Deposition and diagenesis of the Early Permian volcanic-related alkaline playa-lake dolomitic shales, NW Junggar Basin, NW China

Guo, Pei; Wen, Hao; Gibert, Luís; Jin, Jun; Wang, Jian; Lei, Haiyan

doi:10.1016/j.marpetgeo.2020.104780

Cited by 16 publications

(18 citation statements)

References 56 publications

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“…The dolomite aggregates were mainly filled in tepee horizons, shrinkage cracks, and evaporite crystal moulds (Fig. 7A to D; Guo et al ., 2021), which were formed in pedogenetic or shallow water environments that frequently suffered subaerial exposures. During dry seasons, evaporite crusts, nodules and shrinkage cracks were developed, which would be dissolved during following flooding seasons.…”

Section: Discussionmentioning

confidence: 99%

“…The early Cretaceous highly alkaline lakes within South Atlantic rift–sag lacustrine basins are also rich in authigenic Mg‐clays [stevensite (Na,Ca) 0.3 Mg 3 Si 4 O 10 (OH) 2 ·nH 2 O] instead of primary dolomite (Tosca & Wright, 2015; Wright & Barnett, 2015; Mercedes‐Martín et al ., 2019). Under similar hypersaline conditions, Mg‐evaporite‐bearing shales of alkaline hypersaline lakes contain statistically less dolomite (generally <50%: Boak & Poole, 2015; Guo et al ., 2021) than the sulphate‐bearing shales of non‐alkaline hypersaline lakes (easily >50%: Li et al ., 2018; Guo et al ., 2019). Apparently, the formation of dolomite in low‐T alkaline environments is constrained by the authigenesis of other Mg‐rich minerals, including Mg‐Na‐carbonate evaporites (tychite, northupite, etc.…”

Section: Introductionmentioning

confidence: 99%

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Constraints on dolomite formation in a Late Palaeozoic saline alkaline lake deposit, Junggar Basin, north‐west China

2023

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Alkaline lakes (pH > 9) are among the few modern sedimentary environments that are hydrochemically favourable for low‐temperature dolomite formation. While Mg‐clays and Mg‐evaporites also form more easily in alkaline environments, few studies have focused on how the kinetically inhibited dolomite wins the competition for Mg2+. Here, a basin‐wide survey of the distribution, paragenesis and stable C, O and Mg isotopes of main Mg‐rich minerals in the Late Palaeozoic saline alkaline lake deposit of the north‐west Junggar Basin, north‐west China, is conducted to study the influence of the formation and diagenesis of eitelite, northupite and Mg‐clays on dolomite formation. Large, isolated dolomite crystals (20 to 70 μm in diameter), show positive δ13C values (ranging from +1 to +7‰) and a restricted distribution in the mudstones of the lake‐transitional zone. These crystals have been interpreted as organogenic dolomite driven by methanogenesis via fermentation of organic substrates. The δ18O values of dolomitic mudstones (from −7.4 to +3.4‰), calcitic mudstones (from −15.1 to −3.3‰) and bedded Na‐carbonate evaporites (from +0.08 to +3.7‰), together with their Mg isotopic compositions, suggest that dolomite was not enriched in the most concentrated environments or during stages with most Mg sources, but in the organic‐rich deposits containing few other authigenic Mg‐rich minerals. Dolomite is at a competitive disadvantage for Mg2+ ions compared to Mg‐evaporite and Mg‐clay minerals due to its slow crystallization rates and the deficiency of micritic calcium carbonate precursors. However, it can nucleate and progressively grow into large crystals (>20 μm) if bacterial methanogenesis could effectively lower porewater pH (<8.5) and induce the dissolution of generated eitelite, northupite or Mg‐clays. These findings suggest that high salinity and/or high alkalinity are not always favourable conditions for dolomite formation and highlight the active role of pH fluctuations in inducing low‐temperature dolomite formation.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Constraints on dolomite formation in a Late Palaeozoic saline alkaline lake deposit, Junggar Basin, north‐west China

2023

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“…Modern and Neogene microcrystalline dolomites occur mainly in high‐salinity environments, such as sabkha flats, hypersaline lagoons and playa lakes, where evaporation is intense (Arvidson & MacKenzie, 1999; Petrash et al ., 2017). However, the Lucaogou dolostones do not contain any evaporite minerals and their pseudomorphs, as well as evaporitic sedimentary structures, such as dewatering‐induced shrinkage cracks and subaerially exposed and transported sediments with ripple marks (Alsharhan & Kendall, 2003; Garcia‐Fresca et al ., 2018; Guo et al ., 2021).…”

Section: Discussionmentioning

confidence: 99%

“…Hence, it is speculated that evaporation and later dolomitization are required for the formation of ancient massive dolostones. In those lakes, some of them record strong volcanic–hydrothermal activity; they are termed alkaline lakes and have been reported frequently in recent years (Cangemi et al ., 2016; Newell et al ., 2017; Yu et al ., 2018; Cao et al ., 2020; Guo et al ., 2021). Those lakes have an extremely high primary productivity, unique environmental conditions and associated carbonate deposits (Southgate et al ., 1989; Rothschild & Mancinelli, 2001; Cangemi et al ., 2016; Hammond et al ., 2019; Cao et al ., 2020).…”

Section: Introductionmentioning

confidence: 99%

Microcrystalline dolomite in a middle Permian volcanic lake: Insights on primary dolomite formation in a non‐evaporitic environment

et al. 2022

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Lacustrine dolomite nucleation commonly occurs in modern and Neogene evaporitic alkaline lakes. As a result, ancient lacustrine microcrystalline dolomite has been conventionally interpreted to be formed in evaporitic environments. This study, however, suggests a non-evaporitic origin of dolomite precipitated in a volcanic-hydrothermal lake, where hydrothermal and volcanic processes interacted. The dolomite occurs in lacustrine fine-grained sedimentary rocks in the middle Permian Lucaogou Formation in the Santanghu intracontinental rift basin, north-west China. Dolostones are composed mainly of nano-sized to micron-sized dolomite with a euhedral to subhedral shape and a low degree of cation ordering, and are interlaminated and intercalated with tuffaceous shale. Non-dolomite minerals, including quartz, alkaline feldspars, smectite and magnesite mix with the dolomite in various proportions. The 87 Sr/ 86 Sr ratios (0.704528 to 0.705372, average = 0.705004) and d 26 Mg values (À0.89 to À0.24&, average = À0.55&) of dolostones are similar to those of mantle rocks, indicating that the precipitates mainly originated from fluids that migrated upward from the mantle and were subject to water-rock reactions at a great depth. The d 18 O values (À3.1 to À22.7&, average = À14.0&) of the dolostones indicate hydrothermal influence. The trace and rare earth element concentrations suggest a saline, anoxic and volcanic-hydrothermally-influenced subaqueous environment. In this subaqueous environment of Lucaogou lake, locally high temperatures and a supply of abundant Mg 2+ from a deep source induced by volcanic-hydrothermal activity formed favourable chemical conditions for direct precipitation of primary dolomite. This study's findings deepen the understanding of the origin and processes of lacustrine primary dolomite formation and provide an alternative possibility for environmental interpretations of ancient dolostones.

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“…The Jimsar Sag is a dustpan depression formed on the lower Carboniferous folded basement [29,36]. According to geological records, the Jimsar Sag underwent several distinct tectonic events in the Hercynian, Indosinian, Yanshan, and Himalayan periods [37,38].…”

Section: Geological Settingmentioning

confidence: 99%

Permian Cyanobacterial Blooms Resulted in Enrichment of Organic Matter in the Lucaogou Formation in the Junggar Basin, NW China

et al. 2023

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The Permian Lucaogou Formation in the Junggar Basin, NW China is the target layer for shale oil exploration, but its hydrocarbon precursors have remained the focus of debate. In this study, we investigated the Lucaogou source rocks throughout Well J10025 by conducting detailed petrological, paleontological, and geochemical analyses for the purpose of revealing the occurrence of cyanobacterial blooms as specific hydrocarbon events in the upper Lucaogou Formation. The morphological characteristics of the microfossils and the geochemical signatures of the microfossil-bearing layers support a biological affinity with Microcystis, a kind of cyanobacteria. Microcystis observed as colonial forms embedded in the upper Lucaogou Formation are of great abundance, indicating the presence of cyanobacterial blooms. They were further evidenced by cyanobacteria-derived biomarkers including low terrestrial/aquatic ratio, high 2α-methylhopane index values, and high abundance of 7- and 8-monomethyl heptadecanes. The blooms occurred in a semiarid and brackish paleoenvironment with anoxic to suboxic water conditions and intermittent volcanic eruptions. Permian Microcystis blooms contributed to the enrichment of organic matter in the upper Lucaogou Formation in two main ways: by directly promoting the accumulation of algal biomass and by creating an oxygen-depleted environment for better preservation of organic matter. This study adds a new record to the geological occurrences of cyanobacterial blooms in the Permian, and provides unique insight into the hydrocarbon generation of Jimsar shale oil in the Junggar Basin.

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Deposition and diagenesis of the Early Permian volcanic-related alkaline playa-lake dolomitic shales, NW Junggar Basin, NW China

Cited by 16 publications

References 56 publications

Constraints on dolomite formation in a Late Palaeozoic saline alkaline lake deposit, Junggar Basin, north‐west China

Constraints on dolomite formation in a Late Palaeozoic saline alkaline lake deposit, Junggar Basin, north‐west China

Microcrystalline dolomite in a middle Permian volcanic lake: Insights on primary dolomite formation in a non‐evaporitic environment

Permian Cyanobacterial Blooms Resulted in Enrichment of Organic Matter in the Lucaogou Formation in the Junggar Basin, NW China

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