Cretaceous (Albian–Coniacian) dinoflagellate biostratigraphy of the Vomb Trough, southern Sweden

ABSTRACT

We document diverse and well-preserved dinoflagellate cyst assemblages from Cretaceous successions in the Kullemölla 1 drill core (640.0 m–590.0 m), Vomb Trough, southern Sweden. Palynology reveals a nearshore marine environment. Dinoflagellate index taxa indicate an Albian to Coniacian age, thus spanning the Early–Late Cretaceous boundary. The lower part of the core is Albian, based on the presence of the index dinocyst taxa Pareodinia and Callaiosphaeridium asymmetricum. The First Appearance Datum (FAD) of Oligosphaeridium prolixispinosum, together with the presence of Achomosphaera ramulifera, Heterosphaeridium difficile and Oligosphaeridium pulcherrimum, reveals a Cenomanian age for the interval 635– m–617 m. The Turonian interval is characterized by an increase in the dinocysts Chatangiella spectabilis and Florentinia spp., in combination with the FAD of Senoniasphaera rotundata, whereas the youngest samples are dated to the Coniacian as defined by the appearance of Glaphyrocysta sp.

We show that Cenomanian and Turonian strata are indeed represented by a relatively condensed section between 635 m and 612 m in the Kullemölla 1 core showing that the apparent hiatus recorded by calcareous microfossils elsewhere is likely a result of post-depositional dissolution of calcareous tests and limestone, a process that did not affect the organic-walled plankton. This is further supported by the presence of hardgrounds and dissolution features.

This updated, detailed biostratigraphical assessment based on dinoflagellates provides a framework for correlation with zonations based on other marine fossil groups, useful, for e.g., correlating aquifers in subsurface successions and, further, provides opportunities for linking marine and continental biotas.

KEYWORDS:

• Dinoflagellate cysts
• Albian
• Cenomanian
• Turonian
• Coniacian
• biostratigraphy
• southern Sweden

Introduction

The Cretaceous 143.3 Ma (Wimbledon et al. 2020a, 2020b) to 66 Ma (Renne et al. 2018) is characterized by major tectonic events, including the breakup of Pangea, which led to the formation of new continents and ocean basins (Wimbledon et al. 2020a, 2020b). These major changes in the Earth’s plate tectonics affected climate and biodiversity globally and the period ended with a mass extinction that resulted from an asteroid impact in Yucatan, Mexico (Schulte et al. 2010). The Cretaceous is formally divided into the Early Cretaceous (Berriasian–Albian) and the Late Cretaceous (Cenomanian–Maastrichtian), whereas the Lower Cretaceous successions in Europe are mainly characterized by alternating non-marine and marine mudstones, limestones and sandstones (Norling 1981, 1982; Vajda & Wigforss-Lange 2006; Wimbledon 2020a, 2020b), the Upper Cretaceous is characterized by vast deposits of chalk (Brotzen 1936; Hallam 1992; Hart et al. 2012; Einarsson et al. 2016; Fio Firi et al. 2017; Khrushchev et al. 2022). The Albian–Turonian interval coincided with maximum surface-water temperatures, and the mid-Cretaceous transgression led to the expansion of relatively shallow and warm epicontinental seas (Hallam 1992; Larsson et al. 2000; Gale et al. 2008) that resulted in extensive carbonate deposition during the later part of the Cretaceous (Hallam 1992). These chalks were formed primarily by the remains of calcareous phytoplankton, such as coccolithophorids, zooplankton including foraminifera (Brotzen 1945; Norling 1981; Larsson et al. 2000; Slater et al. 2022), and calcareous dinoflagellates (belonging to the family Thoracosphaeraceae; Elbrachter et al. 2008; Montresor et al. 1994; Dubicka & Peryt 2012). The tectonic setting that resulted in the formation of sizable shallow shelves, favourable for life in the photic zones along the shorelines, and the generally warm global climate contributed to the proliferation of these groups. In addition, volcanism, a result of the enhanced tectonic activity, contributed significant quantities of greenhouse gases, such as carbon dioxide, into the atmosphere (Vajda & Bercovici 2014). Importantly, organic-walled dinoflagellate cysts were abundant in the Cretaceous marine environments, comprising the resting stages of both autotrophs and heterotrophs, and fossil assemblages of these organisms today provide an excellent tool for detailed stratigraphical assessments of these marine Cretaceous successions. The Late Cretaceous continental fringes also hosted the earliest mangroves and seagrasses (McLoughlin et al. 2018; van der Ham et al. 2017), which resulted in dramatic changes to marginal marine communities, nutrient cycling, and sediment stability. Due to the high productivity in the Cretaceous oceans, the rich nutrient supply from adjacent landmasses, and the extensive populations of planktonic organisms, faunal groups at higher trophic levels also proliferated and diversified during the Cretaceous (Einarsson et al. 2016, Einarsson 2018). A rich and diverse mollusc fauna existed, and importantly, both belemnites and ammonites underwent rapid evolution and inhabited extensive tracts of the oceans (Christensen 1985; Christensen & Schultz 1997). When combined with the diverse plankton, their ranges provide a robust biostratigraphical framework for detailed global correlation (Christensen 1985; Christensen & Schultz 1997). Here, we aim to provide a detailed dinoflagellate cyst biostratigraphy for the mid-Cretaceous successions of the Vomb Trough and to compare this zonation with existing biostratigraphical schemes established for northern Europe.

Geological setting

Mesozoic basin formation on the Baltic Shield was controlled by continental rifting in association with tectonic activity along the Sorgenfrei-Tornquist Zone, a tectonic feature forming a continuous fault zone from Ukraine in SE Europe to the central North Sea (Sorgenfrei & Buch 1964; Bergström 1984; Larsson et al. 2000; Vajda 2001; Vajda & Wigforss-Lange 2009). The Sorgenfrei-Tornquist Zone forms part of the Fennoscandian Border Zone and represents the border between the East European Craton and the Variscan terranes to the south. Southern Sweden hosts thick Mesozoic successions typified by mainly non-marine Upper Triassic and Jurassic successions, but with marine conditions becoming predominant from the early Late Cretaceous with deposition of glauconitic sandstones and chalks. Cretaceous successions occur throughout the southern province of Skåne (Fig. 1) and are represented within several basins; the Båstad Basin, the Höllviken Halfgraben, the Kristianstad Basin, and the Vomb Trough (Larsson et al. 2000)

Figure 1. A map outlining the present distribution of Mesozoic strata in southern Sweden. The Kullemölla 1 core is marked with a red dot. Modified after Kruger et al. (2021).

The Vomb Trough initiated during the Triassic as a rift basin (Norling 1982; Chatziemmanouil 1982; Vajda 1998; Lindström & Erlström 2011). Maximum sedimentary accumulations occurred in the southern part of the trough, near Köpingsberg, where the succession reaches about 1500 m in thickness (Norling 1982; Vajda, 1988, 1999; Lindström & Erlström 2007, 2011). The Lower Cretaceous succession is ∼200 m thick and generally comprises sandstones (some glauconitic) and shales; the Upper Cretaceous mostly comprises weakly consolidated carbonate rocks, with some representation of claystones and sandstones. Tectonic activity late in the period led to higher deposition rates, and the Upper Cretaceous successions reached a thickness of ∼1100 m (Bergström et al. 1982).

The Kullemölla 1 core was drilled in the southeastern part of the Vomb Trough (Fig. 1) and, in 1919, as part of a coal prospecting initiative by the Swedish government targeting the Jurassic coal-bearing strata underlying the Cretaceous sedimentary successions in the area. Lower Cretaceous successions rest unconformably on Lower Jurassic “Liassic” strata, and the boundary between the Jurassic and Cretaceous has been placed previously at 644 m in the Kullemölla 1 core, where it is characterized by a major hiatus (Gaveling 1919; Lundegren 1935; Guy Ohlson 1982). The present study is the first detailed dinocyst biostratigraphic analysis of Early and mid-Cretaceous deposits from Sweden.

Material and Methods

Twenty-four samples were collected for palynological analyses from a 50-metre interval (640 m–590 m) within the Kullemölla 1 drill core. Around 20 g of sedimentary rock was processed according to standard palynological procedures at Global Geolab Limited in Medicine Hat, Canada. The organic residue was sieved using a 5-μm mesh and mounted in epoxy resin on two microscope slides per sample. All dinoflagellate cysts in the two slides per sample were identified taxonomically. Additionally, one kerogen slide was prepared from each sample, i.e., the residue was not exposed to oxidation or sieving processes. Only dinoflagellate cysts have been treated taxonomically in this study, as these are the most valuable for stratigraphic purposes, with several Cretaceous index taxa present. We follow the dinocyst taxonomy of Lentin and Williams (1993) with some more recent amendments (Fensome & Williams 2004). Palynomorphs were studied in permanent preparations using an Olympus BX51 biological modular light microscope. The illustrated palynomorphs are identified by NRM numbers and marked with England Finder coordinates. The slides and residues are stored in the Palaeobiology collections of the Department of Palaeobiology at the Swedish Museum of Natural History, Stockholm, Sweden.

Dinoflagellate cyst biostratigraphy

All the studied samples from the Kullemölla 1 drill core yielded well-preserved and abundant dinoflagellate cyst assemblages, and over 100 taxa were identified. The range of selected taxa is presented in Fig. 2, and micrographs of selected dinoflagellate cysts and algae in Figs. 3–6.

Figure 2. Stratigraphic ranges of selected important dinocysts. Legend 1–3%

4–6%

7–10%

.

Figure 3. Micrographs of selected dinoflagellate cysts identified in the samples from the Kullemölla 1 drill core. A. Muderongia sp., 614_S202689_S45. B. Muderongia simplex, 637.5_W38. C. Vesperopsis nebulosa, 610.95_M41-3. D. Muderongia sp., 610.95_M41-3. E. Cribroperidinium muderongense, 598_S202703_U36-1. F. Cribroperidinium tenuiceras, 637.5_T29-4. G. Cribroperidinium confossum, 640_O28-4. H. Palaeoperidinium cretaceum, 616_S202711_J27-4. I. Apteodinium sp., 616_S202711_K29. J. Apteodinium granulatum_610.95_L9-4. K. Pareodinia sp., 640_Q15-4. L. Gonyaulacysta sp., 614_S202689_M23-1. M. Leptodinium sp., 637.5_W21-4. N. Chatangiella granulifera, 614_S202689_F17-3. O. Chatangiella sp., 637.5_J32. P. Chatangiella spectabilis, 614_S202689_G25-2. Q. Spinidinium sp., 610.95_D20. R. Endoceratium sp., 592_S202484_U17-4. S, Alterbidinium sp., 590_S202719_H12. T, Trichodinium castanea, 590_S202719_E29. U. Batiacasphaera sp., 614_S202689_D21-1. V. Stephodinium coronatum, 614_S202689_M19-3. W. Dinogymnium sp., 610.95_J14-2. Scale bars = 10 μm.

Figure 4. Micrographs of selected dinoflagellate cysts identified in the samples from the Kullemölla 1 drill core. A. Odontochitina operculata, 610.95_H39. B. Odontochitina operculata, 640_E15-3. C. Cyclonephelium filoreticulatum, 598_S202703_G17. D. Leberidocysta sp., 614_S202689_O23-4. E. Canningia sp., 616_S202711_M27-3. F. Senoniasphaera rotundata, 614_S202689_L21. G. Pteridodinium cingulatum, 616_S202711_D28. H. Callaiosphaeridium asymmetricum, 637.5_P20. I. Hystrichosphaeridium tubiferum, 614_S202689_P45-2. J. Florentina laciniata, 590_S202719_E48-2. K. Occisucysta sp., 592_ S202717_P16-2. L. Conosphaeridium striatoconum, 610.95_D37-2. M. Florentinia deanei, 610.95_F36-2. N. Florentinia sp., 590_S202719_K29-4. O. Florentinia ferox, 616_S202711_M27-3. P. Callaiosphaeridium asymmetricum, 592_S202717_H46. Q. Florentinia laciniata, 598_S202703_O25-1. Scale bars = 10 μm.

Figure 5. Micrographs of selected dinoflagellate cysts identified in the samples from the Kullemölla 1 drill core. A. Hystrichodinium solare, 640_X15-2. B. Pervosphaeridium pseudhystrichodinium, 610.95_J14-3. C. Hystrichodinium sp., 640_N14-1. D. Exochosphaeridium phragmites, 590_S202719_R13-2. E, Oligosphaeridium pulcherrimum, 640_G19-4. F. Oligosphaeridium complex, 640_T13-2. G. Hystrichoshaeridium bowerbankii, 610.95_T32. H. Spiniferites ramosus, 592_S202717_B28. I. Systematophora sp., 590_S202719_K24-4. J, Cyclonephelium sp., 640_F35-1. K. Circulodinium distinctum, 590_S202719_P28-2. L. Circulodinium distinctum, 598_S202703_V32. M. Glaphyrocysta sp., 610.95_U47-2. N. Achomosphaera ramulifera, 610.95_H13. O. Achomosphaera sagena, 610.95_V36-2. P. Exochosphaeridium sp., 610.95_C27-1. Q. Circulodinium distinctum, 610.95_S25. R. Cleistosphaeridium sp., 640_K45-2. S. Operculodinium sp, 637.5_P20-4. T. Coronifera striolata, 614_S202689_O-22-3. U. Tanyosphaeridium sp., 640_T19-2. Scale bars = 10 μm.

Figure 6. Micrographs of selected acritarchs, prasinophtes, cuticles, and other microfossils from the Kullemölla 1 core. A. Tasmanites sp. 1, 590_S202719_U21-2; B. Tasmanites sp. 2, 640_K39; C. Botryococcus braunii, 640_S22-4; D. Leiofusa filifera, 590_S202719_O25-4; E. Micrhystridium fragile, 610.95_R24; F. Schizosporis reticulatus, 640_T31-2; G. Plant cuticle, 637.5_G37.

Biostratigraphy

Based on dinoflagellate key-taxa, we interpret the studied interval to span the Albian to Coniacian. Reworking is rather extensive at the base of the core.

Interval 640 m–637.5 m, age Albian (samples 640 m and 637.5 m)

The interval 640 m–637.5 m comprises dark grey calcareous siltstones. The dinoflagellate assemblages are diverse and an Albian age is inferred for this interval based on the presence of the age-diagnostic dinocyst taxa (Fig. 3–5) Pareodinia spp., species belonging to the Chatangiella group, and Odontochitina singhii, together with the first occurrence of Callaiosphaeridium asymmetricum (Fig. 4. H, P). Other characteristic dinocysts of this interval include Spiniferites ramosus, Hystrichosphaeridium spp., Cleistosphaeridium spp., Downiesphaeridium sp., Odontochitina operculata, Oligosphaeridium complex and single Cleistosphaeridium polypes, Chytroeisphaeridia sp., Ascodinium sp., Endoscrinium sp., Valensiella sp., Tanyosphaeridium sp., and Sentusidinium sp.

Reworked palynomorphs occur in low numbers <1% represented by 1–2 specimens per taxon throughout the succession. These include typical Early Cretaceous (Berriasian–Barremian) dinocysts, such as Cribroperidinium confossum (Fig. 3G), Cribroperidinium tenuiceras (Fig. 3F), Oligosphaeridium albertense, Muderongia simplex (Fig. 3B), and Scriniodinium sp.

Fresh to brackish-water algae present in this interval include Botryococcus braunii (Fig. 6C), Tasmanites (Fig. 6B), Schizosporis reticulatus (Fig. 6F) and the acritarchs – Micrhystridium fragile and Baltisphaeridium.

Results are compared and contrasted with those from coeval sections elsewhere in the western and eastern Europe (Heilmann-Clausen 1987; Tocher & Jarvis 1996; Sánchez-Pellicer et al. 2018; Shevchuk and Pustovoitova 2021).

Interval 635 m–617 m, age Cenomanian (samples 635, 632.6, 629, 626, 624, 622, 620 and 617 m)

The interval 635 m–617 m comprises mainly dark-grey calcareous mudstones. The Cenomanian age of this interval is based on the first occurrences of Achomosphaera ramulifera, Heterosphaeridium difficile, Hystrichosphaeridium bowerbankii, Circulodinium distinctum and Oligosphaeridium prolixispinosum. Taxa dominating in terms of relative abundance include Pervosphaeridium pseudhystrichodinium, Achomosphaera spp., Batiacasphaera spp., Chlamydophorella sp., Chlamydophorella nyei, Oligosphaeridium spp. Pterodinium sp., Spiniferites scabrosus, Spiniferites spp., Hystrichosphaeridium spp., Heterosphaeridium sp., Cleistosphaeridium spp., Downiesphaeridium sp. and Odontochitina spp. Single occurrences of dinocyst taxa in this interval include Systematophora sp., Litosphaeridium sp., Cyclonephelium sp., Hystrichodinium sp., Hystrichodinium solare, Oligosphaeridium pulcherrimum, Trithyrodinium sp., among others (Figs. 3–5).

Fresh to brackish-water algae present in this interval include Botryococcus, Palambages, Tasmanites and acritarchs – Micrhystridium fragile, M. singulare, Veryhachium reductum, and Baltisphaeridium.

Interval 616 m–612 m, age Turonian (samples 616, 614, 612 m)

The interval 616 m–612 m comprises weakly consolidated, light-grey marlstones. The Turonian assemblages are characterized by an increase in diversity and relative abundance of dinocysts. Florentinia ferox is a zonal taxon, appearing at 616 m, marking the lower Turonian. Senoniasphaera rotundata (appearing at 614 m) is also a zonal species with a First Appearance Datum in the middle Turonian (Ogg et al. 2008; Ivanik et al. 2012; Shevchuk 2016, 2018, 2020). Heterosphaeridium difficile and Florentinia buspina are also stratigraphically important middle Turonian species within this interval (Radmacher et al. 2014). Chatangiella spectabilis, Chatangiella granulifera and single occurrences of the dinocysts Alterbidinium sp., Alterbidinium acutulum are also represented in this interval. The assemblages are dominated by the following taxa: Coronifera oceanica, Chlamidophorella nyei, Spiniferites spp., Spiniferites ramosus, Achomosphaera ramulifera, Hystrichosphaeridium spp., Cleistosphaeridium spp., Oligosphaeridium spp., Membranosphaera sp., Circulodinium distinctum, and Odontochitina spp. Typical taxa that are age-diagnostic for the Turonian include Pterodinium cingulatum, Coronifera striolata, Callaiosphaeridium asymmetricum, Hystrichosphaeridium tubiferum, Xenikoon australis, Stephodinium coronatum, Canningia sp. and Leberidocysta sp. Single occurrences of the dinocysts Trichodinium castanea, Palaeoperidinium cretaceum, Microdinium sp., Microdinium ornatum, Senoniasphaera sp., Prolixosphaeridium sp., and Pyxidiella sp. occur in this interval along with the last occurrence of Gonyaulacysta sp. Examples of reworked dinocysts include Apteodinium spp., Apteodinium granulatum, Apteodinium maculatum, and Sirmiodinium sp. (Figs. 3–5).

Fresh-brackish water algae present in this interval include Botryococcus, Palambages, Tasmanites, and the acritarch taxa Micrhystridium fragile.

Interval 610.95 m–590 m, age Coniacian (samples 610.95, 608, 606, 604.5, 602, 600, 598, 596, 594, 592, and 590 m)

The interval 610.95 m–590 m comprises mainly light grey mudstones. This Coniacian interval is characterized by the first appearance of the dinocysts Glaphyrocysta sp. (Fig. 5M) and Dinogymnium sp. (Fig. 3W). Other typical Coniacian index taxa include the dinocysts Achomosphaera sagena, Hystrichodinium pulchrum, Pterodinium spp., Pterodinium pterotum, Pterodinium cingulatum, Cauveridinium membraniphorum, Amphidiadema sp., Conosphaeridium striatoconum, Odontochitina costata, and Odontochitina operculata. Cleistosphaeridium spp. and Downiesphaeridium sp. show little variation in relative abundance throughout the section. An acme of Spiniferites spp. and Spiniferites ramosus is noted in this interval. Single occurrences of stratigraphically important dinocysts include Exochosphaeridium phragmites, Exochosphaeridium sp., Cyclonephelium filoreticulatum, Trichodinium castanea, Spinidinium sp., Occisucysta sp., Cribroperidinium sp., Cribroperidinium edwardsii, Cribroperidinium muderongense, Hystrichodinium sp., Oligosphaeridium pulcherrimum, Operculodinium sp., Ginginodinium spinulosum, Florentinia laciniata, Florentinia deanei, and Tehamadinium coummia (Figs. 3–5). There are also numerous Kallosphaeridium sp. and Pervosphaeridium pseudhystrichodinium.

Fresh to brackish-water algae present in this interval include Shizosporis reticulatus, Tasmanites, Zygnemaceae and acritarchs – Leiofusa filifera (Fig. 6D) and Micrhystridium fragile (Fig. 6E), Veryhachium reductum, Baltisphaeridium.

Remains of plant cuticles and tracheids are found uniformly throughout all intervals (Figs. 6G).

Discussion and conclusions

We have provided a detailed palynostratigraphy for the successions from the Vomb Trough, accessed through the Kullemölla 1 core, southern Sweden. This study significantly expands on pioneer palynological investigations (Guy-Ohlson 1982) concerning both taxonomic richness and refined stratigraphy across the Lower–Upper Cretaceous boundary in Skåne. We show that the 50 m sedimentary succession within the Kullemölla 1 core reflects a relatively condensed interval spanning the Albian to Coniacian, thus representing a time interval of about 25 million years, indicating a sedimentation rate of 2 m/million years or 2 millimetres per 1000 years, which signifies a slow depositional rate. The studied assemblages can further be placed in a broader biostratigraphical context and linked to dinoflagellate, ammonite and foraminifera zonations from elsewhere in the Boreal and Tethys regions (Fig. 7).

Figure 7. Overview of Cretaceous zonation scheme, modified after Ogg et al., 2008.

Based on the dinoflagellate assemblages, the environment was marine during the entire time of deposition in this part of the Vomb Trough. In fact, sea levels were rising throughout this time interval, as previously demonstrated by others (Hart 1980; Solakius and Larsson 1985; Larsson et al. 2000). Important findings herein include the identification of species belonging to the Chatangiella dinocyst complex very low in the studied interval (within the Albian successions). Importantly, Glaphyrocysta was also recorded within the Coniacian part of the analyzed succession. Elsewhere these genera generally characterize the Coniacian–Santonian (Westin 1992; Riding and Crame 2002; Shevchuk and Vajda 2014). Another notable observation is the presence of Dinogymnium dinocysts in the Coniacian interval of the Kullemölla 1 core. The Dinogymnium complex is an important component of Upper Cretaceous dinoflagellate cyst assemblages from marine palaeoenvironments worldwide (Evitt et al. 1967).

Previous studies from Cretaceous basins in southern Sweden have revealed a complex depositional history, with a hiatus across the Cenomanian–Turonian boundary (Solakius and Larsson 1985; Larsson et al. 2000; Hart et al. 2012; Ezampanah et al. 2021). According to Hart et al. (2012) upper Cenomanian to lower Coniacian strata are lacking in the basin altogether, with no evidence of Turonian microfossils such as calcareous plankton. In other parts of the European Platform, such as in Ukraine, there is evidence of blooms of calcareous dinoflagellate cysts with an acme during the Albian to the Turonian (Dias Brito 2000; Ciurej et al. 2023), also coinciding with the highest recorded Cretaceous seawater surface temperatures (Ciurej et al. 2023). Around the Cenomanian–Turonian boundary, a sea level rise reached a maximum for the Cretaceous, estimated to have reached ∼ 250 m above the present-day maximum (Haq 2014). After reaching its Cretaceous maximum, sea level fell by the end of the Turonian, remaining relatively stable for almost 20 Myr during the Coniacian and the Campanian (Haq 2014).

Based on organic-walled dinoflagellate cysts we show that Cenomanian and Turonian strata are indeed represented by a relatively condensed section between 635 and 612 m in the Kullemölla 1 core. The diversity of cysts is characteristic of the Cenomanian / Turonian boundary interval, which is typified by the dominance of tolerant cosmopolitan forms, such as Circulodinium distinctum, Hystrichosphaeridium bowerbankii, Oligosphaeridium complex, Odontochitina costata, and O. operculata (Bruce and Jarvis 1995; Shevchuk 2020). Turonian assemblages are characterized by diverse Florentinia spp. dinocysts and important Senoniasphaera rotundata and Heterosphaeridium difficile. The Coniacian is characterized by the first appearance of the dinocysts genera Glaphyrocysta and Dinogymnium. This indicates that the apparent hiatus recorded from calcareous microfossils and strata was likely the result of post-depositional dissolution of carbonate tests, which did not affect organic-walled remains.

Post-depositional dissolution and consequent mixing of fossil assemblages complicate the stratigraphical assessment. The complex lithologies and slow depositional rates are further evidenced by the many hardgrounds, intensely bioturbated intervals, formation of glauconitic sands, and phosphatization (Vajda et al. 1999; Larsson et al. 2000; Hart et al. 2012). These factors have resulted in reworking or alteration of faunal remains hampering the identification of discrete events (Hart et al. 2012).

An example of reworking within the studied dinoflagellate assemblages is the record of Muderongia simplex and other single occurrences within the transgressive bed at the base of the core at 637.5 m. These species are characteristic of successions not younger than Valanginian (Riding et al. 2000); however, in the base of the Kullemölla 1 core they co-occur with large numbers of Odontochitina operculata typical of Albian assemblages (Ogg et al. 2008).

Dinocyst biozonation provides a high-resolution means of dating marine strata in southern Sweden that can be useful for correlating aquifers in subsurface successions, along with evaluating the rates of subsidence and sea-level fluctuations in the Cretaceous. Spore-pollen zonations and carbon isotope data from the studied marine successions of southern Sweden are at a preliminary stage but future work on these terrestrial microfossil assemblages will provide an opportunity to link the neritic and continental biozonations and the global carbon isotope stratigraphy.

Acknowledgements

Funding is acknowledged from the Swedish Foundation for Strategic Research, grant UKR22-0020 (O. Shevchuk). We are further thankful to the Knut & Alice Wallenberg Foundation KAW 2020.0145 (V. Vajda), and the Swedish Research Council VR grants 2019–4061 (V. Vajda). Olena Shevchuk wishes to thank the Swedish Government, the Swedish Museum of Natural History, and Stockholm University for hosting her under the Temporary Protection Directive, following the Russian invasion of Ukraine. We thank the reviewers for their constructive comments.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by Swedish Foundation for Strategic Research, grants for Ukrainian scientists 2022 [grant number UKR22-0020].

Appendix

List of dinocyst species and genera recorded from the Kullemölla 1 drill core. The taxa are arranged alphabetically by genus and species:

Achomosphaera ramulifera (Deflandre 1937b) Evitt, 1963

Achomosphaera sagena Davey & Williams, 1966a

Achomosphaera sp.

Alterbidinium sp.

Alterbidinium acutulum (Wilson, 1967b) Lentin and Williams, 1985

Amphidiadema sp.

Apteodinium sp.

Apteodinium granulatum Eisenack 1958a

Apteodinium maculatum Eisenack & Cookson, 1960

Ascodinium sp.

Batiacasphaera sp.

Callaiosphaeridium asymmetricum (Deflandre and Courteville, 1939) Davey and Williams, 1966b

Canningia sp.

Cauveridinium membraniphorum (Cookson and Eisenack, 1962b)

Conosphaeridium striatoconum (Deflandre and Cookson 1955) Cookson and Eisenack 1969

Chatangiella sp.

Chatangiella granulifera (Manum, 1963) Lentin and Williams, 1976

Chatangiella spectabilis (Alberti, 1959b) Lentin and Williams, 1976

Chlamydophorella sp.

Chlamydophorella nyei Cookson and Eisenack, 1958

Chytroeisphaeridia sp.

Circulodinium distinctum (Deflandre and Cookson, 1955) Jansonius, 1986

Cleistosphaeridium polypes (Cookson and Eisenack, 1962b) Davey, 1969a

Cleistosphaeridium sp.

Coronifera oceanica Cookson and Eisenack, 1958

Coronifera striolata (Deflandre, 1937b) Stover and Evitt, 1978

Cribroperidinium sp.

Cribroperidinium confossum (Duxbury, 1977) Helenes, 1984

Cribroperidinium edwardsii (Cookson and Eisenack, 1958) Davey, 1969a

Cribroperidinium tenuiceras (Eisenack, 1958a) Poulsen, 1996

Cribroperidinium muderongense (Cookson and Eisenack, 1958) Davey, 1969a

Cyclonephelium sp.

Cyclonephelium filoreticulatum (Slimani, 1994) Prince et al., 1999

Dinogymnium sp.

Downiesphaeridium sp.

Endoscrinium sp.

Exochosphaeridium sp.

Exochosphaeridium phragmites Davey et al., 1966

Florentinia sp.

Florentinia deanei (Davey and Williams, 1966b) Davey and Verdier, 1973

Florentinia ferox (Deflandre, 1937b) Duxbury, 1980

Florentinia laciniata Davey and Verdier, 1973

Florentinia buspina (Davey and Verdier, 1976)

Ginginodinium spinulosum Cookson and Eisenack, 1960a

Glaphyrocysta sp.

Gonyaulacysta sp.

Heterosphaeridium sp.

Heterosphaeridium difficile (Manum and Cookson, 1964) Ioannides, 1986

Hystrichodinium sp.

Hystrichodinium pulchrum Deflandre, 1935

Hystrichodinium solare Pestchevitskaya, 2009

Hystrichosphaeridium bowerbankii Davey and Williams, 1966b

Hystrichosphaeridium tubiferum (Ehrenberg, 1837b) Deflandre, 1937b

Hystrichosphaeridium sp.

Kallosphaeridium sp.

Leberidocysta sp.

Leptodinium sp.

Litosphaeridium sp.

Membranosphaera Samoilovitch and Mtchedlishvili, 1961

Microdinium sp.

Microdinium ornatum Cookson and Eisenack, 1960a

Muderongia sp.

Muderongia simplex Alberti, 1961

Occisucysta sp.

Odontochitina sp.

Odontochitina costata Alberti, 1961

Odontochitina operculata (Wetzel, 1933a) Deflandre and Cookson, 1955

Odontochitina singhii Morgan, 1980

Oligosphaeridium albertense (Pocock, 1962) Davey and Williams, 1969

Oligosphaeridium complex (White, 1842) Davey and Williams, 1966b

Oligosphaeridium prolixispinosum Davey and Williams, 1966b

Oligosphaeridium pulcherrimum Deflandre and Cookson, 1955

Oligosphaeridium sp.

Operculodinium sp.

Palaeoperidinium cretaceum (Pocock, 1962 ex Davey, 1970) Lentin and Williams, 1976

Pareodinia sp.

Pervosphaeridium pseudhystrichodinium (Deflandre, 1937b) Yun Hyesu, 1981

Prolixosphaeridium sp.

Pterodinium sp.

Pterodinium cingulatum (Wetzel, 1933b) Below, 1981a

Pterodinium pterotum (Cookson and Eisenack) Pavlishina, 1990

Pyxidiella sp.

Scriniodinium sp.

Senoniasphaera sp.

Senoniasphaera rotundata Clarke and Verdier, 1967

Sentusidinium sp.

Sirmiodinium sp.

Spinidinium sp.

Spiniferites ramosus (Ehrenberg, 1837b) Mantell, 1854

Spiniferites scabrosus (Clarke and Verdier, 1967) Lentin and Williams, 1975

Spiniferites sp.

Stephodinium coronatum Deflandre, 1936a

Systematophora sp.

Tanyosphaeridium sp.

Tehamadinium coummia (Below, 1981a) Jan du Chêne et al., 1986b

Trichodinium castanea Deflandre, 1935 ex Clarke and Verdier, 1967

Trithyrodinium sp.

Valensiella sp.

Vesperopsis nebulosa Bint, 1986

Xenikoon australis Cookson and Eisenack, 1960a

Green algae

Botryococcus braunii Kutzing, 1849

Palambages sp.

Shizosporis reticulatus Cookson & Dettmann 1959 emend. Pierce 1976

Tasmanites sp.

Zygnemaceae sp.

Аcritarchs

Baltisphaeridium sp.

Cymatiosphaera sp.

Micrhystridium fragile Deflandre, 1947

Micrhystridium singulare Firtion, 1952

Leiofusa filifera Downie, 1959

Veryhachium reductum Deunff, 1958

Other HF-resistent organic matter

Tracheids

Cuticles

Structured wood fragments

Opaque phytoclast

Microforaminifera