Proterozoic-Cambrian Boundary Research Articles

Environmental covariation of metazoans and microbialites in the Lower Ordovician Boat Harbour Formation, Newfoundland

An antagonistic view of the relationship between microbialites and metazoans has long been inferred, in part because of the large scale anticorrelation of these two groups through geologic time. The nexus of this relationship occurs in the Early Paleozoic Era: stromatolites declined in abundance as complex animals and algae diversified, but thrombolites, a type of microbialite little known before the Proterozoic-Cambrian boundary, proliferated for the first time. Well-preserved parasequences in the basal portion of the Lower Ordovician Boat Harbour Formation, western Newfoundland, contain a succession of stromatolites and thrombolites that permit an investigation into the role metazoans played in shaping the nature and abundance of microbialites in Early Paleozoic carbonate seas. Sessile benthic animals colonized thrombolite surfaces, but are nearly absent from stromatolites. Bioturbation rarely co-occurred with microbialites, but is widespread in clastic carbonates that lack microbialites. Our results, thus, support the hypothesis of ecological antagonism between microbial communities and motile benthic animals, but also demonstrate biological facilitation between thrombolites and both sessile benthic animals and nekton.

Basin evolution of the margin of Gondwana at the Neoproterozoic/Cambrian transition: the Puncoviscana Formation of Northwest Argentina

The Puncoviscana Formation is an important stratigraphic sequence of sandstones and shales, with intercalated conglomerates, limestones and volcanic rocks. It was deposited at the Neo- proterozoic-Cambrian boundary in a shallow basin on the western margin of Gondwana. Trace fos- sils of the Puncoviscana Formation s.l. are represented by the Oldhamia and Nereites ichnoassocia- tions and the geographical distribution of assemblages are aligned as parallel belts, with a shallower eastern Nereites association and a deeper western Oldhamia association. They represent different temporal levels on the evolution of the basin. Detrital zircon spectra display a wide range of ages, which indicate their provenance from Neoproterozoic to Cambrian source areas as well as Meso- to Paleoproterozoic basement units, exhibiting typical Gondwanan ages. Thick limestone banks are interbedded in the Puncoviscana Formation s.l. that included the siliciclastic sequences, with very low- to medium-metamorphic grade. δ13C values for the limestones vary from -1.57 to +3.40/00 VPDB, while in the Sierras Pampeanas, δ13C values vary from +2.6 to +80/00 VPDB. Also, reported 87Sr/86Sr values allow a clear differentiation of two rock sets. The limestones interbedded with the Puncoviscana Formation yielded values typical for the Neoproterozoic-Cambrian transition, between 0.70868 and 0.70896; while the limestones of the Sierras Pampeanas display values between 0.70748 and 0.70756, common in the middle to late Ediacaran. Deformation and uplift of the Puncoviscana Formation is clearly represented in northern Argentina by the Tilcarian unconformity, which is over- lain by the Meson Group.

Evolutionary History of Cambrian Spiculate Sponges: Implications for the Cambrian Evolutionary Fauna

Broad-scale analyses of Cambrian spiculate sponges are scarce. The apparent differences between Cambrian and Ordovician sponge faunas were included in Sepkoski's concept of evolutionary faunas; in these, sponges were regarded as minor contributors to the Paleozoic and modern faunas and insignificant in the Cambrian Evolutionary Fauna. More recent published occurrences of Cambrian and Ordovician spiculate sponges and the inclusion of archaeocyaths in the phylum Porifera, however, have altered our understanding of the significance of sponges among Cambrian faunas. The majority of Cambrian occurrences appear to be segregated into two major associations: lower Cambrian sponges in China, and middle Cambrian sponges in North America, primarily British Columbia and Utah. The main associations of spiculate sponges are in siliciclastic deposits from middle-to-deep muddy shelf and basin environments, whereas orchoclad demosponges are associated with shallow carbonate environments. Four main aspects of sponge biology are considered potential factors dictating the distribution of sponges in the Cambrian: their trophic requirements, hydrodynamic constraints, possible biogeochemical constraints, and the sponge-sediment relationship. A series of critical steps in sponge evolutionary history occurred during the interval from the Proterozoic-Cambrian boundary to the middle– late Ordovician. The lower–middle Cambrian faunas are considered to be a Cambrian evolutionary sponge fauna, with archaeocyaths and diverse monaxonid demosponges as distinctive components. There was a transitional fauna in the upper Cambrian–Lower Ordovician, with orchoclad lithistids dominating shallow environments. Hexactinellids began to colonize nearshore siliciclastic settings during this time. The third interval, Middle–Upper Ordovician, corresponds to the Paleozoic Evolutionary Fauna, which is the interval during which lithistids diversified in several suborders and families and the stromatoporoid and sphinctozoan calcified sponges experienced their first radiation.

Geology and petrology of the Hormuz dolomite, Infra-Cambrian: Implications for the formation of the salt-cored Halul and Shraouh islands, Offshore, State of Qatar

Geological investigations of the Halul and the Shraouh islands, offshore Qatar, indicate that most of their calcareous rocks, which display abundant stromatolitic bedding, belong to the Infra-Cambrian Hormuz Series. Mineralogical, petrological, and geochemical analyses show that these calcareous rocks consist dominantly of dolomite and have formed in a reducing depositional environment. Faint laminations and small streaks of organic matter furnish evidence for the involvement of algal mats in their genesis and indicate their formation in an intertidal to supratidal setting. The Halul and Shraouh dolomites experienced extensive recrystallization and sulfatization during the emplacement of the Halul and Shraouh salt domes that form the cores of the islands. During mobilization and ascent of the salt, the dolomite recrystallized, and its Sr initial ratios were abnormally enhanced by the incorporation of 87Sr from a source, which is more radiogenic than the attendant seawater at the time of the dolomite formation near the Proterozoic–Cambrian boundary. Geochemical analysis show that Si, Al, Ti Zr, and % of insoluble residue are highly correlative, suggesting the presence of detrital minerals such as rutile and zircon. A paleosabkha model may well agree with this chemical signature. However, the Infra-Cambrian age of the Hormuz rocks and the presence of stromatolitic layers containing organic materials in the studied rocks, suggest that organogenic dolomitization could be an alternative dolomitization model.

Oxygen, carbon and strontium isotope records of Ediacaran carbonates in Central Iberia (Spain)

Carbon, oxygen and strontium isotope ratios were determined in the uppermost Neoproterozoic carbonate rocks at Pastores (SW Salamanca, Central Iberia). The low δ 18O V-SMOW values (15–19‰) indicate an alteration of the whole stratigraphic series. However, the alteration process does not seem to have affected most of the δ 13C and 87Sr/ 86Sr data. These data reveal that only one particular level of the carbonate section shows simultaneous relative increases in Mn/Sr and 87Sr/ 86Sr ratios, decreases in the δ 18O and δ 13C values, and covariations between 87Sr/ 86Sr and the other parameters. Despite this, most of the carbonate section lacks these covariations and its δ 13C (−4.6 to −0.5‰) and 87Sr/ 86Sr (0.70845–0.70853) ratios probably reflect the seawater isotopic features at the time of sedimentation. The data for C and Sr isotopes at the Pastores carbonate section and the presence of Cloudina in platform carbonates together with an overlying major sub-Cambrian unconformity following a relative sea level fall are features comparable to those frequently shown by Proterozoic–Cambrian boundary series across the world. Thus, our results consistently support the notion that the carbonates at Pastores are equivalent to other carbonates deposited close to the Ediacaran/Cambrian boundary.

The Fish River Subgroup in Namibia: stratigraphy, depositional environments and the Proterozoic–Cambrian boundary problem revisited

The Fish River Subgroup of the Nama Group, southern Namibia, is restudied in terms of lithostratigraphy and depositional environment. The study is based on partly fine-scaled sections, particularly of the Nababis and Gross Aub Formation. The results are generally in accordance with earlier studies. However, braided river deposits appear to be less widely distributed in the studied area, and a considerable part of the formations of the middle and upper subgroup apparently were deposited under shallowest marine conditions including upper shore-face. Evidence comes partly from sedimentary features and facies distribution, and partly from trace fossils, particularly Skolithos and the characteristic Trichophycus pedum. Environmental conditions represented by layers with T. pedum suggest that the producer favoured shallow marine habitats and transgressive regimes. The successions represent two deepening-upward sequences, both starting as fluvial (braided river) systems and ending as shallow marine tidally dominated environments. The first sequence includes the traditional Stockdale, Breckhorn and lower Nababis formations (Zamnarib Member). The second sequence includes the upper Nababis (Haribes Member) and Gross Aub formations. As a result, the Nababis and Gross Aub formations require emendation: a new formation including the Haribes and Rosenhof and possibly also the Deurstamp members. In addition, four distinct sequence stratigraphic units are deter-minable for the Fish River Subgroup in the southern part of the basin. The Proterozoic–Cambrian transition in southern Namibia is most probably located as low as the middle Schwarzrand Subgroup. The environmentally controlled occurrence of Trichophycus pedum undermines the local stratigraphic significance of this trace fossil which is eponymous with the lowest Cambrian and Phanerozoic trace fossil assemblage on a global scale. However, occurrences of such trace fossils have to be regarded as positive evidence for Phanerozoic age regardless of co-occurring body fossils. Other suggestions strongly dispute the concept of the formal Proterozoic–Cambrian and Precambrian–Phanerozoic boundary. Carbon isotope excursions and radiometric datings for the Nama Group do not help to calibrate precisely the temporal extent of the Fish River Subgroup. Fossil content, sequence stratigraphy and inferred depositional developments suggest that this subgroup represents only a short period of late orogenic molasse sedimentation during the early sub-trilobitic Early Cambrian.

Vestiges of a beginning? Paleontological and geochemical constraints on early animal evolution

Inspired by molecular clock estimates, some biologists have proposed that animals in general, and bilaterian metazoans in particular, began to diverge substantially earlier than fossils indicate. Balavoine and Adoutte [Science 280 (1998) 397] specifically hypothesized that the Cambrian explosion documents parallel radiations within three major bilaterian clades that diverged from one another relatively early in the Neoproterozoic Era. The geological record is permissive with respect to such hypotheses, but not encouraging. The earliest evolution of animals certainly took place before the initial appearance of phosphatic animal microfossils or Ediacaran macrofossils, but bilaterian clades need not have been part of this metazoan “pre-history”. Alternatively, the evolution of large size, made possible by late Neoproterozoic oxygen increase, may have provided the selective environment in which stem bilaterians differentiated. Cambrian events per se appear to have begun in the wake of environmental perturbation and accompanying extinction near the Proterozoic–Cambrian boundary.

Dynamics of the Neoproterozoic carbon cycle.

The existence of unusually large fluctuations in the Neoproterozoic (1,000-543 million years ago) carbon-isotopic record implies strong perturbations to the Earth's carbon cycle. To analyze these fluctuations, we examine records of both the isotopic content of carbonate carbon and the fractionation between carbonate and marine organic carbon. Together, these are inconsistent with conventional, steady-state models of the carbon cycle. The records can be well understood, however, as deriving from the nonsteady dynamics of two reactive pools of carbon. The lack of a steady state is traced to an unusually large oceanic reservoir of organic carbon. We suggest that the most significant of the Neoproterozoic negative carbon-isotopic excursions resulted from increased remineralization of this reservoir. The terminal event, at the Proterozoic-Cambrian boundary, signals the final diminution of the reservoir, a process that was likely initiated by evolutionary innovations that increased export of organic matter to the deep sea.

Burrowing below the basal Cambrian GSSP, Fortune Head, Newfoundland

The range of Treptichnus pedum, the index trace fossil for the Treptichnus pedum Zone, extends some 4 m below the Global Standard Stratotype-section and Point for the base of the Cambrian Period at Fortune Head on the Burin Peninsula in southeastern Newfoundland. The identification of zigzag traces of Treptichnus isp., even further below the GSSP than T. pedum in the Fortune Head section, and in other terminal Proterozoic successions around the globe, supports the concept of a gradational onset of three-dimensional burrowing across the Proterozoic–Cambrian boundary. Although T. pedum remains a reasonable indicator for the base of the Cambrian Period, greater precision in the stratotype section can be achieved by a detailed re-evaluation of the stratigraphic ranges and the morphological variation of ichnotaxa included in the T. pedum Zone.

Environmental interpretation and a sequence stratigraphic framework for the terminal Proterozoic Ediacara Member within the Rawnsley Quartzite, South Australia

Fossils of the Ediacara biota are confined to preservational windows in the Ediacara Member of the Rawnsley Quartzite (Pound Subgroup), from the Flinders Ranges, South Australia. The base of the Ediacara Member is a type 1 sequence boundary incised into the partly lithified Chace Quartzite Member of the Rawnsley Quartzite. The Ediacara Member and the upper half of the Rawnsley Quartzite comprise the Rawnsley depositional sequence, bounded above by the Early Cambrian Uratanna depositional sequence. The Rawnsley sequence developed over an erosional surface cut into the Chace Quartzite Member, with some 250 m of relief. Southeasterly directed palaeovalleys are filled with a sequence of massive sandstone and laminated siltstone passing up into well bedded sandstone. At lowstand, local channels were filled by massive sand. The highstand systems tract consists of several parasequence sets of laminated siltstone and channelized, amalgamated massive sandstone overlain by interbedded siltstone and sandstone grading up into well-bedded, clean sandstone. Impressions of soft-bodied Ediacaran organisms are preserved above the valley fill facies on sandstone partings within upward-shoaling, delta-front environments between storm- and fairweather wavebase. The Ediacara Member is, in places, conformably overlain by up to 500 m of sandstone of the same lithofacies as those underlying the member. Distinctive microbial-mat-bound sand laminae, described as petee structures, are characteristic of the Rawnsley Quartzite above and below the Ediacara Member. During deposition of the Ediacara Member, colonization of the deeper substrates by microbial mat communities enabled preservation of the Ediacara biota. The erosional surface at the base of the Ediacara Member is one of at least four incision events recorded in the terminal Proterozoic and Early Cambrian succession of the Adelaide Geosyncline. The similarity of incision-filling lithofacies of the Rawnsley and Uratanna sequences has, in condensed sections, led to confusion between the depositional sequences. Characteristic trace and body fossil assemblages provide the only clear distinction between depositional cycles spanning the Proterozoic–Cambrian boundary.

Early animal evolution: emerging views from comparative biology and geology.

The Cambrian appearance of fossils representing diverse phyla has long inspired hypotheses about possible genetic or environmental catalysts of early animal evolution. Only recently, however, have data begun to emerge that can resolve the sequence of genetic and morphological innovations, environmental events, and ecological interactions that collectively shaped Cambrian evolution. Assembly of the modern genetic tool kit for development and the initial divergence of major animal clades occurred during the Proterozoic Eon. Crown group morphologies diversified in the Cambrian through changes in the genetic regulatory networks that organize animal ontogeny. Cambrian radiation may have been triggered by environmental perturbation near the Proterozoic-Cambrian boundary and subsequently amplified by ecological interactions within reorganized ecosystems.

A composite reference section for terminal Proterozoic strata of southern Namibia

Integrated sequence stratigraphic and chemostratigraphic data yield a framework for correlations of stratigraphic units in the terminal Proterozoic to Cambrian Witvlei and Nama Groups of Namibia. Coupled with precise U-Pb zircon age constraints, these correlations make it possible to construct a composite reference section for use in calibrating terminal Proterozoic chronostratigraphy. The Namibian reference section starts with two distinct glacial horizons and extends up to within 1 million years of the Proterozoic-Cambrian boundary. The two glacial horizons may represent each of two distinct Varanger-age glaciations better known from the North Atlantic region. From the higher of the two glacial horizons up, the composite stratigraphy preserves one of the thickest and most complete available records of carbon-isotope variability in post-Varanger terminal Proterozoic seawater. Four carbon-isotope chemostratigraphic intervals are recognized: (1) a postglacial negative delta 13C excursion (Npg interval); (2) a rising interval (Pr interval) of increasing positive delta 13C values; (3) a falling interval (Pf interval) characterized by decreasing positive delta 13C and culminating in near zero or negative values; and (4) an interval of moderately positive, relatively invariant delta 13C values (I interval) that extends up to the unconformity that contains the Proterozoic-Cambrian boundary. Each of these chemostratigraphic intervals can be recognized in widely separated correlative sections around the world. By comparing sediment accumulation rate in the radiometrically calibrated Namibian stratigraphy with sediment accumulation rates in correlative sections in Arctic Canada and Oman, a maximum age of 564 Ma is estimated for the end of the younger Varanger glaciation, 25 m.y. younger than previous estimates.

The nature of the Proterozoic-Cambrian transition in the northern Amadeus Basin, central Australia

The nature and completeness of the rock record is vital in determining whether evolution across the Proterozoic-Cambrian boundary was episodic or explosive. This study documents the nature of the Neoproterozoic-Cambrian transition within the Arumbera Sandstone of the northern Amadeus Basin, central Australia and demonstrates an erosional surface close to this major stratigraphic boundary. We are not able to recognise major facies differences between the largely unfossiliferous lower units I and II and the rich trace fossil bearing beds of units III–IV. Horizons above and below the purported Neoproterozoic-Cambrian transition were traced laterally and searched for both trace and body fossils in an attempt to locate a local boundary between the Neoproterozoic and Cambrian. A single nonconformity bound sedimentary cycle above the major erosional surface dividing Arumbera II and III is recognised. The lowest sedimentary cycle of Arumbera III is devoid of trace fossils, but at correlative levels in the Ooraminna Sub-Basin, this horizon bears elements of the Ediacara fauna, reported for the first time herein. We, therefore, suggest that, contrary to previous studies, the Neoproterozoic-Cambrian transition is not coincident with the widespread seismic reflector but, in areas of more complete sedimentation, actually lies above it. This study also presents new stratigraphic, ichnological and palaeontological information from the Neoproterozoic-Cambrian transition in central Australia, identifies areas of potentially continuous sedimentation and provides a basis for correlation of the Arumbera Sandstone with other Neoproterozoic-Cambrian successions within Australia and beyond.

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 Manykal 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.

Temporal relationship between cratonization and glaciation: The Vendian-early Cambrian glaciation in Western Gondwana

Cratonization has long been said to favour glaciation: cratons would be ideal cradles for the development of continental ice-caps. In Western Africa, the widespread continental glaciation, developed near the Proterozoic-Paleozoic boundary, can no longer be considered as Vendian. According to the recent discovery of an Early Cambrian microfauna in Eastern Senegal, it extends until Atdabanian time (530-520 Ma). The age of its beginning lacks in precision: it can be Vendian or Early Cambrian. In the Hoggar fold belt, it constitutes the lower part of molassic intermontane sequences where it is about 30–40 Ma younger than the Pan-African cratonization dated at 600 Ma. This extensive cratonization closed the old long-lived Pan-African-Brasiliano cycle (∼1000-600 Ma) and is responsible for the aggregation of West Gondwana. In the South American Paraguay belt, glacial sequences also postdate Brasiliano cratonization dated at 590 Ma. They are probably Vendian, overlain by an Ediacara-like fauna and slightly older than those of Western Africa. The glaciation was restricted to the shoulders of rifted basins initiating the young, short-lived Pan-African-Brasiliano cycle (600, or slightly younger, to 520 Ma). World distribution of continents indicate that the widespread Late Vendian (?)- Early Cambrian ice cap glaciation of West Africa developed at middle latitudes (50-30°), whereas the probably Vendian glaciation of the Paraguay belt, restricted to local highlands, occupied high to middle latitudes (60°). In Western Gondwana, glaciation and cratonization occurring near the Proterozoic-Cambrian boundary appear as distinct and temporary unrelated phenomena.

Tectonic Development of the Eastern Officer Basin, Central Australia

Interactive interpretation of some 3000 km of seismic section, combined with the integration of well, potential field, subcrop and surface geological data with satellite imagery have shown that the structure of this prospective basin is controlled by compressive tectonics. Four stages of basin development between the late Proterozic and Carboniferous can be identified on calibrated seismic sections from the eastern Officer Basin. Recent work suggests that the Officer Basin was originally part of an extensional Centralian Superbasin which included the present Amadeus, Ngalia, Savory and Georgina Basins. Justification for this hypothesis comes from the broadly similar stratigraphy in each of these basins. A model is proposed which explains the deformation of the Centralian Superbasin into those basins recognised today, separated by basement uplifted to shallow depth and exposure by compression. The main deformation is believed to have commenced at or around the Proterozoic-Cambrian boundary, with a substantial pulse at the end of the Cambrian. As a result, the original broad platform depositional system was deformed into a series of foreland-style basins. Stacked thrust sheets of late Proterozic to Cambrian sediments, containing the best known source rocks in the basin, illustrate the intensity of the compressive deformation. lnterbedded reservoir units have been folded to form traps suitable for hydrocarbon retention. This model will enhance the probability of prospect generation and the search for commercial hydrocarbons.

Neoproterozoic of the Mackenzie Mountains, northwestern Canada

Neoproterozoic strata of the Mackenzie Mountains are more than 10 km thick, and comprise two supergroups of differing style. The Mackenzie Mountains Supergroup (4–6 km thick), an epicratonic succession of mainly shallow-water siliciclastic and carbonate strata, contains detrital zircons dated at 1080 Ma and is cut by dykes and plugs dated at 780-778 Ma. The overlying Windermere Supergroup (5–7 km thick) comprises basal rift deposits and glacial diamictites that are overlain by three, kilometre-scale, siliciclastic to carbonate “Grand Cycles” deposited on a passive margin. “Grand Cycles” in the eastern Mackenzie Mountains were deposited in a shallow-water setting, whereas contemporaneous cycles in the western Mackenzies were deposited on a continental slope; mapping of successive shelf-margin positions reveals southwestward progradation. The Windermere Supergroup is unconformably overlain by the Ingta Formation, which contains the Proterozoic-Cambrian boundary. Subtrilobite Cambrian strata are approximately 1.5 km thick, and are overlain by trilobite-bearing Lower Cambrian carbonates of the Sekwi Formation. Fossils are abundant and biostratigraphically useful in these Neoproterozoic strata. Acritarchs, carbonaceous megafossils, and microbial structures occur throughout the entire succession, but are especially well preserved in the Little Dal Group of the Mackenzie Mountains Supergroup. Edicara-type fossils first appear below the second Windermere glacial unit (Ice Brook diamictite), and range upwards through 2.5 km of strata to near the top of the Windermere; three distinctive assemblages corresponding to the Twitya, Sheepbed, and Blueflower formations can be recognized. The Neoproterozoic-Cambrian boundary occurs within the Ingta Formation, and is marked by an abrupt change from simple, subhorizontal burrows ( Planolites-Torrowangea Assemblage) to complex feeding burrows of the Phycodes Assemblage. Small shelly fossils of Nemakit-Daldyn aspect occur in the upper part of the Ingta Formation, slightly above the basal Cambrian boundary. The presence of two glacial diamictites in the Windermere and numerous sequence boundaries and flooding surfaces throughout the succession provides opportunities for regional and global event correlation. The palaeomagnetic record shows three hair-pin curves that should be recognizable in continents attached to Laurentia. Carbonate interbeds throughout the succession typically yield relatively unaltered carbon isotope ratios; C and Sr chemostratigraphy shows marked excursions that appear to be globally correlatable.

Integrated approaches to terminal Proterozoic stratigraphy: an example from the Olenek Uplift, northeastern Siberia

In the Olenek Uplift of northeastern Siberia, the Khorbusuonka Group and overlying Kessyusa and Erkeket formations preserve a significant record of terminal Proterozoic and basal Cambrian Earth history. A composite section more than 350 m thick is reconstructed from numerous exposures along the Khorbusuonka River. The Khorbusuonka Group comprises three principal sedimentary sequences: peritidal dolomites of the Mastakh Formation, which are bounded above and below by red beds; the Khatyspyt and most of the overlying Turkut formations, which shallow upward from relatively deep-water carbonaceous micrites to cross-bedded dolomitic grainstones and stromatolites; and a thin upper Turkut sequence bounded by karst surfaces. The overlying Kessyusa Formation is bounded above and below by erosional surfaces and contains additional parasequence boundaries internally. Ediacaran metazoans, simple trace fossils, and vendotaenids occur in the Khatyspyt Formation; small shelly fossils, more complex trace fossils, and acritarchs all appear near the base of the Kessyusa Formation and diversify upward. The carbon-isotopic composition of carbonates varies stratigraphically in a pattern comparable to that determined for other terminal Proterozoic and basal Cambrian successions. In concert, litho-, bio-, and chemostratigraphic data indicate the importance of the Khorbusuonka Group in the global correlation of terminal Proterozoic sedimentary rocks. Stratigraphic data and a recently determined radiometric date on basal Kessyusa volcanic breccias further underscore the significance of the Olenek region in investigations of the Proterozoic-Cambrian boundary.

Sizing up the sub-Tommotian unconformity in Siberia.

Sedimentary rocks in the western Anabar region, northwestern Siberia, preserve an exceptional record of evolution and biogeochemical events near the Proterozoic-Cambrian boundary. Carbon isotopic data on petrographically and geochemically screened samples collected at 1 to 2 m intervals support correlation of the lower Anabar succession (Staraya Reckha and lower Manykai Formations) with sub-Tommotian carbonates of the Ust'-Yu-doma Formation in southeastern Siberia. In contrast, the upper Manykai and most of the overlying Medvezhya Formation appear to preserve a sedimentary and paleontological record of an evolutionary important time interval represented in southeastern Siberia only by the sub-Tommotian unconformity. Correlation of the Anabar section with other northern Siberian successions that contain well-dated volcanic rocks permits the estimate that the sub-Tommotian unconformity in southeastern Siberia spans approximately 3 to 6 m.y. Diverse small shelly fossils (but not archaeocyathans) previously thought to mark the base of the Tommotian Stage evolved sequentially throughout this earlier interval.

Calcified Microbes in Neoproterozoic Carbonates: Implications for Our Understanding of the Proterozoic/Cambrian Transition

Tidal flat and lagoonal dolostones of the Neoproterozoic Draken Formation, Spitsbergen, exhibit excellent preservation of carbonate fabrics, including heavily calcified microfossils. The crust-forming cyanobacterium Polybessurus is preserved locally by carbonate precipitated on and within sheaths in mildly evaporitic upper intertidal to supratidal environments. In contrast, calcified filaments in columnar stromatolites reflect subtidal precipitation. Filament molds in dolomicrites independently document extremely early lithification. The presence of heavily calcified cyanobacteria in Draken and other Proterozoic carbonates constrains potential explanations for the widespread appearance of calcified microorganisms near the Proterozoic-Cambrian boundary. We propose that the rarity of Proterozoic examples principally reflects the abundance and wide distribution of carbonate crystals precipitated on the sea floor or in the water column. Cyanobacterial sheaths would have competed effectively as sites for carbonate nucleation and growth only where calcitic and/or aragonitic nuclei were absent. In this view, the Proterozoic-Cambrian expansion of calcified microfossils primarily reflects the emergence of skeletons as principal agents of carbonate deposition.