Molar-tooth Structures Research Articles

Folded calcite cracks in noncalcareous shales: a window into shale diagenesis and hydrothermal influence

ABSTRACT Shale diagenesis is not well understood, and cracks in shale contain important information about diagenetic conditions. The way these cracks open reveals physical changes in the sediment, and the infilling minerals provide insight into the chemical conditions of the formation water. Typically, the authigenic minerals filling the folded cracks are consistent with the chemical composition of the host rocks. For example, folded calcite cracks are found in limestone. This paper, however, focuses on a set of folded calcite cracks in noncalcareous black shales. The goal is to improve our understanding of shale diagenesis by deciphering the origins of these cracks. The cracks are sinuously to ptygmatically folded in a vertical view and weakly sinuous on the bedding plane. They are filled with calcite, bitumen, pyrite, or a combination of them. Evidence of bioturbation and low redox-sensitive trace-element ratios suggest suboxic to oxic depositional conditions of the shale. The cracks were likely opened by gas expansion in unconsolidated mud. The main mineral filling the cracks, calcite, was sourced from hydrothermal fluid that passed through the underlying dolomite. Hydrothermal influences are indicated by the presence of bornite and microcrystalline pyrite in the cracks, as well as Fe and Mn enrichment in the host sediments. Hydrothermal activity can also explain the presence of buddingtonite, an ammonium feldspar in the shale. The results of this study suggest that folded cracks filled by minerals, gradually narrowing towards the top, and lacking internal detrital matrix are likely formed during early diagenesis. The inconsistency between the chemical compositions of the infilling material and the host sediment requires further exploration to identify the source. Hydrothermal fluid passing through the underlying dolomite may be the source of folded calcite cracks in noncalcareous sedimentary rocks. These cracks resemble molar-tooth structures (MTS), which are sinuous cracks filled with microcrystalline calcite mostly found in Precambrian limestone and calcareous shales. If these cracks are indeed MTS, they may be an exception to the age and host-rock lithology constraints of MTS.

Formation of molar tooth structures in low sulfate Precambrian oceans

Molar tooth structures (MTS) comprise calcite microspar-filled voids in fine-grained shallow-water carbonate, and were commonly formed in the Mesoproterozoic and early Neoproterozoic. However, the origin of MTS and links between the temporal distribution of MTS and contemporaneous seawater chemistry remains poorly understood. Here we report elemental and isotopic data for MTS and host rocks from the Mesoproterozoic Gaoyuzhuang Formation (∼1,600–1,550 Ma), North China Craton. The results reveal similar C, S and Sr isotope signatures between MTS and host rocks, which are close to the isotopic compositions of contemporaneous global seawater, suggesting an early diagenetic, seawater-buffered origin for MTS. A small sulfur isotopic fractionation between seawater sulfate and pyrite (Δ34SCAS-Py) of 4.1 ± 1.5‰ in host rocks is consistent with previously reported data, providing support for sub-millimolar sulfate concentrations in Mesoproterozoic seawater. Our observations suggest that the widespread occurrence of MTS through the Mesoproterozoic to early Neoproterozoic was broadly linked to sulfate scarcity in the ocean. We further propose that in Proterozoic oceans with sub-millimolar seawater sulfate concentrations, where aerobic and anaerobic methane oxidation was likely inhibited, methane produced via methanogenesis may have been more prone to accumulate in sediments, creating voids during escape. The absence of MTS across periods of higher sulfate concentrations during the Palaeoproterozoic and after the mid-Neoproterozoic, suggests that elevated sulfate concentrations promoted consumption of methane via anaerobic methane oxidation, thus preventing methane accumulation and the formation of sediment voids. Rapid lithification of the substrate as a result of elevated carbonate saturation may have also hindered the formation of MTS during these intervals. The link between MTS and changes in both oceanic sulfate levels and benthic methane fluxes gives a new perspective on temporal fluctuations in Earth’s redox state through time.

Mesoproterozoic Molar Tooth Structure Related to Increased Marine Oxygenation

AbstractMarine carbonate fabrics are intrinsically related to ocean chemistry, physical processes and biological activity. Molar tooth structure (MTS), a globally distributed structure in Proterozoic carbonate sediments, has been widely studied for more than a century; yet its connections with physical and biological processes remain unclear. Using multiple techniques, we studied ∼1.57 Ga MTS and identified a connection between its occurrence and increased marine oxygenation. In our samples, the matrix surrounding MTS is typically dominated by carbonate mud with early diagenetic dolomite crystals. High I/(Ca + Mg) ratios (up to 4.1 μmol/mol) and negative Ce anomalies (∼0.8) detected in the matrix are consistent with the oxidative removal of inhibitors such as Fe2+ and Mn2+ in the water‐column that permitted carbonate “whiting” mud precipitation stimulated by cyanobacterial photosynthesis. This cohesive but not rigid seafloor carbonate mud was a prerequisite for synsedimentary MTS crack formation. Systematically higher carbon isotope (δ13C) values in MTS microspars, relative to host sediment, support origination of the cracks by methane degassing in the organic‐rich carbonate mud. Low, but non‐zero, I/(Ca + Mg) values of the MTS microspar suggest that the precipitation of the microspar that filled the MTS cracks was triggered by oxidative removal of residual Fe2+ and Mn2+ in porewater through mixing with overlying oxygenated seawater. We therefore propose that MTS formed under moderately oxygenated conditions and that its sporadic occurrence prior to ∼1.2 Ga reflects episodes of pulsed marine oxygenation in an overall anoxic setting.

Molar-Tooth Structure as a Window into the Deposition and Diagenesis of Precambrian Carbonate

Molar-tooth structure (MTS) is an unusual carbonate fabric that is composed of variously shaped cracks and voids filled with calcite microspar. Despite a century of study, MTS remains enigmatic because it juxtaposes void formation within a cohesive yet unlithified substrate with the penecontemporaneous precipitation and lithification of void-filling carbonate microspar. MTS is broadly restricted to shallow marine carbonate strata of the Mesoproterozoic and Neoproterozoic, suggesting a fundamental link between the formation of MTS and the biogeochemical evolution of marine environments. Despite uncertainties in the origin of MTS, molar-tooth (MT) microspar remains a popular target for geochemical analysis and the reconstruction of Precambrian marine chemistry. Here we review models for the formation of MTS and show how detailed petrographic analysis of MT microspar permits identification of a complex process of precipitation and diagenesis that must be considered when the microspar phase is used as a geochemical proxy. ▪ Molar-tooth fabric is an enigmatic structure in Precambrian sedimentary rocks that is composed of variously shaped cracks and voids filled with carbonate microspar. ▪ Time restriction of this fabric suggests a link between this unusual structure and the biogeochemical evolution of marine environments. ▪ Petrographic analysis of molar-tooth fabric provides insight into fundamental processes of crystallization.

Sedimentary sequence and evolutionary history of the Proterozoic basin at the southern margin of the North China Craton

Aulacogen/Rift system deposits and tectonic-magmatic activities can be linked to the breakup of the Columbia supercontinent and have developed in the North China Craton (NCC). The Xiong'er Basin at the southern margin of the NCC consists of late Palaeoproterozoic-Neoproterozoic sedimentary strata and can be divided into three sedimentary sequences. Sedimentary Sequence I (1.83–1.75 Ga) consists of bimodal volcanic-sedimentary rocks of the Xiong'er Group with continental-marine facies and alluvial fan deposits of the Bingmagou Formation with local development. It represents the evolution of the intracontinental fault from initiation to decline. Sedimentary Sequence II (1.75–1.6 Ga) developed after the southern margin of the NCC entered a stable continental margin stage, is dominated by clastic rocks interbedded with local carbonate deposits, and fills the interior of the basin. As continuous transgression led to a transition in the sedimentary environment, sedimentary Sequence III (1.6–1.4 Ga) is dominated by carbonate strata that began to develop afterwards. This sequence is the caprock deposit in the Xiong'er Basin. Subsequently, tectonic uplift events occurred at the southern margin of the NCC, along with the start of exposure and denudation, which lasted for hundreds of millions of years. Moreover, abundant microbially induced sedimentary structures (MISS) with different morphological characteristics developed in clastic strata across different areas of the Xiong'er Basin. These structures effectively indicate sedimentary facies and their evolution, which facilitates the establishment of regional stratigraphic isochronous correlation and connection. For example, based on the time limit for the development of molar-tooth structures (MTS), the Hejiazhai Formation belongs to the Neoproterozoic strata, with a deposition time of 1.0–0.72 Ga. In conclusion, the study of the Precambrian sedimentary succession in the Xiong'er Basin, in response to the supercontinental breakup, can be useful in exploring the coupling mechanisms between various geological events.

Sedimentology and geochemistry of the Uk Formation, Upper Riphean, the Southern Urals

Research subject. This article presents new data on carbonate facies of the upperUk subformation, Upper Riphean of theSouth Urals. A particular attention is paid to the distribution of rare-earth elements (REE) and yttrium (Y) in stromatolitic and intraclastic limestones and calcareous shales (bulk samples and their acetic acid leachates).Materials and methods. We have studied lithological features of limestones in the geological section and in thin sections. The contents of trace elements in rocks were determined by the ICP-MS method at the IGG UB RAS (Yekaterinburg), the composition of organic matter was determined at the IPGG SB RAS (Novosibirsk) by gas-liquid chromatography and chromatography-mass spectrometry.Results. The most representative section of the Upper Uk Subformation located along the eastern edge of the town Ust-Katav can be subdivided into several members of different lithology and thickness: (1) biohermdominated member comprising large microbialitic build-ups and inter-bioherm sediments (intraclastic limestones, calcareous biolaminites); (2) transitional member characterised by small bioherms alternating with other carbonates; (3) interbedded coarseand fine-grained limestones. The presence of molar-tooth structures in the carbonate rocks of Uk Formation made it possible to constrain the age of thisUpper Riphean formation to pre-CryogenianConclusions. There are similarities in REE and Y distribution in both clean (devoid of siliciclastic component) bulk limestone samples and in their acetic acid leachates. PAAS-normalised REE + Y patterns demonstrate positive La, Gd, Y anomalies and negative Eu, Ce anomalies. The latter suggest marine depositional environments for theUpper Uk stromatolitic limestones. The results of the pioneering research into the composition of bitumens and biomarkers from the Upper Uk Subformation have shown that carbonates and shales are depleted in the organic matter. The source material for the organic matter was provided by two types of communities comprising both eukaryotes and prokaryotes and inhabiting well aerated environment, perhaps with lowered salinity of marine water.

The role of microbial iron reduction in the formation of Proterozoic molar tooth structures

Molar tooth structures are poorly understood early diagenetic, microspar-filled voids in clay-rich carbonate sediments. They are a common structure in sedimentary successions dating from 2600–720 Ma, but do not occur in rocks older or younger, with the exception of two isolated Ediacaran occurrences. Despite being locally volumetrically significant in carbonate rocks of this age, their formation and disappearance in the geological record remain enigmatic. Here we present iron isotope data, supported by carbon and oxygen isotopes, major and minor element concentrations, and total organic carbon and sulphur contents for 87 samples from units in ten different basins spanning ca. 1900–635 Ma. The iron isotope composition of molar tooth structures is almost always lighter (modal depletion of 2‰) than the carbonate or residue components in the host sediment. We interpret the isotopically light iron in molar tooth structures to have been produced by dissimilatory iron reduction utilising Fe-rich smectites and Fe-oxyhydroxides in the upper sediment column. The microbial conversion of smectite to illite results in a volume reduction of clay minerals (∼30%) while simultaneously increasing pore water alkalinity. When coupled with wave loading, this biogeochemical process is a viable mechanism to produce voids and subsequently precipitate carbonate minerals. The disappearance of molar tooth structures in the mid-Neoproterozoic is likely linked to a combination of a decrease in smectite abundance, a decline in the marine DIC reservoir, and an increase in the concentration of O2 in shallow seawater.

Tonian-Cryogenian boundary sections of Argyll, Scotland

The Tonian-Cryogenian System boundary is to be defined at a GSSP (Global Boundary Stratigraphic Section and Point) beneath the first evidence of widespread glaciation. A candidate lies within the Dalradian Supergroup of Scotland and Ireland, which is least deformed and metamorphosed in Argyll, western Scotland. We present new stratigraphic profiles and interpretations from the Isle of Islay and the Garvellach Islands, update the chemostratigraphy of the Appin Group Tonian carbonates underlying the thick (ca. 1km) glacigenic Port Askaig Formation (PAF) and demonstrate an environmental transition at the contact.The Appin Group forms a regionally extensive, >4km-thick, succession of limestones, shales and sandstones deposited on a marine shelf. On Islay, the upper part of the lithostratigraphy has been clarified by measuring and correlating two sections containing distinctive stratigraphic levels including molar tooth structure, oolite, stromatolitic dolomite and intraclastic microbial mounds. Significantly deeper erosion at the unconformity at the base of the overlying PAF is demonstrated in the southern section. Carbonate facies show a gradual decline in δ13CVPDB from +5 to +2‰ upwards.In NE Garbh Eileach (Garvellach Islands), a continuously exposed section of Appin Group carbonates, 70m thick, here designated the Garbh Eileach Formation (GEF), lies conformably beneath the PAF. The GEF and the GEF-PAF boundary relationships are re-described with new sedimentological logs, petrological and stable isotope data. Interstratified limestone and dolomicrosparite with δ13C of −4 to −7‰ (a feature named the Garvellach anomaly, replacing the term Islay anomaly) are overlain by dolomite in which the isotope signature becomes weakly positive (up to +1‰) upwards. Shallow subtidal conditions become peritidal upwards, with evidence of wave and storm activity. Gypsum pseudomorphs and subaerial exposure surfaces are common near the top of the GEF. The basal diamictite (D1) of the PAF is rich in carbonate clasts similar to slightly deeper-water parts of the underlying succession. D1 is typically several metres thick with interstratified sandstone and conglomerate, but dies out laterally. Scattered siliciclastic coarse sandstone to pebble conglomerate with dropstones associated with soft-sediment deformation is interbedded with carbonate below and above D1. Dolomite beds with derived intraclasts and gypsum pseudomorphs are found above D1 (or equivalent position, where D1 is absent).Published and new Sr isotope studies, including successive leach data, demonstrate primary Tonian 87Sr/86Sr values of 0.7066–0.7069 on Islay, decreasing to 0.7064–0.7066 in the younger GEF limestones on the Garvellachs, with 1700–2700ppmSr. Other typically Tonian characteristics of the carbonates are the Sr-rich nature of limestones, molar tooth structure, and dolomitized peritidal facies with evidence of aridity. Seabed surveys suggesting uniformly-dipping strata and shallow borehole core material illustrate the potential for extending the Tonian record offshore of the Garvellachs.A candidate Tonian-Cryogenian GSSP is proposed on Garbh Eileach within the smooth δ13C profile at the cross-over to positive δ13C signatures, 4m below the first occurrence of ice-rafted sediment and 9m below the first diamictite. Although lacking radiometric constraints or stratigraphically significant biotas or biomarkers, the Scottish succession has a thick and relatively complete sedimentary record of glaciation, coherent carbon and strontium chemostratigraphy, lateral continuity of outcrops and 100% exposure at the proposed boundary interval.

New Discovery of Molar Tooth Structures in the Kangjia Formation at Benxi, Liaoning and its Significance

Acta Geologica Sinica - English EditionVolume 91, Issue 3 p. 1139-1140 Research Advances New Discovery of Molar Tooth Structures in the Kangjia Formation at Benxi, Liaoning and its Significance Hao WU, Corresponding Author Hao WU wuhaogeo@163.com Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037 ChinaCorresponding author. E-mail: wuhaogeo@163.comSearch for more papers by this authorXiaozhong DING, Xiaozhong DING Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037 ChinaSearch for more papers by this authorYanxue LIU, Yanxue LIU Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037 ChinaSearch for more papers by this authorHongwei KUANG, Hongwei KUANG Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037 ChinaSearch for more papers by this author Hao WU, Corresponding Author Hao WU wuhaogeo@163.com Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037 ChinaCorresponding author. E-mail: wuhaogeo@163.comSearch for more papers by this authorXiaozhong DING, Xiaozhong DING Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037 ChinaSearch for more papers by this authorYanxue LIU, Yanxue LIU Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037 ChinaSearch for more papers by this authorHongwei KUANG, Hongwei KUANG Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037 ChinaSearch for more papers by this author First published: 05 July 2017 https://doi.org/10.1111/1755-6724.13332AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume91, Issue3June 2017Pages 1139-1140 RelatedInformation

Molar tooth structures and origin of peloids in proterozoic carbonate platforms (Middle Riphean of the Turukhansk Uplift, Siberia)

Structures and textures of the peloidal wackestones, as well as size, shape, and composition of peloidal grains, from the Mesoproterozoic (Middle Riphean) Sukhaya Tunguska carbonate platform in the Turukhansk Uplift (Siberia) are considered. It is shown that these grains formed in the course of diagenesis were closely associated with the microsparitic replacement and the formation of molar tooth (MT) structures. Diagenetic transformations of rocks were related to the activity of anaerobic microbial communities inside the buried carbonate silt layers. The microbial activity during diagenesis was governed by the carbonate sediment composition and conservation mechanism of the high-molecular organic matter of primary producers therein, since this organic matter was the nutritious substrate for the primary anaerobe communities.

Molar tooth carbonates and benthic methane fluxes in Proterozoic oceans.

Molar tooth structures are ptygmatically folded and microspar-filled structures common in early- and mid-Proterozoic (∼2,500–750 million years ago, Ma) subtidal successions, but extremely rare in rocks <750 Ma. Here, on the basis of Mg and S isotopes, we show that molar tooth structures may have formed within sediments where microbial sulphate reduction and methanogenesis converged. The convergence was driven by the abundant production of methyl sulphides (dimethyl sulphide and methanethiol) in euxinic or H2S-rich seawaters that were widespread in Proterozoic continental margins. In this convergence zone, methyl sulphides served as a non-competitive substrate supporting methane generation and methanethiol inhibited anaerobic oxidation of methane, resulting in the buildup of CH4, formation of degassing cracks in sediments and an increase in the benthic methane flux from sediments. Precipitation of crack-filling microspar was driven by methanogenesis-related alkalinity accumulation. Deep ocean ventilation and oxygenation around 750 Ma brought molar tooth structures to an end.

Review of molar tooth structure research

For more than a century, molar tooth structure (MTS) has been studied. The study developed in three stages. During the first stage (before 1980), researchers described three basic morphologies of MTS, mainly from the Belt Supergroup in North America, and they provided several hypotheses for the origin of MTS. During the second stage (1980–1999), the frequent discoveries of MTS on all continents resulted in many detailed descriptions of their shape and in several hypotheses concerning the origin of MTS. Notably, hypotheses of MTS’s origin such as seismic activity and biological activity were developed. Since 2000, research has progressed into a new stage (the third stage). This is due to discoveries of MTS in the Meso–Neoproterozoic of China and elsewhere, and the ongoing debate on the seismic or biological origin is replaced by a hypothesis that involves gas expansion and chemically-controlled carbonate precipitation (both of them possibly affected by biological activities). This latter idea has gradually been commonly recognized as the mainstream theory. Despite continued disagreements, researchers now agree that microsparry calcite played a controlling role regarding the development and the global distribution of MTS in time and space during the Proterozoic, the morphological diversity, and the impact on the sedimentary environment. The present contribution analyses the three major hypotheses regarding the origin of MTS; it also discusses the shortcomings of the hypotheses regarding a seismic or biologic origin, and it details the modern hypothesis that links formation of cracks to the precipitation of sparry calcite. It is deduced that important questions dealing with the Precambrian can be answered, among other aspects regarding the depositional palaeogeography and stratigraphic correlations.

Influence Of Increase Of The Occlusal Contact Area On The Tension Generation On Natural Teeth And Adjacent Structures By Finite Element Analisys (FEA)

Background: The increase in the occlusal contact area can generate significant stresses on the occlusal surface of a natural tooth as well as the cervical region and in the alveolar crest around this same tooth edge. Objective: The objective of this study was to study the influence of the increase of the occlusal contact area on the tension generation on a molar tooth and adjacent structures by three-dimensional finite element models. Methods: A tridimensional model composed of an inferior molar tooth and surrounding structures was fabricated, and a load of 100 N was applied over the occlusal surface on pre-set regions according to an ideal occlusal standard that was previously defined for the study. The diameters of the contact spots on the tooth were increased gradually and varied from 0.5 mm to 1.0 mm to 1.5 mm. The generated tension over those contact spots was analyzed and quantified on pre-set regions of the occlusal marginal crest: the cervical region and the alveolar crestal bone. Results: The results showed that occlusal contact diameter causes a decrease in the tension level of 33.8% on the marginal crests, a decrease of 20.7% on the alveolar rim interface, and an increase of 44.1% on the cervical region, near the cement enamel junction. Conclusion: The occlusal contact diameter on the tooth surface had an influence on the tension generated on all analyzed regions.

Cover

Enigmatic molar tooth structure in a dolomitic stromatolite from the approximately 1.4 Ga Belt Supergroup, Glacier National Park. The formation mechanism of these typically Mesoproterozoic structures has long been a mystery, but likely involves the unique geomicrobiological processes of the time. Photo by Carrine Blank. [Vol. 49, No. 6, pp.1040–1055]

Molar‐Tooth Structure from the Mesoproterozoic Wumishan Formation in Lingyuan, Yanshan Region, North China, and Geological Implications

Abstract:Although its origin has not yet reached a consensus so far, MTS (Molar‐Tooth Structure) has been documented for more than 100 years. Current study reports a discovery of MTS from the Mesoproterozoic Wumishan Formation, Lingyuan, Yanshan Region, North China, and the features and geological implications of MTS are further discussed. Here, straitigraphic horizons of MTS's occurrences show that it was mainly located within the top part of the Wumishan Formation, i.e., limestone unit. Four kinds of morphology of MTS, i.e., fine fusiform, debris, ribbon, ptigmatic and nodular (irregular), were recognized and thought to be highly related to the sedimentary environments and facies. Geochemistry of MTS including oxides, trace elements and C, O and Sr isotopes indicates that the horizons of MTS‐bearing is of higher Sr/Ba and Ca/Mg ratios, lower positive δ13C and highly negative δ18O values than the adjacent stratigraphic levels of rare MTS. Lithology, morphology and geochemistry of MTS in the Wumishan Formation suggest that MTS occurs mainly in shallow subtidal near the storm wave base, which is typically characterized by warm temperature, oversaturated calcium carbonate seawater and high organic productivity. Furthermore, occasional enrichment of algae bacteria here is more favorable for the calcification of calcium oozes and catalytic for MTS. C isotope composition of the Wumishan Formation and MTS of this study is well correlated with that of the Mesoproterozoic Belt Supergroup, North America and Riphean, Siberia, suggesting that MTS acts as a sedimentary record responding to global changes and is a perfect indicator in Precambrian stratigraphic correlation worldwide.

Molar Tooth Structure: a Contribution from the Mesoproterozoic Gaoyuzhuang Formation, Tianjin City, North China

Abstract:Molar‐tooth (MT) structure is an enigmatic sedimentary structure consisting of variously‐shaped cracks and voids filled with a characteristically uniform, equant calcite microspar. It is globally distributed but temporally restricted to rocks from Neoarchean to Neoproterozoic age. The origin of MT structures has been debated for more than a century and the topic continues to be highly contentious. Some features of MT structure occurring in micritic limestones of the Mesoproterozoic Gaoyuzhuang Formation (ca. 1500 Ma to ca. 1400 Ma), Jixian section, Tianjin City, North China show that: 1) there is a definite interface or lining, rich in organic material and pyrite, between the MT crack‐filling calcite microspar and the micritic host rock, which is also rich in organic matter; 2) the micritic host rocks are notable for the absence of stromatolites and microbial laminites; 3) distinctive conglomeratic lag deposits made up of intraclasts of MT microspar result from storm reworking of the MT structures; 4) the MT structure is associated with possible algal megafossils such as Chuaria; 5) the MT microspar is made up of the larger calcite crystal and the MT crack is marked by the diversity of configurations; 6) both the TOC content and the carbon‐isotopic value (δ13CPDB) among the host rock, the MT microspar and the possible algae fossil are obviously different. For the forming mechanism of the Gaoyuzhuang MT structure, these features can still indicate that: A) the MT microspar was formed by rapid precipitation and lithification; B) the MT microspar precipitated directly within the cracks; C) the decomposition of organic matter within the host micrite might be the chief mechanism producing gas bubbles; D) microscale gas‐sediment interaction led to the generation of the MT cracks and the precipitation of microspar therein; E) the MT cracks might represent the track of migration and expansion of gas bubbles, and that the recrystallization of host micrites cannot be eliminated during forming process of the MT microspar; F) the MT structure is occurred in early diagenetic period; and G) the formation of MT microspars is a complex diagenetic process. Therefore, model of the microbially‐induced gas‐bubble expansion and migration is the best interpretation for the formation of the MT structure. Effectively, MT structures are a type of sedimentary structure that is formed in the early diagenetic period and is related to microbial activities and organic matter degradation.

Molar Tooth Structure: a Contribution from the Mesoproterozoic Gaoyuzhuang Formation, Tianjin City, North China

Molar Tooth Structure: a Contribution from the Mesoproterozoic Gaoyuzhuang Formation, Tianjin City, North China

Molar tooth structures: Formation and specificity of carbonate diagenesis in the Late Precambrian, Middle Riphean Sukhaya Tunguska Formation of the Turukhansk Uplift, Siberia

A new way of formation of the problematic Molar Tooth (MT) structures, which, along with stromatolites, could be considered as a “visiting card” of the Riphean, was examined on the example of carbonate sediments of the Riphean Sukhaya Tunguska Formation from the Turukhansk Uplift, Siberia. These structures were formed due to the consequent replacement of oozy constituent of carbonate sediments with calcitic microsparite in the course of diagenesis with substantial shift of the dynamic equilibrium toward the solution in the sediment—porous solution—microsparite system. An excess of soluble magnesium significantly hampering precipitation of crystal cores of the future calcitic microsparite could be one of the possible causes of the shift. It is suggested that magnesium mole fraction of the porous solutions was related to the early dissolution of the metastable high-Mg calcite of the silty sediment brought from the areas of active stromatolite formation. The facies occurrence of the MT-structures indicates that they occurred in descending trails of the most productive zones of carbonate platforms tapering toward the basin. The synchronism of stratigraphic trends of the MT-structures and stromatolites in the Precambrian resulted from the evolution of carbonate-productive microbe-mineral systems which were the immediate suppliers of fine-grained carbonate sediment into external zones of carbonate platforms.

Sedimentary talc in Neoproterozoic carbonate successions

Mineralogical, petrographic and sedimentological observations document early diagenetic talc in carbonate-dominated successions deposited on two early Neoproterozoic (~ 800–700 million years old) platform margins. In the Akademikerbreen Group, Svalbard, talc occurs as nodules that pre-date microspar cements that fill molar tooth structures and primary porosity in stromatolitic carbonates. In the upper Fifteenmile Group of the Ogilvie Mountains, NW Canada, the talc is present as nodules, coated grains, rip-up clasts and massive beds that are several meters thick. To gain insight into the chemistry required to form early diagenetic talc, we conducted precipitation experiments at 25 °C with low-SO 4 synthetic seawater solutions at varying pH, Mg 2+ and SiO 2(aq). Our experiments reveal a sharp and reproducible pH boundary (at ~ 8.7) only above which does poorly crystalline Mg-silicate precipitate; increasing Mg 2+ and/or SiO 2(aq) alone is insufficient to produce the material. The strong pH control can be explained by Mg-silica complexing activated by the deprotonation of silicic acid above ~ 8.6–8.7. FT-IR, TEM and XRD of the synthetic precipitates reveal a talc-like 2:1 trioctahedral structure with short-range stacking order. Hydrothermal experiments simulating burial diagenesis show that dehydration of the precipitate drives a transition to kerolite (hydrated talc) and eventually to talc. This formation pathway imparts extensive layer stacking disorder to the synthetic talc end-product that is identical to Neoproterozoic occurrences. Early diagenetic talc in Neoproterozoic carbonate platform successions appears to reflect a unique combination of low Al concentrations (and, by inference, low siliciclastic input), near modern marine salinity and Mg 2+, elevated SiO 2(aq), and pH > ~ 8.7. Because the talc occurs in close association with microbially influenced sediments, we suggest that soluble species requirements were most easily met through microbial influences on pore water chemistry, specifically pH and alkalinity increases driven by anaerobic Fe respiration.

Genesis types of the Neoproterozoic Molar tooth structures in the southeastern Jilin and eastern Liaoning provinces and its research significances

Molar tooth (briefly called MT) carbonate is one of the Proterozoic carbonates with enigmatic sedimentary structure. According to the morphology of MT, it can be divided into two main genesis-morphology types, i.e., autochthonous and allochthonous, and each type can be further divided into a series of subtypes. The autochthonous MT can be divided into filamentous (MF1), ribbon, nodular and dotted type (MF4), in which the ribbon one can be subdivided into simple vertical (MF2) and complex ribbon (MF3), broken ribbon by storm in situ (MF5) and MT within nodular limestone (MF6). Allochthonous MT includes conglomerate clasts and horizontal detritus. Studying on the links between MT and the host rocks on five stratigraphic sections in the southeastern Jilin and eastern Liaoning indicates that the morphology of MT is closely related to host rocks, and seven genetic types by relationships between MT and the host rocks with facies interpretations are classified and discussed in this paper. The sedimentary environments of MT formation will be diverse if their morphologies are different.