Middle Cambrian Research Articles

The Atlas-Meseta Red Beds basin (Morocco) and the Lower Ordovician rifting of NW-Gondwana

AbstractThe transition from the Cambrian to Ordovician in Morocco is known to be characterized by a frequent Furongian hiatus, restricted extension of the Tremadocian marine deposits, and frequent unconformities at the base of the transgressive upper Floian deposits. In the present work, we first highlight the occurrence of Fe- and mica-rich, red silty/sandy formations in the Central and Eastern High Atlas between the Middle Cambrian and Upper Floian sequences. In the Tislyt type-locality, a synsedimentary hemigraben structure is defined, within which the red beds show frequent slump folds, debris flows and internal unconformities. The correlation with several coeval series of the Meseta domain allows us to define a shallow marine, ferruginous clastic Atlas-Meseta Red Beds (AMRB) basin during the Tremadocian-early Floian. The AMRB basin extended between the Meseta coastal block and the Anti-Atlas domain, being limited by the fault zones that became the West Meseta shear zone and the South Meseta fault, respectively, in the Variscan orogen. The AMRB basin compares with the coeval rifted basins of the central Iberian and Armorican massifs. The red beds were likely sourced from the east, from both the Precambrian basement and Early Ordovician magmatic rocks, contrary to the Ordovician deposits of the Sahara platform sourced from the south. Subsidence of the AMRB and central Iberian-Armorican basins of the NW-Gondwana border aborted during the Floian, whereas the opening of the Rheic ocean went on more to the west.

Acritarchs from the Hanford Brook Formation, New Brunswick, Canada: new biochronological constraints on theProtolenus elegansZone and the Cambrian Series 2–3 transition

AbstractDiverse and well-preserved acritarchs are reported from the type section of the Cambrian Hanford Brook Formation at Hanford Brook, southern New Brunswick. This section fills an important gap in acritarch studies by providing the first detailed picture of changing acritarch associations close to the traditional lower–middle Cambrian boundary in Avalonia. Acritarchs from the St Martins Member, at the base of the succession, includeSkiagia ciliosa, Heliosphaeridium notatum, H. longumandLiepaina planaand suggest attribution to Cambrian Stage 4. Acritarchs from the Somerset Street Member, in the middle of the formation, includeEliasum llaniscumandComasphaeridium silesiense. This information adds new biochronological context to an ash bed in the Somerset Street Member previously dated asc. 510 Ma or 508 Ma, and to the endemic trilobites from the same member, includingProtolenus elegans. It also places absolute ages on the basal range of stratigraphically important acritarchs. Both the acritarch assemblage and the radiometric age are consistent with a position very close to the traditional lower–middle Cambrian transition and likely within Cambrian Stage 5. Acritarchs from the Long Island Member, at the top of the succession, include additional taxa demonstrating assignment to Cambrian Stage 5. Both the Somerset Street and Long Island members probably correlate with theMorocconus notabilisZone. The new acritarch speciesRetisphaeridium striatumPalacios is described. New data are presented on acritarchs from the upper part of the Hell's Mouth Formation, Wales, and correlation proposed with the Long Island Member.

The Cambrian trilobite fauna from the Shian (Saybang) section, Pin Valley (Spiti) and its biostratigraphic significance

The Cambrian trilobite fauna, illustrated for the first time from the Pin Valley (Spiti, Northwest Himalaya, India), comprises Oryctocephalus indicus, Pagetia significans, Kunmingaspis pervulgata, and Bhargavia prakritika and they ranges from the top part of the Cambrian Series 2 (Stage 4) to the basal most part of the Series 3 (Stage 5). On the basis of the trilobite fauna and their stratigraphic order of occurrence we recognize the Oryctocephalus indicus Zone (5.4m), interval 1 of no zonation, and the Bhargavia prakritika Level (Cambrian Series 3, Stage 5). The Oryctocephalus indicus Zone is based on the FAD and LAD (local range) of the eponymous species in the Pin Valley section. The occurrence of fossiliferous section immediately below the Cambro-Ordovician angular unconformity in the Pin Valley indicates, in comparison to the Parahio Valley, nearly 1160m of the Cambrian strata (deposited within about 2Ma, i.e., 509 to 507Ma) further eroded prior to the deposition of the Thango Formation.

Cambrian to Lower Ordovician reefs on the Yangtze Platform, South China Block, and their controlling factors

The Yangtze Platform, South China Block, maintained an epicontinental sea in the Cambrian and Ordovician with oldlands surrounding its northern and western margins and a southeastern slope belt. Many Cambrian to Lower Ordovician reef complexes are known from the Yangtze Platform for which an updated spatiotemporal framework is provided, their biotic composition and architecture is presented, and the reef development is discussed in a global context. The biotic structures of reef units on the Yangtze Platform largely match the evolutionary pattern in terms of composition and architecture. Microbes and calcareous algae are key components in all reefs but metazoan reef-builders are better suited to identifying macroevolutionary patterns. Large-scale dolomitization in Cambrian Series 4 on the one hand prevents recognition of metazoan reefs and on the other hand may indicate hypersaline environments, which prevented metazoan reef growth and only permitted small-scale stromatolites in a few regions. Ordovician metazoans appear to have played substantial roles in reef-building earlier on the Yangtze Platform than elsewhere. Specifically bryozoan and Calathium-lithistid sponge reefs diversified during the Early Ordovician. Climate and eustatic sea-level changes and their related impact on water energy, turbidity, and salinity determined the growth and demise of reefs. The morphology and arrangement of reefs were controlled by the topography of the sea floor.

Palaeoscolecid worms from the Lower Ordovician Fezouata Lagerstätte, Morocco: Palaeoecological and palaeogeographical implications

The Lower Ordovician Fezouata Lagerstätte from Morocco (central Anti-Atlas, Zagora area) has yielded abundant and diverse soft-bodied fossils. Most described taxa were epibenthic or pelagic, so that little is known about the endobenthic components of the fauna. Here we report the discovery of a locally abundant element of the biota, a palaeoscolecid worm, which may have played an important ecological role in the community. Palaeoscolecids had a long, annulated body with an eversible tooth-bearing pharynx. They probably represent stem priapulids, but unlike them, they were protected by biomineralized (phosphatic) microscopic plates covering most of the body. These sclerites are commonly found isolated within SSF assemblages, thus considerably extending the fossil record of the group that ranges from the Cambrian Series 2 (Stage 3) to the upper Silurian. Like priapulids, palaeoscolecids might have been important bioturbators, contributing to the substrate colonization in the lower Palaeozoic. The discovery of abundant material (38 specimens) in the Fezouata Shale (Lower Ordovician, Morocco) allows us to reconsider their possible lifestyle, habitat and feeding habits. All the specimens belong to Palaeoscolex? tenensis, a species previously only known from a fragment of cuticle from the Floian of Bohemia (Czech Republic). Some exhibit remains of the gut and aboral spines, features that are highly informative with regard to the ecology of these extinct organisms, but rarely preserved. The phosphatic nature of the plates is demonstrated by compositional analyses and interpreted with regard to the diagenetic context of the Fezouata Shale as primary. We also hypothesize that (at least) some palaeoscolecids were makers of Tomaculum-type ichnofossils, which are elongate clusters of faecal pellets, and show that they ingested a notable amount of sediment during the normal course of feeding. The presence of Palaeoscolex? tenensis in the Lower Ordovician of Morocco further documents the great faunal similarities between Perunica and northern Gondwana at that time.

The Ocean redox state evolution and its controls during the Cambrian Series 1–2: Evidence from Lijiatuo Section, South China

Well-exposed Lijiatuo Section was chosen to explore the temporal evolution and controls of the oceanic redox state, primary productivity and seawater sulfate levels during the Cambrian Series 1–2, South China. This section consists of Xiaoyanxi Formation (Fm.) mudstones and Liuchapo Fm. cherts that deposited in the slope and basin environment. Five oxic-anoxic cycles were identified based on V/Sc, Th/U and the enrichment factors of Mo, U, V, Ni and Cu. The Middle-Upper Liuchapo Fm. and the Middle Xiaoyanxi Fm. were deposited under oxic-suboxic conditions, and the rest of the strata were under anoxic conditions. The Re/Mo ratio demonstrated that the oxic-suboxic conditions in the Middle Xiaoyanxi Fm. were accompanied by transient sulfidic conditions, and the rest of the section was underanoxic and non-sulfidic conditions. All the TOC and the enrichment factors of Ba, Ni, Cu, Zn and Cd demonstrated that both the sinking and burial flux of organic matter (OM) in Liuchapo Fm. were lower than that in the overlying Xiaoyanxi Fm. The highest sinking and burial flux of OM in the Xiaoyanxi Fm. appeared at its lower parts; however, the lowest sinking and burial flux of OM in the Xiaoyanxi Fm. appeared in its middle parts. TOC/TS, TS and the vertical trend of δ34Spy demonstrated that the seawater was dominated by low oceanic sulfate levels, which resulted in the absence of free H2S. The rise of the atmospheric oxygen content may be the principal driver for the associated, transient suboxic-oxic and nearly sulfidic environment in the middle Xiaoyanxi Fm.

Early–Middle Cambrian Palynomorph microfossils and related geochemical events in South China

The earliest Cambrian acritarch ‘Asteridium–Heliosphaeridium–Comasphaeridium’ assemblage and the Early Cambrian (‘Chiungchussuan’) acritarch ‘Skiagia ornate–Fimbriaglomerella membranacea’ assemblage in South China can be correlated with assemblages from synchronous strata elsewhere in the world. Recent geochemical study and biomarker evidence further confirm a biostratigraphic change between Cambrian Series 2 and Series 3 and support the recognition of a major geological and biotic event during this time interval.

Trilobites and biostratigraphy of the Kuonamka Formation, northern Siberian Platform (Olenek River)

A study of triobite distribution allowed a biostratigraphic subdivision of the Kuonamka Formation section exposed in the middle reaches of the Olenek River (18 km downstream of the village of Olenek). Through detailed description of the section, a biostratigraphic subdivision down to the level of biozones and faunal beds was performed. These are beds with Calodiscus–Triangulaspis annio, Delgadella ultima–Bergeroniellus expansus Zone, Lermontovia dzevanovskii–Neopagetina orbiculata Zone, Anabaraspis Zone, Cheiruroides arcticus– Pagetia horrida Zone, Kounamkites Zone, Triplagnostus gibbus Zone, and beds with Tomagnostus fissus–Paradoxides sacheri. These biostratigraphic subdivisions were correlated with regional zones of the Siberian Platform. The study also provides analysis of trilobite assemblages from the Lower/Middle Cambrian boundary interval, which was identified in the studied section and other previously described sections (Nekekit, Molodo, Boroulakh Rivers). In the studied section, the base of the Middle Cambrian is defined by the FAD of Cheiruroides arcticus, i.e., at the base of the Cheiruroides arcticus–Pagetia horrida Zone. This level correlates with a presumed lower boundary of Stage 5 of a new International Stratigraphic Chart for the Cambrian System.

Cambrian–early Ordovician volcanism across the South Armorican and Occitan domains of the Variscan Belt in France: Continental break-up and rifting of the northern Gondwana margin

The Cambrian–lower Ordovician volcanic units of the South Armorican and Occitan domains are analysed in a tectonostratigraphic survey of the French Variscan Belt. The South Armorican lavas consist of continental tholeiites in middle Cambrian–Furongian sequences related to continental break-up. A significant volcanic activity occurred in the Tremadocian, dominated by crustal melted rhyolitic lavas and initial rifting tholeiites. The Occitan lavas are distributed into five volcanic phases: (1) basal Cambrian rhyolites, (2) upper lower Cambrian Mg-rich tholeiites close to N-MORBs but crustal contaminated, (3) upper lower–middle Cambrian continental tholeiites, (4) Tremadocian rhyolites, and (5) upper lower Ordovician initial rift tholeiites. A rifting event linked to asthenosphere upwelling took place in the late early Cambrian but did not evolve. It renewed in the Tremadocian with abundant crustal melting due to underplating of mixed asthenospheric and lithospheric magmas. This main tectono-magmatic continental rift is termed the “Tremadocian Tectonic Belt” underlined by a chain of rhyolitic volcanoes from Occitan and South Armorican domains to Central Iberia. It evolved with the setting of syn-rift coarse siliciclastic deposits overlain by post-rift deep water shales in a suite of sedimentary basins that forecasted the South Armorican–Medio-European Ocean as a part of the Palaeotethys Ocean.

Petrophysical and numerical seismic modelling of CO 2 geological storage in the E6 structure, Baltic Sea, offshore Latvia

Time-lapse numerical seismic modelling based on rock physics studies was for the first time applied to analyse the feasibility of CO 2 storage monitoring in the largest Latvian offshore geological structure E6 in the Baltic Sea. The novelty of this approach was the coupling of the chemically induced petrophysical alteration effect of CO 2 -hosting rocks measured in laboratory with time-lapse numerical seismic modelling. Synthetic seismograms were computed for the E6 structure, where the sandstone reservoir of the Deimena Formation of Cambrian Series 3 (earlier Middle Cambrian) was saturated with different concentrations of CO 2 . The synthetic seismograms obtained after CO 2 injection were compared with the baseline. The following four scenarios were considered: (1) a uniform model without the alteration effect; (2) a uniform model with the alteration effect; (3) a plume model without the alteration effect; and (4) a plume model with the alteration effect. The presence of CO 2 in the reservoir layers can be detected by direct comparison and interpretation of plane-wave synthetic seismic sections, and is clearly observed when one displays the difference between the baseline and post-CO 2 injection synthetics. The normalized root-mean-square imaging techniques also clearly highlight the time-lapse differences between the baseline and post-injection seismic data. The laboratory-conducted alteration of the petrophysical properties of the reservoir had a strong influence on the reflected signals in the seismic sections. The greatest difference was revealed on seismic sections with 1% CO 2 saturation, increasing the detectability of the stored CO 2 . The difference decreased with an increase in CO 2 content. The saturation of CO 2 could be qualitatively estimated up to a value of 5%. Higher saturation produced a strong signal in the repeatability metrics but the seismic velocity varied so slightly with the increasing gas content that the estimation was challenging. A time shift or push-down of the reflectors below the CO 2 storage area was observed for all scenarios. According to changes in the amplitude and two-way travel times in the presence of CO 2 , reflection seismics could detect CO 2 injected into the deep aquifer formations even with low CO 2 saturation values. Our data showed the effectiveness of the implemented time-lapse rock physics and seismic methods in the monitoring of the CO 2 plume evolution and migration in the E6 offshore oil-bearing structure. The new results obtained could be applied to other prospective structures in the Baltic region.

A fresh look at Nisusia Walcott, 1905 from the Cambrian Kaili Formation in Guizhou

NisusiaWalcott, 1905 is an early genus of kutorginid brachiopod of the Subphylum Rhynchonelliformea. Originating in the Cambrian Series 2 and becoming extinct in the late Cambrian Series 3, Nisusia existed for a relatively short period of geologic time, and is a brachiopod genus that exhibited a slow rate of morphological evolution. This paper details the brachiopod Nisusia from the middle Cambrian Kaili Konservat-Lagerstätte. Nisusia, in terms of abundance, is one of the most important brachiopods from the Kaili Biota. Two Nisusia species from the Kaili Formation of the Cambrian Series 2–3 in the eastern part of Guizhou Province are described, Nisusia granosa and Nisusia guizhouensis n. sp. This study will give valuable insights into the morphological variability of Nisusia species, and provides new details on the composition of the brachiopod community in the Kaili Biota.

Self-regulation of trilobite diversity in Murero (middle Cambrian, Spain) due to compensatory extinction

As species accumulate in a community, competition for available ecospace is expected to prevent the addition of new species and to facilitate species extinction, thus producing a dynamic equilibrium of diversity. This mechanism remains under debate since its empirical support comes mainly from indirect or partial evidence, with very few direct tests at the species level. Here a new method is described to detect the presence of selfregulation feedbacks between species richness and turnover rates. It consists of Monte Carlo simulations which randomize the distribution of species ranges among stratigraphic intervals, providing predictions which allow the detection of genuine self-regulation feedbacks in the real data. Since the simulations include any potential bias due to preservation, sampling, or change in depositional environment, and these biases would also affect the real dataset, they are thus ruled out as explanations for any difference found. This method is applied to one of the best known fossiliferous sequences worldwide, the Rambla de Valdemiedes in Murero (RV1 section, middle Cambrian, Spain), a classic locality that has been studied for more than 150 years and which stands out due to its excellent sampling density, continuous deposition, and homogeneous fossil preservation. The results show that trilobite species richness was self-regulated due to positive feedback with extinction rate, which implies that compensatory extinction regulated this fauna in spite of the on-going Cambrian radiation. The lack of evidence of any origination feedback suggests ecological opportunities were not limiting for new species to colonize this Cambrian community

The metamorphic sole of the western Tasmanian ophiolite: New insights into the Cambrian tectonic setting of the Gondwana Pacific margin

The Cambrian Ross–Delamerian Orogeny records the first phase of accretional tectonics along the eastern margin of Gondwana following breakup of the supercontinent Rodinia. Western Tasmania represents a key area for understanding the Cambrian tectonic setting of the eastern margin of Gondwana as it is one of the few places where a Tethyan-type ophiolite is preserved and contains the only known exposures of a sub-ophiolitic metamorphic sole associated with the Ross–Delamerian Orogen. This paper presents an integrated study of the field, petrographic, geochemical, and metamorphic characteristics of the metamorphic sole to the western Tasmanian ophiolite. The structurally highest levels of the metamorphic sole consist of granulite–upper amphibolite facies metacumulates and metagabbros. A transition to amphibolite and epidote–amphibolite facies conditions is recorded by metadolerites and metabasalts towards the base of the metamorphic sole. Kinematic indicators in mylonitic amphibolites suggest the metamorphic sole formed in an east-dipping subduction zone located to the east of the Proterozoic continental crust of Tasmania. Major and trace element whole rock and relict igneous spinel geochemistry indicates that the protoliths to the metamorphic sole formed at a back arc basin spreading centre. Our new data supports a model in which east-dipping subduction in Tasmania was driven by collapse of a back arc basin developed above an earlier west-dipping subduction zone outboard of the eastern margin of Gondwana. The proposed model may help to resolve a controversy related to apparent along-strike variations in subduction zone polarity during the Ross-Delamerian Orogeny and suggests a complex geodynamic setting had developed along the eastern margin of Gondwana by the Middle Cambrian. This study highlights the importance of considering the role of multiple subduction zones in generating metamorphic soles and emplacing ophiolites, which are key events associated with the construction of many orogenic belts worldwide.

The earliest pelagic jellyfish with rhopalia from Cambrian Chengjiang Lagerstätte

Modern cnidarian medusae generally show a triphasic life cycle with the succession of a larva, a sessile polyp and a pelagic medusa stage. The debate around the metagenesis of sessile polyps into pelagic medusae has lasted for more than 100years. When pelagic forms originated is not clear. Hitherto, the earliest crown-group medusae have been found at Cambrian Stage 5 (traditional Middle Cambrian, 509Ma) in Utah, while diverse stem-group medusozoans were found in the basal Cambrian Fortunian Stage. No reliable medusae have been found from Cambrian Series 2 Stage 3 (ca. 521Ma), although the marine benthic community teemed with many phyla of bilaterians, sponges and ctenophores. Here, we reinterpret Yunnanoascus haikouensisHu et al. (2007), originally described as a ctenophore, as a pelagic, predatory, crown-group medusozoan, based on the presence of rhopalia, possible radial canals and marginal tentacles. The medusae were a predatory member of the pelagic food web at the middle level of the ocean at Cambrian Stage 3.

How quiet was the epeiric sea when the Middle Cambrian Zhangxia Formation was deposited in SW Beijing, China?

The Middle Cambrian Zhangxia Formation, 65 km southwest of Beijing City, was deposited in an epeiric sea that extended over almost the entire North China palaeo-plate (including Korea). The depositional conditions under which the limestones were deposited are commonly considered to have been quiet, although oolites indicating current and wave activity occur fairly frequently. The presence of a large (6 m) lens of breccia, embedded in oolite, raises the question, however, how quiet the depositional conditions actually were. The lens is composed of a limestone breccia which occurs only rarely in the Zhangxia Formation, but that is common in the Late Cambrian Chaomidian Formation.It appears that the large lens was deposited as an entity. This does not fit the commonly presumed quiet depositional conditions of its host rock. Therefore it may be concluded that the lens was put in place by a special event. This event is interpreted to have been a form of mass movement, more particularly sliding of a consolidated but not yet completely lithified breccia mass.Only once such a feature has been described before from an epeiric sea, remarkably enough from the Chaomidian Formation, at a distance of several hundreds of kilometres from the present study site. Although this feature thus seems to be extremely rare, it is important as it indicates instable conditions. Such instable conditions in epeiric seas have not commonly been recognized thus far, which implies that our idea of the conditions in this environment need to be re-considered.

Reefs in the Early Paleozoic Taebaek Group, Korea: A Review

AbstractVarious early Paleozoic (Cambrian Series 3–Middle Ordovician) reefs are found in the Taebaek Group, eastern Korea, located in the eastern margin of the Sino‐Korean Block. They occur in every carbonate‐dominant lithostratigraphic unit of the group, but their morphology and composition differ markedly. The Daegi Formation (middle Cambrian: Cambrian Series 3) contains siliceous sponge‐Epiphyton reefs formed in a shallow subtidal environment, which is one of the earliest metazoan‐bearing microbial reefs after the archaeocyath extinction. The Hwajeol Formation (upper Cambrian: Furongian) encloses sporadic dendrolites consisting of Angulocellularia, which developed in a relatively deep subtidal environment, representing a rare deeper water example. The onset of the Ordovician radiation resulted in the formation of microbialite–Archaeoscyphia–calathiid patch reefs in shallow subtidal deposits of the Lower Ordovician Dumugol Formation. Subsequent late Early Ordovician relative sea‐level fall established extensive peritidal environments, forming microbial mats and stromatolites of the Lower–Middle Ordovician Makgol Formation. Ensuing Ordovician radiation resulted in one of the earliest metazoan skeletal reefs of the Middle Ordovician Duwibong Formation, constructed by stromatoporoid Cystostroma and bryozoan Nicholsonella, and developed around shallow shoals. These reefs reflect ongoing evolution and sea‐level change during the early Paleozoic, and exemplify a rare glimpse of peri‐Gondwanan records of reef evolution, which warrant detailed investigations and comparison with their counterparts in other regions.

Furongian (upper Cambrian) Guole Konservat‐Lagerstätte from South China

AbstractThe Guole biota contains common shelly fossils and some exceptionally well‐preserved soft‐bodied fossils. Stratigraphically, it is located in the mudstones and siltstones of the Sandu Formation near Guole Town, Jingxi County, Guangxi, South China. It is the first Furongian (late Cambrian) Burgess Shale‐type biota found in the world, thereby filling the gap between middle Cambrian and Lower Ordovician Burgess Shale‐type Lagerstätten. Preliminary studies suggest that the Guole biota includes approximately seven metazoan groups as well as algae. These will provide important new evolutionary and ecological information.

Tectonic evolution from Late Sinian to Early Paleozoic and natural gas exploration in northwestern Sichuan Basin, SW China

Abstract Based on the field outcrops, drilling and seismic data of northwestern Sichuan Basin, this paper systematically discusses tectonic evolution characteristics from the Late Sinian to the Early Paleozoic, analyzes the petroleum geological significance, and points out the recent favorable exploration directions in this area. The area experienced mainly three times of tectonic evolution during the Late Sinian to the Early Paleozoic: (1) During the Late Sinian to the early period of Early Cambrian, this area was affected by tension action and formed two sets of nearly north-south trending continental rifts, and two sets of platform margins in the Sinian Dengying Formation. (2) During the middle period of the Early Cambrian to the Middle Ordovician, this area was eroded strongly (stronger in the northwest than in the southeast) because of multi-period uplifting of the northwestern Bikou paleo-land. (3) During the Late Ordovician to the Late Silurian, this area was eroded strongly again because of the compression of the Kwangsian Orogeny, and the erosion was stronger in the southwest than in the northeast. During the Late Sinian to the Early Cambrian, the formation of continental rifts controlled the development of bioherm beach facies reservoirs on the platform margins in the Dengying Formation and the high quality source rock in the Lower Cambrian series. During the middle of the Early Cambrian, the uplifting of the Bikou paleo-land controlled the distribution of the Kongmingdong Formation oolitic beach facies reservoirs around the paleo-land. It is suggested that, vertically, the fourth Member of the Dengying Formation and the Kongmingdong Formation be the main exploration targets, and, horizontally, the Jiulongshan structural trap be the most beneficial exploration belts recently due to its matching with petroleum migration and accumulation.

A new family of Cambrian rhynchonelliformean brachiopods (Order Naukatida) with an aberrant coral-like morphology

Tomteluva perturbata gen. et sp. nov. and Nasakia thulensis gen. et sp. nov., two new rhynchonelliformean brachiopod taxa, are described from carbonate beds from the lower middle Cambrian (Series 3, Stage 5) basinal Stephen Formation, Canada, and the upper lower Cambrian (Series 2, Stage 4) Henson Gletscher Formation, North Greenland, respectively. The two taxa are characterized by an unusual coral-like morphology typified by a high conical ventral valve with an anteriorly curved umbo and a tube-like structure inside the ventral valve, interpreted as pedicle tube. Both resemble the problematic late middle Cambrian (Drumian) species Anomalocalyx cawoodi Brock from Australia, whose systematic affiliation is controversial. Together, the three genera are interpreted as representatives of a new family of rhynchonelliformean brachiopods, the Tomteluvidae fam. nov., which is interpreted as an aberrant or derived taxon within the Order Naukatida. Convergence between the Tomteluvidae and the coralla of small solitary Cambrian coralimorphs, as well as the late Palaeozoic reef-building richthofenioid brachiopods, might indicate adaptation to a similar life habits and environments. However, their small size (length 4 mm), well-developed pedicle and perfect morphological symmetry make it more likely that tomteluvids lived attached to frondose algae or sponges, above the seafloor, in a similar fashion to the acrotretoid brachiopods with which they show a high degree of morphological convergence. Morphological features of the pedicle tube of N. thulensis suggest that the tomteluvid pedicle is homologous to that in modern rhynchonelliformean brachiopods. This is the first evidence of the pedicle type within the Naukatida and represents the oldest confirmation of a rhynchonellate pedicle.

Growth mechanisms and geochemistry of carbonate concretions from the Cambrian Wheeler Formation (Utah, USA)

AbstractCarbonate concretions provide unique records of ancient biogeochemical processes in marine sediments. Typically, they form in organic‐rich mudstones, where a significant fraction of the bicarbonate required for carbonate precipitation is supplied from the decomposition of organic matter in the sediments. As a result, carbonates that comprise concretions are usually characterized by broad ranges in δ13C and include values that are significantly depleted relative to seawater. This article reports results from a physical, petrographic and geochemical analysis of 238 concretions from the Wheeler Formation (Cambrian Series 3), Utah, USA, which are unusual in several respects. Most prominently, they formed in organic‐poor mudstones (total organic carbon = 0·1 to 0·5%) and are characterized by a narrow range of δ13C that onlaps the range of contemporaneous seawater values. Subtle centre to edge trends in δ13C demonstrate that concretion precipitation was initiated by local chemical gradients set up by microbial activity in the sediments, but was sustained during growth by a large pool of inorganic bicarbonate probably derived from alkaline bottom waters. The large inorganic pool appears to have been important in facilitating rapid precipitation of the concretion matrix, which occurred via both displacive and replacive carbonate precipitation during early diagenesis. Stable isotope data from cogenetic pyrite (δ34S) and silica (δ18O) phases provide insight into the evolution of biogeochemical processes during concretion growth, and suggest that concretions were formed almost entirely during sulphate reduction, with only minor modification thereafter. Concretions of the Wheeler Formation appear to represent an end‐member system of concretion formation in which rapid growth was promoted by ions supplied from sea‐water. As such, they offer insight into the spectrum of processes that may influence the growth of carbonate concretions in marine sediments.