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General geology |
Instituto Superior de Correlación Geológica (Consejo Nacional de Investigaciones Científicas y Técnicas, Universidad Nacional de Tucumán), Miguel Lillo 205, 4000 San Miguel de Tucumán, Argentina (e-mail: insugeo{at}csnat.unt.edu.ar)
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Abstract |
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 Top Abstract The Neoproterozoic-Early...The Puncoviscana Formation of...Fossils and ageFinal remarks and conclusionsReferences |
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More than one hundred years of research resulted in the definition of the base of the Phanerozoic at Fortune Head, Newfoundland, Canada (Gradstein et al. 2004 with references). Analysis of the discussions held over the last 30 years clearly highlights that most debates concerning this boundary and others, involved data from North America, Africa, Asia and Australia, but lacked information from the Neoproterozoic–Early Cambrian sequences of South America (Glaessner 1984; Fedonkin 1987, 1994). Thus further research is required on the sedimentary and metamorphic rocks of this region.
The Neoproterozoic–Early Cambrian sequence in the central Andean Basin of South America is best exposed in northwestern Argentina, where it is dominated by thick, heterogeneous slightly metamorphosed and predominantly siliciclastic successions of the Puncoviscana Formation. This unit represents the regional basement (Turner 1960; Aceñolaza & Toselli 1981; Omarini et al. 1999a), but its stratigraphy has been obscured by several deformational events. The strata of the Puncoviscana Formation represent multiple sedimentary environments (Jezek 1990; Aceñolaza & Aceñolaza 2003). As a consequence, lithological correlation, dating and precise identification of sequences in the Puncoviscana basin are highly problematic, with the only clear data points provided by trace fossils and rare geochronological input. Based on these studies, it is clear that at least part of the Puncoviscana Formation is Early Cambrian in age (Aceñolaza et al. 1999; Omarini et al. 1999b with references) (Figs 1, 2, 3, 4).
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This contribution summarizes the current knowledge of the Neoproterozoic–Early Cambrian transition in the Central Andean Basin: the Puncoviscana Formation and related units. It focuses on NW Argentina, where exposures are best developed, and the only region where enough data is available to allow a well-defined lithostratigraphic framework centred on the Proterozoic–Phanerozoic transition.
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The Neoproterozoic–Early Cambrian in the South American Central Andean Basin |
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TopAbstractThe Neoproterozoic-Early...The Puncoviscana Formation of...Fossils and ageFinal remarks and conclusionsReferences |
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In Argentina, the northern outcrops are dominated by very low- to low-grade metasedimentary rocks, whereas southwards gneisses and migmatites are more common, all having been affected by Middle Cambrian to Early Ordovician plutonism (Rapela et al. 1992, 1998; Keppie & Bahlburg 1999). Ramos (1999) noted that there is no consensus in interpretation of the tectonic setting of the Puncoviscana Basin. Ideas range from: it being a passive margin (Jezek et al. 1985; Jezek 1990); to a closing ocean with collision and subduction (Ramos 1988); rifting processes (Omarini & Sureda 1993; Suárez Soruco 1989, 2000; Aceñolaza & Alonso 2001); a foreland basin (Kraemer et al. 1995; Keppie & Bahlburg 1999); or even part of an island arc (Omarini et al. 1999).
In addition, a spectrum of environmental settings has been identified in the Puncoviscana Formation, with an eastern ‘Nereites belt’ characterized by a particular trace fossil associated with current-rippled sandstones, turbidites and conglomerates from a shallower setting; and a western ‘Oldhamia belt’, with relatively deeper-water facies associated with turbidites, shales and pelagic clays (Salfity et al. 1975; Jezek et al. 1985; Jezek 1986, 1990; Willner 1990; Durand & Aceñolaza 1990; Durand 1993; Aceñolaza & Aceñolaza 2001). Neither belts is characterized by archetypal ichnofacies. Contrary to recent suggestions by Buatois & Mángano (2003b), neither Jezek (1990) nor Aceñolaza et al. (1999) documented an exclusive deep water setting for the Puncoviscana Formation. Locally, turbidites and sandstones are associated with limestone units (e.g. Volcán and Las Tienditas Fm), whose depositional setting is contentious; they have been interpreted as olistostromal or autochthonous deposits on submarine swells (e.g. Jezek 1986). Aceñolaza (2005) noted that limestones, conglomerates and shales, to mention only some facies represented in the unit, do not form under the same palaeoenvironmental conditions. The Puncoviscana Formation is considered to be a continental margin wedge succession that was deposited during Neoproterozoic–Cambrian times, consisting mostly of shelf, shelf-edge and slope deposits, with a general deepening trend towards the west (Borrello 1969; Aceñolaza et al. 1999; Aceñolaza & Alonso 2001 with references).
The facies complexity with tectonically-obscured relationships makes interpretation of the stratigraphic succession difficult (Mon & Hong 1996). In addition, the existence of igneous material interbedded in the Puncoviscana Formation at several localities in Salta and Jujuy (Toselli & Aceñolaza 1984; Chayle & Coria 1987; Manca et al. 1987; Toselli & Rossi 1990; Coria et al. 1990; Omarini et al. 1993) records eruptive events that occurred during the deposition of the unit. Unfortunately, dates for these different magmatic rocks are controversial, highlighting the need of more detailed geochemical investigation (see Omarini et al. 1999).
Several attempts have been made to order the sequences in the Puncoviscana Basin (Salfity et al. 1975; Baldis & Omarini 1984; Moya 1998), but most of the studies are local to the Valle de Lerma in Salta Province and should not be applied to the basin as a whole.
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The Puncoviscana Formation of NW Argentina |
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TopAbstractThe Neoproterozoic-Early...The Puncoviscana Formation of...Fossils and ageFinal remarks and conclusionsReferences |
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Even though the term of Puncoviscana Formation (s.l.) in northern Argentina has been applied to the Neoproterozoic–Lower Cambrian metasedimentary basin in the region, several other questions and inconsistencies arise as detailed work proceeds (Aceñolaza et al. 1999; Durand & Aceñolaza 1990; Ramos 1999; Sial et al. 2001; Buatois & Mángano 2003a, b) (Fig. 2).
Taking into account that the highly deformed siliciclastic rocks of the type area are depleted of biogenic structure and fossils, and that similar sequences located to the south and west (e.g. Valle de Lerma, San Antonio de los Cobres in Salta and Choromoro in Tucumán), contain fossils, correlation is rather complex. Any correlation must be based on detailed and precisely located information. Lithology changes laterally and rapidly, with common interbeds of conglomerates, limestones and vulcanites; their stratigraphic position is unclear due to the strong deformation of the whole complex. The mentioned differences, far from contributing to a clear undertanding of the unit, only increase the complexity of this regional Neoproterozoic–Cambrian transition panorama further.
Omarini (1983), Omarini & Baldis (1984) and Jezek (1986) discussed sedimentary characters of the Puncoviscana Basin, distinguishing lithological types based on texture and internal structures: (a) coarse-grained flysch-like turbiditic sandstones; (b) thick and monotonous sequences of argillites, siltstones, and sandstones; (c) minor diamictites and polymictic conglomerates; and (d) isolated shallow water micritic limestones, reflecting a wide spectrum of sedimentary settings for the Puncoviscana Basin.
In addition, Omarini et al. (1999) distinguished at least three episodes within the succession: a basal sequence formed primarily by immature epiclastic sediments associated with ultra-potassic dykes (with interbedded mantle-related volcanic flows); shelf deposits with near-shore sandstones (associated with alkalic and tholeiitic lavas); and an upper sequence with flysch-like, siliciclastic deeper water facies (associated to minor pyroclastites, volcanic layers, breccias and granites). Jezek (1990) provided a detailed sedimentological analysis of the Puncoviscana Formation in Tucumán and Salta provinces, distinguishing several sedimentary facies: Facies 1: ‘conglomerate facies’ described from various localities in Salta Province, are characterized by pebbly sandstones/mudstones and organized/disorganized conglomerates representing different detritic flows in the basin; Facies 2: ‘proximal facies’ characterized by dominance of psammites, representing braided river channels, common in Tucumán Province; Facies 3: ‘intermediate’ with mid- to outer fan prograding lobes including a wide spectrum of sedimentary structures; Facies 4: ‘distal facies’ with low density and low-velocity turbidites; and Facies 5: dominated by hemipelagic slates. Trace fossils and wrinkle structures are common in facies 2, 3, 4; and all are well represented in the northern sector of the Puncoviscana Basin.
Interbedded dark grey limestone facies are known to occur in the region, with notable outcrops at Volcán, León, Tumbaya (Jujuy) and Las Tienditas (Salta). Recent study of these carbonate bodies (Toselli et al. 2005) indicate similar 
13C signatures for Las Tienditas and Tumbaya limestones and dolomites, with –1.33 to 2.28% PDB for the latter ones; whereas Volcán and León carbonates display higher values, between –6.11 to 4.58% PDB, probably representing older ages (possibly Sturtian?). Toselli et al. (2005) note the possibility that the Las Tienditas and Tumbaya limestones may represent ‘cap-carbonates’, highlighting that in the latter locality dolomites are deposited above basalt. In both localities, underlying strata have been considered as likely glacial (Sial et al. 2001; Toselli et al. 2005).
Volcanic rocks associated with these carbonate-rich bodies are subalkaline basalts, transitional basalts and alkaline basalts associated with minor dacites and leucitites, either intruding or as lava flows in the sequence. These have been interpreted as a product of rifting associated with the formation of the Puncoviscana Basin, evolving into a passive margin volcanism and a magmatic arc environment (Omarini & Sureda 1999; Omarini et al. 1999).
The regional metamorphism which affected the sequences of the Puncoviscana Basin has been referred to the anchizone with a maximum depth of 6 km, developed between Tucumán Province southwards to the Sierras Pampeanas of Catamarca, La Rioja, Córdoba and San Luis (Toselli 1990; Willner 1990). Contact metamorphism with development of hornfels has been described in different areas of the basin, along edges of plutons intruding the Puncoviscana Formation in Salta and Jujuy, at Cañaní, Tipayoc, Fundición, Santa Rosa de Tastil and Cachi (Kilmurray et al. 1974; Toselli 1990; Lork et al. 1990; Rossi et al. 1992; Omarini et al. 1999).
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Fossils and age |
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TopAbstractThe Neoproterozoic-Early...The Puncoviscana Formation of...Fossils and ageFinal remarks and conclusionsReferences |
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Preservation varies from good to fairly good, depending on lithology. Tectonism has played a primary role in preservation. It is almost impossible to distinguish any biological signature where cleavage strongly superimposes primary bedding. Ichnofossils occur throughout the basin, except in the northernmost outcrops, where tight folds and the near-absence of traces precludes dating and correlation of strata. Body fossils are located within the shallower eastern belt (‘Nereites belt’ of Aceñolaza & Durand 1973, 1986; Aceñolaza 1990), which implies better environmental conditions and preservation patterns for organisms in these facies.
More than twenty localities in Jujuy, Salta, Tucumán, Catamarca and La Rioja provinces have yielded a moderately diverse ichnofauna, with locomotion traces, feeding and grazing furrows, trails and tracks, denoting significant biological activity in the Puncoviscana Sea during the Neoproterozoic– Early Cambrian (Fig. 1). The fossils that occur in sediments deposited during this time include several whose taxonomic validity is debated, so the status of several trace fossils described previously have been subject to re-evaluation (i.e. Aceñolaza & Aceñolaza 2001, 2003). Abundance of trace fossils varies, according to facies, and the fossils themselves do not represent typical ichnofacies. The entire sequence in NW Argentina has undergone metamorphism and coeval deformation.
A summary of the ichnofauna is mentioned below, with comments on the taxonomy where possible. Early attempts, with different goals, have been published before (Buatois & Mángano 2003a, b), with assignments broadly recognized but not universally accepted (Aceñolaza & Aceñolaza 2001, 2003; Aceñolaza 2005).
The Puncoviscana ichnofauna includes: Archaeonassa fossulata, Asaphoidichnus sp., Cochlichnus anguineus, Didymaulichnus lyelli, Dimorphichnus obliquus, Diplichnites sp., Glockerichnus sp., Helminthoraphe sp., Helminthopsis abeli, H. tenuis, Helminthoidichnites tenuis, Monomorphichnus lineatus, M. isp., cf. Multipodichnus,Nereites saltensis(non-Psammichnites saltensis of Seilacher et al. 2005; Aceñolaza & Aceñolaza 2006), Neonereites uniserialis, N. biserialis, Oldhamia alata, O. antiqua,O. curvata,O. flabellata, O. geniculata, O. radiata, cf. Thalassinoides isp., Palaeophycus tubularis, Palaeophycus isp., Protichnites isp., Protovirgularia isp., Tasmanadia cachii, Treptichnus isp., Treptichnus cf. aequalternus and T. pollardi, and a variety of undetermined arthropod-related scratch marks and isolated imprints (Durand & Aceñolaza 1990; Aceñolaza et al. 1999; Aceñolaza & Tortello 2003; Buatois & Mángano 2003a, b; Aceñolaza 2004; Seilacher et al. 2005).
The poorly preserved material assigned to Circulichnis montanus (Buatois & Mángano 2003a) displays ambiguous morphological characters and lacks diagnostic characters, precluding precise taxonomic assignment. Even though the ichnogenus Treptichnus is recognized in the unit (Aceñolaza & Alonso 2001; Aceñolaza 2004), the taxonomic status of Phycodes/Treptichnus/Trichophycus is still a matter of debate, and the taxonomy of these three forms should take into account early interpretations of open burrows or feeding structures (Erdogan et al. 2004).
The presence of large-sized, irregular, three-dimensional burrow systems with few dichotomous bifurcations allows the identification of possible trapping or farming networks in the bases of sandstone layers associated with turbidites in San Antonio de Los Cobres (Salta Province). These networks are comparable to Thalassinoides isp., where as individual burrows can be identified as large-sized Palaeophycus tubularis (Figs 4, 11–13). Branching can be falsely assumed when in reality the traces are superimposed or crossing traces, but undoubted dichotomizations were recorded after a detailed pattern analysis of a single surface of over 20 square metres at one locality. The strata at San Antonio de los Cobres are characterized by a slightly-reworked sedimentary fabric, with a low overall degree of bioturbation restricted to the sandstone–mudstone interval (Durand & Aceñolaza 1990; Buatois & Mángano 2003b). These traces may represent an early attempt to develop possible trapping methods as feeding strategies. Traces penetrate the substratum up to 8 cm, a remarkable depth when compared to the associated traces restricted to the upper few millimetres (Cochlichnus, Oldhamia, Helminthopsis, Diplichnites and small Palaeophycus).
In general, the ichnological record preserved in the Puncoviscana Formation includes diverse ethological variants, ranging from grazing to feeding and locomotion structures possibly made by vermiform organisms and arthropods. The possible presence of trapping structures in the locality of San Antonio de los Cobres reflects a particular feeding strategy not recorded before in the Early Cambrian of South America.
As stated by Fedonkin (1992), the systematic position of Ediacaran metazoans is controversial. Several interpretations have been presented on their relationships. Glaessner (1984) placed Precambrian animals in a framework of Phanerozoic invertebrates, but Seilacher (1984, 1989) stressed the uniqueness of Precambrian organisms with no modern analogues. Fedonkin (1983, 1985, 1987) supported a different taxonomic arrangement for Precambrian metazoans based on body plans and further suggested that the metazoan fossil record of the Late Proterozoic, with its much lower diversity than today might be, a preservational artefact (Fedonkin 1992).
Aside from the relatively good record of trace fossils, two undoubted body fossils have been recognized in the Puncoviscana Formation after almost 30 years of palaeontological research: Selkirkia sp. (=Sphenothallus? sp., in Aceñolaza 2004) occurs in outcrops near Choromoro village in Tucumán Province (García Bellido & Aceñolaza 2005), and well preserved samples of Beltanelloides sp., were recovered at Purmamarca village in Jujuy Province. Early references to ‘Beltanelliformis, Sekwia and Nemiana’ have been re-assigned to Beltanelloides sp., following the detailed revision of Leonov (2007) (Aceñolaza et al. 1999, 2005, with references).
Other biologically induced structures
The knowledge of Neoproterozoic–Early Phanerozoic siliciclastic environments and their biological signature has dramatically increased during the last fifteen years. The Puncoviscana Formation displays some unique surface morphologies that were historically referred as to wrinkle structures associated to certain lithofacies (i.e. Durand et al. 1994; Aceñolaza et al. 1999 with references). During the 1990s, these structures were re-evaluated as microbial mat-related structures, following ideas of Seilacher & Pflüger (1994), Seilacher (1999), Noffke et al. (2001, 2002) and Pflüger (1999). In the case of the Puncoviscana Formation, Omarini et al. (1999) were the first to propose these ideas, followed by Buatois et al. (2000), Aceñolaza & Aceñolaza (2001, 2003) and Aceñolaza (2004), supporting a microbial mat-related lifestyle of the fauna.
So far, the only reliable chronological indicators in the Puncoviscana Basin are the Early Cambrian fossils restricted to certain strata, as well as some limited geochronological data (for summaries, see Aceñolaza & Durand 1986; Do Campo 1999; Omarini et al. 1999; Sureda et al. 1999). Lork et al. (1990) noted a 530 to 560 Ma date for detritic zircons in the Puncoviscana Formation, and Bachmann et al. (1987) presented 536 ± 7 Ma and 534 ± 9 Ma dates for plutonic bodies intruding Puncoviscana Formation, therefore as a minimum date. In addition, Adams et al. (1990) recorded metamorphic events between 530 to 540 million years ago, where as Cordani et al. (1990) suggested an age ranging from 520 to 538 million years ago, all pointing to a Neoproterozoic–Early Cambrian age for the sedimentation and metamorphism of the unit. Assuming C-isotope stratigraphy is a powerful temporal tool in Precambrian stratigraphy, especially for sediments lacking recognizable fossils (Kaufman 1988), the recent record of a strong C-isotope peak in the upper part of Las Tienditas Formation suggests a Proterozoic–Phanerozoic datum in the upper part of this calcareous unit in the Puncoviscana Formation s.l. (Sial et al. 2001; Toselli et al. 2005). In addition, recent work on these limestone units of Salta and Jujuy (Toselli et al. 2005) suggests that possible Sturtian sequences with ‘cap-carbonates’ at Las Tienditas and Tumbaya, supporting early and ignored interpretations of glacial-related sediments associated to the Puncoviscana Formation (Loss & Giordana 1952) (Fig. 2). This, however, needs a more precise age control.
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Final remarks and conclusions |
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Distinctive facies within this basin, including limestones and conglomerates, should be noted under their original designation (i.e. Volcán Formation, Las Tienditas Formation, El Coro Member, etc.), and the use of the Lerma Group (Salfity et al. 1975) should be restricted to the Valle de Lerma in Salta Province.
New palaeontological finds and new geochronological dating are the most reliable sources that provide a more accurate age definition and better correlation of rock units discussed in this paper. The Early Cambrian trace fossils and body fossils are clearly restricted to certain lithologies within the basin, and the geochronological data points to a Neoproterozoic–Early Cambrian age. Until now, there are no definite indicators, neither palaeontological nor isotopic, that allow a sharper chronological dating of the unit.
The lack of distinct or reference fossil remains (macro- and micro-) in the Neoproterozoic/Cambrian transition interval in the Early Palaeozoic Central Andean Basin, does not allow a biozonation to be defined, allowing correlation to other reference sections (Newfoundland, Siberia, Baltica or China). As recently stated by García Bellido and Aceñolaza (2005), miscorrelation by previous workers occurred because of forcing of unreliable data into a chronological system (systemic divisions, stages, biozones, etc).
The detailed chronostratigraphic and biostratigraphic correlation of the Puncoviscana rock sequence to the Siberian section is not really possible at present due of the lack of palaeontological data that characterize biostratigraphic subdivisions in the Argentine strata. Linkage of the Neoproterozoic–Early Cambrian strata of NW Argentina to other reference sections around the world must be carried out carefully. A global chronostratigraphy for the lowermost Cambrian has been discussed often (Landing 1992). Interregional correlation based on fossiliferous data at a stage-level (Nemakit–Daldynian, Atdabanian, etc.) is highly speculative in NW Argentina, and names are irrelevant because of the lack of index fossils in the Andean Margin of South America. As noted by Landing (1992), Siberian stages reflect a long-term evolutionary development and immigration of species and communities that define a warm-water faunal province for the Siberian Basin: a very different setting that that of the Argentine sequences and other reference sections around the world (i.e. Avalon).
Early records of Beltanelliformis, Sekwia and Nemiana in the Puncoviscana Formation should be synonymized with Beltanelloides sp., and a new fossiliferous locality noted in the province of Jujuy. Material there is preserved in highly tectonized sandstones and shales, that crop out near the village of Purmamarca (Jujuy Province), and yield well-preserved samples of Beltanelloides sp. (an element that characterizes Vendian (Ediacaran age) biota of Siberia and those of similar age from other regions of the world (Figs 3, 8–9)).
The presence of microbially-induced structures, mostly restricted to finer-grained lithologies in the Puncoviscana Formation, stresses the importance of sediment binding in the earliest Cambrian, prior to the general substrate revolution (matgrounds being replaced by mixgrounds).
The occurrence of dichotomous traces assigned to cf. Thalassinoides network systems in the sediments of San Antonio de los Cobres (Salta Province), adds an interesting new feeding strategy in the Early Cambrian on the seafloor of the Puncoviscana Sea, with possible primitive trapping structures not previously reported in this unit.
Further efforts are necessary to integrate structural, geochemical, sedimentological and palaeontological data known from the Puncoviscana Formation. Systematic studies and re-evaluation of published material from the in the Puncoviscana Basin, are the only way to obtain reliable data for an in-depth understanding of the Neoproterozoic–Cambrian transition in the Andean margin of South America.
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Acknowledgments |
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References |
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