Rishtan Fm

Includes in Fergana Valley, Isfara River

Synonym: Rishtan layers (Риштанские слои)

Reference section:

In Alai-Fergana basin, it is red mudstones with thin beds of red or grey siltstone and sandstone, some interbeds of marl and limestone with bivalves. 5-60 m.

Sandy_claystone. In the South Gissar and Dushanbe districts are mainly represented by sandstones, rarely gypsiferous clays and siltstones, mainly light gray in color with admixtures of volcanic ash. The thickness is 1-15 m. In most of the Kafirnigan district, the Rishtan layers are two-membered (gray sandstones and siltstones at the bottom, red-colored clays, siltstones, and sandstones at the top), or they are represented by alternately interbedded sandstones, siltstones, and clays. Locally, there is ash material. The thickness ranges from several meters to 75 m. In the eastern part of the Kafirnigan district, as well as in the Vakhsh and Zaalaik districts, the Rishtan layers have a three-membered structure. At the base of the section, gray sandstones and siltstones are distinguished. The middle part is composed of gypsums, and the upper part is red-colored clays, siltstones, and sandstones with interbeds of greenish-gray carbonate sandstones and marls. The thickness of the described layers is 70-185 m.

From the remains of the fauna, in the deposits of the Rishtan layers, Ostrea simplex Desh., Platygena asiatica Rom., Cordiopsis incrassata Sow., C. tenuis Alek., Pitar heberti Desh., Gari fischeri Bosq., Nucula aralensis Luk., Corbula rischtanensis Ivan., Panope heberti Bosq., Calyptraea operta Sol., C. lamellosa Desh., Athleta nodosa Sow., Turritella angulata Sow., T. uniangularis Lam. and foraminifera of the «Gavelinella vialovi» zone (Davidzon, 1970).