ON THE POSSIBILITY OF USING GEOCHEMICAL DATA FOR CORRELATION AND FACIES ANALYSIS OF JURASSIC DEPOSITS

A. N. Kondakov

(Presented by Prof. A. M. Kuzmin)

In this work, by comparing the geochemical background of monotypic, heterogeneous sediments, clear geochemical differences are established for a number of elements in the deposits of the Itat Formation (Middle Jurassic), in the interval down to the Itat–Berezovka bed, and of the over-Itat member (Middle–Upper Jurassic), including the sediments of the Tyazhin Formation and the top of the Itat Formation, developed in the area of the Arga Range, Kansk–Achinsk Basin. On the basis of the behavior in hypergene processes of each of the elements considered, a conclusion is drawn concerning the dependence of the geochemical background of rocks on the stage of formation of the weathering crust, under particular climatic conditions, which supplied the suspended and dissolved material of the streams.

The conclusions obtained make it possible to recommend the use of geochemical data for stratigraphic subdivision of the sequence according to the method described. In order to eliminate the influence of superimposed processes that distort the original geochemical background, certain features of the infiltration precipitation of elements in coals and carbonaceous rocks are considered.

The geochemical constructions make use of data from 200 semiquantitative spectral analyses. The analyses of the samples were carried out in the Central Laboratory of one of the geological organizations of the city of Novosibirsk. The practical accuracy of the analyses is no lower than 1% for potassium, 0.01% for magnesium and calcium, 0.001% for iron, manganese, zinc, barium, strontium, vanadium, lead, cobalt, nickel, and chromium, and 0.0001% for molybdenum. The locations of the boreholes whose cores were subjected to geochemical sampling are shown in Figs. 2–4 [6].

Geochemical characterization of the deposits of the Upper and Middle Jurassic

Accumulation of the Upper and Middle Jurassic sediments took place in similar facies settings [6]. Sandy–aleuritic sediments are characteristic mainly of channel–floodplain deposits, whereas clayey sediments formed in lake or swamp-lake bodies of water of the floodplain.

Differences in the average content of elements in monotypic sedimentary rocks of continental origin, accumulated under similar facies conditions, are due chiefly to climatic and tectonic causes acting together. A definite

climatic conditions there corresponds a particular weathering profile; the level of denudation is connected with tectonic causes, in accordance with which weathering processes either outstrip erosional removal, reaching the final stages of formation of the weathering crust, or lag behind, stopping at intermediate stages.

According to N. M. Strakhov [9], it is precisely the possibilities of chemical weathering of drainage areas, and not a chain of rewashings and redepositions, that explain the polymictic or oligomictic character of sediments; moreover, in continental deposits a correlation is established in the composition of sandy-aleuritic and clayey sediments. An oligomictic sandy-aleuritic unit is matched by clayey kaolinitic products of the final stages of formation of the weathering crust. Until quite recently, the literature widely held the opinion that clay minerals reflect the physicochemical environment of the sedimentation basin. In his works N. M. Strakhov [8, 9] convincingly showed that in modern water bodies clay minerals, just like sandy-aleuritic sediments, are for the most part carried into the basin in suspension, and that in its basic features the course of sedimentation of detrital material in ancient water bodies proceeded analogously to sediment accumulation in modern water bodies. Thus, the background contents of elements in units of different ages but of the same facies type characterize the stage of weathering and the level of denudation of the geological formations whose erosion forms the suspended and dissolved material of streams.

As a starting point we assume that the average contents of elements in sandy rocks characterize the share of transport of elements in the lattices of minerals of the detrital fraction. The readily mobile part of a migrating element, in the form of true and colloidal solutions, reaches the terminal water bodies of runoff and is precipitated, being sorbed by clays and other sorbents, and also by chemical or biochemical means. The mass migration of each individual element begins at a definite stage in the formation of the weathering crust and under definite physicochemical conditions of the transporting medium.

As is evident from Table 1, the sandy and clayey rocks of the Naditatka member, relative to the corresponding rocks of the coal-bearing subformation of the Itatka Formation, are characterized by higher average contents of a number of elements; however, the ratios of the contents of different elements for particular rocks are not proportional, and, in increasing order of the ratios, the elements are arranged in the following sequence:

for sandstones

$$ \begin{array}{cccccccc} \mathrm{Mo}, & \mathrm{Ba}, & \mathrm{V}, & \mathrm{Mg}, & \mathrm{K}, & \mathrm{Fe}, & \mathrm{Na}, & \mathrm{Mn} \\ 0.90, & 0.90, & 0.95, & 1.00, & 1.13, & 1.22, & 1.26, & 1.33, \end{array} $$

$$ \begin{array}{ccccc} \mathrm{Ca}, & \mathrm{Cu}, & \mathrm{Zn}, & \mathrm{Sr}, & \mathrm{Pb} \\ 1.38, & 1.38, & 1.43, & 1.64, & 1.80; \end{array} $$

for clays (argillites)

$$ \begin{array}{cccccccc} \mathrm{Pb}, & \mathrm{Zn}, & \mathrm{V}, & \mathrm{K}, & \mathrm{Cu}, & \mathrm{Ba}, & \mathrm{Mg}, & \mathrm{Fe} \\ 0.55, & 0.70, & 0.72, & 0.80, & 0.90, & 1.35, & 1.37, & 1.55, \end{array} $$

$$ \begin{array}{ccccc} \mathrm{Mn}, & \mathrm{Ca}, & \mathrm{Na}, & \mathrm{Mo}, & \mathrm{Sr} \\ 1.60, & 1.66, & 1.68, & 2.60, & 2.72. \end{array} $$

Table 1

Average contents of elements in Middle and Upper Jurassic rocks of the Berezovka and Nazarovo troughs
(based on results of semiquantitative spectral analyses)

Note: n/o—not detected.

In the course of sediment accumulation, the deposited clayey material is chiefly a product of the weathering crust and, consequently, reflects the maximum level reached by the weathering processes. The maximum level of weathering ultimately determines the moment at which mass migration of any of the elements begins. Migration of elements in the lattices of minerals of the detrital fraction is passive in character, whereas in the clayey fraction, along with the passive component, the active part of the migrating component is accumulated mainly. Taking, as an approximation, the difference in the weathering level of the detrital material of clays by suites for a given element to be equal to the ratio of the element contents in sandy rocks, we derive the coefficient of active migration by dividing the ratios of element contents in clayey rocks, respectively, by the ratios of elements in sandy rocks. This method makes it possible to take into account, to some extent, the difference in the material composition of sandy and clayey rocks. We obtain the following series:

$$ \begin{array}{cccccccc} \mathrm{Pb} & \mathrm{Zn} & \mathrm{Cu} & \mathrm{K} & \mathrm{V} & \mathrm{Ca} & \mathrm{Fe} & \mathrm{Na} \\ 0.30, & 0.48, & 0.66, & 0.70, & 0.75, & 1.20, & 1.28, & 1.35, \\ & & \mathrm{Mg} & \mathrm{Ba} & \mathrm{Sr} & \mathrm{Mo} & & \\ & & 1.37, & 1.50, & 1.68, & 1.69. & & \end{array} $$

It follows from this that the deposits of the Naditatka member are enriched in alkaline and basic metals, which indicate an interruption of weathering processes at the alkaline stage. Potassium is an exception to the rule, since it accumulates in residual and soil products of the weathering crust. The sharp decrease in the content of lead, zinc, and copper in argillites, in the opinion of A. I. Perel’man [3], A. P. Vinogradov [1], and others, indicates the alkaline stage of formation of the weathering crust, as well as the neutral or basic character of the waters that brought these elements into the basin.

In contrast to the Naditatka member, the weathering processes during the accumulation of the coal-bearing subsuite were completed by the acid stage, with the removal of lead, zinc, and copper; and the transport conditions were apparently characterized by the acidic environment of bog waters, favorable for the migration of molybdenum, vanadium, and also of lead, zinc, and copper.

The clear differentiation of the deposits of the coal-bearing subsuite of the Middle Jurassic and the Naditatka member, which includes undifferentiated deposits of the Middle and Upper Jurassic, by the content of a number of elements makes it possible to recommend the use of geochemical-research material in resolving questions of stratigraphy. For these purposes it is most advisable to use data on the contents of lead, zinc, copper, vanadium, calcium, iron, sodium, strontium, and molybdenum. Samples must be taken from the reduced part of the section, since in the oxidation zone there is infiltration enrichment of the rocks by certain elements.

Some features of infiltration enrichment of carbonaceous rocks and coals by elements

In the oxidation zone a change in the geochemical background of the rocks is detected (Table 2); in particular, there is an influx of a number of elements entering a new physicochemical environment.

A distinct enrichment in molybdenum, vanadium, nickel, chromium, lead, and copper is established in coals; however, in sandstones and argillites as well the content of these elements is also elevated. As a rule,

the highest contents are established in rocks enriched with coalified plant remains, which makes it possible to assume a similar nature of sedimentation.

In samples from a shaft exposing sooty coals, a definite sequence of element precipitation with depth is revealed (Fig. 1). In the oxidation zone and in gley environments, in sandstone samples with signs of infiltration enrichment, an indistinct spatial differentiation in the contents of these elements is also established; moreover, only molybdenum reaches the deepest horizons (down to 1—

Fig. 1. Sequence of precipitation (from left to right) of elements in sooty coal at the outcrop of the Berezovsky coal seam beneath Quaternary deposits (results of sampling of the shaft wall).

1.5 km down the dip). In relation to the maximum content in the sooty coal and to the average content of elements in unaltered coals, the degree of enrichment of the sooty coal is different for different elements, and, in decreasing order, the elements are arranged in the following series:

\[ \mathrm{Mo} > \mathrm{V} > \mathrm{Co} = \mathrm{Sr} > \mathrm{Ni} > \mathrm{Ba} > \mathrm{Mn} > \mathrm{Cr} > \mathrm{Ca} = \mathrm{Fe} > \mathrm{Mg} > \mathrm{Cu}. \]

\[ >250 \quad 250 \quad 17 \quad 17 \quad 10 \quad 9 \quad 6.5 \quad 3.5 \quad 3 \quad 3 \quad 2.5 \quad 2 \]

By composition, the coals of the basin belong to humic brown coals with a high content of humic and fulvic acids (up to 20%). In natural processes, solid acids, owing to the dissociation of the carboxyl group with cleavage of the ion \( \mathrm{H}^{+} \), act as typical cation exchangers [5], with the exception of amino acids, which belong to ampholytes. The mechanism of element precipitation in brown coals is sufficiently complex to make it difficult to reduce it to any single aspect of the process; however, numerous studies in this direction make it possible to assume a leading role for polar exchange sorption, up to chemical interaction with the formation of stable organometallic compounds.

Table 2

Average contents of elements in oxidized and unaltered rocks of the Middle and Upper Jurassic
(based on the results of semiquantitative spectral analyses)

Continuation of Table 2

District of the village of Glinka

Note: n/o—not detected; +—traces.

Most of the elements listed, in the hypergenesis zone, according to A. P. Vinogradov [1], migrate in the form of anions or anionic carbonate complexes, bearing the same sign as organic sorbents. In this connection, a necessary condition for sorption is the preliminary destruction of ions with their conversion into the cationic form; for this, in some cases a change in the pH of the medium is sufficient, while in others a lowering of the oxidation–reduction potential is required. Natural waters are characterized by rather narrow limits of fluctuation in the pH of the medium (5.5–7.2 for our conditions); therefore, the influence of the concentration of the hydrogen ion on the sequence of precipitation of elements is limited, and for elements characterized by variable valence and a high geochemical enrichment factor, the observed sequence (Fig. 1) is connected with the limits of existence of oxidized–reduced pairs of ions.

A similar sequence of arrangement for elements with the highest enrichment factor of the natural series can be derived from the diagrams of Krauskopf and Garrels [3], from high to low values of the oxidation–reduction potential within real pH limits.

Some discrepancies are apparently explained by particular causes, which include concentration phenomena, different activity and hydration of ions, superposition of physical adsorption, etc. These same causes apparently explain a certain discrepancy between the natural sorption series and that derived from experimental data by G. B. Gapon [2], V. L. Zolotavina, and A. I. Kraeva [4] for sulfocoal:

\[ \mathrm{Hg}^{2+} > \mathrm{Ag}^{+} = \mathrm{Bi}^{3+} = \mathrm{Pb}^{2+} > \mathrm{UO}_{2}^{2+} > \mathrm{Cu}^{2+} = \mathrm{Fe}^{3+} = \mathrm{Cr}^{3+} = \mathrm{Cd}^{2+} = \]

\[ = \mathrm{Mg}^{2+} = \mathrm{VO}^{2+} > \mathrm{CO}^{2+} = \mathrm{Ni}^{2+} = \mathrm{Zn}^{2+}. \]

There is no doubt that, under conditions favorable for migration, the sorption of elements characterized by variable valence is governed by the laws of oxidation–reduction equilibria, which is favored by the long duration of geological processes.

In searching for dispersion halos associated with sorption processes, it is recommended (E. V. Rozhkova and M. B. Serebryakova [7]) that sorption series be compiled for different sorbents. The derivation of sorption series for sorbents with a sharply changing oxidation–reduction environment (coals, peat, soils) must be accompanied by additional investigations to determine the optimal conditions for sorption of an element. As is evident from Fig. 1, samples taken at random during metallometry may lead to an erroneous idea of the sorption capacity of a sorbent with respect to one or another element.

As has already been said, distortions of the geochemical background of rocks due to infiltration precipitation are significant, and when identifying it for stratigraphic correlations, rocks outside the oxidation zone should be sampled. According to the materials of our investigations, the width of the oxidation zone ranges from several tens of meters in coal seams to 2–3 km in alluvial, weakly cemented sandstones.

LITERATURE

1. A. P. Vinogradov. Geochemistry of Rare and Dispersed Chemical Elements in Soils. Publishing House of the Academy of Sciences of the USSR, 1950.

2. E. N. Gapon, G. M. Shuvaeva. DAN SSSR, 70, 1007, 1950.

1. L. S. Evseeva and A. I. Perel’man. The Geochemistry of Uranium in the Hypergenesis Zone. Gosatomizdat, 1962.

2. V. L. Zolotavina and A. I. Kraeva. Determination of the position of the vanadyl ion in sorption series on certain sorbents. Transactions of the Commission on Analytical Chemistry, vol. VI/IX. Publishing House of the Academy of Sciences of the USSR, 1955.

3. S. P. Kravkov. Soil Science. Sel’khozgiz, 1937.

4. A. N. Kondakov. Facies conditions of sediment accumulation of the Jurassic deposits of the western Kansk-Achinsk Basin. Proceedings of TPI, vol. 151, 1965.

5. E. V. Rozhkova and M. B. Serebryakova. The possible role of sorption in the formation of dispersion halos. Soviet Geology, no. 4, 1958.

6. N. M. Strakhov et al. Formation of Sediments in Modern Water Bodies. Publishing House of the Academy of Sciences of the USSR, 1954.

7. N. M. Strakhov. Fundamentals of the Theory of Lithogenesis (vols. I, II). Publishing House of the Academy of Sciences of the USSR, 1962.