ON INCIPIENT OOLITE FORMATION IN THE CRETACEOUS DEPOSITS OF THE BEREZOVKA TROUGH (KANSK–ACHINSK COAL BASIN)

A. N. Kondakov

(Presented by Prof. A. M. Kuzmin)

In the Cretaceous deposits of the central part of the Berezovka trough, lying 60–70 m above the roof of the Berezovsky coal seam (boreholes 899 and 902), associated works revealed segregations of gibbsite filling individual cavities, as well as microlites of the composition: gibbsite–iron oxide hydrates.

The gibbsite-bearing deposits are represented by greenish-gray massive siltstones and silty argillites with unsorted, unevenly distributed silty material. In adjacent parts of the section, a freshwater fauna has been found, making it possible to assume subaqueous conditions of sediment accumulation.

Fig. 1. Gibbsite segregations and the morphology of oolites in the sediment.
a) cavities filled with gibbsite: 1 — main mass of the rock, 2 — limonite, 3 — gibbsite. Magnification 15×; b) increase in the content of iron oxide hydrate (dark) toward the center of the oolitic structure. Magnification 300×; c) asymmetric distribution of iron oxide hydrate in concenters (the arrow indicates the direction of diffusion of the external component); d) gibbsite segregations in the clayey mass of the rock. Magnification 900×;

In the siltstones exposed by borehole 899 at a depth of 14 m, gibbsite was encountered filling voids and fractures. The segregations consist of aggregates of cryptocrystalline structure and six-sided crystals of gibbsite (Fig. 1, a).

Of definite interest for understanding the mode of formation of oolitic structures are oolites composed of gibbsite and iron hydroxides. Oolites of this composition have been established in siltstones penetrated by borehole 902 at depths of 36 and 52 m. The oolites have either a predominantly gibbsitic or a predominantly iron-hydroxide composition. The oolites are small in size (0.04–0.01 mm and smaller). The larger oolitic grains consist chiefly of iron hydroxides. The distribution of the latter within the oolitic structure is regular. The content of iron oxide hydrate increases toward the center, whereas the content of gibbsite increases toward the periphery of the structure (Fig. 1, б). On the outside, the oolitic grains are, as a rule, bordered by a dark, thin shell of iron oxide hydrate. The first outer concentric zone of gibbsite contains almost no iron hydroxides; the next concentric zone of gibbsite has a faint brownish coloration, and from the outside separate tongues of iron oxide hydrate penetrate into it; the third concentric zone is found in a limited number of oolitic grains owing to the ring-shaped clearing of the brown iron-hydroxide substance in the inner part of the oolitic grains. At their center is a dark, opaque mass of iron oxide hydrate. The intermediate concentric zones of the latter have an asymmetric structure, with the maximum precipitation of iron oxide hydrate shifted within the concentric zones closer to their outer part (Fig. 1, в). The small bodies are composed of gibbsite, sometimes monocrystalline; however, even in them, in the central parts, there are weakly expressed light concentric zones of iron oxide hydrate with an asymmetric structure (Fig. 1, г).

The oolites are unevenly distributed in the main aleuritic-clayey mass of the rock and are sometimes bound by a rarely occurring spotted leptochlorite cement, which indicates the reducing character of the pore waters of the final stages of transformation of the sediment. Individual oolitic grains are bound by a single common outer shell.

The oolites undoubtedly formed in place in the sediment during the course of a periodic Liesegang reaction in a closed contour, according to the principle set forth by A. M. Kuz’min [1]. Pore solutions are characterized by a varied physicochemical environment and by uneven impregnation of the sediment, which leads to the formation in the sediment of closed contours whose substance differs, at least, by undersaturation with respect to individual components, and sometimes by entirely different physicochemical conditions. Diffusion of external components of the surrounding solution into the contour is accompanied by periodic precipitation of matter (in the present case, iron hydroxide), which is caused, on the one hand, by zonal sorption, as established by G. L. Pospelov and T. I. Kaushanskaya [2], and, on the other, by periodic disturbance of ionic equilibrium in the diffusing solution. The precipitating substance displaces the components that composed the substance of the medium of the closed contour in the ionic state [2], which is accompanied by the emergence of counter-diffusion. Part of the substance of the counter-flow may be deposited in parts of the structure not occupied by iron hydroxides, in minerals existing under the new physicochemical conditions (gibbsite). The growth of the latter also leads to ionic replacement of the minerals of the medium. Ultimately, the structure may be made up entirely of the “regular” mineral, while the displaced components are either removed by the pore solution or deposited there in the form of concretions and fill cracks and pores of the surrounding mass of sediment.

The oolite formation described above in Cretaceous sediments has a limited and incomplete character; however, it vividly illustrates oolite formation in sedimentary iron-ore processes. The oolites of another mineralogical composition that arise in the sediment, but in...

principle set forth above create only the prerequisites for deposits, which are realized through the constant washing-out of ooliths from the oolite-forming medium, with the subsequent concentration of oolitic material in areas determined by the hydrodynamic properties of the transporting current.

In conclusion, it should be noted what an enormous role is played by processes of oolite formation of the Liesegang-reaction type in the general transformation of the sedimentary substance. The specific conditions of opposing currents in a developing oolitic structure lead to metasomatic displacement and ionic decomposition of the minerals of the medium of the closed contour, even those stable in the physicochemical environment of the sediments. In mass oolite formation, the process leads to complete reworking of the original parent material, as is observed in bauxite deposits.

REFERENCES

1. A. M. Kuz’min. Periodic-rhythmic phenomena in mineralogy and geology. Dissertation, TPI, 1947.

2. G. L. Pospelov, P. I. Kaushanskaya. Sorption and chromatographic processes in ore formation. Kolloidnyi zhurnal, vol. XXV, No. 2, 1963.