A mid neritic-upper bathyal Ypresian section at Aktulagay, western Kazakhstan, has been analyzed palynologically. A number of key dinoflagellate cyst events are directly calibrated with published calcareous nannofossil data from the same section. The events are used to identify eight dinoflagellate cyst zones from a recently established zonation, used elsewhere in the eastern Peri-Tethys, and to calibrate these zones with the standard nannofossil zonation (NP zones). The events include the lowermost occurrences of Deflandrea oebisfeldensis (∼1%), Dracodinium simile, Eatonicysta ursulae, Dracodinium varielongitudum, Charlesdowniea coleothrypta, Ochetodinium romanum, Charlesdowniea columna, Samlandia chlamydophora, Areosphaeridium diktyoplokum, and Wetzeliella eocaenica. An important regional unconformity separates the Ypresian section from overlying non-calcareous strata with the age-diagnostic species Enneadocysta arcuata, Wetzeliella ovalis, Wilsonidium echinosuturatum, and Rhombodinium draco, indicating the Rhombodinium draco Zone of latest Lutetian–Bartonian age. Based on fluctuations of ecological groups of dinoflagellate cysts, a series of different depositional environments are interpreted and related to the existing sequence stratigraphic model of the section. In most cases dinoflagellate cyst agree with, or supplement, calcareous micro- and nannofossil indications, and support the sequence stratigraphic model. Impagidinium wardii sp. nov. is atypical for the otherwise oceanic genus as it bloomed in a mid-neritic environment. The first cooling at the end of the Early Eocene Climatic Optimum (EECO) is suggested to have caused a strong acme of Eatonicysta ursulae and distinct lowering of the sea level in the NP13 zone. Four new species are formally described: Cribroperidinium cavagnettiae sp. nov., Dracodinium robertknoxii sp. nov., Impagidinium wardii sp. nov., and Samlandia chriskingii sp. nov. The Aktulagay Formation of King et al. (2013) is renamed the Kulsary Formation.
The historical type areas of most Paleogene stages are situated in northwest Europe. As a result, most stratigraphic studies of dinoflagellate cysts have been carried out in this region, making the Eocene dinoflagellate cyst succession of northwest Europe the best-documented worldwide. The numerous Eocene dinoflagellate cyst zonations include, among others, Costa and Downie (1976); Châteauneuf and Gruas-Cavagnetto (1978); Bujak et al. (1980), Costa and Manum (1988); De Coninck (1991), Heilmann-Clausen (1988), Heilmann-Clausen and Costa 1989, Köthe (1990, 2003, 2012); Powell (1992); Bujak and Mudge (1994); Mudge and Bujak (1994, 1996); Heilmann-Clausen and Van Simaeys (2005); Köthe and Piesker (2007), Powell and Brinkhuis in Vandenberghe et al. (2012). Subsequently King (2016), who reviewed all previous zonations and provided a new zonation scheme.
A similar detailed knowledge of the dinoflagellate cyst succession is not available from other regions, but, if this were provided, may be used to elucidate Eocene paleooceanography and paleobiogeography. In particular, the dinoflagellate cyst succession in the Peri-Tethys may shed light on problematic Eocene marine connections, such as the possible direct marine connection between the Peri-Tethys and the North Sea Basin (King et al. 2013; Deprez et al. 2015). Such paleogeographic studies evidently necessitate a detailed regional bio- and chronostratigraphy of the eastern Peri-Tethys successions. Organic-walled dinoflagellate cysts are widely distributed from inner neritic to oceanic settings and are preserved in both calcareous and non-calcareous sediments. They may therefore be particularly useful to improve the chronostratigraphic understanding of marine Eocene deposits in the Peri-Tethys.
Dinoflagellate cyst studies from the Eocene deposits undertaken in the Caspian area are scarce. The earliest publications (Benyamovsky et al. 1990; Akhmetiev and Zaporozhets 1992; Vasilyeva 1994; Iakovleva 1998, 2000) recognized broad dinoflagellate cyst zones from northwest Europe. Iakovleva et al. (2001) presented the first detailed study of late Paleocene–earliest Eocene dinoflagellate cysts from the Sokolovsky Quarry section in Northern Kazakhstan. This high-resolution quantitative dinoflagellate cyst analysis demonstrated the close resemblance of dinoflagellate cyst successions in coeval North Sea Basin and Peri-Tethys sections and identified several Thanetian–earliest Ypresian zones in Kazakhstan, previously established in the North Sea Basin. During the last 10 years, new detailed dinoflagellate cyst studies have been published (Vasilyeva and Musatov 2010, 2012; Vasilyeva 2013; Oreshkina et al. 2015). These studies are partially calibrated with the nannofossil zones of Martini (1971) and are based on a limited number of Paleocene–Eocene boreholes from the northern Caspian area (the Novouznensk and Elton boreholes, Russia; boreholes no. SP-1 and 57, northern Kazakhstan). Recently, King et al. (2013) published a high-resolution Eocene record from the Aktulagay outcrop section in Western Kazakhstan (Figure 1). This study includes a detailed description of the site and its sedimentary facies and lithostratigraphy, and integrated the biostratigraphy of calcareous nannofossils, dinoflagellate cysts, foraminifers, pteropods, and shark teeth. The well-exposed, mid to outer neritic Aktulagay section now serves as a reference section for the eastern Peri-Tethys. Although dinoflagellate cysts were included in the King et al. (2013) study, their distribution and paleoecology were not fully documented, and photographs and taxonomy were not included.
Consequently, the aim of our present study is: (i) to present a detailed palynostratigraphic relative abundance analysis of the Aktulagay section, including a taxonomic treatment with photographic documentation of the excellently preserved material, and formal descriptions of four new dinoflagellate cyst species; (ii) to accentuate the first-order calibrations of dinoflagellate cyst and calcareous nannofossil events; and (iii) to interpret the paleoenvironment based on the dinoflagellate cyst associations.
2. Study site
The Aktulagay outcrops (a group of hills; 47°32′31.47″N, 55°09′13.75″E) are located in western Kazakhstan, east of the Caspian Sea and southwest of the Aral Sea, 150 km northeast of the town of Kulsary and 35 km north of the Embi River. The outcrops represent an isolated outlier of the Ustyurt Plateau (Figure 1).
The Aktulagay section was discovered by V.I. Zhelezko in 1967, but the first published description was given by Benyamovsky et al. (1990), who divided the Early Eocene strata of these outcrops into two new units: the Alashen and Tolagaysor sviti (∼formations) with their stratotypes at Aktulagay. Zhelezko and Kozlov (1999) gave a more detailed and in some respects differing description of the Aktulagay section. New detailed logging and sampling of the Aktulagay section was made between 2000 and 2003 by Chris King and David J. Ward (King et al. 2013).
According to Naidin and Beniamovski (2006), Maastrichtian chalks outcrop on the lower slopes of the Aktulagay hills and are extensively exposed on their western margin. King et al. (2013) observed a thin, and apparently localized, upper Paleocene chalky limestone (upper NP8 nannofossil zone interval) between the Eocene sediments and the Maastrichtian chalk several hundred meters east of the Aktulagay section studied in the present paper. The section is the same as studied by King et al. (2013) and is exposed at the western boundary of the hills. Here, lowermost Eocene strata directly overlie Maastrichtian chalk. In turn, the Eocene strata underlie ‘Sarmatian’ (Serravallian) limestones.
King et al. (2013) divided the Eocene part of the Aktulagay section into four major lithostratigraphic units with well-defined boundaries (units A, B, C and D). Here we present a short summarized lithostratigraphy of the Eocene section (from base to top; Figure 2), based on the detailed description of King et al. (2013).
Unit A (relatively homogeneous, light gray to gray-green marl and calcareous clay):
Unit A1 (0–0.20 m): A very thin but lithologically distinct unit of gray-green clay.
Unit A2 (0.20–10.80 m): Mainly calcareous clay and marly clay. The base is marked by an abrupt change to calcareous clay with small dispersed phosphatic granules. Thirteen omission surfaces have been identified (see King et al. 2013, fig. 10).
Unit A3 (10.80–13.45 m): Mainly marl.
Unit B (heterogeneous, with four interbedded lithologies including light gray marly clay/marl, blocky light gray-brown clay, brown shaly non-calcareous clay and black sapropelic clay):
Unit B1 (13.45–16.22 m): Slightly calcareous to non-calcareous clay, with four thin sapropelic clay beds.
Unit B2 (16.22–23.57 m): Mainly interbedded marly clay, with thin non-calcareous clay interbeds. The base is a prominent omission surface with concentrations of phosphatized coprolites and fish debris. Four black sapropelic clay beds have been identified within the marly clay units.
Unit C (silty clay and silt):
Unit C1 (23.57–∼27.25 m): Sandy clayey silt; many thin lenticular layers of very fine silty sand.
Unit C2 (∼27.25–56.20 m): The basal part is a silty clay/clayey silt rich in foraminifers and small mollusks. It grades upward to sandy clayey silt and clayey silt with thin, very fine sand partings. Widely spaced calcareous siltstone concretions are present in the middle part. Very fine-grained glauconite is present in the upper part.
Unit D: (silty clay and silt):
Unit D1 (56.2–56.45 m): Sandy clayey silt with abundant phosphatized sandstone pebbles and with abraded shark teeth. The base is sharply defined and marks a disconformity.
Unit D2 (56.45–∼78.0 m): Homogeneous, silty, non-calcareous clay.
In the studied section only the lowermost 3 m of Unit D is present, unconformably overlain by Miocene limestones. One kilometer north of the main Aktulagay Plateau, Unit D is much thicker.
Based on their detailed study of the Aktulagay section, King et al. (2013) noted that the previous division of the Eocene section into the Alashen and Tolagaysor sviti of Benyamovsky et al. (1990) needed to be discussed. The Alashen Svita was divided by Benyamovsky et al. (1990) into units 1 and 2. According to King et al. (2013), Unit 1 corresponds to Unit A of King et al. (2013), and Unit 2 to the lowest 2 m of Unit B. The overlying Tolagaysor Svita corresponds to the majority of Unit B together with Unit C (Figures 2, 3). Consequently, the Tolagaysor Svita of Benyamovsky et al. (1990) included two distinct lithological entities. Benyamovsky et al. (1990) divided the Tolagaysor Svita into four lithological units, but it was difficult to relate these to the section logged by Chris King and David J. Ward in 2000–2003 (King et al. 2013). The highest unit (4) of Benyamovsky et al. (1990) was described as green calcareous clay with benthic and planktonic foraminifers, and is probably a part of Unit C. To achieve a better agreement between formations and lithologies, King et al. (2013) proposed a revision of the lithostratigraphy, as follows (Figures 2, 3): the Alashen Svita was slightly modified as the Alashen Formation; it corresponds to Unit A (following Zhelezko and Kozlov 1999) and is a relatively homogeneous unit of gray calcareous clay and marls. The new Aktulagay Formation was defined, corresponding to the heterogeneous Unit B. The Tolagaysor Svita was redefined as the Tolagaysor Formation, which represents the coarser-grained Unit C. Benyamovsky et al. (1990) did not recognize Unit D and apparently considered it a part of the overlying Sarmatian sediments. Zhelezko and Kozlov (1999) recognized a 7 m clay unit above the Tolagaysor Svita (with a layer of phosphatized pebbles at the base) that corresponds to Unit D. Unit D was tentatively assigned by King et al. (2013) to the Sangryk Formation.
A recent study of the Eocene section at Aktulagay was undertaken by Baraboshkin et al. (2015). These authors broadly agree with the lithologic description of the succession given by King et al. (2013). They noted, however, that the name ‘Aktulagay Formation' is already being used for an earlier described Upper Cretaceous formation (Koltypin 1957) and the name cannot, therefore, be used for a different unit. Baraboshkin et al. (2015) again revised the lithostratigraphy. They followed King et al. (2013) concerning the definition of the Tolagaysor and tentative Sangryk formations. They considered the Alashen and Aktulagay formations of King et al. (2013) to constitute ‘a singular sequence' (Baraboshkin et al. 2015, p. 149). Despite their well-described and clearly different lithologic characters and their significant thicknesses, the two formations were referred to a single, redefined Alashen Formation, thus eliminating the Aktulagay Formation of King et al. (2013). We disagree with the concept of ‘a singular sequence’ for the two formations, and we find that the recognition of different, mappable lithologic characters in the Alashen and Aktulagay formations of King et al. (2013) merits a continued subdivision of this interval into two separate formations. As the name Aktulagay is already used for an Upper Cretaceous unit we here propose to rename the Aktulagay Formation of King et al. (2013) the Kulsary Formation, named after the nearby town of Kulsary (Figure 3).
3. Material and methods
Thirty-nine samples were analyzed palynologically. The samples were collected by Chris King and David J. Ward during their field work in 2000, 2001, and 2003. All samples were prepared at the Department of Geosciences, Aarhus University, using standard techniques and with sieving on 20-µm filters (King et al. 2013). In general, dinoflagellate cyst taxonomy follows Williams, Fensome, et al. (2017), except for the subfamily Wetzelielloideae. For the subfamily Wetzelielloideae a new systematics, introducing new genera, was proposed by Williams et al. (2015) and has initiated a major discussion within the dinoflagellate cyst specialist community (Bijl et al. 2017; Williams, Damassa, et al. 2017). The authors of the present work adhere to opposite points of view concerning the new systematics of Williams et al. (2015). In this paper we decided to keep the old generic names of wetzelielloids and follow the taxonomy of Fensome et al. (2008).
The dinoflagellate cyst zonation used in the study of the Aktulagay section by King et al. (2013) was modified from that of Heilmann-Clausen (1988) and is shown in figure 15 in King et al. (2013). Since then, a new Eocene dinoflagellate cyst zonation has been established for the eastern Peri-Tethys (Iakovleva 2017b). This zonation, published in Russian, is applied here and for convenience is shown in Figure 4. The generic names of wetzelielloidean species in this zonation followed the systematics of Williams et al. (2015). Calcareous nannofossil data used for calibrations are from E. Steurbaut in King et al. (2013).
Palynological preparations are stored at the Department of Geoscience, Aarhus University, Denmark. The types of Cribroperidinium cavagnettiae sp. nov., Impagidinium wardii sp. nov. Dracodinium robertknoxii sp. nov., and Samlandia chriskingii sp. nov. are housed in the type collection of the Geological Museum, Øster Voldgade 5–7, Copenhagen, Denmark, under the catalogue numbers MGUH 32065 to MGUH 32072.
Among the 39 studied samples, 24 were studied for relative abundance of dinoflagellate cysts, whereas 15 additional samples were investigated qualitatively (presence/absence of dinoflagellate cyst taxa). For proportional analysis at least 200–250 dinoflagellate cysts per sample were counted.
Since the 1970s, numerous studies have contributed to the current understanding of the paleoecology of Paleogene dinoflagellate cysts. For some dinoflagellate cysts, the paleoecology is now well known, based on several studies from both hemispheres, whereas the signals of others are still uncertain. For the present study, we have based our interpretation of the paleoenvironment on a number of groups, the taxa of which are thought to share environmental preferences (‘eco-groups'). Most of these groups were established in previous studies, primarily Brinkhuis (1994). The groups and their presumed paleoenvironmental signals are shown in Table 1.
4. Results and discussion
Palynomorph assemblages from the Aktulagay section are strongly dominated by dinoflagellate cysts with rare acritarchs, prasinophytes and sparse bisaccate pollen grains (samples sieved on 20-µm filters; see Methods). Dinoflagellate cyst assemblages in nearly all samples are diverse and well preserved. In total ∼190 largely cosmopolitan dinoflagellate cyst taxa have been identified. The stratigraphic distribution of the dinoflagellate cysts is shown in Figure 5. The most important dinoflagellate cyst events and their calibration with calcareous nannofossil events are shown in Figure 6. All nannofossil data are from King et al. (2013). The relative abundance of dinoflagellate cyst eco-groups in proportionally studied samples is presented in Figure 7. The most characteristic dinoflagellate cyst species are illustrated in Plates 1–13.
Large changes of the dinoflagellate cyst assemblages are recorded in the section (Figure 7). The Spiniferites-group attains high numbers or dominance in the Alashen and lower Kulsary (Unit B1) formations. The upper Kulsary Formation (B2 Unit) revealed assemblages with common wetzelielloids, deflandreoids, Homotryblium-group and Thalassiphora pelagica. A peak (25%) of Impagidinium wardii sp. nov. is noted in one sample within Unit B2. An extreme dominance (up to 96%) of Eatonicysta ursulae is found in the Tolagaysor Formation (Unit C). The paleoecology of the latter two species is discussed below. The upper part of the Tolagaysor Formation includes common deflandreoids, Cleistosphaeridium diversispinosum, and the Spiniferites-group.
4.1.1. Alashen Formation
The dinoflagellate cyst assemblage in the single sample from the 20 cm thick Unit A1 (lowermost Alashen Formation) is relatively diverse: dominated by the Spiniferites-group and containing common long-ranging Oligosphaeridium complex, Thalassiphora delicata, and Lanternosphaeridium lanosum. Deflandrea oebisfeldensis accounts for ∼1% of total dinoflagellate cysts, and Hystrichosphaeridium tubiferum and Glaphyrocysta ordinata are rare. This assemblage somewhat resembles that of the D. oebisfeldensis acme in the North Sea Basin, except that D. oebisfeldensis and G. ordinata are generally more common in the North Sea Basin (Heilmann-Clausen 1985, 1988). The assemblage corresponds to the D. oebisfeldensis Zone in Northern Caucasus (Iakovleva 2017b; Shcherbinina et al. 2020). Based on calcareous nannofossils (King et al. 2013), Unit A1 can be referred to Subzone NP10b of Aubry (1996).
Dinoflagellate eco-groups and their inferred paleoenvironmental indications.
The dinoflagellate cyst assemblage from the lower-middle part of Unit A2 is taxonomically diverse and includes a number of stratigraphically important lowermost occurrences (LOs). In the lower part of Unit A2 (0.65 m) are recorded the LOs of Dracodinium simile, Achilleodinium biformoides, Biconidinium longissimum, Hystrichostrogylon holohymenium, Wetzeliella meckelfeldensis, Wetzeliella lunaris, and Wetzeliella samlandica. The LO of Dr. simile, which marks the base of the Dr. simile Zone in the eastern Peri-Tethys (Iakovleva 2017b), coincides at Aktulagay with the LO of the calcareous nannofossil Tribrachiatus orthostylus and is just below the LO of the nannofossil Imperiaster obscurus and the highest occurrence (HO) of the nannofossil Discoaster multiradiatus. The HO of Alisocysta sp. 2 sensu Heilmann-Clausen (1985) coincides with the LOs of the calcareous nannofossils Chiphragmalithus calathus (base of middle NP11) and Discoaster pacificus. The dinoflagellate cysts Cleistosphaeridium diversispinosum, Deflandrea phosphoritica, and Turbiosphaera galatea occur within the Dr. simile Zone.
The LO of Dr. simile occurs in the middle of NP11 and is below the LOs of W. meckelfeldensis and W. lunaris, suggesting an important hiatus at the base of Unit A2, corresponding to the interval of the Dracodinium (= Wetzeliella) astra and Stenodinium (= Wetzeliella) meckelfeldense zones of Iakovleva (2017b). Calcareous nannofossil data suggest that the hiatus corresponds to the upper part of NP10 (King et al. 2013). The dinoflagellate cyst assemblage in the sample at the base of Unit A2 (0.2–0.25 m) resembles that in A1. This assemblage may be largely reworked, as for the nannofossils (King et al. 2013).
The LO of Eatonicysta ursulae at ∼4.6 m marks the base of the E. ursulae Zone (mid NP11 zone interval), ∼1.2 m above the LOs of the calcareous nannofossils Ch. calathus and D. pacificus. The LOs of several stratigraphically significant species, including aff. Heslertonia heslertonensis, Cannosphaeropsis utinensis, Cleistosphaeridium polypetellum, Diphyes ficusoides, and Homotryblium abbreviatum are recorded in the E. ursulae Zone.
The LOs of Dracodinium varielongitudum, Pentadinium laticinctum, and Wetzeliella aff. articulata-group are at 6.89 m in the middle of Unit A2. The LO of Dr. varielongitudum defines the base of the Dracodinium varielongitudum Zone and occurs ∼2 m below the base of NP12 (LO of D. lodoensis) and the HO of Ellipsolithus macellus.
The LOs of Dracodinium robertknoxii sp. nov. and Diphyes pseudoficusoides almost coincide with the LO of Discoaster lodoensis. The LO of the stratigraphically significant Rhombodinium? glabrum occurs at ∼11.9 m within Unit A3, ∼1 m above the LCO of D. lodoensis.
The LO of Charlesdowniea coleothrypta is recorded at 13.25 m in the topmost Unit A3, 0.15 m below the base of Unit B1, and defines the base of the Ch. coleothrypta Zone. The LO of Ch. coleothrypta at Aktulagay is in the middle NP12, immediately above the LOs of Micrantholithus mirabilis and Chiphragmalithus barbatus and coincides with the HO of Pontosphaera exilis.
4.1.2. Kulsary Formation
Dinoflagellate cyst diversity is very high in both Unit B1 and Unit B2, and a number of important LOs are recognized within the formation. The LOs of Corrudinium incompositum and Diphyes brevispinum occur at the base of Unit B1 and coincide with the LO of the calcareous nannofossil Lophodolithus reniformis.
The LOs of the stratigraphically important Dracodinium politum and Dracodinium condylos occur at 14.7 m and are very close to the LO of the calcareous nannofossil Helicosphaera seminulum (14.45 m). The HO of Apectodinium quinquelatum (15.5 m) is slightly below the HO of the calcareous nannofossil Micrantholithus mirabilis.
The LOs of Ochetodinium romanum and Samlandia chlamydophora occur at the base of Unit B2 at 16.27 m, while the LOs of Charlesdowniea columna and Thalassiphora dominiquei are at 17.0 m. The LO of Ochetodinium romanum defines the base of the Och. romanum Zone of De Coninck (1991) in Belgium, and of the Och. romanum/Sam. chlamydophora Zone in Western Siberia (Iakovleva and Aleksandrova 2013) and the eastern Peri-Tethys (Iakovleva 2017b). In Western Siberia (Iakovleva and Heilmann-Clausen 2010), Northern Caucasus (Shcherbinina et al. 2020), and the North Sea (Iakovleva unpublished data), Och. romanum, Charlesdowniea columna, and S. chlamydophora occur successively. The simultaneous LOs of Och. romanum and S. chlamydophora at Aktulagay support the presence of a substantial hiatus at the base of Unit B2, as indicated by calcareous nannofossil data (King et al. 2013).
The LO of the prominent marker Areosphaeridium diktyoplokum defines the base of Ar. diktyoplokum Zone in Northern Germany and Denmark (Heilmann-Clausen 1988; Heilmann-Clausen and Costa 1989), Western Siberia (Iakovleva and Heilmann-Clausen 2010; Iakovleva and Aleksandrova 2013), and the eastern Peri-Tethys (Iakovleva 2017b). At Aktulagay the LO of Ar. diktyoplokum is at 17.95 m, very close to the NP12/NP13 zonal boundary, defined by the HO of T. orthostylus (18.05 m).
The LO of Areosphaeridium michoudii occurs at 19.25 m, slightly above the LO of the calcareous nannofossil Discoaster praebifax (18.95 m). Finally, the LOs of Hystrichosphaeropsis cf. costae and Wetzeliella coronata and the HO of Apectodinium homomorphum are at 23.5 m, near the top of Unit B2 and coincide with the LO of the calcareous nannofossil Helicosphaera lophota.
4.1.3. Tolagaysor Formation
The base of Unit C1 (sample at 23.8 m) is marked by the beginning of an extreme dominance (acme) of Eatonicysta ursulae (up to 96% of total dinoflagellate cysts; see Figure 5). The LOs of Wetzeliella articulata subsp. brevicornuta and Deflandrea truncata occur at 26.9 m, in topmost part of Unit C1, coinciding with the LO of the calcareous nannofossil Lanternithus minutus and shortly above the LOs of the calcareous nannofossils Chiphragmalithus acanthodes and Dictyococcites chriskingii.
The LOs of Wetzeliella eocaenica, Areoligera tauloma, and Rhombodinium? pentagonum are recorded in the highest sample in Unit C1 (55.5 m), ∼4 m above the LOs of the calcareous nannofossil Discoaster sublodoensis and Nannotetrina cristata, which define the base of NP14 (King et al. 2013). The LO of W. eocaenica defines the base of the W. eocaenica Zone of Heilmann-Clausen (1988) and of Iakovleva (2017b, as Dr. eocaenicum). The zone is recorded in the North Sea Basin (Heilmann-Clausen and Costa 1989; Schnetler and Heilmann-Clausen 2011), Western Siberia (Iakovleva and Heilmann-Clausen 2010; Iakovleva and Aleksandrova 2013), and eastern Peri-Tethys (Iakovleva 2017b).
4.1.4. Unit D (?Sangryk Formation)
Only two samples of the non-calcareous Unit D were taken, one near the base and one near the top. The sample near the top was palynologically barren. The rich dinoflagellate cyst assemblage in the sample near the base includes the stratigraphically important Enneadocysta arcuata, Rhombodinium draco, Wetzeliella ovalis, and Wilsonidium echinosuturatum. The combined occurrence of these taxa indicates the Rhombodinium draco Zone of latest Lutetian–Bartonian age, demonstrating a major hiatus between Units C and D.
4.2. Paleoenvironmental interpretations
As suggested by King et al. (2013), the regional context and paleogeographic reconstructions indicate a predominantly mid- to outer neritic environment for the Aktulagay section, with the nearest land area located probably ∼200 km to the east and northeast. Dinoflagellate cysts dominate the palynomorph assemblages, while acritarchs and prasinophytes are rare, and bisaccate pollen are scarce. This composition of the microplankton is typical for a mid- to outer neritic environment and thus supports the paleogeographic reconstruction. The low frequency of bisaccate pollen does not conflict with this interpretation. Similar low percentages of bisaccate pollen are recorded elsewhere, for example in the bathyal-outer neritic Søvind Marl Formation of Denmark (Heilmann-Clausen and Van Simaeys 2005) and in the mid-neritic Eocene Suvlu-Kaya section, Crimea (King et al. 2018).
Based primarily on sedimentology, and additionally on micropaleontologic criteria, King et al. (2013) proposed a sequence stratigraphic model for the Eocene part of the Aktulagay section, emphasizing that only major environmental shifts may be recognized in this relatively distal environment. Here, we interpret the fluctuations of dinoflagellate cyst eco-groups in relation to depositional changes in more detail (Figure 7).
4.2.1. Alashen Formation
220.127.116.11. Unit A1.
The dinoflagellate cyst assemblage in this thin unit is dominated by the open-marine Spiniferites-group. According to King et al. (2013), a high proportion of benthic foraminifers relative to planktonic foraminifers may indicate a deeper, but somewhat more restricted environment, than the overlying units.
18.104.22.168. Unit A2.
A stratigraphic hiatus between Units A1 and A2 and a concentration of phosphate granules at the base of Unit A2 indicate a sequence boundary (SB). The high proportion of the Areoligera-group close to the base of Unit A2 is interpreted to reflect high-energy settings at the beginning of a transgressive phase (e.g. Brinkhuis 1994; Sluijs and Brinkhuis 2009). The progressive decrease of the Areoligera group and an increase of the Spiniferites-group suggest a gradually more offshore setting. King et al. (2013) tentatively placed the maximum flooding surface (MFS) at ∼7 m, based on the highest proportion of the Spiniferites-group near this level, and supported by a progressive decrease in planktonic foraminifers above ∼7 m. Based on combined lithological and microfossil information, King et al. (2013) identified the lower part of Unit A2 (0–7 m) as part of a transgressive systems tract (TST).
The upper part of Unit A2, and Unit A3, are characterized by a relative decrease, but still dominance, of the Spiniferites-group, and a very small increase in peridinioid wetzelielloids and deflandreoids and in the gonyaulacoid Homotryblium group. These changes indicate a slightly more nutrient-rich (i.e. probably somewhat coastally influenced) water mass. This supports King et al. (2013), who interpreted this interval as part of a highstand systems tract (HST). According to Deprez et al. (2015), foraminifer data suggest outer neritic environments (100–200m) during deposition of the Alashen Formation.
4.2.2. Kulsary Formation
22.214.171.124. Unit B1.
King et al. (2013) interpreted an SB at the lithologic break from the marls of Unit A3 to the interbedded clay and marly clay with several sapropelic clay beds of Unit B1. The first sapropelic bed, marking the base of Unit B1, shows an increase and high proportion of the Spiniferites-group, (60% of total dinoflagellate cysts). At higher levels in B1, a drop in the Spiniferites-group and a relative increase in wetzelielloids, deflandreoids, and the Homotryblium (Polysphaeridium zoharyi) and Areoligera-groups take place. These assemblage changes point to an originally outer neritic situation followed by an inshore shift. This is in accordance with King et al. (2013), who interpreted a marked shallowing of the entire Unit B1, based on a strong fall in diversity and abundance of benthic foraminifers and ostracods, and an abrupt decrease in nannofossil productivity.
According to King et al. (2013), the sapropelic beds were formed in dysoxic environments, due to restricted water circulation, nutrient influxes, or increased precipitation. The low proportion of deflandreoids and the Homotryblium-group and the high proportion of Spiniferites in the basal sapropelic bed indicates oligotrophic surface waters. This suggests that the dysoxia here was caused by restricted circulation rather than increased nutrients.
126.96.36.199. Unit B2.
According to King et al. (2013), the base of Unit B2 is a prominent omission surface with the reappearance of carbonate rich sediments, and corresponding to a ∼55 ky hiatus. The hiatus is supported by the simultaneous LOs of Och. romanum and S. chlamydophora (see above). This boundary was interpreted by King et al. (2013) as a combined SB and TST. The dinoflagellate cyst assemblage at ∼18.95 m includes a remarkably high (25%) influx of mono-specific Impagidinium-group (Impagidinium wardii sp. nov.). The genus Impagidinium is usually a reliable oceanic marker (e.g. Sluijs et al. 2005 and references therein); however, Impagidinium wardii sp. nov. seems to be an exception. First, the remaining assemblage shows only an intermediate abundance of Spiniferites (ca. 32%), ca. 12% deflandreoids, and ca. 5% of the Homotryblium-group, together suggesting a nutrient-enriched mid-neritic setting. Second, in a true oceanic water mass, the other species of Impagidinium (e.g. I. elegans recorded below and above) would be expected to occur too. The level with the I. wardii acme is just above the NP12/NP13 zonal boundary (King et al. 2013). This dates the sample to the warmest part of the Early Eocene Climatic Optimum (EECO; Vandenberghe et al. 2012; Deprez et al. 2015). Thus, the cysts of Impagidinium wardii sp. nov. may have been produced in a warm, nutrient-enriched, mid-neritic environment.
The lower-middle part of Unit B2 is interpreted by King et al. (2013) as part of a TST with the MFS tentatively placed at ∼21 m, based on a peak in the proportions of planktonic foraminifers. The dinoflagellate cyst signal, which occurs slightly below at ∼20.25 m, is mixed with a moderate increase in the Spiniferites-group, but also with a significant influx (20%) in the Homotryblium-group. The upper part of Unit B2, above ∼21 m, is interpreted as part of an HST (King et al. 2013). At ∼21.5 m, peridinioid dinoflagellate cysts and the Homotryblium-group practically disappear, while Thalassiphora pelagica (22%) and the Cordosphaeridium group (27%) show high influxes and the Spiniferites-group decreases. This is in accordance with foraminiferal and nannofossil data (high proportions of coastal taxa) suggesting a progressive shallowing. The strong influx of T. pelagica and the concomitant drop in the Homotryblium-group perhaps indicate a shift from higher to lower salinity (Pross and Schmiedl 2002).
4.2.3. Tolagaysor Formation
188.8.131.52. Unit C1.
The base of Unit C1 corresponds to a sharp decrease in carbonates and the first significant influx of silt-grade quartz. Neither an omission surface nor a biostratigraphic hiatus has been detected at the base of this unit. King et al. (2013) interpreted the base of the unit as an SB. They noted the presence of many diffuse, very thin laminae and layers of very fine silty sand, interpreting them as storm-generated units, which suggests an abrupt decrease in water depths compared with underlying intervals. Based on these lithological characteristics and the occurrence of relatively shallow-water ostracods and foraminifers, King et al. (2013) tentatively interpret Unit C1 as part of a lowstand systems tract (LST). The base of Unit C1 is marked by an extreme acme of Eatonicysta ursulae (up to 96% of total dinoflagellate cysts). The paleoecology of this taxon is not well understood, but there is some evidence from its paleogeographic distribution that E. ursulae may have been a cool-water species (Molina et al. 2011; King et al. 2013, 2018). The acme of E. ursulae and the probable decrease in water depth may both have been caused by a cooling climate. Unit C1 is dated to an interval within NP13, below the LO of Wetzeliella eocaenica. This points to Chron 22r (Vandenberghe et al. 2012) and consequently to the first cooling phase after the culmination of the EECO (Vandenberghe et al. 2012, fig. 28.11). The coincidence with a cooling event does not, however, exclude that another mechanism, such as regional uplift, may have caused the decreased water depth.
184.108.40.206. Unit C2.
The base of Unit C2 is marked by a decrease in grain size; King et al. (2013) tentatively interpreted the base as a TST. A strong, gradual decrease in E. ursulae takes place toward the top of the unit, accompanied with a significant influx of Cleistosphaeridium diversispinosum (up to 30% in the upper part) and re-occurrence of wetzelielloids and deflandreoids. The Spiniferites-group varies in the unit between 5% and 40%. A decrease in the proportion of planktonic foraminifers above ∼40.0m suggests shallowing, which culminated with the first significant sand-grade quartz at 56.0m (topmost sample of Unit C2) (King et al. 2013). This part of Unit C2 may be interpreted as part of an HST. A similar increase of C. diversispinosum (as S. placacanthum) is observed in coeval North Sea Basin strata (Heilmann-Clausen 1988).
4.2.4.? Sangryk Formation (Unit D)
The base of Unit D is sharply defined; it is characterized by abundant phosphatized sandstone pebbles and indicates a sequence boundary. Dinoflagellate cyst data suggest an important stratigraphic hiatus (probably spanning most of the Lutetian). A sample near the base of Unit D (∼56.5 m) includes common Spiniferites-group (20%) and Enneadocysta arcuata (15%). Other eco-groups (i.e. wetzelielloids, deflandreoids, Homotryblium-group, Cleistosphaeridium, Cribroperidinium) attain between 5 and 10%. This uppermost part of the Aktulagay section is tentatively interpreted as part of a TST because of the rich dinoflagellate cyst assemblage and relatively high proportion of Spiniferites.
5. Systematic paleontology
For the systematic classification, we follow Fensome et al. (1993) for all new species, and Williams, Fensome, et al. (2017) for Cribroperidinium cavagnettiae sp. nov., Samlandia chriskingii sp. nov., and Impagidinium wardii sp. nov. For the wetzelielloidean species Dracodinium robertknoxii sp. nov., we follow the previous classification of Fensome et al. (2008) (see Methods above).
Division DINOFLAGELLATA (Bütschi 1885) Fensome et al. 1993
Class DINOPHYCEAE Pascher 1914
Subclass PERIDINIPHYCIDAE Fensome et al. 1993
Order PERIDINIALES Haeckel 1894
Family PERIDINIACEAE Ehrenberg 1831
Subfamily WETZELIELLOIDEAE (Vozzhennikova 1961) Bujak & Davis 1983
Genus Dracodinium (Gocht 1955) Bujak et al. 1980
Type. Dracodinium condylos (Williams & Downie 1966)
Dracodinium robertknoxii sp. nov.
Plate 6, figures 1–4
Holotype. Slide 2826-F2, England Finder coordinates C25; MGUH 32065.
Paratype. Slide 2809-E1, England Finder coordinates S44/1; MGUH 32066.
Type stratum and locality. Sample 9.45 m, Alashen Formation, Aktulagay outcrop section, Kazakhstan.
Diagnosis. A circumcavate to cornucavate peridinioid cyst with a pericyst which is broadly square-shaped, reduced apical pericoel and poorly developed lateral and antapical horns.
Description. A circumcavate to cornucavate wetzelielloidean cyst with a rounded square-shaped to subcircular ambitus of the pericyst. The apical horn is rounded and weakly developed or entirely absent. Occasionally the pericoel in the apical zone may be completely invisible. The lateral horns are short and greatly reduced. The lengths of the epipericyst and hypopericyst are almost equal. In the antapical region the pericyst usually forms a slightly toothed swelling with a weak initiation of only the left antapical horn. The endocyst is circular in ambitus. The endophragm is relatively smooth and the periphragm is slightly chagrinate. The paratabulation is indeterminate. The pericingulum may be expressed only at the extremities of the lateral horns. The archeopyle is intercalary and corresponds to paraplate 2a.
Dimensions. Holotype: pericyst length 102 µm; pericyst width 110 µm; endocyst length 78 µm; endocyst width 80 µm. Paratype: pericyst length 89 µm; pericyst width 93 µm; endocyst length 69 µm; endocyst width 78 µm. Dimensions of measured specimens: pericyst length 81–114 µm (mean 95 µm); pericyst width 84–114 µm (mean 97 µm); endocyst length 65–87 µm (mean 76 µm); endocyst width 66–84 µm (mean 75 µm); 16 specimens measured.
Comparison. Dracodinium robertknoxii sp. nov. differs from the type species (Dr. condylos) by the absence of the intratabular ornamentation and by the extremely short and blunt lateral horns. It also differs from two closely related species, Dracodinium laszczynskii Gedl 1995 and Dracodinium waipawaense (Wilson 1967) Costa and Downie 1979, by a less developed or almost absent apical pericoel and horn, and less developed (or even absent) lateral horns, by a more cornucavate than circumcavate pericyst, and by the slightly chagrinate periphragm.
Discussion. The new wetzelielloidean systematics proposed by Williams et al. (2015) is based primarily on the archeopyle type and only secondarily on the wall morphology and ornamentation. One of us (Iakovleva 2017a) has already applied this new systematics in the description of 14 new earliest Eocene species. In the morphologic study of Dracodinium robertknoxii sp. nov. we also tried to clearly recognize the archeopyle type of this new taxon according to the systematic concepts of Williams et al. (2015). However, we ran into a problem with correctly identifying the archeopyle of Dracodinium robertknoxii sp. nov. The holotype of this species (Plate 6, figure 4) seems to have an equiepeliform archeopyle; and its paratype (Plate 6, figure 3) has either an equiepeliform or hyperepeliform archeopyle. The specimen illustrated in Plate 6, figure 1, however, seems to demonstrate the latiepeliform archeopyle, while the archeopyle of the specimen in Plate 6, figure 2 is not clearly recognizable. Additionally, the specimen of Dracodinium robertknoxii sp. nov. from Western Siberia, published as Dracodinium sp. 1 (pl. 5, fig. 1 in Iakovleva and Heilmann-Clausen 2010) also seems to have a latiepeliform archeopyle. The first specimen, also published as Dracodinium sp. 1 from the NW Germany (pl. 1, fig. 1 in Heilmann-Clausen and Costa 1989), seems to have an equiepeliform archeopyle, while the second one (pl. 1, fig. 2 in Heilmann-Clausen and Costa 1989) more likely has a latiepeliform archeopyle. This variability in archeopyle geometry requires without doubt additional investigations both for Dracodinium robertknoxii sp. nov. and for the wetzelielloidean taxa in general, and will be considered in future publications.
Stratigraphic range. Mid-upper Ypresian.
Derivation of name. In honor of the late British geologist Robert W.O'B. Knox.
Order GONYAULACALES Fensome et al. 1993
Suborder GONYAULACINEAE Fensome et al. 1993
Family GONYAULACACEAE Lindemann 1928
Subfamily CRIBROPERIDINIOIDEAE Fensome et al. 1993
Genus Cribroperidinium (Neale & Sarjeant 1962) Helenes 1984
Type. Cribroperidinium sepimentum Neale & Sarjeant 1962
Cribroperidinium cavagnettiae sp. nov.
Plate 4, figures 4, 5, 9–17
Holotype. Slide 2810-E2, England Finder coordinates R25/3; MGUH 32067.
Paratype. Slide 2827-E1, England Finder coordinates U29/1; MGUH 32068.
Type stratum and locality. Sample 13.46m, Kulsary Formation, Aktulagay outcrop section, Kazakhstan.
Diagnosis. A medium-sized subspherical to ellipsoidal proximate cyst with reticulate intratabular areas and a perforated blunt apex.
Description. A subspherical to ellipsoidal gonyaulacoid cyst. The autophragm is relatively thick with finely reticulate intratabular areas and perforate parasutural septa. Sometimes these parasutural septa are only slightly visible. The most prominent reticulate network is observed over the paracingular area. The relatively short apical horn terminates in a prominent perforate blunt apex. The reflected tabulation is expressed by the formula: 4′, 6″, 6c, 6′″, 1p, 1″″. The archeopyle is relatively large and corresponds to plate 3″ (Type P). The operculum is free.
Dimensions. Holotype: total length 75 µm; total width 75 µm; length of apical horn 14 µm; maximum height of septa 5 µm. Paratype: total length 75 µm; total dorsoventral dimension 60 µm; length of apical horn 15 µm, maximum height of septa 3 µm. Dimensions of measured specimens: total length 66–87 µm (mean 74 µm); total width 56–78 µm (mean 65 µm); total dorsoventral dimension 60–69 µm (mean 64 µm); length of apical horn 9–15 µm (mean 11 µm); maximum height of septa 1–5 µm (mean 2.5 µm); 13 specimens measured.
Comparison. Cribroperidinium cavagnettiae sp. nov. differs from other species of Cribroperidinium by the distinctive reticulation of intratabular areas and by the prominent perforate blunt apex.
Stratigraphic range. Ypresian.
Derivation of name. In honor of the French palynologist Carla Cavagnetto.
Genus Samlandia Eisenack 1954
Type. Samlandia chlamydophora Eisenack 1954
Samlandia chriskingii sp. nov.
Plate 11, figures 8–12
Synonymy. Samlandia sp. 1 in Iakovleva and Heilmann-Clausen, 2010, pl. 10, figs 4, 6.
Holotype. Slide 2828-F2, England Finder coordinates N29/3-4; MGUH 32069.
Paratype. Slide 2828-F2, England Finder coordinates N28/4; MGUH 32070.
Type stratum and locality. Sample 17.0m, Kulsary Formation, Aktulagay outcrop section, Kazakhstan.
Diagnosis. Small subspherical gonyaulacoid cyst with precingular archeopyle and without apical and antapical protrusions.
Description. A small subspherical cyst without apical and antapical protrusions. The cyst consists of an autophragm surrounded by an ectophragm. The autophragm is thick and its outer surface bears densely spaced protuberances connecting with the fragile membrane-like ectophragm, which completely surrounds the autocyst. The paratabulation is indicated by the archeopyle only. The archeopyle is precingular, relatively large, type P (3″). The operculum is free. Paracingulum and parasulcus are not recognized.
Dimensions. Holotype: ectocyst length 57µm; ectocyst width 54µm; autophragm thickness 3µm; total wall thickness 9µm. Paratype: ectocyst length 48µm; autophragm thickness 3µm; total wall thickness 6µm. Dimensions of measured specimens: ectocyst length 41–57µm (mean 49µm); ectocyst width 36–54µm (mean 48µm); ectocyst dorsoventral dimension 35–53µm (mean 43µm); autophragm thickness 1.5–3µm; total wall thickness 5–11µm; 12 specimens measured.
Comparison. Samlandia chriskingii sp. nov. differs from other species of this genus by its smaller size and the absence of apical and antapical protrusions, as well as by the more densely spaced protuberances of the autophragm.
Stratigraphic range. Ypresian.
Derivation of name. In honor of the late British geologist and micropaleontologist Chris King who, together with David J. Ward, studied the Aktulagay section and provided the samples for the present study.
Subfamily GONYAULACOIDEAE Fensome et al. 1993
Genus Impagidinium (Neale & Sarjeant 1962) Helenes 1984
Type. Impagidinium dispertitum Cookson & Eisenack 1965
Impagidinium wardii sp. nov.
Plate 9, figures 5–6
Synonymy. Impagidinium sp. A in Iakovleva and Heilmann-Clausen, 2010, pl. 7, figs 14, 17.
Holotype. Slide 2802-G2. England Finder coordinates N36/4; MGUH 32071.
Paratype. Slide 2802-G2. England Finder coordinates N25; MGUH 32072.
Type stratum and locality. Sample 18.95m, Kulsary Formation, Aktulagay outcrop section, Kazakhstan.
Diagnosis. A small gonyaulacoid proximochorate, finely granulated cyst with low and smooth septa and a precingular archeopyle.
Description. A small, relatively thick-walled species of Impagidinium. The central body is egg-shaped to subspherical. The epicyst is slightly longer than the hypocyst. The paracinculum is rather broad (ca. 5 µm). Septa are low and smooth (hyaline). The distal margin of the septa varies (within an individual cyst) from ragged in most parts, to locally entire, and locally finely denticulate. A prominent feature of the cyst is a fine and dense granulation of the central body caused by closely spaced minute but rather high (up to ca. 0.5 µm high) granules. A gonyaulacacean paratabulation is indicated by parasutural septa, with the possible formula 3′, 5–6″, 5c, 5″′, 1p, 1″″. The archeopyle is precingular and formed by the removal of paraplate 3″.
Dimensions. Holotype: total cyst length 32 µm; total cyst width 30 µm; central body length 29 µm; central body width 26 µm; height of septa 2 µm (average); autophragm thickness 1 µm. Paratype: total cyst length 38 µm; total cyst width 36 µm; central body length 34 µm; central body width 31 µm; height of septa 2.5–3.5 µm; autophragm thickness 0.8 µm. Dimensions of measured specimens: total cyst length 32–40 µm (mean 37 µm); total cyst width 30–40 µm (mean 35 µm); central body length 29–35 µm (mean 32 µm); central body width 26–35 µm (mean 31 µm); height of septa 1.8–3.6 µm (mean 2.5 µm); autophragm thickness 0.7–1 µm (mean 0.9 µm); 9 specimens measured.
Comparison. Impagidinium wardii sp. nov. differs from other species of Impagidinium mainly by its fine and dense granulation, as well as by the small size and subspherical habitus of the cyst.
Stratigraphic range. Ypresian.
Derivation of name. In honor of the British paleontologist and field geologist David J. Ward, who, together with Chris King, studied the Aktulagay section and provided the samples for the present study.
A relative abundance analysis of dinoflagellate cysts from the isolated lower Eocene Aktulagay key site in the eastern Peri-Tethys, Kazakhstan, is presented. The analysis shows that this outer neritic-upper bathyal section includes a high diversity of excellently preserved dinoflagellate cysts. Eight dinoflagellate cyst zones of the Peri-Tethys zonation have been recognized, showing that the section, except for the missing Wetzeliella (= Dracodinium) astra and Wetzeliella (= Stenodinium) meckelfeldensis zones, is without major hiatuses. The zones present at Aktulagay include the D. oebisfeldensis Zone, Dr. simile Zone, Dr. varielongitudum Zone, Ch. coleothrypta Zone, Och. romanum/S. chlamydophora Zone, Ar. diktyoplokum Zone, and Wetzeliella (= Dracodinium) eocaenica Zone. The lowermost occurrences of a number of Ypresian key species have been directly calibrated to calcareous nannofossil events. The section is unconformably overlain by strata with a rich latest Lutetian–Bartonian dinoflagellate cyst assemblage.
The distribution of dinoflagellate cyst eco-groups is presented and discussed in relation to the established sequence stratigraphy of the section. The paleoecological signals of the assemblages are generally in agreement with, or supplementary to, calcareous micro- and nannofossil indications and the sequence stratigraphic model. The paleoecology of Impagidinium wardii sp. nov. appears to be atypical for the genus as I. wardii flourished in a warm mid-neritic environment. The decrease in water depth and acme of Eatonicysta ursulae in NP13 is suggested to result from the cooling event in Chron 22r at the end of the EECO.
In the taxonomic section we formally describe four new species: Cribroperidinium cavagnettiae sp. nov., Dracodinium robertknoxii sp. nov., Impagidinium wardii sp. nov., and Samlandia chriskingii sp. nov. Comments are given on a number of taxa in open nomenclature, and photographs are included of the most important taxa recorded at Aktulagay. The Aktulagay Formation of King et al. (2013) is renamed the Kulsary Formation.
This paper is dedicated to the late Chris King who invited us to study the excellent material from the Aktulagay section and with whom we had many interesting and constructive discussions. Many thanks also to Etienne Steurbaut for his calcareous nannofossil data. The late Kirsten Rosendal skillfully made the palynological preparations. Søren Bo Andersen is kindly thanked for his technical work with the photographic plates. Joanna Davies kindly corrected the English text. Jörg Pross and Peter Bijl are thanked for their careful and constructive reviews, which improved the paper. We also thank James Riding for his very helpful editorial suggestions.
The research of AI was supported by the Danish Natural Science Research Council (Grant no. 21-04-0298) and the Russian State program no. 0135-2019-0044 (Geological Institute, Russian Academy of Sciences).