Paleoclimatic variability in the southern Tethys, Egypt: Insights from the mineralogy and geochemistry of Upper Cretaceous lacustrine organic-rich deposits

Fathy, Douaa; Wagreich, Michael; Ntaflos, Theodoros; Sami, Mabrouk

doi:10.1016/j.cretres.2021.104880

Cited by 33 publications

(19 citation statements)

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“…The high abundance values of gammacerane and the corresponding gammacerane index (GI = gammacerane/17α(H), 21β(H) C 30 hopane) usually serve as indicators of saline-hypersaline depositional environments and waterbody stratification. , The presence of low-abundance gammacerane and very low GI values (0.01–0.06, average 0.03) imply no development of saline water settings and waterbody stratification during the deposition of the Chang7 3 submember. The ratio of Sr/Ba can be used as an indicator of water salinity, with Sr/Ba < 0.6 signifying freshwater; 0.6 < Sr/Ba < 1.0, brackish water; and Sr/Ba > 1.0, saline water. , However, it should be noted that this parameter is not suitable for samples with a high concentration of CaO, which benefits the substitution of Sr 2+ for Ca 2+ and leads to unreliable Sr/Ba ratios . A relatively low concentration of CaO (mostly less than 2.0%) in studied samples and Sr/Ba ratio values of generally <0.6 (Table and Figure a) indicate a fresh-brackish water environment for the study sections.…”

Section: Discussionmentioning

confidence: 99%

“…The ratio of Sr/Ba can be used as an indicator of water salinity, with Sr/Ba < 0.6 signifying freshwater; 0.6 < Sr/Ba < 1.0, brackish water; and Sr/Ba > 1.0, saline water. 36,37 However, it should be noted that this parameter is not suitable for samples with a high concentration of CaO, which benefits the substitution of Sr 2+ for Ca 2+ and leads to unreliable Sr/Ba ratios. 38 A relatively low concentration of CaO (mostly less than 2.0%) in studied samples and Sr/Ba ratio values of generally <0.6 (Table 3 and Figure 10a) indicate a freshbrackish water environment for the study sections.…”

Section: Left)mentioning

confidence: 99%

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Controls on Organic Matter Accumulation of the Triassic Yanchang Formation Lacustrine Shales in the Ordos Basin, North China

et al. 2021

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Lacustrine shales in the third submember of the Chang7 (Chang7 3 ) of the Triassic Yanchang Formation have the highest oil and gas generation potential in the Ordos Basin, North China. To unravel factors governing organic enrichment within this submember, Rock-Eval pyrolysis, major and trace elemental analyses, and molecular composition of extractable organic matter were applied for redox condition, paleosalinity, dilution effect by terrestrial input, paleoproductivity, and paleoclimate condition investigation. The total organic carbon (TOC) contents of the Chang7 3 organic-rich lacustrine shales show a tripartite feature and can be divided into the upper organic-rich section (UORS, average TOC 6.8 wt %), the middle organic-lean section (MOLS, average TOC 3.5 wt %), and the lower organic-rich section (LORS, average TOC 6.7 wt %). The variation of the productivity-related paleoclimate is likely the main driving force leading to the change of organic richness within the Chang7 3 submember. The MOLS was deposited under a relatively hot and arid climate (high Sr/Cu but low Rb/Sr values) with lower paleoproductivity (low P org /Ti and P org values). Additionally, clastic dilution may further reduce the TOC content to a certain extent in the MOLS. The UORS and LORS, however, were deposited under a warm and humid climate, which leads to enhancement of chemical weathering (high Ln(Al 2 O 3 /Na 2 O) values), increased nutrient input, and elevated paleoproductivity. Furthermore, paleoproductivity of UORS and LORS was further boosted by additional key nutrients, such as Fe and P 2 O 5 , provided by syn-depositional volcanic ash. Both paleoredox (U/Th, C org /P, and Pr/Ph) and paleosalinity (Sr/ Ba, gammacerane index) proxies suggest no noteworthy variation of redox and salinity conditions throughout the Chang7 3 interval.

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Section: Discussionmentioning

confidence: 99%

Section: Left)mentioning

confidence: 99%

Controls on Organic Matter Accumulation of the Triassic Yanchang Formation Lacustrine Shales in the Ordos Basin, North China

et al. 2021

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“…The formation and distribution of source rocks is mainly controlled by sedimentary basin, palaeoclimate, palaeohydrodynamic conditions, productivity, and preservation of organic matter [6]. Therefore, geochemical characterization of the source rocks is a key to understanding their type and origin of organic matter, thermal maturity, capacity for hydrocarbon generation, palaeoceanographic, and paleoclimatic conditions [7,8].…”

Section: Introductionmentioning

confidence: 99%

Geochemical Characterization of the Eocene Coal-Bearing Source Rocks, Xihu Sag, East China Sea Shelf Basin, China: Implications for Origin and Depositional Environment of Organic Matter and Hydrocarbon Potential

et al. 2021

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Eocene coal-bearing source rocks of the Pinghu Formation from the W-3 well in the western margin of the Xihu Sag, East China Sea Shelf Basin were analyzed using Rock-Eval pyrolysis and gas chromatography–mass spectrometry to investigate the samples’ source of organic matter, depositional environment, thermal maturity, and hydrocarbon generative potential. The distribution patterns of n-alkanes, isoprenoids and steranes, high Pr/Ph ratios, abundant diterpanes, and the presence of non-hopanoid triterpanes indicate predominant source input from higher land plants. The contribution of aquatic organic matter was occasionally slightly elevated probably due to a raised water table. High hopane/sterane ratios and the occurrence of bicyclic sesquiterpanes and A-ring degraded triterpanes suggest microbial activity and the input of microbial organisms. Overwhelming predominance of gymnosperm-derived diterpanes over angiosperm-derived triterpanes suggest a domination of gymnosperms over angiosperms in local palaeovegetation during the period of deposition. The high Pr/Ph ratios, the plot of Pr/n-C17 versus Ph/n-C18, the almost complete absence of gammacerane, and the distribution pattern of hopanes suggest that the samples were deposited in a relatively oxic environment. Generally, fluctuation of redox potential is coupled with source input, i.e., less oxic conditions were associated with more aquatic organic matter, suggesting an occasionally raised water table. Comprehensive maturity evaluation based on Ro, Tmax, and biomarker parameters shows that the samples constitute a natural maturation profile ranging from marginally mature to a near peak oil window. Hydrogen index and atomic H/C and O/C ratios of kerogens suggest that the samples mainly contain type II/III organic matter and could generate mixed oil and gas.

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“…Sediment color is considered one of the best indicators of climate. 6 − 10 It is used to identify rock types and divide and compare the strata, and it is an important index of climate and environmental change. 11 , 12 According to the genetic sediments, the color of the sediments can be categorized into inherited color, authigenic color, and secondary color.…”

Section: Introductionmentioning

confidence: 99%

“…The color of clastic rocks is mainly caused by dyeing substances such as iron-containing compounds and free carbon, and the color can be divided into two broad categories: “red” and “gray”. 11 Red represents the oxidizing environment, and gray represents the reducing environment; 10 yellow to orange, brown, maroon, deep purple, and red belong to the “red” category due to the rock’s iron oxide or hydroxide staining. 14 Dark gray to light gray, brown, bluish-gray, and green belong to the “gray” category, and the color usually darkens as the content of organic carbon or dispersed iron sulfide increases.…”

Section: Introductionmentioning

confidence: 99%

Controlling the Iron Migration Mechanism for the Cretaceous Sediment Color Variations in Sichuan Basin, China

Jiang

Xia

et al. 2021

ACS Omega

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Cretaceous continental sediments in Sichuan Basin, China, have different colors, and the reasons for their formation are not determined. Based on mineralogical and geochemical characteristics, red beds and nonred beds in the Upper and Lower Cretaceous sedimentary strata in the western Sichuan Basin are described and tested in this study. The test and analysis of the mineral composition, element content, and iron speciation of mudstone samples with gray-green, gray, and red colors in Cangxi, Bailong, and Guankou formations found that the change in hematite content directly causes the color difference of samples. For red mudstone, the average chemical index of alteration, chemical index of weathering, weather eluviation index (Ba), Ca/(Mg*Al), and Al 2 O 3 /SiO 2 index are 67.75, 79.94, 2.07, 0.26, and 0.26, respectively, indicating that chemical weathering is the most intense. The geochemical indexes corresponding to gray samples are 64.41, 74.91, 2.08, 0.19, and 0.24, respectively. Those corresponding to the gray-green samples are 62.30, 70.68, 2.17, 0.21, and 0.24, with the weakest chemical weathering. The ratio of Cu/Zn and the enrichment factor of V show that red and nonred bed samples are formed in weak oxidation and weak reduction environments, respectively. The red sample contains the highest content of hematite iron. The gray-green sample mainly represents paramagnetic ferrous in clay minerals. The geochemical contents of the gray sample’s three iron elements are slightly different, mainly trivalent iron. The change in iron speciation content in different color samples shows that the Fe element forming hematite in red bed samples may come from the weathering of source rock and clay minerals subjected to secondary weathering. At present, it is confirmed that different colors of samples are related to different weathering degrees of source rocks, which can be related to hot, dry/humid climates. It is necessary to distinguish the climate type in combination with other indicators.

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Paleoclimatic variability in the southern Tethys, Egypt: Insights from the mineralogy and geochemistry of Upper Cretaceous lacustrine organic-rich deposits

Cited by 33 publications

References 62 publications

Controls on Organic Matter Accumulation of the Triassic Yanchang Formation Lacustrine Shales in the Ordos Basin, North China

Controls on Organic Matter Accumulation of the Triassic Yanchang Formation Lacustrine Shales in the Ordos Basin, North China

Geochemical Characterization of the Eocene Coal-Bearing Source Rocks, Xihu Sag, East China Sea Shelf Basin, China: Implications for Origin and Depositional Environment of Organic Matter and Hydrocarbon Potential

Controlling the Iron Migration Mechanism for the Cretaceous Sediment Color Variations in Sichuan Basin, China

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