Vendian Early Cambrian Research Articles

Paleogeotectonic position of the Kyzyk Chadr porphyry copper ore field, Republic of Tyva

Regional-scale localization conditions of the Kyzyk-Chadr molybdenum-copper porphyry ore field in the southern folded frame of the Siberian Platform are considered. The porphyry mineralization of the field formed at the Salairian (Early Caledonian) phase of development of the Altai-Sayan segment of the
 Central Asian Orogenic Belt (CAOB), in connection with small intrusions of the gabbro-diorite-granodioritegranite Kyzykchadr Complex (Є2–3). The formation of the ore-bearing complex was associated with an accretionalcollisional magmatic belt superimposed on a Vendian–Early Cambrian volcanic-sedimentary island-arc substrate. The complex is only known within the local Ozhu uplifted block with a “batholitic” massif of the gabbrotonalite-plagiogranite Tannu-Ola Complex (Є2), but it is suggested that there could existed some analogous accretional-collisional Cambrian intrusions productive for Mo-Cu porphyry mineralization in some other areas, that were later overlapped by Ordovician–Silurian and Early Devonian volcanosedimentary deposits or broken by
 intrusive massifs of the Caledonian collisional volcano-plutonic belt. The rocks of the Salairian and Caledonian magmatic belts were deformed during subsequent geotectonic stages, primarily the Late Paleozoic collision, and overlain by younger sedimentary deposits. The position of economic-grade Mo and Mo-Cu porphyry deposits in the southern Altai-Sayan segment of the CAOB is assumed to be confined to areas of spatial superposition
 (overlapping) of magmatic belts of the three geotectonic stages: 1) island-arc volcanic belts (V–Є1), 2) “batholithic” intrusions and small intrusions of the magmatic belt of the accretional-collisional stage (Є2–3), 3) the collisional volcano-plutonic belt (O–D1).

Chemical Weathering Indices and their Use for Paleoclimatic Reconstructions (on the Example of the Vendian‒Lower Cambrian Section of Podolsk Transnistria)

The article provides an overview of various indices/indicators (Vogt, Parker, CIA, CIW, PIA, MIA, etc.) used in the study of weathering profiles/crusts and reconstruction of paleoclimatic environments of sedimentary sequence accumulation. Their possibilities are demonstrated by the example of Vendian-Lower Cambrian terrigenous deposits of Podolsk Transnistria (southwestern slope of the Ukrainian Shield). The distribution of the ba1 index values in this section indicates the presence of the material most transformed by the processes of chemical weathering in the mudstones of the Nagoryany Formation, the lower part of the Danylivka and the middle part of the Studenytsya formations. For mudstones of the Danylivka–Zbruch interval, the HM values are close to the HMPAAS. For rocks of the Yaryshiv–Nagoryany interval, the HM values are slightly higher, and for mudstones in the lower part of the Yaryshiv Formation, they are comparable to those inherent in continental clays of a hot tropical climate. The average SA index for mudstones of the entire section is 5.6 ± ± 0.7. Mudstones of the Hrushka–Nagoryany interval, where SA SAPAAS, are composed of more weathered material. The WIP values in the mudstones of the Mohyliv and Yaryshiv formations, as well as in the upper part of the Zbruch Formation, correspond to the interval of their values between PAAS and the average Archean granite. Clay rocks of other formations have WIP ≤ WIPPAAS values. The average CIA value for mudstones is 71 ± 4, which practically corresponds to the CIA value (70), which separates the sediments of cold/arid and warm/humid climates. Variations in the CIW index along the section are symbate with variations in the CIA. The vast majority of mudstones are characterized by PIA PIAPAAS values. The average CPA value is 91 ± 4, which is also typical for PAAS. The above and other data show that on the basis of a “direct” interpretation of the values of various indices of chemical weathering inherent in fine-grained clastic rocks, the paleoclimate that existed in the Vendian‒Early Cambrian on the territory of Podolsk Transnistria was rather moderate or warm humid. Comparison of the CIA values of mudstones with the values of this indicator for the particulate suspended matter of modern rivers suggests that the climate in the Vendian‒Early Cambrian resembled dry and humid subtropical or dry tropical with elements of humid.

New data on the trace element composition and Lu-Hf isotopic system of zircons from the Early Cambrian granites of the Ufaley block (Middle Urals): a step to the correction of geodynamic concepts

Research subject. Composition and isotopic parameters of zircons and their host granites of the Bitim complex, localised in the eastern part of the Ufaley block (Middle Urals).Materials and methods. The determination of trace element composition and Lu-Hf isotope system in zircons was carried out by LA-ICP-MS (the “Geoanalitik” shared research facilities of the IGG UB RAS, Ekaterinburg).Results and conclusions. The images and geochemical features of the main zircon population from granites confirm their magmatic genesis and minimal alteration, which proves previously defined Vendian- Cambrian (520 ± 9 Ma) age for the granites. Younger zircons (401–459 Ma) differing in composition from the older ones, were formed during subsequent tectonic-thermal activity, possibly under the fluid action. The mixed mantle-crustal nature of substrate for the granite melting is indicated by specific isotopic parameters (87Sr/86Sr = 0.703389, εHf(t)aver = +6.3) and a number of characteristic ratios for zircons and granites. The geodynamic concepts of the granite formation settings of the Bitim complex are corrected: in the Vendian–Early Cambrian, a transform-type margin has developed in this sector, the divergent movements on which have been accompanied by a break in the continuity of the crust and the intrusion of a deep mantle diapir; the participation of the slab substance in the magma generation might also be assumed.Conclusions. The results obtained can be used for the geological mapping and correction of a general geodynamic scheme for the junction zone between the Ural Mobile Belt and the East European platform.

Palаeomagnetism of the Vendian traps of Volyn, southwestern margin of the East European platform. P. 1: palаeomagnetic directions and poles

In Earth’s geological evolution the Vendian is significant because: 1) it preceded the «Cambrian Evolutionary explosion» when the multicellular organisms became suddenly much more diverse; 2) it is associated with the global tectonic and palaeogeographic restructuring of supercontinents, a change in geomagnetic field generation and other global processes affecting different layers of the Earth. At the same time, recent publications indicate an extremely irregular behavior of the geomagnetic field in the Vendian, which significantly differs from the recent regime of the geomagnetic field generation. New information on the configuration and magnitude of the geomagnetic field is crucial for understanding the reasons that can cause different modes of geodynamo. The article describes the new results of palaeomagnetic studies of the Volyn traps — basalts and tuffs of the Volyn series with ages about 560—580 My. The determined palaeodirections and palaeopoles are in good agreement with the previously published data of other authors and substantially complement them. The ChRM-component of «low-titanium» basalts was isolated in high-temperature (>500 °C) and characterized as primary. The same samples were used for palaeointensity determinations which showed the extremely low values of the geomagnetic field — an order of magnitude lower than the present one. The ChRM-component of «high-titanium» basalts was yielded in the temperature range of 200—400 °C. No results were obtained from palaeointensity experiments, and the complex nature of remanent magnetization remains unclear. The new data are compared with the palaeo-magnetic data of the Vendian—Early Cambrian poles of the East European Platform determined by other authors. To test the hypothesis of an anomalous Ediacaran magnetic field characterized by a low dipole moment and a high inversion frequency, more data are necessary. These data can be obtained from the studies of a stratigraphically more complete section of the Volyn traps opened by boreholes. These results will be presented in the next part of the article.

Isotope stratigraphy and U-Pb dating of detrital zircons from the Vendian-Cambrian deposits of the North Muya block

U-Pb dating of detrital zircons from the sandstones of the Mamakan Formation has been made. Geochemical and isotope parameters of the carbonate deposits of the Yanguda Formation in the Vendian-Cambrian cover of the North Muya continental block have been estimated. It has been established that only the Neoproterozoic (630-915 Ma) rocks of the North Muya block were the provenances of terrigenous material. In the least altered carbonate rocks of the Yanguda Formation, the 87Sr/86Sr ratio is within 0.70814-0.70879 and δ13C varies from -0.4 to + 1.9‰. Comparison of the evaluated isotope parameters with those of carbonate rocks of typical Vendian-Cambrian sections shows that the carbonate deposits of the Yanguda Formation accumulated in the Early Cambrian, about 520 Ma. Sedimentation of the Mamakan and Yanguda Formations took place in the local sedimentary basin in the Vendian-Early Cambrian, in the absence of tectonic activity within the North Muya block. Detrital material that formed during the destruction of the rocks of the Siberian Platform basement and cover was not supplied into the basin.

Features of magmatim-related metallogeny of Gorny Altai and Rudny Altai (Russia)

The Rudny Altai and Gorny Altai regions had different geologic histories and differ in metallogenic patterns. The Vendian-Early Cambrian to Permian-Triassic multistage evolution of Gorny Altai included subduction, accretion-collision, and rifting events accompanied by magmatism and related mineralization. Metallogeny evolved in discrete pulses, with especially abundant Late Paleozoic-earliest Mesozoic mineralization. The Devonian-Carboniferous pulse produced diverse mineral deposits (iron, mercury, gold, silver, molybdenum, tungsten, cobalt, polymetallic ores, and rare earths), some of considerable economic value. The territory of Gorny Altai includes several large ore districts that belong to different zones. They are the Beloretsk-Kholzun iron district in the west, the Kayancha-Sinyukha fluorine-gold district in the northeast, the Kurai gold-mercury and Yustyd rare-metal-silver districts in the southeast, and the Kalguty rare-metal-tungsten and Ulandryk U-REE-Cu districts in the south. The largest mineral deposits are Kholzun (Fe, P2O5), Karakul (Co, Bi), Sinyukha (Au), Aktash and Chagan-Uzun (Hg), Ozernoe and Pogranichnoe (Ag), Kalguty (Mo, W), Alakha (Li, Ta), Rudnyi Log (Y,Fe-specularite), and Urzarsai (W-scheelite). Mineralization in Rudny Altai is mainly pyritic: copper-pyrite, pyrite-polymelallic ore, and barite-polymelallic ore. It resides in suprasubduction basalts and rhyolites and in Emsian to Frasnian island-arc volcanics at different stratigraphic levels of Devonian volcanosedimentary sequences in six ore districts. The Kurchum high-grade metamorphic block hosts copper-pyrite and gold-quartz mineralization related to Hercynian volcanism.

GEOCHRONOLOGY OF THE METAMORPHIC ROCKS IN THE KURAI ACCRETIONARY PRISM (SOUTH-EASTERN GORNY ALTAI)

The paper provides a review of the published and new geological and geochronological data on the metamorphic rocks in the Kurai accretionary prism, considering the evolution of the Kuznetsk-Altai island-arc paleosubduction channel of the Siberian continent. The following two stages are distinguished by 40 Ar/ 39 Ar and U/Pb dating: (1) 636–619 Ma and earlier: sinking of ophiolites into the subduction zone; (2) 604–585 Ma: sinking of the large bo­dies of oceanic uplifts into the subduction zone. These processes led to the exhumation of the high-pressure rocks and the hot Chagan-Uzun peridotites. The dynamo-thermal effect of the latter on the basalts is reflected in the inverted metamorphic zoning and the occurrence of garnet amphibolites and plagiogranite migmatites. It is probable that the paleo-seamounts collided with the island arc during the Vendian–Early Cambrian. The data reviewed in this paper give evidence of an active margin of the West Pacific type in the western Altai-Sayan folded region in the Vendian–Cambrian.

The Early Cambrian bimodal magmatism in the northeastern Siberian Craton

We present new data on geochemistry, isotopic geochemistry, and geochronology of the Early Cambrian igneous rocks of the northeastern Siberian Craton (Kharaulakh anticlinorium, contact between the Siberian Platform and the West Verkhoyansk sector of the Verkhoyansk fold-and-thrust belt united into an Early Cambrian bimodal complex. This complex comprises trachyrhyolites forming pebbles in conglomerates near the base of the Cambrian succession, overlying trachybasalts, and mafic sills and dikes cutting Neoproterozoic strata. According to chemical composition, the felsic rocks are high-alkali rhyolites and correspond to A-type granites. The high contents of Ta, Nb, Hf, Tb, and Zr in these rocks suggest the presence of enriched mantle material in their magmatic sources. The mafic volcanics are high-Ti trachybasalts and trachydolerites with similar geochemical characteristics corresponding to alkali basalts or OIB. The high (Tb/Yb)PM ratios in these volcanics evidence that their magmatic source was the garnet peridotite mantle located at depths more than 90 km and characterized by a low degree of melting. However, the rhyolites, trachybasalts, and trachydolerites show high positive εNd(T) values (4.2–4.7, 7.5–8.9, and 7.2–8.2, respectively) indicating a depleted mantle source and no crustal contamination. The high (Nb/Yb)PM ratio points to the mixing of magmas from enriched and depleted mantle sources. Mafic magmas might have been generated from a heterogeneous source or interacted with the depleted mantle before intrusion. Both the felsic and the mafic rocks formed in within-plate environments. U–Pb zircon dating yielded concordant ages of 525.6 ± 3.9 and 537.0 ± 4.2 Ma, corresponding to the Early Cambrian age of the rhyolites. The date of 546.0 ± 7.7 Ma obtained for one sample points (with regard to the error) to the Late Vendian–Early Cambrian age. Thus, at the Vendian–Early Cambrian boundary, the northeastern Siberian Platform was subjected to continental rifting accompanied by bimodal magmatism of antidrome evolution. According to paleotectonic reconstructions, this part of the Siberian Craton might have been connected with the eastern margin of Laurentia in the Late Neoproterozoic (Late Riphean–Late Vendian), and continental rifting that started at the Vendian–Cambrian boundary led to their separation. The obtained isotope-geochronological data suggest that the studied bimodal complex began to form at the Vendian–Cambrian boundary and this process terminated no earlier than the end of the Terreneuvian (Tommotian), i.e., the complex formed during rifting for about 20 Myr.

Nd isotope systematics of the Vendian–Early Cambrian sedimentary ores in the northern segment of the Paleoasian Ocean

Isotope–geochemical studies of Mn, P, and Ba ores were performed in order to establish the influence of submarine hydrotherms on the formation of Early Cambrian sedimentary rocks of the southern environs of the Siberian Platform. Based on study of the geochemical and isotope (eNd) characteristics of the shallow-water Mn and Ba ores and phosphorites of southern environs of the Siberian Platform with similar ages, two types of sedimentary basins of the different geodynamic origins were distinguished: intraplate oceanic and those of the active continental margin, for which the sources of ore materials differ by the proportions of the mantle and contaminated crustal matter.

Vendian–Early Cambrian island-arc plagiogranitoid magmatism in the Altai–Sayan folded area and in the Lake Zone of western Mongolia (geochronological, geochemical, and isotope data)

We generalize results of geological, geochronological, geochemical, and isotope-geochemical studies of the Vendian–Early Cambrian island-arc plagiogranitoid magmatism in the Altai–Sayan folded area and in the Lake Zone of western Mongolia. Based on these data, we analyzed the scales of development of plagiogranitoid magmatism, studied the petrologic composition and isotope characteristics of granitoids, and established the main sources of plagiogranitoid-generating melts and the leading mechanisms of formation of Early Caledonian juvenile crust.

Geothermobarometry of very low-grade metamorphic pelites of the Vendian–Early Cambrian Puncoviscana Formation (NW Argentina)

The Vendian–Early Cambrian Puncoviscana Formation is a pelite-greywacke turbidite sequence affected by polyphase deformation cropping out extensively in the Cordillera Oriental of northwestern Argentina. Previous X-ray diffraction and analytical high-resolution TEM studies on southern locations found anchizonal grade and proposed medium–high pressure metamorphism followed by higher thermal conditions. We have determined the chemical composition of mica and chlorite with EDX on small uncontaminated areas selected using SEM in backscattered electron mode. The application of mica-chlorite geothermobarometry to mineral grains defining two different foliations has produced consistent pressure/temperature results based on the intersection of six reactions: 9 kbar/250 °C peak conditions were followed by an isothermal decompression to 3 kbar and a successive increase of temperature to nearly 350 °C at 1.5 kbar. The shales from the overlying Meson and Santa Victoria Cambro–Ordovician groups do not contain chlorite, thus chlorite-mica geothermobarometry could not be applied. According to the range of KICIS obtained (0.41–0.68Δ°2θ) and their mineralogical assemblage (illite ± kaolinite ± corrensite + Qtz + Pl ± Kfs), these rocks have not surpassed diagenetic conditions, which implies a minimum temperature difference with the Puncoviscana Formation of 100 °C, consistent with the sharp angular unconformity between the two units.

The early stages of island-arc plagiogranitoid magmatism in Gornaya Shoriya and West Sayan

The structure, composition, and age of Vendian–Early Cambrian plagiogranitoid associations composing the Kshta and Taraskyr massifs of the Yenisei pluton in the Altai–North Sayan island-arc belt are considered. We have established that these associations formed within 550–520 Ma and differ in petrographic composition and sources. Two stages of island-arc plagiogranitoid magmatism are recognized: early (550–540 Ma, formation of plagiogranitoids of the Kshta (545 ± 8 Ma) and Taraskyr (545 ± 7 Ma) massifs) and late (525–520 Ma, formation of plagiogranitoids of the Maina complex of the Yenisei (524 ± 2 Ma) and Tabat plutons). By petrochemical composition and geochemical characteristics, the rocks of the Kshta massif are high-alumina plagiogranitoids similar to adakites. They might have been produced through the melting of metabasites compositionally similar to N-MORB in equilibrium with garnet-containing restite during the subduction of oceanic slab at ≥15 kbar. The rocks of the Taraskyr massif are low-alumina plagiogranites. They formed through the melting of metabasites located in the lower layers and(or) the basement of the island-arc system in equilibrium with plagioclase-containing restite at 3–8 kbar. The low-alumina plagiogranitoids of the Yenisei pluton melted out under the same conditions. Isotope-geochemical studies showed that the Vendian–Early Cambrian plagiogranitoids formed at the early stage are characterized by high positive ∑Nd(T) values (7.5–4.9), Late Riphean model Nd-age (TNd(DM) = 0.64–0.98 Ga), and Sr isotope ratio varying from 0.7040 to 0.7053. These data point to the juvenile parental melts of the rocks and the varying content of ancient crustal material in the magma generation zone.

The petroleum potential of the Riphean-Vendian succession of southern East Siberia

Abstract The Siberian Platform covers an area of c. 4.5 million km 2 in the East Siberia region of Russia, up to 3.5 million km 2 of which is prospective for hydrocarbons. We review the Archaean to Neoproterozoic evolution of the Siberian Platform and the potential oil and gas resources of Riphean, Vendian and Infracambrian sediments. The Riphean was dominated by passive margin sedimentation and was intensely deformed during the Baikalian orogeny. Vendian strata record a clastic transgressive sequence and the eventual re-establishment of carbonate platform sedimentation. The late Vendian–early Cambrian is characterized by carbonate deposition including thick salt horizons, which form a regional seal. Hydrocarbon maturation and migration from Riphean and Vendian source rocks occurred during the late Neoproterozoic and Early Palaeozoic, indicating that hydrocarbon reservoirs on the Siberian Platform may have hosted their reserves over a remarkable period of geological time. Despite many years of hydrocarbon exploration in East Siberia, many regions remain under-explored, and aspects of the proven hydrocarbon systems are poorly understood. There are undoubtedly more major discoveries to be made in the region, and the Infracambrian succession of the southern Siberian Platform therefore represents an irresistible target for further hydrocarbon exploration.

The tectonomagmatic evolution of structure-lithologic complexes in the Tannu-Ola zone, Tuva, in the Late Vendian–Early Cambrian ( from geochemical, Nd isotope, and geochronological data)

In the Tannu-Ola zone, there are island-arc (Late Vendian) and subduction-accretionary (first half of the Early Cambrian) complexes whose mantle sources were generated from melts produced in suprasubduction conditions as a result of the melting of mantle wedge peridotites. The island arc, whose fragment is present in the Tannu-Ola zone, formed tentatively at 570 Ma and ceased as an individual structure at the Vendian–Cambrian boundary, though the subduction continued till the middle Early Cambrian. From the Vendian–Cambrian boundary to middle Early Cambrian, the accretion of island-arc, back-arc, oceanic, and metamorphic structure-lithologic complexes took place at this segment of the Central Asian Fold Belt. From middle to late Early Cambrian, the Tannu-Ola zone was an active continental margin, where basalts with island-arc characteristics, quartz microdiorites of adakite composition, basalts, dolerites, and quartz microdiorites of intraplate type, and diorite-tonalite-plagiogranite association of island-arc type formed. To explain the spatial and temporal conjugation of different magmatism manifestations on the active margin, we applied the slab–window model.

Late Vendian-Early Cambrian formations of the north-west Siberian Craton

The composition and structure of principal key-sections for the Tira (Late Vendian) and Danilovo (Late Vendian-Early Cambrian) Horizons were characterized on the basis of deep-drilling data and studies of natural outcrops along the peripheries of the Kureika syneclise. The typical sedimentary formations/associations were recognized, their vertical and lateral successions were distinguished, and their deposition environments were interpreted. A distribution pattern scheme of thicknesses and depositional environments of Upper Vendian-Lower Cambrian deposits was compiled at the 1: 1000000 scale. The reservoir characteristics of the formations are discussed and forecasted for poorly studied areas of the northwestern Siberian platform.

Upper mantle roots of Siberian craton basement structures along the Rift DSS profile

We investigate the upper mantle velocity structure through processing first arrival data from peaceful nuclear explosions. The reported 2D model has been obtained by ray tracing for a spherical Earth, unlike the classical plane-approximation approach with subsequent spherical symmetry corrections, which is not always applicable to a laterally heterogeneous subsurface. The upper mantle velocity highs and lows imaged to 200–220 km depths show obvious correlation with major structures of the craton basement. Namely, low-velocity zones are observed beneath basins, the largest (to 8.0–8.1 km/s) under the Vendian–Early Cambrian Sayan–Yenisei syneclise. A discontinuous high-velocity layer (8.6–8.7 km/s) at depths between 150 and 240 km is underlain by a zone of lower velocity (8.50–8.55 km/s) down to the 410 km discontinuity, where the velocity at the top of the transition zone is 9.4–9.5 km/s.

Dzhida island-arc system in the Paleoasian Ocean: structure and main stages of Vendian-Paleozoic geodynamic evolution

We propose a model of the geodynamic evolution of the Dzhida island-arc system of the Paleoasian Ocean margin which records transformation of an oceanic basin into an accretion-collision orogenic belt. The system includes several Vendian-Paleozoic complexes that represent a mature oceanic island arc with an accretionary prism, oceanic islands, marginal and remnant seas, and Early Ordovician collisional granitoids. We have revealed a number of subunits (sedimentary sequences and igneous complexes) in the complexes and reconstructed their geodynamic settings. The tectonic evolution of the Dzhida island-arc system comprises five stages: (1) ocean opening (Late Riphean); (2) subduction and initiation of an island arc (Vendian-Early Cambrian); (3) subduction and development of a mature island arc (Middle-Late Cambrian); (4) accretion and formation of local collision zones and remnant basins (Early Ordovician-Devonian); and (5) postcollisional strike-slip faulting (Carboniferous-Permian).

Fragments of oceanic islands in accretion–collision areas of Gorny Altai and Salair, southern Siberia, Russia: early stages of continental crustal growth of the Siberian continent in Vendian–Early Cambrian time

The Altai-Salair area in southern Siberia is a Caledonian folded area containing fragments of Vendian–Early Cambrian island arcs. In the Vendian–Early Cambrian, an extended system of island arcs existed near the Paleo-Asian Ocean/Siberian continent boundary and was located in an open ocean realm. In the present-day structural pattern of southern Siberia, the fragments of Vendian–Early Cambrian ophiolites, island arcs and paleo-oceanic islands occur in the accretion–collision zones. We recognized that the accretion–collision zones were mainly composed of the rock units, which were formed within an island-arc system or were incorporated in it during the subduction of the Paleo-Asian Ocean under the island arc or the Siberian continent. This system consists of accretionary wedge, fore-arc basin, primitive island arc and normal island arc. The accretionary wedges contain the oceanic island fragments which consist of OIB basalts and siliceous—carbonate cover including top and slope facies sediments. Oceanic islands submerged into the subduction zone and, later were incorporated into an accretionary wedge. Collision of oceanic islands and island arcs in subduction zones resulted in reverse currents in the accretionary wedge and exhumation of high-pressure rocks. Our studies of the Gorny Altai and Salair accretionary wedges showed that the remnants of oceanic crust are mainly oceanic islands and ophiolites. Therefore, it is important to recognize paleo-islands in folded areas. The study of paleo- islands is important for understanding the evolution of accretionary wedges and exhumation of subducted high-pressure rocks.

Possible REE constraints on the depositional and diagenetic environment of Doushantuo Formation phosphorites containing the earliest metazoan fauna

The Neoproterozoic Doushantuo Formation, South China, preserves a unique assemblage of what are probably the earliest metazoan fossils (sponges, cnidarians, and bilaterians) in the world that could contribute to a better understanding of early faunal evolution on Earth. The formation spans an interval of dramatic biological, chemical, and climatic change. In Weng'an mine in southwest China, extensive paleontological investigations show embryonic, larval, and adult animal fossils occur in the upper ore bed of the Doushantuo phosphorite, but no animal forms have been found in the lower ore bed. Taphonomy and experiments on fossilization suggest that no fauna lived on or in the lower ore bed as it was deposited. Rare-earth element (REE) data for the Doushantuo phosphorites in Weng'an document the paleoenvironmental condition under which the phosphorites were deposited. The shale-normalized REE patterns of the upper phosphorite bed (UPB) are characterized by “old” (Vendian–Early Cambrian) phosphorite patterns. The REE patterns of the lower phosphorite bed (LPB) are significantly different with less negative Ce anomalies and less LREE and HREE depletion. The REE characteristics suggest that the Doushantuo phosphorites record seawater condition at the time of sediment deposition and are not affected by subsequent diagenesis. The REE compositions of the Doushantuo phosphorites show that the marine depositional environment changed from anoxic in the lower ore bed to oxic in the upper ore bed. The uniquely diverse and abundant Doushantuo eukaryote fossil accumulations at Weng'an thus may be related to a change from anoxic to oxic bottom-water conditions.

Early Cambrian palaeogeography and the probable Iberia–Siberia connection

The end of the Proterozoic–beginning of the Cambrian is marked by some of the most dramatic events in the history of Earth. The fall of the Ediacaran biota, followed by the Cambrian Explosion of skeletonised bilaterians, a pronounced shift in oceanic and atmospheric chemistry and rapid climatic change from ‘snowball earth’ to ‘greenhouse’ conditions all happened within a rather geologically short period of time. These events took place against a background of the rearrangement of the prevailing supercontinent; some authors view this as a sequence of individual supercontinents such as Mesoproterozoic Midgardia, Neoproterozoic Rodinia and Early Cambrian Pannotia. Assembled in the Mesoproterozoic, this supercontinent appears to have existed through the Neoproterozoic into the Early Cambrian with periodic changes in configuration. The final rearrangement took place during the Precambrian–Cambrian transition with the Cadomian and related phases of the Pan-African orogeny. The distribution of Early Cambrian molluscs and other small shelly fossils (SSF) across all continents indicates a close geographic proximity of all major cratonic basins that is consistent with the continued existence of the supercontinent at that time. Subsequently, Rodinia experienced breakup that led to the amalgamation of Gondwana, separation of Laurentia, Baltica, Siberia and some small terranes and the emergence of oceanic basins between them. Spreading oceanic basins caused a gradual geographic isolation of the faunal assemblages that were united during the Vendian–Early Cambrian.