Organic matter in Upper Devonian deposits of the Chernyshev Ridge

The article deals with the lithology and geochemistry of organic matter of Upper Devonian-Lower Carboniferous deposits (Carbonate Devonian) of the eastern part of the South Tatar arch. Rocks of this age have a high generative potential due to increased concentrations of organic matter (OM) and its phytogenic and zoophytogenic composition. They refer to non-traditional sources of hydrocarbons. The generative potential depends on the facial environment and the transformation of OM. The highest potential have carbonate-siliceous and siliceous-carbonate rhythmites domannic horizon.

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Composition, structure and model of the formation of conodont lamellar tissue

LITOSFERA ◽

10.24930/1681-9004-2020-20-2-184-195 ◽

2020 ◽

Vol 20 (2) ◽

pp. 184-195

Author(s):

A. V. Zhuravlev

Keyword(s):

Organic Matter ◽

Light And Electron Microscopy ◽

Carbon Isotopic Composition ◽

Carbon Isotope Ratio ◽

Growth Cycle ◽

Structural Features ◽

Major Elements ◽

Upper Devonian ◽

Thin Sections ◽

East European

Research subject. This research was focused on the most common mineralized tissue that composes conodont elements. The aim was to investigate the characteristics of the composition and structure of this tissue and to reconstruct its formation process. Materials and methods. The work was based on a collection of well-preserved conodont elements from the Upper Devonian of the East European Platform and the Upper Devonian – Lower Carboniferous of the east of the Pechora Plate. Oriented and polished thin sections made from some of the elements were studied using light and electron microscopy, as well as a microhardness tester. Energy dispersive spectroscopy was used to determine the chemical (elemental) composition of the lamellar tissue. In addition, the carbon isotope ratio was determined for organic matter. Results. The study showed that the lamellar tissue in conodont elements consists of fluorohydroxylapatite crystallites of various morphology, surrounded by organic matter, which makes up 2–3% of the tissue. Variations in the composition of major elements incorporated in fluorohydroxylapatite of the lamellar tissue are insignificant. Organic matter is represented by a collagen-like protein, likely to be of a non-fibrillar type, with a light carbon isotopic composition (–26.2 ‰ PDB). The lamellar tissue has an average microhardness of 2.6 GPa, the variations of which are due to textural and structural features and the distribution of organic matter. In conodont elements, the lamellar tissue is in contact with other types of tissue. Transitions between tissues are relatively sharp at the borders of the lamellae and gradual within the same lamella. Conclusions. A model was developed, according to which the growth cycle of a conodont element covered the sequential formation of two lamellae preceded by the resorption of one external lamella. In the structures formed by the lamellar tissue, both lamellae consisted of this tissue. The lamellar tissue is of interest as a natural model of an organic-mineral composite based on protein and calcium phosphate.

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New insights into organic matter accumulation from high-resolution geochemical analysis of a black shale: Middle and Upper Devonian Horn River Group, Canada

Geological Society of America Bulletin ◽

10.1130/b36091.1 ◽

2021 ◽

Author(s):

Haolin Zhou ◽

Nicholas B. Harris ◽

Tian Dong ◽

Korhan Ayranci ◽

Jilu Feng ◽

...

Keyword(s):

Organic Matter ◽

High Resolution ◽

Second Order ◽

Upper Devonian ◽

Redox Conditions ◽

Geochemical Data ◽

Third Order ◽

Data Set ◽

Organic Matter Accumulation ◽

Geochemical Proxies

Organic matter (OM) accumulation in organic matter-rich mudstones, or black shales, is generally recognized to be controlled by combinations of bioproductivity, preservation, and dilution. However, specific triggers of OM deposition in these formations are commonly difficult to identify with geochemical proxies, in part because of feedbacks that cause geochemical proxies for these controls to vary synchronously. This apparent synchronicity is partly a function of sample spacing, commonly at decimeter to meter intervals, which may represent longer periods of time than is required for the development of feedbacks. Higher resolution data sets may be required to fully interpret OM accumulation. This study applies a novel combination of technologies to develop a high-resolution geochemical data set, integrating energy-dispersive X-ray fluorescence (EDXRF) and infrared imagery analyses, to record proxies for redox conditions, bioproductivity, and clastic and carbonate dilution in millimeter-resolution profiles of 133 core slabs from the Middle and Upper Devonian Horn River shale in the Western Canada Sedimentary Basin, which provides decadal-scale temporal resolution. A comparison to a more coarsely sampled data set from the same core results in substantially different interpretations of variations in bioproductivity, redox, and dilution proxies. Stratigraphic distributions of organic matter accumulation patterns (bioproductivity-control, siliciclastic/carbonate-dilution, and redox conditions-control) show that organic enrichment events were highly varied during deposition of the shale and were closely related to second- and third-order sea-level changes. High-resolution profiles indicate that bioproductivity was the predominant trigger for organic matter accumulation in a second-order highstand, particularly during deposition of third-order transgressive systems tracts. Organic matter accumulation was largely controlled by dilution from either carbonate or clastic sediments in a second-order lowstand. Bioproductivity-redox feedbacks developed on timescales of decades to centuries.

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