Depositional Conditions of Cretaceous Ironstones Deposit in the Chulym-Yenisey Basin (Western Siberia)

This study reconstructs the depositional conditions of ironstones within the Chulym-Yenisey basin and assesses the iron source. The detrital minerals of the studied deposits include quartz and feldspar. The authigenic minerals are goethite, siderite, aragonite, dolomite, calcite, apatite, barite, and pyrite. The clay components include minerals of the chlorite group (possible chamosite), nontronite, kaolinite, illite, and beidellite. Local bacterial sulfate reduction led to the formation of pyrite framboids in siltstone layers. The subsequent diagenetic iron reduction promoted the formation of chamosite from siderite. The goethite precipitation occurred in an oxidic aqueous environment. The Cretaceous continental sediments of the Ilek and Kia Formations of the Chulym-Yenisei depression consist of fine- and medium-grained, cross-stratified, poorly sorted litho-feldspatho-quartzose sandstones of fluvial channel origin alternating with bluish-gray siltstones and ironstones of floodplain–lacustrine–bog origin. Thin layers of iron-bearing rocks within siltstones formed in meromictic waters. The changes in geochemical proxies demonstrate fluctuations of paleoenvironmental conditions within the Cretaceous sequence. Siltstones and sandstones formed under humid and arid conditions, respectively. The primary iron source for sediments of the Chulym-Yenisey depression was determined as volcanogenic and igneous rocks of the Altai-Sayan mountainous region.

The Late Jurassic and Early Cretaceous of the Chulym-Enisey basin witnessed reduced tectonic activity, the formation of a peneplain, and the development of deep chemically weathered crusts, including the formation of an iron duricrust. Sedimentation within the Chulym-Yenisey took place in a continental setting. Variegated fine-grained sediments of the Ilek Formation were deposited during the Early Cretaceous. The deposition of the Kiya Formation took place in the Albian-Cenomanian period after the tectonic uplift of the region. Cretaceous sediments of Ilek and Kiya Formations contain a variety of continental fossils, including the following remains: Psittacosaurus sibiricus, Evgenavis nobilis, Hemicorbicula elegans, Musculiopsis dolosus, Darwinula contracta, Cypridea  [38]. (b) Early Cretaceous paleogeographic map of the region studied between the West Siberian Sea and Altai-Sayan [36]. (c) Simplified geological map of the studied Chulym-Yenisey basin and north-western part of Altai-Sayan region [52].
The Late Jurassic and Early Cretaceous of the Chulym-Enisey basin witnessed reduced tectonic activity, the formation of a peneplain, and the development of deep chemically weathered crusts, including the formation of an iron duricrust. Sedimentation within the Chulym-Yenisey took place in a continental setting. Variegated fine-grained sediments of the Ilek Formation were deposited during the Early Cretaceous. The deposition of the Kiya Formation took place in the Albian-Cenomanian period after the tectonic uplift of the region. Cretaceous sediments of Ilek and Kiya Formations contain a variety of continental fossils, including the following remains: Psittacosaurus sibiricus, Evgenavis nobilis, Hemicorbicula elegans, Musculiopsis dolosus, Darwinula contracta, Cypridea faveolata, Ginkgo digitata, Cladophlebis longifotia, Sequoia reichehbachii, Sphaerium tasaranica, etc. [34,[53][54][55][56]. The interval from the Cenomanian to the Eocene witnessed a period of tectonic dormancy, during which the peneplanation took place.

Materials and Methods
Twenty-five samples were collected from the Shestakovo outcrop (55 • 54 26 N; 87 • 57 13 E), Kemerovo Province, Western Siberia, Russia ( Figure 1C). For the petrographic investigation of the samples under the optical light microscope, polished thin sections were prepared using standard methods. The bulk mineralogical composition of the samples was determined by a Rigaku Ultima IV (Rigaku, Tokyo, Japan) X-ray diffractometer (XRD) equipped with a Cu-Kα radiation source and operated at a current of 30 mA and a voltage of 40 kV. The finely powdered samples were examined from 3-65 • 2θ using a step of 0.02 • at a scanning rate of 1 s per step. Electron microscopic investigations were carried out on carbon-coated (15 nm), polished thin sections using a TESCAN VEGA 3 SBU (Tescan Orsay Holding, Brno, Czech Republic) scanning electron microscope (SEM) equipped with an OXFORD X-Max 50 (OXFORD instruments, Abingdon, UK) energy dispersive spectroscopy (EDS). The accelerating voltage for the SEM study was set to 20 kV, with a probe current intensity varying between 3.5 and 12.2 nA. The chemical composition of different minerals was determined by repeated measurements of more than 10 points per mineral by SEM-EDS. The transmission electron microscopy (TEM) study was carried out on ten samples using a JEOL JEM-2100F (JEOL Ltd., Tokyo, Japan) instrument. A drop of a fine particle suspension was transferred to a copper grid (300 mesh, 3.05 mm in diameter) covered with a carbon film before TEM investigation at 200 kV.
Major element concentrations of the powdered samples (20 samples) were determined by a HORIBA XGT 7200 (Horiba, Kyoto, Japan) X-ray fluorescence (XRF) microscope operated at a tube current of 1 mA, beam diameter of 1.2 mm, and a voltage of 50 kV. Pressed and fused pellets were prepared for XRF analysis. The average value was calculated from five analytical points evenly distributed on the pellet surface for each sample. Loss on ignition (L.O.I.) was obtained by heating sample powders to 900 • C for 9 h. The detection limit for major elements was better than 0.01 wt. %. Concentrations of trace elements (TEs), including rare earth elements (REEs), were measured on 20 samples using inductively coupled plasma-mass spectroscopy (ICP-MS), conducted on a NexION 300D (PerkinElmer Inc., Waltham, MA, United States) instrument. About 0.5 g of the powdered sample was fused at 1050 • C for about 15 min using a mixture of LiBO 2 /Li 2 B 4 O 7 (0.8 g) as the fluent agent. The glass beads were dissolved in a mixture of 5:4:1.5 HF, HNO 3 and HClO 4 solutions at 120 • C in a platinum crucible for 6 h. The liquid extract was allowed to evaporate at 160 • C. Afterwards, the sample residue was dissolved in 10 mL of 5 M HNO 3 solution, filtered, and analyzed for the REEs concentrations.
Elemental concentrations were normalized to Al content to remove the effect of variable terrigenous input [57,58]. Ce*, Eu*, Y sn /Ho sn are the theoretical Eu, Ce, Y and Ho values derived from a post-Archean Australian shale (PAAS) normalized rare earth element (REE) pattern. The enrichment factor (EF) was calculated for each sample using the standard formula [58]. The Al EF was calculated as follows: Al EF =Al sample /Al PAAS . The Chemical Index of Alteration (CIA) was calculated according to the relation 100 × (Al 2 O 3 /(Al 2 O 3 + CaO + Na 2 O + K 2 O)) [59,60].
The stable carbon and oxygen isotope ratios were obtained on seven carbonate-bearing rock samples of the studied area. Powdered samples were reacted with phosphoric acid at 70 • C using a Gasbench II connected to a Thermo Finnigan Five Plus mass spectrometer (Thermo Fisher Scientific, Waltham, MA, United States) [61]. Carbon and oxygen isotope values are reported per mil relative to V-PDB (NBS19 and LSVEC, respectively). Reproducibility was checked by replicate analysis of laboratory standards and was ±0.07‰ (1σ) for both carbon and oxygen isotope analyses.

Lithology and Mineralogy
Iron-bearing sedimentary rocks occur as thin bands within the Lower Cretaceous Ilek and Upper Cretaceous Kiya Formations in the Chulym-Enisey depression ( Figure 2). The Ilek Formation consists of greenish-gray sandstones, brick-red or bluish-gray sandy siltstones, silty ironstones, and bluish-gray claystones. The thickness of the Ilek Formation ranges up to 10-18 m within the studied area, increasing in the north-western corner of the Chulym-Enisey depression. The fine-grained ironstone shows reddish or brick color, and it consists mainly of iron-rich chlorite, calcite, siderite, and terrigenous silty components ( Figure 3A,B). Sandstone is brownish or greenish-gray with rare cross-bedding ( Figure 3C). Siltstones are brick-red or bluish-gray. They consist of carbonate, often forming nodules, with calcareous and chloritic matrix ( Figure 3E). Siltstone layers consist of an admixture of sand, as well as cm-scale fragments of lignite. The thickness of sandstones and siltstones in the Ilek Formation varies from 1.8 to 11 m and from 0.2 to 1.5 m, respectively. The thickness of iron-bearing siltstones and ironstones varies from 0.3 to 1 m.

Lithology and Mineralogy
Iron-bearing sedimentary rocks occur as thin bands within the Lower Cretaceous Ilek and Upper Cretaceous Kiya Formations in the Chulym-Enisey depression ( Figure 2). The Ilek Formation consists of greenish-gray sandstones, brick-red or bluish-gray sandy siltstones, silty ironstones, and bluish-gray claystones. The thickness of the Ilek Formation ranges up to 10-18 m within the studied area, increasing in the north-western corner of the Chulym-Enisey depression. The fine-grained ironstone shows reddish or brick color, and it consists mainly of iron-rich chlorite, calcite, siderite, and terrigenous silty components ( Figure 3A,B). Sandstone is brownish or greenish-gray with rare crossbedding ( Figure 3C). Siltstones are brick-red or bluish-gray. They consist of carbonate, often forming nodules, with calcareous and chloritic matrix ( Figure 3E). Siltstone layers consist of an admixture of sand, as well as cm-scale fragments of lignite. The thickness of sandstones and siltstones in the Ilek Formation varies from 1.8 to 11 m and from 0.2 to 1.5 m, respectively. The thickness of iron-bearing siltstones and ironstones varies from 0.3 to 1 m.  common features within the sandstone. Siltstones and clays are gray, greenish-gray, and yellowish-gray with parallel lamination. Siltstones consist of quartz, feldspars, kaolinite, and illite with siderite micro-concretions. A thin layer of gritstone (up to 0.5 m thick) occurs at the base of the Kiya Formation. It contains rounded and sub-angular fragments up to 3-8 mm diameter and carbonate (calcite) cement ( Figure 3F). The thickness of sandstones and siltstones in the Kiya Formation vary 4-14 m and 0.5-6 m, respectively. Ironstones layers have a thickness ranging from 0.5 to 1.5 m.  The Kiya Formation includes cross-bedded sandstones with intervening gray siltstones, gray clays, lignite lenses, ironstones, and gritstones. The thickness of this Formation varies from 10 to 40 m within the study area. Ironstones are characterised by lenticular geometry, brown color, finely laminated structure, consisting mainly of siderite. Sandstone is cross-bedded, gray to greenish-gray or brownish-gray, and medium-grained ( Figure 3D) with a kaolinite-nontronite matrix. Siderite and claystone nodules are common features within the sandstone. Siltstones and clays are gray, greenish-gray, and yellowish-gray with parallel lamination. Siltstones consist of quartz, feldspars, kaolinite, and illite with siderite micro-concretions. A thin layer of gritstone (up to 0.5 m thick) occurs at the base of the Kiya Formation. It contains rounded and sub-angular fragments up to 3-8 mm diameter and carbonate (calcite) cement ( Figure 3F). The thickness of sandstones and siltstones in the Kiya Formation vary 4-14 m and 0.5-6 m, respectively. Ironstones layers have a thickness ranging from 0.5 to 1.5 m.
Distinctive lithofacies characterize the Cretaceous deposits of the Chulym-Yenisey basin. The Kiya river basin shows two main lithofacies of litho-feldspatho-quartzose sandstone (F1) and siltstone (F2). The medium-to fine-grained sandstone is cross-stratified (F1b). Pebbly sandstone may occur at the bottom of medium-grained sandstone. The fine-grained sandstone (F1a) is overlain by siltstone. The siltstone beds incorporate reddishbrown ironstone and are calcareous in places. The fining-upward grain size variation from medium-to fine-grained sandstone, alteration with siltstone, occasional pebbly base, and the nature of cross-stratifications indicate the fluvial channel origin of the Kiya Formation (see also [20,21]). The siltstones alternating within the sandstones with thin layers of ironstones and lenses of lignite show floodplain-lacustrine-boggy facies.

Geochemistry
The iron-bearing rocks, including ironstones of the Ilek and Kiya Formations, have Fe2O3(total) content varying from 8.6% to 18.9% and from 8.5% to 16.4%, respectively ( Table  1). The bulk chemical composition of sandstones is characterised by the Fe2O3(total) below 5.2%, with SiO2 49.0-67.0% and Al2O3 10.8-13.7%. Carbonate cemented gritstones display an increased CaO content up to 13.6%. Calcite nodules up to 20 cm in diameter have the following average composition: CaO 23.6%, SiO2 36.3%, Al2O3 8.6%, Fe2O3(total) 3.0%.  Figure 5D) in the matrix of ironstones and terrigenous rocks. Goethite is less common among authigenic minerals and forms micro-clusters up to 40 µm in diameter ( Figure 5E) or thin crusts ( Figure 6). Pyrite occurs mainly in bluish-gray siltstones of both Ilek and Kiya Formations as sub-isometric aggregates ( Figure 5F) ranging in length from 25 to 2000 µm similar to "sunflowers" with elongated cores. Barite occurs in the gritstone matrix as aggregates up to 120-µm long. They occur within the gritstone matrix ( Figure 5G). Native silver crystals are recorded in the form of irregular grains in the illite matrix of brownish-gray sandstones of the Ilek Formation ( Figure 5I).

Geochemistry
The iron-bearing rocks, including ironstones of the Ilek and Kiya Formations, have Fe 2 O 3(total) content varying from 8.6% to 18.9% and from 8.5% to 16.4%, respectively ( Trace element concentrations are presented in Figure 7 and Table 2  Three groups of elements can be distinguished among the trace metals in ironstones and terrigenous rocks of studied strata based on their distribution (Figure 8). The first group comprising the iron-bearing rocks shows enrichment of Ba, Sb, As, and Ag compared with the average contents in the earth's upper continental crust (UCC) [62,66]. The second group displays lower contents of Cr, Ni, Zr, Hf, Th, Sn, Cs, W, and Tl than those of UCC. The third group (Co, Cu, Sr, Nb, Mo, Ta, Pb, Bi, and U) has abundances in iron-bearing rocks that are similar to UCC. Ironstones of the Chulym-Yenisey basin are distinguished by the higher content of Ba and lower contents of Cr, Zn, Mo, Bi, Th, U, and Sb compared to marine ooidal ironstones of the Bakchar deposit in Western Siberia (Figure 8).   The total contents of rare earth elements and yttrium (REY; Table 3) are higher in ironstones (up to 141 ppm) and terrigenous rocks of the Ilek Formation than in the rest of the succession (Figure 7, Table 4). A weak positive Eu* is noted in the siltstones with carbonates of the Ilek Formation. Ce* shows robust average values 1.0. The HREE content increases relative to light LREE in rocks containing carbonates. Geochemical proxies reflect changing sedimentation conditions (Figure 7, Table 2) [63][64][65]. The Mo EF (enrichment factor) and U EF values do not exceed 2.5. Mo EF is greater than 1 in calcareous siltstones of the Kiya Formation. The ironstone layers exhibit a slight increase in proxies of the bioproductivity (P EF, Cu EF). The P EF and Cu EF values exceed 1 in the intervals of iron-bearing rocks with siderite. Proxies of the paleosalinity (Sr/Ba and Ca/(Ca + Fe)) increase at two intervals. The Sr/Ba ratio may vary from 0.8 to 1.2. These two layers also exhibit changes in paleoclimate proxies (CIA (Chemical Index of Alteration) and Sr/Cu). CIA increases to 70.9 in siltstones at the top of the Ilek Formation, while Sr/Cu decreases to 6.5. Proxies of sedimentation influx (Ti EF, Si EF) have relative maximum values of 1.4-1.5 and 1.0-1.3, respectively, in sandstones of the Ilek Formation and gritstones of the Kiya Formation. The Mn EF increases to 5.2-6.9 and is up to 2.2 in ironstones and siltstones containing carbonates of the Ilek Formation, respectively. Redox-sensitive proxies (Mo EF, U EF, U/Th) indicate a slight depletion of oxygen during the deposition of siltstones. Proxies of the paleosalinity correspond to siltstones with carbonates or pyrite in the Ilek Formation. These proxies change by the reduced CIA and slightly elevated Sr/Cu at the base of the Kiya Formation. The Fe EF and Mn EF vary simultaneously but remain at the average concentrations of bulk continental crust level [62,66] in calcareous siltstones of the Ilek Formation. The Fe EF in iron-bearing rocks varies from 1.5 to 3.6.
Three groups of elements can be distinguished among the trace metals in ironstones and terrigenous rocks of studied strata based on their distribution (Figure 8). The first group comprising the iron-bearing rocks shows enrichment of Ba, Sb, As, and Ag compared with the average contents in the earth's upper continental crust (UCC) [62,66]. The second group displays lower contents of Cr, Ni, Zr, Hf, Th, Sn, Cs, W, and Tl than those of UCC. The third group (Co, Cu, Sr, Nb, Mo, Ta, Pb, Bi, and U) has abundances in iron-bearing rocks that are similar to UCC. Ironstones of the Chulym-Yenisey basin are distinguished by the higher content of Ba and lower contents of Cr, Zn, Mo, Bi, Th, U, and Sb compared to marine ooidal ironstones of the Bakchar deposit in Western Siberia (Figure 8).  [62,66]. Data of the Bakchar marine ironstone deposit are plotted for comparison [47].
The average content of REY in the Cretaceous sedimentary deposits of the Chulym-Yenisey basin is 87-141 ppm, less than the average content of the post-Archean Australian shale (PAAS). The low content of REY in studied samples clearly distinguishes them from the Bakchar marine ironstone deposit (Figure 9). The absence of the Ce* and the positive Eu* are also characteristic of the studied sediments.   [62,66]. Data of the Bakchar marine ironstone deposit are plotted for comparison [47].
The average content of REY in the Cretaceous sedimentary deposits of the Chulym-Yenisey basin is 87-141 ppm, less than the average content of the post-Archean Australian shale (PAAS). The low content of REY in studied samples clearly distinguishes them from the Bakchar marine ironstone deposit (Figure 9). The absence of the Ce* and the positive Eu* are also characteristic of the studied sediments.  [62,66]. Data of the Bakchar marine ironstone deposit are plotted for comparison [47].
The average content of REY in the Cretaceous sedimentary deposits of the Chuly Yenisey basin is 87-141 ppm, less than the average content of the post-Archean Australi shale (PAAS). The low content of REY in studied samples clearly distinguishes them fro the Bakchar marine ironstone deposit ( Figure 9). The absence of the Ce* and the posit Eu* are also characteristic of the studied sediments.  [47]. Normalization factors are from published data [66].
The δ 13 C and δ 18 O values of the carbonate-bearing samples in the studied area display ranges from −5.4‰ to −12.6‰ V-PDB and −11.2‰ to −15.0‰ V-PDB, respectively. The siderite and calcite of the Ilek Formation exhibit δ 13 C ratios from~−5.4 to −6.7‰ V-PDB and δ 18 O ratios from −11.2‰ to −11.7‰ V-PDB. Carbonate samples of the Kiya Formation of the Shestakovo area show δ 13 C ratios from −7.1‰ to −12.6‰ V-PDB and δ 18 O ratios from −12.8‰ to −15.0‰ V-PDB.

Metal Sources of the Chulym-Yenisey Deposits and West-Siberian Iron-Ore Basin
The Chulym-Yenisey depression in the West Siberian plate is distinctive from similar basins because of its paleogeographic conditions [36]. The epicontinental basin of the ancient West Siberian Sea [67][68][69] occupied a large area of Western Siberia during the Upper Mesozoic. A continental environment with a well-developed hydrographic network prevailed within the Chulym-Yenisei depression at that time [36,38,69]. Marine ooidal ironstone deposits accumulated across the eastern and southeastern shallow marine zones: Bakchar, Kolpashevo, Laskinsk, Kargasok, Eloguy-Turukhansk, Kulundinsk [39,48,49,70,71]. Previous studies considered that weathered magmatic and metamorphic rocks of the Altai-Sayan mountain region and the Yenisey ridge adjacent to the West Siberian plate represented the source of iron to the basin [39,70,72,73]. Alternately, the presence of iron was linked to the transfer from the ancient river system and precipitation in the zone of mixing of acidic river waters with alkaline sea waters. A few researchers [6,[74][75][76][77] considered the ironstone deposits as a product of low-temperature seeps with mobilization of iron-saturated fluids into the ancient seabed, as described in modern sediments. Therefore, depositional conditions of continental ironstones of the Chulym-Yenisey depression, which is contemporaneous with the West Siberian iron ore basin, remain unexplored.
The mineralogical and geochemical data of the studied iron-bearing strata (Table 5) indicate different metal sources for these regions. The most suitable source for the deposits is the adjacent mountainous Altai-Sayan region which includes the volcanic rocks, basic and felsic magmatic complexes [50,51]. Angular and sub-rounded fragments of quartz and feldspars, the presence of heavy detrital minerals (zircon, monazite, ilmenite) in sediments and the rare earth elements pattern, including positive europium anomaly, supports this interpretation. We infer the late Caledonian volcanogenic rocks of the Altai-Sayan region [50,51] as the main iron source. The weathering of iron-bearing minerals in these provinces could have served as primary iron sources. Table 5. Main features of studied continental ironstones of the Chulym-Yenisei depression and marine ironstones of the West Siberian basin [39,47,48].

Depositional and Diagenetic Conditions
Thin layers of ironstones in the Cretaceous strata of the Chulym-Yenisei depression are confined to siltstone deposits, which indicates iron immobilization within the environment of meromictic waters [18]. This environment promotes the accumulation of ironstones in the floodplain-lacustrine-boggy facies. The presence of siderite and chamosite within ironstones indicates oxygen-depleted porewater conditions of the mineral formation. Stable isotope composition of studied siderite and calcite (δ 13 C −5.4 to −12.6‰ V-PDB and δ 18 O −11.2‰ to −15.0‰ V-PDB) indicate continental freshwater limestones [78]. Carbon dioxide produced by the bacterial decomposition of organic matter contributed to the concentration of carbonate minerals and possible iron concentration. Depleted carbon isotope ratios (−12.6‰ V-PDB) indicate bacterial carbon dioxide production [79]. Calcite, siderite, and aragonite formed within the ferruginous lakes depending on the depositional conditions [18].
Association of calcite and aragonite ( Figure 6) in sedimentary rocks reflects two possible mechanisms: (i) sequential precipitation of calcite and aragonite or (ii) conversion of aragonite to calcite. The co-occurrence of minerals ( Figure 6) in some grains excludes their sequential precipitation in these cases. A diagenetic process in an aqueous environment with an admixture of Mg and/or biomolecules [80][81][82] led to the transformation of aragonite to calcite. This transformation was the predominant mechanism leading to the formation calcite-aragonite microaggregates ( Figure 6). For example, these features in the sedimentary environment were documented in hot spring travertines [83]. Siderite was transformed to goethite with the active participation of humic acids with the local formation of hydroxyapatite in lacustrine-bog conditions [84]. The occasional presence of pyrite framboids in siltstones suggests the local saturation of the pore water with sulfide ions due to bacterial sulfate reduction [85]. The interaction of sulfide ions with reactive iron led to the formation of pyrite framboids below the water-sediment interface, which is rarely recorded in freshwater environments [18,86,87], including basal loams of peat bogs [88,89]. The accumulation of siltstones with ironstones took place at the bottom of swampy lakes. Layers with carbonates are indicators of the mesotrophic and eutrophic conditions at the bottom of lakes. Subsequent diagenetic reduction of mineral phases promoted iron transfer [73] to the coastal-channel facies, where metal oxidation could occur with partial accumulation of chamosite.
The mobilized iron was primarily precipitated in the meromictic waters of lakes and bogs in the area of the paleo-river system that coincides with the modern Kiya river. The organic matter decomposition resulted in the production of carbon dioxide and/or sulfide ions and the formation of carbonates or pyrite. During the diagenetic transformation with the additional input of ferric oxide water, part of the iron was immobilized under sub-oxic to oxic conditions within floodplain lacustrine to coastal channel facies, which contributed to the accumulation of ferruginous phyllosilicates and/or goethite.

Paleo-Environmental Conditions
The elevated enrichment factors for Fe and Mn suggest immobilization of these metals in the biogeochemical cycle of the river system as authigenic mineral formation. Iron concentrates as carbonate (siderite), phyllosilicate (chamosite), or hydroxide (hydrogoethite). Moreover, hydrogoethite probably forms as precipitates in an oxidic aqueous environment by diagenetic alteration of siderite and by the oxidation of chamosite. An enrichment of Fe EF and Mn EF is accompanied by enhanced bioproductivity (Cu EF and P EF) proxies, suggesting organic matter decomposition and generation of HCO 3 2− . These processes facilitated the predominance of carbonates as the authigenic minerals.
The variation of the sedimentation flux proxies (Ti EF, Si EF) for the deposits preceding the carbonate layers indicates periods of flooding and the subsequent development of the bog-lacustrine environment. Paleo-salinity proxies (Sr/Ba and Ca/(Ca + Fe)) of the sedimentation environment reflect weak aridization and reduced weathering. Siltstones at the top of the Ilek Formation accumulated under conditions of humidization and weak weathering, revealed by the changes in Sr/Cu and CIA, respectively. The environment was favorable for the accumulation of organic matter. Changes in environmental conditions correspond to the transgression of the West Siberian Sea [36] and the humidification of climate in the Chulym-Yenisey depression.
The sedimentary record of the Ilek and Kiya Formations [34,90] includes known oceanic anoxic events [91][92][93], such as the Early Aptian anoxia (OAE Ia) and Cenomanian-Turonian anoxia (OAE 2). A relationship was established between the hypoxia of the bottom water and global oceanic anoxia during the Cretaceous period in coeval marine ooidal ironstone deposits in Western Siberia [47]. The global geological events possibly influenced regional geological processes within the Chulym-Yenisey basin. This could be expressed in an increase in bioproductivity and mobilization of nutrients, humidification, and increased weathering of the magmatic rocks of the Kuznetsk Alatau as a result of OAE Ia and/or OAE 2 [29,32,94,95].

Conclusions
Detailed petrographical, mineralogical, and geochemical studies made it possible to highlight the following conclusions.

1.
Continental sediments of Cretaceous Ilek and Kia Formations of the Chulym-Yenisei depression consist of two lithofacies: (i) cross-stratified litho-feldspatho-quartzose sandstones and siltstones and (ii) bluish-gray siltstones with ironstones. These two facies document fluvial channel and floodplain-lacustrine-boggy deposits, respectively. Thin layers of ironstones within siltstones indicate deposition in meromictic waters.

2.
The detrital minerals of studied deposits are represented by quartz and feldspars. The main iron-rich authigenic minerals of ironstones are chlorite group (possible chamosite), goethite, siderite, pyrite, nontronite, and illite. Non-iron minerals are kaolinite, beidellite, aragonite, dolomite, calcite, apatite, and barite. Local bacterial sulfate reduction led to the formation of pyrite framboids in siltstone layers. The subsequent diagenetic iron reduction, induced by microbial activity, promoted iron diffusion transfer and the formation of chamosite. Goethite precipitated in an aqueous system due to the additional input iron-rich water.

3.
The changes in geochemical proxies in the studied Cretaceous sequence demonstrate fluctuations in paleoenvironmental conditions. Siltstones accumulated under humid conditions, while sandstones were deposited in weakly arid conditions. This could be linked with iron mobilization due to increased weathering of the rocks of the Kuznetsk Alatau as a possible result of intense humidification during oceanic anoxic events.

4.
During the Cretaceous, the Chulym-Yenisey depression was a part of a distinctive continental environment, unlike the West Siberian plate, where marine ooidal ironstones were deposited. Most irons within the Chulym-Yenisey depression were supplied from the Altai-Sayan mountainous region. Mineralogical and geochemical characteristics of these ironstones are clearly distinguished, indicating different iron sources for the Chulym-Yenisey basin and the ancient West Siberian Sea.