**4. Discussion**

The stratigraphic scale developed for Jurassic sediments of Western Siberia has been followed in the comparison of Jurassic sediments of the Irkutsk Basin with the adjacent regions of Western Siberia [3]. This scheme is comprehensively reasonable and can be considered as a comparative standard for large stratigraphic correlations and age dating of continental sec‐ tions within Siberia.

Summarizing paleobotanical review of the aforementioned sections, it should be noted that plant assemblage identified in Lower Cheremkhovskaya Subformation is characterized by uncommon ferns *Cladophlebis haiburnensis* (L. et H.) Sew. and conifers *Podozamites eichwaldii* Pryn. var. *minor* Pryn., *Schizolepis* sp. Ginkgoales and Leptostrobales are presented by species *Sphenobaiera* ex gr. *czekanowskiana* (Heer) Flor. and *Czekanowskia* ex gr. *rigida* Heer, the latter one dominates among them. The taxonomic composition of the flora of the lower part of the Lower Cheremkhovskaya Subformation is comparable with Jagel'nyi fossil plant assemblage from the Lower Jurassic sediments of Western Siberia [21] (**Table 2**). The age of the Lower Cheremkhovskaya Subformation is determined within the framework of the second half of the Early Jurassic (Pliensbachian).

Various representatives of genus *Equisetites*: *E. lateralis* (Phill.) Phill. and *E. asiaticus* Pryn. are revealed in all studied sections of Middle and Upper Cheremkhovskaya Subformations. Genus *Cladophlebis* is presented by species *C. williamsonii* Brongn. and *Cl. haiburnensis* (L. et H.) Sew. that are abundant. *Czekanowskia rigida* Heer, *Cz. baikalica* Kiritch., *Sphenobaiera czekanowskiana* (Heer) Flor., *S. vigentis* Kiritch. et Bat., *Pityophyllum* ex gr. *nordenskioldii* (Heer) Nath. Peжe вcтpeчaютcя *Lycopodites* sp., *Lobifolia nana* A. Frol., *Ginkgo* ex gr*. sibirica* Heer, *Pseudotorellia paradoxa* Dolud., *Phoenicopsis* ex gr. *angustifolia* Heer, *Leptostrobus laxiflorus* Heer, *Elatocladus manchuricus* (Yok.) Uabe, *Carpolithes cinctus* Nath., *C. minor* Pryn., *Carpolithes* sp., *Ixostrobus heeri* Pryn., *Ix. grandis* Tesl., and *Schizolepis follinii* Nath. play the role of cosmopolites among gymnosperms. *Hausmannia crenata* (Nath.) Maell., *Raphaelia diamensis* Sew., *Ginkgo concinna* (Heer) Sew., *Podozamites* cf*. lanceolatus* (L. et H.) Schimp., and *P.* cf. *eichwaldii* Pryn. var. *major* Pryn. are presented by several finds.

In whole, the flora composition of Middle and Upper Cheremkhovskaya Subformations is very consistent in outcrops that allow us to consider it as Cheremkhovo fossil plant assemblage [7]. *Equisetites lateralis* (Phill.) Phill., *E. asiaticus* Pryn., *Cladophlebis haiburnensis* (L. et H.) Sew., *Sphenobaiera czekanowskiana* (Heer) Flor., *S. vigentis* Kiritch. et Bat., *Czekanowskia baikalica* Kiritch. et Samyl., and *Cz. rigida* Heer whose remains dominate in the sections of Cheremkhovskaya Formation are typical of the assemblage (**Table 1**). Section of Middle

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*

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

The stratigraphic scale developed for Jurassic sediments of Western Siberia has been followed in the comparison of Jurassic sediments of the Irkutsk Basin with the adjacent regions of Western Siberia [3]. This scheme is comprehensively reasonable and can be considered as a comparative standard for large stratigraphic correlations and age dating of continental sec‐

Summarizing paleobotanical review of the aforementioned sections, it should be noted that plant assemblage identified in Lower Cheremkhovskaya Subformation is characterized by uncommon ferns *Cladophlebis haiburnensis* (L. et H.) Sew. and conifers *Podozamites eichwaldii* Pryn. var. *minor* Pryn., *Schizolepis* sp. Ginkgoales and Leptostrobales are presented by species *Sphenobaiera* ex gr. *czekanowskiana* (Heer) Flor. and *Czekanowskia* ex gr. *rigida* Heer, the latter one dominates among them. The taxonomic composition of the flora of the lower part of the Lower Cheremkhovskaya Subformation is comparable with Jagel'nyi fossil plant assemblage from the Lower Jurassic sediments of Western Siberia [21] (**Table 2**). The age of the Lower Cheremkhovskaya Subformation is determined within the framework of the second half of

Various representatives of genus *Equisetites*: *E. lateralis* (Phill.) Phill. and *E. asiaticus* Pryn. are revealed in all studied sections of Middle and Upper Cheremkhovskaya Subformations. Genus *Cladophlebis* is presented by species *C. williamsonii* Brongn. and *Cl. haiburnensis* (L. et H.) Sew. that are abundant. *Czekanowskia rigida* Heer, *Cz. baikalica* Kiritch., *Sphenobaiera czekanowskiana* (Heer) Flor., *S. vigentis* Kiritch. et Bat., *Pityophyllum* ex gr. *nordenskioldii* (Heer) Nath. Peжe вcтpeчaютcя *Lycopodites* sp., *Lobifolia nana* A. Frol., *Ginkgo* ex gr*. sibirica* Heer, *Pseudotorellia paradoxa* Dolud., *Phoenicopsis* ex gr. *angustifolia* Heer, *Leptostrobus laxiflorus* Heer, *Elatocladus manchuricus* (Yok.) Uabe, *Carpolithes cinctus* Nath., *C. minor* Pryn., *Carpolithes* sp., *Ixostrobus heeri* Pryn., *Ix. grandis* Tesl., and *Schizolepis follinii* Nath. play the role of cosmopolites among gymnosperms. *Hausmannia crenata* (Nath.) Maell., *Raphaelia diamensis* Sew., *Ginkgo concinna* (Heer) Sew., *Podozamites* cf*. lanceolatus* (L. et H.) Schimp., and *P.* cf. *eichwaldii* Pryn. var. *major*

In whole, the flora composition of Middle and Upper Cheremkhovskaya Subformations is very consistent in outcrops that allow us to consider it as Cheremkhovo fossil plant assemblage

Nath., and *C. minor* Pryn. are found in mudstones [13, 26].

**4. Discussion**

tions within Siberia.

the Early Jurassic (Pliensbachian).

Pryn. are presented by several finds.


Note: Numbering of fossil plan occurrences is given in **Figure 1**. **□**—Species dominates in burials, +—species is present in burials.

**Table 1.** Taxonomic composition and stratigraphic distribution of fossil plant assemblages within Jurassic sediments of the Irkutsk Basin.


Note: FPA—Fossil plant assemblage.

**Table 2.** Correlation scheme of Jurassic sediments of the Irkutsk Basin and adjacent areas on paleobotanical data.

Cheremkhovskaya Subformation of the Cheremkhovo hard coal deposit is a key for layers of the assemblage. Degree of development of the Cheremkhovo assemblage floras is comparable with floras of Novogodnii (Western Siberia) and Kamzasskii (Kuznetsk Basin) assemblages (**Table 2**). Thus, the age of Middle and Upper Cheremkhovskaya Subformations is deter‐ mined in the range of Toarcian.

The performed paleobotanical analysis of fossil plants found within Prisayanskaya and Kudinskaya Formations suggests that the time of their accumulation concurred with the flourishing of ferns of the genera *Conipteris* (*C. maakiana* (Heer) Pryn. emend. Kiritch. et Trav., *C. murrayana* (Brongn.) Brongn., *C. spectabilis* Brick, *C.* cf. *sachsii* Tesl.), *Cladophlebis* (*Cl. argutula* (Heer) Font., *Cl. haiburnensis* (L. et H.) Brongn., *Cl. kanskiensis* Kost., *Cl. nebbensis* (Brongn.) Nath., and *Cl. williamsonii* Brongn.) and *Raphaelia* (*R. diamensis* Sew. and *R. tapkensisi* (Heer) Pryn. emend Kost.). *Coniopteris maakiana* (Heer) Pryn. emend. Kiritch. et Trav., *Raphaelia diamensis* Sew., and *Cladophlebis nebbensis* (Brongn.) Nath. are dominant among them. *Coniopteris murrayana* (Brongn.) Brongn. и *Raphaelia tapkensis* (Heer) Pryn. emend Kost. occur rarely. Genus *Phoenicopsis*, presented by six species: *Ph. angustifolia* Heer, *Ph. samylinae* Kiritch. et Moskv., *Ph. irkutensis* Dolud. et Rasskaz., *Ph. cognata* Kiritch., *Ph. dentata* Pryn., and *Ph. markovitchiae* Kiritch. et Schischk. presented in majority of occurrences, is the most diverse among gymnosperms (**Table 1**). *Czekanowskia* genus is presented by species *Czekanowskia curta* Kiritch. et Samyl, *Cz. irkutensis* Kiritch. et Samyl, and *Cz. rigida* Heer. *Ginkgo* ex gr. *sibirica* Heer. and *G. tapkensis* Dolud. et Rasskaz. are frequent. Besides them, *Hepaticites arcuatus* (L. et H.) Harris, *Lycopodites baikalensis* A. Frol., *L. subulifolius* A. Frol. et Mash., *Phyllotheca sibirica* Heer*, Hausmannia crenata* (Nath. ) Mael., *Lobifolia lobifolia* (Phill.) Rasskaz. et E. Leb., *Anomozamites lindleyanus* Schimp., *Sphenobaiera* ex gr. *czekanowskiana* (Heer) Florin, *Pseudotorellia* cf. *ensiformis* (Heer) Dolud., *P.* cf. *paradoxa* Dolud., *Taxocladus ketovae* Tesl., *Elatocladus manchuricus* (Yok.) Yabe, *Pityophyllum* ex gr. *nordenskioldii* (Heer) Nath., *Ixostrobus heeri* Pryn., *Carpolithes cinctus* Nath., *C. deplanatus* Pryn., *C. heeri* Tur.‐Ket., *C. minor* Pryn., *Schizolepis follinii* Nath., *Samaropsis rotundata* Heer, and *Stenorachis* (?) *clavata* Nath. are found within the Prisayanskaya Formation.

The flora composition of Prisayanskaya and Kudinskaya Formations shows good horizon‐ tal consistency, and we consider it as Prisayan fossil plant assemblage. Species *Coniopteris maakiana* (Heer) Pryn. emend. Kiritch. et Trav., *C. murrayana* (Brongn.) Brongn., *C. spectabilis* Brick., *Cladophlebis nebbensis* (Brongn.) Nath., *Raphaelia diamensis* Sew., *R. tapkensis* (Heer) Pryn. emend Kost., *Phoenicopsis angustifolia* Heer, *Ph. cognata* Kiritch., and *Ph. irkutensis* Dolud. et Rasskaz., the remains of which prevail in the sections of Prisayanskaya and Kudinskaya Formations (**Table 1**), are characteristic of this assemblage [7, 13]. Composition of Prisayan assemblage allows it to compare to Verkhnepeshkovskii (Western Siberia), Rybinskii (Kansk Basin) and Etapskii (Kuznetsk Basin) fossil plant assemblages and date including sedi‐ ments to the beginning of Middle Jurassic (Aalenian) (**Table 2**). Outcrops situated near the Smolenshchina settlement and on the left bank of the Irkutsk water reservoir serve as key sections for Prisayan assemblage.

Plant remains from the sections of Upper Prisayanskaya Subformation opened within the Topka river were previously compared, according to the level of flora development, with Azharminskii fossil plant assemblage traced in Tyumen'skaya Formation of Ob'‐ Tazovskaya area of Western Siberia [21]. Azharminskii assemblage is characterized by renewal of species composition of genera *Coniopteris*, *Czekanowskia*, and *Phoenicopsis*. Genus *Coniopteris* is replenished with species *C. burejensis* (Zaless.) Sew., *C. (Birissia?) depensis*

Cheremkhovskaya Subformation of the Cheremkhovo hard coal deposit is a key for layers of the assemblage. Degree of development of the Cheremkhovo assemblage floras is comparable with floras of Novogodnii (Western Siberia) and Kamzasskii (Kuznetsk Basin) assemblages

**Table 2.** Correlation scheme of Jurassic sediments of the Irkutsk Basin and adjacent areas on paleobotanical data.

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

Note: FPA—Fossil plant assemblage.

E. Leb. Species *C. hymenophylloides* (Brongn.) Sew., *C. simplex* (L. et H.) Harris, *C. vsevolodii* E. Leb. Occur everywhere. Genus *Phoenicopsis*, presented by 10 species, is the most diverse among Leptostobales. Species *Ph. taschkessiensis* Krasser and *Ph. mogutchevae* Kiritch. et Trav. are the first to occur among them. The most abundant are species *Ph. samylinae* Kiritch. et Moskvin., *Ph. sibirica* Kiritch. et Trav., and *Ph. varia* Kiritch. et Trav. The age of sediments including Azharminskii fossil plant assemblage is determined in the range of Bajocian stage. Analysis of flora taxonomic composition from the sections of the Topka river revealed in it the lack of all new species of genera *Coniopteris* and *Phoenicopsis* typical of Azharminskii assemblage (**Table 1**). Thus, according to the available data, there is no Bajocian flora within the Irkusk Basin.
