Cambrian Formation Research Articles

Hydrogeothermal resources of Middle Lithuania

The main geothermal prospects are related to West Lithuania, while the potential of Middle Lithuania was little considered before. The Cambrian sandstone reservoir was identified as a potential hydrothermal resource in Middle Lithuania. Other large-scale geothermal aquifers have too low temperatures. The study indicates that the effective thickness of the Cambrian geothermal aquifer of Middle Lithuania ranges within 15–58 m, the average porosity of sandstones is 13–20% and permeability averages 370 mD; therefore, these sandstones are classified as a good reservoir. The geothermal potential was calculated for geothermal well doublet. The productivity of the geothermal well doublet is estimated to be as high as 53–175 m 3 /h in the southern part of the study area, while reaching only 16–43 m 3 /h in the north. Accordingly, the geothermal energy production of the well doublet varies from 0.44 MWh to 1.37 MWh in the northern area, ranging from 1.69 MWh to 6.02 MWh in the south. The water of the Cambrian formation is of the NaCl type with a high salinity (98–130 g/l). The thermochemical modelling indicates that depending on water composition, either the precipitation of gypsum or carbonates and hydroxides should be expected in the geothermal system, while other major mineral phases remain unsaturated in the geothermal loop. Analysis of the available geothermal resources and their comparison with the heat energy needs of Middle Lithuanian customers indicate that one or two doublet installations exploiting the Cambrian geothermal aquifer may cover the basic needs in the central heating of towns in the region. It is concluded that the Cambrian geothermal aquifer is highly promising for district heating in Middle Lithuania.

Detrital zircon geochronology and provenance of the Neoproterozoic to Late Devonian Franklinian Basin, Canadian Arctic Islands

More than 1800 detrital zircon uranium-lead (U-Pb) ages collected from Franklinian Basin sedimentary strata of the Canadian Arctic Islands provide important insights into the depositional and tectonic evolution of the northern margin of Laurentia from the late Neoproterozoic to the Late Devonian. The Franklinian Basin succession is composed of strata with three distinctly different U-Pb age provenance signatures, which have implications for the tectonic and paleogeographic evolution of the entire Arctic region. Neoproterozoic and Lower Cambrian formations contain detrital zircon populations of 1750–1950 Ma and 2650–2800 Ma, which are consistent with derivation from Archean to Paleoproterozoic gneisses and granites of the west Greenland–northeast Canadian Shield. The Lower Silurian to Lower Devonian Danish River Formation contains a dominant population of 900–2150 Ma detrital zircons with scattered Archean ages. The 900–2150 Ma zircons were likely transported axially along the foreland basin of the East Greenland Caledonides (Caledonian orogen) and deposited in a deep-water basin between the Pearya terrane and northern Laurentian margin. Middle Devonian to Upper Devonian strata contain detrital zircon populations of 900–2150 Ma, similar to the Danish River Formation, but these units also contain 370–450 Ma and 500–700 Ma detrital zircons. The 900–2150 Ma zircons were likely derived from the East Greenland Caledonian Mountains, the uplifted foreland of the East Greenland Caledonides, and the Pearya terrane. The population of 370–450 Ma detrital zircons potentially comes from uplifting granites in the Caledonian Mountains and Pearya terrane. The 500–700 Ma detrital zircons were likely derived from the continental landmass responsible for the Ellesmerian orogen. The 500–700 Ma age of the zircons suggests that the northern landmass likely had a connection to rocks of the Timanide orogens, located in the Timan Range of northwestern Russia. A dominant population of 365–450 Ma and 500–700 Ma ages in Upper Devonian strata suggests that the Pearya terrane and the northern continental landmass became the dominant source by the end of Franklinian Basin sedimentation. Because detrital zircons are often recycled from older strata into younger deposits, these data provide the basis for understanding the sedimentary provenance of younger units of the Sverdrup Basin and sedimentary wedges along the present Arctic continental margin.

APPLICATION OF PALYNOFACIES TO THE STUDY OF THE PALEOZOIC SOURCE ROCKS IN LOWER YANGTZE REGION

We discussed the origin,organic-matter type and the formation environments of the source rocks occurred in the Lower Cambrian Mufushan Formation and the Lower Permian Chihsia Formation in the lower Yangtze region upon palynofacies.High quality source rocks with high TOC occur in the Mufushan Formation.They were dominated by over 90% of amorphous organic matter in most of the cases,and predominated by the type I of organic matter mainly from aquatic organisms formed in suboxic-anoxic lagoonal or estuarine environments.The organic matters in the Chihsia Formation are also dominated by amorphous organic matters,but with obvious variations in organic components,so it is of a mixed-type.The frequent change in sedimentary environments and oxygen-deficiency level made the source rocks distributed in a heterogeneous manner with lower TOC and lower quality.

Molybdenum isotopic records across the Precambrian-Cambrian boundary

Molybdenum (Mo) isotopic data from early Cambrian formations in southern China demonstrate the importance of non-euxinic sediments as a promising archive for coeval seawater Mo records. For the first time, in analogue to noneuxinic, nonskeletal carbonates, we report that the pristine phosphorites also can preserve the ambient fluid, which make the pristine phosphorites a new potential fingerprinting tool to record the coeval paleoseawater. Pristine phosphorite and dolomite samples of two early Cambrian formations in southern China suggest that early Cambrian seawater may have had δ 97/95 Mo values of at least 1.4‰, similar to modern oceans. Low δ 97/95 Mo (as low as −0.5‰) values were obtained from reworked phosphorite, but evidence suggests that these are reflecting Mo related to Fe oxyhydroxide minerals rather than the coeval seawater. In contrast, late Precambrian dolomite just below the transition to the Cambrian would indicate a δ 97/95 Mo of coeval seawater slightly heavier than that of the Mesoproterozoic (0.9‰–1‰ versus 0.8‰), suggesting that oceanic circulation patterns may have been thoroughly reorganized by that time, and may have triggered biological diversification from the Ediacaran to the early Cambrian.

The Bouznika Cambrian barite deposit (Morocco), an early mineralization on the Iapetus margin

Most of Morocco’s barite production comes from deposits of epigenetic origin emplaced during Mesozoic time and hosted by Cambrian formations. Located in the Hercynian Meseta, the Bouznika deposit is associated with the Cambrian Oued Rhebar bimodal volcanic complex. Barite occurs as both stockworks and stratiform lenses within felsic tuffs and epiclastites, and is associated with a potassic alteration characterized by the presence of sericite. Lead and sulphur isotope analyses demonstrate that emplacement occurred during Cambrian time when barite-enriched hydrothermal fluids, likely of volcanic origin, mixed with sulphur from seawater. The Cambrian formations hosting the Bouznika, Jbel Irhoud and Seksaoua barite deposits are located near the faulted limit of the Meseta’s lower Palaeozoic Iapetus rift on the northwestern edge of the Gondwana continent, suggesting that they formed during the Mesozoic reworking of early Cambrian concentrations.

Olenelloid trilobites from Cambrian Series 2 of Devon Island, Nunavut, Arctic Canada

Four species of olenelloid trilobites characterized by advanced or robust genal spines are described from the lower Cambrian Rabbit Point Formation (Cambrian Series 2, Stage 4) of eastern Devon Island, representing the most diverse assemblage of olenelloids yet described from Arctic Canada. New taxa: Bristolia kurtzi n. sp., Paranephrolenellus cowiei n. sp., Siciliolenellus fritzi n. gen. and n. sp.

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.

Closing the Clymene ocean and bending a Brasiliano belt: Evidence for the Cambrian formation of Gondwana, southeast Amazon craton

Research Article| March 01, 2010 Closing the Clymene ocean and bending a Brasiliano belt: Evidence for the Cambrian formation of Gondwana, southeast Amazon craton E. Tohver; E. Tohver * 1School of Earth and Environment, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia *E-mail: etohver@cyllene.uwa.edu.au. Search for other works by this author on: GSW Google Scholar R.I.F. Trindade; R.I.F. Trindade 2Instituto de Astronomia e Geofísica, Universidade de São Paulo, Rua Maria Antônia, 294 São Paulo, SP 01222010, Brazil Search for other works by this author on: GSW Google Scholar J.G. Solum; J.G. Solum 3Shell Exploration, Bellaire Technology Center, Shell International Exploration and Production, 3737 Bellaire Blvd, Houston, Texas 77025, USA Search for other works by this author on: GSW Google Scholar C.M. Hall; C.M. Hall 4Department of Geological Sciences, University of Michigan, 2534 C.C. Little Bldg., Ann Arbor, Michigan 48104, USA Search for other works by this author on: GSW Google Scholar C. Riccomini; C. Riccomini 5Instituto de Geociências, Universidade de São Paulo, Rua Maria Antônia, 294 São Paulo, SP 01222010, Brazil Search for other works by this author on: GSW Google Scholar A.C. Nogueira A.C. Nogueira 6Instituto de Geociências, Universidade de Pará, Rua Augusto Corrêa, 01-Guamá, CEP 66075-110, Brazil Search for other works by this author on: GSW Google Scholar Geology (2010) 38 (3): 267–270. https://doi.org/10.1130/G30510.1 Article history received: 08 Jul 2009 rev-recd: 19 Oct 2009 accepted: 20 Oct 2009 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation E. Tohver, R.I.F. Trindade, J.G. Solum, C.M. Hall, C. Riccomini, A.C. Nogueira; Closing the Clymene ocean and bending a Brasiliano belt: Evidence for the Cambrian formation of Gondwana, southeast Amazon craton. Geology 2010;; 38 (3): 267–270. doi: https://doi.org/10.1130/G30510.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We report new paleomagnetic and geochronological data from Ediacaran rift-drift carbonates in the Paraguai belt at the southern end of the suture zone between the Amazon craton and the São Francisco and Rio de Plata cratons, South America. Early thrusting resulted in remagnetization ca. 528 ± 36 Ma or later; the mean age is established by 40Ar/39Ar encapsulation dating of mixed authigenic and detrital illite from remagnetized carbonates from the unmetamorphosed fold-thrust belt. This remagnetization overlaps with a 525 Ma Gondwana reference pole. Metamorphic illite from the slate belt yields 40Ar/39Ar ages of 496–484 Ma, the timing of peak regional metamorphism. Oroclinal bending of the Paraguai belt was caused by a 90° clockwise rotation of the east-west limb after ca. 528 Ma, probably reflecting the irregular margin of the southeast Amazon craton. The age of the Paraguai belt overlaps with that of the Pampean orogeny farther south along the western margin of the Rio de Plata craton, suggesting a coeval closure for the Clymene ocean separating the Amazon craton from the São Francisco and Rio de Plata cratons. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Borings in Quartzite Surf Boulders from the Upper Cambrian Basal Deadwood Formation, Black Hills of South Dakota

Shallow, semi-spherical borings occur in clusters with densities of 1–3.5/cm2 in quartzite boulders and in vein quartz from the localized basal conglomerate of the Cambrian-Ordovician Deadwood Formation in the east-central Black Hills of South Dakota. Some borings are superimposed on primary but enigmatic semi-circular structures, 2.5–5 cm in diameter, which are soft-sediment trace fossils formed prior to lithification. The macroborings are the first to be recorded from quartzite and vein quartz. Host boulders were eroded from near vertically dipping Paleoproterozoic quartzites from several different stratigraphic units. The thin boulder conglomerate grades were abruptly changed to sandstone through a layer ≤2 m thick in the Marjuman transgression (regionally correlated to the Cedarina dakotaensis trilobite zone). This transgression occurred prior to the start of the globally recognized Upper Cambrian Paibian Stage. Physically similar rocky-shore settings are widely known from quartzite islands of Cambrian age in Wisconsin, Middle Ordovician age on Ontario's Manitoulin Island, Ordovician-Silurian age in Manitoba, and Devonian age in western Australia. Erosion of quartzite surf boulders of equal or larger size occurred in all those regions, but the Black Hills of South Dakota is the only region where borings in quartzite are documented.

Hopanes and hopenes of the Lower Cambrian Sinyaya Formation (northern slope of the Aldan Anteclise)

Hopane hydrocarbons contained in organic matter from the Lower Cambrian Sinyaya Formation, southeast of the Siberian platform, were analyzed by gas chromatography/mass spectrometry. This study first identified C28 28, 30-bisnorhopane and its homologues present in high concentrations in autochthonous organic matter from East Siberian sedimentary basins. These biomarkers can be used in distinguishing between the sources of naphthides on the northern slope of the Aldan anteclise. Two saturated naphthenic hydrocarbons, C31 homohopene and C31 28-norhomohopene, were also detected. This study provides some details on hopane distribution, organic matter accumulation conditions, and the degree of OM catagenesis.

Heteroaromatic compounds of bitumenoids from the Cambrian oil shale formation

The molecular composition of NSO heterocyclic aromatic compounds in the lube stock fractions of chloroform-extractable bitumenoids isolated from Cambrian shale formation rocks of Kuonam oil shale, which differ in their organic carbon content, was studied. Of the sulfur compounds, C3–C4 alkylbenzothiophenes; dibenzothiophene, its C1–C5 alkyl and naphtheno derivatives; naphthobenzothiophene; and methylnaphthobenzothiophenes were identified. Among the oxygen compounds, unsubstituted dibenzofuran, unsubstituted naphthobenzofuran, and their C1–C4 and C1–C2 homologues, respectively, were detected. The nitrogen compounds present in the test fractions were supposed to be 7H-benzo[c]- and 7H-benzo[b]acridin-12-ones and methylbenzoacridin-12-ones.

Mo isotopes in the Lower Cambrian formation of southern China and its implications on paleo-ocean environment

Over the past decade, as one of nontraditional stable isotopes, Mo isotope has developed rapidly and now become an important geochemical proxy to trace paleo-oceanic and atmospheric evolution through geological history. In this paper, Early Cambrian formations in southern China are investigated. The results indicate that δ97/95Mo values of Early Cambrian seawater may have been larger than 1.4%, values that are close to those of the modern ocean. It was also found that the variations in Mo isotope composition in samples from two sections (Huangjiawan and Gezhongwu in Guizhou) were closely related to changes in redox conditions during sedimentary processes. Combining our results with existing data, a preliminary model for the evolution of seawater Mo isotope composition through geological history was provided. It indicated that Mo isotopic variations were generally consistent with the evolution of atmospheric oxygen.

Traces of brine and hydrocarbon migration in carbon, oxygen, and sulfur isotopic compositions of reservoir rocks in the Talakan petroleum field, Southwestern Yakutia

Dolomites from the productive Osa horizon (upper subformation of the Lower Cambrian Bilir Formation) in the Talakan petroleum field show a prominent 1–2‰ decrease in δ18O (from 23–24 to 21–22‰), which presumably marks a zone of relatively high water/rock ratios. Productive boreholes are characterized by moderate δ34S values (from 25.1 to 30.6‰) and negative correlation between δ34S in anhydrite and δ18O in associated dolomite, which points to a partial sulfate reduction during catagenesis. In nonproductive borehole, δ34S values increase significantly (from 31.4 to 35.6‰) and show positive correlation with δ18O in dolomite. Rocks recovered by nonproductive borehole possibly recrystallized during early diagenesis, and, correspondingly lost their permeability and capacity to form pores. Limestones and dolomites of the Osa horizon have a carbon isotopic composition within the range of normal marine carbonates (δ13C = 0 ± 1 ‰), which does not indicate a significant role of organic matter in postsedimentary recrystallization of carbonate sediments. A positive δ13C excursion up to 4.5‰ recorded in the lower subformation of the Bilir Formation presumably occurred at the sedimentation stage under conditions of high rates of bioproductivity and organic matter burial in sediments.

Re-interpretation of a Lower–Middle Cambrian West Gondwanan ramp depositional system: a case study from the Cantabrian Zone (NW Spain)

Detailed litho- and biofacies investigations of the Lower–Middle Cambrian carbonate Lancara Formation resulted in its subdivision into nine lithofacies types: (1) claystone, (2) recrystallized mudstone, (3) laminated mudstone with laminoid-fenestral fabrics, (4) stromatolite, (5) laminated aggregate grainstone, (6) non-laminated aggregate grainstone, (7) oolitic-bioclastic floatstone, (8) echinodermal packstone, and (9) bioclastic grainstone. The thicknesses of lithofacies 1–7 (lower member of the Lancara Formation) decrease from south to north. Lithofacies types 8–9 (upper member of the Lancara Formation) are characterized by similar thicknesses and low facies and faunal gradients and are thus indicative of deposition on a carbonate ramp. From palaeoecological, palaeo(bio)geographical, palaeomagnetic, and tectonic considerations, the depositional environment of the Lancara Formation is re-interpreted as an eastward/north-eastward sloping, low morphology carbonate ramp. The Cantabrian Zone, with a primary lateral extension of about 300 km, is further construed to be an element of a widespread and connected, discontinuous drowned Perigondwanan depositional system.

Source Rock Characteristics, Organic Maturity, and Hydrocarbon Potential of the Lower Paleozoic Sequences in the Taurus Belt of Turkey

Like elsewhere in the world, Lower Paleozoic (Cambrian-Ordovician-Silurian) formations in Turkey are also exposed in limited areas. Lower Paleozoic sequences are exposed at various parts of the Taurus belt in southern Turkey. In this study, organic matter content, type, and maturity of organic matter and their hydrocarbon potential were investigated. In addition, the Lower Paleozoic is very restricted with respect to species and their diversity. Therefore, it is very important to investigate type and content of organic matter within the Lower Paleozoic sediments. In general, total organic carbon (TOC) values of Lower Paleozoic sequences are extremely low. TOC could not be measured in Cambrian, Ovacık, and Çaltepe limestones. Shales in the Emirgazi region have very low TOC values. Ordovician and Silurian units also have low TOC values. Total organic carbon in these sequences is residual carbon. In the S 2-TOK kerogen diagram, all organic materials are plotted in the Type III kerogen field. All samples were composed dominantly of residual organic matter and lesser amounts of Type III kerogen. According to T max values, the Cambrian, Ordovician, and Silurian formations are over maturity character. In addition, these sequences have very low S 1, S 2, and HI values.

The birth of the Rheic Ocean — Early Palaeozoic subsidence patterns and subsequent tectonic plate scenarios

New plate-tectonic reconstructions of the Gondwana margin suggest that the location of Gondwana-derived terranes should not only be guided by the models, but should also consider the possible detrital input from some Asian blocks (Hunia), supposed to have been located along the Cambrian Gondwana margin, and accreted in the Silurian to the North-Chinese block. Consequently, the Gondwana margin has to be subdivided into a more western domain, where the future Avalonian blocks will be separated from Gondwana by the opening Rheic Ocean, whereas in its eastern continuation, hosting the future basement areas of Central Europe, different periods of crustal extension should be distinguished. Instead of applying a rather cylindrical model, it is supposed that crustal extension follows a much more complex pattern, where local back-arcs or intra-continental rifts are involved. Guided by the age data of magmatic rocks and the pattern of subsidence curves, the following extensional events can be distinguished: – During the early to middle Cambrian, a back-arc setting guided the evolution at the Gondwana margin. Contemporaneous intra-continental rift basins developed at other places related to a general post-Pan-African extensional phase affecting Africa – Upper Cambrian formation of oceanic crust is manifested in the Chamrousse area, and may have lateral cryptic relics preserved in other places. This is regarded as the oceanisation of some marginal basins in a context of back-arc rifting. These basins were closed in a mid-Ordovician tectonic phase, related to the subduction of buoyant material (mid-ocean ridge?) – Since the Early Ordovician, a new phase of extension is observed, accompanied by a large-scale volcanic activity, erosion of the rift shoulders generated detritus (Armorican Quartzite) and the rift basins collected detrital zircons from a wide hinterland. This phase heralded the opening of Palaeotethys, but it failed due to the Silurian collision (Eo-Variscan phase) of an intra-oceanic arc with the Gondwana margin. During this time period, at the eastern wing of the Gondwana margin begins the drift of the future Hunia microcontinents, through the opening of an eastern prolongation of the already existing Rheic Ocean. The passive margin of the remaining Gondwana was composed of the Galatian superterranes, constituents of the future Variscan basement areas. Remaining under the influence of crustal extension, they will start their drift to Laurussia since the earliest Devonian during the opening of the Palaeotethys Ocean.

Simulation Study of Acid Gas Injection Into an Aquifer

Abstract A series of simulation runs using a compositional simulator were conducted to determine the area of influence that results from injecting acid gas (H2S) for 30 years at 330 e3m3 into a 75 m thick high permeability sandstone aquifer. In this study, the three-phase (gas-oil-water) equilibrium calculation was used. Initially, a 5 Township by 5 Range grid model was used to determine the area of influence; subsequently, a smaller grid model 1 Township by 1 Range was used. Boundary producers were required to properly simulate an infinite aquifer. This study found that buoyancy and relative permeability, more than other parameters such as the perforation interval and the gas-water capillary pressure, affected the area of influence. A refined 75-layer model, one-metre thick, was constructed to check the gravity effect. The results of this refined layer model agree closely with a 13-layer model after 11 years of acid gas injection. A blowdown simulation was also conducted to determine the time to significant water inflow from perforations at the bottom of the zone. The effect of a 50 year shut-in after 30 years of injection was also simulated. Introduction The objective of the study was to understand the reservoir volume and the area of influence that would result from the injection of H2S into the Basal Sand aquifer of the Cambrian age Deadwood Formation. This was accomplished through the use of a compositional simulator (CMG's GEM). The Husky Lloydminster Upgrader (HLU) produces large amounts of acid gas: mainly H2S. The existing sulphur plant removes the sulphur from the H2S. Implementation of H2S disposal into the Deadwood Formation would initially reduce the load on the existing HLU sulphur plant, and ultimately it might be shut-in. Any future expansion of the HLU would not require an additional sulphur plant if acid gas disposal was proven to be effective. For safety reasons and regulatory requirements, it was required to know where the injected H2S would migrate to within the aquifer over time. The resulting area of influence would determine the penetrating wells and the mineral rights owners that might ultimately be affected by H2S injection. In this study, a commercial compositional simulator (CMG's GEM) was used as the model as it can take H2S water solubility into account. In the simulation results, the area of influence is defined as the area that has an H2S concentration greater than 0.00001 mole fraction. This value is based on the numerical accuracy of the simulation. The selected simulator was developed for oil-gas-water three-phase and multi-components usage. To simulate an H2S-water system, a small amount of methane (0.0005 gas saturation) was used as an oil phase for initialization. To distinguish the initial methane gas from H2S in the gas phase, all the gas saturation graphs used a minimum scale of 0.0005. Deadwood Aquifer Formation and Reservoir Model References (1) and (2) provide a detailed geological appraisal and a mineralogy study of the Cambrian Deadwood Formation Basal Sand.

Soft anatomy of sunellid arthropods from the Chengjiang Lagerstutte, Lower Cambrian of southwest China

Two bivalved arthropods of the family Sunellidae Huo, 1965 are described from the Lower Cambrian Helinpu Formation of southwestern China: Sunella cf. shensiensis Huo, 1965 and Combinivalvula chengjiangensis Hou, 1987. Both taxa preserve soft anatomies, described here for the first time, including a pair of lateral eyes and a possible median eye, a trunk carrying flaplike appendages, and a simple gut with caeca. The family Sunellidae includes three genera: Sunella Huo, Combinivalvula Hou, and Jinningella Huo and Shu. In contrast to most other Cambrian arthropods with a larger bivalved carapace, especially those from the Burgess Shale–type deposits, sunellids lack a limbless abdomen protruding posteriorly beyond the carapace. This, coupled with a combination of a number of snared features (e.g., the presence of cardinal spines and distinctive anterodorsal sulcus, elongated valves, and median eye), appears to support sunellids as a clade. The distinctive anterodorsal sulcus extending from the anterodorsal corner to the anteromedian part of the carapace is regarded as an autapomorphy for this clade. Sunellids resemble Isoxys Walcott, 1890 to which they may be closely related; both possess a bivalved, elongated carapace with cardinal spines that almost entirely covers the body. However, neither cephalic appendages nor proximal portions of trunk limbs are visible in sunellids, and thus, their systematic position remains uncertain.

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

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

Isotopic evidence for postsedimentary transformation of rocks of the Lower Cambrian Angara Formation in the Irkutsk Amphitheater

The section of the Angara Formation (upper portion of Lower Cambrian) is composed of a complex combination of dolomites, anhydrites, marlstones, and salinastones. The occurrence of solution breccia, various types of metasomatism, and other indications testify to the substantial postsedimentary reworking of these rocks. Sulfate reduction, one of the most important processes of reworking, is clearly expressed in the sulfur isotopic composition. The 87Sr/86Sr ratio in carbonate and sulfate materials is more stable owing to the relatively high Sr content in these rocks. Nevertheless, most sulfate and carbonate rocks are contaminated with the radiogenic Sr. The carbon isotope ratio in carbonates is close to the “normal” value despite the evaporitic character of sedimentation basins and the possible participation of organic carbon, which is a product of the oxidation of hydrocarbons by sulfates, in the transformation of carbonates. The Rb-Sr analysis of the clayey component of marlstones and the K-Ar analysis of mudstones from the lower portion of the Lower Cambrian-Vendian section indicate that the rocks were mainly transformed during a time span corresponding to the Caledonian folding dramatically manifested at the platform margin.