Integrated chronostratigraphy of Proterozoic-Cambrian boundary beds in the western Anabar region, northern Siberia

Carbonate-rich sedimentary rocks of the western Anabar region, northern Siberia, preserve an exceptional record of evolutionary and biogeochemical events near the Proterozoic/Cambrian boundary. Sedimentologically, the boundary succession can be divided into three sequences representing successive episodes of late transgressive to early highstand deposition; four parasequences are recognized in the sequence corresponding lithostratigraphically to the Manykai Formation. Small shelly fossils are abundant and include many taxa that also occur in standard sections of southeastern Siberia. Despite this coincidence of faunal elements, biostratigraphic correlations between the two regions have been controversial because numerous species that first appear at or immediately above the basal Tommotian boundary in southeastern sections have first appearances scattered through more than thirty metres of section in the western Anabar. Carbon- and Sr-isotopic data on petrographically and geochemically screened samples collected at one- to two-metre intervals in a section along the Kotuikan River, favour correlation of the Staraya Reckha Formation and most of the overlying Manykai Formation with sub-Tommotian carbonates in southeastern Siberia. In contrast, isotopic data suggest that the uppermost Manykai Formation and the basal 26 m of the unconformably overlying Medvezhya Formation may have no equivalent in the southeast; they appear to provide a sedimentary and palaeontological record of an evolutionarily significant time interval represented in southeastern Siberia only by the sub-Tommotian unconformity. Correlations with radiometrically dated horizons in the Olenek and Kharaulakh regions of northern Siberia suggest that this interval lasted approximately three to six million years, during which essentially all 'basal Tommotian' small shelly fossils evolved

The Genesis and Time Distribution of Two Distinctive Proterozoic Stromatolite Microstructures

Stromatolites are commonly viewed as sedimentary proxies for microbial communities. In consequence, secular variation in stromatolite form has been attributed to evolutionary change in mat organisms and/or their interactions with global environments. This interpretation requires that one be able to identify features of stromatolite macrostructure or microstructure that are under direct biological influence and use them to document the sedimentological or petrological consequences of microbial evolution. Well-preserved stromatolites in Proterozoic carbonates of the Siberian Platform contain true distinctive types of microstructure that enable us to address such issues. Preserved microstructures in Baicalia lacera and Tungussia confusa from lower Neoproterozoic (lower Upper Riphean) platform carbonates of several widely separated regions clearly reflect the biology of underlying mat communities; mm-scale laminae of densely interwoven calcified. filaments alternate with filament-poor microspar. Comparable carbonate microstructures are known from a number of other Neoproterozoic stromatolites, but are as yet unreported from older successions. Filamentous cyanobacteria formed mats throughout the Proterozoic Eon; thus, the temporal distribution of this microstructure appears principally to reflect secular and environmental variations in carbonate cementation and diagenesis. Omachtenia omachtensis is the characteristic stromatolite of Mesoproterozoic (Lower and, occasionally, Middle Riphean) successions of the Siberian. Platform and elsewhere. Its distinctive microstructure consists of mechanically deposited event laminae separated by thin organic films that served as nucleation sites for micritic and, less commonly, fibro-radiate carbonate precipitates. Microbial mats may have stabilized sediments between events, but there is little evidence that mats played an active role in the trapping and binding or precipitation of laminae. Thus, physico-chemical factors must also be responsible for the distribution. of these stromatolites in time and space. Analysis of these two microstructures suggests that, in general, secular trends in stromatolite microstructure may encrypt important information about environmental change through the Proterozoic Eon. Microbial evolution may also play a role in determining the stratigraphic distributions of particular Proterozoic stromatolites, but this remains to be demonstrated. Evolution. may be most important in driving the progressive environmental restriction of stromatolite-forming microbial communities through time.Organismic and Evolutionary Biolog

Late Mesoproterozoic magnetostratigraphic results from Siberia: Paleogeographic implications and magnetic field behavior

International audienceWe present a magnetostratigraphic study of the late Mesoproterozoic Malgina and Linok Formations, located along the southeastern (Uchur-Maya region) and northwestern (Turukhansk region) margins of the Siberian craton, respectively. Biostratigraphic, radiometric, and chemostratigraphic data indicate that these formations are likely coeval between 1050 and 1100 Ma. Paleomagnetic analyses reveal a high-temperature component carried by magnetite and/or hematite. This component yields positive fold and reversal tests, together with a positive conglomerate test for the Malgina Formation, which indicates that the magnetization was acquired during or soon after sediment deposition. The mean paleomagnetic direction obtained from the Uchur-Maya region, which is unambiguously representative of the Siberian craton, indicates that it could not have been part of Rodinia at that time if Siberia was located in the Southern Hemisphere and if we assume that Laurentia and Siberia were connected along their present northern shorelines. We emphasize that Siberia could have been part of Rodinia during the late Mesoproterozoic if southern Siberia was joined to the northern part of Laurentia as recently proposed by Rainbird et al. [1998]. If true, placing the Siberian craton in the Southern Hemisphere implies that the magnetic polarity of the ∼1000 Ma Laurentian paleomagnetic poles must be switched. Our data also show the occurrence of at least 15 symmetric geomagnetic field reversals, indicating that the paleomagnetic results from the late Mesoproterozoic Keweenawan lavas do not reflect a worldwide and persistent asymmetric field during the Proterozoic

A Vendian-Cambrian Boundary Succession from the Northwestern Margin of the Siberian Platform: Stratigraphy, Palaeontology, Chemostratigraphy and Correlation

Siberia contains several key reference sections for studies of biological and environmental evolution across the Proterozoic-Phanerozoic transition. The Platonovskaya Formation, exposed in the Turukhansk region of western Siberia, is an uppermost Proterozoic to Cambrian succession whose trace and body fossils place broad limits on the age of deposition, but do not permit detailed correlation with boundary successions elsewhere. In contrast, a striking negative carbon isotopic excursion in the lower part of the Platonovskaya Formation permits precise chemostratigraphic correlation with uppermost Yudomian successions in Siberia, and possibly worldwide. In addition to providing a tool for correlation, the isotopic excursion preserved in the Platonovskaya and contemporaneous successions documents a major biogeochemical event, likely involving the world ocean. The excursion coincides with the palaeontological breakpoint between Ediacaran- and Cambrian-style assemblages, suggesting a role for biogeochemical change in evolutionary events near the Proterozoic-Cambrian boundary.Organismic and Evolutionary Biolog

Sizing up the sub-Tommotian unconformity in Siberia - Reply

The occurrence of an unconformity beneath Tommotian rocks in southeastern Siberia is not controversial. Landing implies that we presented this observation as new, but as one of us (Semikhatov and Serebryakov, 1983) first documented this feature more than a decade ago, we would hardly make such a claim; four references to previous research (two by Landing) attend its first mention in our text. Interpretation of the unconformity is more contentious, and Landing has, indeed, swum against the current for years in asserting that the hiatus beneath the Tommotian stratotype lasted a long time. We regret that in trimming our paper to meet the page limits of Geology we omitted reference to this view. Assertion, however, is one thing; demonstration is another

Global Events Across the Mesoproterozoic-Neoproterozoic Boundary: C and Sr Isotopic Evidence from Siberia

Thick, unmetamorphosed successions of siliciclastic and carbonate rocks in eastern and western Siberia preserve a record of Middle Riphean to Early Upper Riphean sedimentary environments and geochemistry. Consistent with data from other continents, our studies in the Uchur-Maya region in southeastern Siberia and the Turukhansk Uplift in northwestern Siberia suggest a first-order shift in delta C-13 from values near 0 parts per thousand in the early Mesoproterozoic to values near +3.5 parts per thousand after about 1300 Ma. Over this same interval, primary Sr-87/Sr-86 values decrease from > 0.7060 to < 0.7053. Combining lithologic, biostratigraphic, and geochemical data sets with available geochronologic constraints, we present a refined correlation between these two key Proterozoic successions in Siberia and add this dataset to a growing body of C and Sr isotopic data from this time interval, Carbon isotope chemostratigraphy from these regions supports the occurrence and timing of a first-order, similar to 3.5 parts per thousand positive shift ca. 1250-1300 Ma, approximately coeval with the onset of active margin activity that predates the main phase of Rodinia assembly. Sr isotopic data may also be interpreted within the context of the evolving Mesoproterozoic tectonic regime. Available data suggest that no dramatic rise in Sr-87/Sr-86 heralds the main phase of Rodinia assembly in the terminal Mesoproterozoic, suggesting that significant juvenile crust was involved in mountain building, that relative hydrothermal flux from mid-ocean ridges remained high throughout the assembly of Rodinia and/or that increased continental runoff related to intense erosion of Grenvillian mountain belts terminated shortly after orogeny.Earth and Planetary SciencesOrganismic and Evolutionary Biolog