Early Cambrian Interval Research Articles

Enrichment mechanism of selenium in Lujiaping formation in central South Qinling: Controls from mixed sources, sedimentary environment, primary productivity and paleo-climate during Ediacaran to early Cambrian period

The Ediacaran to lower Cambrian Lujiaping Formation in South Qinling exhibited extremely high level of Se enrichment, which has provoked extensive concerns since the critical role of Se in paleo-biological evolution and diversification as well as its bidirectional effects on current ecological environment and human health. Because of complex geological setting within South Qinling during the Ediacaran–early Cambrian interval, distribution pattern and mechanism of Se enrichment in Lujiaping Formation have not been fully understood. In this study, therefore, stratigraphic variations of Se concentration and enrichment level together with enrichment mechanism in Lujiaping Formation in the central part of South Qinling were systematically explored. The result showed that, in the whole Lujiaping Formation, Se concentrations varied from 0.19 to 129 μg/g, with an average of 13.28 μg/g. According to the enrichment factor, most of samples can be classified as strong enrichment and anomaly levels (EF = 10–1000) which mainly occurred in organic matter-enriched siliciclatic rocks including mudstone, shale and stone coal seams. Meanwhile, Se enrichment was accompanied with enrichment of Ag, Mo, As, Cd and V in Lujiaping Formation. Various geochemical proxies were collectively employed to identify potential mechanism of Se enrichment. The results indicated that Se was derived from both terrestrial detrital materials and deep hydrothermal fluids. Strong Se anomaly was intimately linked to authigenic enrichment of U and Mo, manifesting advantageous role of euxinic benthic condition to Se accumulation. Besides, weak watermass restriction, intermittent upwelling, high primary productivity and warm and humid climate were also beneficial to ultimate Se enrichment in Lujiaping Formation. This study complemented fundamental data of Se concentration and enrichment degree in strata of the Ediacaran–early Cambrian period in central South Qinling. Meanwhile, preliminary insight of correlation among Se enrichment, sedimentary environment, paleo-climate and biological blooming during the Ediacaran to early Cambrian interval was obtained, which laid the foundation for further researches.

Global sea-level fall triggered Ediacaran–Cambrian unconformity in North China craton

The Great Unconformity that separates the Cambrian and the underlying Proterozoic strata has been interpreted as recording a composite of multiple diachronous unconformities, among which the Ediacaran–Cambrian unconformity represents the last stage of these erosional processes. Whilst the major denudational episode of the Great Unconformity was strictly restricted to happen before or during the Cryogenian and considered to be associated with the Rodinia supercontinent cycle and “Snowball Earth” event, the cause(s) for the relatively temporary exhumation in late Neoproterozoic remains elusive. Here we present a synthetic geochronological study for the late Ediacaran to early Cambrian strata on the southern North China craton (NCC), where the Ediacaran–Cambrian unconformity occurs. The absence of the latest Ediacaran to earliest Cambrian age zircons of these strata may suggest a significant tectonic quiescence in NCC. A similar phenomenon can also be recognized in other blocks isolated from the Gondwanaland during the late Neoproterozoic–early Cambrian interval. This world-wide Ediacaran–Cambrian unconformity thus was more likely caused by a global sea-level fall, possibly associated with the assembly of the Gondwana supercontinent during the Precambrian–Cambrian transitional period.

The marine redox evolution and the formation model for the early Cambrian Gongxi-Tianzhu barite deposits in the South China Block

Late Ediacaran to early Cambrian is marked as a vital ore-formation interval of the Earth’s history, particularly for the bedded barite deposits hosted in cherts and shales. Although plenty of work has been carried out, controversies still exist, such as how the marine paleoenvironment changed in this critical interval, and where the origin of ore-formation materials (i.e., barium ions) was. To address these questions, here, we conducted systematic geochemical research on Gongxi-Tianzhu barite deposits in the South China Block, using redox proxies (MoEF-UEF, Mo-TOC, U-TOC patterns, and elemental ratios), REY patterns, and Sr isotopes. Redox proxies were used to reconstruct the paleoenvironment evolution in the Gongxi-Tianzhu region during the early Cambrian interval. Redox proxy results revealed that the bottom water was oxygen-depleted during most of the depositional time of the Niutitang Formation. The bedded and nodular barites in the Niutitang Formation were formed in the Sulfate-Methane Transition Zone (SMTZ) during the synsedimentary and early diagenetic processes. Formation of the barite ores depended greatly on the redox state variation of the bottom water and the barium remobilization in the sediments. The ore-formation event ceased when the supply of oxygen and sulfate from the upper water stopped, and the pre-stored Ba ions in the anoxic bottom seawater were exhausted and the SMTZ rose back into the water again. REY patterns and 87Sr/86Sr values recorded coeval seawater signals without hydrothermal activities during the studied interval. Based on the elemental and isotopic evidence, a three-stage redox-diagenetic model was established for the formation of the barite deposits in South China.

Coupling of ocean redox and animal evolution during the Ediacaran-Cambrian transition

The late Ediacaran to early Cambrian interval witnessed extraordinary radiations of metazoan life. The role of the physical environment in this biological revolution, such as changes to oxygen levels and nutrient availability, has been the focus of longstanding debate. Seemingly contradictory data from geochemical redox proxies help to fuel this controversy. As an essential nutrient, nitrogen can help to resolve this impasse by establishing linkages between nutrient supply, ocean redox, and biological changes. Here we present a comprehensive N-isotope dataset from the Yangtze Basin that reveals remarkable coupling between δ15N, δ13C, and evolutionary events from circa 551 to 515 Ma. The results indicate that increased fixed nitrogen supply may have facilitated episodic animal radiations by reinforcing ocean oxygenation, and restricting anoxia to near, or even at the sediment–water interface. Conversely, sporadic ocean anoxic events interrupted ocean oxygenation, and may have led to extinctions of the Ediacaran biota and small shelly animals.

The early history of the metazoa—a paleontologist’s viewpoint

The molecular biology success, which became influential for a revision of principal views on the relationships and evolutionary pathways of major groups ("phyla") of multicellular animals, were much more appreciated by palaeontologists rather than by neontologists. This is not surprising because this is the fossil record that serves as a firm test for molecular biological hypotheses. The fossil record indicates that the different "phyla" united nowadays into the Ecdysozoa, namely, arthropods, onychophorans, tardigrades, priapulids, nematomorphs, comprise a number of transitional forms having become extinct already in the early Palaeozoic. The very appearance of those organisms fits entirely to hypothetical ancestral forms which have to exist in accordance with some data on ontogeny. There are no even tentative intermediates between arthropods and annelids in the fossil record. The oldest Deuterostomia, which is the only branch of the Bilateria being agreed upon by all the biological disciplines, allow us, although in less details, to present their early history and point to agile bilaterally-symmetrical forms at the dawn of chordates, hemichordates, and echinoderms. The interpretation of the early history of the Lophotrochozoa is even more difficult because in comparison to other bilaterians, their oldest fossils are preserved as mineralized skeletons only. However, a unity of microstructures of molluscs, brachiopods, and bryozoans, absent in other metazoans, is indicative of the presence of close relatives among different earliest lophotrochozoans some of which are sedentary filterers while others are motile epibenthic detritophages. In the aggregate, modern data of molecular biology, palaeontology, and comparative embryology/morphology, having got a second wind with an introduction of new microscopy techniques, imply that the suggestion of a planktotrophic gastraea-like common ancestor is the least possible among diverse suggestions on the Metazoa origins. The common ancestor of the Bilateria had to be a motile epibenthic animal and the explosive metazoan diversification embracing the late Ediacaran--early Cambrian interval (c. 40 Ma) was probably a real event, which was predated by a long (c. a billion years) assembly of the metazoan genome within unicellular and colonial common ancestors of the Opisthokonta and even the Unikonta as a whole.

Geochemical changes in the Early Cambrian interval of the Yangtze Platform, South China: Implications for hydrothermal influences and paleocean redox conditions

The possible triggering mechanisms of the Ediacaran–Cambrian transition in South China have been debated for many years. In this study, 80 samples from Liuchapo, Niutitang and Mingxinsi Formations were collected from Well YK-1, located in southeastern Chongqing. A high-resolution chemostratigraphic study, including evaluation of rare earth elements (REEs) and trace elements, was used to elucidate the paleoenvironments of the Upper Yangtze Platform margin. The Mingxinsi gray shale and the Liuchapo Formation exhibit flat Post-Archean Australian-normalized REE patterns with weakly negative Ce anomalies, which is typical of detritus. Obvious positive Eu anomalies in chert samples of the basal Niutitang Formation imply a short-term influence of hydrothermal fluid. The REE patterns of the basal Niutitang phosphatic rocks show significant enrichment of heavy REEs, significantly negative Ce anomalies, and positive Y anomalies, similar to the REE-distribution pattern in seawater, demonstrating that these sediments were produced under oxic seawater conditions. Moderately positive Eu anomalies and positive Y anomalies of the lower Niutitang shales may indicate mixing of hydrothermal fluids and seawater. The flat REE patterns with weak Eu anomalies of the upper Niutitang shale may have been generated by hydrothermal inputs diluted by detrital materials. Dramatic increases of Ce anomalies in the basal Niutitang Formation reveal a significant decrease of the oxygen level in seawater. Trace elements, such as Ba, U, V, U, Cu, Cr, Zn, Ni and Sr, are commonly enriched in these formations compared to the upper continental crust. Authigenic abundances of Al-normalized U and Al-normalized V as well as ratios of Th/U and V/Sc are used as indicators of redox conditions. These distributions demonstrate that the redox conditions of the bottom water during the Ediacaran–Cambrian transition abruptly changed from oxic in the Nemakit–Daldynian age to anoxic in the early Tommotian age, and gradually became less reducing in the late Tommotian age and the Atdabanian age. A strong correlation between total organic carbon (TOC) concentrations and ratios of U/Al and V/Al in the entire section (except in the phosphatic rocks) implies non-sulfidic conditions. Most trace elements cannot be used as proxies for redox conditions associated with the phosphatic rocks in the basal Niutitang Formation because abundant trace elements were especially absorbed in apatite. The TOC concentrations correlated well with the redox conditions of bottom water. We infer that the section deposited in the platform margin was influenced by episodic hydrothermal activities that began at the beginning of the Tommotian age, and these activities may have terminated at the beginning of the Atdabanian age. Enrichments of organic matter that were mainly derived from the photic zone were controlled by the redox conditions of the bottom water.

Trace element chemostratigraphy of two Ediacaran–Cambrian successions in South China: Implications for organosedimentary metal enrichment and silicification in the Early Cambrian

A trace element study is presented of two Ediacaran–Early Cambrian sedimentary successions in South China: the Shatan section, Sichuan Province, which represents a shallow platform setting, and the Songtao section, Guizhou Province, which corresponds to a more basinal, depositional environment. Across South China, sedimentary strata of this age exhibit characteristic trace metal enrichments in generally organic-rich, stratigraphically correlative, transgressive units. Trace element (including REE) data are used here to determine the palaeoenvironmental and diagenetic conditions during this Early Cambrian interval of metal scavenging that resulted in enrichments over grey shale of Mo > Cd > V > U >Ni > Ag > Zn > Cu > Pb. At Shatan, high organic carbon contents (up to 5% TOC) correlate with Mo enrichment, which indicates that metal scavenging was linked to organic productivity and decay. Low Th/U ratios are consistent with highly reducing conditions during metal scavenging, while extremely high V contents imply that bottom waters were possibly euxinic at times of extreme anoxia. At Songtao, very low Th/U ratios are strongly associated with high V/Sc and low δ 13C org, implying that anoxia, organic decomposition and metal sequestration were associated in the deepwater facies, too. There is no unambiguous trace element evidence that hydrothermal activity contributed to metal enrichment in South China during the Early Cambrian. The systematic relationship with ocean transgression suggests that enrichment was caused largely by oceanographic changes to organic productivity and burial rather than post-depositional fluid flow. Negative Ce anomalies and seawater-like REE distribution patterns are a feature of the Liuchapo chert and lower black shales of the Jiumenchong Formation at Songtao. Ce depletion indicates that authigenic components of these sedimentary units formed under oxic conditions, which contradicts the other geochemical redox proxies. We argue that this chert derived primarily from biogenic silica from siliceous plankton, which formed above a redox boundary within the water column. Neither silicification nor hydrothermal fluids, contrary to some published models, played a significant role in metal transport.

Post-Marinoan marine siliciclastic sedimentation: The Masirah Bay Formation, Neoproterozoic Huqf Supergroup of Oman

The Huqf Supergroup of Oman contains an excellently exposed succession from the presumed Marinoan-age Fiq glaciation (ca. 635 Ma) to the Precambrian–Cambrian boundary (542 Ma). Within this time interval, two major siliciclastic-to-carbonate cycles are present, starting with the transgression of basin margins following the deposition of the glacigenic, probably rift-related Fiq Member. The dominantly siliciclastic portion of the first cycle is termed the Masirah Bay Formation. In the Huqf region of east-central Oman, two quartz-arenitic sandstone bodies crop out, below the transition into the overlying Khufai Formation carbonates. In the Jabal Akhdar of northern Oman, only deep marine shales and siltstones are found. The Masirah Bay Formation in the Huqf area is divided into a number of members with constituent units containing distinctive facies assemblages. In Member 1, wave-rippled shoreface deposits are overlain by trough cross-stratified, coarse-grained sandstones deposited in proximal tidal sandsheets or estuarine tidal shoals. Member 2 represents essentially a repeat of this coarsening- and shallowing-up trend via a basin-wide flooding event. A second major flooding surface is overlain by the lower shoreface to offshore sedimentary rocks of Member 3, which pass gradationally upwards into the distal carbonate ramp of the Khufai Formation. The two major progradational cycles can be identified in nearby subsurface well penetrations such as Masirah-1 (SMP-1). The tidal sandstones of the Masirah Bay Formation were deposited as regressive-transgressive pulses in a longer term transgressive stratigraphic trend associated with both the demise of continental glaciation and the end of active extensional tectonics. As basin margins were flooded, compositionally and texturally mature but coarse-grained quartzose sand was swept by currents into tidal sandsheets and estuary-fills along a broad littoral margin situated in the eastern fringe of the study area. The development of meso-macrotidal conditions over extensive sandy shelves and coastlines appears to be a common but unexplained feature of the Ediacaran and Early Cambrian interval.

δ 13 C carb and Ce anom excursions in the post-glacial Neoproterozoic and Early Cambrian interval in Guizhou, South China*

Abstract Secular δ13Ccarb and Ceanom profiles in the post-glacial Neoproterozoic and Early Cambrian interval are reported from Guizhou on the southeasten border of the Yangtze platform. South China. Overall, the δ13Ccarb profile drifts to negative values in the postglacial Nantuo and lower to middle Doushantuo Formations, and then to positive values in the upper Doushantuo and Dengying Formations of Neoproterozoic, followed by negative values in the Lower Cambrian Gezhongwu and Niutitang Formations. A detailed investigation of the relationship between the δ 13Ccarb and Ceanom profiles reveals that the main anoxic and oxic episodes are coupled with negative and positive δ 13Ccarbvalues, respectively. This may suggest a control of alternation between ocean stratification and mixing on variations in 13C abundance in the ancient ocean of the investigated areas.

Stratigraphic implications of Sinian-Early Cambrian volcanic ash beds on the Yangtze Platform *

Abstract Volcanic ash beds from shallow- to deep-water fades strata of the Sinian-Early Cambrian (Meishucunian) on the Yangtze Platform consist of Bentonites and tuffites which are readily recognized in the field by their physical features and confirmed by geochemical analyses. Geochemistry suggests that the volcanic ash beds in Meishucunian time are rhyolite and rhyodacite while those in the Qiongzhusian and Sinian are andesite and trachyandesite. The ash beds in the time-equivalent strata, even in different areas display rather similar geochemical features, whereas the ash beds in different strata even in the same areas show large chemical difference. The results suggest that these ash beds can be used for intra- and extra-basinal correlations of the Sinian- Early Cambrian interval on the Yangtze Platform. Additionally, these ash beds suggest high potentials for further U-Pb dating strategies.

Lower Cambrian (Branchian) trilobites and biostratigraphy of the Hanford Brook Formation, southern New Brunswick

The Hanford Brook Formation, one of the classic Cambrian units of Avalonian North America, contains at least eight species of endemic trilobites, including Berabichia milleri Westrop n. sp., that are assigned to seven genera. The vertical succession of faunas is far more complex than has been recognized previously, with each member containing a lithofacies-specific assemblage. These are, in ascending order: a bradoriid-linguloid Association without trilobites in the nearshore St. Martin's Member, a Protolenus Association in dysaerobic siltstones and sandstones of the Somerset Street Member, and a Kingaspidoides-Berabichia Association in hummocky cross-stratified sandstones of the Long Island Member that overlie a parasequence boundary at Hanford Brook. Due to the breakdown of biogeographic barriers in the late Early Cambrian, two new species-based zones, the Protolenus elegans and Kingaspidoides cf. obliquoculatus zones, share trilobite genera with the Tissafinian Stage of Morocco. This generic similarity has been the basis for correlation of this upper Lower Cambrian interval on the Avalon continent with the West Gondwanan lowest Middle Cambrian. However, the clear facies control on the occurrence of genera in the Hanford Brook Formation and the presence of an abrupt faunal break and unconformity at the base of the Tissafinian in Morocco makes this correlation questionable. The Hanford Brook Formation may represent a late Early Cambrian interval unknown in Gondwana. Sequence-stratigraphic criteria even raise the possibility that the Protolenus Association is the biofacies equivalent of Callavia broeggeri Zone faunas of the Brigus Formation of Newfoundland, Nova Scotia and Massachusetts.

Transpressive deformation in the southwestern part of the Sierra de San Luis (Sierras Pampeanas, Argentina)

In the southwestern part of the Sierra de San Luis, clastic sediments of the Phyllite Group are assigned to the Late Precambrian–Early Cambrian interval and contain acid meta-mamatic dykes. Deposits of the Micaschist Group are interpreted as deeper crustal equivalents. Lower Ordovician intrusions of the Bemberg and Las Verbenas Tonalites, as well as the Lower Devonian La Escalerilla Granite pluton, led to a thermal overprint in the Phyllite Group. A first deformation, pegmatitic injections, and amphibolite facies metamorphism in the Micaschist Group are of Famatinian (Ordovician) age. Post-Famatinian (Devonian) ∼WNW–ESE compression affected all units under greenschist facies metamorphism, which locally reached amphibolite facies conditions, and is the second event in the Micaschist Group. Mostly sinistral, ductile shear zones within the granite, Las Verbenas Tonalite, and parts of the Phyllite Group were formed together with folds in the meta-clastic successions and an eastern mylonite zone along which the Micaschist Group was obliquely thrust northwestwards over the granite. They are related to the sinuous bending of the granite under sinistral transpression which probably was an important mechanism in this part of the sierra. Possible implications with respect to the collision of the Precordillera (Cuyania) Terrane are briefly discussed.

Earliest skeletal fossils from Precambrian–Cambrian boundary strata, Elburz Mountains, Iran

AbstractThe lithological and biotic succession across the late Precambrian to early Cambrian interval is outlined for the Dalir and Valiabad successions of the Elburz Mountains of Iran. The Lower Dolomite Member contains an assemblage of phosphatic tubes and other poorly preserved remains. The succeeding Lower Shale Member bears macroscopic chuariamorphid algae. Early skeletal fossil diversity rises through the Middle Dolomite Member, with the successive appearance ofProtohertzina anabarica, Cambrotubulus decurvatusandAnabarites trisulcatus, culminating near the top of the dolomites with the appearance ofPurellasp.,Maikhanella multa, Tiksitheca licisand circothecids. This succession is compared with lower to upper parts of the Nemakit-Daldyn Formation of Siberia. The overlying Upper Shale Member bears phosphatic beds at its base with allathecids and an uncoiled pelagiellid (?Aldanellasp.) that suggest comparison with lower Tommotian strata and the Precambrian–Cambrian boundary phosphorite event of southern and central Asia. A rich assemblage of molluscs appears high in the Upper Shale Member, including theLatouchella korobkovigroup and thePelagiella lorenzigroup. The succession is broadly homotaxial with those from the Siberian Platform and Mongolia and those platforms bordering Gondwana (India, Kazakhstan, South China).