Cambrian Rocks Research Articles

Crustal Thermal Architecture, Structural Reconstructions, Field Relationships, and Geophysical Data Rule Out Deep Structural Burial of the Footwall of the Northern Snake Range Metamorphic Core Complex (Nevada, USA)

AbstractThermobarometry in the Northern Snake Range metamorphic core complex (Nevada, USA) implies pre‐extensional burial of footwall rocks to 21–30 km depths, while geologic field relationships support 7–13 km pre‐extensional depths. This has fueled a 40‐year‐long debate, which has far‐reaching implications for how pressure data are interpreted in orogenic settings. Here, we test published models for deep burial by integrating regional cross‐section reconstructions with new (n = 95) and published (n = 132) peak temperature measurements, field relationships and published geophysical data. Burial of Neoproterozoic‐Cambrian metasedimentary footwall rocks to 21–30 km depths is incompatible with a regional seismic reflection cross‐section that interprets the top of Precambrian crystalline basement at 17–20 km depths. Two reconstructed cross‐sections define 42 km and 50–65 km of displacement on the master detachment fault and demonstrate that the higher displacement ranges (>66–94 km and >76–102 km, respectively) necessary to exhume rocks from 21 to 30 km depths are not possible without spatially overlapping Cambrian rocks preserved in its footwall and hanging wall. The 22°C/km average Late Cretaceous thermal gradient predicted by thermobarometry is incompatible with the 46 ± 10°C/km Late Cretaceous peak thermal gradient that we calculate down to 15–20 km pre‐extensional depths. Field relationships that rule out large‐magnitude shortening invalidate models for deep footwall burial via thrust or reverse faulting. We conclude that there is no scenario for deep burial that is compatible with structural/geophysical constraints, crustal thermal architecture, and field relationships. This necessitates a non‐lithostatic interpretation for pressures from the Northern Snake Range, similar to recent interpretations for other Cordilleran metamorphic core complexes.

Trilobites from the upper middle Cambrian (Miaolingian, Guzhangian) of the Sierra Los Ajos, northern Sonora, Mexico: Biostratigraphic and paleogeographic considerations

A Cambrian sedimentary sequence with a predominant lithology of limestone, sandy limestone, and quartzite pertaining to the Bolsa and Abrigo formations is exposed in the Sierra Los Ajos to the east of Cananea city (northeastern Sonora, Mexico). Two fossil assemblages from Guzhangian layers of the Abrigo Formation are described herein. The trilobites identified comprise Tricrepicephalus texanus (Shumard), Crepicephalus sp., Coosia ariston (Walcott), Coosia sp., Cedaria cf. eurycheilos Palmer, Coosella sp., Llanoaspis modesta Lochman, and Llanoaspis? sp., representing the Cedaria and Crepicephalus zones (Miaolingian, Guzhangian). The Cambrian rocks of northeastern Sonora (Sierra Los Ajos, El Tule, and Mesteñas hills), southern Arizona, and Texas, were deposited as part of a thick sedimentary sequence on the southwestern margin of Laurentia, associated with an inner shelf marine environment. The distribution of the trilobites identified confirms the existence of a widespread Cambrian faunal province that includes the southeastern United States of America and northwestern Mexico.

Two distinct early Paleozoic subduction zones in the eastern Central Asian Orogenic Belt: Evidence of subduction recycling and arc evolution

This paper presents zircon U–Pb dating and Hf isotopic, and whole-rock geochemical and Sr–Nd isotopic data for early Paleozoic igneous rocks as well as zircon U–Pb dating for the sedimentary rocks of the Songnen Massif within the eastern Central Asian Orogenic Belt (CAOB), aiming to reveal arc magma evolution process. The zircon U–Pb dating results of the siltstone of the Xiaojingou Formation yielded 450 Ma, 471 Ma, 502 Ma, 773 Ma, 818 Ma and 949 Ma age peaks, as well as multi-episodic Paleo-proterozoic ages (2486-1612 Ma), implying its deposition time is younger than ∼ 450 Ma. The zircon U–Pb dating results reveal that the Songnen Massif exists three episodes of volcanic rocks of the middle Cambrian (502 Ma), Middle Ordovician (453-449 Ma) and the Silurian (435-420 Ma). The middle Cambrian basalts are low-K tholeiitic to calc-alkaline series, weakly depleted εHf(t) and εNd(t) values. They were derived from the weakly depleted mantle metasomatized by slab fluid in an immature island arc setting. The Middle Ordovician to Silurian volcanic rocks show SiO2 of 49.4–58.1 wt%, plotting into high-K calc-alkaline series, which are also featured by higher Th/Nd and (Hf/Sm)PM ratios, together with moderate depleted εNd(t) values, suggesting their derivation of the depleted mantle wedge metasomatized by sediment-derived melt. In contrast, coeval early Paleozoic volcanic rocks in the eastern Xing’an Massif with the MORB-like εNd(t) values are characterized by an origin of the depleted mantle wedge and the distinct interaction of oceanic crust melt and mantle peridotite. Combined with the variation of calculated crust thickness, we suggested that there was a continental arc in the eastern Songnen Massif, which developed from immaturity to maturity during middle Cambrian to Silurian, while the early Paleozoic island arc in the eastern Xing’an Massif is characterized by ocean ridge subduction process.

Utilizing compositions of zircon and apatite for prospecting of Cu-Mo-Auporphyry mineralization in the Pekinsky and Tessemsky granitoid massifs of the Taimyr-Severozemelskaya folded area

Research subject. The Tessemsky granite massif is located in the North Taimyr tectonic zone, surrounded by Cambrian rocks. The Pekinsky granite massif is located within the Central Taimyr zone, surrounded by metamorphosed Proterozoic rocks. Aim. To develop a methodology for using the composition of accessory granitoid minerals when prospecting Cu-Mo-Au-porphyry mineralization on the example of the Pekinsky and Tessemsky granitoid massifs of the Taimyr Peninsula. Materials and methods. Accessory zircon and apatite contained in two granitoid samples from the Pekinsky massif (P1, P2) and two granitoid samples from the Tessemsky massif (T2, T3) were studied. Their mineral composition was examined using an EPMA Cameca SX100 instrument. The element content in minerals was determined by LA-ICPMS using an NexION 300S instrument equipped with an NWR 213 attachment. Results. Most of the zircons from the Pekinsky and Tessemsky massifs were formed at T < 738°C in oxidized magma with ΔFMQ of 0.6–2.6, which is a favorable sign for the identification of Cu-Mo-Au-porphyry mineralization. Zircons are characterized by elevated (Eu/Eu*)Y and (Ce/Nd)n/Y ratios, which is also a favorable, though not a strongly reliable, sign for identifying porphyry mineralization. The Eu/Eu* and Sr/Y ratios in the apatites from the Tessemsky massif are higher than those in the apatites from the Pekinsky massif. The rock compositions of both massifs fall within the fields of adakites on the classification diagrams. The estimates of oxygen fugacity (logfO2) calculated from Mn in apatites for four samples agree well within the error limits. Conclusion. Specific features of using the composition of accessory minerals (zircon and apatite) for prospecting the Cu-Mo-Au-porphyry mineralization associated with granitoids were considered. Accessory indicator minerals can be used to rank granitoid massifs in order to assess their ore content. The example of two granite intrusions of the Taimyr Peninsula made it was possible to show that the Tessemsky massif is more promising for the discovery of associated Cu-Mo-Auporphyry mineralization than the Pekinsky massif.

Identifying and characterizing missing source orogens for syn-orogenic basins based on detrital accessory mineral U-Pb geochronology and trace element geochemistry

Abstract Identifying source orogens for syn-orogenic sediments in dispersed fragments of a supercontinent provides key information on the tectonic evolution of the orogens involved in supercontinent assembly and on paleogeographic reconstruction. An approach using U-Pb geochronology and trace elements of multiple detrital minerals including zircon, rutile, apatite, and monazite has major advantages over detrital zircon studies alone. Application of this multi-pronged approach to Ediacaran and Cambrian clastic sedimentary rocks from the West Cathaysia terrane of the South China block demonstrates that these rocks were deposited in two distinct, but overlapping, syn-orogenic basins related to the assembly of eastern Gondwana. The Ediacaran rocks have detritus likely sourced from the Paterson-Petermann orogen (PPO) in Australia, and the dominant 650–600 Ma populations and trace element compositions of detrital rutile and apatite indicate a significant Ediacaran thermal and high-pressure metamorphic event in the orogen. The appearance of Cambrian and Tonian detrital rutile and apatite populations in the Cambrian rocks indicates derivation from a different source orogen characterized by multiple thermal events, most likely the Kuunga-Pinjarra orogen (KPO). Our data suggest that West Cathaysia was located adjacent to both the PPO and the KPO and support the hypothesis that the PPO was a collisional, rather than a long-lived post-Grenvillian intraplate, orogen. Our data also suggest a poly-metamorphic history for the poorly preserved northern part of the KPO and a larger expanse of Greater India.

The Molecular Profile of Soil Microbial Communities Inhabiting a Cambrian Host Rock.

The process of soil genesis unfolds as pioneering microbial communities colonize mineral substrates, enriching them with biomolecules released from bedrock. The resultant intricate surface units emerge from a complex interplay among microbiota and plant communities. Under these conditions, host rocks undergo initial weathering through microbial activity, rendering them far from pristine and challenging the quest for biomarkers in ancient sedimentary rocks. In addressing this challenge, a comprehensive analysis utilizing Gas Chromatography Mass Spectrometry (GC-MS) and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) was conducted on a 520-Ma-old Cambrian rock. This investigation revealed a diverse molecular assemblage with comprising alkanols, sterols, fatty acids, glycerolipids, wax esters, and nitrogen-bearing compounds. Notably, elevated levels of bacterial C16, C18 and C14 fatty acids, iso and anteiso methyl-branched fatty acids, as well as fungal sterols, long-chained fatty acids, and alcohols, consistently align with a consortium of bacteria and fungi accessing complex organic matter within a soil-type ecosystem. The prominence of bacterial and fungal lipids alongside maturity indicators denotes derivation from heterotrophic activity rather than ancient preservation or marine sources. Moreover, the identification of long-chain (>C22) n-alkanols, even-carbon-numbered long chain (>C20) fatty acids, and campesterol, as well as stigmastanol, provides confirmation of plant residue inputs. Furthermore, findings highlight the ability of contemporary soil microbiota to inhabit rocky substrates actively, requiring strict contamination controls when evaluating ancient molecular biosignatures or extraterrestrial materials collected.

Multi-stages of Paleozoic deformation of the fault system in the Tazhong Uplift, Tarim Basin, NW China: Implications for hydrocarbon accumulation

The Tazhong Uplift is an important petroliferous secondary structure unit in the Tarim Basin and has undergone multiple structural deformation. In the current study, we use 3D and 2D seismic data to investigate the nature of the fault system in the Tazhong Uplift. The deformation of the Cambrian sedimentary rocks in the Tazhong Uplift is generally neglected but noted in this study. We found that the Lower–Middle Cambrian rocks are involved in thrust faults. NE-trending weak zones acted as accommodation faults formed at the connective positions and bends of the deep thrust faults. The Middle Cambrian compressional stress stemmed from the northeastern margin of the Tarim Block, pointed SW, and caused the uprising of the Tabei Uplift. The second deformation occurred in the Late Ordovician. In this instance, the compressional stress stemmed from the SW direction of the Tazhong Uplift, which originated from the amalgamation of the Western Kunlun Terrane and Tarim Block. The main strike–slip fault surfaces and subordinate faults developed along the weak zones generated in the Middle Cambrian. The collision that occurred in the southeast part of the Tarim Block in the Middle Silurian to Middle Devonian resulted in the third deformation. The representative structures that occurred during this compressive deformation were the intensive NEE-trending thrust faults in the southeast of the Tazhong Uplift. The three stages of deformation played different and important roles in hydrocarbon accumulation. These strike–slip faults holding the high-productivity hydrocarbon wells generally experienced early-stage Cambrian deformation, had wide fracture damage zones in the Ordovician carbonate rock strata, and had weak en echelon faults in the Silurian to Middle Devonian clastic rocks.

The Cabrières Biota (France) provides insights into Ordovician polar ecosystems

Early Palaeozoic sites with soft-tissue preservation are predominantly found in Cambrian rocks and tend to capture past tropical and temperate ecosystems. In this study, we describe the diversity and preservation of the Cabrières Biota, a newly discovered Early Ordovician Lagerstätte from Montagne Noire, southern France. The Cabrières Biota showcases a diverse polar assemblage of both biomineralized and soft-bodied organisms predominantly preserved in iron oxides. Echinoderms are extremely scarce, while sponges and algae are abundantly represented. Non-biomineralized arthropod fragments are also preserved, along with faunal elements reminiscent of Cambrian Burgess Shale-type ecosystems, such as armoured lobopodians. The taxonomic diversity observed in the Cabrières Biota mixes Early Ordovician Lagerstätten taxa with Cambrian forms. By potentially being the closest Lagerstätte to the South Pole, the Cabrières Biota probably served as a biotic refuge amid the high-water temperatures of the Early Ordovician, and shows comparable ecological structuring to modern polar communities.

Transgression–regression cycles drive correlations in Ediacaran–Cambrian rock and fossil records

AbstractStrata of the Ediacaran Period (635–538.8 Ma) yield the oldest known fossils of complex, macroscopic organisms in the geologic record. These “Ediacaran-type” macrofossils (known as the Ediacaran biota) first appear in mid-Ediacaran strata, experience an apparent decline through the terminal Ediacaran, and directly precede the Cambrian (538.8–485.4 Ma) radiation of animals. Existing hypotheses for the origin and demise of the Ediacaran biota include: changing oceanic redox states, biotic replacement by succeeding Cambrian-type fauna, and mass extinction driven by environmental change. Few studies frame trends in Ediacaran and Cambrian macroevolution from the perspective of the sedimentary rock record, despite well-documented Phanerozoic covariation of macroevolutionary patterns and sedimentary rock quantity. Here we present a quantitative analysis of North American Ediacaran–Cambrian rock and fossil records from Macrostrat and the Paleobiology Database. Marine sedimentary rock quantity increases nearly monotonically and by more than a factor of five from the latest Ediacaran to the late Cambrian. Ediacaran–Cambrian fossil quantities exhibit a comparable trajectory and have strong (rs > 0.8) positive correlations with marine sedimentary area and volume flux at multiple temporal resolutions. Even so, Ediacaran fossil quantities are dramatically reduced in comparison to the Cambrian when normalized by the quantity of preserved marine rock. Although aspects of these results are consistent with the expectations of a simple fossil preservation–induced sampling bias, together they suggest that transgression–regression and a large expansion of marine shelf environments coincided with the diversification of animals during a dramatic transition that is starkly evident in both the sedimentary rock and fossil records.

Resource potential of the Proterozoic–Paleozoic Carrara depocentre, South Nicholson region, Australia: Insights from stratigraphic drilling

A comprehensive geochemical program was carried out on rock samples collected in the NDI Carrara 1 drill hole, the first stratigraphic test of the newly discovered Carrara Sub-basin located in the South Nicholson region of northern Australia. The drill hole recovered continuous core from 284 m to total depth at 1750 m and intersected approximately 1120 m of Proterozoic sedimentary rocks, unconformably overlain by 630 m of Cambrian Georgina Basin carbonate-rich rocks. Total organic carbon (TOC) contents from Rock-Eval pyrolysis highlight the potential for several thick black shales to be a source of petroleum for conventional and unconventional plays. Cambrian rocks contain an organic-rich section with TOC contents of up to 4.7 wt.% and excellent oil-generating potential. The Proterozoic section is overmature for oil generation but mature for gas generation, with potential for generating gas in carbonaceous mudstones showing TOC contents up to 5.5 wt.% between 680 and 725 m depth. A sustained release of methane (up to 2%) recorded during drilling from 1150 to 1500 m suggests potential for an unconventional gas system in the Proterozoic rocks from 950 to 1415 m depth, which exhibit favourable organic richness and thermal maturity. The Proterozoic rocks, which are comparable in age to the sediment-hosted deposits of the Century Mine, contain local occurrences of lead, zinc and copper sulfide minerals providing hints of mineralisation. The combined geochemical results offer the promise of a potential new resource province in northern Australia.

The influence of synorogenic extension on the crustal architecture of North Gondwana during the assembly of Pangaea (Ossa–Morena Zone, SW Iberia)

Abstract We present a new structural study of a D 2 –M 2 tectono-thermal structure in SW Iberia (Ponte de Sor–Seda gneiss dome) characterized by a spatial distribution of telescoping isograds providing a record of Buchan-type metamorphic conditions. The gneiss dome comprises an infrastructure made up of a lower gneiss unit (LGU) and an intermediate schist unit (ISU), separated by early D 2 ductile extensional shear zones. The LGU and the ISU are composed of Ediacaran–Cambrian rocks that experienced the highest-grade M 2 metamorphic conditions (amphibolite facies). Late Ediacaran–Early Terreneuvian and Late Miaolingian–Early Furongian protolith ages for LGU (496 ± 3 Ma) and ISU (539 ± 2 Ma) orthogneisses are reported. A superstructure made of Cambrian–Devonian rocks (Upper Slate Unit, USU) deformed under M 2 greenschist facies conditions, tectonically overlies the ISU across a D 2 extensional shear zone. Kinematic criteria associated with D 2 –M 2 fabrics indicate top-to-ESE–SE sense of shear. A late-D 2 brittle-ductile high-angle extensional shear zone (Seda shear zone) crosscuts the gneiss dome. D 3 upright folds, thrusts and transpressive shear zones caused the steepening of D 2 structures and the local crenulation of S 2 foliation. The Mississippian D 2 –M 2 event recorded in the Ossa–Morena Zone may be regarded as a regional-scale phenomenon that markedly influenced the crustal architecture of North Gondwana during the assembly of Pangaea.

Biostratigraphy, Lithology and Geochemistry of the Lower and Middle Cambrian Rocks in the Kyulenke River Basin: First Results of Borehole Core Study

Abstract —We have studied biostratigraphy, lithology and geochemistry of drill cores from the Kyulenke River basin. The study of fossil remains and rock compositions, the analysis of trilobite distributions and changes in the lithological and geochemical characteristics of the sediments made it possible to identify structural features of the marker horizons, to correlate borehole sections and outcrops, to compare individual Cambrian intervals and the regional zones of the Yudoma–Olenek facies region of the Siberian Platform. The study has shown rocks of the Kuonamka Fomation to be enriched in uranium (U) and revealed a direct relationship between the contents of U and organic carbon (Corg) in the analyzed rocks, typical for the lower and middle Cambrian black shale deposits sampled from the outcrops in the eastern parts of the Siberian Platform. For the first time, a relationship of rock radioactivity by gamma-ray logging with the contents of U and Corg in Cambrian rocks was established.

Li-rich pegmatites and related peraluminous granites of the Fregeneda-Almendra field (Spain-Portugal): A case study of magmatic signature for Li enrichment

Based on field and petrographic observations, mineralogical, geochemical (whole-rock by ICP-MS, ICP-OES, ISE and PGNAA) and geochronological (ArAr on mica and UPb on columbite) data, ten pegmatite types and one type of cassiterite-rich quartz hydrothermal veins have been distinguished in the Fregeneda-Almendra field (FA) (Central Iberian Zone), some of them highly enriched in Li (included in petalite, spodumene and Li-rich mica). The pegmatites and the veins mostly intrude Neoproterozoic to Cambrian rocks from the Schist-Greywacke Complex. Felsic igneous magmatism during syn- to late-D3 stages (≈ 320–295 Ma) of the Variscan orogeny was important and nearly continuous in the area, with the overlap of different granitic units of highly peraluminous S-type composition. The most important in outcrop corresponds to the Mêda-Escalhão-Penedono-Lumbrales granitic complex (MEPL), which belongs to a two-mica leucogranitic series. The other granitic units (Saucelle, Feli and East-MEPL granites) are younger and belong to a P-rich, Ca-poor monzogranitic series. Spatial relationships, together with chemical, mineralogical and geochronological data, indicate that all the pegmatite types and the cassiterite-rich quartz hydrothermal veins are not cogenetic, being related to three different magmatic events: (i) (≈ 319–313 Ma) includes the syn-D3 MEPL granite and three types of barren pegmatites (intragranitic, quartz+andalusite layers and simple concordant pegmatites); (ii) (≈ 305–300 Ma) corresponds to the late to post-D3 Feli granite, related to a group of cassiterite-rich quartz veins; and, (iii) (≈ 300–298 Ma) represents the late- to post-D3 East-MEPL and Saucelle granites, likely parental of some barren aplitic-pegmatitic apophyses and the discordant intermediate and Li-rich pegmatites. These latter form part of a general granite-pegmatite fractionation trend.

Multiscale Structural Analysis of Ediacaran–Cambrian Rocks on the Northeastern Edge of the Saghro Inlier (Eastern Anti-Atlas): Relevance of Post-Cambrian Deformation

The Ediacaran–Cambrian rocks on the northeastern edge of the Saghro inlier experienced polycyclic tectono-thermal events, which are reported here based on a multiscale structural analysis, from field measurements to fluid inclusion planes. Three striking populations were identified, cutting across both the Ediacaran and Cambrian formations. These tectonic structures were generated during four tectonic events. (i) E-W-striking structures that host ore mineralized bodies (sulfide, oxide, quartz, and barite). They display a polyphase tectonic history, caused by a dextral movement in response to a NW–SE-oriented shortening, leading to the formation of quartz gashes and veins. This tectonic event took place during the Neovariscan. These E–W-striking structures were subsequently reactivated during the Mesozoic time under a sinistral strike-slip regime as a result of NE–SW shortening syn-kinematic with barite mineralization. (ii) NE–SW-striking strike-slip structures (dextral or sinistral) crosscut the E–W-striking veins. These faults are related to the NW–SE-oriented shortening that occurred during the Neogene. (iii) The last tectonic episode, related to the N–S shortening, took place during the late Neogene to the Quaternary period. It resulted in NW–SE to N–S-striking structures that were related to dextral and sinistral strike-slip movements, which crosscut the preexisting E–W structures.

The use of rare earth element profiles as a proxy for a fractionation source and mine-waste provenance

Rare earth elements (REEs) have been determined in acid mine drainage samples from the Wiśniówka area, south-central Poland. Two benchmark acid pit ponds, i.e., Podwiśniówka (PwIIb) and Wiśniówka Duża (WD), have shown diverse contents of sulfates, phosphates, REEs and metal(loid)s. Moreover, these ponds exhibit different NASC-normalized REE concentration patterns: (i) a positive middle REE anomaly in PwIIb and (ii) a positive heavy REE anomaly in WD, regardless of sampling time. This MREE anomaly has also been highlighted in a small tailings pile pool showing high contents of metal(loid)s, including As (3.86 g/L) and REEs (90.1 mg/L). In contrast, the light REE (LaEu)-rich profiles are recorded in all Upper Cambrian rock series of the study area. However, the Pw geologic section is distinctly enriched in pyrite, goethite/hematite and carbonaceous clayey-silty shales compared to its WD counterpart that contains a lesser amount of these components, but many more quartzite/sandstone beds. The Pw mineral-lithologic pattern favors selective partitioning of heavy REEs to abundant Fe- and Al-oxyhydroxides and organic matter. Both very short solute transport from sheer rock faces to pit ponds and a low pH of acid waters (mean of 2.3 to 3.0) indicate that scavenging/adsorption and mineral co-precipitation of REEs in a water column may be negligible. This inference is also backed up by overlapping REE profiles at different depths of acid pit ponds. Taken together, this implies that preferential fractionation of REEs takes place primarily during on-site weathering of pyrite and REE-bearing minerals in different rock media thus leading to changes in the Pw- and WD shale-normalized REE concentration patterns at source. The characteristic Pw roof-shaped (convex-up) profile in water samples has been used as a proxy for tracing the most detrimental Podwiśniówka As-bearing mine-waste that were scattered within the mining area a couple of years ago.

Preservation of Soft-Bodied Organisms in Early Cambrian Carbonates

Abstract —Preservation of soft-bodied organisms as casts is common in the Ediacaran fossil record and extremely rare in Cambrian rocks. Among the factors, limiting fossilization of soft tissues, there is bioturbation–sediment disturbance by burrowing organisms. It is the emergence of burrowing metazoans and associated sediment bioturbation that is thought to be one of the major causes for the disappearance of Ediacaran soft-bodied organisms from the fossil record around ~540 Ma. Here, we study an assemblage of fossils preserved as casts in the Fortunian carbonates of the Olenek Uplift (northeastern Siberian Platform) in association with a typical Fortunian ichnoassemblage. The overall morphology and preservation of the fossils reveal that they comprise microbially induced sedimentary structures and soft-bodied holdfasts. The latter are vaguely reminiscent of some Ediacaran holdfasts, though it is unlikely that they include their phylogenetic descendants. Three-dimensional preservation of the studied fossils was caused by authigenic crystallization of calcite and its further early-diagenetic dolomitization. Our study confirms the critical importance of specific environmental conditions ensuring preservation of soft-bodied organisms as three-dimensional molds and casts. This unique interplay of environmental factors became rare in the Cambrian Period, which was caused by intensification and expansion of bioturbation in marine basins.

Ordovician of the Bohemian Massif

Abstract The lower Paleozoic succession of central Europe exposed in the Bohemian Massif is a classic area of geology with a long-standing tradition of research dating back to the eighteenth century. The Ordovician rocks form parts of sections in several units that sit on the Cadomian basement. These sedimentary and volcano-sedimentary fills of partial depressions in the basement are relics of the system of rift basins in the Gondwanan margin reflecting the rifting of the Rheic Ocean. The Ordovician sections are related to the subsidence period during the extensional regime accompanied by volcanism. They are underlain by Neoproterozoic or Cambrian rocks and continue up usually without breaks. After closure of the Rheic Ocean owing to the Gondwana–Laurussia collision, the Ordovician successions were incorporated into the Variscan Orogen belt and preserved in denudation relics such as the Bohemian Massif and its units. Ordovician strata with Gondwanan shelf affinities can be traced along the Variscans from Spain to central Europe, and are reflected in the regional stratigraphic scale based mainly on the succession in the Prague Basin. The Ordovician fill of this accumulation centre, together with relics of another preserved in the Schwarzburg Anticline, represents the main exposures in the Bohemian Massif. The individual features of the Ordovician successions, such as facies developments, fossil associations and volcanism, make them model areas both for understanding the palaeogeographic and geotectonic evolution of the peri-Gondwanan margin and a stratigraphic standard for a cool-water regime.

Conditions for the formation of siliceous rocks of the Lower-Middle Cambrian in the southeast of the Siberian Platform

Based on the study of microstructural and geochemical features of bedded cherts of the Inikan Formation (Є 1–2 in ) from coastal outcrops of the river Yudoma found that the key role in their formation apparently belongs to siliceous organisms — radiolarians and sponges. The Ge/Si ratio indicates that there is no direct contribution of fluids to the formation of cherts in this part of the paleobasin. The studied chalcedony-quartz rocks were formed from primary biogenic silica subjected to diagenetic redistribution and post-diagenetic processes.

Distribution of Cambrian Source Rock Controlled by the Inherited Paleotopography on the Precambrian Basement in the Tarim Basin, NW China

The Cambrian source rock is a key indicator that will guide future hydrocarbon explorations in ultra-deep strata. Despite its importance, the characteristics of paleotopography on the Precambrian basement and the distribution of source rocks above it remain unclear. The seismic and borehole data, isopach data, and the unconformity analysis are used to study the relations of the Cambrian source rock distribution to the paleotopography. The consistency of the strata thickness distribution maps of the Nanhua–Cambrian sequences show that the extensive thermal subsidence lasted the whole Sinian period, and makes the paleotopography of the Precambrian basement inherited from the landform after the Nanhua continental rifting. A major transgression caused by rapid subsidence of the Tarim Basin took place during the Cambrian. Shales were deposited offshore along the slopes of a paleo-uplift on the inherited paleotopography and formed the onlap in seismic profiles. The onlap on the inherited paleotopography reflects the distribution range of Yuertusi source rocks in the Cambrian.

Early Ediacaran and Middle Ediacaran Granitoids in the Provenances of Early Cambrian Coarse-Grained Rocks of the Bayan-Kol Formation of the Systyg-Khem Depression (Tuva)

Abstract —The Tuva segment of the Central Asian Orogenic Belt is characterized by the ubiquitous presence of conglomerates few tens of meters to a kilometer in thickness in early Paleozoic volcanosedimentary sequences. We present the first results of geochemical, isotopegeochemical (Sm–Nd and Rb–Sr), and U–Pb geochronological studies of granitoid boulders and pebbles from the conglomerate sequence of the early Cambrian Bayan-Kol Formation of the Systyg-Khem depression. These studies made it possible to establish several sources of clastic material as a result of the destruction of granitoids of different ages and isotope-geochemical compositions. At least two complexes of granitoids were denuded in the pre-Ediacaran tectonic block in the early Cambrian: (1) middle Ediacaran (~590 Ma) and (2) early Ediacaran (~630 Ma); the latter resulted from the melting of pre-Ediacaran island arc crust formed from a depleted mantle source (εNd(T) = +8.0 to +8.6). At present, no granitoids of this age and with such isotope-geochemical characteristics have been found within the Tuva segment. Probably, the granitoid complexes reconstructed from the results of study of clastic conglomerates are eroded or buried beneath younger deposits and do not expose. Thus, the study of clastic conglomerates from the Bayan-Kol Formation provided the first information about the Precambrian history of the tectonic block whose destruction led to the accumulation of this terrigenous sequence.