Florencio G. Aceñolaza

Proterozoic–Early Paleozoic evolution in western South America—a discussion☆

Abstract The hypothesis of exotic terranes in Peru, Bolivia, Argentina and Chile generated discussions on the mode of transfer and extent of accretional events that may have occurred in the southern Andes during the Late Proterozoic–Early Paleozoic. Initially, a tectogenesis based on autochthonous mobile fold belts was discussed. Following ideas emphasised the fragmentation of the supercontinent Rodinia, Laurentia moving along the West Gondwana border and colliding with the Gondwana western margin. The most important effect of this Laurentia/Gondwana relationship was attributed to the Argentine Precordillera (or Cuyania) terrane splitting off from Laurentia and docking to Gondwana in the Early Paleozoic. In this study, the most cited arguments for this Laurentia/Precordillera relationship are discussed, emphasising paleontological considerations. It is shown that these arguments do not exclude a close original vicinity of the Precordillera terrane to Gondwana. The Precordillera terrane is suggested to be part of a hypothetical platform, which developed between South America, Africa and Antarctica (SAFRAN platform), and which was displaced to its actual position by transcurrent faults. The collisional events in the Sierras Pampeanas ensued from strike–slip movements and were responsible for the S and I type transpressional magmatism along the Pampean and Famatinian terranes. The final result of this continent-parallel movement of terrane slices is similar to that of a terrane split off from Laurentia, but the first-named way of formation easier explains the general continuity of plate convergence at the western border of Gondwana than the Laurentia/Precordillera connection does.

The Puncoviscana trough — a large basin of Late Precambrian to Early Cambrian age on the pacific edge of the Brazilian shield

During the Late Precambrian to Early Cambrian a large basin with a homogeneous psammitic-pelitic sediment fill existed in the area of the NW-Argentine Andes. It is now exposed in different tectonic levels. This basin of meridional elongation was situated on a stable continental margin at the western edge of the Brazilian shield. It was underlain by segmented older continental crust. According to the modal and chemical composition of the greywackes and subgreywackes a predominant metasedimentary source may be supposed. Four clastic facies types indicate a transport by gravity currents within a submarine fan system, while massive red pelites represent cut-off periods, and rare carbonates deposition on rises. The gravity currents derived from easterly directions. The sedimentation age is proved by a rich ichnofauna and some medusoid impressions.ZusammenfassungIm Jungpräkambrium bis Unterkambrium existierte im Bereich der NW-argentinischen Anden ein ausgedehnter Trog mit homogener psammitisch-pelitischer Füllung, die heute in verschiedenen Gebirgsstockwerken aufgeschlossen ist. Dieses Becken lag an einem stabilen Kontinentalrand im Westen des Brasilianischen Schildes und war von segmentierter älterer, kontinentaler Kruste unterlagert. Nach der modalen und chemischen Zusammensetzung der Grauwacken und Subgrauwacken sind überwiegend metasedimentäre Herkunftsgesteine anzunehmen. Vier klastische Faziestypen lassen auf Transport durch Trübeströme in submarine Fächer schließen, während mächtige Rotpelite Ruhezeiten und die seltenen Karbonate Ablagerungen auf Schwellen anzeigen. Die Schüttungen stammen aus östlichen Richtungen. Das Sedimentationsalter ist durch eine reichhaltige Ichnofauna und einige Medusenabdrücke dokumentiert.RésuménDurante el Precámbrico superior hasta el Cámbrico inferior en los Andes del NW Argentino se extendía una larga cuenca rellena de sedimentos psamo-pelíticos homogéneos, que hoy afloran en distintos pisos tectónicos. Esta cuenca de elongación meridional estaba situada en un margen continental estable al lado occidental del escudo brasileño, sobre corteza continental segmentada más antigua. Según su composición modal y química, las grauvacas y subgrauvacas derivaron predominantemente de rocas metasedimentarias. Cuatro tipos de facies clásticas indican su transporte en corrientes de gravedad, provenientes del este, y sedimentación en conos submarinos. Pelitas rojas macizas representan períodos de sedimentación pelágica, mientras que los escasos carbonatos se depositaron en dorsales. Una icnofauna amplia y algunas impresiones de medusoides documentan la edad de sedimentación.Краткое содержаниеВ регионе северо-запа да аргентинских Анд о т позднего докембрия до нижнего кембрия существовал обширный трог, заполн енный гомогенными псаммито-пелитовыми отложениями, которые сегодня обнажаются в составе различных тектониче ских этажей. Этот бассейн н аходился на стабильн ом материковом крае на западе бразил ьского щита и его подстилала более дре вняя осалочная матер иковая кора. По модальному и х имическому составу г раувак и субграувак они явля ются породами преиму щественно осадочно-метаморфич еского происхождени я. Наличие четырех типо в кластических отлож ений позваляет предполаг ать перенос их мутевн ыми потоками, расходивши мися под водой веером, в то время, как мощные крас ноцветные пелиты ука зывают на период спокойстви я, а редке отложения ка рбонатов на порогов. Область сн оса, вероятно, находил ась на востоке. Возраст от ложений определяют здесь с помощью богат ой ихнофауны и по отде льным отпечаткам медуз.

Permian nonmarine invertebrate trace fossils from southern Patagonia, Argentina: Ichnologic signatures of substrate consolidation and colonization sequences

Floodplain deposits of the Permian Laguna Polino Member (La Golondrina Formation) of southern Patagonia, Argentina, contain a moderate diversity invertebrate ichnofauna. This member consists of up to 500 m of ripple cross-laminated and trough cross-bedded, medium-grained sandstone and mudstone, with some conglomerate. It records sedimentation in braided fluvial environments with moderate channel sinuosity. The system was characterized by the development of multistory channels, transverse or linguoid bars, and levees. At the Laguna Grande locality, trace fossils were recovered from current-rippled, very fine-grained muddy sandstone beds formed in a pond adjacent to the levee. The ichnofauna consists of a mixed pascichnia, repichnia, and domichnia assemblage composed of Cochlichnus anguineus, Ctenopholeus kutscheri, Helminthoidichnites tenuis, Helminthopsis abeli, and Palaeophycus striatus. These low-energy ponds were more suitable for development of benthic fauna and preservation of ichnofossils than were fluvial channels and point bars. Accordingly, preservation of distinct trace fossil-bearing pond horizons within an otherwise unfossiliferous fluvial succession may be regarded as taphonomic and colonization windows. The colonization history of these beds can be traced through the study of cross-cutting and substrate relationships. Detailed ichnologic analysis suggests the following succession: 1) large Cochlichnus tracemakers colonizing transitional soupgrounds to softgrounds; 2) large and small Cochlichnus, Helminthopsis and Helminthoidichnites formed in softgrounds; and 3) Palaeophycus and Ctenopholeus emplaced in transitional soft to firm substrates. The colonization sequence suggests an ecologic replacement of a surface mobile fauna dominated by nematodes, nematomorphs, and larval insects by a more stationary infauna consisting of adult insects as a result of increasing consolidation. Presence of Ctenopholeus is significant because it represents the second report of this ichnogenus and the first from nonmarine settings.

Nonmarine perigondwanic trace fossils from the late Paleozoic of Argentina

The upper Paleozoic nonmarine ichnofauna from Puna, Paganzo and San Rafael basins (northwestern and western Argentina) is divided into two major associations: Carboniferous deep‐glacio‐lacustrine (Didymaulichnus assemblage) and Permian fluvial/shallow‐lacustrine trace fossil assemblages (Ancorichnus, Umfolozia and Isopodichnus ichnocoenoses). The Didymaulichnus assemblage is dominated by arthropod locomotion traces, grazing trails being common also. It is present in laminated siltstones deposited in a deep, oxygenated, glacial lake. The Ancorichnus ichnocoenosis is dominated by arthropod feeding structures in a floodplain situation. The Umfolozia ichnocoenosis is dominated by arthropod trackways in a shallow lake. The Isopodichnus ichnocoenosis consists of resting and locomotion traces of arthropods and bivalves in shallow ephemeral pools. This ichnocoenosis is a common feature of shallow ephemeral lake, channel and floodplain deposits in the stratigraphic record. Similar ichnofossils have been reported f...

Sedimentary facies and environmental ichnology of a ?Permian playa-lake complex in western Argentina

Abstract A moderately diverse arthropod ichnofauna occurs in ?Permian ephemeral lacustrine deposits of the Paganzo Basin that crop out at Bordo Atravesado, Cuesta de Miranda, western Argentina. Sedimentary successions are interpreted as having accumulated in a playa-lake complex. Deposits include three sedimentary facies: (A) laminated siltstone and mudstone; (B) current-rippled cross-laminated very fine grained sandstone; and (C) climbing and wave-rippled cross-laminated fine-grained sandstone deposited by sheet floods under wave influence in the playa-lake complex. Analysis of facies sequences suggests that repeated vertical facies associations result from transgressive-regressive episodes of variable time spans. The Bordo Atravesado ichnofauna includes Cruziana problematica, Diplocraterion isp., cf. Diplopodichnus biformis, Kouphichnium ? isp., Merostomichnites aicunai, Mirandaichnium famatinense, Monomorphichnus lineatus, Palaeophycus tubularis, Umfolozia sinuosa and Umfolozia cf. U. longula . The assemblage is largely dominated by arthropod trackways and represents an example of the Scoyenia ichnofacies. Trace fossils are mostly preserved as hypichnial ridges on the soles of facies C beds, being comparatively rare in facies A and B. Ichnofossil preservation was linked to rapid influx of sand via sheet floods entering into the lake. Four taphonomic variants (types 1–4) are recognized, each determined by substrate consistency and time averaging. Type 1 is recorded by the presence of low density assemblages consisting of poorly defined trackways, which suggests that arthropods crawled in soft, probably slightly subaqueous substrates. Type 2 is represented by low to moderate density suites that include sharply defined trackways commonly associated with mud cracks, suggesting that the tracemakers inhabited a firm, desiccated lacustrine substrate. Type 3 displays features of types 1 and 2 and represents palimpsestic bedding surfaces, resulting from the overprint of terrestrial ichnocoenoses over previously formed softground suites. Type 4 differs from type 2 only in that assemblages display a high density of traces, recorded by numerous superimposed trackways, which suggests a major time gap of subaerial exposure before sheet flood entrance. Therefore, type 4 surfaces are mostly interpreted as track imprinted omission surfaces.

Chapter 6 The Pampean Orogen: Ediacaran-Lower Cambrian Evolutionary History of Central and Northwest Region of Argentina

Abstract The Ediacaran-Lower Cambrian Pampean Orogen comprises a large area in northwest Argentina, which formed the western margin of Gondwana. This orogen evolved from an aulacogenic structure, which determined the outline of the large ‘Puncoviscana Basin’. This basin was bounded by the Meso- to Neoproterozoic cratonic areas of the Rio de la Plata, Guapore (West Amazonia), and the ‘Arequipa Terrane’. A thick siliciclastic succession with subordinate carbonates and conglomerates was deposited. Body and trace fossils occurring in the siliciclastic sequence allow to assign it to the Ediacaran-Lower Cambrian. Carbon isotopes show that limestones represent different stratigraphic levels. Radiometric ages show that the deformational event F1 and the metamorphism M1 and M2 range from Lower to Middle Cambrian. Detrital zircon ages indicate a close relationship with the peripheral shield rocks. Current evidence supports an autochthonous origin for the Pampean Orogen, contrasting with other ideas supporting an assumed exotic block that collided with West Gondwana.

Chapter 9.3 Trace Fossils and the Agronomic Revolution at the Neoproterozoic-Cambrian Transition in Southwest Gondwana☆

Abstract The Ediacaran-Early Cambrian transition in the southwestern margin of Gondwana is represented in the Andean Margin of South America by the thick sequence of the Puncoviscana Basin. In southern Africa, the Nama and Vanrhynsdorp groups display partially equivalent successions. Trace fossil assemblages are well known from both regions, with relatively simple, low diversity forms in the lower stratigraphic levels. An increase in complexity is recorded in the upper sector of the Andean and African sequences, with a variety of Treptichnus -type traces that denotes a more gradual diversity increase towards the Cambrian than generally accepted. Trace fossil associations indicate that the fossiliferous levels of the Puncoviscana Basin may predate the uppermost lower Cambrian (pre-trilobite) successions of the Nama and Vanrhynsdorp groups, and lasted into the ‘trilobitic’ Cambrian. Intergradations between Treptichnus and different ichnogenera cast doubts on using related ichnospecies in Ediacaran-Early Cambrian biozonation.

Basin evolution of the margin of Gondwana at the Neoproterozoic/ Cambrian transition: the Puncoviscana Formation of Northwest Argentina

The Puncoviscana Formation is an important stratigraphic sequence of sandstones and shales, with intercalated conglomerates, limestones and volcanic rocks. It was deposited at the Neo- proterozoic-Cambrian boundary in a shallow basin on the western margin of Gondwana. Trace fos- sils of the Puncoviscana Formation s.l. are represented by the Oldhamia and Nereites ichnoassocia- tions and the geographical distribution of assemblages are aligned as parallel belts, with a shallower eastern Nereites association and a deeper western Oldhamia association. They represent different temporal levels on the evolution of the basin. Detrital zircon spectra display a wide range of ages, which indicate their provenance from Neoproterozoic to Cambrian source areas as well as Meso- to Paleoproterozoic basement units, exhibiting typical Gondwanan ages. Thick limestone banks are interbedded in the Puncoviscana Formation s.l. that included the siliciclastic sequences, with very low- to medium-metamorphic grade. δ13C values for the limestones vary from -1.57 to +3.40/00 VPDB, while in the Sierras Pampeanas, δ13C values vary from +2.6 to +80/00 VPDB. Also, reported 87Sr/86Sr values allow a clear differentiation of two rock sets. The limestones interbedded with the Puncoviscana Formation yielded values typical for the Neoproterozoic-Cambrian transition, between 0.70868 and 0.70896; while the limestones of the Sierras Pampeanas display values between 0.70748 and 0.70756, common in the middle to late Ediacaran. Deformation and uplift of the Puncoviscana Formation is clearly represented in northern Argentina by the Tilcarian unconformity, which is over- lain by the Meson Group.