Lower Eocene sedimentary succession and microfossil biostratigraphy in the central northern Caucasus basin

 E. Shcherbinina, A., Iakovleva, Yu., Gavrilov, O., Golovanova, N., Muzlöv, 2020 CC BY-SA E . S h c h e r b i n i n a e t a l . G e o l o g i c a A c t a , 1 8 . 1 , 1 1 5 , I X V I I I ( 2 0 2 0 ) D O I : 1 0 . 1 3 4 4 / G e o l o g i c a A c t a 2 0 2 0 . 1 8 . 1 Lower Eocene biostratigraphy in the Caucasus basin 2 dinocysts have only been studied in some specific intervals of the Kheu section: the uppermost Thanetian (Radionova et al., 2009), Paleocene-Eocene Thermal Maximum (PETM) (Shcherbinina et al., 2016) and the middle Eocene (Iakovleva et al., in press; Zaporozhets, 2001). The Paleocene-Eocene sediments of the Kheu section host a complete record of the global and regional paleoecological events (Muzylöv, 1994). They include intervals rich in organic matter which represent the sedimentary response to the early Eocene climatic perturbations. The lower Eocene (Ypresian) interval of the Kheu section encompasses the uppermost Nalchik, Abaza and Cherkessk formations. Selandian to lowermost Ypresian greenish marly sediments (named Nalchik Formation (Fm.) in the studied area), cover a large area from Crimea to eastern Caucasus. The intercalation of the sapropelitic bed in the uppermost part of this formation corresponds to PETM (Gavrilov et al., 2000; Gavrilov et al., 2003; Shcherbinina et al., 2016). The geographic range of the lower Ypresian siliciclastic Abaza Fm. is restricted to the small area of the central northern Caucasus between Kuban and Cherek Rivers having the maximum thickness (ca. 20m) in the Kheu section. In the most complete successions of the deepest part of the Peri-Tethyan Basin, the upper Ypresian interval (NP12 to lower part NP13 nannofossil zones) contains intercalations of sediments rich in organic matter in varying extents, which likely display the regional response to the Early Eocene Climatic Optimum (EECO), encompassing this time span. The upper Ypresian marly Cherkessk Fm. exposed along the Kheu R. contains the highest number of sapropelitic intercalations forming a cyclically built succession. Our recent study of the lowermost Eocene interval of the Kheu section revealed the specificities and dynamics of the PETM in the deepest part of the NE Peri-Tethys (Shcherbinina et al., 2016). Here we present a highresolution lithostratigraphy and nannofossil and dinocyst biostratigraphy of the upper part of the lower Eocene succession. With this results we aim to build the basis for forthcoming paleoecological reconstructions of the Caucasian Basin during the early Eocene. MATERIAL AND METHODS The lower Eocene succession is exposed along the Kheu R. stream crossing the village of Gerpegezh (43o22’22.0”N 43o39’04.2”E, see Fig. 1C). This part of the section was revisited during several field trips and the total of 105 samples were collected from the ca. 47m thick succession with resolution from few centimeters within sapropelitic beds in the upper part of the studied section (Cherkessk Fm.) to 1m in the low-calcareous Abaza Fm. The nannofossils were examined from the smear-slides made of raw sediments of all samples collected using standard technics described in Bown and Young, 1998. The study of smear-slides and nannofossil pictures were made using the light microscope Olympus BX41 (x1200) and video-camera Infinity X. The semi-quantitative estimation C a u c a s u s Kheu section


INTRODUCTION
In the early Eocene, the huge basin of the NE Peri-Tethys was widely connected with the Tethyan realm by a system of the gateways and had restricted connection with the Boreal realm via the shallow Turgay Strait (Fig. 1A). The central Caucasus basin was the deepest part of this marine region with mostly calcareous and siliciclastic-calcareous sedimentation. The most continuous Paleogene succession is exposed 15km southeastward from the city of Nalchik, along the middle reaches of the Kheu River, Kabardino-Balkaria, central northern Caucasus (Fig. 1B). This classical perfectly exposed section lacking recognizable hiatuses and containing diverse microfossils (planktonic and benthic foraminifera, nannofossils, dinocysts) became a subject of detailed stratigraphic research since the 40s of the last century. The most of the works were focused on the biostratigraphical subdivision of the Paleogene sediments of the Kheu section based on regional planktonic foraminiferal zonation (e.g. Beniamovsky, 2001;Bugrova et al., 1988;Leonov and Alimarina, 1964;Shutskaya, 1960;Subbotina, 1947, and others). Later, the foraminiferal zones were correlated with the nannofossil zones (Krasheninnikov and Muzylöv, 1975) and some intervals of this section were studied again in better detail (Beniamovski et al., 2003;Beniamovskiy, 2011;Shcherbinina et al., 2016). By now, dinocysts have only been studied in some specific intervals of the Kheu section: the uppermost Thanetian (Radionova et al., 2009), Paleocene-Eocene Thermal Maximum (PETM) (Shcherbinina et al., 2016) and the middle Eocene (Iakovleva et al., in press;Zaporozhets, 2001).
The Paleocene-Eocene sediments of the Kheu section host a complete record of the global and regional paleoecological events (Muzylöv, 1994). They include intervals rich in organic matter which represent the sedimentary response to the early Eocene climatic perturbations. The lower Eocene (Ypresian) interval of the Kheu section encompasses the uppermost Nalchik, Abaza and Cherkessk formations. Selandian to lowermost Ypresian greenish marly sediments (named Nalchik Formation (Fm.) in the studied area), cover a large area from Crimea to eastern Caucasus. The intercalation of the sapropelitic bed in the uppermost part of this formation corresponds to PETM (Gavrilov et al., 2000;Gavrilov et al., 2003;Shcherbinina et al., 2016). The geographic range of the lower Ypresian siliciclastic Abaza Fm. is restricted to the small area of the central northern Caucasus between Kuban and Cherek Rivers having the maximum thickness (ca. 20m) in the Kheu section. In the most complete successions of the deepest part of the Peri-Tethyan Basin, the upper Ypresian interval (NP12 to lower part NP13 nannofossil zones) contains intercalations of sediments rich in organic matter in varying extents, which likely display the regional response to the Early Eocene Climatic Optimum (EECO), encompassing this time span. The upper Ypresian marly Cherkessk Fm. exposed along the Kheu R. contains the highest number of sapropelitic intercalations forming a cyclically built succession.
Our recent study of the lowermost Eocene interval of the Kheu section revealed the specificities and dynamics of the PETM in the deepest part of the NE Peri-Tethys (Shcherbinina et al., 2016). Here we present a highresolution lithostratigraphy and nannofossil and dinocyst biostratigraphy of the upper part of the lower Eocene succession. With this results we aim to build the basis for forthcoming paleoecological reconstructions of the Caucasian Basin during the early Eocene.

MATERIAL AND METHODS
The lower Eocene succession is exposed along the Kheu R. stream crossing the village of Gerpegezh (43º22'22.0"N 43º39'04.2"E, see Fig. 1C). This part of the section was revisited during several field trips and the total of 105 samples were collected from the ca. 47m thick succession with resolution from few centimeters within sapropelitic beds in the upper part of the studied section (Cherkessk Fm.) to 1m in the low-calcareous Abaza Fm.
The nannofossils were examined from the smear-slides made of raw sediments of all samples collected using standard technics described in Bown and Young, 1998. The study of smear-slides and nannofossil pictures were made using the light microscope Olympus BX41 (x1200) and video-camera Infinity X. The semi-quantitative estimation of the abundances of individual species is made with following gradations: a: abundant (>5 specimens per field of view (f.v.) of the microscope), c: common (1-5 specimens per f.v.), r: rare (one specimen per 2-10 f.v.), f: few (one specimen per more than 10 f.v.).
Palynomorphs were concentrated according to the standard palynological techniques of the Russian Academy of Sciences. Samples were i) processed with 10% hydrochloric acid until the calcium carbonate was dissolved; ii) processed with 10% tetrasodium pyrophosphate (Na 4 P2O7×10H 2 O) to disperse the clay; iii) washed several times to eliminate clayey compounds; iv) centrifuged with a heavy liquid (K 2 CdJ 4 ) with a specific gravity of 2.25; v) washed in water and then treated with 10% hydrofluoric acid until the siliceous matter was dissolved and vi) boiled with 10% hydrochloric acid to remove fluoro-silicate compounds. The material was not sieved. The residues were mounted on glass slides using glycerine jelly. A minimum of 200 palynomorphs were counted and grouped into 10 broad categories (dinocysts, acritarchs, prasinophytes, green algae, fungi, angiosperms, conifer bisaccate pollen, spores and unknown palynomorphs). Next, a minimum of 200 dinocysts (if possible) were counted. The remaining material was scanned for rare dinocyst taxa. Dinoflagellate cyst taxonomy follows Williams et al. (2017).
The calcium carbonate composition was determined using X-ray diffractometry. Chemical analysis of CaCO 3 was made using a gravimetric technique; its content was calculated after the treatment of sediments with HCl and CO 2 absorption in Ascarite. Total Organic Carbon (TOC) was analyzed gravimetrically after oxidation of organic matter with chromium trioxide.

Lithology and sedimentation
In the Kheu section, the base of the Ypresian corresponds to the SaproPelitic Bed (SPB), representing the regional expression of the global event of the PETM (Gavrilov et al., 2003(Gavrilov et al., , 2009Shcherbinina et al., 2016). The accumulation of the dark thinly laminated low-calcareous SPB was preceded by the large scale regression forming the erosional surface and stratigraphic hiatus in the shallower areas of the NE Peri-Tethys (Gavrilov et al., 2003), which mark the Sequence Boundary (SB1, Fig. 2). The event of PETM significantly affected the CaCO 3 accumulation. The cyclicity of calcium carbonate accumulation within SPB and the impact of calcareous microbiota are discussed in Shcherbinina et al., 2016. Although the CaCO 3 concentrations are similar below and above the SPB (Fig.  3), it has different composition in these intervals. Below the SPB, calcium carbonate is mainly composed of calcareous nannofossils and weathered and redeposited carbonate in form of pelitic particles (micarb). Above the SPB, the CaCO 3 content became partially restored (up to 23%) in the 2m thick topmost part of the Nalchik Fm. (see Fig. 3) due to the restoration of nannofossil abundance, but micarb is almost absent from this sediment. The occurrence of glauconite and coarser siliciclastic component in this part of formation likely indicates regressive trend. The transition from bluish-greenish-gray calcareous sediments of Nalchik Fm. to khaki-colored Abaza Fm. is characterized by decline of calcareous microbiota and occurrence of phosphatic nodules and individual siliceous concretions (Fig. 4A). This transition corresponds to the switch from mainly calcareous to siliceous sedimentation that persisted for ca. 2.5Ma in this part of the Caucasian basin.
The Abaza Fm. is composed by ca. 20m thick succession of dark non-laminated greenish-gray mudstone lacking organic matter and containing 10-30cm thick intercalations of more compact cherty layers. CaCO 3 content varies from 0 to 4% in the lower part of the formation and progressively increases up to 20% toward its top due to gradual restoration of calcareous microfossils, which form the main calcareous component in these sediments. Radiolarians and diatoms (Fig. 4B) are important component of microbiota assemblage in the sediments of Abaza Fm. (Agarkov et al., 1989;Radionova and Khokhlova, 1994) and they are suggested to be the main silica suppliers for the formation of cherty intercalations concurrently with enhanced supply of dissolved silica from the neighboring land. The sediments of Abaza Fm. are extensively borrowed that testifies normal oxic bottom conditions and, thus, oxygen exchange in the water column. The progressive restoration of calcareous microbiota in the upper Abaza Fm. (Fig.  4C) evidences the slow deepening of the basin during its accumulation, although the occurrence of glauconite and particles of terrestrial organic matter indicate still shallow environment.
The Abaza and Cherkessk fms. have gradational contact characterized by gradual increase of calcium carbonate value and total abundance of calcareous microfossils. The few cm thick layer enriched in clayey and sandy material found within the interval of NP10d Subzone (see below) corresponds to the regional hiatus in the uppermost part of the NP10 or CP9a nannofossil Zone/Subzone (base of Cherkessk Fm.), documented in many areas of the Former USSR from Crimea to Central Asia (Muzylöv, 1980). This hiatus marks the sequence boundary SB2, followed by new deepening of the basin and return to mostly calcareous sedimentation.
The Cherkessk Fm. is characterized by generally high CaCO 3 content, which dramatically drops at the few specific  Soft bluish-grey marl with 9 intercalations of 5-20 cm thick dark thinly laminated low-calcareous clays rich in organic matter, ca. 13.5 m   Fig. 3), foraminifera and occurrence of relatively high amount of micarb (Fig. 4C). The calcium carbonate content (68%), nannofossil abundance (see Fig. 3) are still high at the level 24.4m (sample 743), but foraminifera are hardly visible in the thin-section (Fig. 4D  Radiolarians persist to be present in the Cherkessk Fm., but show much lower abundance and had minor impact into the sediment composition. Few meters above, foraminifera recover their abundance, forming clayey foraminiferal limestone at the level 31.5m (sample 744) while nannofossils become less abundant (see Fig. 3).
The Unit 2 (ca. 13.5m thick) is composed of soft bluish-gray coarsely bioturbated marl with 7-30cm thick intercalations of dark thinly laminated TOC-rich calcareous clays. CaCO 3 content varies from 55 to 70% in the embedding marls and significantly drops (up to 15%) in the TOC-rich sediments (see Fig. 3). The dark intercalations are SPBs with various TOC concentrations (from 0.7 to 3.8%), containing small pyrite concretions, tiny wood debris, fish scales and shark teeth (Fig. 4F). The SPBs of Unit II have very distinct bottom and gradational contacts with embedding sediments at their top, showing progressive decrease in TOC content. The SPBs contain fine borrowing at the base, which become coarser to their tops, which evidences mostly normal oxic conditions during their accumulation. Clay mineral composition is similar in the SPBs and underlying and overlying sediments: the main clay composing mineral are mixed-layered smeсtite and hydromica, kaolinite and chlorite are present in minor values (Gavrilov and Muzylev, 1991). The recent analytical study (Gavrilov and Muzylev, 1991) revealed their enrichment in many trace elements (S, Co, P, Ni, V, Mo, Cu, Cr, among others), while the concentrations of Mn and Ti dramatically drops comparing to surrounding sediments.
The rhythmical intercalation of SPBs represents, more likely, the climatic response to Milankovich orbital cycles. At the same time, the TOC is spread unevenly in the light marls of Unit II: the thicker SPBs (20-25cm) with higher TOC concentrations occur close to the middle part of the Unit (see Figs. 2; 3), where they are separated by thin marly intervals (30-50cm). In the lower and upper parts of the Unit II, the SPBs occur in thicker intervals (up to few meters); they are thinner (5-15cm) and enriched in TOC in a minor extent being compared with the middle part of the Unit. These likely indicate the progressive onset and termination of environmental instability in the basin of northern Caucasus.
The increase of coarser clastics and glauconite toward the top of the Unit II and reduce in calcareous microplankton total abundance, possibly, indicate the beginning of the progressive the shallowing of the basin culminated at the Ypresian/Lutetian transition.

Calcareous nannofossils
The high-resolution study of nannofossil stratigraphic distribution in the Kheu section shows the large variations in total nannofossil abundance and species diversity in the early Eocene sediments. The dramatic decline of nannofossils and reorganization of nannofossil assemblage during the PETM corresponding to the SPB (Shcherbinina et al., 2016) were followed by short-time recovery of total abundance in the topmost Nalchik Fm., where the last Paleocene taxa (Fasciculithus, Neochiastozygus, Heliolithus, among other) became extinct. Few meters above, at the Nalchik/Abaza Fm. transition, the nannofossil assemblage underwent a new phase of reduced abundance due to facies changes. Nannofossils are absent from the lowermost part of the Abaza Fm. (5.0-7.8m, samples 724-732) and their occurrence remain scarce to rare in the rest of the Formation, progressively increasing their total abundance and species diversity toward the top of Formation, where they appear to be the main producers of the CaCO 3 . This low-diversity assemblage is characterized by small size of all coccoliths and wide domination of Toweius and Coccolithus (Appendix, Fig. I; II). The small-size Toweius (mainly T. pertusus) dominate over Coccolithus in the lower part of the formation; the consistent medium-size T. callosus appear in its upper part keeping this ratio in spite of increasing Coccolithu abundance. The warm-water Discoaster and Sphenolithus (mainly S. moriformis) are rare throughout Abaza Fm., but increase their abundance toward the upper part of the Formation. Only few coolwater Chiasmolithus were found in the interval of Abaza Fm.
The low-diversity assemblage of the lowermost part of Abaza Fm. (see Appendix, Table I), lack both in the Paleocene and characteristic Eocene species, corresponds to NP9 zone of Martini (1971). It occurs above the markers of NP9c subzone of Aubry and Salem (2012) and CP8b subzone of Okada and Bukry (1980), the Lowest Occurrences (LOs) of Discoaster mahmoudii and Campylosphaera eodela, respectively and the Highest Occurrence (HO) of Fasciculithus tympaniformis, which also corresponds to the base of CNE2 zone of Agnini et al. (2014) (Fig. 5).
The early diversification of nannofossils after PETM decline in the Kheu section (Shcherbinina et al., 2016) is related to Coccolithus and Tribrachiatus radiations. Besides, the large occurrence of the first Cenozoic rhabdoliths Blackites solus and appearance of first representatives of pontosphaerids and helicosphaerids is the peculiarity of nannofossil assemblage from the upper part of the Abaza Fm. The LOs of C. foraminis and C. latus occur prior to the LO of T. bramlettei corresponding to the base of NP10 (sample 323), but they are very scarce in the early Ypresian sediments. The LO of Discoaster diastypus in the sample 327 marks the base of CP9 zone. The NP10b subzone corresponds to the range of Tribrachiatus digitalis, which occurs between samples 328 and 331. After a slight recovery of the nannofossil abundance and species diversity roughly corresponding to the interval of NP10a subzone, the assemblage of the NP10b subzone becomes scarce again and only very few specimens of the marker species were found. This causes lesser precision of its stratigraphic range in the Kheu section and lesser accuracy of the NP10b subzone boundaries. The interval between the HO of T. digitalis and LO of T. contortus (sample 340) is identified as the NP10c subzone. It is characterized by farther increase of nannofossil species diversity and wide occurrence of small Toweius. The Abaza and Cherkessk fms. boundary falls into the middle of NP10d subzone and, possibly, corresponds to the short hiatus marked by thin sandy layer. The sedimentary hiatus, if any, cannot be detected biostratigraphically in the Kheu section. However, it could cause the small thickness of the sediments corresponding to NP10d subzone. The LO of T. orthostylus in the NP10d subzone defines the base of CNE3 zone of Agnini et al., 2014 in the sample 343. The peculiarity of nannofossil assemblage of the lower Unit II is the consistent occurrence of Scyphosphaera, holococcoliths and micrantoliths. The important upper Paleogene taxa Reticulofenestra and Cyclicargolithus appear in this interval, and the nannofossils assemblage displays its highest species diversity of the studied interval. Nannofossil total abundance significantly drops in the sapropelitic beds, but many LOs are found in these layers. Blackites solus, forming the markable component of earlier nannofossil assemblage, disappears in this interval. The HO of Tribrachiatus orthostylus in the middle of Unit II (sample 20) defines the bases of NP13 and CNE5 zones. Above this level, nannofossils decrease their total abundance and species diversity. Toweius spp. exhibits farther decline: small-sized taxon (T. pertusus) gradually decreases its abundance, medium-sized T. callosus becomes eliminated, and only large-sized T. magnictassus remains to be constantly present. Generally, the high rate of nannofossil diversification, occurrence of short-lived species (Blackites truncatus, Lophodolithus reniformis, Scyphosphaera tubicena, Nannoturba jolotteana, among others) and the large variations in total nannofossil abundance likely suggest the high environmental dynamics during the time span of Cherkessk Fm.

Palynology
In the scope of the present study, 96 samples from the lower Eocene part of the Kheu section have been analysed palynologically. In general, the most part of the samples revelead quantitatively representative assemblages of dinocysts and other palynomorphs. Dinocysts mostly dominate the assemblage, especially in the lower Abaza Fm. The sapropelitic beds of the Cherkessk Fm. are characterised by higher abundance of acritarchs and angiosperms pollen grains. Additionally, the Cherkessk Fm. is marked by significant fluctuations in the ratio of gonyaulacoidean and peridinioidean dinocysts. Stratigraphic distribution of dinocyst taxa is done on Tables II and III. The most characteristic dinoflagellate cyst species are illustrated on Figures III-VI.
According to our previously published results (Shcherbinina et al., 2016), the lowermost part of the SPB, corresponding to the onset of the Carbon Isotope Excursion (CIE) associated with the PETM, is marked by the LOs of species Axiodinium augustum, Epelidinium brinkhuisii and Epelidinium pechoricum (sample 714A). The HO of Axiodinium augustum is recognised in the sample 721 (~1.75m above the SPB and slightly after the end of the CIE). Consequently, the part of the Nalchik Fm. between samples 714A and 721 corresponds to the interval of the Axiodinium (=Apectodinium) augustum Zone, known from different Eurasian dinocyst zonations (Iakovleva and Aleksandrova, 2013;Iakovleva, 2017;Heilmann-Clausen, 1994;Mudge and Bujak, 1994;Powell, 1992) and associated with the PETM event worldwide (Crouch et al., 2001) o l o g i c a A c t a , 1 8 . 1 , 1 -1  Lower Eocene biostratigraphy in the Caucasus basin 10 are observed within the SPB between the LO of nannofossil Rhomboaster spp. and the LOs of Campylosphaera eodela and Discoaster araneus. Besides, the LOs of Homotryblium tasmaniense, H. tenuispinosum and Muratodinium fimbriatum are recognized within the SPB. The successive LOs of Homotryblium abbreviatum, Hystrichostrogylon holohymenium and Biconidinium longissimum, in their turn, are revealed above the SPB and the LO of Discoaster mahmoudii nannofossil. Finally, the HO of Axiodinium augustum (sample 721, ~1.75m above the SPB) coincides with the HO of Fasciculithus richardii group. The uppermost part of the Nalchik Fm. (samples 722-725) is characterized by the disappearance of the most Apectodinium spp. and the HO of Epelidinium pechoricum (sample 723) close to the HO of nannofossil Fasciculithus tympaniformis. At the same time, the dinocyst assemblage does not contain the younger key-species Dracodinium astra, while the abrupt influx of the Areoligera eco-group is observed at the base of this interval that indicates the Deflandrea oebisfeldensis Zone, which was identified in the eastern Peri-Tethys (Iakovleva, 2017) and well known in the NW European basins (Powell, 1992;as Viborg-7 Zone in Denmark, Heilmann-Clausen, 1985; as E1b-c Subzone in the North Sea, Mudge and Bujak, 1996; as Glaphyrocysta ordinata Zone in southern England, Powell et al., 1996) and Western Siberia (Iakovleva and Aleksandrova, 2013).
The LO of Deflandrea phosphoritica is recognized in the sample 729, ~ 3.5m below the LO of nannofossil Tribrachiatus bramlettei. Unfortunately, the relatively poor dinocyst assemblage of the lower Abaza Fm. mudstone did not reveal the stratigraphically important species Stenodinium meckelfeldense and Axiodinium lunare. Nevertheless, based on the records of Deflandrea phosphoritica from the base of the Stenodinium (=Wetzeliella) meckelfeldense Zone in the eastern Peri-Tethys (Iakovleva, 2017), Western Siberia (Iakovleva and Aleksandrova, 2013), North Sea (E2a subzone, Mudge and Bujak, 1994) and NW Europe (Powell, 1992), we suggest that the lower Abaza Fm. (samples 729-330) possibly corresponds to the interval of Stenodinium meckelfeldense Zone. The HO of Palaeotetradinium minusculum is observed below the LO of T. bramlettei and the HOs of A. paniculatum and A. homomorphum are above the LO of T. bramlettei.
A noteworthy fact is that the LO of Eatonicysta ursulae marks the base of the Eatonicysta ursulae Zone in the eastern Peri-Tethys (Kazakhstan) bounded by the Dracodinium simile and Dracodinium varielongitudum Zones (Iakovleva, 2017;King et al., 2013). However, in the Kheu section, E. ursulae is revealed belatedly and sporadically only within the Dracodinium varielongitudum Zone interval and its true lowermost occurrence seems to be missed. The next part of the Cherkessk Fm. between samples 744 and 745A contains taxonomically poor dinocyst assemblage, dominated by Achomosphaera ramulifera and Spiniferites ramosus, and did not reveal stratigraphically important species Charlesdowniea coleothrypta, Petalodinium condylos, or Rhadinodinium politum. Consequently, this interval of the section remains unclear: it corresponds either to the Charlesdowniea coleothrypta Zone or to the upper part of the Dracodinium varielongitudum Zone. Taking into account that the nannofossil record does not suggest the occurrence of any significant stratigraphical hiatus, it is most probably that the true LO of Ch. coleothrypta is missed due to some facies unfavorability that is indicated by very low abundance of wetzelielloids.
The key-species Ochetodinium romanum as well as Cerebrocysta bartonensis and Memranilarnacia compressa first occur in the sample 745B. The LO of Och. romanum is found here in the middle CP10 zone. The LO of stratigraphically important Samlandia chlamydophora and the HO of Deflandrea oebisfeldensis are revealed in the sample 746B and coincide with the LOs of Coccolithus crassus and the base of CP11 zone. It is noteworthy that the lowermost for this section (but not the true LO) common occurrence of Sophismatia crassoramosa (up to ~7% of the dinocyst assemblage) is documented slightly above the LO of S. chalmydophora and its HO is recognized in the sample 759 (the upper part of NP12 zone). It was largely considered previously that S. crassoramosa, occurring first during the early Ypresian, has a very short stratigraphic range and disappears within the NP10 Zone interval. Our record from the Kheu section suggests a wider stratigraphical interval for this species. The LOs of species Pentadinium laticinctum and Membranilarnacia glabra (sample 7) are recognized in the upper parts of NP12 and CNE4 nannofossil. Stratigraphically important species Piladinium columna, which true LO is usually between the LOs of Och. romanum and S. chamydophora (Iakovleva and Aleksandrova, 2013;Iakovleva, 2017), has a belated first occurrence in the Kheu section (sample 6). Successive LOs of Och. romanum and S. chlamydophora allow us to attribute this part of the Cherkessk Fm. to the Ochetodinium romanum/Samlandia chlamydophora Zone of eastern Peri-Tethys (Iakovleva, 2017), which is also known in Western Siberia (Iakovleva and Aleksandrova, 2013). It corresponds to both Ochetodinium romanum and Kisselevia aff. clathrata zones interval in Belgian Basin (De Coninck, 1991) and to the D8b-c subzones of NW Europe (Costa and Manum, 1988).
The succession of the LOs of dinocyst key-species is recognized within the upper Cherkessk Fm. Thus, the LO of Areosphaeridium diktyoplokum (sample 21) corresponds to the HO of nannofossil Tribrachiatus orthostylus, which defines the base of NP13 Zone. This bioevent is followed by LOs of Thalassiphora dominiquei and Dracodinium coronatum (sample 22), Diphyes brevispinum (sample 23), and Areosphaeridium michoudii (sample 24). The HO of Deflandrea denticulata is observed in the sample 32. The LO of Areosphaeridium diktyoplokum defines the base of the Areosphaeridium diktyoplokum Zone in the eastern Peri-Tethys (Iakovleva, 2017), Western Siberia (Iakovleva and Aleksandrova, 2013) and Denmark (Heilmann-Clausen and Costa, 1989). The Peri-Tethyan Areosphaeridium diktyoplokum Zone corresponds to the part of the NW European D9 Zone (Costa and Manum, 1988), the interval of three zones in Belgium (Areosphaeridium diktyoplokum, Paucilobimorpha triradiata, Litosphaeridium mamellatum;De Coninck, 1991), the part of the E2c Subzone in the North Sea (Mudge and Bujak, 1994), and the part of the В-2 Zone of southern England (Bujak et al., 1980). Similarly to the dinocyst record of the key lower Eocene Peri-Caspian Aktulagay section (Kazakhstan, King et al., 2013) and in the West European sections (Steurbaut, 1998), the LO of Ar. diktyoplokum almost coincides with the base of NP13 Zone in the Kheu section, while the LO of Thalassiphora dominiquei is very close to this level.
The LOs of stratigraphically important species Duosphaeridium nudum and Dracodinium? brevicornutum are recognized in the uppermost part of the studied interval of Cherkessk Fm., within the last sapropelitic bed. At Aktulagay section, these species occur slightly before the first occurrence of key-species Dracodinium eocaenicum, which characterizes the uppermost Ypresian (the base of NP14 Zone), while in Western Siberia D. nudum and Dr.? brevicornutum are revealed simultaneously with Dracodinium eocaenicum (Iakovleva and Aleksandrova, 2013). In the Kheu section, the lowermost occurrences of D. nudum and Dr.? brevicornutum are constrained to within the NP13 Zone interval, similarly to the record found in Kazakhstan (King et al., 2013).

DISCUSSION AND CONCLUSIONS
The lower Eocene sediments of the Kheu section consist of two lithostratigraphic units (Abaza Fm. and Cherkessk Fm.). Their sedimentological peculiarities can be interpreted in response to regional paleogeographic changes. The accumulation of Abaza Fm. represents an interruption of the carbonate shelf sedimentation of the earliest Eocene (the uppermost Nalchik Fm.). It was caused by the extensive input of clayey material, likely due to the significant sea-level fall of the lowermost Eocene (see Fig. 3) and dramatically increased river discharge from the neighboring island. These paleoenvironmental changes negatively affected calcareous microbiota -the main CaCO 3 supplier into the sediment-which displays very gradual restoration in abundance after their dramatic elimination from the lower part of Abaza Fm. Nannofossils recovered earlier than foraminifera restoration and formed the main calcareous component of the mostly siliciclastic Abaza Fm. (see Fig. 4B), while foraminifera G e o l o g i c a A c t a , 1 8 . 1 , 1 -1  Lower Eocene biostratigraphy in the Caucasus basin 12 reappeared in the upper part of this Formation only. The combining foraminifera-nannofossil component is the main constituting elements of the calcium carbonate in the Cherkessk Fm. These variations in microfossil abundance and diversity likely reflect the variations in the basin depth, changes of the source area of eroded and transported into the basin sediments and, thus, the supply of nutrients for microorganisms during the early Eocene. The maximum depth of this part of N Caucasian basin during Ypresian was likely likey achieved during the formation of the highly calcareous compact pink marl in the middle of the Unit 1 between 28 and 31m (see photographs in Figs. 2; 4) marked by the highest nannofossil and foraminifera abundance. Many shallow basins of the NE Peri-Tethys margin display hiatus or shallowing upward sequences ranging the upper Ypresian interval (Muzylöv, 1994). The sediments of the Unit 2 of Cherkessk Fm. are supposed to be accumulated during the regressive trend corresponding to the long-lived eustatic sea-level fall (Miller et al., 2005), that is supported by coarsening of siliciclastic component in this part of section and reduced abundance of planktonic microbiota.
The different number of TOC-rich layers is documented throughout a wide territory of the NE Peri-Tethys in the upper part of the Ypresian (Muzylöv, 1994). The Kheu section, located in the deepest part of this paleo-basin, displays the highest number of TOC-rich layers and highest TOC enrichment showing the most complete record of the paleoenvironmental changes during this time span (upper NP12-lower NP13 nannofossil zones) evidently related to the Early Eocene Climatic Optimum. Short regressive pulses suggested by the increase of glauconite concentrations and reduced nannofossil abundance preceded the formation of each TOC-rich sapropelite interval ( Fig. 4E; 5) during the following short transgression pulses (see Fig. 3).
The high-resolution study of the lower Eocene microfossils enabled the direct calibration between nannofossil and dinocyst bioevents. Despite the wide variations in the nannofossil abundance throughout the section, the representativeness of nannofossil assemblages enabled the use of three complimentary nannofossil zonations (Agnini et al., 2014;Martini, 1971;Okada and Bukry, 1980) that provided a detailed biostratigraphy of the studied succession. The series of dinocyst bioevents revealed in the lower Eocene of the Kheu section led to recognize eight dinocyst zones previously established in the NE Peri-Tethys and NW Europe: Axiodinium augustum, Deflandrea oebisfeldensis, Dracodinium astra, Dracodinium simile, Stenodinium meckelfeldense, Draconidium varielongitudum, Ochetodinium romanum/Salmandia chlamydophora and Aerosphaeridium diktyoplokum (see Fig. 5). The true LOs of nominative species for two Ypresian Peri-Tethyan zones (Eatonicysta ursulae and Charlesdowniea coleothrypta) have not been recognized in the Kheu section, probably due to the peculiarities of facies and dinocyst assemblages. Nevertheless, dinocyst data indicate quite continuous character of the early-mid Ypresian sedimentation.
Generally, both nannofossils and dinocysts show very high dynamics in the species first appearances/disappearances around the SPB corresponding to the global PETM event, and just above it in the interval of the late phase of the CIE (Shcherbinina et al., 2016). Above this level, the renovation of nannofossil assemblage is mostly related to the radiation of short-lived species (Tribrachiatus and Coccolithus linages), reflecting the environmental specificity worldwide during the earliest Eocene. In the basin of the central northern Caucasus, the large-scale supply of siliciclastic material oppressed calcareous microbiota and caused the bloom of siliceous (radiolarian and diatoms) and organicwalled (dinoflagellates) microorganisms, sustained by the increased nutrient availability. The palynomorph assemblage from the post-PETM interval (part of NP9c) shows the absolute domination of dinocysts (~100%) within the uppermost Nalchik and the whole Abaza fms. In its turn, the dinocyst assemblage within this whole interval (Deflandrea oebisfeldensis-Dracodinium astra-Stenodinium meckelfeldense zones) is clearly dominated by Spiniferites/Achomosphaera-group, indicating the restoration of the open-marine shelf environments after the PETM and during the post-PETM time. This suggests small-scale, albeit long-lived, sea-level fall in the basin and evidences that facies change has much more crucial effect on calcareous microbiota than water depth changes. Wide occurrence of siliceous microorganisms and dinoflagellates and domination of mesotrophic/euthrophic Toweius in nannofossil assemblage of Abaza Fm. both indicate high nutrition level during the early Ypresian time. The high ratio of warm-water Discoaster and cool-water Chiasmolithus suggests rather warm conditions.
In the middle Ypresian (NP11-12), the high turnover rate in nannofossil assemblage of the northern Caucasus is mainly derived from the extensive radiation in Sphenolithus lineages and first appearance of new species of Discoaster, Toweius, Helicosphaera and Girgisia (14 new nannofossil taxa), while many Paleocene survivors become extinct. This increased dynamics of the changes in the nannofossil assemblage more likely reflects initial environmental variations possibly corresponding to the minor lower Eocene hyperthermals (Nicolo et al., 2010;D'Onofrio et al., 2016;Sexton et al., 2011;among others) occurred during this time-span. The dinocyst taxonomic diversity and prominent fluctuations of different dinocyst eco-groups (wetzelielloids, deflandroids, Homotryblium-, Areoligera-, Spiniferites-groups, Impagidinium) increases from the beginning of the Cherkessk Fm. (Dracodinium varielongitudum Zone, NP11 Zone), that also confirms Lower Eocene biostratigraphy in the Caucasus basin 13 environmental fluctuations in the basin. The next part of the Cherkessk Fm. (conceivably Charlesdowniea coleothrypta Zone interval), is characterized by a very scarce dinocysts and, therefore, suggests the maximal deepening of the basin, already confirmed by highest nannoplankton and foraminifera abundance in the calcareous marl of this interval.
The very high diversification rate is detected in the interval of NP12 nannofossil Zone and Ochetodinium romanum/Samlandia chlamydophora dinocyst Zone, where many nannofossil and dinocyst taxa have their LOs, including the LOs of important upper Cenozoic nannofossil genera Reticulofenestra and Cyclicargolithus. This bloom in microfossil diversity during the mid-Ypresian, recognized in the Kheu section, is typical for many other regions of marine sedimentation (North Sea Basin, Peri-Tethys, Western Siberia) and was more likely governed by the paleoenvironmental instability during the EECO that caused variations in the sedimentary environment and occurrence of multiple ecological niches. In the topmost part of studied interval (NP13 zone), both microfossil groups display low turnover dynamics in the microfossil assemblages that likely suggests environmental stabilization also known for many other West Eurasian regions.

ACKNOWLEDGMENTS
We thank G.N. Aleksandrova (Geological Institute of the Russian Academy of Sciences) for her help in chemical palynological preparation and anonymous reviewer for important comment, helping to improve the manuscript. This work was made following the plans of scientific research of the Geological Institute of RAS (projects nos. 0135-2019-0062, 0135-2019-0044 and 0135-2019-0070) and was partially supported by the Presidium of RAS Program no. 0135-2018-0051.   l o g i c a A c t a , 1 8 . 1 , 1 -1 Lower Eocene biostratigraphy in the Caucasus basin    l o g i c a A c t a , 1 8 . 1 , 1 -1   2 2 4 1 90 6 1 4 1 0 1 5 1 2 1 4 3 1 10 9 2 3 21 1 1 0  C h e r k e s s k F m.