Hydrogeochemistry of Jurassic Sedimentary and Igneous Formations in the Central Part of Kuzbass

G. A. Plevako

(Presented by the scientific seminar of the Department of Hydrogeology and Engineering Geology)

Jurassic deposits in the central part of Kuzbass form a central trough, elongated along the long axis of the basin for 125 km and measuring 12–48 km across. The total thickness of the sediments ranges from 350 to 880 m [1]. The Jurassic formations in this area of Kuzbass are represented by weakly cemented coal-bearing sandy-clayey rocks, predominantly sandstones (35–67%) and siltstones (18–37%). Brown coals make up 2.6–8.0% of the section of the coal-bearing suites.

Igneous formations in this area of Kuzbass are developed most widely. The basalts of the Saltymakovo Ridge, the Taradanovo swell, the Kailot and Naryk mountains, and the Karakan Ridge, according to A. M. Kuzmin [1], are parts of a monoclinal asymmetric laccolith-sill. This entire complex of uplands forms a horseshoe-shaped outcrop of igneous rocks, surrounding on almost all sides the southern part of the central trough—the Bungarai depression, filled with Mesozoic sediments. Therefore the formation of groundwater in the complex of Jurassic deposits of the Bungarai depression is closely connected with the surrounding igneous formations, which partly constitute recharge areas for the groundwater of the subartesian basin of the southern part of the Central trough.

The Jurassic formations in the Central trough, owing to weak diagenesis, considerable porosity, and fracturing in the upper part of the section to depths of 100–150 m, are characterized by rather high water abundance. The most water-bearing are beds of sandstones and gravelites in river valleys and depressions. Specific well yields exceed 2–2.5 l/s; the yields of many flowing wells are 15–48 l/s, and the productivity of individual vertical water intakes reaches 250–300 m³/h [3]. The waters of the Jurassic deposits are widely used for the water supply of small settlements and nearby towns of Kuzbass.

Igneous rocks are watered along weathering cracks and zones of crushing. The zone of intense fracturing in basalts is developed to a depth of 40–50 m, and its maximum thicknesses are confined to watershed areas. Below the thalweg of the Tom River, regional fracturing in the basalts sharply dies out, and their water abundance is possible only along zones of disturbance. Spring discharges within the area of development of igneous rocks rarely exceed 0.3–0.5 l/s, while surface watercourses receive groundwater inflow of up to 15–20 l/s per 1 km of channel.

Thus, Jurassic and igneous rocks possess increased water abundance only in the zone of intensive water exchange, the formation of which depends above all on local physicogeographical conditions and on the features of the geological and geomorphological development of the area. The natural resources of groundwater in Jurassic and igneous rocks are formed within the area of distribution of these rocks. The chemical composition of the waters is fully determined by the lithological-mineralogical composition of the water-bearing rocks and by the degree to which they have been leached and drained.

In recharge areas associated with watershed spaces, the chemical composition of groundwater is very close to that of atmospheric precipitation. In terms of chemical composition, the waters of Jurassic formations in the zone of intensive water exchange are predominantly calcium bicarbonate and calcium-magnesium waters, with a mineralization of 0.5–0.8 g/l. The pH value varies within the range 6.6–7.4. A slightly acidic environment is most characteristic of the waters of recharge areas, studied from springs emerging at the heads of ravines. A neutral and slightly alkaline environment is usually established in discharge areas. In addition, an increase in water alkalinity is observed in the deeper parts of the section. In the same direction there is an increase in total mineralization and a decrease in the water abundance of the rocks.

Groundwaters of the Jurassic deposits in the upper parts of the section of the Central trough, uniform in their general chemical composition, are characterized by a certain diversity in the distribution and content of microcomponents in them.

At the northwestern and northeastern ends of the Central trough, in the forest-steppe zone, the Jurassic deposits are complicated by additional plicative folding and are characterized by increased coal-bearing capacity in the upper parts of the section, where coal seams pass beneath the Quaternary cover. In addition, the Mesozoic formations are in contact with industrial coal-bearing sediments of the Erunakovskaya suite \((P_2)\). Owing to intensive weathering of the coal-saturated strata of the Jurassic and Permian formations and to active drainage of groundwater by the river systems of the Inya, Northern Un’ga, and Chusovitina, the waters in the zone of intensive drainage are enriched with arsenic, antimony, zinc, copper, chromium, strontium, titanium, barium, and vanadium, which are characteristic of the zone of oxidized coals.

When studying groundwater in the noted areas for drinking-water supply, special attention should be paid to the concentration of arsenic and antimony, which almost reach the inadmissible standard.

In the area described, high contents of sulfate and chloride ions are recorded in groundwater; these enter the water during the leaching of coals.

In weakly drained areas of the central part of the trough, the predominant place in groundwater is occupied by nickel, titanium, and manganese, which are widely distributed in sediments of Quaternary age and in Mesozoic formations. Closer to the marginal parts of the structure, where Jurassic and Permian coal seams pass beneath the Quaternary cover, in well-drained areas of the Jurassic deposits—especially at the headwaters of the Inya, Southern Un’ga, and Urop rivers—elevated contents of barium, strontium, arsenic, copper, zinc, and silver appear in groundwater, with their concentrations several times higher than background values.

In the southern dampened part of the Central trough (the Bungaraptinskaya depression), characterized by increased moisture and more intensive leaching of rocks in the zone of active water exchange, calcium bicarbonate waters have lower mineralization (0.2–0.4 g/l) in comparison with the northern and central territories of the trough. Sla-

low mineralization, low chlorine concentration, and the complete absence of sulfates are characteristic even in areas with deep discharge of groundwater, indicated by large ascending springs in the headwaters of the Bungarap River and by the opening of confined waters by boreholes (Karalda village).

In the indicated discharge centers, highly diverse microcomponents have been found in the groundwaters, pointing to a very active migration capacity of elements in the taiga zone [2]. High contents of zinc and somewhat elevated concentrations of barium, copper, and lead have been established here. Closer to the contact with the Triassic and the effusive rock complex (the Karakanskii Ridge and the Naryk Mountains), chromium, vanadium, titanium, zirconium, zinc, and lead occur more often in the waters of the Jurassic deposits; the previously noted arsenic, silver, copper, and barium also remain present. It should be assumed that magmatic activity manifested in the Triassic may have contributed to the enrichment of groundwater within the Bungarap depression with a very broad complex of microcomponents. Tectonic movements of this period and younger ones determined the discharge of carbon dioxide waters of the Makarikhinskoe deposit in the valley of the Upper Ters River; these waters carry high concentrations of arsenic, antimony, strontium, barium, zinc, nickel, zirconium, and others.

Within the widely developed basalts and Triassic deposits on the Taradanov Upland and the Saltymakovskii Ridge, which are relatively elevated above the surrounding terrain, groundwaters are characterized by the following features.

Because of the considerable elevation of the Saltymakovskii Ridge and its spur, the Taradanov Upland, groundwaters forming on watershed areas, as they move toward discharge areas (the valleys of the Tom and Taidon rivers), are only weakly metamorphosed and only slightly increase their mineralization. The rocks within these uplifts are exceptionally well washed; therefore a close relationship is established here between the chemical composition of groundwater and that of atmospheric precipitation. In the upper reaches of the river network originating on the Saltymakovskii Ridge, groundwaters and surface waters have the very same composition as atmospheric precipitation. These are acidic (often pH less than 6.0) chloride and bicarbonate-chloride sodium waters with mineralization of 0.05–0.15 g/l. Lower on the slopes and in discharge areas, bicarbonates begin to predominate over chlorides, and alkaline-earth elements over sodium; the waters become predominantly bicarbonate-chloride magnesium and magnesium-calcium waters with mineralization of 0.3–0.5 g/l. The noted zonality is traced rather clearly.

The complex of microcomponents in atmospheric precipitation and groundwaters is qualitatively almost identical. Low concentrations of barium, titanium, manganese, zinc, lead, copper, and, less often, silver, vanadium, and chromium occupy the dominant position in both kinds of water.

In the valley of the Tom River near the settlement of Bogdanovo and somewhat farther downstream, where mainly surface watercourses were studied in the area of basalt development, the presence in the latter of products of hydrothermal activity, and along large fractures, elevated contents of cobalt, nickel, vanadium, arsenic, strontium, chromium, and sulfate ion up to 500 mg/l were established in the waters. The most characteristic microcomponents in the groundwaters of the Saltymakovskii Ridge are barium, and in the area of the Taradanov Upland, barium, nickel, and vanadium. The remaining elements occur within background contents.

Thus, the conditions for the formation of the chemical composition of groundwaters in the Jurassic deposits of the central trough and in the tuffaceous-sedimentary and magmatic formations of the “melaphyre basement” are quite complex. Groundwaters of this region, uniform in their general chemical composition, are very diverse in their contents of ...

Table 1

Background, anomalous, and frequently occurring contents of microcomponents in Jurassic sedimentary and igneous waters of the central part of Kuzbass

metals, whose nature depends on a number of factors: the geological-structural features of individual areas, the geomorphology and natural landscapes, which determine the general hydrodynamic plan, the degree of leaching of the water-bearing rocks, and the intensity of removal of chemical elements by groundwater.

Factual material on the distribution of microcomponents in the groundwater of Jurassic deposits and igneous formations in the central part of Kuzbass is given in Table 1.

REFERENCES

1. Geology of Deposits and Oil Shales of the USSR. Vol. 7. Nedra, Moscow, 1969.
2. A. I. Perelman, Geochemistry of the Landscape. Vysshaya Shkola Publishing House, Moscow, 1966.
3. G. M. Rogov, G. A. Plevako. On the hydrogeology of the Central Jurassic artesian basin of Kuzbass.—In: Materials on the Geology and Mineral Resources of Western Siberia. TSU Publishing House, 1964.