Crystal Fans Research Articles

Sedimentology and Geochemistry of the 2930 Ma Red Lake–Wallace Lake Carbonate Platform, Western Superior Province, Canada

AbstractChemical sedimentary rocks in the Red Lake–Wallace Lake area of the Canadian Shield form the Earth's oldest known carbonate platform and, as such, provide a unique opportunity to explore the floor of a 2930 Myr old warm, shallow sea. Peritidal depositional features dominate the platform top, ranging from colloform crusts, teepee structures, and evaporate pseudomorphs in the supratidal; pseudomorph crystal fans and laterally linked domal stromatolites, associated with stromatactis‐like structures, sheet cracks and liminoid fenestral fabrics extending from the lower supratidal through the intertidal; and herring‐bone cross‐stratification, isolated domal stromatolites and herring‐bone calcite cement in tidal channels. These lithofacies indicate a low energy, restricted evaporitic environment. Limited subtidal platform top deposits are characterized by laterally linked domal stromatolites and pseudomorph crystal fans on a larger scale than those found in the intertidal areas. A transitional lithofacies to deeper water were deposited further offshore. It consists of ribbon rock (mixed laminae and beds of carbonate, slate and iron oxide sediments), slump structures composed of intraclastic carbonate lithoclasts in a marl matrix and carbonate‐associated iron formation. Basinal deposits consist of chert and chert‐oxide facies iron formation. Peritidal lithofacies are composed of ferroan dolomite, whereas deep subtidal to upper slope lithofacies are composed of calcite. The dolomites have O and Sr isotopic ratios which were probably reset during dolomitization, whereas only O is altered in the limestones. The δ13C values for all the carbonate samples were 0 ± 1·1‰V‐PDB, with samples formed in deeper water having lighter δ13C values, suggesting the deeper open ocean had a lighter δ13C budget than water in the platform interior. Post Archean Australian Shale normalized rare earth element patterns for the carbonate samples have positive La and Eu anomalies, suprachondritic Y/Ho ratios and slight heavy rare earth element enrichment in most samples. Basin lithofacies are characterized by heavy rare earth element enrichment and positive Eu anomalies. One sample had a significant negative Ce anomaly. These data probably indicate restricted circulation in the supra and intertidal areas and possibly the development of spatially limited areas where oxygen production could move the redox boundary out into the water column.

Aragonite Crystal Fans In Neoproterozoic Cap Carbonates: A Case Study From Brazil and Implications For the Post-Snowball Earth Coastal Environment

Abstract Neoproterozoic cap carbonates are characterized by “anomalous” sedimentary features, including sea-floor precipitates, represented by aragonite pseudomorph cement crusts and crystal fans. These features are found in restricted periods of geological history, and their generation in the context of the Neoproterozoic has been attributed to very specific postglacial conditions leading to alkalinity oversaturation. New sedimentary and geochemical results on well-preserved Neoproterozoic deposits of aragonite pseudomorph crystal fans at the base of the Sete Lagoas Formation (Bambui Group, central Brazil) are reported. The micrite-settling facies shows the highest abundance of crystal fans, arranged in centimeter-scale layers interfingered with the micrite matrix; this facies is interpreted as a result of postglacial calcium carbonate oversaturation in restricted areas characterized by marine dysoxic waters. A numerical model constrains the kinetics of formation of aragonite crystals or micrite by calculating the induction times and precipitation rates of both carbonate species for postglacial conditions of elevated temperature and high atmospheric pCO2. In these conditions, the Mg/Ca ratio of the Neoproterozoic seawater is calculated at ca. 1.2. Alkalinity oversaturation is monitored by a variable evaporation degree. Aragonite formation is kinetically favored, and the discrepancy between calcite and aragonite precipitation rate is greater as atmospheric pCO2, hence alkalinity, is higher. These modeling conditions do not preclude the contribution of incomplete organic-matter degradation to alkalinity as suggested by negative carbon isotope signatures. In addition, the large number of aragonite–micrite pairs suggests a seasonal or paleoclimatic forcing. Other cases of abiotic aragonite precipitation through time are also briefly examined by comparison.

Microbialite concretions in a dolostone crust at the Permian–Triassic boundary of the Xishan section in Jiangsu Province, South China

Carbonate concretions with structures and fossil groups associated with microbialite developed in a dolostone crust at the Permian–Triassic boundary of the Xishan section in Jiangsu Province, South China. These structures include clotted fabrics and laminated carbonate needles, as well as abundant carbonate crystal fans. Fossil groups associated with microbialite include microconchids, small gastropods, and small foraminifers. These fabrics and fossils suggest that the concretions are carbonate microbialite blocks developed in the dolostone crust. On the basis of the analysis of the microfabrics and the fossil groups together with a comparison to modern analogues, we attribute the formation of the micritic patches in the microbialite concretions to the calcification of cyanobacterial mats via carbonate nanoparticles and we attribute the carbonate crystal fans to the direct recrystallization of micritic carbonates. The sparitic patches were interpreted as either the direct recrystallization of micritic carbonates or the precipitation of carbonate spars in the inter-/intra-spaces of metazoan shells together with the recrystallization of these shells. The similarities to modern stromatolites, both in morphology and in internal texture, suggest that the laminated carbonate needles are stromatolite laminae built by filamentous cyanobacteria. The preservation of these microbialite microfabrics indicates that early lithification by carbonate precipitation was widespread and intense following the end-Permian boundary events. The weak development of microbialites as small concretions may be attributed to the deeper water depth and the lower water energy in the Xishan area during the earliest Triassic.

Biogeochemical formation of calyx-shaped carbonate crystal fans in the subsurface of the Early Triassic seafloor

The Kuh e Dena sedimentary section in Iran developed on the southwestern margin of the Neotethys Ocean. The fossil- and bioturbation-lean Early Triassic section (Late Dienerian to Early Smithian) is typified by finely laminated calcareous shales that are interbedded with laterally extending carbonate beds, which are mostly composed of calyx-shaped carbonate crystal fans. The calyxes consist of fibrous crystals that apparently formed within the subseafloor in soft, water-saturated sediment by displacive growth. Rare earth element and yttrium patterns indicate that calyx-shaped crystals precipitated from anoxic pore waters. Biomarker patterns of the carbonate beds reveal major input of lipids from prokaryotes that typically occur within layered benthic microbial mats (i.e., cyanobacteria, anoxygenic phototrophic bacteria, sulphate-reducing bacteria, and methanogenic archaea). Along their length, the fibrous crystals of the calyxes reveal a trend of increasing δ13Ccarb values (from 2.7 to 3.3‰), suggesting that archaeal methanogenesis affected the carbonate pool. Apart from the putative benthic prokaryotes, molecular fossils of halophilic, most likely planktic, archaea were detected in the carbonate beds. Low pristane to phytane ratios (≤0.8) and the presence of halophilic archaea are in accord with the lack of bioclasts and bioturbation, which likely reflects increased salinities. The former presence of a benthic microbial mat with cyanobacteria underlain by anoxygenic phototrophic bacteria, sulphate-reducing bacteria, and methanogenic archaea is in accordance with studies on living microbial mats in hypersaline, low-oxygen marine environments. The Early Triassic subseafloor seems to have been a sink for carbon, driven by biologically-induced formation of secondary, diagenetic carbonate.

Synsedimentary deformation and the paleoseismic record in Marinoan cap carbonate of the southern Amazon Craton, Brazil

Event Layers in Neoproterozoic cap carbonates of Brazil's southwestern Amazon Craton record post-Marinoan synsedimentary seismicity. The 35 m-thick cap carbonates overlie glaciogenic sediments related to the Marinoan glaciation (635 Ma) and are comprised of two units: the lower cap consists of dolomite (∼15 m thick) and the upper cap is limestone (∼25 m thick). The cap dolomite includes pinkish crystalline dolostone with even parallel lamination, stratiform stromatolites, eventual tube structures and megaripple bedded peloidal dolostone interpreted as shallow (euphotic) platform deposits. The cap limestone onlaps the cap dolomite and consists of red marl, gray to black bituminous lime mudstone, bituminous shale with abundant calcite crystal fans (pseudomorphs after aragonite) and even parallel lamination interpreted as moderately deep to deep platform deposits. Five successive events of synsedimentary deformation were recognized in the cap carbonates exposed at Mirassol d'Oeste and Tangará da Serra, in Central Brazil: Event 1 – large to small-scale load cast structures in the contact between dolostones and glaciogenic sediments; Event 2 – stromatolitic lamination truncated by tube structures; Event 3 – vertical to subvertical fractures and faults, and large-scale synclines and anticlines with chevron folds; Event 4 – conglomerate and breccia filling neptunian dykes limited by undeformed beds; and Event 5 – slump and sliding deposits found only in the upper part of the cap limestone. Event 1 was produced by hydroplastic dynamics likely induced by isostatic rebound during ice cap melting in the final stages of the Marinoan glaciation. Events 2 and 5 are autocyclic in nature, and related to depositional processes. Event 2 is linked to fluid and methane escape from organic degradation of microbial mats and domes that formed tubestones; Event 5 is associated to collapse and sliding/slumping in the platform and slope. The reliable orientations of synsedimentary faults, and fractures and folds of events 3 and 4 are consistent with regional extensional tectonics associated with earthquakes that triggered sediment deformation. The 200 km that separate the occurrences of cap carbonates suggest that important seismic events took place during the early Ediacaran in the southern Amazon Craton.

BIOLOGICAL INFLUENCES ON SEAFLOOR CARBONATE PRECIPITATION

The sedimentary record reveals first-order changes in the locus of carbonate precipitation through time, documented in the decreasing abundance of carbonate precipitation on the seafloor. This pattern is most clearly recorded by the occurrence of seafloor carbonate crystal fans (bladed aragonite pseudomorphs neomorphosed to calcite or dolomite), which have a distinct temporal distribution, ubiquitous in Archean carbonate platforms, but declining through Proterozoic time and extremely rare in Phanerozoic basins. To understand better the potential influences on this pattern, we built a mathematical framework detailing the effects of organic matter delivery and microbial respiratory metabolisms on the carbonate chemistry of shallow sediments. Two nonunique end-member solutions emerge in which seafloor precipitation is favorable: enhanced anaerobic respiration of organic matter, and low organic matter delivery to the sediment-water interface. This analysis suggests that not all crystal fans reflect a unique set of circumstances; rather there may have been several different geobiological and sedimentary mechanisms that led to their deposition. We then applied this logical framework to better understand the petrogenesis of two distinct crystal fan occurrences—the Paleoproterozoic Beechey Formation, Northwest Territories, Canada, and the middle Ediacaran Rainstorm Member of the Johnnie Formation, Basin and Range, United States—using a combination of high-resolution petrography, micro X-ray fluorescence and wavelength dispersive spectroscopy, C isotopes, and sedimentary context to provide information on geobiological processes occurring at the sediment-water interface. Interestingly, both of these Proterozoic examples are associated with iron-rich secondary mineral assemblages, have elevated trace metal signatures, and sit within maximum flooding intervals, highlighting key commonalities in synsedimentary geobiological processes that led to seafloor carbonate precipitation.

On the unique isotropic aragonitic tube microstructure of some serpulids (Polychaeta, Annelida)

A Scanning electron microscopy study of peculiar serpulid tube microstructures was carried out to determine whether this structure is unique to serpulids and to understand its formation and evolution. The isotropic aragonitic tube microstructure of some serpulids, termed spherulitic irregularly oriented prismatic structure (SIOP), is unknown in other animal skeletons. After nucleation, initial spherulitic prismatic aragonite crystals presumably grow in length and width at one end, collide with the surrounding growing crystal fans, and then continue intertwined growth until the secreted carbonate-charged mucus is completely crystallized. Spherulitic irregularly oriented prismati structure presumably evolved either from the IOP or homogeneous rounded crystal microstructures.

Sr isotope geochemistry and Pb–Pb geochronology of the Neoproterozoic cap carbonates, Tangará da Serra, Brazil

The Mirassol d'Oeste Formation (cap dolomites) and the base of the Guia Formation (cap limestones) represent the cap carbonates of the Neoproterozoic Araras Group, located along the southern border of Amazonian Craton, central Brazil. Petrographical and microfacies descriptions together with geochemical and mineralogical data of the top of the Mirassol d'Oeste and the base of the Guia formations in the Tangará da Serra area allowed us to identify the main diagenetic features (neomorphism, chemical compaction, and fractures) and to evaluate the terrigenous contribution. Dolomitization has been documented in a limestone horizon of the lower part of Guia Formation through petrography, X-ray diffractometry, and geochemistry. Despite diagenetic alteration, sedimentary structures of the Tangará da Serra cap carbonate are well preserved (laminar bedding and crystal fans). Pb–Pb dating of the Guia limestones yielded an age of 622 ± 33 million years. This early Ediacaran age is considered as the deposition age and constitutes further evidence that the cap carbonates of the Araras Group are related to the Marinoan glaciation, which took place at the end of the Cryogenian. The assessment of the primary 87Sr/86Sr signature of the Guia limestones has been improved by sequential leaching of diluted acetic acid, which allowed the elimination of highly radiogenic Sr contributions due to terrigenous grains. The 87Sr/86Sr ratios of 0.70709–0.70729 are regarded as the primary Sr isotopic signature when the carbonates precipitated. The terrigenous contribution can explain the discrepancy with the more radiogenic 87Sr/86Sr ratios previously obtained on the cap carbonate. At a global scale, the Sr isotopic signature around 0.7071–0.7073 of the cap carbonates of the Araras Group is compatible with Sr marine evolution curves at the end of the Marinoan glaciation. However, such low Sr values suggest that the abrupt rise could have happened asynchronously and heterogeneously in the early Ediacaran oceans.

Diagenesis or dire genesis? The origin of columnar spar in tufa stromatolites of central Greece and the role of chironomid larvae

AbstractTwo Greek Pleistocene tufa stromatolites were examined petrographically and with stable isotope geochemistry to determine whether calcite spar is of primary or diagenetic origin. The younger (ca 100 ka) tufa from Zemeno primarily is micritic, with primary columnar calcite spar restricted to areas immediately above chironomid larval tubes. This relationship suggests that chironomid larval feeding behaviour is responsible for the development of Zemeno tufa columnar calcite, probably involving biological substances smeared onto the tufa surface. Most micritic crystals are not suitably oriented to allow later post‐depositional growth resulting in columnar fabrics. The older (ca 1 Ma) predominantly sparry tufa from Nemea contains some chironomid tubes and organic cyanobacterial filaments preserved in crystal fans but also contains many fabrics found in primary speleothem spar. Columnar spar here is unlikely to be the result of post‐depositional crystal growth. A comparison of stable isotopic trends between the two tufa deposits suggests that both contain interpretable seasonal trends and implies little or minor post‐depositional alteration of either tufa. Consequently, there is no basis for the common assumption that sparry tufa fabrics must be of diagenetic origin.

Chapter 5 Chemical sediments associated with Neoproterozoic glaciation: iron formation, cap carbonate, barite and phosphorite

Abstract Orthochemical sediments associated with Neoproterozoic glaciation have prominence beyond their volumetric proportions because of the insights they provide on the nature of glaciation and the records they hold of the environment in which they were precipitated. Synglacial Fe formations are mineralogically simple (haematite jaspilite), and their trace element spectra resemble modern seawater, with a weaker hydrothermal signature than Archaean–Palaeoproterozoic Fe formations. Lithofacies associations implicate subglacial meltwater plumes as the agents of Fe(II) oxidation, and temporal oscillations in the plume flux as the cause of alternating Fe- and Mn-oxide deposits. Most if not all Neoproterozoic examples belong to the older Cryogenian (Sturtian) glaciation. Older and younger Cryogenian (Marinoan) cap carbonates are distinct. Only the younger have well-developed transgressive cap dolostones, which were laid down during the rise in global mean sea level resulting from ice-sheet meltdown. Marinoan cap dolostones have a suite of unusual sedimentary structures, indicating abnormal palaeoenvironmental conditions during their deposition. Assuming the meltdown of ice-sheets was rapid, cap dolostones were deposited from surface waters dominated by buoyant glacial meltwater, within and beneath which microbial activity probably catalysed dolomite nucleation. Former aragonite seafloor cement (crystal fans) found in deeper water limestone above Marinoan cap dolostones indicates carbonate oversaturation at depth, implying extreme concentrations of dissolved inorganic carbon. Barite is associated with a number of Marinoan cap dolostones, either as digitate seafloor cement associated with Fe-dolomite at the top of the cap dolostone, or as early diagenetic void-filling cement associated with tepee or tepee-like breccias. Seafloor barite marks a redoxcline in the water column across which euxinic Ba-rich waters upwelled, causing simultaneous barite titration and Fe(III) reduction. Phosphatic stromatolites, shrub-like structures and coated grains are associated with a glacioisostatically induced exposure surface on a cap dolostone in the NE of the West African craton, but this appears to be a singular occurrence of phosphorite formed during a Neoproterozoic deglaciation.

Chapter 29 The Tsagaan Oloom Formation, southwestern Mongolia

Abstract The Tsagaan Oloom Formation (Fm.) in southwestern Mongolia contains two Neoproterozoic glacial deposits, with diamictite in the Maikhan Ul Member (Mb.) and in the Khongoryn Mb., which are separated by over 500 m of limestone. The Maikhan Ul Mb. ranges in thickness between 5 m and greater than 300 m, expanding in deeper-water sections towards the SW, where it is composed of two massive diamictites separated by over 100 m of sandstone, siltstone and shale. The basal 10 m of the overlying Tayshir Mb. of the Tsagaan Oloom Fm. consists of a fine-laminated, dark grey limestone. The Khongoryn Mb. is composed primarily of limestone clasts in a shale matrix, and is between 0 and 23 m thick. The overlying Ol Mb. contains sedimentary structures characteristic of basal Ediacaran cap carbonates including micropeloids, tubestone stromatolites, giant wave ripples and former aragonite crystal fans. U–Pb evaporation ages from zircons in the underlying Dzabkhan Volcanics constrain the Tsagaan Oloom Fm. to <773 Ma, and tuffs within the Maikhan Ul and Tayshir members testify to the potential for additional geochronology. The Cryogenian organic-rich limestone of the Tayshir Mb., which lies between the two glacial deposits, is ideally suited for geochemical studies and has been the subject of several carbon, strontium and rare earth element investigations. Limited palaeomagnetic studies suggest a mid- to low-latitude position of the Dzabkhan platform during deposition of the glaciogenic strata, and additional studies are in progress.

Earliest Triassic microbialites in Çürük Dag, southern Turkey: composition, sequences and controls on formation

AbstractThe Permian–Triassic Boundary sequence at Çürük Dag, near Antalya, Turkey, begins with a major erosion surface interpreted as being the Late Permian lowstand, on which lies ca 0·4 m of grainstone/packstone composed of ooids, peloids and bioclasts. Most ooids are superficial coats on fragments of calcite crystals presumed to be eroded from crystal fans which are no longer present. The erosion surface is smooth and shows no evidence of dissolution; the grainstone/packstone contains intraclasts of the underlying wackestone, proving erosion. Next are 15 m of microbialite comprised of interbedded stromatolites, thrombolites, plus beds of planar limestones with small‐scale erosion. The latter comprise a complex interlayering of stromatolitic, thrombolitic and peloidal fabrics and precipitated crystal fans, which form a hybrid of microbialite and inorganic carbonate, together with bioclastic debris and micrite. The Çürük Dag microbialite sequence is repetitious; the lower part is more complex, with abundant stromatolites and hybrid microbialites. Some of the stromatolites are themselves hybrids composed of peloids and crystal fans. In the upper part of the sequence stromatolites are missing and the rock is composed mostly of recrystallized thrombolites that develop upwards from tabular to domal form. The domes form directly below small breaks in microbialite growth where very thin shelly micrites and grainstones/packstones are deposited. Repetition of facies may be controlled by sea‐level change; a deepening‐up model is consistent with the evidence. Stromatolites (with abundant crystal fans) dominate in shallower water, deepening through hybrid microbialite and interlayered sediments to thrombolite, probably no more than a few tens of metres deep, followed by breaks and renewal of microbialite growth. An interpretation of open marine fully oxygenated waters for microbialite growth is consistent with ongoing parallel work that has identified Bairdioid ostracods in the microbialite, a group known to be open marine. However, other researchers have proposed low oxygen conditions for Permian–Triassic boundary facies globally, so work continues to confirm whether the Çürük Dag microbialite grew in dysoxic or normally oxygenated conditions. The principal stimulus for post‐extinction microbialites is likely to be carbonate supersaturation of the oceans. The microbialite sequence is overlain by a further 25 m of grainstone/packstone (without microbialite), followed by Early Triassic shales. Overall, microbialites form a thin aggradational sequence during an overall relative sea‐level rise, consistent with global eustatic rise following the Late Permian lowstand.

Neoproterozoic glaciation on a carbonate platform margin in Arctic Alaska and the origin of the North Slope subterrane

Research Article| March 01, 2009 Neoproterozoic glaciation on a carbonate platform margin in Arctic Alaska and the origin of the North Slope subterrane Francis A. Macdonald; Francis A. Macdonald † 1Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA †E-mail: fmacdon@fas.harvard.edu Search for other works by this author on: GSW Google Scholar William C. McClelland; William C. McClelland 2Department of Geological Sciences, University of Idaho, Moscow, Idaho 83843, USA Search for other works by this author on: GSW Google Scholar Daniel P. Schrag; Daniel P. Schrag 3Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA Search for other works by this author on: GSW Google Scholar Winston P. Macdonald Winston P. Macdonald 4Biology Department, Boston University, 5 Cummington Street, Boston, Massachusetts 02215, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Francis A. Macdonald † 1Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA William C. McClelland 2Department of Geological Sciences, University of Idaho, Moscow, Idaho 83843, USA Daniel P. Schrag 3Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA Winston P. Macdonald 4Biology Department, Boston University, 5 Cummington Street, Boston, Massachusetts 02215, USA †E-mail: fmacdon@fas.harvard.edu Publisher: Geological Society of America Received: 04 Jan 2008 Revision Received: 27 Apr 2008 Accepted: 27 May 2008 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2009 Geological Society of America GSA Bulletin (2009) 121 (3-4): 448–473. https://doi.org/10.1130/B26401.1 Article history Received: 04 Jan 2008 Revision Received: 27 Apr 2008 Accepted: 27 May 2008 First Online: 02 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 Francis A. Macdonald, William C. McClelland, Daniel P. Schrag, Winston P. Macdonald; Neoproterozoic glaciation on a carbonate platform margin in Arctic Alaska and the origin of the North Slope subterrane. GSA Bulletin 2009;; 121 (3-4): 448–473. doi: https://doi.org/10.1130/B26401.1 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 SocietyGSA Bulletin Search Advanced Search Abstract The rotation model for the opening of the Canada Basin of the Arctic Ocean predicts stratigraphic links between the Alaskan North Slope and the Canadian Arctic islands. The Katakturuk Dolomite is a 2080-m-thick Neoproterozoic carbonate succession exposed in the northeastern Brooks Range of Arctic Alaska. These strata have previously been correlated with the pre–723 Ma Shaler Supergroup of the Amundson Basin. Herein we report new composite δ13C profiles and detrital zircon ages that test this connection. We go further and use stratigraphic markers and a compilation of δ13C chemostratigraphy from around the world, tied to U-Pb ages, to derive an age model for deposition of the Katakturuk Dolomite. In particular, we report the identification of ca. 760 Ma detrital zircons in strata underlying the Katakturuk Dolomite. Moreover, a diamictite present at the base of the Katakturuk Dolomite is capped by a dark-colored limestone with peculiar roll-up structures. Chemostratigraphy and lithostratigraphy suggest this is an early-Cryogenian glacial diamictite-cap carbonate couplet and that deposition of the Katakturuk Dolomite spanned much of the late Neoproterozoic. Approximately 500 m above the diamictite, a micropeloidal dolomite, with idiosyncratic textures that are characteristic of basal Ediacaran cap carbonates, such as tubestone stromatolites, giant wave ripples, and decameters of pseudomorphosed former aragonite crystal fans, rests on a silicified surface. Chemostratigraphic correlations also indicate a large increase in sedimentation rate in the upper ~1 km of the Katakturuk Dolomite and in the overlying lower Nanook Limestone. We suggest that the accompanying increase in accommodation space, along with the presence of two low-angle unconformities within these strata, are the product of late Ediacaran rifting along the southern margin of the North Slope subterrane. There are no strata present in the Amundson Basin that are potentially correlative with the late Neoproterozoic Katakturuk Dolomite, as the Cambrian Saline River Formation rests on the ca. 723 Ma Natkusiak Formation. Detrital zircon geochronology, chemostratigraphic correlations, and the style of sedimentation are inconsistent with both a Canadian Arctic origin of the North Slope subterrane and a simple rotation model for the origin of the Arctic Ocean. If the rotation model is to be retained, the exotic North Slope subterrane must have accreted to northwest Laurentia in the Early to Middle Devonian. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Seafloor-precipitated carbonate fans in the Neoproterozoic Rainstorm Member, Johnnie Formation, Death Valley Region, USA

Cm-sized carbonate seafloor fans occur in the Neoproterozoic Rainstorm Member of the Johnnie Formation, Death Valley, USA. The fans formed in a mixed carbonate-clastic succession near storm wave base at the base of parasequences on a storm-dominated ramp. Petrographic observations indicate that the fans were originally precipitated as aragonite and later inverted to calcite during diagenesis. Although not directly dated, the Rainstorm Member preserves a large magnitude negative carbon isotopic anomaly (down to − 11‰ PDB) tentatively correlated to the largest known carbon isotope excursion found in many stratigraphic successions around the world between 585 Ma and 550 Ma. Thus, the age distribution of seafloor aragonite fans in Neoproterozoic strata appears more widespread than previously thought, occurring in strata significantly younger than the last widespread Neoproterozoic glaciation. Rainstorm Member carbonate fans and oolitic units (representing time-correlative shallower environments) record similar carbon isotope ratios during the negative carbon isotopic anomaly. The carbon isotopic homogeneity displayed between fans and other carbonate sediments implies that waters across the shelf were well-mixed rather than stratified during the late Neoproterozoic isotopic anomaly. In addition, the similarity of carbon isotope ratios shared among fans along a stratigraphic horizon (on a m- to cm-scales) suggests that the local source of alkalinity required for fan growth was derived from a well-mixed reservoir, likely seawater, rather than local diagenetic fluids. Increased alkalinity and the presence of inhibitors to carbonate nucleation (perhaps Fe 2+ under anoxic conditions) likely fostered precipitation of aragonite crystal fans on the seafloor.

Varying growth rates in bamboo corals: sclerochronology and radiocarbon dating of a mid-Holocene deep-water gorgonian skeleton (Keratoisis sp.: Octocorallia) from Chatham Rise (New Zealand)

A branched mid-Holocene bamboo coral skeleton of the isidid gorgonian genus Keratoisis (Octocorallia) recovered at southwestern Chatham Rise (New Zealand) from an average water depth of 680 m is described with respect to sclerochronology and age determination. Growth rates of the Mg-calcitic internodal increments were investigated by the counting of colour bands and radiocarbon dating. Growth banding is produced by varying orientations of crystal fan bundles towards the image plane. The skeleton shows three growth interruptions, which are documented in all branches. AMS 14C ages decrease from base to top of the trunk and from the central axes to the margins of the branches, documenting a simultaneous vertical and lateral growth. The data provide a maximum age of 3,975 ± 35 years BP, and a record spanning 240 ± 35 years. While calculated longitudinal growth rates amount to an average of 5 mm year−1 during a 55-year record, average lateral linear extension rates of 0.4 mm year−1 are an order of magnitude lower, still allowing for a seasonal to annual resolution of colour bands on a macroscopic scale and for a daily to monthly resolution on microscales of individual crystal generations to fascicle bundles. Hence, the isidid skeleton provides a high-resolution archive of paleoceanographic dynamics in deeper water masses. Concentric incremental accretion around the central axis in the early growth stages changed into a unilaterally asymmetric growth during late-stage evolution, probably triggered by the establishment of a stable system of unidirectional currents and nutrient flux. While colour band counts, related to the AMS 14C ages, support a seasonal to annual accretion of macroscopic growth bands in the inner concentric and complete outer parts of the skeleton, incremental growth rates at the condensed side are highly variable, as documented by hiatuses and unconformities. Thus the specimen proves that growth rates of bamboo corals may vary within individual skeletons and strongly deviate from the annual mode, hence showing implications on paleoceanographic proxy analyses.

Erosional truncation of uppermost Permian shallow-marine carbonates and implications for Permian-Triassic boundary events

On shallow-marine carbonate buildups in south China, Turkey, and Japan, uppermost Permian skeletal limestones are truncated by an erosional surface that exhibits as much as 10 cm of topography, including overhanging relief. Sedimentary facies, microfabrics, carbon isotopes, and cements together suggest that erosion occurred in a submarine setting. Moreover, biostratigraphic data from south China demonstrate that the surface postdates the uppermost Permian sequence boundary at the global stratotype section and truncates strata within the youngest known Permian conodont zone. The occurrences of similar truncation surfaces at the mass-extinction horizon on carbonate platforms across the global tropics, each overlain by microbial buildups, and their association with a large negative excursion in δ 13 C further suggest a causal link between erosion of shallow-marine carbonates and mass extinction. Previously proposed to account for marine extinctions, the hypothesis of rapid carbon release from sedimentary reservoirs or the deep ocean can also explain the petrographic observations. Rapid, unbuffered carbon release would cause submarine carbonate dissolution, accounting for erosion of uppermost Permian skeletal carbonates, and would be followed by a pulse of high carbonate saturation, explaining the precipitation of microbial limestones containing upwardgrowing carbonate crystal fans. Models for other carbon-release events suggest that at least 5 × 10 18 g of carbon, released in <100 k.y., would be required. Of previously hypothesized Permian-Triassic boundary scenarios, thermogenic methane production from heating of coals during Siberian Traps emplacement best accounts for petrographic characteristics and depositional environment of the truncation surface and overlying microbial limestone, as well as an associated carbon isotope excursion and physiologically selective extinction in the marine realm.

Chemostratigraphy and lithological characters of Neoproterozoic cap carbonates from the Kuruktag Mountain, Xinjiang, western China

The Neoproterozoic Era includes some of the most largest ice ages in the geological history. The exact number of glaciations is unknown, though there were at least two events of global glaciation. Neoproterozoic glacial deposits in the Kuruktag Mountain, Xinjiang, western China have proven that there had occurred three discrete Neoproterozoic glaciations. Diamictite units occurred in the Bayisi, Tereeken, and Hankalchough formations, carbonate units were recognized among the diamictites and immediately overlied the Bayisi, Tereeken and Hankalchough diamictites. Carbonates at the top of the Bayisi Formation are characterized by the dolo-sility stones with negative δ13C values ranging from −4.10‰ to −8.17‰ (PDB), comparable to the Sturtian cap carbonates that overlie the Sturtian glacial deposits from other Neoproterozoic sequences. Carbonates overlying the Tereeken Formation are characterized by the pinkish cap dolostones (ca. 10 m thick) with negative δ13C values ranging from −2.58‰ to −4.77‰ (PDB), comparable to the Marinoan cap carbonates. The cap is also characterized by tepee-like structures, barite precipitates and pseudomorphous aragonite crystal fan limestones. Carbonates at the top of the Hankalchough Formation are characterized by subaerial exposure crust (vadose pisolite structure, calcareous crust structure) dolostones with negative δ13C values ranging from −4.56‰ to −11.45‰ (PDB) and the calcareous crust dolostones, implying that the Hankalchough cap carbonates differ from either the Sturtian or Marinoan cap carbonates in sedimentary environment and carbon isotopic composition. In addition, it is suggested the Hankalchough glaciation belongs to a terrestrial glaciation and it is the third largest glaciation during the Neoproterozoic period on the Tarim platform.

Earliest Triassic microbialites in the South China block and other areas: controls on their growth and distribution

Earliest Triassic microbialites (ETMs) and inorganic carbonate crystal fans formed after the end-Permian mass extinction (ca. 251.4 Ma) within the basal Triassic Hindeodus parvus conodont zone. ETMs are distinguished from rarer, and more regional, subsequent Triassic microbialites. Large differences in ETMs between northern and southern areas of the South China block suggest geographic provinces, and ETMs are most abundant throughout the equatorial Tethys Ocean with further geographic variation. ETMs occur in shallow-marine shelves in a superanoxic stratified ocean and form the only widespread Phanerozoic microbialites with structures similar to those of the Cambro-Ordovician, and briefly after the latest Ordovician, Late Silurian and Late Devonian extinctions. ETMs disappeared long before the mid-Triassic biotic recovery, but it is not clear why, if they are interpreted as disaster taxa. In general, ETM occurrence suggests that microbially mediated calcification occurred where upwelled carbonate-rich anoxic waters mixed with warm aerated surface waters, forming regional dysoxia, so that extreme carbonate supersaturation and dysoxic conditions were both required for their growth. Long-term oceanic and atmospheric changes may have contributed to a trigger for ETM formation. In equatorial western Pangea, the earliest microbialites are late Early Triassic, but it is possible that ETMs could exist in western Pangea, if well-preserved earliest Triassic facies are discovered in future work.

Identification of a Sturtian cap carbonate in the Neoproterozoic Sete Lagoas carbonate platform, Bambuí Group, Brazil

A sedimentological and C–O isotopic study has been carried out in nine sections of the Sete Lagoas Formation at its classical outcropping area, in the southern tip of the São Francisco craton (central Brazil), with the objective of refining its stratigraphic position within the Neoproterozoic. At the study area, the Neoproterozoic Sete Lagoas Formation comprises two shallowing-upward megacycles, corresponding to more than 200 m in thickness. Each cycle is limited by a flooding surface amalgamated with a third-order sequence boundary. The first megacycle presents deep-platform deposits with abundance of crystal fans (aragonite pseudomorphs). These deposits are characterized by negative C-isotope values (–4.5‰). They grade upward to storm-wave and tide-influenced layers with δ 13C values around 0‰. In the second megacycle, a new transgression drowned the platform, depositing a thick, mixed sub-storm wave-base succession. This megacycle comprises deposits of lime mudstone-pelite rhythmite, which grade to crystalline limestone rich in organic matter, both with unusually positive δ 13C values (up to + 14‰). Regional correlation of Sete Lagoas deposits indicate that they rest atop glaciomarine rocks of the Macaúbas Group and basal strata show seafloor precipitates with negative δ 13C values. Therefore, it is possible to characterize the Sete Lagoas carbonate as a cap carbonate sequence. The very high δ 13C in the second megacycle together with geochronologic data suggest that this unit correlates better with post-Sturtian sequences. Some differences in the depositional record are observed between Sete Lagoas and the other post-Sturtian units previously described in North America, Australia, and Namibia. Those differences may in part be due to deposition in shallower settings of the Sete Lagoas carbonates, thus preserving a thick record of storm- and wave-influenced sedimentation not found elsewhere. Alternatively, they may also be attributed to diachronic deposition of the so-called post-Sturtian cap carbonate sequences.

Carbon and strontium isotope fluctuations and paleoceanographic changes in the late Neoproterozoic Araras carbonate platform, southern Amazon craton, Brazil

The late Neoproterozoic Araras Group is exposed along the southern border of the Amazon craton and at the northern Paraguay belt, central Brazil. It comprises carbonate rocks over 600 m thick overlying Marinoan glaciogenic diamictites, and includes the first well-documented cap carbonate sequence in South America. δ 13C values for Araras carbonates, interpreted as representative of the original seawater ( δ 18O > − 10‰), exhibit a trend from strongly negative δ 13C values around − 9‰ at the base to enriched 13C compositions towards the top of the succession reaching values above 0‰. The cap carbonate succession is composed of deep platform deposits of dolomite (Mirassol d'Oeste Formation) covered by limestone rich in crusts and cements (Lower Guia Formation). These rocks show δ 13C values around − 5‰ and anomalous sedimentary structures such as vertical tubes, megaripple bedding (tepee-like structure), and crystal fans of aragonite pseudomorphs. The anoxic deep-platform limestone succession (Guia Formation) that overlies the cap succession presents a homogeneous C-isotope curve, with δ 13C values around − 2.5 to − 1‰. Upsection, values switch to positive (+ 0.1 to + 0.3‰) in platform dolomites and shoreface to shallow-platform deposits (Serra do Quilombo Formation), but exhibit oscillations to slightly negative values in the overlying peritidal facies (Nobres Formation). 87Sr/ 86Sr values follow the general C-isotope trend, with values increasing upsection from 0.7074 to 0.7088. Striking shifts in δ 13C and 87Sr/ 86Sr values are observed close to stratigraphic surfaces. The correlation between the isotope record and the relative sea-level curves supports the close association between paleoceanographic changes and paleoenvironmental evolution during the terminal Neoproterozoic on the Araras carbonate platform of Amazon craton.