No 3 (2023)

Illite-smectite (I-S) minerals from the Upper Jurassic oil-bearing rock shales of Denmark and the North Sea were studied by a complex of diffraction and spectroscopic methods. Detailed structures were identified to reveal the mechanism of diagenetic transformation of these shales. Usually, in oil-bearing rocks of sedimentary basins, oil generation occurs simultaneously with the I-S diagenetic transformation. The results obtained demonstrate the relationship between these two reactions: NH₃ molecules released from kerogen during ma-ximum oil formation are fixed as NH₄ ammonium cations in smectite or vermiculite interlayers, forming mica or tobelite structural fragments. As a result of this solid-phase transformation, mixed-layer structures are formed, consisting of layers of illite, tobelite, smectite and vermiculite (I-T-S-V).

The article provides an overview of various indices/indicators (Vogt, Parker, CIA, CIW, PIA, MIA, etc.) used in the study of weathering profiles/crusts and reconstruction of paleoclimatic environments of sedimentary sequence accumulation. Their possibilities are demonstrated by the example of Vendian-Lower Cambrian terrigenous deposits of Podolsk Transnistria (southwestern slope of the Ukrainian Shield). The distribution of the ba₁ index values in this section indicates the presence of the material most transformed by the processes of chemical weathering in the mudstones of the Nagoryany Formation, the lower part of the Danylivka and the middle part of the Studenytsya formations. For mudstones of the Danylivka–Zbruch interval, the HM values are close to the HM_PAAS. For rocks of the Yaryshiv–Nagoryany interval, the HM values are slightly higher, and for mudstones in the lower part of the Yaryshiv Formation, they are comparable to those inherent in continental clays of a hot tropical climate. The average SA index for mudstones of the entire section is 5.6 ± ± 0.7. Mudstones of the Hrushka–Nagoryany interval, where SA < SA_PAAS, are composed of more weathered material. The WIP values in the mudstones of the Mohyliv and Yaryshiv formations, as well as in the upper part of the Zbruch Formation, correspond to the interval of their values between PAAS and the average Archean granite. Clay rocks of other formations have WIP ≤ WIP_PAAS values. The average CIA value for mudstones is 71 ± 4, which practically corresponds to the CIA value (70), which separates the sediments of cold/arid and warm/humid climates. Variations in the CIW index along the section are symbate with variations in the CIA. The vast majority of mudstones are characterized by PIA > PIA_PAAS values. The average CPA value is 91 ± 4, which is also typical for PAAS. The above and other data show that on the basis of a “direct” interpretation of the values of various indices of chemical weathering inherent in fine-grained clastic rocks, the paleoclimate that existed in the Vendian‒Early Cambrian on the territory of Podolsk Transnistria was rather moderate or warm humid. Comparison of the CIA values of mudstones with the values of this indicator for the particulate suspended matter of modern rivers suggests that the climate in the Vendian‒Early Cambrian resembled dry and humid subtropical or dry tropical with elements of humid.

The article presents the results of biostratigraphic and U-Pb geochronological (detrital zircons) studies of the volcanic-terrigenous lacustrine member of the upper subformation of the Lower Paleozoic Oldyndinskaya Formation, which contains pyrite-polymetallic ores of the Ozernoye deposit (Kurbino-Eravninsky ore region, Western Transbaikalia). The first, second and “crystal tuff” productive horizons of the first ore-bearing level of the ozernaya member were studied. It is represented by an alternation of tuffs, calcareous, siliceous, carbonaceous tuffites, pelitomorphic limestones, limestone gravelstones with interlayers and lenses of mine-ralized tuff-conglobreccia and layers of banded siderite pyrite ores. For the first time, bryozoans, algae, and palynoflora were found in calcareous tuffaceous siltstones and limestones of the second and “crystalline tuff” productive horizons. These data indicate the Early Carboniferous, Tournaisian time of sediment accumulation. The results of U-Pb geochronological studies of detrital zircons from the mineralized tuff-conglobreccia of the third productive horizon limit the lower limit of the rock formation time as not older than the Late Cambrian.