Akchin Fm

It is identified in 1939 by Ye.A. Kochnev.

Synonym: Akchinsk Fm

Reference section:

This suite begins the section of the powerful upper Paleozoic-Triassic volcanic series of Karamazar, to which other suites also belong: Nadak (C2-3? nd), Oyayskaya (C3 os), Shurabskaya (P1 sr), Ravashskaya (P2? гv), and Kyzyl-Nurinskaya (P2-T1 kz). All of them do not form continuous coverings, but lie in isolated graben-synclinal depressions and in volcano-tectonic depressions of subsidence.

Graben-synclines are characteristic of the Western Karamazar. They are narrow (2-5 km) structures, stretched for tens of kilometers in a sublatitudinal direction, bounded or bounded by faults. Volcano-tectonic depressions of subsidence are more typical of the Central and Eastern Karamazar. These are isometric structures in plan, occupying areas of hundreds of square kilometers. Graben-synclines are primarily filled with relatively older formations of the Karamazar volcanic series, while the depressions of subsidence are younger. However, there are cases of superimposition of the depressions of subsidence on graben-synclines. On the uplifts separating individual grabens and depressions, volcanics have been preserved only in the form of subeffusive or subvolcanic facies.

To date, the sedimentary-volcanogenic formations of each graben-syncline and depression of subsidence studied by geologists in Karamazar have been subdivided into a series of thicknesses characteristic of each such structure. In the thicknesses, not only stratified formations but also subvolcanic and subeffusive ones have been included, since they cannot always be distinguished under conditions of similarity of rocks, significant secondary changes, and manifestations of numerous faults. The units used to correlate the thicknesses are suites of the N.P. scheme. Vasilyevsky (1952). However, the attribution of specific layers to one or another suite is often controversial and varies among different researchers, as in the absence of a sufficient amount of organic remains, it is based on features used by different researchers differently: petrographic and petrochemical properties of rocks and layers, their positions relative to intrusive complexes, fault systems, etc. We do not describe individual layers and do not consider issues of their correlation. The scheme adopted in this monograph is used by the team of geologists of the Geological Management of the Council of Ministers of the Tajik SSR, studying Karamazar.

The most complete sections of the Akchinsk suite (акчинской свиты) are observed in the Tokmak graben and in the southern part of the Adrasman depression. The generalized section of the suite for the Karamazar region is as follows (from bottom to top):

1 - conglomerates, sandstones, tuffs, liparite-dacites and their clastolava, thickness 100-400 m (Chashlinskaya and Sassyksay layers) (ашIинская и сассыксайская толши);

2 - tuff sandstones, tuffs, andesites, their clastolava, thickness 200 - 700 m (Myshikol, Tokmak, Doloninskaya layers) (мышиккольакая, токмакская, доонинская толщи);

3 - fluidal quartz latites and their ignimbrites, thickness 200-500 m (Tabosharskaya and Pzamanskaya layers) (табошарская и пазманская толщи);

4 - gravels, sandstones and clay slates, thickness 100 - 300 m (Chetsuiskaya layer) (четсуйская толща);

5 - gray and greenish-gray andesite-dacites, less often their tuffs, thickness about 500 m (Djaman Saiskaya and Dzeltimesskaya layers) (джамансайская и джелтииесская толщи);

6-gray and lilac dacites, their clastolava, less often their tuffs, thickness 200-950 m (Jangalikskaya, Dzhanbulakskaya, Ishumskaya layers) (джангалыкская, джанбулакская, ишумская толщи);

7 - lilac and purple coarse-porphyritic andesites, thickness 100-200 m (Khojiulgenskaya, Burybulakskaya and other layers) (ходжиу:ьгенская, бурыбулакская и другие толщи);

8 – gray and reddish-gray liparites, sometimes with sandstones and conglomerates at the base, thickness about 600 m (Karkhaninskaya layer) ( карханинская толща);

9 - pinkish-gray and reddish-brown liparites and their clastolava, above liparite-dacites and their clastolava, thickness 500--1000 m (Utkemsuiskaya layer) (уткемсуйская толща).

The thickness of the Akchin suite is 900 - 2400 m.

In the clay slates of the pack № 4 (Chetsuiskaya layer) (четсуйская толща), remains of algae have been found, which, according to K. B. Qorde's definition, have a Middle Carboniferous appearance. Ortonella artemovae Korde, Ort. upensis Birind, Ort. furcata Garwood. No other organic remains have been found in the suite. In determining the age of the Akchinsk suite, in addition to these remains, the following is taken into account. The age of the older granitoids of the Karamazar complex is, according to radiological data, on average 320 million years, which corresponds to the Middle Carboniferous, and in the overlying Nadak suite, Middle Carboniferous organic remains are also known.

V. N. Baykov, V. V. Mikhailov, and V. A. Moskvitzev distinguished in 1968 the upper part of the described formations, starting with the mentioned Chetsuy thickness, into a separate Jamansay suite (джамансайскую свиту) based on their occurrence with a deep erosion on the preceding rocks and manifestations of the preceding post-magmatic activity. The Jamansay suite is well distinguished in structures that have already been covered by large-scale geological mapping. The results of the ongoing geological mapping of the Eastern Karamazar will show whether the Jamansay suite can be distinguished for the entire Karamazar region.