SOME POST-SEDIMENTARY CHANGES IN CARBONATE DEPOSITS OF THE CHINGASAN SERIES

(YENISEI RIDGE)

V. D. Gavrilenko

(Presented by Professor A. M. Kuzmin)

To determine the geochemical setting of sediment accumulation, in addition to the structural features and conditions of occurrence of rocks, it is necessary to know their primary material composition and structural-textural features without those superimposed secondary features that arise in the process of post-sedimentary transformation of deposits. In this connection, the recognition and study of the character and degree of secondary changes in rocks acquires considerable importance.

The present note describes the most widespread post-sedimentary changes in the carbonate deposits of the Chingasan Series in the northeastern part of the Yenisei Ridge (the Uvolga River basin). Carbonate deposits are most widely developed here at two stratigraphic levels, composing the upper part of the Lopatino Formation and the greater part of the Pod’emnaya Formation. The variegated carbonate deposits of the Lopatino Formation are subdivided into two members: a lower terrigenous-limestone member and an upper dolomite member. The lower member consists of dark-gray, almost black dolomitic limestones, containing intervals of frequent alternation of light-gray or dark-gray platy limestones, calcareous aleurolites, and marls. The deposits of the upper member are represented exclusively by pure dolomitic (chemogenic and biochemogenic) rocks.

The red-colored carbonate deposits of the Pod’emnaya Formation are also subdivided into two members: a lower dolomite member, composed of cherry-red aleuritic-clayey dolomites, and an upper sandy-limestone member, represented by alternating variegated sandstones, oncolitic limestones, and aleurolites. Practically all carbonate deposits of the series prove to be secondarily altered to one degree or another. The following types of alteration are distinguished: dolomitization and silicification.

Dolomitization of the limestones of the Chingasan Series is a rather widespread phenomenon. It is manifested on different scales. Weakly dolomitized limestones (Pod’emnaya Formation) macroscopically preserve all structural-textural features of the primary rocks. Under the microscope, the development of relatively uniformly disseminated dolomite crystals of rhombohedral form, or of isometric grains, is clearly observed. Their characteristic feature is a fairly distinctly expressed zonal structure and the constantly observed brownish-red rims. S. G. Vishnyakov [2] relates the formation of the latter to phenomena of sorption of iron hydroxides.

surfaces of dolomite crystals. With a broader development of dolomitization phenomena (the lower part of the terrigenous–limestone member of the Lopatina Formation), indistinctly separated bodies of dolomitized limestones are formed, having the most varied shapes and an unusual variability in the degree of dolomitization, up to the formation of weakly calcareous dolomites. The rocks are characterized by spotted and banded textures and a sharply expressed inequigranular structure.

Silicification. Siliceous formations in the carbonate deposits of the Chingasan Series are not widely distributed, but they are confined to certain stratigraphic levels (the dolomite member of the Lopatina Formation), which, as a result, may be used as marker horizons. Silicification is developed mainly in oncolitic dolomites. Such confinement of silicification to rocks possessing elevated porosity may be regarded as an indication of its connection with the circulation of groundwater [1]. Among the siliceous formations of the dolomite member of the Lopatina Formation, two types are distinguished: unevenly silicified portions of dolomites, and secretions, geodes, and cross-cutting veinlets.

In outward appearance, silicified portions of dolomites differ from unaltered ones by a somewhat lighter color and a distinctly inequigranular fabric. Their fracture is splintery or conchoidal. Macroscopically, the uneven, spotted-banded or spotted-patterned character of silicification is quite clearly established. It is known [3] that the structure of silicified rocks is determined chiefly by the structure of the rocks being replaced and by the degree of recrystallization of silica. Depending on the primary microstructure of the nodules and of the cementing mass of the oncolitic dolomites, two varieties of silicified rocks are distinguished, differing in mineral composition and structural-textural features.

1. Oncolitic dolomites with nodules consisting of micro- to fine-grained dolomite underwent silicification; the cementing mass is represented by fine- to small-grained material. The coarser-grained carbonate material of the cement is replaced first, whereas the fine-grained dolomite of the nodules is replaced last. As a result, the nodules prove to be composed mainly of micrograined chalcedony, against the background of which more recrystallized areas sometimes appear, while the cementing mass is composed of distinctly inequigranular fine-platy chalcedony, sometimes mixed with quartz. The size of the chalcedony plates ranges from hundredths of a millimeter to 0.65 mm. The original carbonate material is preserved as a pelitomorphic or fine-grained rim outlining the nodules, and more rarely as relicts within the nodules. In some cases the rim proves to be completely replaced, and the nodules merge with one another, being only indistinctly discernible by the more recrystallized chalcedony of the pore spaces.

2. Oncolitic dolomites with nodules consisting of fine-grained material underwent silicification; the pore, more rarely basal, cement is represented by pelitomorphic or micrograined dolomite. The process begins with replacement of the fine-grained dolomite of the nodules by platy chalcedony (sometimes mixed with quartz), with simultaneous recrystallization of the pelitomorphic–micrograined material of the cement into a radially fibrous aggregate, forming encrustation rims around the nodules and practically completely filling the pore space. Characteristically, in this case the silicification process does not proceed further than the nodules. In some cases, despite complete recrystallization of the cementing

masses and nodules, the siliceous material very clearly records the internal microstructure of the rocks. Thus, in oncolites of the Radiosus group, the outer radial shell and the granular inner core are clearly visible. In oncolites of the Osagia group, the primary concentric-layered structure is also distinctly expressed.

In addition to silicified areas, small geodes and secretional formations are quite often encountered in the dolomites; these represent the results of complete or partial filling of large pores or fractures. Secretional formations in the dolomites of the Lopatin Formation are, as a rule, confined to fine-grained and clotted-pelitomorphic dolomites. Their form is highly varied—from round and ellipsoidal to irregularly angular—and probably depends entirely on the morphology of the space being filled. Their sizes vary widely—from microscopic to 3–5 cm along the long axis. The process of their formation was multistage. At first the cavities of the pores were filled with gel-like silica, which, upon dehydration, formed a comb-like, radial, or rosette-like aggregate lining the pore cavities, as well as, in favorable places (recesses), buds with a radial-rayed structure. Later, the central parts of the geodes that had formed were filled by subsequent portions of solution, with precipitation of coarse-lumpy chalcedony, quartz, and calcite. The latter, in addition to filling cavities, together with quartz rather often forms veinlets cutting both the main mass of the rocks and the quartz-chalcedony secretions.

The material presented indicates a fairly broad manifestation of processes of post-sedimentary alteration of the carbonate rocks of the Chingasan Series, and the need to take this phenomenon into account when deciphering the conditions of sedimentogenesis in Chingasan time.

REFERENCES

1. Z. L. Bogdanova. On the lithology of carbonate rocks of the Lower Carboniferous of the Moscow Basin. In: Collection in Memory of S. F. Malyavkin. All-Union Geological Institute, Leningrad, 1940.

2. S. G. Vishnyakov. Genetic types of dolomite rocks of the northwestern margin of the Russian Platform. Transactions of the Geological Institute, Academy of Sciences of the USSR, issue 4, 1956.

3. S. G. Vishnyakov. Siliceous formations in carbonate rocks of the Lower and Middle Carboniferous of the northwestern flank of the Moscow Basin. Proceedings of the Academy of Sciences of the USSR, Geological Series, No. 4, 1953.