Paleocene Glauconite of the Near-Volga Monocline of Submarine Volcanic Sedimentary Genesis

Abstract

A new concept of the formation of glauconite from the Paleocene deposits of the sedimentary cover of the southwestern part of the Near-Volga monocline is presented. It was formed as a result of halmyrolysis of ash material in lake conditions. Glauconitization is confined to the Paleogene thermal maximum and the periphery of active volcanism along the entire mobile belt. In this connection, it is widely distributed in the Paleogene deposits. Structural and crystal-chemical features of glauconite, stages of crystallization of its aggregates, paragenetic associations with smectite, kaolinite, and zeolite, as well as the presence of ore minerals indicate its authigenic origin. The authigenic origin is confined to the formations of temporary, episodically submerging shallow marine basins. An applied scheme of discrimination of glauconites by their crystallochemical features has been developed. The discrimination diagram can be used to determine the specific conditions of the sedimentation basin, which are expressed in the progradation cycles of glauconite accumulation. The cycles were controlled by induced depressions within the Russian plate.

INTRODUCTION

Glauconite is a group of dioctahedral micas with the general formula K_0.6–0.85(Fe³⁺,Mg,Al)₂[(Si, Al)₄O₁₀](OH)₂ consisting of a single-layer polymorphic modification 1M–1Md and complex compositions of octahedra that can include three main elements: Al, Fe, and Mg [1]. It is a 2 : 1 hydrous mica, where 50% of the octahedral positions are replaced by iron. It is considered by convention as an analogue of illite, i.e., a dioctahedral mica with (K + Na) < 0.85 and (Mg + Fe) > Al and Fe³⁺ > Al in the octahedral positions, which are mainly interlayer defects corresponding to the subgroup of celadonite [2]. Since glauconite is a group of highly siliceous minerals containing Si from 3.5 to 3.8 f.u., this leads to a potassium deficit compared to true mica, and as in Al-hydromicas, this potassium deficit is accompanied by greater hydration [3]. Genetically, glauconite is generally considered to be a result of marine authigenesis associated with transgressive deposits and condensed sections. It occurs as aggregates of green-colored layered silicates (globules) ranging from 0.06 to 1.00 mm in size and within the cavities of shells in sedimentary rocks ranging aged from the Late Paleoproterozoic to the Holocene. Various authors associate their formation with a wide range of initial substrates from biogenic to colloidal in their works [4]. The stratigraphic levels of glauconitization are well studied and are usually confined to hiatus in sediment accumulation. However, the stage character of formation and the trend of glauconitization over the geological time are almost unknown, and debates about the indicators of globule maturity (whether it is ferrous iron, potassium oxide, or alumina) have been quite urgent until recently. The occurrence of modern sediments with glauconite in much deeper shelf and slope areas is often extrapolated to interpret ancient sedimentary conditions, i.e., in most cases, redeposited sedimentary layers containing glauconite. However, ancient glauconite varieties are encountered in various paleoenvironments [5]. Although some researchers have tried to associate the composition of glauconite with the sedimentary environment, they did not take factors that affect its composition into account [6, 7]. Recent studies have revealed an exceptionally high glauconite content in the deposits corresponding to the Paleogene warm climate intervals at the Paleocene–Eocene nonconformable boundary [8]. In the Russian literature, the relationship between the Paleocene–Eocene temperature maximum and the high glauconite content in the deposits was barely discussed.

In view of the above, the question of determining the genetic affiliation of glauconitization to a particular sedimentary environment remains relevant. This work presents the facts indicating confinedness of glauconite-bearing deposits to the zones of volcanic degassing that affected the global temperature maximum upon their activation.

GEOLOGICAL FEATURES OF THE AREA

The research area is located at the junction of the Voronezh anteclise, Scythian Plate, Near-Volga monocline, and Great Donbass (Fig. 1). The Paleogene deposits rest on the eroded surface of the lower Maastrichtian Gremyachinskaya Formation and are overlaid by Neogene formations. They are represented by marine, primarily terrigenous, rocks of all three divisions. The depositional sequence was classified primarily according to the unified stratigraphical sectional plane for the Paleogene of the Volga and Caspian regions, using the sectional planes of the neighboring regions in southern European Russia, approved by the Bureau of the Interdepartmental Stratigraphic Committee (ISC) in January 2000. Paleocene deposits were identified as part of the Danian, Selandian, and Thanetian stages, which in the Volgograd district of the Volga region are composed of the Sysranian (Danian–Selandian), Saratov, and Kamyshin (Thanetian) formations. However, in well 13, the Danian–Selandian interval is represented by rock complexes that sharply contrast with the non-carbonate, siliceous–terrigenous Syzranian Formation of the Near-Volga monocline, but are fully consistent with the Yeyskaya Formation in the eastern Azov part of the Great Donbass. The Danian–Selandian deposits (well D71, the interval of 307.0–216.6 m) within the Yeyskaya (Syzranian) Formation (₽₁sz) consist of gray aleuritic fine-grained clayey sands that grade upward through the section into greenish dark gray quartz–glauconite and highly micaceous, clayey, and calcareous aleurolites. The aleurolites contain beds of aleurolites having the same composition and greenish gray inequigranular glauconite–quartz and quartz–glauconite calcareous sandstones on siliceous and anhydrite–calcite cement. At the top of the formation is a layer of sands and sandstones with mass accumulation of broken mollusk shells. Thanetian deposits in the Saratov (₽₁sz) and Kamyshin formations are identified within the interval of 279–260 m (well 13) and 271–256 m (well 23 (CC)) and are represented by a homogeneous stratum of fine-grained sands and quartz–glauconite micaceous aleurolites that grade into dark-gray strong glauconite–quartz micaceous siltstone on clayey–siliceous cement at the top of the stage. Based upon the dinocyst complex, the deposits in the interval of 279–267 m (well 13) belong to the early Thanetian Saratov Formation (₽₁sz) (the layers with Alisocysta margarita). The overlying sequence of siltstones and aleurolites (267–260 m) is characterized by dinocysts of the Late Thanetian Apectodinium hyperacanthum zone (Kamyshin Formation ₽₁km) [9].

Fig. 1.

The scheme of factual points of study on the lithofacial basis of the Upper Paleocene and Lower–Middle Eocene, according to [10], supplemented by structural elements. (1) Sorted, zeolite–glauconite sands of coastal currents; (2) sands, siltstones, and clays of shallow stagnant water bodies; (3) clay sands and clays of weak currents; (4) absence of these deposits. Main structures: (5) the Near-Volga monocline; (6) Karpinskii Ridge; (7) Donets Ridge.

The Paleocene layers in the study area are developed constantly and sufficiently heavily with quite a deep-water character in well D71 and are replaced with shallow water deposits in well CC [10]. To the north, the sedimentation basin becomes shallow, starting from well 13 [11]. The direct shore of the Donetsk protrusion was located slightly southwestward (see Fig. 1b). The Paleocene deposits characterized sufficiently by microfauna rest upon the upper Cretaceous eroded layers [9]. They are in contact with Eocene clayey rocks on top. The upper contact is eroded according to significant evidence, which is confirmed by the swampy–lagoon character of Eocene clays in their lower layers and by the presence of gravelstone–pebble material in the near-contact zones. The occurrence depth of the Paleocene deposits varies from 200 to 350 m. Their thickness in this area is significantly reduced and does not exceed 35–40 m in the right-bank area of the Tsimlyansk Reservoir, occasionally increasing to 75–80 m in the area of the town of Kotelnikovo, mainly due to an increase in the thickness of the upper sandy horizon.

OBJECTS AND METHODS OF STUDY

The studies were conducted on the Paleocene glauconite deposits represented by the lithologically homogeneous stratum of fine-grained quartz–glauconite micaceous sands that grade into dark-gray strong glauconite–quartz micaceous siltstones on clayey–siliceous cement at the top of the stage. The Paleocene stratum comprises two horizons: (1) the lower horizon corresponding to detrital calcareous–chert layers and (2) the upper horizon representing the extension of the upper horizon of glauconitized sands and sandstones (Fig. 2).

Fig. 2.

Lithological columns of wells and their test sites. Lithological composition: (1) sands; (2) clay sands; (3) sandstones; (4) siltstones; (5) clays; (6) aleuritic clays; (7) diatomaceous clays; (8) diatomites; (9) marls; (10) aleuritic marls. Correlation lines: (11) stratigraphic subdivisions; (12) test sites. The visual rock colors are highlighted.

We studied four samples (SS–270.8, 13–277, D71–268.1, D71–269.1) 350 g each taken from the well core and the skip shaft, as well as two bulk samples (up to 15 kg) from the skip shaft (SS in the scheme) and the cage shaft (Ca in the scheme) of the Mining and Processing Plant under construction.

The bulk chemical composition of the samples was determined using an ARL 9900 X-ray fluorescence spectrometer with a 3.6 kW semiconductor high-frequency generator (the maximum voltage/current is 60 kV—60 mА or 30 kV—120 mA).

The grain-size analysis was carried out by separating the samples into 13 fractions and by washing the fractions finer than 0.005 mm with a centrifuge according to the methodology of the first author [12]. The isolated fractions were studied under a binocular microscope in immersion fluids. The monomineral fraction for X-ray diffraction analysis was selected under a binocular microscope with some tolerance for the presence of other minerals. Ore minerals were also examined using a binocular microscope.

For X-ray diffraction analysis, the samples finer than 0.01 mm were prepared by dry grinding to an optimal size of approximately 50 μm in the sample to obtain a sufficient number of crystallites and ensure a random orientation. The samples were ground in an agate mortar. Afterwards the samples with ethyl alcohol were applied to a silicon substrate with “zero background.”

The X-ray diffraction patterns were recorded using an ARL X’tra diffractometer (Switzerland) at Voronezh State University (Russia). ARL X’tra is a full-sized θ–θ geometry powder diffractometer with a goniometer radius of 260 mm and a source, a narrow focus tube with the power of 2200 W (Cu-anodes). The instrument uses an energy-dispersive solid-state detector with a Peltier cooler to eliminate passive elements (beta-filters/monochromators) from the optical scheme of the instrument due to program separation of Kβ and fluorescent radiation. The instrumental resolution of the instrument is 0.04° 2θ. The ICDD PDF-2 database was used for qualitative phase analysis. The analysis was performed manually and/or using the Oxford Crystallographica software. Quantitative X-ray diffraction analysis by the Rietveld method was performed using the Siroquant Sietronics Pty Ltd software.

The polished samples of a conditionally monomineral fraction embedded in an epoxy resin polished puck were scanned using the JSM-6480LV scanning electron microscope (Jeol (Japan) with a combined Oxford Nordlys Max2/Inca Wave 500 microanalysis system) and an electron-probe microanalyzer (JXA-8230 Superprobe) (Moscow State University, Moscow). The Jeol JSM-6480LV electron microscope has a tungsten thermionic cathode with an accelerating voltage range from 200 V to 30 kV and is capable of obtaining electron images in secondary (with a resolution of up to 10 nm) and backscattered electrons. The complex is equipped with a combined microanalysis system that combines energy-dispersive and wave diffraction spectrometers (“Oxford Instruments,” Great Britain) with an ability to perform local electron microprobe analysis of chemical elements from B to U inclusive. The measurement locality is up to 3–5 µm, with detection thresholds up to 0.01 wt %. Local electron microprobe analysis was performed at 2–4 points in the center and at the periphery.

The thermal behavior of glauconite upon heating was studied by differential gravimetry and scanning calorimetry using a STA 449F1 Jupiter synchronous thermal analyzer at a heating rate of 10°C/min in the air atmosphere (Germany, Netzsch) (Analitika Center for Collective Use, Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, Moscow).

RESULTS

The grain-size composition of Paleocene deposits is presented in the diagram (Fig. 3). The Paleocene deposits in the selected interval of 270.5–279.0 m are represented by greenish gray, fine to medium grain, micaceous, clayey, quartz-glauconite sands. The bulk chemical composition of sands varies within the following limits (wt %): 17.75–18.18 of Al₂O₃, 0.98–2.17 of CaO, 63.73–52.97 of SiO₂, 6.25–9.19 of Fe₂O₃, 0.50–0.37 of Na₂O, 2.67–2.98 of K₂O, 1.46–1.79 of MgO, 0.18–0.52 of SO₃, and 0.029–0.110 of Cl^–.

Fig. 3.

The grain size distribution of Paleocene glauconitic deposits: (1) well 13, depth is 277.0 m; (2) well D71, 269.1 m; (3) D71, 268.0 m; (4) SS, 270.8 m; and (5) bulk samples.

The maximum content of glauconite, up to 45 wt %, was found in the fraction of 0.01–0.04 mm (Fig. 4). The grains are typical globules of round, oval, drop-like, and subangular shape (Fig. 5). Under a binocular microscope, the largest grains are dark green, and the smaller ones are light pistachio in color (Fig. 5). Ore mineralization is represented by pyrite, altered ilmenite, rutile, hematite–hydrohematite, and zircon. Pyrite (up to 5%) occurs as a thin inclusion in individual quartz grains. The size of the impregnations does not exceed 10 µm, mostly 3–5 µm. The pyrite contains nickel impurities ranging from 0.8 to 1.77 wt %. Ilmenite is recorded as round-shaped individual grains among quartz fragments in the nonmetallic matrix. The grain size reaches 200 µm. Ilmenite undergoes leucoxenization, which manifests itself in a nonstoichiometric chemical composition with predominance of Ti. Rutile forms scarce grains in the nonmetallic matrix. The size of the rutile grains is 50–100 µm. The shape is unrounded, angular, and sometimes elongated. The hematite–hydrohematite aggregate forms lace-like segregations, which were presumably formed during the replacement of existing sulfides. The sizes of the segregations are 50–100 µm. Zircon occurs in individual grains, up to 20 µm in size. The shape of the grains is isometric, round. Zircon also forms inclusions in quartz grains.

Fig. 4.

Distribution of glauconite by fractions (left columns display the sample from well 13, depth 277.0 m; right columns show the sample from well D71, depth 269.1 m).

Fig. 5.

SEM images of glauconite grains in the siltstone fraction. (a) D71-269.1; (b) and (c) 13-277.0; (d) SS-270.8.

All samples contain carbonaceous detritus in the form of fragments in the rock and represent the organic remnants.

Electron microprobe analysis. The data of the electron microprobe analysis of glauconite globules indicate a wide range of their chemical composition both along the lateral of the study area and across the section (Table 1). The measurements were made in the center and at the periphery of the grains; however, no significant changes were found in the composition, except for variation in Na₂O and TiO_2. The content of K₂O varies from 8.35 to 9.57 wt % in dark green glauconite and is usually less than 8.15 wt % in light olive globules; Fe₂O_3(tot.) is 19.82–25.45 wt %; CaO varies slightly (0.36 wt %, on average); MgO is recorded from 3.24 to 4.97 wt %.

Table 1. Chemical composition of Paleocene glauconite (wt %)*

Some glauconite grains exhibit zoning associated with variation in the content primarily of Na₂O from the values below detection limit (BDL) to 0.22 wt % and TiO₂ from the values below the minimum detection limit to 0.33 wt % in the center and at the periphery, respectively. The potassium content in glauconite globules increases from the center to the periphery and reaches 9 wt %. Other grains contain mineral inclusions that correspond in composition to minerals having a similar composition to apatite, sulfides, and plagioclases, which are mentioned above. The increased content of alumina (more than 9 wt %) is associated with the presence of minerals from the smectite group, which is confirmed by X-ray diffraction analysis upon saturation of samples with glycerin, as well as by a positive correlation of potassium with iron and a negative correlation with silicon and aluminum.

X-ray diffraction analysis. In the X-ray diffraction patterns of the almost monomineral fraction, glauconite is identified by the basal reflections d₀₀₁ (SS-270.8, 10.86; D71-269.1, 10.65; D71-268.1, 10.76; 13-277.0, 10.70 Å), as well as 4.99–5.08 and 3.32 Å (Fig. 6). The most intense nonbasal reflections of hkl are d₀₂₀ ≈ 4.51, d₀₀₃ ≈ 3.32, and d₁₁₂ ≈ 3.03 Å. The hkl (001)/(020) ratio indicates good crystallographic orientation of glauconite [13]. The asymmetric shape of d₀₀₁ corresponds to the thickness of the structural package of the mineral, which includes a 2 : 1 layer and an interlayer gap. The very weak reflection with d₀₆₀ = 1.50 Å belongs to a mixed-layer mineral in glauconite globules, and the adjacent d₀₆₀ = 1.54 Å glauconitic is represented by an even smaller intensity.

Fig. 6.

X-ray diffraction patterns of the glauconite fraction: (a) in the air-dry (natural) state, (b) in the glycerin-saturated state, and (c) heated to 550°C. Gl is glauconite, Chl is chlorite, Qtz is quartz, Zl is zeolite. (1) Deep-water, sample D71-269.1; (2) shallow-water, sample 13-277.0.

Upon saturation of the samples with glycerin, there is a common tendency toward a decrease in the values of d_hkl of the first basal reflection to 10.08, 10.0, 10.04, and 10.55 Å, respectively (Fig. 6b). After annealing for five minutes at a temperature of 550°С, they shift to 10.1, 10.3, 10.1, and 10.14 Å (Fig. 6c). These deformations of the structural inhomogeneity of glauconite indicate mixed-layer formation in its globules. This is consistent with the general opinion that all dark green glauconite grains contain some amount of mineral with swelling layers, rather than a mixture of layered silicates in one globule. Zeolites are identified by the appearance of reflections d₀₂₀ = 8.89‒8.92 Å in the X-ray patterns.

Thermal analysis. The DTG curve (Fig. 7) shows four endothermic effects. The first one is associated with initial dehydration (from 25°C to ~200°C), which corresponds to the release of absorbed moisture and some amount of weakly bound water, possibly from the external surfaces of the sample. In this case, the weight loss is 0.79%. The second and third endo-effects in the temperature range of 200–600°С and 600‒780°С, with maxima at 428 and 670°С, correspond to the stepwise expulsion of structurally bound water. In this case, the weight loss is 1.79 and 0.56%. The fourth endo-effect in the temperature range from 780 to ~1050°C, with the maximum at 852°C, is likely to correspond to the dolomite decomposition. The weight loss is 0.36% of the initial sample weight. This effect does not manifest itself in the DSC curve since it is overlapped by an exo-effect associated with organic matter dissociation. An endo-effect related to the phase transition in quartz is recorded at a temperature of 568°C.

Fig. 7.

Thermal curves of glauconite, sample D71-269.1.

Crystal chemical formulas. The crystal chemical formula of glauconite is calculated based on the total content of tetrahedral cations (Si, Al, Cr)₄ and the anion part, which is close to О₁₀(ОН)₂; possible slight deviations should not decrease the number of oxygen atoms below ten since these atoms are included in the coordination of tetrahedral cations:

$${{\left( {{{{\text{K}}}_{{{\text{0}}{\text{.80}}}}}{\text{N}}{{{\text{a}}}_{{{\text{0}}{\text{.01}}}}}{\text{C}}{{{\text{a}}}_{{{\text{0}}{\text{.03}}}}}} \right)}_{{0.84}}}{{\left( {{\text{S}}{{{\text{i}}}_{{{\text{3}}{\text{.63}}}}}{\text{A}}{{{\text{l}}}_{{{\text{0}}{\text{.36}}}}}{\text{C}}{{{\text{r}}}_{{{\text{0}}{\text{.01}}}}}} \right)}_{{4.0}}}{{({\text{Fe}}_{{1.55}}^{{3 + }},{\text{M}}{{{\text{g}}}_{{0.45}}})}_{{2.0}}}{{{\text{O}}}_{{10.04}}}{{\left( {{\text{OH}}} \right)}_{{1.99}}}.$$

This method is the most justified because, in the crystal structure of minerals, the number of oxygen atoms per cell always remains constant regardless of the character of isomorphic substitutions, and structures with a deficient number of oxygen atoms are hardly found.

According to this formula, the water content is 3.98 wt %, which coincides with the theoretically possible value of 4.0 wt % for the idealized formula of glauconite (K,Na)_1.0(Si_3.0Al_1.0)₄Fе$_{2}^{{3 + }}$O₁₀(OH)₂ and with a water content scatter within 3.8–4.2% for earlier studied numerous samples of glauconite [14]. As evidenced from the thermal analysis data, the water content of 5.4 wt % in glauconites is obviously the result of the incorporation of a small number of illite fragments containing interlayer water between the glauconite layers. This is also confirmed by the values of interplanar spacings (d₀₀₁ 10.6–10.7 Å) in X-Ray powder diffraction patterns, while interplanar spacing for pure glauconite does not exceed 10.1 Å.

DISCUSSION

In the Paleogene deposits and especially during the Eocene, glauconite is widely distributed in all continents but is always confined to shelf zones of sedimentation. In the area studied, the glauconite mineralization belongs to the Paleocene terrigenous-(clayey)-quartz formation [15]. In our study, we found that neither quartz nor glauconite shows signs of redeposition, and their weak rounding and good sorting indicate their in-situ formation in the sedimentation basin, which represents shallow connected lakes [16]. The Paleogene deposits in the Near-Volga monocline cover also have minerals of igneous origin (pyrite, altered ilmenite, rutile, hematite–hydrohematite, individual grains of zircon, carbonaceous matter, zeolite, etc.) without signs of transport and with a negligibly small number of clays. These fine-grained sands and siltstones are characterized by an extremely poor association of phytoplankton, which may indicate a common stage of the paleobasin development [9]. Globules and angular grains in glauconite are often associated with syneresis cracks in the 0.01–0.04 mm fractions in fine-grained sands and siltstones. In such a sedimentation system, potassium feldspar first disappears from fine fractions, while their corroded relics are preserved in coarse-grained sandstones until metamorphism.

The described features of glauconite can be explained as follows. The most common and reliably established potassium donor is potassium feldspar, which undergoes intense intralayer dissolution and plagioclase in the volcanic series losing noncoherent potassium cations during albitization. The grains often contain mineral inclusions with the composition corresponding to minerals similar to apatite, sulfides, and plagioclases. We note the high contents of TiO₂ to 0.33 and Cr₂O₃ to 0.26 wt % in glauconite, which is associated with basitic pyroclastics as the source material.

According to the data of basal reflections, the structural inhomogeneities of glauconite are associated with swelling layers corresponding to smectite in their characteristic. The growth of glauconite on iron-rich smectite was accompanied by an increase in the interlayer charge and potassium saturation due to the loss of swelling layers and octahedral iron reduction [17]. This is also confirmed in the cases of oxidation that contributed to the formation of Fe³⁺ in smectite when it is still contained in high amounts in glauconite grains. Iron oxidation also indicates galmirolysis, which resulted in potassium leaching from potassium feldspar and possibly biotite (or phlogopite), muscovite, etc. Consequently, the increase in potassium and the reduction of iron-rich smectite layers in the structure are recorded under moderately reducing (post-oxygen) conditions. The development of silica in foraminiferal shells indicates the control of oxidation of iron in smectite by calcite dissolution. This may relate the deposition of glauconite on the South Ocean shelf with the abundant distribution of primary biogenic opal. The discovery of the first few percentage points of zeolite and traces of dolomite suggests a high temperature of the source minerals that were readily incorporated into the sedimentation basin.

According to the crystal chemical discrimination method for layered silicates proposed by the first author [18, 19], the studied grains of glauconite occupy an intermediate position between conditionally iron-rich and aluminum hydromicas (Fig. 8). It differs from illite in the presence of a large amount of trivalent iron and from celadonite in the significant replacement of silicon by aluminum. The trend in crystalline evolution is traced in the diagram from highly smectitized glauconite as the initial stage of its formation to minerals with a significant potassium content that reflect the long-term conditions of its formation, which can be seen from the geological conditions of its occurrence in various water bodies. The change in the geological character of sedimentation is caused by tectonically induced subsidence.

Fig. 8.

Discrimination diagram of glauconite grains and their paragenetic associations according to the procedure in [18, 19] with additions. (1) SS-270.8; (2) D71-269.1; (3) D71-268.1; (4) 13-277.0; (5) glauconite and berthierine from the encyclopedia (Huggett, 2013; six analyses); (6) chemical composition covariance fields: (1) Paleogene glauconite grain field (Bansal, 2018; 32 analyses; Simakova, 2020; nine analyses); (2) ferruginous illite (Bandopadhyay, 2007; 73 analyses); (3) Al-glauconite-skolite (Busharina, 2002; five analyses); (4) odinite (Harding-Nash, 2014; three analyses); (5) nontronite (Dir, 1966; Dodatko, 1969; Drits and Kossowskaya, 1990; Nikulin, 2008; Chi Ma; 1998; 19 analyses); (6) celadonite (Dorfman, 1965; three analyses). Atomic proportions: “Si” = SiO₂, “Al” = Al₂O₃ + TiO₂, “Mg” = MgО + MnО.

Thus, detailed study of the evolution of the composition and properties of glauconite in biostratigraphically limited Paleogene deposits sheds light on the source of material for such synchronous glauconite formation in the Paleogene from the Atlantic Ocean to Indonesia. The Mediterranean (Alpine–Himalayan) mobile belt (with active volcanism at that time) was developed from the Pyrenees through the Eastern Atlas, Northern Apennines, Alps, Carpathians, Hellenides, Black Sea–Anatolian zone, Caucasus, Turkestan–Iranian, and Afghan–Pakistan segments to the Himalayas and the folded structures of Burma and Indonesia. When glauconite sediments began to deposit throughout the mobile belt, there was a paleoclimate temperature maximum caused by the greenhouse effect from the widespread active volcanoes. The Paleocene–Eocene thermal maximum (PETM) was recorded ~56 Myr when the peak of global warming for about 5 kyr was caused by the emission of as many as 10 000 gigatons of carbon into the atmosphere from volcanic sources [20]. As is known, volcanic ash material is highly specialized in terms of mineral and grain-size composition. It corresponds in a remarkable manner in Near-Volga characteristics to Paleocene glauconites in the Volga monocline and from all other basins of glauconite sedimentation adjacent to the mobile belt of the territories. In the Late Eocene sections, there was a boom in the development of biogenic silica, which led to high species diversity and an increased burial of biogenic opals in the Southern Ocean. Currently the Southern Ocean also plays a key role in global biogenic silica production. In this regard, flooding in the Paleocene might have been caused by periodic melting of the snow cover. The progradation cycles of glauconitized layers mark the transition from suboxic to oxic conditions, conforming to tectonically induced subsidence and subsequent filling of the paleobasin.

In the Paleogene section of the Near-Volga monocline, at least three layers of glauconitization development are identified [18], indicating the important role of rapidly changing climate conditions in its formation. Sediments were deposited near the fissure volcanism areas in subaerial or very shallow water conditions. This is also evidenced by the overlying sediments represented by argillites with remnants of shallow-water fauna. At the base of the argillites lie glauconitic sands, which were formed as a result of ash expulsions (andesibasalt tephra) and possibly their ashfall in snow covers (at negative temperatures) in orogenic systems at the periphery of the mobile belt. Thus, the initial material was ash, which was initially well-sorted, having a fine sand but more often silt size. The main component of ash is fragments of basaltic glass, fresh and altered (palagonite, montmorillonite, celadonite). The crystal-vitreous ash ranges from red- and yellow brown to green of different shades in color. As a result of galmyrolysis during sharp temperature changes, amphiboles and basaltic glass were recrystallized (as in the case of bentonite formation on the slopes of paleovolcanos) to form glauconites. The trend of ash material transformation is expressed well in the distribution diagram of glauconites and associated minerals. The good sorting of the quartz component without signs of wave-cut conditions, as evidenced by the angular shape of the quartz grains and the absence of layering in the deposits, confirms the in-situ formation of glauconite. In a shallow-water basin or a coastal part of the subsiding basin, the recrystallization of ash material consisted of significant development of a mixed-layer mineral of smectite and glauconite. With the subsidence of the sedimentation basin, glauconite was saturated with potassium. The variations of glauconite in the diagram make it possible to determine the dynamics of the development of the paleowater body and the number of cycles of shallowing or subsidence.

CONCLUSIONS

The compared section of the study area indicates that the deposits were formed as a result of galmyrolysis of ash material under lake conditions.

Glauconitization is confined to the PETM and the periphery of active volcanism across the whole mobile belt, and therefore has a planetary distribution in the Paleogene deposits.

The structural and crystal-chemical features of glauconite, the stage-wise crystallization of its aggregates, paragenetic associations with smectite, zeolite, and kaolinite, and the presence of ore minerals indicate that its formation is confined to the formations of temporary, occasionally shallowing marine facies and has an authigenic origin.

The application scheme developed for discrimination of glauconites from other hydromicas by their crystal-chemical features can be used to determine the specific character of its formation and further transformations depending on the conditions of the sedimentation basin.