Lithology and Mineral Resources最新文献

This paper provides the first modern day overview of fossil resin finds in western Central Asia, which is located far beyond the traditional resin-bearing regions both in Russia and in northern West Europe. Based on little-known facts and random references in the geological literature, the main locations of fossil resins were revised and described, and the level of their study is assessed. The first FTIR study of the molecular structure of fossil resins in the Aral region showed that their classification as amber (in modern understanding – succinite assigned to the group of viscous resin) is not correct. They are represented by exclusively fragile varieties (retinite and gedanite) and, therefore, have no practical significance as jewelry raw material.

Based on the lithological and gas-geochemical studies and a comprehensive interpretation of the available materials, the main factors of the formation and distribution of grain size distribution, water-physical properties, organic saturation, as well as concentrations and geochemical parameters of hydrocarbon (HC) gases in seafloor sediments of geostructures in the western part of the East Siberian Sea are summarized and analyzed. It has been established that the sediments in the northern and central parts of the studied region are represented by aleurite–pelites; in the southern part, by aleurite–pelite–psammites with the following parameters: C_org—0.6–2.0%, natural moisture content (hereafter, wetness)—18–43%, density—1.5–2.0 g/cm³, open porosity—17–33%, concentrations of methane and its homologs—0.001–5.934 and 0.00003–0.0312 cm³/kg, respectively, molecular mass of the HC fraction—16.05–22.6 g/mol, coefficients of “wetness”, “dryness”, and transformation—0.2–51, 1–1999, and 0.2–50.8%, respectively, and δ¹³С‒СН₄ varying from –82.7 to –38.4‰. Based on the values of gas-geochemical parameters, eleven types of gas sources were identified in bottom sediments. The results showed that the formation of HC anomalies in sediments is governed mainly by their reservoir properties, C_org content, sampling depths, integrated influence of geological factors, such as gas saturation of the underlying sediments, type of gas sources, fault and fold tectonics, position of geostructures, thickness of Quaternary sediments, content of coal and gas, and age of the folded basement. Anomalies of HC gases are associated to a lesser extent with the lithological composition and density parameters of sediments.

New data on the mineral composition of Lower Triassic terrigenous rocks in the northern Pechora oil- and gas-bearing basin are presented. The relevance of the study is due to the fact that the terrigenous natural reservoirs confined to this part of the section have heterogeneous structure. The purpose of the article was to identify the features of the formation and distribution of minerals that fill the void space of the reservoirs. The optical microscopic, X-ray phase, and electron microscopic study revealed diverse postsedimentary transformations of sandy reservoirs. It is shown that the most widespread are quartz regeneration, transformation of mica-group minerals, as well as the formation of calcite of various generations and authigenic clay minerals. Diagenetic changes in terrigenous rocks play a significant role in the formation of reservoir properties and promote the heterogeneity of reservoirs. The large amount of calcite in sandstone cement leads to a significant decrease in filtration and capacity parameters. The pore cement of chlorite‒smectite composition in the fine-grained sandstones also causes a decrease in the reservoir quality. The coarse- and medium-grained sandstones with a low content of cement (predominantly of the kaolinite or chlorite composition) are characterized by the higher reservoir properties. The chloritic crustification and kaolinitic pore-filling cement facilitate the formation of higher quality reservoirs.

The comparative characteristics of sedimentary successions corresponding to the paleoecological event OAE 2, which are spaced over 1000 km apart within the Crimean–Caucasian region, are present. It is shown that the sediments accumulated during this time span under different sedimentary conditions are featured by the similar structures. The sedimentological, geochemical, and paleoecological analyses, indicate that (1) stable oxygen deficient conditions were not spread in the water column of the paleobasin; they occurred locally in a thin layer of bottom water owing to the oxidation of organic matter accumulated on the seafloor; (2) geochemical anomalies inherent in the TOC-rich horizon extend to the embedding layers that indicate their involving to the paleoecological event. The carbonaceous sediments of OAE 2 formed due to the enhanced nutrients influx from the land during the eustatic transgression caused the dramatic increase in the plankton productivity. The transgression was complicated by frequent sea-level fluctuations of minor magnitude associated with the Milankovitch precession cycles.

The results of analyzing litho- and isotopic geochemical indicators of the composition of paleocatchment areas, paleoclimate, and paleobioproductivity in rocks of the Mariinsk–Nokhtui interval (Upper Riphean?–Vendian–Lower Cambrian) in the northern Patom Highland (Lena–Zhuya region) are presented. It was found that the Th/Sc, Th/Co, and (La/Yb)_N values in the fine-grained clastic/clayey rocks, as well as the age of detrital zircons in sandstones of the Mariinsk–Barakun interval and the overlying deposits, are different, indicating the emergence of new sources of fine-grained aluminosiliciclastics in the pre-Ura or Ura periods. This is consistent with the conclusions in previous studies. Significant increase in ε_Nd (t) values and decrease in Nd model ages, recorded at the base of the Nikol’sk Formation, can be traced upsection to the Early Cambrian Nokhtui Formation. Based on the geochemical data on sediments in large modern rivers, the Vendian and Early Cambrian paleocatchment areas were likely composed of rocks influenced by the humid subtropical and tropical climate. The absence of a prominent negative correlation between the EF_P value and the ratios of several trace elements reflecting the rock composition in the paleocatchment areas, and, presumably, sedimentary basin types, suggests the lack of any significant relationship between the paleoproductivity and geodynamic settings in the Vendian.

Improper use of alluvial aquifers and declining water levels in these aquifers have led to significant attention to the search for other groundwater sources in karst areas and hard formations. Iran is one of the countries that has a very high percentage of karst in the world and the study and tracing of karst are very important in this country. In this research, eight effective factors in karstification are identified and categorized into two groups and the relevant information layers are prepared. The first group includes average annual rainfall, type of geological formations, the density of fractures, and density of the fracture site. The second group, which is of secondary importance, includes the average annual temperature, the density of canals, slope, and type of vegetation. After preparing the information layers, quantitative parameters based on fuzzy logic and qualitative parameters were standardized by the rasterization method. The importance and weight of each of the effective factors were determined from two methods viz. Analytic Hierarchy Process (AHP) and the Fuzzy Hierarchical Analysis Process (FAHP). After multiplying the standard maps with their respective weights, they were overlaid, and finally, karst potential maps were generated. Maps were generated from the two weighting methods based on the subsurface and surface hydrogeological-geomorphological diagrams. Karst features such as Karen, dissolution cavities, depressions, caves, dry valleys, reservoir rocks of springs and water exploitation wells, and Calcareous limbs were ascertained, which indicates that the results of the FAHP method are more consistent with the existing conditions in the region. The final map was divided into five classes in terms of the karst potential, viz. very high, high, medium, low, and no potential.

The C–O–Sr isotope composition of carbonates and the S–Sr isotope composition of gypsum were studied in the Ordovician section and the Upper Cambrian–Lower Silurian boundary horizons of the Vilyui structural-facies zone located in the central Siberian Platform. The upsection decrease of the ⁸⁷Sr/⁸⁶Sr ratio within 0.70920‒0.70795 and the value of δ³⁴S ~ 25‒28‰, typical for the reference sections of this stratigraphic interval, indicates a good connection between the basin and the World Ocean in the Ordovician and Early Silurian, but its isolation in the Late Cambrian (Kholomolokh Formation) when the ⁸⁷Sr/⁸⁶Sr ratio (0.7085) was lower than the oceanic one. The sharp ⁸⁷Sr/⁸⁶Sr ratio decrease from ~0.7087 to 0.7080 in the Khar’yalakh Formation (~45 m thick) can be correlated with the interval from the upper Darriwilian-to-upper Sandbian stages of the ICC (Middle Ordovician Volgian horizon–Upper Ordovician Baksanian or Dolborian horizon of the Russian GSS). Based on this benchmark and lithological features of the section, we discuss the causes of wide variations in δ¹³С (‒7.2 to 1.6‰) and δ¹⁸О (16.9‒29.8‰) in the carbonates, as well as a possible correlation of carbon isotope excursions in the Vilyui section with the global carbon isotope events.

Detailed sedimentological and sequence stratigraphic analysis of the upper Paleocene–lower Eocene shallow-marine limestones exposed in the Eastern Gafsa basin in southern Tunisia provides a new insight into the sedimentary response to climate and sea-level changes revealed in the southern Tethysian margin. The 81 m thick Ayaycha Formation is composed of three units. The lower unit is built up of channelized fossiliferous limestones superposed by the sequence of alternated thin–to medium-bedded limestone and marls. The second unit is dominated by marls, and the third unit is formed by channelized limestones lying within bioclastic limestones. Based on their lithological features and strata geometries, the Ayaycha Formation exhibits seven facies evolving from offshore to intertidal environments. Clay paragenesis shows that the lower unit was accumulated under the warm and seasonally contrasting climate, or probably, repeated change of dry and humid seasons. The lower unit corresponds to the upper Paleocene deposits. The upper Paleocene-lower Eocene, middle and upper units were accumulated under warm climate and cover the time interval of global paleoecological crisis known as the Paleocene–Eocene Thermal Maximum (PETM). The consequence of this global warming was the sea level rise, which correspond to the transgressive pulse occurred prior to the Paleocene–Eocene boundary. Integrated sequence stratigraphic analysis showed that the Ayaycha Fm is formed by the stacking of thirteen third-order depositional sequences. Each depositional sequence results from a transgressive-regressive cycle in shallow marine environments.

The lower Maeotian carbonate encrustation of bryozoan bioherms and local problematic carbonate buildups at Cape Kazantip (Kerch Peninsula) were studied to elucidate their genesis. Analytical (lithological and mineralogical, X-ray diffractometry, scanning electron microscopy, energy dispersive spectrometry, and isotopy) studies have shown that hardness of the bryozoan framework is related to the syndepositional, biologically induced cement around bryozoans and carbonate encrustation of bioherms. In addition to fossilized traces of products of the microbiotic vital activity (bacteriomorphic structures, mineralized biofilms, glycocalyx—an exopolymer substance (IPS), and framboidal pyrite), the carbonate crusts on bryozoan bioherms and mollusk–polychaete minibioherms contain abundant bitumen, strontianite, barite, celestine, Mn-rich calcite (kutnohorite), Mg-calcite, aragonite, dolomite are widespread. Mineralized biofilms include trace elements Fe, Si, Mg, Al, K, Na, Cl, Ba, S, Ni, and Co. The isotopic composition of different carbonate rock types is marked by wide variations of carbon (–2.76…7.17‰) and oxygen (24.20–33.01‰) and manifested in fluctuations of water salinity (16.67–39.74‰). The chemical composition and mineral specificity of rocks, confinement of carbonate crusts and minibioherms to saline waters, and local pattern of their formation suggest the manifestation of near-bottom cold gas-fluid seeps, probably, of a complex chloride-sodium-sulfate-magnesium composition or various modifications of these components in a shallow sea basin, whereas the bryozoan biohermal complex is most likely a near-hydrothermal oasis.