Taxonomic composition and stratigraphic distribution of fossil plant assemblages within Jurassic sediments of the Irkutsk Basin.
Abstract
This paper presents the results of comprehensive lithologic and paleobotanical study of Jurassic sediments of the Irkutsk Coal Basin revealed in outcrops and sections within operating coal deposits. The lithologic characteristics of the main stratigraphic units of the Irkutsk Basin: Cheremkhovskaya, Prisayanskaya, and Kudinskaya Formations are given. Two uneven‐aged fossil plant assemblages: Cheremkhovo for middle and upper subformations of Cheremkhovskaya Formation and Prisayan for Prisayanskaya and Kudinskaya Formations have been identified. 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 species are typical of the Cheremkhovo assemblage. The age of sediments including the Cheremkhovo assemblage is the end of Early Jurassic (conventionally, Toarcian). 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 species are characteristic of the Prisayan assemblage. Prisayan assemblage sediments are dated at the beginning of Middle Jurassic (conventionally, Aalenian). The stratigraphic correlation of Jurassic sediments of the Irkutsk Coal Basin with the sedimentary basins of Western Siberia has been carried out based on paleobotanical data.
Keywords
- Jurassic sediments
- stratigraphy
- fossil and flora assemblages
- Irkutsk Basin
- Siberian platform
1. Introduction
Lower and middle Jurassic continental sediments of Siberia are abundant. They are exposed along river valleys and gorges, within quarries and mine workings of the Kuznetsk, Kansk, Irkutsk, and other Coal Basins. Lower and Middle Jurassic sediments are rich in plant remains, which are important in stratigraphy and correlation of continental complex of Jurassic sediments where large coal deposits are concentrated. The Irkutsk Basin situated within the southern Siberian platform is one of them (Figure 1). Three structural and facies zones: Prisayan piedmont trough, Platform limb, and Angara‐Koty intermountain area are clearly distinguished within it [2]. According to the regional stratigraphic scheme, the Jurassic sediments of the Platform limb and Prisayan piedmont trough are subdivided into three formations: Cheremkhovskaya, Prisayanskaya, and Kudinskaya. Cheremkhovskaya Formation is composed of three subformations: lower, middle, and upper and Prisayanskaya includes lower and upper subformations. The stratigraphic sequences of these formations and their lithologic characteristics were confirmed by lithostratigraphic data in different boreholes and natural sections [1, 28].
Since the second half of the nineteenth century, the Early‐Middle Jurassic flora of the Irkutsk Basin were repeatedly studied in detail. Data on Jurassic flora of the Basin are cited in various works [14–16, 22, 27, 29]. The results of the studies on some groups of fossil plants are published in several papers [4–6, 8–12, 18–20, 25]. Despite the large number of publications, there is no unified view on stratigraphy of Jurassic sediments of the Irkutsk Basin. Against the background of well‐studied Jurassic floras of the Kuznetsk [17], the Kansk Basins [24], and Western Siberia [21], Jurassic flora is still poorly studied. Paleobotanical characteristic of formations needs clarification, and stratigraphic importance of some species of fossil plants requires revision. For example, species
The aim of this work is to suggest a solution for the above‐listed problems by the implementation of comprehensive lithologic and paleobotanical study of key sections of Jurassic sediments within present‐day active coal deposits rich in paleobotanical material.
2. Methods
Starting in 2008 and in 2016, the authors carried out quite a number of field works in order to explore lithologic and paleobotanical peculiarities in all active coal deposits within the Irkutsk Basin (see Figure 1). Opencasts of three coal‐producing quarries: Cheremkhovo, Mugun, and Glinki were studied in detail. In addition, 13 exposures which are stratotypes of regional stratigraphic units were studied. The leaves of Ginkgoales and Leptostrobales were investigated by the epidermal‐cuticular method. Leaf cuticles were macerated by the standard technique in Schultze mixture. Microslides were examined using the Olympus BX41TF light microscope and electronic scanning microscope Philips’ SEM 525‐M. About one hundred pieces of ore and more than 300 microslides with unfolded pattern of leaves’ compressions were studied.
3. Lithostratigraphy of the formations of the Irkutsk Basin
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):
Thickness, m | ||
---|---|---|
1. | Conglomerates interstratifying with coarse‐grained sandstones | 0.65 |
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 | |
6. | Sandstones, gray, medium‐fine‐grained with horizontal bedding and abundant plant detrital matter | 0.70 |
Disappearance of outcrop | 3.50 | |
7. | Sandstones, pale gray, coarse‐grained, massive | 0.60 |
8. | Sandstones, gray, medium‐grained with horizontal bedding | 0.60 |
Disappearance of outcrop | 1.39 | |
9. | Sandstones, red, medium‐fine‐grained with horizontal bedding | 1.10 |
10. | Alternation of red, burnt rocks presented by sandstones and siltstones with horizontal bedding and impressions of | 3.54 |
11. | Speckled sandstones, coarse‐medium‐grained with horizontal bedding | 0.75 |
Developed thickness | 13.12 |
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
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):
Thickness, m | ||
---|---|---|
Middle Cheremkhovskaya Subformation | ||
1. | Siltstones, gray | 0.60 |
2. | Coal hard | 0.50 |
3. | Mudstones, coaly with horizontal bedding | 0.20 |
4. | Coal | 0.30 |
5. | Mudstones, coaly with horizontal bedding | 0.20 |
6. | Coal | 0.50 |
7. | Mudstones, coaly with horizontal bedding | 0.25 |
8. | Coal | 0.80 |
9. | Sandstones, gray, medium‐fine‐grained with horizontal bedding, contain plant remains of | 1.00 |
10. | Siltstones, gray | 0.40 |
11. | Sandstones, gray, medium‐fine‐grained | 1.45 |
12. | Siltstones, gray, with horizontal bedding | 0.30 |
13. | Sandstones, gray, medium‐fine‐grained | 0.50 |
14. | Mudstones, coaly, with horizontal bedding | 0.20 |
15. | Sandstones, gray, fine‐grained with horizontal bedding | 2.00 |
16. | Siltstones, gray, with horizontal bedding, contain plant remainsof | 1.20 |
17. | Coal | 1.00 |
18. | Mudstones, coaly with horizontal bedding, contain fossil plant remains of | 0.20 |
19. | Coal | 0.78 |
20. | Mudstones, coaly with horizontal bedding | 0.10 |
21. | Coal | 0.45 |
22. | Mudstones, coaly with horizontal bedding | 0.14 |
23. | Coal | 0.40 |
24. | Siltstones, gray, rich in plant remains of | 0.64 |
25. | Coal | 0.20 |
26. | Siltstones, gray, with burst plant detritus | 0.30 |
27. | Sandstones, yellow, medium‐fine‐grained with impressionsof | 2.00 |
28. | Coal | 0.20 |
29. | Sandstones, yellowish‐gray, coarse-grained | 1.65 |
30. | Sandstones, gray, medium‐fine‐grained with leaf impressions of | 0.80 |
31. | Coal | 0.20 |
32. | Sandstones, yellowish‐gray, medium‐fine‐grained, poorly cemented, flat‐bedded with abundant plant detritus | 0.80 |
33. | Coal | 0.20 |
Upper Cheremkhovskaya Subformation | ||
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 |
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, siltstones, 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:
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 themselves. Productive coal seams (thickness from 3.5 up to 10 m) are confined to this part of opencast. We distinguished the following plant remains:
Thickness of the upper lens reaches 5.8 m, and its length is about 120 m. Siltstones and mudstones of the lens contain numerous impressions of insects and fewer those of fishes and shells of limnetic myarians. Plant remains of
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:
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).
Thickness, m | ||
---|---|---|
1. | Sandstones, yellowish‐gray, fine‐ and medium‐grained, contain plant remains of | 2.00 |
2. | Siltstones and silty sandstones, yellowish‐gray, with impressions of plants: | 0.20 |
3. | Coal | 0.40 |
4. | Sandstones, yellowish‐gray, fine‐grained, contain plantimpressions of | 0.20 |
Opened thickness | 2.80 |
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.
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.
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:
Thickness, m | ||
---|---|---|
1. | Sandstones, gray medium‐grained, horizontally bedded micaceous | 0.14 |
2. | Sandstones, silty, gray, plant detritus is exposed, plant detritus is found in the upper part of the layer | 0.46 |
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 |
8. | Mudstones, coaly, horizontally bedded, contain abundant plant detritus of | 0.11 |
9. | Sandstones, gray, medium‐grained with plant detritus | 0.07 |
10. | Sandstones, gray medium‐grained with distinct cross bedding and impressions of trunks. Leaf mats of | 1.10 |
11. | Sandstones, gray, horizontally bedded with plant detritus | 0.10 |
12. | Mudstones, gray, horizontally bedded. Impressions of | 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 |
17. | Mudstones, gray, with impressions of fossil plant remains: | 0.55 |
18. | Sandstones, medium‐grained, massive | 0.60 |
19. | Sandstones, horizontally bedded with impressions of | 0.25 |
20. | Coal | 0.02 |
21. | Sandstones, gray, fine‐grained with thin slab parting | 0.23 |
22. | Sandstones, gray, coarse‐grained, cross‐bedded with trunks of trees | 4.00 |
23. | Sandstones, gray, medium‐grained with two coal bands (0.02 and 0.03 m) | 0.35 |
24. | Siltstones, gray, with plant remains: | 0.10 |
25. | Sandstones, gray, fine‐grained, cross‐bedded, with impressions of | 2.00 |
26. | Mudstones, coaly, with coal band (0.03 m) | 0.23 |
27. | Mudstones, gray, with impressions of ancient plants: | 0.15 |
28. | Sandstones, medium‐grained, horizontally bedded | 1.70 |
Thickness of the section opened part | 13.21 |
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 mudstones with thin (0.1–0.3 m) coal bands.
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
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 sections 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
Various representatives of genus
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].
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
The flora composition of Prisayanskaya and Kudinskaya Formations shows good horizontal consistency, and we consider it as Prisayan fossil plant assemblage. Species
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
5. Conclusion
The results of lithologic and paleobotanical investigations of Lower and Middle Jurassic sediments of the Irkutsk Basin allowed to draw the following conclusions.
Two uneven‐aged fossil plant assemblages: Cheremkhovo for the Middle and Upper Cheremkhovskaya Subformations and Prisayan for the Prisayanskaya and Kudinskaya Formations were identified.
Species
Species
Acknowledgments
The study was financially supported by the Russian Foundation for Basic Research (16‐35‐60005).
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