ON THE STRATIGRAPHY OF THE MIDDLE CAMBRIAN DEPOSITS IN THE AREA OF OKTYABRSKY SETTLEMENT (BATENEVSKY RIDGE)

A. K. SEMASHKO, T. V. YANKAUSKAS

(Presented by Professor A. G. Sivov)

The purpose of this article is to refine the geological structure of one of the sectors of the Batenevsky Ridge, located in the area of Oktyabrsky Settlement, 1.5 km southeast of the Yuliya mine.

Carbonate, sandy-shale, and effusive rocks take part in the geological structure of the area.

The age of the carbonate and terrigenous sequences has been established by various authors conditionally and often incorrectly, which was due to the absence of paleontological data.

For example, V. D. Tomashpolskaya [3] correlated the deposits of the terrigenous sequence with the Yuliya Formation, while R. T. Bognibova [1] regards the same rocks as the most ancient Cambrian formations of the area.

As can be seen from the geological map (Fig. 1), structurally the area is an anticlinal fold of northeastern strike, complicated by a tectonic disturbance and a small syenite-diorite intrusion.

In the closure of the anticline a dolomite sequence is exposed, forming the sides of the subsequent valley of the Sukhoi Erba River. The dolomites are a fine-grained, dense, light-gray rock with a characteristic weathering surface and a shiny sandstone-like fracture.

Lens-shaped bodies of gray and light-gray silicilites with abundant ferruginization along fractures are associated with the dolomite sequence. Admixtures of iron hydroxides give the rock a characteristic brownish color. The lenses of siliceous rocks extend in a northeastern direction, repeat the general structure of the area, and evidently are confined to a definite stratigraphic level within the light-colored massive dolomites.

Over the described dolomites (which we correlate with the dolomites of the Martyukha Formation of Riphean age), gray, massive, weakly bituminous, microspotted limestones of fine-grained, locally pelitomorphic structure occur with a major break. In these limestones on Mount Sarainaya (right bank of the Sukhoi Erba River, point 282), a fauna of trilobites and archaeocyaths was found. The trilobites are represented by the following forms: Kooteniella cellulifera Lerm., Kooteniella sp., Bonnia globosa Tomashp., Kootenia sp., Edelsteinaspis ornata Lerm., Solontzella enorma sp. nov., Chondranomocare sp.; archaeocyaths: Tegerocyathus sp., Cribricyathus Vologd. The presence of the genus Chondranomocare indicates a younger age of the enclosing rocks than the limestones of Mount Dolgii Mys.

(the stratotype of the Obruchev horizon). The presence of trilobites Solontzella enorma sp. n. indicates a broader range of vertical distribution for this genus, which was previously considered characteristic of the “Solontsovsky” horizon.

SECTION ALONG LINE I–I

Fig. 1. Geological map of the area of Oktyabrsky settlement: 1—gray and light-gray dolomites; 2—silicilite lenses; 3—light massive limestones; 4—sandy-clayey deposits with limestone interbeds; 5—oncolitic limestones; 6—dark-gray platy, oncolitic limestones; 7—effusives; 8—intrusion of syenites and granosyenites; 9—tectonic faults; 10—fauna collection points: a—archaeocyaths, b—trilobites.

We determine the age of the enclosing sequence as the very lowermost part of the Middle Cambrian. Thus, the hiatus in time corresponds to the entire Lower Cambrian.

Upon the gray massive limestones there lies a sandy-shaly sequence with interbeds of almost black bedded limestones. Being present in subordinate quantity on the northwestern limb of the anticline, limestones predominate on the southeastern one; the reverse picture is observed with respect to the terrigenous rocks.

The sandstones are represented by fine- and very fine-grained varieties of brownish or greenish-brown color. Bedding is clearly expressed on the weathered surface.

Under the microscope a fine-grained structure is observed; the rock fragments are weakly rounded and consist of intensely sericitized and pelitized plagioclase, quartz, and limestone grains.

Within the sequence the predominant importance belongs to siltstones, similar in composition to the sandstones described above.

The sandy-shaly sequence lies upon the underlying massive limestones with a break and an azimuthal unconformity, which is distinctly expressed in the area of Mt. Skalistaya (west of the settlement of Oktyabrsky).

On Mt. Balykhtag [2], above deposits corresponding in age to the gray massive limestones of Mt. Sarainaya, there occur limestones of members “B” and “V,” absent from the section being described.

In the area studied, in its southwestern part, the contact of the two sequences is exposed by a ditch, where an abrupt change is observed from the massive limestones of the carbonate sequence to the bedded sandstones and siltstones of the younger carbonate-terrigenous sequence.

In one of the limestone interbeds of the upper sequence (left bank of the Sukhaya Erba River), 30 meters stratigraphically above its base, trilobites were found: Oryctocephalops frischenfeldi Lerm., Prohedinella erbiensis Sivov, Gaphuraspis cf. gaphuri Ivsh., Pachyaspis sp., Olenoides sp., Bathynotus sp., and some new forms.

The paleontological data indicate the undoubtedly Middle Cambrian age of the enclosing deposits.

In the sandy-shaly rocks of the southeastern limb of the anticline, in interbeds of black limestones (point 27), trilobites were found whose generic and species composition corresponds completely to the composition of the trilobites of the northwestern limb. The thickness of the terrigenous-carbonate sequence in the southeastern part of the area is considerably reduced, since within the sequence there is a tectonic disturbance of northeastern strike. In the upper part of the sequence, in dark-gray platy oncolitic limestones (point 291), trilobites Tankhella sp. and Oryctocephalidae were found.

The relationship of the terrigenous-carbonate sequence described here with the underlying archaeocyathid-trilobite limestones is probably tectonic, since near the contact strongly brecciated varieties of terrigenous rocks are observed.

The terrigenous-carbonate sequence is overlain by light-colored massive, strongly metamorphosed limestones, upon which effusive formations of Devonian (?) age lie with angular unconformity [4].

REFERENCES

1. R. T. Bognibova. Stratigraphic position and age of the archaeocyathid-trilobite limestone member in the area of Mt. Arkheotsiatovaya and Sladkie Korenya (Batenevsky Ridge). Transactions of SNIIGGiMS, issue 34, 1965.

2. A. K. Semashko, V. A. Shipitsyn. On the biostratigraphy of Cambrian deposits in the area of Mt. Balykhtag (Batenevsky Ridge). Izvestiya TPI, vol. 166, 1967.

3. V. D. Tomashpolskaya. Author’s abstract of dissertation, 1964.

4. V. M. Yaroshevich. Stratigraphy of Sinian and Cambrian deposits of the Batenevsky Ridge, the Azyrtal Range, and the basin of the Bely Iyus River. Transactions of the Institute of Geology and Geophysics, Siberian Branch, Academy of Sciences of the USSR, issue 17, 1962.