Neoproterozoic-Cambrian Transition Research Articles

Oceanic and atmospheric oxygenation during the late Ediacaran based on multiple isotope records from South China

The Neoproterozoic–Cambrian transition was a key period in Earth's history, particularly because changes in the oceanic redox environment were crucial to the Cambrian explosion of life. However, the redox state of the ocean during this transition is unclear. In this study, the carbonate strata of the Dengying Formation in Yunnan Province, South China, were subjected to δ98/95Mo, δ13C, 87Sr/86Sr, and trace element analyses. The low δ98/95Mo results (0‰ ∼ +2‰) of most samples indicate that a shallow-marine sedimentary environment dominated by low-O2 conditions during the late Ediacaran period (Dengying Formation), but a brief period of marine oxidation existed at this time, reflected by elevated δ98/95Mo values (+2.07‰ ∼ +2.43‰). At this time, δ13C values were increasing, which possibly indicate an increase in primary productivity and photosynthesis in shallow waters, causing oxidation of the shallow seawater. However, two anomalously high δ98/95Mo values (>2.65‰) could have resulted from additional isotope fractionation due to the upwelling of euxinic bottom waters abruptly. In addition, an increase in 87Sr/86Sr ratios lagged the δ13C and δ98/95Mo changes, suggesting that atmospheric changes appear to have lagged oceanic oxidation, but more evidence are needed to prove the relationship. The dynamic changes in the redox conditions of the shallow sea during deposition of the Dengying Formation provided favorable conditions for changes in the atmosphere and the Ediacaran biota.

U-Pb and Ar-Ar geochronological constraints on timing of deformation and peak metamorphism in the Central Africa Orogenic Belt, Yaoundé Domain, Cameroon

ABSTRACT The Central Africa Orogenic Belt (CAOB) and the Transaharan and Sergipano belts form a major West Gondwana suturing orogen affected by D1–D4 compressional deformation. Microstructural observations, mineral chemistry, zircon, and titanite laser ablation ICP-MS U-Pb dating, and biotite, muscovite, and K-feldspar 40Ar-39Ar age data are used to constrain the timing of rocks formation and D1–D4 events in the Yaoundé Domain of the CAOB, as well as characterize the Neoproterozoic-Cambrian transition. The Yaoundé Domain rocks were deposited on a pre-Neoproterozoic structure that was overprinted by a c. 1000–700 Ma mega arc/back-arc system and related Tonian to Ediacaran magmatism. Older U-Pb dates (1000 up to 2700 Ma) are inheritance from a pre-existing crust. The D1–D4 compressional events relate to the inversion of the c. 1000–700 Ma mega-rift system. New dates at c. 700 Ma, c. 630 Ma, c. 580–555 Ma constrain the timing of D1, D2, D3+D4, respectively. The peak of accompanying metamorphism occurred at 614 ± 4 Ma during D2. This was followed by a c. 115 Ma period of deformation-driven crustal exhumation and cooling that extended into the Cambrian. New data document short episodes of rapid denudation and cooling (up to 0.9 km/Ma and 40°C/Ma at c. 578 Ma in gneiss and up to 5 km/Ma and >125°C/Ma at c. 555 Ma in micaschist). It is proposed that the CAOB and correlative belts are parts of the orogenic hinterland formed from accretion and collision between the West African Craton and the Sahara Metacraton precursor. The correlated belts appear to represent the result of a protracted tectono-metamorphic and magmatic process that started in late Mesoproterozoic (c. 1167 Ma) and continued through the Neoproterozoic to the Cambrian (c. 500 Ma).

Seawater silica cycling and chert formation at the Neoproterozoic–Cambrian transition: Insights from δ30Si and Ge/Si systematics of hydrothermal cherts from the Bohemian Massif

The genesis of cherts, rocks with >90 wt% SiO2, is suggested to be linked with dissolved seawater silica and its elevated precipitation in the Phanerozoic due to massive biological activity in the Earth's oceans compared to the Precambrian where hydrothermal fluids played a more dominant role. Because cherts may record a history of global seawater Si cycle, their Si isotope systematics may serve as a proxy for their origin and depositional environment. In this study, Si isotope compositions, combined with Ge/Si ratios, were determined for a suite of shallow-water, deep-water and stromatolitic cherts from the Blovice accretionary complex (Bohemian Massif, Czech Republic), representative of deep marine and subduction settings, to establish the nature of cherts and processes of their formation, identify the sources of Si and Ge, and discuss the possible effect of the Neoproterozoic–Cambrian transition. The predominantly high δ30Si values > +0.23‰, associated with low Ge/Si, in deep-water cherts suggest their precipitation from seawater-derived low-temperature fluids at the seawater–sediment interface with subordinate admixture of terrigenous material. Moreover, the tight relationship between δ30Si and organic matter contents in these deep-water cherts is indicative of either equilibrium precipitation of isotopically lighter Si as the organic content of the medium increases or global decrease of δ30Si values of seawater during the Meso–Neoproterozoic related to coupled Si and C cycles. On the other hand, a δ30Si variation of ∼2.5‰ (−0.94‰ to +1.60‰) in shallow-water cherts argues for precipitation from seawater interacting with hydrothermal fluids of variable temperature. Organic matter, Fe-(oxy)-hydroxides and clays represent important sinks for Ge (and Si in case of organic matter) and thus play a determining role in global marine Si−Ge cycling. This, together with the presence of abundant cherts at the Neoproterozoic–Cambrian transition, appears to be associated with redox-controlled Fe cycle which itself is related to microbial reduction and the increasing presence of organic matter towards the Phanerozoic.

Phosphatized microbial fossils from the lowest Cambrian of South China and their ecological and environmental implications for the Kuanchuanpu biota

The early Cambrian Fortunian-age Kuanchuanpu Formation exposed in the northwestern margin of South China is well known for recording unique aspects of the “Cambrian explosion”, notably the coexistence of Small Shelly Faunas and soft-bodied meiofaunas. Although microorganisms, both primary producers and decomposers, are important biotic components of marine ecosystems in which they play a crucial ecological role in matter cycling and energy flow, Cambrian microbial records have not been brought into attention so much as their intimately associated faunas. To extend these records here we report a diverse assemblage of cellularly preserved microbial fossils from cherts of the Kuanchuanpu Formation. Nine species comprising six genera and two undescribed taxa have been identified. Among taxa interpreted as primary producers, are Gloeodiniopsis and unnamed form B that are morphologically analogous to coccoidal chroococcacean cyanobacteria, and Megathrix and Siphonophycus to filamentous oscillatoriacean cyanobacteria. In contrast, co-occurring members of endobiotic Endoconchia ? are inferred to be biodegraders. The high preservational fidelity of this microbial assemblage is here documented by use of optical microscopy, scanning electron microscopy, energy dispersive spectrometry, back-scattered electron microscopy and Raman spectroscopy. These analyses results demonstrate that the microbial fossils were organic remains (Kerogen) permineralized by pre-decay phosphatization and then sealed within the encompassing siliceous matrix. This mode of fossilization differs from typical chert-embedded silica-permineralized Proterozoic microfossils in the presence of apatite permineralization, which is regarded as a response to the phosphogenic event during the Neoproterozoic-Cambrian transition.

Application of the apatite oxygen paleobarometer (AOP) across the Neoproterozoic-Cambrian transition

Fluorescence signatures of samarium3+ substituted into apatite for calcium2+ have been shown experimentally to correlate with oxygen availability, such substituted apatite being a principal diagenetic mineral emplaced in shallow-water environments during the Neoproterozoic-Cambrian global phosphogenic event coeval with the “Cambrian Explosion of Life.” The occurrence of Sm3+-substituted apatite, encrusting, infilling and permineralizing components of shallow water microbial communities suggested that quantification of these fluorescence signatures might provide a widely applicable mineral-based paleobarometer by which to measure oxygen concentrations in the environment in which such apatite precipitated. To test this hypothesis, fluorescence spectroscopic analyses have been carried out on the Sm3+-substituted apatite of photic-zone microscopic fossils and fossil-associated ooids of near-shore facies of the Ediacaran Doushantuo (South China) and Early Cambrian Chulaktau (South Kazakhstan) Formations, and of the open ocean apatite-biomineralized phytoplanktonic “scale fossils” (Characodictyon) of the mid-Neoproterozoic (Tonian) Fifteenmile Group (Yukon Canada). Results obtained – quantified by the newly devised apatite oxygen paleobarometer metric – indicate a low (<5% O2) oxygen concentration in the depositional environment of the Ediacaran Doushantuo, a similarly low but slightly higher O2 level in the Early Cambrian Chulaktau, and are consistent with a low oxygen concentration in the scale fossil-evidenced mid-Neoproterozoic photic zone of the open marine environment. By documenting the rise of oxygen across the Neoproterozoic-Cambrian transition these results confirm the efficacy of this newly devised technique and suggest its value for future global application.

Copper, zinc, chromium and osmium isotopic compositions of the Teplá-Barrandian unit black shales and implications for the composition and oxygenation of the Neoproterozoic-Cambrian ocean

Two Neoproterozoic-Cambrian shale suites that consist of normal (NBS) and metal-rich (HMBS) black shales from the Teplá-Barrandian Unit, Bohemian Massif (Czech Republic) are studied in terms of their Cu, Zn, Cr and Re–Os isotopic compositions to improve our knowledge of elemental and isotopic mass balance of these elements in Earth's oceans in the past and the extent of surface oxygenation at the Neoproterozoic–Cambrian boundary. The rock compositions indicate that the inputs of Cu and Zn have been controlled by variable proportions of metal-rich authigenic and low-metal terrigenous sources, the latter having exceptionally low δ65Cu and partially δ66Zn. In this respect, our data confirm previous studies conclusions that organic-rich sediments with high a terrigenous flux play an important role in the sinking of light Zn in the oceans and provide new evidence that these types of rocks also represent the currently unidentified burial of very light Cu in the ocean. The chromium isotopic composition reveals a complex history of Cr uptake and fractionation through the dominant Cr input from different terrigenous sources with variable δ53Cr followed by Cr fractionation that was controlled by redox states (NBS). On the other hand, mixing and/or continuous equilibrium isotopic exchange between seawater and hydrothermal Cr was the dominant process for the HMBS. The Re–Os contents of the NBS were controlled by hydrogenous components, which yield a poorly defined Re–Os age for the least disturbed samples of 555 ± 60 Ma, indicating an open-system behavior, while the HMBS Re–Os composition was largely influenced by hydrothermal fluids during deposition. The calculated authigenic δ65Cu, which is similar to that in the present-day ocean, and highly positive δ53Cr values of the HMBS suggest high levels of surface oxygenation during the Neoproterozoic–Cambrian transition. Furthermore, the authigenic δ66Zn, which is indistinguishable from that in the present-day ocean, may imply a constant Zn isotopic composition of the oceans from the Neoproterozoic because of the well-balanced cycle of phosphates, which were strongly adsorbed by Fe-hydroxides. The radiogenic nature of the initial 187Os/188Os (~0.6), although with a high associated error, seems to not confirm previous indications of an abrupt increase of seawater 187Os/188Os during the late Neoproterozoic due to the large-scale radiogenic Os flux into the ocean.

Bioturbation and directionality in Earth's carbon isotope record across the Neoproterozoic–Cambrian transition

Mixing of sediments by moving animals becomes apparent in the trace fossil record from about 550 million years ago (Ma), loosely overlapping with the tail end of the extreme carbonate carbon isotope δ13 Ccarbonate fluctuations that qualitatively distinguish the Proterozoic geochemical record from that of the Phanerozoic. These Precambrian-scale fluctuations in δ13 Ccarbonate (PSF-δ13 Ccarbonate ) remain enigmatic, due to their high amplitude and inclusion of global-scale negative δ13 Ccarbonate values, below anything attributable to mantle input. Here, we note that different biogeochemical-model scenarios plausibly explaining globally synchronous PSF-δ13 Ccarbonate converge: via mechanistic requirements for extensive anoxia in marine sediments to support sedimentary build-up of 13 C-depleted carbon. We hypothesize that bioturbation qualitatively reduced marine sediment anoxia by exposing sediments to oxygenated overlying waters, which ultimately contributed to decreasing the carbon cycle's subsequent susceptibility to PSF- δ13 Ccarbonate . Bioturbation may also have reduced the quantity of (isotopically light) organic-derived carbon available to contribute to PSF- δ13 Ccarbonate via ocean crust carbonatization at depth. We conduct a comparative modelling exercise in which we introduce bioturbation to existing model scenarios for PSF- δ13 Ccarbonate : expressing both the anoxic proportion of marine sediments, and the global organic carbon burial efficiency, as a decreasing function of bioturbation. We find that bioturbation's oxygenating impact on sediments has the capacity to prevent PSF- δ13 Ccarbonate caused by authigenic carbonate precipitation or methanogenesis. Bioturbation's impact on the f-ratio via remineralization is partially offset by liberation of organic phosphate, some of which feeds back into new production. We emphasize that this study is semiquantitative, exploratory and intended merely to provide a qualitative theoretical framework within which bioturbation's impact on long-term, first-order δ13 Ccarbonate can be assessed (and it is hoped quantified in more detail by future work). With this proviso, we conclude that it is entirely plausible that bioturbation made a decisive contribution to the enigmatic directionality in the δ13 Ccarbonate record, from the Neoproterozoic-Cambrian boundary onwards.

Late Cambrian magmatic arc activity in peri-Gondwana: geochemical evidence from metagranitoid rocks of the Basal Allochthonous Units of NW Iberia.

The North African section of the Gondwana margin was the site of voluminous, arc-related magmatism during the Late Neoproterozoic (Avalonian–Cadomian orogen). The lower (and older) metasedimentary sequence that constitutes the Basal Units of the Allochthonous Complexes of NW Iberia was deposited in that setting. In these units, sedimentation was followed by the intrusion of tonalites and granodiorites in the late Cambrian ( ca. 493–489Ma). In the Late Paleozoic, the collision of Gondwana and Laurussia (Variscan orogeny) deformed and metamorphosed the whole ensemble. New whole rock geochemical analysis performed in seven samples of metatonalites and fourteen samples of metagranodiorites are characterized by: i) slight enrichment in incompatible elements (Rb, Ba, Th, U), ii) negative anomalies in Nb, Ta, P, and Ti, and iii) negative anomalies in Eu. These chemical features are in agreement with a subduction-related setting for the genesis of both types of magma, which is also supported by chemical discrimination using tectonic setting diagrams. Positive anomalies of Pb suggest a crustal component. The new geochemical data reveal that the convergent orogen that ruled the paleogeography of the Gondwana periphery during the Neoproterozoic (Cadomian orogen) remained active beyond the Neoproterozoic-Cambrian transition, up to at least late Cambrian times.

SIMS U–Pb zircon geochronological constraints on upper Ediacaran stratigraphic correlations, South China

AbstractFossiliferous Ediacaran successions of South China, the Doushantuo and Dengying formations and their equivalents, are key to understanding bio- and geological evolution at the Neoproterozoic–Cambrian transition. However, their absolute ages, especially the upper Ediacaran successions, are poorly constrained. SIMS zircon U–Pb dating results in this study suggest that ash beds at the basal and middle parts of the Jiucheng Member (middle Dengying Formation) in eastern Yunnan Province were deposited at 553.6 ± 2.7/(3.8) Ma and 546.3 ± 2.7/(3.8) Ma, respectively. These new dates indicate that the age for the base of Dengying Formation in eastern Yunnan Province is similar to the 550.55 ± 0.75 Ma date, which is from an ash bed at the top of the Miaohe Member and has been regarded as the age for the base of Dengying Formation in Yangtze Gorges area. These dates do not permit a clear test of the two correlation models for the chronostratigraphic position of the Miaohe Member (uppermost Doushantuo Formation vs. middle Dengying Formation), implying that further integrated intra-basinal stratigraphic correlations and more high-resolution chronological data from the upper Ediacaran deposits of South China are required. New dates of the Jiucheng Member constrain the age of the fossil biotas in the middle Dengying Formation and extend the stratigraphic range ofRangea,HiemaloraandCharniodiscusto 546.3 ± 2.7/(3.8) Ma. The geochronology of the Dengying Formation implies that Ediacaran-type fossils preserved in this formation are younger than the White Sea Assemblage and temporally overlapping with the Nama Assemblage.

Reconstruction of early Cambrian ocean chemistry from Mo isotopes

The Neoproterozoic–Cambrian transition was a key time interval in the history of the Earth, especially for variations in oceanic and atmospheric chemical composition. However, two conflicting views exist concerning the nature of ocean chemistry across the Precambrian–Cambrian boundary. Abundant geochemical evidence suggests that oceanic basins were fully oxygenated by the late Ediacaran, while other studies provide seemingly conflicting evidence for anoxic deep waters, with ferruginous conditions [Fe(II)-enriched] persisting into the Cambrian. Here, two early Cambrian sedimentary platform and shelf-slope sections in South China were investigated to trace early Cambrian ocean chemistry from Mo isotopes. The results reveal that early Cambrian sediments deposited under oxic to anoxic/euxinic conditions have δ98/95Mo values ranging from −0.28‰ to 2.29‰, which suggests that early Cambrian seawater may have had δ98/95Mo values of at least 2.29‰, similar to modern oceans. The heaviest and relatively homogeneous δ98/95Mo values were recorded in siltstone samples formed under completely oxic conditions, which is considered that Mn oxide-free shuttling was responsible for such heavy δ98/95Mo value. Further, combined with Fe species data and the accumulation extent of Mo and U, the variation of δ98/95Mo values in the two studied sections demonstrate a redox-stratified ocean with completely oxic shallow water and predominantly anoxic (even euxinic) deeper water having developed early on, which eventually became completely oxygenated. This suggests that oceanic circulation at the time became reorganized, and such changes in oceanic chemistry may have been responsible for triggering the “Cambrian Explosion” of biological diversity.

Early Cambrian palaeobiogeography of the Zhenba–Fangxian Block (South China): Independent terrane or part of the Yangtze Platform?

Early Cambrian small skeletal fossils (SSFs) are studied and revised from the Zhenba–Fangxian Block of the transitional zone between the Yangtze Block and the South Qinling Terrane. The study reveals a diverse fauna with 47 species of various biological affinities, including the new species Gapparodus gapparites sp. nov. The SSFs are assigned to the newly defined Cambroclavus fangxianensis–Rhombocorniculum cancellatum Assemblage Zone. Based on the investigated SSF fauna from Zhenba County, Southeast Shaanxi of China and published data, a palaeobiogeographic study is carried out for the Cambrian Stage 3 (equivalent to the Atdabanian–Botoman of Siberia). A hierarchical Pearson similarity cluster analysis of 295 species from 32 regions of the world indicates a distinct palaeobiogeographic pattern with seven faunal provinces. The result is mostly consistent with existing palaeogeographic reconstructions for the early Cambrian. However, it is also shown that the SSF assemblages of the Zhenba–Fangxian Block have low similarity with those of the Yangtze Block. Instead, they share high similarity with those from Armorica, Tarim and the Karatau–Naryn terranes (South Kazakhstan/North Kyrgyzstan). The Yangtze Block has a unique SSF assemblage dissimilar to most of other regions. The Terreneuvian–Cambrian Stage 3 sedimentary sequence of the Zhenba–Fangxian Block is more consistent with that of the South Qinling Terrane. Besides, sedimentary Ediacaran manganese ore deposits and Cambrian barite/witherite deposits have unique distribution pattern on the Zhenba–Fangxian Block. Derived from the profound dissimilarities in faunal composition, sedimentary sequence and distribution of sedimentary ore deposits, we hypothesize that during the Neoproterozoic–Cambrian transition, the Zhenba–Fangxian Block might have been an independent terrane and more distant from the Yangtze Block. The palaeobiogeographic analysis of SSFs also indicates a closer alliance between Avalonia and Siberia. It corroborates the palaeogeographic reconstruction of North China at the margin of Gondwana, in the vicinity of Australia, Antarctica, and Armorica.

The Recognition of Hailstone Impressions In Clay-Rich Sediment: Experimental Results and Relation To the Neoproterozoic Case

Abstract Based on experimental impacts of hailstone and rain droplets on clayey sediment, distinct differences between their imprints are shown. The key difference that unequivocally differentiates these impact structures is that raindrop imprints are always part of a sphere, even after different degrees of compaction, whereas hailstone imprints never have such a shape, forming a more or less regular funnel, terminated by a “nipple-like” structure that usually retains the original shape of the hailstone. We show and discuss the reasons why hailstone imprints have never been reported in the sedimentary record, albeit their fossilization potential seems to be much higher than that of raindrop imprints, which are commonly reported from various types of sedimentary rocks of widely differing ages. Inferred hailstone imprints are illustrated from the fossil record for the first time, from the Neoproterozoic–Cambrian transition of western Africa. The origin of paleohailstones required specific dynamic conditions of the atmosphere similar to those of the Recent atmosphere. Present-day hailstorms are generally restricted to specific geographic regions, e.g., mid-latitude, along mountain ranges, and they are largely absent from circum-polar and circum-equatorial areas. Correspondingly, hailstone impact structures preserved in the sedimentary record may possess interpretational value, regarding fundamental questions concerning the dynamics of the paleoatmosphere and perhaps paleogeography of ancient continents.

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.

SHRIMP zircon U–Pb age constraints on Neoproterozoic Quruqtagh diamictites in NW China

The Neoproterozoic Quruqtagh Group in the Tarim Block, NW China, contains multiple diamictites in the Bayisi, Altungol, Tereeken, and Hankalchough formations. These diamictites may represent three or possibly four discrete glaciations, although evidence for a glacial origin of the Bayisi and Altungol diamictite is ambiguous. To constrain their age and duration, we dated three volcanic beds (V1, V2, and V3) in the Quruqtagh Group using the SHRIMP (sensitive high-resolution ion microprobe) zircon U–Pb method. Volcanic bed V1 near the base of the Bayisi diamictite yields a 740 ± 7 Ma age, volcanic bed V2 near the top of the Bayisi Formation gives a 725 ± 10 Ma age, and volcanic bed V3 between the Tereeken and Hankalchough diamictites yields a 615 ± 6 Ma age. V1 and V2 have overlapping ages, and together these dates suggest that the Bayisi diamictite was deposited at around 730 Ma. The Tereeken and Altungol diamictites were deposited between 725 ± 10 Ma and 615 ± 6 Ma, and the Hankalchough diamictite between 615 ± 6 Ma and ∼542 Ma (i.e., the Neoproterozoic–Cambrian transition). These dates and previously published chemostratigraphic data are consistent with (but doe not require) the correlation of the Tereeken and Hankalchough diamictites with the 635 Ma Nantuo and 582 Ma Gaskiers glaciations, respectively. However, the new dates are inconsistent with a single and globally synchronous Sturtian glaciation that occurred in the pre-Nantuo Neoproterozoic Era. Instead, currently available data necessitate that either multiple glaciations occurred, or a globally diachronous glacial event developed during a protracted period between ∼750 Ma and ∼650 Ma.

Late Neoproterozoic carbonate productivity in a rifting context: the Adoudou Formation and its associated bimodal volcanism onlapping the western Saghro inlier, Morocco

Abstract An interval of episodic carbonate productivity, lithostratigraphically recognized as the ‘Calcaires inférieurs’ (upper member of the Adoudou Formation), took place across the Neoproterozoic–Cambrian transition onlapping the western Saghro inlier, Morocco. Sedimentation of the ‘Calcaires inférieurs’ was highly variable: in relatively stable substrates, a peritidal-dominated mixed platform is recorded where deposition was primarily controlled by autocyclic processes and accommodation space availability, whereas, in unstable substrates, the tectonic activity associated with the inherited block-faulting basement led to deposition of complex slide sheets composed of penecontemporaneous isoclinal folds and disrupted strata. The uppermost part of the ‘Calcaires inférieurs’ displays a negative δ 13 C shift reaching values of −6.5‰. This shift may represent the δ 13 C excursion to −6‰ that marks the Neoproterozoic–Cambrian boundary in the western Anti-Atlas. Two volcanic episodes bracketed the carbonate productivity. They consist of lower basaltic flows and an upper rhyolitic ignimbrite, with a SiO 2 gap between 52 and 74 wt%. The basic rocks resemble those of tholeiitic magmas in continental rifts. The felsic rocks show high light to heavy rare earth element abundances and negative Nb, Ta, P and Ti anomalies, and were probably generated as a result of either fractional crystallization coupled with relative crustal contamination, or from a different magmatic source. The lower basic flows of tholeiitic affinity predated and geochemically differ from the alkaline magmatism of the Alougoum volcanic complex (Boho jbel) that surrounds the neighbouring Bou-Azzer inlier.

Salterella and Volborthella from the Early Cambrian of Spitsbergen: the evolution of agglutinating organisms during the Neoproterozoic-Cambrian transition

The co-occurrence of the small agglutinated, conical fossils Volborthella and Salterella is documented from the Lower Cambrian Tokammane Formation of Ny Friesland, Spitsbergen. Microstructural investigations of well-preserved Volborthella tenuis Schmidt (1888), collected from the fine-grained Topiggane Shale, indicate the presence of an organic cementing matrix that may have extended around the edges of the test to form an outer organic sheath. This is clearly distinct from the carbonate cements observed in specimens of Salterella maccullochi (Murchison 1859), supporting the taxonomic validity of Volborthella and Salterella . The occurrence of both these forms in this succession may indicate a biogeographic link between the Laurentian province, where both Salterella and Volborthella are found, and the Baltic and East European Platform, where only Volborthella is reported to occur. The exclusive presence of Volborthella in the older siliciclastic Topiggane Shale Member and Salterella in the younger carbonates of the Ditlovtoppen Dolomite provides both a testable age-relation between these two forms and suggests that there may be a significant facies control on their distribution. These organisms are significant in the early history of agglutinated skeletons, especially in their use of both organic and carbonate cements significantly earlier than other Paleozoic agglutinating organisms, such as the foraminifera.

Insights in the Neoproterozoic–Early Cambrian transition of NW Argentina: facies, environments and fossils in the proto-margin of western Gondwana

Abstract In NW Argentina, over 3000 metres of highly tectonized and metamorphosed siliciclastics of the Puncoviscana Formation underlie the more fossiliferous Cambro–Ordovician strata of the Mesón and Santa Victoria groups. Historically regarded as the non-fossiliferous ‘basal Precambrian shield’ of the region, its age was later found to be in part Phanerozoic, with the discovery of Early Cambrian trace fossils, and more recently refined with geochronological data. Widespread siliciclastic deposits characterize this sequence. A review of trace fossils in the Puncoviscana Formation has added new taxa. Trace fossil assemblages denote? geographical belts, indicating shallower marine conditions to the east ( Nereites association) and deeper to the west ( Oldhamia association). Such assemblages related to the palaeomorphology of the basin also represent different temporal levels, and in so doing reflect the overall evolution of the Puncoviscana Sea through time. The use of taxa that characterize Siberian stages in order to date parts of the Puncoviscana Formation should be abandoned as Siberian stages are defined by a specific set of fossils that are absent in the strata of NW Argentina. Thus, careful systematic studies and re-evaluation of the strata and fauna in the Puncoviscana Basin are the only way to improve understanding of the Neoproterozoic–Cambrian transition in the Andean margin of South America.

Trends in oolite dolomitization across the Neoproterozoic–Cambrian boundary: A case study from Death Valley, California

Changes in dolomite fabric within the Neoproterozoic–Cambrian stratigraphic succession in Death Valley, California preserve evidence for a first-order biogeochemical change in early diagenetic environments through the Neoproterozoic–Cambrian transition. Oolite forms the primary focus of the study so that the general paleoenvironmental setting and presumed susceptibility to early diagenesis and fluid flow can be kept relatively constant across geologic time. Two key generalizations can be made: (1) late Neoproterozoic dolomitization in Death Valley occurred very early in the paragenetic sequence, was volumetrically significant, and preserved fine detail of the original aragonite or calcite precursor; and (2) Early Cambrian dolomitization in Death Valley occurred late in the paragenetic sequence with burial, was commonly incomplete, and obliterated details of the calcite or aragonite precursor; early diagenetic dolomitization of any kind was absent. The dolomitization fabrics in Death Valley parallel the presumed pattern of seawater Mg/Ca with respect to mimetic dolomitization in the Paleozoic (low Mg/Ca, little/no mimetic dolomitization) and Cenozoic (higher Mg/Ca, more common mimetic dolomitization). However, the volume of Neoproterozoic mimetic dolomitization is immense compared to the Cenozoic examples. We speculate that a fundamental shift in the early diagenetic environment occurred in late Neoproterozoic time, perhaps in association with secular variation in the sulfate concentration of seawater and the advent and proliferation of penetrative bioturbation that destroyed sediment-capping microbial mats. An oscillatory trend in the fabric and volume of dolomite through the Phanerozoic has been suggested, but the termination of volumetrically significant mimetic dolomitization across the Neoproterozoic–Cambrian transition, a critical time in Earth history for many reasons, dwarfs the oscillatory trends noted in the Phanerozoic.

The Early-Cambrian Boho volcano of the El Graara massif, Morocco: Petrology, geodynamic setting and coeval sedimentation

A major volcanic episode is recorded across the Neoproterozoic-Cambrian transition in the Moroccan Anti-Atlas. Several volcanic cones are still preserved in the El Graara massif, laterally correlatable with volcanic flows dated as Early Cambrian (U/Pb date of 534 ± 10 Ma). Volcanic ashes and flows are interbedded with the uppermost part of the Adoudou dolostones, whereas the best-preserved volcano (the Boho Jbel) is onlapped by the overlying Lie-de-vin strata. Available petro-geochemical data from the Boho volcano suggest an alkaline magmatism probably derived from low-grade melting of a garnet–lherzolite mantle source, followed by fractional crystallization. The silica-undersaturated basaltic liquid evolved to form oversaturated rocks: the fractionation of a ferromagnesian phase with high-Ca and low-Al contents is suggested as the main process to cut across the critical plane of silica undersaturation in this geochemical series. Although the Boho geochemical patterns are similar to those of some rift emplacements, more data from other coeval magmatic eruptions are necessary to constrain their geodynamic setting. Erosion of the Boho volcano favoured formation of a slope-apron composed of four sedimentary facies belts: chaotic megabreccia (related to downslope mass movements of rigid blocks), amalgamated breccia sheets (emplaced by viscous debris flows), a heterogeneous terrigenous belt (representing offshore substrates interrupted by channels intersected by cross-bedded shoals), and variegated shales and stromatolitic dolostones (typical of the Lie-de-vin Formation). Sharp changes in sedimentation rate were associated with modifications in paleorelief sloping and transport mechanisms from subaerial (?) rock fall at the foot of the cone escarpment to sheet-like debris flow on the slopes, and the replacement by sedimentation under wave and storm influence. The presence of an active carbonate productivity, recorded in the primary porosities of the Boho slope-apron, is suggested by widespread development of a robust, coelobiontic, microbial carbonate factory resilient to poisoning by terrigenous influx. Diagenesis in these deposits includes marine, meteoric and deep-burial cementation of calcite, dolomite, iron oxides, quartz, feldspar, and celestine, the latter indicating precipitation from hypersaline pore fluids.

Facies distribution of the Lower Cambrian cryptic microbial and epibenthic archaeocyathan‐microbial communities, western Anti‐Atlas, Morocco

AbstractMarine microbial communities recorded in the Moroccan Anti‐Atlas were unaffected across the Neoproterozoic–Cambrian transition. A stromatolite‐dominated consortium was replaced at the beginning of the Atdabanian (ca 20 Myr after the Neoproterozoic–Cambrian boundary) by shelly metazoan and thromboid consortia, which contain the oldest biostratigraphically significant fossils of the Moroccan Cambrian. The associated collapse of microbial mat (stromatolitic) growth appears to coincide with a change from pre‐Atdabanian shallow‐water restricted conditions into Atdabanian deeper, open‐sea conditions. It is postulated that this environmental change led to an episode of improved water circulation over carbonate platform interiors, promoting shelly metazoan immigration into the region. The Tiout/Amouslek lithostratigraphic contact in the early Atdabanian marks the end of an episodically unstable seafloor as suggested by the abundance of slumping and sliding structures, and synsedimentary microfaults and cracks recorded in the underlying Tiout Member. Concurrent with the transition is the occurrence of a network of cryptic fissures and cavities that provided habitats for a coelobiontic chemosynthetic–heterotrophic microbial community composed of stromatolitic crusts, Renalcis–Epiphyton–Girvanella intergrowths, and Kundatia thalli. In the overlying Amouslek Formation, archaeocyathan–thromboid reefs were constrained by substrate stability, water depth and subsidence rate. Four reef geometries are distinguished: (i) patch reefs surrounded by shales, (ii) bioherms in which flank beds intercalate laterally with carbonate and shale inter‐reef sediments, (iii) biostromes or low‐relief structures formed as a result of lateral accretion of patch reefs, and (iv) kalyptrate complexes that nucleated because of a marked tendency for aggregation, and in which patch reefs and bioherms occur stacked together bounded by clay–marl–silt seams.