Proceedings of the Tomsk Order of the Red Banner of Labor Polytechnic Institute named after S. M. Kirov

Vol. 127, no. 1 — 1964

On the Composition of Cambrian Limestones from Some Sections of the Sayan-Altai Region

A. F. Senakolis

(Presented by Prof. A. M. Kuzmin)

The study of carbonate rocks of the Sayan-Altai region in the following Cambrian sections: 1) the Sukhie Solontsy tract, 2) the Sanashtykgol spring, 3) the Babakov spring, 4) the Gerasimov spring, 5) the Kizas River, was carried out with the aim of determining the possibility of distinguishing lithological rock types, characterizing their composition and structure, clarifying their genesis, and determining the content and distribution of minor elements in the different lithological types. Despite the difficulty of studying these rocks owing to their massive occurrence and apparent homogeneity, we have distinguished the following lithological types:

A. Biohermal limestones: 1) epiphyton limestones, 2) limestones with remains of creeping algae.

B. Clastic limestones—products of reef destruction: 1) detrital algal limestones, 2) carbonate breccias.

C. Rocks of complex genesis: 1) oolitic limestones, 2) oncolitic limestones.

A. Biohermal Limestones

Limestones formed with the participation of reef-building algae are assigned to this group of rocks.

1. Epiphyton limestones have a light-gray and light-coffee coloration and possess a homogeneous structure. However, on the polished surface a mottled coloration is visible, caused by the fact that the algal colonies are colored in somewhat lighter tones than the calcite aggregate cementing them. These rocks consist of the finest branches of algae of the genus Epiphyton, 0.08–0.1 mm thick. The algal colonies are arranged in life position. The algal branches are composed of microgranular calcite, which almost does not polarize light, and usually contain no mechanical impurities. Individual algal branches are cemented by micro- and fine-grained calcite (0.01–0.02 mm).

The space between the colonies is filled in the following manner: directly adjoining the algal tufts is a thin crust of microgranular calcite, fragments of branches of the same algae, and small lumps of microgranular calcite. The remaining interstices

between the colonies are overgrown with coarse-crystalline bladed calcite. The number of crystals is greatest in the immediate vicinity of the colonies; toward the middle of the cavity it decreases, and its central part is overgrown with relatively coarse-grained parallel-bladed calcite. In a number of cases the overgrowth of the spaces between colonies by bladed calcite is incomplete. The remaining voids are usually filled with fine-grained calcite containing a large amount of iron hydroxides, displaced during recrystallization to the margins of the grains. Such overgrowth of the free space between algal colonies by coarse-crystalline (0.2–0.3 mm) bladed calcite is a characteristic feature of the biohermal rocks of the sections described. In the literature on epiphytic limestones a different character of algal burial is noted. Thus, K. K. Zelenov writes that the space between algal colonies in Lower Cambrian limestones of the Siberian Platform is filled with “microcrystalline light-gray carbonate” [1].

1. Limestones with remains of encrusting algae have a dark-gray or variegated coloration; the latter is caused by the filling of pores with fine-grained calcite containing a large amount of iron hydroxides. The rocks are formed by carbonate encrusting algae and therefore have a parallel-fibrous, sometimes tangled-fibrous structure. Branches extend upward from the thallus. Interlayers with encrusting algae are usually overlain by micro- and fine-grained calcite, small fragments of archaeocyath cups, lumps of microgranular calcite, rare trilobite fragments, and bushes of Epiphyton. The limestones usually contain a large number of macropores, 0.5 to 2 cm in size, of irregular shape with numerous offshoots and “bays.” The larger cavities are filled with fine-grained calcite containing a large amount of iron hydroxides. The distribution of ferruginous formations in the cavities is uneven; for this reason the coloration of the filled voids is not uniform. Usually toward the periphery of a cavity it is lighter; toward the center, and sometimes also at its bottom, it is dark cherry-colored. Smaller cavities are filled with recrystallized calcite with an accumulation of ferruginous products in the center. In the thallus of the encrusting algae there are cavities elongated along the bedding of the organisms and filled with bladed calcite arranged perpendicular to the fibers.

B. Clastic limestones—products of reef destruction

1. Detrital algal limestones have varied coloration. Thus, in the Sukhie Solontsy stream they are gray and brown; the cavities and cracks formerly present in them are filled with siliceous material and iron hydroxides. Similar limestones of the Babakov spring and the Sanashtykgol spring are usually dark-gray, almost black, with pores filled either with transparent or black calcite. Archaeocyath cups are not uncommon in detrital algal limestones. Rocks of this type are composed mainly of various products of algal destruction: fragments of Epiphyton colonies, tubular algae, encrusting algae, lumps of microgranular calcite, oncolites, and also archaeocyath cups; more rarely, remains of trilobites, brachiopods, and ostracods occur. Fragments of Epiphyton algae occur both as individual twigs and as fragments of colonies; the latter usually have cementing them fine-grained calcite.

Fragments of tubular algae in thin section appear as hollow cylinders, the cavities of which are filled with fine- and microgranular

calcite. The walls are composed of darker cryptocrystalline calcite. The tubes are straight or slightly curved. The external diameter of the tubes ranges from 0.04 to 0.08 mm, the internal diameter from 0.02 to 0.03 mm, and the wall thickness from 0.02 to 0.04 mm. Encrusting algae occur as rare fragments and short interbeds that overlap the clastic material. Lumps of microgranular calcite have an oval and rounded form; not infrequently they are covered by a thin crust of cryptocrystalline calcite, which was apparently formed by enveloping algae; in this case such lumps are more properly called oncolites. Archaeocyaths in these limestones occur as fragments or entire cups. The walls and septal partitions of the archaeocyaths are composed of microgranular calcite; the interseptal chambers and central cavity are filled with coarse-crystalline fibrous calcite growing perpendicular to the wall of the organism.

Fragments of trilobite carapaces in thin section have the form of fantastically curved, sinuous bands. Ostracod remains occur both as whole shells and as individual valves. Their dimensions amount to fractions of a millimeter. Ostracod shells have an elliptically oval form, with coarse-crystalline (0.1–0.3 mm) calcite inside. Trilobite remains and ostracod valves are composed of thin calcite crystals that are difficult to distinguish, arranged perpendicular to the outer surfaces of the shells; as a result, under crossed nicols and upon rotation of the microscope stage, an extinction wave passes along the shell. Brachiopod valves have a fine-prismatic structure, characterized by distinct prisms that extinguish individually and are usually oriented at an angle or perpendicular to the surface of the shell.

All this clastic material is cemented to varying degrees by recrystallized fine- and small-grained calcite.

2. Carbonate breccias. In the present work we consider only syngenetic carbonate breccias. They are composed of fragments of biogenic, detrital-algal, and other types of rocks. Mechanical destruction of reefs and partial dissolution of fragments take part in the formation of the terrigenous material. The size of the latter ranges from 1 to 6 mm across, sometimes up to 10 mm. As a rule, their form is unrounded, with slightly smoothed edges. The fragments are in point contact and are embedded in carbonate cement. The latter is represented by an aggregate of coarse-crystalline calcite, while the fragments themselves are surrounded by a crustification rim of the same calcite. In addition, there are breccias whose clastic material is cemented by coarse-crystalline calcite with barite. The latter is colorless in thin section, \(N_p^1 = 1.636\), \(+2V = 37^\circ\); the crystals are tabular in form, although radial-fibrous intergrowths are also not rare. The barite content in individual cases reaches 17%. In some cases, as a result of the preferential dissolution of microgranular calcite cementing the coarse-grained fibrous calcite, the latter is released and rocks are formed that consist of acutely angular fragments composed of coarse-crystalline fibrous calcite. This clastic material is cemented by microgranular calcite.

B. Rocks of Complex Genesis

1. Oolitic limestones were encountered on the Kizas River. The rocks have various colors, from light gray to dark gray, and a distinct oolitic structure. The sizes of the ooliths range from 3 to 7, sometimes up to 10 mm. Their form is regular, spherical; flat ones are sometimes encountered

oolites. The form of the latter depends on the shape of the oolite nucleus, which may be:

1) fragments of algal limestones, 2) fragments of the crusts of the oolites themselves; in this case their form is flattened, 3) lumps of microgranular calcite. The oolite crust is composed of an aggregate of thin-platy calcite with plate thickness of about 0.01 mm and length from 0.03–0.04 to 0.1 mm. The calcite plates are arranged tangentially along the circumference of the oolites and have very distinct outlines. The thin-platy calcite is colored a slightly brownish shade by bituminous matter. The thickness of the accretion crust may be uniform or may vary over the surface of the oolite, which indicates uniformity or nonuniformity in its overgrowth by chemogenic material. The character of the accretion of the latter, the degree of sorting and size of the oolites, the number of concentric surfaces, and the character of the cement—all this characterizes the duration of oolite formation and the strength of nearshore wave action (Table 1).

Table 1

Recrystallization is widely developed in the oolites; it develops both in the center and along individual concentric interlayers. The thin-platy crystals, becoming coarser, form grains of irregular shape with serrated boundaries between them. The spherical character of the boundary between the concentric layers is preserved; sometimes individual concentric layers composed of thin-platy calcite remain. Within large grains of calcite from recrystallized concentric interlayers, relics of thin-platy calcite are observed—thin bands, 0.01 mm thick, colored by bituminous matter and arranged along the circumference. Solutions containing siliceous matter took part in the recrystallization of the oolites. This is evident from the fact that individual pores between oolites are filled with microgranular chalcedony-like quartz. The oolites adjoining these pores are most intensely recrystallized and contain small quartz crystals elongated along the circumference.

1. Oncolitic limestones are macroscopically light-colored and of homogeneous structure. In thin section it is seen that they consist of onco-

lites, the sizes of which range from 0.4 to 2–3 mm. Their shape is mainly rounded, sometimes elongated, with a length-to-width ratio of 1:2, 1:3. At the center of such formations are fragments of detrital–algal limestones, lumps of micro- and fine-grained calcite, rarely remains of trilobite carapaces and large grains of calcite. These fragments are overgrown by concentric coatings resembling oolitic crusts. The thickness of such coatings ranges from hundredths of a millimeter to 0.2–0.3 mm. The concentric structure is expressed by an alternation of bands composed of micro- and fine-grained calcite.

The bands of lighter, fine-grained calcite are the product of recrystallization of primary cryptocrystalline calcite. Such recrystallized interlayers alternate with interlayers of microgranular calcite, recrystallized to a considerably lesser degree, and have approximately the same thickness (0.01–0.02 mm). It should be noted that recrystallization also affects the interlayers of microgranular calcite. In this case it is evident how individual grains of calcite from the recrystallized bands penetrate, in the form of microveinlets, into the mass of microgranular calcite. This character of recrystallization gives the oncolites the appearance of a radially fibrous structure. The concentric surfaces grow upon the fragment, copying its form and preserving its projections and irregularities. Therefore even formations of rounded shape have uneven concentric interlayers. Thus, the fragment composing the nucleus of these formations, and having long projections and depressions, preserves them when overgrown by concentric interlayers. Such a structure does not permit the idea that they formed in an environment of agitation and rolling, where smoothing of the relief of the fragments would necessarily have taken place. On the outside, the oncolites are surrounded by a rim, about 0.1 mm thick, of finely crystalline sheaf-like calcite, which grows perpendicular to the surface of the oncolite. The remaining part of the interstices is cemented by coarsely crystalline calcite with grain sizes of 0.2–0.5 mm.

Chemical composition of the limestones

Rocks of different lithological types were subjected to chemical analysis. The analyses, 26 in number, were carried out by A. S. Lozova, chemist of the Geological Problem Laboratory at the S. M. Kirov Tomsk Polytechnic Institute. As can be seen from Table 2, the rocks of the lithological types under consideration are pure limestones. The calcite content ranges from 90.88 to 99.42%, and dolomite from 0.38 to 6.84% (Table 2). The distribution of magnesia oxide and dolomite among the various lithological types of rocks is shown in Fig. 1. The content of insoluble mineral residue ranges from 0.34 to 2.48%. It is represented by small elongated prismatic crystals of colorless quartz, showing no detrital nucleus. During growth, the crystals captured fine carbonate dust, which was arranged zonally within the crystals. Similar newly formed quartz has been noted in the literature by Yu. A. Khodak [3] in Lower Cambrian limestones of the Aldan region, and by L. M. Miropol’skii [2] from Lower Permian deposits of Tataria. In the insoluble residue there are also lumps of fine-grained chalcedony-like quartz, accumulations of iron hydroxides, flakes of clay minerals, and, rarely, fragments of quartz, feldspars, and effusive rocks. Thus, even the small insoluble residue present consists mainly of newly formed silica.

Table 2

Fig. 1. Distribution of magnesium oxide and dolomite in various lithological types.

Labels in the figure:

• MgO content, %
• Dolomite content, %
• epiphyton
• detrital–algal
• oncolitic
• oolitic
• carbonate breccias
• limestones with stylolites and algae
• dol. — dolomite
• MgO

Table 3

Content and distribution of minor elements

The distribution of minor elements is most clearly seen in the limestone massif of the Sukhiye Solontsy stream. Here one part of the massif is composed mainly of biohermal limestones with a small participation of detrital-algal and clastic limestones. This part of the massif corresponds more to the inner part of the reef. The other, marginal, part of the massif consists chiefly of detrital-algal and clastic limestones. Epiphyton limestones and limestones with remains of creeping algae are rare here. The limestones of this section, like the limestones of the other sections studied, contain Ba, Sr, Cu, Mn, and, rarely, traces of Pb, Ti, V (Table 3). The character of the distribution of minor elements in the different lithological types is shown in Fig. 2.

Thus, the carbonate rocks studied indicate the broad

Fig. 2. Distribution of minor elements in different lithological types.

development, in the Cambrian deposits of the Sayan-Altai region, of rocks of reef facies.

REFERENCES

1. N. A. Arkhangel’skaya, V. N. Grigor’ev, K. K. Zelenov. Facies of Lower Cambrian deposits of the southern and western margins of the Siberian Platform. Transactions of the Geological Institute, Academy of Sciences of the USSR, issue 33, 1960.

2. L. M. Miropol’skii. On secondary segregations of quartz in Lower Permian deposits of Tataria. Doklady of the Academy of Sciences of the USSR, vol. 36, No. 2, 1942.

3. Yu. A. Khodak. Secondary minerals of Lower Cambrian deposits of the Aldan region of the Yakut ASSR. Transactions of the Geological Institute, Academy of Sciences of the USSR, issue 5, 1956.