Neoproterozoic-Cambrian Transition Research Articles

Neoproterozoic?Cambrian synsedimentary magmatism in the Central Iberian Zone (Spain): geology, petrology and geodynamic significance

Petrographic, geochemical and field studies in low grade metamorphic areas (Ciudad Rodrigo-Hurdes-Sierra de Gata domain, CRHSG, central-western Spain) show that Neoproterozoic-Lowermost Cambrian series in the Central Iberian Zone (CIZ) record two kinds of provenance sources including: (1) detrital material derived from recycled orogens and (2) a Cadomian coeval juvenile contribution that governs their isotopic signature. Evidence of magmatism contemporaneous with Neoproterozoic-Cambrian sedimentation is provided by the presence of coherent, massive volcanic rocks (metabasalts, metaandesites, and metarhyolites), volcaniclastic shales, sandstones, conglomerates and breccias. The appearance of volcanogenic lithic fragments and crystals mixed in different proportions with siliciclastic constituents and also present within calcareous components in the sedimentary succession, reinforces this evidence. Although most of the selected volcanic and volcaniclastic samples appear to show tholeiitic affinity, some of them display calc-alkaline affinity. Different trace element ratios, such as Sm/Nd, Nb/Yb and Ta/Yb, suggest a magmatic evolution in the same tectonic setting. The geochemical results reported here support the existence of an active geodynamic setting as a direct contributor to the synsedimentary and magmatic content of the Neoproterozoic–Lowermost Cambrian successions in the CIZ. In particular, the relatively high eNd (T) values and the high range of f Sm/Nd ratios are consistent with an active margin during the Neoproterozoic–Early Cambrian. The existence of tectonic activity is also confirmed by the presence of synsedimentary deformation and volcanic rocks. All of these traits favour a geodynamic model in which the Iberian Cadomian segment represented in the CIZ would have been part of an active northern margin of Gondwana, with an associated magmatic arc and related basins during Neoproterozoic–Lower Cambrian times. A proposed link between the Ossa Morena and the Central Iberian Zones might account for late Cadomian pull-apart basins developed on both sides of the magmatic arc, sharing the same scenario and involving similar magmatic activity during the Neoproterozoic–Cambrian transition.

Is the Cambrian basin of the Western High Atlas (Morocco) related either to a subduction zone or a major shear zone?

By its geological and structural position, the Palaeozoic massif of the western High Atlas constitutes a hinge between the Anti-Atlasic domain in the South and the domain of Meseta in the North. The upper Proterozoic and lower Palaeozoic formations of the Anti-Atlas and the Cambrian successions of coastal Meseta are well known, while a synthetic vision of the western High Atlas coeval sequences is lacking. Recent field and analytical work performed on low-grade metamorphic rocks of the Moroccan “High Atlas old Massif” allows us to distinguish several sedimentary sequences and interbedded (essentially effusive) magmatic rocks, the geochemical characterization of which forms the main aim of this paper. By comparison with adjacent areas, the age of the metasedimentary rocks is attributed to the late Neoproterozoic and lower Palaeozoic. The volcanic rocks have calc-alkaline (late Neoproterozoic and lower Cambrian), alkaline and/or tholeiitic (Middle Cambrian) affinities. The results of this study impose some constraints on understanding the palaeogeographic links between the Anti-Atlas and the Moroccan Meseta during the Neoproterozoic–Cambrian transition.

Stratigraphic and structural framework of the Neoproterozoic Paracuellos Group, Iberian Chains, NE Spain

Abstract The Neoproterozoic Paracuellos Group of the Iberian Chains constitutes the core of two disconnected faulted blocks, named the Paracuellos and Codos antiforms. Precise lithostratigraphic correlations between both areas are not possible due to the structural complexity and because marker beds do not persist laterally. This paper presents a crustal cross-section of the Neoproterozoic axial core (the Paracuellos antiform) based on surface geology, boreholes and seismic reflection profiles. Seismic reflection data reveal that the basement was directly involved by a major Hercynian structure, named here the Paracuellos fault, which splits longitudinally the Paracuellos axial core. In seismic profiles this fault occurs as a northeasterly-dipping reflector (60–70° steep), evidencing a bivergent geometry of the lateral crustal elements. The sedimentary evolution of the Neoproterozoic Iberian platform ranges from transgressive, non-cyclic, offshore to hemipelagic, black and green shales (Sestrica Formation) to progradational trends recording shoaling during episodes of rapid sediment influx (Saviñán Formation), presumably in response to a low standing sea-level. The siliciclastic succession is punctuated in the inner platform by deposition of phosphatic limestones (Codos Bed), representing a major shoaling event and demarcating a sharp regional change of sedimentation separating two similar siliciclastic tendencies. A diagenetically induced bedded chert (Frasno Bed) occurs in the outer platform, and is interpreted as being the product of at least two silicification episodes. Both the Codos and Frasno Beds are overlain by the Aluenda Formation, which exhibits nearshore to offshore features. An important sedimentary discontinuity appears across the Neoproterozoic-Cambrian transition. The Cambrian(?) Bámbola Formation is paraconformable with the Paracuellos Group displaying a gradual transition in inner platform areas, whereas an erosive unconformity occurs in outer areas. The horizon of the Neoproterozoic-Cambrian boundary is not identified in the Iberian Chains, where neither Cadomian deformation nor discordances are recognisable.

Towering sponges in an Early Cambrian Lagerstätte: Disparity between nonbilaterian and bilaterian epifaunal tierers at the Neoproterozoic-Cambrian transition

Epifaunal, suspension-feeding bilaterian animals in the Cambrian lived close to the sediment-water interface, and hence their ecological tiering levels were low (<10 cm). Here we report an Early Cambrian (Diandongian or probably Tommotian-Atdabanian) Lagerstatte from the Hetang Formation in Anhui Province, south China. The Hetang biota is characterized by high-tiering (to 50 cm) sponges and small (<0.5 cm) bilaterians (including orthothecid hyoliths and bivalved arthropods). Nonbilaterian suspension feeders (sponges, cnidarians, and archaeocyathids) as high-tiering animals and bilaterian suspension feeders as low-tiering animals also characterize other Neoproterozoic-Cambrian assemblages, such as the Ediacaran, Chengjiang, Burgess Shale, and Sinsk biotas. These data are consistent with medium- to high-tiering levels in Neoproterozoic-Cambrian epifaunal communities, but suggest that nonbilaterians achieved such tiering levels long before bilaterian suspension feeders did so in the Early Ordovician. The disparity between bilaterian and nonbilaterian tierers during the Neoproterozoic-Cambrian transition and the delayed appearance of high-tiering bilaterians demand phylogenetic and ecological explanations. The Cambrian substrate revolution may have triggered a cascade of ecological evolution, including the rise of bilaterian animals in high-tiering levels during the Ordovician radiation of the Paleozoic fauna.

Estratigrafia e correlações da Bacia de Pouso Alegre, transição Neoproterozóico-Cambriano, Minas Gerais, Brasil

Os depósitos sedimentares da região de Pouso Alegre e outros remanescentes de bacias eminentemente terrígenas ocorrem ao longo do Cinturão Ribeira, desde o Estado de Minas Gerais até o Paraná. A análise das fácies sedimentares e dos sistemas deposicionais permitiram discriminar na bacia de Pouso Alegre três unidades estratigráficas representadas por associações de fácies que caracterizam depósitos subaéreos de leques aluviais; depósitos deltaicos (fan-delta)e depósitos de plataforma proximal. Nos ambientes subaquosos, esses depósitos foram retrabalhados pela ação de correntes de marés, por ondas normais e, eventualmente, de tempestades. Nos depósitos prodeltaicos foram identificados os acritarcos Soldadophycus bossii GAUCHER et al. 1996, Soldadophycus major GAUCHER 2000 e Symplassosphaeridium sp., evidenciando a participação de águas marinhas relativamente bem oxigenadas durante a sedimentação. A correlação com as demais bacias que ocorrem ao longo do Cinturão Ribeira e com outras dos continentes sul-americano e africano é proposta, tendo-se em conta parâmetros estratigráficos, paleontológicos e geocronológicos. Adaptou-se um modelo paleotectônico e paleogeográfico para a evolução do Gondwana Ocidental durante a transição Neoproterozóico Cambriano, em cujo cenário interagiram blocos cratônicos, a colagem de terrenos alóctones e a abertura e fechamento dos oceanos. Após a glaciação Varanger (~600 Ma) e com o aumento das temperaturas do planeta, o nível dos oceanos encontrava-se em ascensão. As águas do Oceano Brazilides penetraram pelas regiões subsidentes meridionais do Cinturão Ribeira, constituindo nível de base para a deposição em bacias extensionais nas porções ocidentais do Orógeno Mantiqueira. Concomitante a estes embaciamentos em mares epicontinentais ocorria a sedimentação em bacias de margens passivas, às bordas dos crátons. A conexão entre os oceanos Brazilides e Adamastor permitiu a disseminação de microfósseis similares, notadamente Cloudina, em quase todas elas. Com a convergência dos blocos cratônicos e o fechamento dos oceanos, ao redor de 530 Ma, as bacias sofreram inversão e encurtamento, como assinalado nas assinaturas isotópicas de rochas do embasamento e dos depósitos.

The nature of the Proterozoic-Cambrian transition in the northern Amadeus Basin, central Australia

The nature and completeness of the rock record is vital in determining whether evolution across the Proterozoic-Cambrian boundary was episodic or explosive. This study documents the nature of the Neoproterozoic-Cambrian transition within the Arumbera Sandstone of the northern Amadeus Basin, central Australia and demonstrates an erosional surface close to this major stratigraphic boundary. We are not able to recognise major facies differences between the largely unfossiliferous lower units I and II and the rich trace fossil bearing beds of units III–IV. Horizons above and below the purported Neoproterozoic-Cambrian transition were traced laterally and searched for both trace and body fossils in an attempt to locate a local boundary between the Neoproterozoic and Cambrian. A single nonconformity bound sedimentary cycle above the major erosional surface dividing Arumbera II and III is recognised. The lowest sedimentary cycle of Arumbera III is devoid of trace fossils, but at correlative levels in the Ooraminna Sub-Basin, this horizon bears elements of the Ediacara fauna, reported for the first time herein. We, therefore, suggest that, contrary to previous studies, the Neoproterozoic-Cambrian transition is not coincident with the widespread seismic reflector but, in areas of more complete sedimentation, actually lies above it. This study also presents new stratigraphic, ichnological and palaeontological information from the Neoproterozoic-Cambrian transition in central Australia, identifies areas of potentially continuous sedimentation and provides a basis for correlation of the Arumbera Sandstone with other Neoproterozoic-Cambrian successions within Australia and beyond.