**3. Lithostratigraphy of the formations of the Irkutsk Basin**

**The Cheremkhovskaya Formation** was first identified by Korovin [23] in mines within the Cheremkhovo settlement vicinity. It represents a complete megarhythm and is divided into three subformations: lower, middle, and upper ones with total thickness up to 380 m.

**Lower Cheremkhovskaya (Zalarinskaya) Subformation** occurs with discordance on Cambrian limestones of Angarskaya Group. The subformation base is composed of clastic deposits—conglomerates, gravelites, and coarse‐grained sandstones which are overlapped by siltstones, mudstones with thin coal bands.

On the right bank of the Zalari river, on the opposite side of the Zalari settlement is situated the stratotype of Lower Cheremkhovskaya Subformation (bottom‐up):


Clastic deposits of the lower subformation are traced by us in some natural outcrops along the ranks of the Bol'shaya Belaya river.

Industrially coal‐bearing **Middle Cheremkhovskaya Subformation** is a natural sedimentary continuation of the lower subformation, and it is related to the latter by gradual transitions. This subformation is characterized by abundance in section of siltstones, mudstones, and coal seams with thickness up to 10–15 m. The thickness was cumulated during the epoch of maxi‐ mum regional coal storage within the Basin and is abundant all over. Lower boundary line is drawn in bottom of siltstones and sandstones underlying the first coal‐bearing horizon.

deposits—conglomerates, gravelites, and coarse‐grained sandstones which are overlapped by

On the right bank of the Zalari river, on the opposite side of the Zalari settlement is situated

horizontal bedding and abundant plant detrital matter

**Thickness, m**

0.65

0.70

0.60

1.10

3.54

0.75

Clastic deposits of the lower subformation are traced by us in some natural outcrops along the

and siltstones with horizontal bedding and impressions of *Cladophlebis haiburnensis* (L. et H.) Sew., *Sphenobaiera ex gr. czekanowskiana* (Heer) Flor., *Czekanowskia ex gr. rigida* Heer, *Podozamites eichwaldii* Pryn. var. *minor* Pryn. *Ixostrobus grandis*

Industrially coal‐bearing **Middle Cheremkhovskaya Subformation** is a natural sedimentary continuation of the lower subformation, and it is related to the latter by gradual transitions. This subformation is characterized by abundance in section of siltstones, mudstones, and coal seams with thickness up to 10–15 m. The thickness was cumulated during the epoch of maxi‐ mum regional coal storage within the Basin and is abundant all over. Lower boundary line is drawn in bottom of siltstones and sandstones underlying the first coal‐bearing horizon.

the stratotype of Lower Cheremkhovskaya Subformation (bottom‐up):

84 Seismic and Sequence Stratigraphy and Integrated Stratigraphy - New Insights and Contributions

2. Sandstones, coarse‐grained, massive 1.34 3. Conglomerates, fine‐pebbled 2.00 4. Sandstones, white, coarse‐grained, massive 0.30 5. Conglomerates, fine‐pebbled 0.80 Disappearance of outcrop 3.00

Disappearance of outcrop 3.50 7. Sandstones, pale gray, coarse‐grained, massive 0.60

Disappearance of outcrop 1.39

Developed thickness 13.12

1. Conglomerates interstratifying with coarse‐grained sandstones

6. Sandstones, gray, medium‐fine‐grained with

8. Sandstones, gray, medium‐grained with horizontal

9. Sandstones, red, medium‐fine‐grained with horizontal

10. Alternation of red, burnt rocks presented by sandstones

11. Speckled sandstones, coarse‐medium‐grained with horizontal bedding

bedding

bedding

Tesl.

siltstones, mudstones with thin coal bands.

ranks of the Bol'shaya Belaya river.



Section uncovering within the Cheremkhovo coal deposit situated in the central part of the Irkutsk Basin is a stratotype of Middle Cheremkhovskaya Subformation (bottom‐up) (**Figure 2**):

Middle Cheremkhovskaya Subformation is opened in coal opencasts of the Glinki and Mugun deposits. The Glinki coal‐bearing deposits occur on erosional contacts with a weathered surface of the Upper Cambrian clays (**Figure 2**). Opened thickness of Jurassic deposits is 18.4 m. Middle Cheremkhovskaya Subformation composed of gray, flat bedded, fine‐grained sandstones, silt‐ stones, and mudstones with two coal seams of industrial thickness (2–5.8 m) is exposed in the lower part of the opencast. We revealed the following plant remains: *Equisetites asiaticus* Pryn., *Equisetites* cf. *lateralis* (Phill.) Phill., *Hausmannia crenata* (Nath.) Maell., *Cladophlebis williamsonii* Brongn., *Cl. haiburnensis* (L et H) Sew., *Ginkgo concinna* Heer, *Sphenobaiera czekanowskiana* (Heer) Florin, *Pseudotorellia* cf. *paradoxa* Dolud., *Czekanowskia baikalica* Kiritch. et Samyl., *Cz. rigida* Heer, *Phoenicopsis* ex gr. *angustifolia* Heer, *Leptostrobus laxiflora* Heer, *Podozamites* cf*. lanceolatus* (L. et H.) Schimp., *P.* cf. *eichwaldii* Pryn. var. *major* Pryn., *Pityophyllum* ex gr. *nordenskioldii* (Heer) Nath., *Schizolepis* cf. *moelleri* Sew., *Carpolithes minor* Pryn., and *Ixostrobus heeri* Pryn. in siltstones and mudstones of industrially coal‐bearing part of the opencast.

The Mugun lignite deposit is situated in the northwestern part of the Irkutsk Basin, at 40 km to the south of the Tulun city. The deposit has an irregular shape and a simple geological structure. Lower band of the deposit belongs to Middle Cheremkhovskaya Subformation. It is composed of siltstones, mudstones, and fine‐grained sandstones alternating among them‐ selves. Productive coal seams (thickness from 3.5 up to 10 m) are confined to this part of

Section uncovering within the Cheremkhovo coal deposit situated in the central part of the Irkutsk Basin is a stratotype of Middle Cheremkhovskaya Subformation (bottom‐up)

26. Siltstones, gray, with burst plant detritus 0.30

28. Coal 0.20 29. Sandstones, yellowish‐gray, coarse‐grained 1.65

31. Coal 0.20

cemented, flat‐bedded with abundant plant detritus

33. Coal 0.20

34. Sandstones, yellow, medium‐grained 4.50 35. Conglomerates, fine‐pebbly 0.30 36. Sandstones, yellow, medium‐grained polymictic 4.60 Thickness of the opencast developed part 29.47

*Czekanowskia* ex gr. *rigida* Heer and coalified plant detritus

of *Czekanowskia* ex gr. *rigida* Heer and *Pityophyllum* ex gr. *nordenskioldii*

27. Sandstones, yellow, medium‐fine‐grained with impressions

86 Seismic and Sequence Stratigraphy and Integrated Stratigraphy - New Insights and Contributions

30. Sandstones, gray, medium‐fine‐grained with leaf impressions of

32. Sandstones, yellowish‐gray, medium‐fine‐grained, poorly

(Heer) Nath.

Upper Cheremkhovskaya Subformation

**Thickness, m**

2.00

0.80

0.80

Middle Cheremkhovskaya Subformation is opened in coal opencasts of the Glinki and Mugun deposits. The Glinki coal‐bearing deposits occur on erosional contacts with a weathered surface of the Upper Cambrian clays (**Figure 2**). Opened thickness of Jurassic deposits is 18.4 m. Middle Cheremkhovskaya Subformation composed of gray, flat bedded, fine‐grained sandstones, silt‐ stones, and mudstones with two coal seams of industrial thickness (2–5.8 m) is exposed in the lower part of the opencast. We revealed the following plant remains: *Equisetites asiaticus* Pryn., *Equisetites* cf. *lateralis* (Phill.) Phill., *Hausmannia crenata* (Nath.) Maell., *Cladophlebis williamsonii* Brongn., *Cl. haiburnensis* (L et H) Sew., *Ginkgo concinna* Heer, *Sphenobaiera czekanowskiana* (Heer) Florin, *Pseudotorellia* cf. *paradoxa* Dolud., *Czekanowskia baikalica* Kiritch. et Samyl., *Cz. rigida* Heer, *Phoenicopsis* ex gr. *angustifolia* Heer, *Leptostrobus laxiflora* Heer, *Podozamites* cf*. lanceolatus* (L. et H.) Schimp., *P.* cf. *eichwaldii* Pryn. var. *major* Pryn., *Pityophyllum* ex gr. *nordenskioldii* (Heer) Nath., *Schizolepis* cf. *moelleri* Sew., *Carpolithes minor* Pryn., and *Ixostrobus heeri* Pryn. in siltstones

The Mugun lignite deposit is situated in the northwestern part of the Irkutsk Basin, at 40 km to the south of the Tulun city. The deposit has an irregular shape and a simple geological structure. Lower band of the deposit belongs to Middle Cheremkhovskaya Subformation. It is composed of siltstones, mudstones, and fine‐grained sandstones alternating among them‐ selves. Productive coal seams (thickness from 3.5 up to 10 m) are confined to this part of

and mudstones of industrially coal‐bearing part of the opencast.

(**Figure 2**):

Stratigraphy of Jurassic Sediments of the Southern Siberian Platform (Russia) Studied through Lithologic... http://dx.doi.org/10.5772/intechopen.69572 87

**Figure 2.** Correlation scheme of the key sections of Jurassic sediments of the Irkusk Basin. Numbering of sections is given in **Figure 1**. 1—conglomerates, 2—coarse‐grained sandstones, 3—medium‐ and fine‐grained sandstones, 4—siltstones, 5—mudstones, 6—coals, 7—lenses; plant remains: 8—with no stratigraphic importance, 9—typical of Cheremkhovo plant assemblage, 10—typical of Prisayan plant assemblage.

opencast. We distinguished the following plant remains: *Coniopteris* sp., *Lobifolia nana* A. Frol., *Czekanowskia rigida* Heer, *Pityophyllum* ex gr. *nordenskioldii* (Heer) Nath., and *Carpolithes heeri* Tur.‐Ket. [10] in siltstones and mudstones of this band.

**Upper Cheremkhovskaya (Ust'‐Baleiskaya) Subformation** overlaps in concordance with the Middle Cheremkhovskaya Subformation, and more than a half of the former is composed of fine‐ and medium‐grained sandstones with lenses of siltstones and mudstones. Lower bound‐ ary line is drawn in the bottom of sandstones of channel facies [1] and it is observed within opencasts of developed coal deposits (Cheremkhovo, Glinki, and Mugun; **Figure 2**). Outcrop situated at 2.1 km below the Ust'‐Baley settlement is a stratotype of the upper subformation. Gray, differently grained polymictic sandstones with horizontal cross, uni‐, and diversidirec‐ tional lamination dominate in its lower part. Sandstones contain two lenses composed of silt‐ stones and mudstones with horizontal and sometimes banded lamination. Lower lens is long about 70 m with thickness up to 2 m. Impressions of the following plants: *Lycopodites tenerrimus* Heer, *Czekanowskia rigida* Heer, *Elatides ovalis* Heer, *Ixostrobus* sp., *Carpolithes deplanatus* Pryn., *C. cinctus* Nath., and *Samaropsis rotundata* Heer are found on the surface of mudstone bedding.

Thickness of the upper lens reaches 5.8 m, and its length is about 120 m. Siltstones and mud‐ stones of the lens contain numerous impressions of insects and fewer those of fishes and shells of limnetic myarians. Plant remains of *Lycopodites tenerrimus* Heer, *L. trichiatus* Pryn. emend. A. Frol. et Mash., *Phyllotheca sibirica* Heer, *Coniopteris murrayana* (Brongn.) Brongn., *Sphenobaiera czekanowskiana* (Heer) Florin, *Czekanowskia rigida* Heer, *Leptostrobus laxiflora* Heer, *Elatides ovalis* Heer, *Ixostrobus heeri* Pryn., and *Samaropsis rotundata* Heer are found on several plates of beddings.

Outcrops situated on the opposite side of the Tel'ma settlement (**Figure 2**) and the Tolstyi Cape have similar lithologic structure. The following plant remains: *Cladophlebis* cf. *haiburnensis* (L. et H.) Sew., *Cl. williamsonii* Brongn., *Sphenobaiera czekanowskiana* (Heer) Flor., *S. vigentis* Kiritch. et Bat., *Pseudotorellia longifolia* Dolud., *Czekanowskia rigida* Heer, *Cz. baikalica* Kiritch. et Samyl., *Leptostrobus laxiflora* Heer, *Pityophyllum* ex gr. *nordenskioldii* (Heer) Nath., *Schizolepis* sp., *Samaropsis rotundata* Heer, and *Carpolithes* cf. *minor* Pryn. are revealed in siltstone lens opened by these outcrops.

**Prisayanskaya Formation** is less abundant relative to the Cheremkhovskaya one which is overlapped with conformability by it.

**Lower Prisayanskaya (Idanskaya) Subformation** is predominantly confined to the south‐ eastern part of the Basin, its thickness is of 50–70 m. Characteristic feature of its sections is the prevalence of coarse‐grained and gritty sandstones as well as siltstones containing indeter‐ minable plant remains of poor preservation. Conglomerates and gritstones occur on several areas in the base of subformation. Lower boundary line of subformation is drawn relative to band bottom of rhythmically alternating sandstones, gritstones, conglomerates, and silt‐ stones. It is sufficiently conventional.

**Upper Prisayanskaya (Sukhovskaya) Subformation** is preserved from erosion only in Priirkut depression. This subformation is composed of small and fine‐grained sandstones and siltstones, and coarse‐grained varieties of sandstones are rare. The characteristic feature of this subformation is the significant enrichment of rocks in coaly matter and the presence of coal bands. Thickness of this subformation is about 50 m. Its lower boundary with the lower subformation is not always distinct. Within Priirkut depression we studied outcrops of this Upper Prisayanskaya Subformation situated along the banks of the Angara, Irkur, Kaya, and Topka rivers.

opencast. We distinguished the following plant remains: *Coniopteris* sp., *Lobifolia nana* A. Frol., *Czekanowskia rigida* Heer, *Pityophyllum* ex gr. *nordenskioldii* (Heer) Nath., and *Carpolithes heeri*

**Upper Cheremkhovskaya (Ust'‐Baleiskaya) Subformation** overlaps in concordance with the Middle Cheremkhovskaya Subformation, and more than a half of the former is composed of fine‐ and medium‐grained sandstones with lenses of siltstones and mudstones. Lower bound‐ ary line is drawn in the bottom of sandstones of channel facies [1] and it is observed within opencasts of developed coal deposits (Cheremkhovo, Glinki, and Mugun; **Figure 2**). Outcrop situated at 2.1 km below the Ust'‐Baley settlement is a stratotype of the upper subformation. Gray, differently grained polymictic sandstones with horizontal cross, uni‐, and diversidirec‐ tional lamination dominate in its lower part. Sandstones contain two lenses composed of silt‐ stones and mudstones with horizontal and sometimes banded lamination. Lower lens is long about 70 m with thickness up to 2 m. Impressions of the following plants: *Lycopodites tenerrimus* Heer, *Czekanowskia rigida* Heer, *Elatides ovalis* Heer, *Ixostrobus* sp., *Carpolithes deplanatus* Pryn., *C. cinctus* Nath., and *Samaropsis rotundata* Heer are found on the surface of mudstone bedding. Thickness of the upper lens reaches 5.8 m, and its length is about 120 m. Siltstones and mud‐ stones of the lens contain numerous impressions of insects and fewer those of fishes and shells of limnetic myarians. Plant remains of *Lycopodites tenerrimus* Heer, *L. trichiatus* Pryn. emend. A. Frol. et Mash., *Phyllotheca sibirica* Heer, *Coniopteris murrayana* (Brongn.) Brongn., *Sphenobaiera czekanowskiana* (Heer) Florin, *Czekanowskia rigida* Heer, *Leptostrobus laxiflora* Heer, *Elatides ovalis* Heer, *Ixostrobus heeri* Pryn., and *Samaropsis rotundata* Heer are found on several

Outcrops situated on the opposite side of the Tel'ma settlement (**Figure 2**) and the Tolstyi Cape have similar lithologic structure. The following plant remains: *Cladophlebis* cf. *haiburnensis* (L. et H.) Sew., *Cl. williamsonii* Brongn., *Sphenobaiera czekanowskiana* (Heer) Flor., *S. vigentis* Kiritch. et Bat., *Pseudotorellia longifolia* Dolud., *Czekanowskia rigida* Heer, *Cz. baikalica* Kiritch. et Samyl., *Leptostrobus laxiflora* Heer, *Pityophyllum* ex gr. *nordenskioldii* (Heer) Nath., *Schizolepis* sp., *Samaropsis rotundata* Heer, and *Carpolithes* cf. *minor* Pryn. are revealed in siltstone lens

**Prisayanskaya Formation** is less abundant relative to the Cheremkhovskaya one which is

**Lower Prisayanskaya (Idanskaya) Subformation** is predominantly confined to the south‐ eastern part of the Basin, its thickness is of 50–70 m. Characteristic feature of its sections is the prevalence of coarse‐grained and gritty sandstones as well as siltstones containing indeter‐ minable plant remains of poor preservation. Conglomerates and gritstones occur on several areas in the base of subformation. Lower boundary line of subformation is drawn relative to band bottom of rhythmically alternating sandstones, gritstones, conglomerates, and silt‐

**Upper Prisayanskaya (Sukhovskaya) Subformation** is preserved from erosion only in Priirkut depression. This subformation is composed of small and fine‐grained sandstones and siltstones, and coarse‐grained varieties of sandstones are rare. The characteristic feature of this subformation

Tur.‐Ket. [10] in siltstones and mudstones of this band.

88 Seismic and Sequence Stratigraphy and Integrated Stratigraphy - New Insights and Contributions

plates of beddings.

opened by these outcrops.

overlapped with conformability by it.

stones. It is sufficiently conventional.

In the quarry situated on Kaiskaya Gora at 400 m above the Kaya river mouth in the Irkutsk city the Jurassic deposits of Upper Prisayanskaya Subformation are opened (bottom‐up).


The section of Upper Prisayanskaya Subformation is opened within Sinyushina Gora in the Irkutsk city. Gray, fine‐ and medium‐grained sandstones dominate in the lower part of the section. The section's upper part is composed of alternating medium‐grained sandstones and siltstones, only one thin (0.07 m) coal band is noted. *Coniopteris murrayana* (Brongn.) Brongn., *Cladophlebis nebbensis* (Brongn.) Nath., *Cl*. *williamsonii* Brongn., *Czekanowskia* ex gr. *rigida* Heer, *Phoenicopsis angustifolia* Heer, *Ph. samylinae* Kiritch. et Moskv., *Ph. irkutensis* Dolud. et Rasskaz., and *Pityophyllum* ex gr. *nordenskioldii* (Heer) Nath are revealed among plant remains.

The upper part of Upper Prisayanskaya Subformation of 5.4 m thickness and 400 m length is opened in roadside quarry near the Smolenshchina settlement. The section is of rhythmic structure. In this section, quartz‐feldspathic coarse‐ and medium‐grained sandstones alternate upward with fine‐grained sandstones, siltstones, and mudstones with thin (0.1–0.3 m) coal bands. *Hepaticites arcuatus* (L. et H.) Harris, *Coniopteris murrayana* (Brongn.) Brongn., *C. maakiana* (Heer) Pryn. emend. Kiritch. et Trav., *C. spectabilis* Brick, *Cladophlebis williamsonii* Brongn., *Cl. haiburnensis* (L. et H.) Brongn., *Cl.* cf. *nebbensis* (Brongn.) Nath., *Raphaelia diamensis* Sew., *Sphenobaiera* ex gr. *czekanowskiana* (Heer) Flor., *Ginkgo* ex gr. *sibirica* Heer, *Czekanowskia* ex gr. *rigida* Heer, *Phoenicopsis angustifolia* Heer, *Ph.* cf. *mogutchevae* Kiritch. et Trav., *Ph. cognata* Kiritch., *Pityophyllum* ex gr. *nordenskioldii* (Heer) Nath., and *Stenorachis* (?) *clavata* Nath. are identified among plant remains.

One of the key sections of the Upper Prisayanskaya Subformation is outcrops situated on the left bank of the Irkutsk water reservoir on the opposite side of the Stroitel' settlement (**Figure 2**). The section has a rhythmic structure. Bases of rhythms are presented by quartz fieldspathic coarse‐ and medium‐grained sandstones alternating upward the section with fine‐grained sandstones, siltstones, and mudstones with thin (0.1–0.3 m) coal bands. Near water's edge are exposed the following rocks:



One of the key sections of the Upper Prisayanskaya Subformation is outcrops situated on the left bank of the Irkutsk water reservoir on the opposite side of the Stroitel' settlement (**Figure 2**). The section has a rhythmic structure. Bases of rhythms are presented by quartz fieldspathic coarse‐ and medium‐grained sandstones alternating upward the section with fine‐grained sandstones, siltstones, and mudstones with thin (0.1–0.3 m) coal bands. Near

1. Sandstones, gray medium‐grained, horizontally bedded micaceous 0.14

3. Hard coal 0.12 4. Mudstones, gray 0.04 5. Hard coal 0.16 6. Mudstones, coaly, horizontally bedded with abundant plant detritus 0.13 7. Hard coal 0.07

> of *Coniopteris maakiana* (Heer) Pryn. emend. Kirich. et Trav., *Cladophlebis* cf. *williamsonii* Brongn., *Raphaelia diamensis* Sew., *R. tapkensis* (Heer) Pryn. emend. Kost., *Czekanowskia* ex gr. *rigida* Heer, and *Phoenicopsis* ex gr.

9. Sandstones, gray, medium‐grained with plant detritus 0.07

impressions of trunks. Leaf mats of *Phoenicopsis* ex gr. *angustifolia* Heer are

11. Sandstones, gray, horizontally bedded with plant detritus 0.10

13. Sandstones, gray, medium‐grained with cross bedding and plant detritus 0.90 14. Hard coal 0.06 15. Mudstones with plant remains, poorly preserved 0.03 16. Coal 0.04

18. Sandstones, medium‐grained, massive 0.60

(Heer) Pryn. emend. Kiritch. et Trav., *C. murrayana* (Brongn.) Brongn., *Cladophlebis haiburnensis* (L. et H.) Sew., *Cl. williamsonii* Brongn., *Phoenicopsis* ex gr. *angustifolia*

17. Mudstones, gray, with impressions of fossil plant remains: *Coniopteris maakiana*

Heer, and *Pityophyllum* ex gr. *nordenskioldii* (Heer) Nath.

2. Sandstones, silty, gray, plant detritus is exposed, plant detritus is found in

90 Seismic and Sequence Stratigraphy and Integrated Stratigraphy - New Insights and Contributions

8. Mudstones, coaly, horizontally bedded, contain abundant plant detritus

10. Sandstones, gray medium‐grained with distinct cross bedding and

12. Mudstones, gray, horizontally bedded. Impressions of *Coniopteris maakiana* (Heer) Pryn. emend. Kirich. et Trav., *Cladophlebis nebbensis* (Brongn.) Nath., *Cl. williamsonii* Brongn., *Cl. haiburnensisi* (L. et H.) Sew., *Raphaelia diamensis* Sew., *R. tapkensis* (Heer) Pryn. emend. Kost., *Sphenobaiera* ex gr. *czekanowskiana* (Heer) Florin, *Pseudotorellia* cf. *ensiformis* (Heer) Dolud., *P.* cf. *paradoxa* Dolud., *Czekanowskia* ex gr. *rigida* Heer, *Phoenicopsis angustifolia* Heer, *Elatocladus manchuricus* (Yok.) Yabe, and *Pityophyllum* ex gr. *nordenskioldii* (Heer) Nath. are found in the lower part of the layer

found at 0.4 m from the bottom layer

**Thickness, m**

0.46

0.11

1.10

0.10

0.55

water's edge are exposed the following rocks:

the upper part of the layer

*angustifolia* Heer

The Upper Prisayanskaya Subformation outcrop within the Topka creek valley is presented by 18‐meter‐thick section having a rhythmic structure (**Figure 2**). The bases of rhythms are composed of quartz fieldspathic coarse‐ and medium‐grained sandstones, siltstones, and mud‐ stones with thin (0.1–0.3 m) coal bands. *Coniopteris murrayana* (Brongn.) Brongn., *C. maakiana* (Heer) Pryn. emend. Kiritch. et Trav., *Cladophlebis williamsonii* Brongn., *Cl. haiburnensis* (L. et H.) Sew., *Czekanowskia* ex gr. *rigida* Heer, *Phoenicopsis* ex gr. *angustifolia* Heer, *Phoenicopsis* ex gr. *speciosa* Heer, and *Pityophyllum* ex gr. *nordenskioldii* (Heer) Nath. are revealed among plant remains.

The Upper Prisayanskaya Subformation is exposed on the left bank of the Irkut river, 1.5 km below the Pionersk settlement (**Figure 2**). Outcrop of 800 m length has a rhythmic structure. The rhythm bases are composed of quartz fieldspathic coarse‐ and medium‐ grained sandstones (3–4 m) alternating upward with siltstones and mudstones with thin (0.1–0.2 m) coal bands. The following plant remains of *Coniopteris maakiana* (Heer) Pryn. emend. Kiritch. et Trav., *C.* cf*. spectabilis* Brick, *Coniopteris* sp., *Cladophlebis williamsonii* Brongn., *Cl. williamsonii* Brongn. var. *punctata* Brick, *Cl. nebbensis* (Brongn.) Nath., *Raphaelia diamensis* Sew., *Raphaelia tapkensis* (Heer) Pryn. emend. Kost., *Ginkgo* ex gr. *sibirica* Heer, *Sphenobaiera* ex gr. *czekanowskiana* (Heer) Flor., *Czekanowskia* ex gr. *rigida* Heer, *Phoenicopsis angustifolia* Heer, *Pityophyllum* ex gr. *nordenskioldii* (Heer) Nath., *Ixostrobus heeri* Pryn., *Carpolithes deplanatus* Pryn., and *Stenorhachis* sp. are identified in siltstones and fine‐ grained sandstones.

**The Kudinskaya Formation** is located in the northeast of the Irkutsk city within the Kuda trough, overlaps discordantly with the Prisayanskaya Formation. The formation is charac‐

terized by rubbly pebbled conglomerates and coarse‐grained sandstones, thin interlayers of siltstones and mudstones are rare. Thickness of the formation is 50–80 m. Identifiable remains of plants from the Kudinskaya Formation have been long unknown. In 2014 we studied the stratotype of Kudinskaya Formation which is located at 2 km to the north of the Zherdovka village. Clastic rocks opened within these outcrops contain thin interlayers of siltstones and mudstones. The following fossil plants: *Equisetites* sp., *Coniopteris maakiana* (Heer) Pryn. emend. Kiritch. et Trav., *Czekanowskia curta* Kiritch. et Samyl., *Czekanowskia rigida* Heer, *Leptostrobus laxiflora* Heer, *Pityophyllum* ex gr. *nordenskioldii* (Heer) Nath. *Carpolithes cinctus* Nath., and *C. minor* Pryn. are found in mudstones [13, 26].
