Ediacaran Dengying Formation Research Articles

The primary dolostone in the Meso-Neoproterozoic: Cases study on platforms in China

Meso-Neoproterozoic dolostone is abnormally well-developed in the platform areas in China, detailly, in the Mesoproterozoic Calymmian Wumishan Formation of North China Basin, Neoproterozoic Ediacaran Qigbulak Formation of Tarim Basin, and the Neoproterozoic Ediacaran Dengying Formation of Sichuan Basin. The outcrops and drilling cores from these basins show that these dolostones are mostly thick-bedded to massive, with mud-sized to silt-sized crystalline dolostone reaching 86%–97% of the total dolostone thickness. These dolostones basically suffered no secondary metasomatism, regardless of containing microorganisms (such as algae and bacteria) or not. The analyses of sedimentary palaeogeographic features indicate that: (1) the Meso-Neoproterozoic dolostone nearly covered the entire area basins, and was widely deposited in supratidal to intertidal zones, as well as in open platform environments; (2) the dolostone was subdivided according to the crystalline size, with the distribution controlled by geomorphology, i.e., the pure mud-sized crystalline dolostone developing in depression areas, whereas the grainy mud-sized crystalline dolostone or the mound stromatolitic mud-sized crystalline dolostone developing in uplift areas; (3) deep-water basins developed between the carbonate platforms, and the seismic profiles show that these basins were formed by syngenetic deep faults in the Meso-Neoproterozoic strata. These syngenetic faults increased Mg2+ concentration in the seawater by injecting rich-Mg2+-bearing hydrothermal fluids into the carbonate platforms. The analyses of geochemical and paleoclimatic indexes reveal that there is no obvious difference in carbon and oxygen isotope ratios between dolostones having different textures, with the ratios similar to those of the global seawater of the Meso-Neoproterozoic, indicating that the silt-sized crystalline dolostone has the same sedimentary origin as the mud-sized crystalline dolostone, i.e., the former is from authigenic recrystallization of the latter rather than secondary metasomatism. Therefore, whatever the crystalline sizes are, the dolostones are all originated from sedimentation. All the petrological, sedimentary and geochemical data suggest that the dolostones covering almost the whole Meso-Neoproterozoic typical platform areas of China are of relatively primary sedimentary origin.

CHARACTERISTICS AND ORIGIN OF THE DOLOSTONES OF THE EDIACARAN DENGYING FORMATION IN THE EASTERN SICHUAN BASIN, SOUTH CHINA

The Ediacaran Dengying Formation dolostones were the crucial carrier of the co-evolution between the biota and environment before the Cambrian Explosion and the solid mineral and petroleum resources. Based on the petrological and mineralogical analyses, the Dengying Formation at the Liaojiacao Section consists of micritic, fine-crystalline and medium-crystalline dolostones and algal-bonded or algal-clastic dolostones. Moreover, the micritic and algal-bonded dolostones mostly retain the original sedimentary structure with widespread microbial mats, algal spots, algal traces, and other microbial fossils. Recrystallization, dissolution, and other diagenetic alterations occurred in the fine-crystalline and medium-crystalline dolostones as well as algal-clastic dolostones, and their original sedimentary structures had been significantly altered. Furthermore, in situ geochemical analysis shows that the micritic and algal-bonded dolostones of the Dengying Formation, which retain the original sedimentary structure, have Mg/Ca ratios close to the ideal primary dolomite; Na, Fe, Mn, and Sr concentrations located in the distribution range of calcites and dolomites formed in a normal marine environment, negative Ce anomaies and positive Eu anomaies with the patterns of light rare earth element depletion and medium and heavy rare earth element enrichment. In addition, the whole-rock stable isotopic analyses show that the δ13Cdol values of dolostones that retain the original sedimentary structure are consistent with the distribution range of the original marine carbonate rocks, but the δ18Odol values are slightly lower than those of the original marine carbonate rocks. Therefore, we infer that the dolostones of the Dengying Formation at the Liaojiacao Section with original sedimentary structure are the products of mimetic dolomitization and formed in an environment with extensive microbial activity, calcite as the preexisting mineral, normal salinity, and oxidized seawater as the dolomitization fluid. Moreover, the fine- and medium-crystalline dolostones were the results of recrystallization that retained the original sedimentary structures during diagenesis.

Microbialites of terminal Ediacaran in the Upper Yangtze Platform, China: From mesoscopic to nanoscale

Dolomitic microbialites were developed extensively during the Neoproterozoic and record widespread oxygenation of the oceans and the accompanying rise of animal life. Primary precipitation of dolomite cements has now been recognized in both Cryogenian and Ediacaran periods, although the origin of matrix dolomite is debated. The terminal Ediacaran Dengying Formation (551–541 Ma) in the Upper Yangtze Platform of China contains massive dolomite. Particularly delicate microbial structures are preserved in the second member of the formation (Z 2 dn 2 ). This study aimed to elucidate the origin of matrix dolomite in these microbialites, the main mesoscale features of which are stromatolites, thrombolites and composite microbialites. The stromatolites typically have planar and low-relief planar–domal forms, with distinct laminated microstructure alternated with diffuse laminated microstructures. The thrombolites comprise a variety of mesoclot structures and cavities, which are rimmed by isopachous fringes of fibrous cement and filled by late sparry cement. The composite microbialites have clotted and laminated textures of various styles that are intermingled at the sub-centimeter scale. These microbialites are inferred to have been deposited in a subtidal to intertidal setting. Abundant nanoscale dolomite biomorphs were observed with scanning electron microscopy, including single nanoscale spheroids and aggregates of nanometer filaments with granulated, micro-rose shaped, micro-sized foliage-like, and trumpet-like forms, most of which are presumably of biogenic origin. These nanoscale microbial textures act as archives of ancient microbial activity that may have produced abundant extracellular polymeric substance, providing sites for the nucleation and maturation of dolomite. This process may have been facilitated by Mn–S redox cycling and resulted in precipitation of primary ferroan dolomite in the matrix. • Novel study on the old “dolomite problem” from the view of microbialite; • Stromatolite, thrombolite and composite microbialite are all composed of dolomite; • Microbial features of Ediacaran dolomite were displayed from mesoscopic to nanoscale; • Delicate nanoscale microbial biomorphs of dolomite observed under SEM.

In situ U-Pb dating and geochemical characterization of multi-stage dolomite cementation in the Ediacaran Dengying Formation, Central Sichuan Basin, China: Constraints on diagenetic, hydrothermal and paleo-oil filling events

Improved understanding of subsurface paleo-fluid circulation history can assist in reconstructing associated pore evolution of carbonate reservoirs. Multi-stage dolomite cements generated by basinal fluids over time were investigated using a combination of petrography, geochemistry, fluid-inclusion studies, LA-ICP-MS U-Pb geochronology, and seismic interpretation. This study aims to gain clues about diagenetic, hydrothermal and paleo-oil filling events within the Ediacaran Dengying Formation, Sichuan Basin, China.Four main phases of dolomite cementation were petrographically, geochemically and geochronologically distinguished that can be ascribed to major tectonic events throughout the basin evolution. The earliest dolomite generation occurs as fibrous dolomite crust (FDC) growing along the karst vug wall, interpreted as marine diagenetic cement formed in a submarine environment during the Late Ediacaran, most likely from the influx of seawaters following the uplift and subaerial exposure related to the Tongwang movement. In situ U-Pb ages obtained from the other three generations of cement dolomites are interpreted to coincide with the timing of repeated strike-slip fault movements as indicated by the seismic data, suggesting a direct link between cement emplacement and regional tectonic-thermal events. Sub-vertical extension dolomite veins (EDV), representing the second generation, are considered as being syntectonic, generated from slightly modified connate seawaters at a shallow to moderate burial depth in a transtensional stress regime during the Cambrian taphrogenesis. Two generations of saddle dolomite cements, SD1 and SD2, have recorded two major episodes of hot fluids circulating in the Dengying Formation. The SD1, filling in hydraulic fractures and vugs, formed during the third phase of cementation, as a result of a hydrothermal event associated with Caledonian exhumation. Deep-seated hot brines squeezed into the Dengying reservoirs along the sub-vertical strike-slip faults combined with some meteoric waters that percolated downward, are deemed responsible for precipitating SD1. The SD2 is the youngest cement present, which is interpreted to have formed during the ingress of deeper, hot, hypersaline brines arising from the late Permian hydrothermal activity likely triggered by the eruption of Emeishan flood basalts.Cross-cutting relationships between bitumen phases and different dolomite generations indirectly define the dates of two paleo-oil charging events. The first episode of paleo-oil charging occurred during the Late Ediacaran to Early Cambrian, likely sourced from underlying Ediacaran Doushantuo shales. Late oil emplacement occurred after the Late Permian, in agreement with the reported bitumen Re-Os age and the age inferred through fluid-inclusion analysis. Primary source rocks generating this phase of oil were considered as the Cambrian Qiongzhusi shales, because basin modeling-derived date of oil window matches with the second oil filling time.

Geochemical characteristics of the Ediacaran Dengying Formation in South Qinling: implications for the dolomitization mechanism of dolomites and the terminal Ediacaran paleoenvironment

Geochemical characteristics of the Ediacaran Dengying Formation in South Qinling: implications for the dolomitization mechanism of dolomites and the terminal Ediacaran paleoenvironment

Characteristics and primary mineralogy of fibrous marine dolomite cements in the end-Ediacaran Dengying Formation, South China: Implications for aragonite–dolomite seas

Fibrous marine cements are widely distributed in the terminal Ediacaran Dengying Formation of South China and are an important record for reconstructing paleoseawater chemistry. However, the morphological characteristics and filling sequence of these cements, and their formation mechanism and paleoceanographic significance, are poorly understood. Our investigation of five outcrop sections in the northern upper Yangtze region allows four types of fibrous dolomite cement to be identified: fascicular fast dolomite (FFD), radial fast dolomite (RFD), radial slow dolomite (RSD), and fascicular slow dolomite (FSD), where “fast” and “slow” refer to the optical character of cement crystals. FFD cements possess original aragonitic mineralogy, whereas RFD cements had high-Mg calcitic precursors that subsequently underwent mimetic dolomitization via syntaxial replacement. RSDs and FSDs are primary marine precipitates, whereas fibrous dolomite cements with length-fast optical characteristics (FFDs and RFDs) are of secondary origin. These fibrous cements are widely distributed laterally in the Dengying Formation, although the RSDs and FSDs occur only in the second member of this formation. Therefore, the seawater chemistry of the terminal Ediacaran period in the upper Yangtze area was dominated by an aragonite sea, with a regional short-lived “dolomite sea” during the late depositional stage of the second member of the Dengying Formation, from which dolomite may have directly precipitated.

Distinguishing microbial from thermochemical sulfate reduction from the upper Ediacaran in South China

It is hard to distinguish pyrites generated by microbial sulfate reduction (MSR) from thermochemical sulfate reduction (TSR). The upper Ediacaran Dengying Formation (ca 551–541 Ma) of Sichuan basin is chosen to study for this purpose by means of petrological, bulk and micrometer-scale analyses on barite, pyrite, calcite, and dolomite for carbon and sulfur isotope and rare earth elements and yttrium (REY) compositions. We find that MSR-derived pyrites precipitated from near-surface to shallow burial settings are characterized by seawater-like REY patterns and superchondritic Y/Ho ratios (74.3 ± 9.5), and have δ34Spyrite values in a wide range from −36.8‰ to 34.8‰, positively correlated with arsenic contents, likely through cryptic sulfur cycles driven by arsenic. In contrast, TSR-derived pyrites generated during deep burial settings have REY characterized by non-seawater-like patterns and chondritic Y/Ho ratios (33.6 ± 5.1), and show small sulfur isotopic fractionations (~10.5‰). TSR-pyrites yield both U-shaped and bell-shaped δ34S transects compared with U-shaped distributions of MSR-pyrites. The calcites associated with the TSR-pyrites show LREE enrichment, elevated Y/Ho ratios (85.8 ± 14.2) and negative δ13C values, likely resulting from decomposition of organic matter. This study highlights a combination method from REY patterns, δ13C and δ34S values from analyses on micro-sized pyrite and the associated calcites can be used to effectively distinguish MSR from TSR.

Genetic link between Mississippi Valley-Type (MVT) Zn-Pb mineralization and hydrocarbon accumulation in the Nanmushu, northern margin of Sichuan Basin, SW China

The coexistence of Zn-Pb deposits and oil/gas reservoirs demonstrates that a close genetic link between them. The Nanmushu is a large Mississippi Valley-Type (MVT) Zn-Pb deposit discovered on the northern margin of the Yangtze block in recent years. The deposit is hosted in the Ediacaran Dengying Formation dolostone, accompanied by large amount of bitumen in the orebodies. The MVT Zn-Pb deposit overlaps with the paleo-oil/gas reservoir horizontally, and sandwiched in the paleo-oil/gas reservoirs at depth. The Cambrian Guojiaba Formation may have provided not only the oil for paleo-oil/gas reservoirs, but also ore metals for the Zn-Pb mineralization. With increasing burial depth, the Guojiaba Formation may have become more mature to form metal-rich fluids, and the metals migrated and accumulated with hydrocarbons to form a paleo-oil reservoir. Large-scale Zn-Pb mineralization may have occurred in the destruction process of paleo-gas reservoir. With the deep burial of paleo-oil reservoir, the paleo-oil reservoir may have transformed into a paleo-gas reservoir. Decoupling of metals and hydrocarbons during the paleo-gas reservoir formation may have provided the ore metals. Thermal sulfate reduction (TSR) during the paleo-oil/gas reservoirs formation may have provided the hydrogen sulfide for mineralization. Decompression and cooling during the paleo-gas reservoir destruction may have formed extensive metal sulfide precipitation and mineralization.

Microbial carbonate sequence architecture and depositional environments of Member IV of the Late Ediacaran Dengying Formation, Gaoshiti–Moxi area, Sichuan Basin, Southwest China

The recent discovery of the Anyue gas field, with huge reserves, has great potential for ultradeep gas production in carbonate reservoirs. Previous exploration and exploitation of Member IV of the Dengying Formation in the Gaoshiti–Moxi area deposited during the Late Ediacaran have provided useful insight into palaeo‐gas reservoirs, and geological significance of the Precambrian microbial carbonate strata. In this study, the depositional sequence architecture has been established and typical environments from Member IV of the Dengying Formation in the Gaoshiti–Moxi area have been reconstructed through detailed sedimentological analysis of cores, petrophysical data, well‐logging data and seismic interpretation. One third‐order sequence was identified, which is subdivided into seven fourth‐order sequences and 18–19 fifth‐order sequences. The third‐order sequence is bounded by denudation surfaces or onlap surfaces, and composed of transgressive system tracts and highstand system tracts. The fourth‐order sequences are bounded by instantaneously exposed punctuated surfaces or lithology and lithofacies transition surfaces, featured by specific lithological cycles, such as ‘lower microcrystalline dolomite, middle intraclastic dolomite and sparry thrombolite, and upper intraclastic dolomite’, ‘middle–lower sparry thrombolite and upper microcrystalline dolomite and sparry thrombolite’, ‘lower micritic thrombolite, middle micritic dolomite, and upper sparry thrombolite and laminites’, and ‘lower microcrystalline dolomite, middle intraclastic dolomite and argillaceous or micritic dolomite, and upper intraclastic dolomite’. The sequence characteristics reveal that periodic high‐frequency sea‐level change and palaeogeomorphology are controlling factors affecting sequence development, which lay a significant foundation for reservoir development. During the depositional period, a tidal flat on a restricted carbonate platform is well‐developed, allowing the identification of 16 microfacies, 13 microfacies associations, and 6 seismic facies. Inter‐lower tidal flats are widely developed with band‐shaped mound–bank complex, microbial mound, and shoal on the platform margin. On the inner platform, patchy microbial mounds, mound–flat combinations, and inter‐shoal/microbial mounds are developed in the Gaoshiti area, whereas patchy shoals and shoal–flat combinations are developed in the Moxi area. During the early depositional period of Member IV of the Dengying Formation, the microbial mounds and shoals were developed on the platform margin. The dolomitic flat and inter‐shoal/microbial mounds are mainly developed on the inner platform with little microbial mound and shoal deposition. During the late depositional period of Member IV of the Dengying Formation, large‐scale microbial mounds and shoals are sustainably and stably developed on the platform margin; enlarged dolomitic flat, patchy microbial mounds, and patchy shoals are predominantly developed in the platform interior with inter‐shoal/microbial mounds dying out.

Dickinsonia from the Ediacaran Dengying Formation in the Yangtze Gorges area, South China

Dickinsonia is an iconic fossil of the Ediacara biota (~575–539 Ma). It was previously known from siliciclastic successions of the White Sea assemblage in Australia, Baltica, and possibly India. Here we describe Dickinsonia sp. from the terminal Ediacaran Shibantan Member limestone (ca. 551–543 Ma) of the Dengying Formation in the Yangtze Gorges area of South China. The stratigraphic distribution of Ediacara-type fossils in the Shibantan Member indicates that this biota uniquely preserves both the White Sea and Nama assemblages in stratigraphic succession. The new data presented here suggests that Dickinsonia had wider paleogeographic and paleoenvironmental distributions, implying its strong dispersal capability and environmental tolerance.

Trace elements in sulfides from the Maozu Pb-Zn deposit, Yunnan Province, China: Implications for trace-element incorporation mechanisms and ore genesis

Abstract The Sichuan-Yunnan-Guizhou Pb-Zn metallogenic province (SYGMP) is an important region for Pb-Zn resources in China. However, considerable controversy remains as to whether the Pb-Zn deposits are Mississippi Valley Type (MVT). The Maozu deposit, a typical example of the carbonate-hosted Pb-Zn deposits in the SYGMP, occurs in the late Ediacaran Dengying Formation and its ore bodies are divided into three types: lower layer (LL), vein layer (VL), and upper layer (UL) ore bodies based on their spatial relationship. In this study, laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) was used to systematically analyze the trace-element compositions of sphalerite and galena in these three ore bodies. The results show that sphalerite is characterized by Cd and Ge enrichment; Fe, Mn, and Co depletion; and local In and Sn enrichment. Most of these elements likely appear as solid solutions in sphalerite and show a wide compositional variation, which is probably related to the medium- and low-temperature mixing of the ore-forming fluids. The local enrichment of In and Sn is likely attributed to the long-distance migration of ore-forming fluids through In-Sn-bearing volcaniclastic rocks. In vs. Sn and (Cu + Sb) vs. (Ag + Ge) show strong correlations and similar element distribution in the mapped images, indicating that these elements may be incorporated into sphalerite via a coupled substitution for Zn as 2In3+ + Sn4+ + 2☐ ↔ 5Zn2+ (☐ = vacancies) and 4(Cu+ + Sb3+) + (Ge4+ + 2Ag+) + 2☐ ↔ 13Zn2+. Galena is enriched in Ag and Sb with minor Cd and Se and depleted in Bi, and most of the elements may occur as solid solutions. Ag vs. Sb in galena displays a strong positive correlation, implying the coupled substitution of Ag+ + Sb3+ ↔ 2Pb2+. Notably, the majority of the trace-element concentrations gradually decrease in the order LL → UL except Fe, Co, Cu, and Ge, while Fe, In, and Sn in sphalerite and Ag and Sb in galena have the highest concentration in the VL, indicating that the VL is a secondary migration channel for the ore-forming fluids. Furthermore, the trace-element compositions of the sulfides in the Maozu Pb-Zn deposit are consistent with the typical MVT deposit (hosted in the carbonate sequence) but are markedly different from sedimentary exhalative (SEDEX), volcanogenic massive sulfide (VMS) and skarn-type deposits. Based on these results, as well as the geological and geochemical characteristics of the deposit, the Maozu Pb-Zn deposit is an MVT deposit.

Vase-shaped microfossils from the late Ediacaran Dengying Formation of Ningqiang, South China: taxonomy, preservation and biological affinity

The origin and early evolution of protists are shroud in misty. The oldest evidence of heterotrophic protists may be the vase-shaped microfossils (found in Early Neoproterozoic sedimentary rocks around the world (c. 780–720 Ma). Here we explore the taxonomy, preservational types and biological affinity of three-dimensionally preserved vase-shaped microfossils (VSMs) from the Gaojiashan biota of the Dengying Formation, Ningqiang, China. Five species and two genera are recognized, namely Protolagena limbata, P. gaojiashanensis, P. papillata, Sicylagena formosa and S. latistoma. SEM-EDS-BSE data provide sound evidence of calcareous and agglutinated tests of these VSMs. In addition, Found in siltstone beds and clastic dolomite, sedimentological data suggest these VSMs inhabited a series of preservational types including calcification, silicification, phosphatization and pyritization. The fossiliferous strata have recently been dated with U-Pb age of 548 ± 8 Ma, which indicates they are not coeval with diverse vase-shaped microfossil assemblages described in other regions worldwide, but provides supportive evidence for the early diversification of possible protozoa foraminifera clades follow on the Gaskiers Snowball Earth event (c. 590–580 Ma).

Petrological and geochemical characteristics of the botryoidal dolomite of Dengying Formation in the Yangtze Craton, South China: Constraints on terminal Ediacaran “dolomite seas”

Recent studies suggest that the massive Precambrian dolomite successions were likely the products of specific seawater conditions known as “dolomite seas”. However, whether these so-called “dolomite seas” existed in the terminal Ediacaran ocean has not yet been constrained. The widespread botryoidal dolomite with well-preserved sedimentary fabrics from the Upper Ediacaran Dengying Formation (ca. 551 to 545 Ma), South China can be potentially an ideal target to constrain this issue. Results showed that within the botryoidal dolomite, fibrous dolomite cements are the dominant constituent part, and either grew upward from microbialite layers or occurred in sheet cracks within the microbialites, implying a syn-sedimentary diagenesis. Two different types of fibrous cements are newly redefined: fascicular and radial slow dolomite. Well-preserved chemical and cathodoluminescent zonations and length-slow optical characters are direct evidence for syn-sedimentary primary dolomite precipitation, different with the previous alternative interpretations of mimetic dolomitization of aragonite or high-Mg calcite precursors. In contrast, microbialite matrix consists of finely crystalline dolomicrite and preserved microbial fabrics, reveling an early mimetic dolomitization of aragonitic precursors. The dolomite precipitation and mimetic dolomitization likely occurred in coeval seawater or seawater-type fluids as indicated by comparable δ13C and 87Sr/86Sr data with global changes. In particular, the presence of primary dolomite cement precipitates within botryoidal dolomite can reflect that the specific “dolomite seas”, associated with anoxic or even euxinic shallow water conditions (inferred by redox-sensitive trace elements and REEs) and increased microbial activity, also prevailed in the terminal Ediacaran ocean. However, the occurrence of “dolomite seas” was not one occasional event, but common even if not persistent in the early to terminal Neoproterozoic oceans. This study helps to improve the understanding of “dolomite problem” and ubiquitous distribution of Precambrian dolomite worldwide.

Microbial mineralization of botryoidal laminations in the Upper Ediacaran dolostones, Western Yangtze Platform, SW China

Botryoidal dolostones are abundant in the Second Member of the Upper Ediacaran Dengying Formation in the Western Yangtze Platform. Although they have been extensively discussed as important host rocks for giant gas reservoirs, there is no consensus on the microbial impact on mineralization processes in the botryoidal laminations. Typical botryoidal laminations were studied under optical microscopes and scanning electron microscopes (SEMs) to investigate their origin. There are three macroscopic morphological types of botryoidal lamination: stromatoid lamination, cavity-lining lamination, and spherical-encrusting lamination. These three types of lamination are composed of fibrous and bladed crystals growing perpendicular to the botryoidal laminae, which have hitherto been implicitly considered to be from abiogenic chemical precipitation. There are many lines of new evidence that demonstrate microbial activities in the interbedded dark and bright laminae constituting the laminations. In the dark laminae, euhedral dolomite rhombs are observed sticking to linear and tubular filaments, or wrapped in sheets of extracellular polymeric substances (EPS) with dolomite crystal sizes of generally <5 μm. The bright laminae are dominated by coarser dolomites and connected by radial sheaths and trichomes with dolomite crystal sizes of primarily around 10 μm. Different species of bacteria, which are closely related to the macroscopic morphology of the laminations, are present in both the bright and dark laminae. The results are useful in understanding the origin and formation mechanisms of the Neoproterozoic botryoidal dolostones. The new findings may also shed new light on the occurrence and evolution of early life on Earth in the Precambrian Era.

Early development and late preservation of porosity linked to presence of hydrocarbons in Precambrian microbialite gas reservoirs within the Sichuan Basin, southern China

The ancient Ediacaran Dengying Formation consists of microbialites with unusually high porosity that represent a large proportion of the Weiyuan-Anyue gas reservoirs in southwestern China’s Sichuan Basin. However, a clear model for the development and preservation of these microbialite reservoirs has yet not been determined. We used samples from outcrops and borehole cores to characterize these microbialites and propose a model for early development and subsequent reservation of their porosity. Primary porosity in the microbialite framework was mainly represented by fenestrate vugs and elongated cavities between microbial layers that were formed during initial sedimentation as well as secondary karst vugs and caves formed by meteoric karst resulting from subsequent tectonic uplift; these features represent the early development of the microbialite reservoirs. Subsequent filling of vugs by early-stage radial dolomite (RD) and late-stage saddle dolomite (SD) occurred during the deep burial stage, with δ18O and homogenization temperature patterns suggesting continuous cementation. However, most vugs were not cemented due to combination of (1) a rigid dolomitized framework, (2) a closed burial system restricting the flow of mineralized fluid, (3) the presence of petroleum or pyrobitumen, and (4) acidic fluid conditions related to high concentrations of H2S and CO2. The third and fourth factors were linked to presence of hydrocarbons and had made big contribution to porosity retention during deep burial diagenetic stage.

Terminal Ediacaran carbonate tempestites in the eastern Yangtze Gorges area, South China

The terminal Ediacaran Dengying Formation in the eastern Yangtze Gorges area, South China is well known for hosting diverse Ediacaran-type macrofossils and trace fossils. However, the depositional processes of this carbonate succession are still poorly known. Here we present detailed sedimentological data on the Dengying carbonates in four sections across the eastern Yangtze Gorges area, with an emphasis on tempestites in limestones of the Shibantan Member of Dengying Formation that have largely been overlooked. The Shibantan limestones in the studied sections contain extensive autochthonous microbial mats and storm-related deposits, including calcirudite, individual swaley cross-stratified calcarenites, hummocky cross-stratified fine- to very fine-grained calcarenites, normally graded calcarenites, calcarenite-calcisiltite beds in discrete units, and soft-sediment deformed beds. The spatial variation of facies suggests that the Shibantan Member was deposited on a carbonate ramp with an intra-ramp lagoon. With the upward shallowing of water depth and progradation of storm-generated sediments, the ramp gradually become steeper, as characterized by the occurrence of soft-sediment deformation structures and graded bedding in the middle of the member. The abundance of tempestites in the Shibantan limestones suggests that the Shibantan Basin was located in low-latitude regions frequently affected by tropical storms, consistent with previous paleogeographic reconstructions. Storm events probably brought nutrients into the environment and contributed to the rapid burial and preservation of macrofossils. The Shibantan limestones thus provide another example of a massflow-related taphonomic window for the Ediacaran organisms that is comparable to the fossil preservation style in the Ediacaran siliciclastic rocks.

Sources of the Laoxiongdong carbonate-hosted Pb–Zn deposit in Southwest China: constraints from S–Pb–Zn isotopic compositions

The Sichuan–Yunnan–Guizhou (SYG) Pb–Zn mineral province, which has significant Pb–Zn repositories in China, is situated in the western Yangtze Block. Despite decades of research, the Pb–Zn source of deposits is still disputed between the basement rocks, sedimentary wall rocks, or the Emeishan flood basalts. The newly-discovered Laoxiongdong Pb–Zn deposit is hosted by the Late Ediacaran Dengying Formation in the SYG province. Moreover, the Laoxiongdong orebodies lie between regional deep faults and close to the Proterozoic basement and Emeishan basalts. Hence, this deposit represents a good case study on the ore-forming material source for the regional Pb–Zn mineralization. The Laoxiongdong Pb–Zn ores are massive, vein, or disseminated and have mainly sphalerite, galena, pyrite, quartz, and calcite. In this paper, we present new sulfide S–Pb–Zn isotope data of the deposit. The sulfide δ34SCDT values (+ 11.8 to + 16.5 ‰) suggest that the reduced sulfur was mainly sourced from evaporites in the Late Ediacaran–Cambrian sequences. Pb isotopic ratios (206Pb/204Pb = 18.004–18.107, 207Pb/204Pb = 15.652–15.667, and 208Pb/204Pb = 38.037–38.248) suggest that the lead metal was primarily originated from the basement rocks. The sphalerite δ66Zn values (+ 0.16 to + 0.37 ‰) are also highly similar (within error) to those of basement rocks (+ 0.10 to + 0.34 ‰), suggesting a basement-rocks zinc source with minor contributions from the carbonate host rocks and Emeishan flood basalts. The narrow sphalerite Zn isotopic range (0.21 ‰) also indicates that the Zn isotopic fractionation between the sphalerite and initial fluid was limited during the sphalerite ore precipitation. Therefore, we propose that both the Late Ediacaran Dengying Formation rocks and Proterozoic basement rocks were important ore-forming material source for the Laoxiongdong deposit, whereas the Emeishan flood basalts represent only a minor ore-material source.

RETRACTED: Sedimentary characteristics and origin of dolomitic ooids of the terminal Ediacaran Dengying Formation at Yulin (Chongqing, South China)

Abstract The Ediacaran was a crucial transition period in the evolutionary history of the Earth. Ediacaran ooids in the Dengying Formation from the Yulin section in southwestern China are important archives of paleoenvironmental information. Although these ooids have been totally dolomitized, the cortical fabric characteristics are locally preserved well, indicating that these ooids were mimetically replaced. The common occurrence of concentric–radial fabrics and low level of Sr concentrations (60.3 ± 8.8 ppm) in the studied ooid cortex is more likely suggestive of a primary low-Mg calcite mineralogy and that a calcite sea might exist during deposition of the second member of the Dengying Formation. In situ geochemical data for the ooid cortices obtained by laser ablation inductively coupled plasma mass spectrometry show that these ooids have middle rare earth element enrichment (PrN/SmN = 0.7 ± 0.1 and SmN/YbN = 1.5 ± 0.4) and high Fe contents (8200 ± 1800 ppm). This suggests ferruginous pore waters, which were related to the selective or preferential absorption and release of REEs by Fe-Mn oxyhydroxides.

Isotope geochemistry of Zn, Pb and S in the Ediacaran strata hosted Zn-Pb deposits in Southwest China

The Sichuan–Yunnan–Guizhou (SYG) area is a world-class Zn-Pb metallogenic zone and contains over four hundred of carbonate-hosted hydrothermal Zn-Pb deposits, six of which are super large-scale deposits with significant sources of Cd, Ge, and Ag, including the Huize, Wusihe, Xiaoshifang, Maozu, Tianbaoshan and Daliangzi deposits. Although previous studies focused fully on these deposits, the ore genesis of which are still in controversial. Interestedly, four of those super large scale deposits are hosted in the Ediacaran Dengying Formation, including the Maozu, Wusihe, Tianbaoshan and Daliangzi deposits. Based upon previous studies, we investigate the extent, causes and consequences of Zn and Pb isotope signatures at hand specimen scale and ore genesis of the Daliangzi and Tianbaoshan deposits, associated with published data for the Wusihe deposit. Lead isotope ratios, coupled with published data, demonstrate that Pb in the studied deposits was predominantly derived from two components: basements and sedimentary rocks. Meanwhile, a strong positive linear correlation between δ66Zn and δ34SCDT values was observed in hand specimen D-2084, suggesting the variations may be resulted by fluid mixing. Furthuremore, the Zn isotope and Cd/Zn dataset from the Tianbaoshan, Daliangzi and Wusihe deposits defines a logarithmic curve (R2 = 0.82), which, to our knowledege, is firstly reported and lies between two indentified components: a low Cd/Zn end-member with high δ66Zn values and a high Cd/Zn end-member with low δ66Zn values. Based upon previous studies, we suggest that Zn and Pb in the Wusihe and Tianbaoshan deposits were predominantly derived from basements and Ediacaran sediments; in contrast, Zn and Pb in the Daliangzi deposit were predominantly derived from Ediacaran sediments and upper sedimentary rocks, but we cannot rule out metal contributions of the basements. Furthermore, we suggest that fluid mixing is more propreate to better explain Zn isotope signatures in the studied deposits.In summary, Zn and Pb isotope ratios reflect similar ore genesis and metal sources of the Daliangzi, Tianbaoshan and Wusihe Zn-Pb deposits in the SYG area. The application to the studied deposits with the discovery of Zn, S and Pb isotope compositions for sulfides indicates the potential of Zn isotope, combined with Cd/Zn raito, as new geochemical tracers, which may be useful for distinguishing the metal sources in large hydrothermal systems as suggested previously.

Geo-chromatographic fractionation effect of methyldibenzofuran in dolomite reservoirs and its application in tracing oil filling pathways in the Sichuan Basin

Abstract Dibenzofuran and its methyl substituted homologues are the important oxygen-containing polycyclic aromatic compounds in crude oils. The ratio of 1-/4-methyldibenzofuran has been successfully used in sandstone reservoir tracing oil migration orientation. Here we focus on the geo-chromatographic fractionation effect of methyldibenzofuran (MDBF) in dolomite reservoirs. We calculated the charge density differences between 1-methyldibenzofuran (1-MDBF) and 4-methyldibenzofuran (4-MDBF) based on density functional theory. The E I n between MDBF isomers and dolomite (104) surface was also calculated according to molecular dynamics simulation. The result show that comparing with the 4-MDBF, there is strong interaction between 1-MDBF and dolomite (104) surface leading the ratio of 1-MDBF and 4-MDBF increase with the migration distance. There is no significant correlation between bitumen reflectance and the ratio of 1-MDBF and 4-MDBF in our samples. In addition, molecular simulation results show that the MDBF isomers are thermodynamically stable and dissociation of Ph-CH3 is difficult to occur under geological condition. All these demonstrate that 1-MDBF and 4-MDBF can be survived after oil-cracking to gas and pyrobitumen. Therefore, in theory, the ratio of 1-/4-methyldibenzofuran is also regarded as an index for oil migration orientation in dolomite reservoir. Gas chromatography-mass spectrometry results show that MDBFs occur at relatively higher abundance in the bitumens of the Ediacaran Dengying Formation (Z2dn4) and Cambrian Longwangmiao Formation (Є1l) in the Sichuan Basin. The ratio of 1-/4-methyldibenzofuran has been successfully used to trace the crude oil filling pathway in the Z2dn4 and Є1l dolomite reservoir in the Sichuan Basin in this study. Two preferential filling orientations (i.e. from west to east and from north to south) for paleo-oil have been identified in the contour maps of 1-/4-methyldibenzofuran in the Z2dn4 and Є1l, which is consistent with the geological background.