit was defined by Gaucher et al. (1996), to include thick (>5000 m) marine shelf deposits, which occur in the Nico Pérez Terrane of Uruguay (Fig. 2). The boundaries of the terrane also represent the boundaries of the occurrences of the ASG. To the north, the occurrences of the ASG are covered by Phanerozoic sedimentary units and volcanic rocks of the Paraná Basin. The northernmost outcrops of the Group have been recognized and described by Gaucher (2000) in the so-called Isla Cristalina de Rivera, near the boundary with Brazil. The unit lies with erosional and angular unconformity on: (1) a Mesoproterozoic Todas las contribuciones fueron proporcionados directamente por los autores y su contenido es de su exclusiva responsabilidad. metasedimentary complex (Gaucher et al., 1996), (2) Archean (3.4-2.7 Ga) metasedimentary units (Hartmann et al., 2001); (3) Palaeoproterozoic Rapakivi granites (1784 ± 5 Ma: Heaman, in Bossi et al., 1998) and (4) Puntas del Santa Lucía Batholith (granites, granodiorites and monzogranites) that yielded U/Pb SHRIMP age of 633±12 Ma (Bossi et al., 2001). The stratigraphy after Gaucher (2000) starts with the Yerbal Formation, which is a siliciclastic, deepening-upward sequence, and is characterized by thin conglomerates and arkoses at the base, passing into green siltstones and banded siltstones at the top. Oxide-facies BIF with up to 30 wt % iron oxides occur at the top of this unit. The Yerbal Formation is concordantly overlain with a sharp contact by the Polanco Formation, which marks the development of a large carbonate ramp in the basin. The unit is characterized by bluish grey to black limestone-dolostone rhythmites, frequent carbonatic tempestites, pure calc-siltites and dolosiltites, and rare oolitic calcarenites. In the shallowest areas of the basin, carbonates of the Polanco Formation are concordantly overlain by conglomerates and arkoses of the Barriga Negra Formation, recording a major regression. This unit passes upwards into shales and siltstones of the basal Cerro Espuelitas Formation. In the deepest sections, the Polanco Formation shows a direct transition into the Cerro Espuelitas Formation. The latter unit is made up of an alternation of dark shales, thick chert-deposits and oxide-facies BIF with up to 35% magnetite and/or hematite (Gaucher & Schipilov, 1994; Gaucher et al., 1996, 1998). The Cerro Espuelitas Formation is truncated by an erosive surface that probably marks an important regression. The Cerros San Francisco Formation was deposited above this surface, and is characterized by very mature quartz-arenites with wave and current ripples, hummocky cross-stratification and low-angle cross bedding (comp. in Gaucher, 2000). Finally, the Cerros San Francisco Formation passes upwards into stromatolitic and oolitic limestones of the Cerro Victoria Formation, which contains ichnofossils of probable Cambrian age (comp. in Gaucher, 2000). The ASG has been folded and intruded by granites 540-510 Ma (Bossi and Navarro, 1991), Kawashita et al. (1999)), and syenites during the Cambrian. K-Ar age data of illite-rich pelites belonging to the Group yielded recrystallization ages between 532 ± 16 Ma and 492 ± 14 Ma (in Gaucher, 2000), in accordance with ages obtained for intrusive granites. Considering these ages, the orogenic event that determined deformation of the ASG can be taken as a terminal event of the Brasiliano Megacycle or, more probably, represents a separate orogeny (Río Doce Orogeny), recording the collision of a Brasiliano-block (Cuchilla Dionisio Terrane) with a cratonized block, onto which the Arroyo del Soldado-shelf developed. Biostratigraphy Gaucher (2000) assigns both assemblages of organic-walled microfossils found in the ASG (Leiosphaeridia-Lophosphaeridium and Bavlinella-Soldadophycus) to the upper Vendian Kotlin-Rovno assemblage, characterized worldwide by low diversity, simple leiosphaerids, abundant Bavlinella faveolata, cyanobacterial sheaths, rare small acanthomorphs and vendotaenids of the genera Vendotaenia and Tyrasotaenia. The occurrence of Cloudina riemkeae Germs (1972) in the Yerbal Formation is of great biostratigraphic significance. According to Knoll (1996), the range of Cloudina extends from 565 to 543 Ma, coinciding with the depauperate Kotlin-Rovno acritarch assemblage. U/Pb ages of volcanic ash beds interbedded with fossiliferous, Cloudina-bearing strata of the Nama Group outlined by Grötzinger et al. (1995), determined support the above mentioned upper limit for the biostratigraphic range of Cloudina.Finally, trace fossils occurring in the Cerro Victoria Formation denote quite complicated and diverse behavioural patterns of the organisms that caused them. Above all, burrows are not confined to bedding planes and are often perpendicular to them, indicating, according to Seilacher (1999), a Cambrian age. Biostratigraphic data thus suggest that most of the ASG is upper Vendian in age (595-543 Ma). The uppermost units (Cerro Victoria and possibly also the Cerros San Francisco Formation) were deposited in the lowermost Cambrian. Chemostratrigraphy Gaucher (2000) presents a curve of δ13C variations in carbonates of the Arroyo del Soldado Group, based on the data reported by Boggiani (1998). Kawashita et al (1999) present δ13C data and 87Sr/86Sr for the base of the Polanco Formation. More recently, Gaucher et al. (2002; 2003) report high-resolution C-isotopic profiles of the lower and middle ASG (Yerbal, Polanco and lower Cerro Espuelitas formations). The profiles reveal a series of positive and negative δ13C excursions, which correlate well with published δ13C-curves for the upper Vendian. The upper Yerbal-lower Polanco positive δ13C excursion of +5.3 PDB, along with 87Sr/86Sr ratios of 0.7078, point to an age of ca. 580 Ma for the Yerbal-Polanco transition (Jacobsen & Kaufman 1999, Walter et al. 2000). Finally, Gaucher (2000) suggests that negative values obtained by Boggiani (1998) for the Cerro Victoria Formation could be correlative with the sharp negative excursion recorded in the basal Cambrian worldwide. This has been recently confirmed by a detailed δ13C -curve presented by Gaucher et al. (2003) for the Cerro Victoria Formation, allowing to place the Precambrian-Cambrian boundary in the upper-middle Cerros San Francisco Formation.The location of the Arroyo del Soldado Group makes it a key unit to unravel the relationships between Vendian to lowermost Cambrian units of southern Brazil, Argentina, Namibia, South Africa and Uruguay. Furthermore, its rich fossil content allows relatively precise age constraints and correlations with other units. Provenance studies will aim to confirm the provenance of Arroyo del Soldado sedimentary rocks from old cratonic areas of the Río de la Plata Craton. Comparison of the provenance of the ASG with other Neoproterozoic (volcano)sedimentary successions occurring in Uruguay is another major goal. These successions include: Lavalleja and Rocha Groups, Las Ventanas, Playa Hermosa and Piedras de Afilar Formations. Some of this units, like the Rocha Group, contain zircon populations with a strong component around 1.0 Ga, which could indicate provenance from the Kalahari Craton
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