scholarly journals The complex diagenetic history of discontinuities in shallow-marine carbonate rocks: New insights from high-resolution ion microprobe investigation of δ 18 O and δ 13 C of early cements

Sedimentology ◽  
2017 ◽  
Vol 65 (2) ◽  
pp. 360-399 ◽  
Author(s):  
Simon Andrieu ◽  
Benjamin Brigaud ◽  
Jocelyn Barbarand ◽  
Eric Lasseur
Keyword(s):  
High Resolution ◽  
Carbonate Rocks ◽  
Ion Microprobe ◽  
Shallow Marine ◽  
Marine Carbonate ◽  
History Of ◽  
Diagenetic History ◽  
Marine Carbonate Rocks

scholarly journals The Remagnetization of Marine Carbonate Rocks of the Late Ordovician in Pingliang Section, Southwest Ordos (China)

2019 ◽  
Vol 93 (S3) ◽  
pp. 132-134
Author(s):  
Junhua LUO ◽  
Shuang DAI ◽  
Mark DEKKERS ◽  
Weiguo WANG ◽  
Xiaoke QIANG ◽  
...  
Keyword(s):  
Carbonate Rocks ◽  
Late Ordovician ◽  
Marine Carbonate ◽  
Marine Carbonate Rocks

scholarly journals O CONGLOMERADO SAMBURÁ (GRUPO BAMBUÍ, NEOPROTEROZÓICO) E ROCHAS SEDIMENTARES ASSOCIADAS NO FLANCO LESTE DA SERRA DA PIMENTA, SW DE MINAS GERAIS: UM SISTEMA DE FAN DELTA.

1995 ◽  
Author(s):  
Paulo de Tarso Amorin Castro ◽  
Marcel Auguste Dardenne
Keyword(s):  
Source Rocks ◽  
Petrographic Analysis ◽  
Eastern Region ◽  
Thrust Belt ◽  
Shallow Marine ◽  
Metasedimentary Rocks ◽  
Fan Delta ◽  
Marine Carbonate ◽  
Marine Carbonate Rocks ◽  
Southwest Part

In the southwest part of São Francisco Craton occur neoproterozoic metasedimentary rocks of the Bambuí Group.  Among these rocks the Samburá Conglomerate outcrops nearby the external (eastern) region of the southernmost part of Brasilia Thrust Belt.  Sedimentological studies carried on these rocks at Serra da Pimenta region reveals that they belong to a fan delta system developed at the toes of thrust fault escarpments of Brasilia Thrust Belt. Petrographic analysis of clasts of Samburá Conglomerate pointed to Canastra Group and Pium-í greenstone belt rocks of Serra da Pimenta mountains as source rocks. The fan delta deposits are rested above shallow marine carbonate rocks of the base of Bambuí Group.

Multi-episodic recrystallization and isotopic resetting of early-diagenetic dolomites in near-surface settings

2021 ◽  
Vol 91 (1) ◽  
pp. 146-166
Author(s):  
Brooks H. Ryan ◽  
Stephen E. Kaczmarek ◽  
John M. Rivers
Keyword(s):  
Carbonate Rocks ◽  
Isotopic Exchange ◽  
Early Diagenesis ◽  
Geochemical Data ◽  
Near Surface ◽  
Lower Eocene ◽  
Gypsum Formation ◽  
History Of ◽  
Mineral Replacement ◽  
Diagenetic History

ABSTRACT The lower Eocene Rus Formation in Qatar reflects carbonate deposition in a semirestricted to fully restricted marine setting on a shallow ramp. Petrographic, mineralogical, and geochemical evidence from three research cores show early diagenesis has extensively altered nearly every petrological attribute of these rocks despite not having been deeply buried. In southern Qatar, the lower Rus (Traina Mbr.) consists of fabric-retentive dolomite intervals that preserve mudstone, wackestone, and packstone textures that are interbedded with depositional gypsum beds. In northern Qatar, the same member is dominated by fabric-destructive planar-e dolomite, and evaporites are absent. In both northern and southern Qatar, the upper Rus (Al Khor Mbr.) is composed of fabric-retentive dolomite intervals as well as limestone intervals rich with Microcodium textures that display evidence of dedolomitization. Geochemical analysis reveals that the limestones have an average δ18Ocal of –10.73‰ VPDB and δ13Ccal of –7.84‰ VPDB, whereas average dolomite δ18Odol is significantly higher (–1.06‰ VPDB) but δ13Cdol values (–3.04‰ VPDB; range –10 to 0‰) overlap with δ13Ccal values. Additionally, δ13Cdol trends toward normal marine values with depth away from the calcite–dolomite contact in all three cores. Petrographic observations demonstrate that dolomite crystals are commonly included in calcite and partially to completely replaced by calcite in these intervals and suggests that dolomite formed before calcite in the Microcodium-bearing intervals. Furthermore, the dolomites are commonly cemented by gypsum in the Traina Mbr. in southern Qatar, suggesting that dolomitization may have also occurred before, or concurrent with, bedded gypsum formation and indicates that dolomitization occurred early. Early dolomites were subsequently replaced by Microcodium-bearing limestones at and immediately below paleo-exposure surfaces, and at greater depths recrystallized in mixed marine–meteoric fluids, producing a negative δ13Cdol signature that trends toward more positive values away from the limestone–dolomite contact. Lastly, the dolomites underwent another phase of recrystallization in either marine-dominated fluids or possibly a well-mixed aquifer setting, resulting in a near-0‰ δ18Odol signature but retaining the negative δ13C signature. These findings thus have implications for reconstructing the diagenetic history of carbonate rocks, as they suggest that early diagenesis of carbonates can be extremely complex, resulting in multiple stages of mineral replacement and isotopic exchange in meteoric and shallow marine fluids before significant burial. Furthermore, this study shows that dolomitization of a limestone does not necessarily prevent additional early diagenesis and multiple recrystallization events. Lastly, it emphasizes the importance of incorporating petrographic observations with geochemical data when interpreting the diagenetic history of carbonate rocks.

A Novel Machine-Assisted Technique for Extracting Multiscale Vugs and Fractures in Heterogeneous Carbonates Sequence

2021 ◽  
Author(s):  
Sarvagya Parashar ◽  
Ivan Zhia Ming Wu
Keyword(s):  
High Resolution ◽  
Pore Size ◽  
Carbonate Rocks ◽  
Processing Technique ◽  
Pore System ◽  
Fractured Carbonate ◽  
Percolation Effect ◽  
Laplacian Of Gaussian Filter ◽  
Different Response ◽  
Diagenetic History

Abstract Predicting petrophysical properties in carbonate reservoirs is challenging due to the deposition and diagenetic history, which creates pore-scale features and heterogeneity at multiple-length scale. Non-fractured carbonate rocks with monomodal pore distribution often provide weak transportation properties compared to carbonates with multimodal pore system. The behaviour of such formations is subject to percolation effect where the connectivity of vug clusters control the poro-perm relationship which can be explained with high-resolution microresistivity images and nuclear magnetic resonance (NMR) data. A machine-assisted processing technique, defined as "thresholding," was applied to high-resolution microresistivity images, resolving vugs and fractures with similar resistivity. Other objects of interest are removed using object-oriented filters and thresholding, resulting in a "sculptured image" containing only vugs and fractures. The image is analysed to quantify formation porosity. A Laplacian of Gaussian filter is used to avoid highlighting features of no interest. Step two analyses T1 and T2 relaxations allowing portions of signal from a pore-size group to spill across the discrete boundaries. The pore-size takes on a fuzziness near the discrete relaxation time cut-offs corresponding to pore radii breakover points. High poro-perm layers of grainstone in overall thinly bedded sequences of packstone and wackestone were successfully identified and subsequently shed light upon the ambiguities observed in mobility values obtained from formation tester across the same lithocolumn. This novel technology helps in deciphering high-resolution integrated lithofacies. The histogram from the image porosity binning demonstrates a different response within vugular zones compared to fractured zones. Where the vugs sizes are variable, they exhibit a multi-pore system nature in NMR. For the fractured interval, the images and NMR exhibit weak distribution. The resistivity independent image pixel-based filtration technique helps to define interesting features on images which can be enhanced and measurable at various scales. Machine assisted technique in NMR complement the results in aiding to characterize the heterogeneous carbonate rocks.

The Shibantan Lagerstätte: insights into the Proterozoic–Phanerozoic transition

2020 ◽  
Vol 178 (1) ◽  
pp. jgs2020-135
Author(s):  
Shuhai Xiao ◽  
Zhe Chen ◽  
Ke Pang ◽  
Chuanming Zhou ◽  
Xunlai Yuan
Keyword(s):  
South China ◽  
Carbonate Rocks ◽  
Microbial Mats ◽  
Ecological Interactions ◽  
Benthic Organisms ◽  
Important Data ◽  
Ediacara Biota ◽  
Marine Carbonate ◽  
Fossil Assemblages ◽  
Marine Carbonate Rocks

The Shibantan Lagerstätte (551–543 Ma) in the Yangtse Gorges area in South China is one of the best-known examples of terminal Ediacaran fossil assemblages preserved in marine carbonate rocks. Taxonomically dominated by benthic organisms, the Shibantan Lagerstätte preserves various photoautotrophs, biomineralizing tubular fossils, Ediacara-type macrofossils (including rangeomorphs, arboreomorphs, erniettomorphs, palaeopascichnids, a possible dickinsoniomorph, the mobile bilaterian Yilingia and soft-bodied tubular fossils), abundant ichnofossils and a number of problematic and dubious fossils. Shibantan fossils provide intriguing insights into ecological interactions among mobile bilaterians, sessile benthic Ediacara-type organisms and microbial mats, thus offering important data to test various hypotheses accounting for the decline of the Ediacara biota and the concurrent expansion of bilaterian bioturbation and mobility across the Proterozoic–Phanerozoic transition.

scholarly journals Stable Isotope (S, Mg, B) Constraints on the Origin of the Early Precambrian Zhaoanzhuang Serpentine-Magnetite Deposit, Southern North China Craton

Minerals ◽  
2019 ◽  
Vol 9 (6) ◽  
pp. 377 ◽  
Author(s):  
Jie Meng ◽  
Houmin Li ◽  
Yanhe Li ◽  
Zhaochong Zhang ◽  
Lixing Li ◽  
...  
Keyword(s):  
North China Craton ◽  
Carbonate Rocks ◽  
North China ◽  
Mean Value ◽  
Coarse Grained ◽  
Sulfur Source ◽  
Marine Carbonate ◽  
Isotopic Analyses ◽  
Wide Range ◽  
Marine Carbonate Rocks

The origin of the Zhaoanzhuang serpentine-magnetite deposit in the southern North China Craton (NCC) is highly disputed, with some investigators having proposed an ultramafic origin, whereas others favor a chemical sedimentary origin. These discrepancies are largely due to the difficulty in determining the protolithic characteristics of the highly metamorphosed rocks. Sulfur, magnesium, and boron isotope geochemistry combined with detailed petrography was carried out in this study to constrain the original composition of the Zhaoanzhuang iron orebodies. Anhydrite is present as coarse crystals intergrown with magnetite, indicating that the anhydrite formed simultaneously with the magnetite during metamorphism rather than as a product of later hydrothermal alteration. The anhydrite has a narrow range of positive δ34S values from +19.8 to +22.5‰ with a mean value of +21.1‰. These values are significantly higher than that of typical magmatic sulfur (δ34S = 0 ± 5‰) and deviate away from primary igneous anhydrite towards mantle-sulfur isotopic values, but they are similar to those of marine evaporitic anhydrite and gypsum (~+21‰). The sulfur isotopic compositions of several samples show obvious signs of mass-independent sulfur fractionation (Δ33S = −0.47‰ to +0.90‰), suggesting that they were influenced by an external sulfur source through a photochemical reaction at low oxygen concentrations, which is consistent with the Neoarchean-Paleoproterozoic atmosphere. Coarse-grained tourmaline from the tourmaline-rich interlayers of the orebodies occurs closely with Mg-rich minerals such as phlogopite, talc, and diopside, indicating that it has a metamorphic origin. The δ11B values of the tourmaline range from −0.2‰ to +3.6‰ with a mean value of +2.0‰, which is much positive relative to that of magmatic tourmaline but is consistent with that of carbonate-derived tourmaline. The magnesium isotopic analyses of the serpentine–magnetite ores and the magnesium-rich wall rocks revealed a wide range of very negative δ26Mg values from −1.20‰ to −0.34‰ with an average value of −0.80‰. The value is higher than that of ultramafic rocks (δ26Mg = −0.25‰) and exhibits minor Mg isotopic fractionation. However, these values are consistent with those of marine carbonate rocks, which have lower δ26Mg values and larger Mg isotopic variations (δ26Mg = −0.45‰ to −4.5‰). Collectively, the S–Mg–B isotopic characteristics of the Zhaoanzhuang iron orebodies clearly indicate a chemical sedimentary origin. The protoliths of these orebodies most likely reflect a series of Fe–Si–Mg-rich marine carbonate rocks with a considerable evaporite component, indicating a carbonate-rich superior-type banded iron formation precipitated in an evaporitic shallow marine sedimentary environment.

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