Palynology of the Triassic–Jurassic Transition of the Danish Basin (Denmark): A Palynostratigraphic Zonation of the Gassum–Lower Fjerritslev Formations

ABSTRACT The Upper Triassic–Lower Jurassic succession in the Danish Basin is penetrated by many deep wells that were drilled during former hydrocarbon exploration campaigns, but it is today targeted for geothermal energy and storage of CO2. The Stenlille salt dome on Sjælland sandstones of the Gassum Formation, sealed by the overlying Fjerritslev Formation mudstones, has been used for decades as a seasonal storage for natural gas. With its comprehensive dataset of seismics, geophysical well logs and conventional core data from 20 wells, the Stenlille succession serves as a model for other salt domes currently being evaluated as potential CO2 storage sites in the basin. Over the last decade the cored Triassic–Jurassic boundary succession has contributed to the understanding of environmental and palynological events during the end-Triassic mass extinction. Core, sidewall core and cutting samples from several of the closely situated Stenlille wells are here used to establish a high-resolution palynostratigraphic zonation scheme covering the entire Rhaetian to Sinemurian succession by integrating new analyses with previously published data. The palynological dataset has allowed the recognition of nine formally described spore-pollen zones, of which eight are new, while two previously described dinoflagellate cyst zones are subdivided into three informal subzones each. The palynological zonation is integrated with a sequence stratigraphic framework and will form the basis for the dating of future well sections in the Danish Basin and other basins and for correlation to outcrops. The large palynological dataset further shows that the vegetation around the Danish Basin was remarkably stable during the early to middle Rhaetian, but that events related to the emplacement of the Central Atlantic Magmatic Province accelerated ecosystem changes for c. 175 ky in the late Rhaetian and earliest Hettangian, including ∼25 ky of successional recovery before the terrestrial ecosystem had again stabilised.


Background and objectives
The Triassic-Jurassic succession in the Danish Basin has been the subject of many geological studies since 1935, including exploration activities for hydrocarbons, gas storage, geothermal energy, and carbon capture and storage (Nielsen 2003).Numerous deep wells have provided information on the deeply buried succession of which outcrops occur only along the basin margin in Scania, southern Sweden, and on the Danish island of Bornholm in the Baltic Sea.Correlations between deep wells in different parts of the basin and the outcrops have through the years relied heavily on biostratigraphy, using primarily ostracods and palynology (Michelsen 1973(Michelsen , 1975;;Lund 1977;Bertelsen 1978; Guy-Ohlson 1981; Dybkjaer 1988Dybkjaer , 1991;;Surlyk et al. 1995;Poulsen 1996;Poulsen and Riding 2003).These studies, together with other pioneering studies of palynofloras of the latest Triassic to earliest Jurassic (by e.g.Orlowska-Zwolinska 1967;Schulz 1967;Orbell 1973;Morbey 1975;Lund 1977Lund , 2003;;Koppelhus andBatten 1992, 1996;Batten et al. 1994; Koppelhus and Nielsen 1994) have laid the foundation for modern high-resolution palynological research focused on the Triassic-Jurassic boundary (TJB; 201.35 Gravendyck et al. 2020).The end-Triassic crisis is generally regarded as one of the five most severe biotic crises during the Phanerozoic (Raup and Sepkoski 1982) and is generally believed to have been caused by voluminous greenhouse gas emissions from the Central Atlantic Magmatic Province (CAMP), a large igneous province formed during the initial stages of Pangaea breakup (Marzoli et al. 1999).Global warming due to the large-scale volcanism and subsequent ocean acidification resulted in a marine calcification crisis that severely affected both neritic and pelagic organisms (Hallam and Wignall 1999;Hautmann 2004;Hautmann et al. 2008; van de Schootbrugge et al. 2007), and in major floral and faunal turnovers on land (McElwain et al. 1999(McElwain et al. , 2007(McElwain et al. , 2009; Olsen et al. 2002;Whiteside et al. 2007; Mander et al. 2013).The emissions of light carbon ( 12 C) are reflected in both organic and inorganic carbon isotope records worldwide and show a series of negative excursions indicating a highly unstable carbon cycle across the TJB (e.g.Hesselbo et al. 2002;Guex et al. 2004 The Danish Basin succession and especially the cored wells of the Stenlille structure have proved to be instrumental for the understanding of the environmental and biotic effects of the end-Triassic crisis.However, the palynological zonation which, together with organic C-isotopes, has provided a foundation for the stratigraphic framework of the succession and allowed correlation with other TJB successions worldwide has been informal and not clearly defined (Nielsen 2003 Here, we describe and formalise this new spore-pollen zonation, and we also present a subdivision of two previously defined dinoflagellate cyst zones based on the data from the conventionally cored succession at Stenlille in the Danish Basin.In particular, we describe the detailed palynology of the Stenlille-1 and -4 well sections across the TJB that have been used in several of the previously mentioned papers and complement these with Dybkjaer's (1991) previously published data from Stenlille-2 for the Hettangian to Sinemurian part of the succession.In addition, we incorporate new data from primarily conventional cores, complemented by sidewall cores (SWCs) and cuttings (CUs), from several other Stenlille wells (Stenlille abbreviated St: St-5, St-6, St-15, St-17, St-18, St-19) encompassing deeper parts of the Rhaetian succession and which were obtained during a recent investigation of the CO 2 storage potential of the succession.The many high-quality cores from wells located close together have enabled the construction of a unique composite sedimentary succession that spans the Rhaetian-Pliensbachian, and this has allowed calibration between sequence stratigraphic surfaces and the palynological zonation, which provides a well-constrained framework for dating future well sections.

Geological setting
The Danish Basin is bounded to the south-west by the Ringkøbing-Fyn High, while its north-eastern margin follows the NW-SE oriented Sorgenfrei-Tornquist Zone (STZ), a tectonic lineament characterised by extensive block-faulting along the south-western margins of the Baltic Shield (Sorgenfrei and Buch 1964;Liboriussen et al. 1987; Michelsen and Nielsen 1991;Erlstr€ om et al. 1997) (Figure 1).Palaeogeographic reconstructions place southern Scandinavia around 45 N at the Triassic-Jurassic transition (Ziegler 1990) (Figure 1), and at that time the Danish Basin constituted a shallow, low-gradient marine embayment without a shelfslope break on the northern margin of an epicontinental sea that covered large parts of northern Europe (Nielsen 2003).The basin experienced post-rift thermally controlled subsidence, and large amounts of sediments were deposited (Liboriussen et al. 1987;Vejbaek 1989;Erlstr€ om et al. 1997).In the subsurface of onshore and nearshore Denmark, the latest Triassic (Rhaetian) to earliest Jurassic (Sinemurian) succession is represented by the Gassum and Fjerritslev formations, except for the island of Bornholm.Their boundary is highly diachronous, being of latest Rhaetian age in deep and distal parts of the basin including the Stenlille area, whereas it is of early Sinemurian age along the basin margin in northern Jylland and Sjaelland (Michelsen 1975, Bertelsen 1978, Michelsen et al. 2003;Nielsen 2003).
The Stenlille structure is located in the eastern part of the basin and it constitutes a low-relief anticlinal structure on the Top Gassum Formation surface formed over a gentle salt pillow (Figure 1).It includes one of the most complete cored Upper Triassic-Lower Jurassic successions known, as 20 wells have been drilled to evaluate the reservoir quality of the Gassum sandstones, and the seal quality of the Fjerritslev mudstones, and for monitoring purposes as the structure is used for seasonal storage of natural gas.The succession constitutes a unique dataset that have been studied by means of seismic methods, sedimentology and sequence stratigraphy, based on seismic data, well log patterns, core interpretations, and biostratigraphic analyses (Pedersen 1985;Dybkjaer 1991; Hamberg and Nielsen 2000;Nielsen 2003;Lindstr€ om et al. 2012Lindstr€ om et al. , 2015)).Because of this, the interpretations of the Gassum and Fjerritslev formations are used as reference for interpretation of contemporaneous successions elsewhere in the basin, and the Stenlille storage complex is used as a model for similar anticlinal structures in other parts of the Danish Basin, that are of potential interest as storage sites for CO 2 or energy.

Gassum Formation
The Gassum Formation (uppermost Norian-lower Sinemurian) varies in thickness from 50-150 m in the central parts of the basin to as much as 300 m locally within the STZ (Michelsen and Nielsen 1991;Nielsen and Japsen 1991).It consists of interbedded fine-to medium-grained, occasionally coarse-grained and pebbly sandstones, heteroliths, mudstones and few thin coaly beds formed in shallow marine to marginal marine environments (Michelsen et al. 2003;Nielsen 2003).At Stenlille the top of the Gassum Formation is Rhaetian in age and located at Transgressive Surface 7, i.e.TS7, just a few metres below MFS7 (Nielsen 2003; Lindstr€ om et al. 2012).

Fjerritslev Formation
The overlying Lower Jurassic Fjerritslev Formation is dominated by marine claystones and mudstones (Michelsen 1978(Michelsen , 1989)).The transition from the Gassum Formation to the Fjerritslev Formation occurred in several depositional steps ranging from the latest Rhaetian in the central parts of the basin and the Stenlille area to the early Sinemurian along its north-eastern margin, reflecting the overall but stepwise Early Jurassic eustatic sea-level rise shown by a series of transgressive surfaces from TS7 through TS11 defined by shifts from sandstones to overlying mudstones and claystones (Nielsen 2003).At Stenlille, the lowest part of the Fjerritslev Formation consists of dark grey mudstone/shale around Maximum Flooding Surface 7, i.e.MFS7 (Nielsen 2003).This is succeeded by a grey siltstone unit, which contains several soft-sediment deformation horizons that are interpreted as seismites (Lindstr€ om et al. 2015).

Sequence stratigraphy
The present sequence stratigraphic framework is based on the sequence stratigraphic model presented by Nielsen   2).The succession from the upper part of SQ1 to the lower part of SQ11 is presented in Figure 2.
A prominent sequence stratigraphic marker is the maximum flooding surface MFS7, which is associated with the deposition of dark grey claystones and mudstones that are widely recognised all over the basin based on log patterns, sedimentology and biostratigraphy (Nielsen 2003).In the more proximal parts of the basin this late Rhaetian flooding event is recognised as a marine incursion in an otherwise predominantly terrestrial environment (Lindstr€ om and Erlstr€ om 2006).The late Rhaetian flooding event can also be used for correlation outside the Danish Basin, e.g. in Germany where it appears to correspond roughly to the upper part of the Contorta Beds, and in St Audrie's Bay, Great Britain, where it is correlated with the middle Westbury Formation (Hesselbo et al. 2004 flank of the North German Basin is deeply buried, and therefore the majority of the biostratigraphic studies were focused on micropalaeontology and palynology carried out on material from deep wells.Descriptions of some macrofossils can be found in Sorgenfrei and Buch (1964) and Pedersen (1986).Bertelsen (1978Bertelsen ( , 1980) ) included palynology in his description of the Triassic-Lower Jurassic lithostratigraphy and depositional development.Michelsen (1975) erected an ostracod biozonation for the Lower Jurassic succession of the Danish Basin, but during the last three decades palynology has been used to a greater extent.Previous palynological studies of the uppermost Triassic-lower Jurassic succession in the Danish Basin have provided biostratigraphic data for Rhaetian-Sinemurian strata, as well as important information on the depositional environment and palaeoclimate (Nilsson 1958;Lund 1977;Guy-Ohlson 1981;Dybkjaer 1988Dybkjaer , 1991;;Koppelhus 1991; Koppelhus and Batten 1996;Poulsen 1996; Lindstr€ om and Erlstr€ om 2006; Larsson 2009).The spore-pollen zonation erected by Lund (1977) was primarily based on core material from the Danish Rødby-1 well, located in the northern margin of the North German Basin.Lund (1977) divided the succession from the Norian to the early Sinemurian into six zones, five formal and one informal: the Norian-earliest Rhaetian Corollina-Enzonalasporites Zone, the early Rhaetian Ricciisporites-Conbaculatisporites Zone, the middle Rhaetian Rhaetipollis-Limbosporites Zone, the late Rhaetian

Ricciisporites-Polypodiisporites
Zone, the Hettangian Pinuspollenites-Trachysporites Zone, and a then unnamed zone with Cerebropollenites macroverrucosus in the Sinemurian.The latter was later formalised as the Cerebropollenites macroverrucosus Zone by Dybkjaer (1991).As Cerebropollenites macroverrucosus has recently been transferred to Sciadopityspollenites macroverrucosus (Thiergart) Iljina emend.Gravendyck 2023 (in Gravendyck et al. 2023), this zone is herein referred to as the Sciadopityspollenites macroverrucosus Zone.In addition to these zones Dybkjaer (1991) also erected the late Rhaetian Corollina-Ricciisporites Zone, which encompassed assemblages from wells in Jutland that could not be assigned to the zones erected by Lund (1977) (Figure 3).Poulsen and Riding (2003) presented a dinoflagellate cyst zonation for the Danish Basin, in which three zones encompass the Rhaetian to lower Sinemurian interval, namely (in ascending order) the Rhaetian Rhaetogonyaulax rhaetica Zone, the late Rhaetian to earliest Sinemurian Dapcodinium priscum Zone, and the early Sinemurian Liasidium variabile Zone (Figure 3).

Stratigraphic constraints across the TJB
A high-resolution bulk organic C-isotope record of the middle Rhaetian to lower Hettangian succession in the Stenlille-1 well, complemented by intervals from Stenlille-2 and -5, displays three marked shifts to more negative C-isotope values in ascending order: the Marshi, the Spelae and the top-Tilmanni C-isotope excursions (CIEs)which are separated by two intervals with more positive values (Lindstr€ om et al. 2012; Lindstr€ om, van de Schootbrugge et al. 2017) (Figure 1C).These CIEs are well constrained by palynostratigraphy, which allows correlations with the Rødby-1 record in the North German Basin.In addition, two stratigraphically important ammonoids recovered from the Danish Rødby-1 well located in the North German Basin were re-evaluated taxonomically by Lindstr€ om, van de Schootbrugge et al. (2017) and helped constrain the correlation to the global stratotype section and point (GSSP) for the base of the Jurassic at Kuhjoch in the northern Calcareous Alps (Hillebrandt et al. 2013; Lindstr€ om, van de Schootbrugge et al. 2017).
The succession is further constrained by several palynostratigraphic events, both marine and terrestrial (Figure 3).Amongst the marine palynological markers, the last common occurrence (LCO) of the dinoflagellate cyst Rhaetogonyaulax rhaetica usually occurs within the marine black claystone associated with MFS

Sedimentological description, geophysical log correlation, sequence stratigraphy and sampling
The Stenlille wells studied herein are closely located on the Stenlille structure (Figure 1).The wells were conventionally cored across the transition from the Gassum Formation to the Fjerritslev Formation.The cores, which are stored at the Geological Survey of Denmark and Greenland, GEUS, are c. 10 cm in diameter and generally of good quality, and several have been slabbed for easy inspection.The depositional units can easily be correlated between the wells by means of the detailed well-log patterns (Figure 2; Supplementary Figure 19).Core depths (in metres below Kelly Bushing) were (when necessary) adjusted to log depths by closely comparing core lithology to well-log signatures for each well.Highresolution sampling was carried out on the cores from Stenlille-1 and Stenlille-4, with generally 2-4 samples per metre across the Rhaetian-Hettangian interval (Lindstr€ om

Palynological processing and analysis
For palynology, c. 20 g of bulk rock was treated in alternating steps with hydrochloric (38%) and hydrofluoric acid (40%) to remove carbonate and silicate mineral phases, respectively.After washing to neutrality, residues were sieved with 11 lm mesh-size sieves and mounted as strew slides.In all, >300 samples were processed for palynology.Generally, up to 300 palynomorphs or more were counted per slide with a compound microscope at 650Â magnification.Abundance data for the total composition of the palynoflora were calculated as percentages of total palynomorph assemblage and divided into dinoflagellate cysts, marine and freshwater microalgae and acritarchs, as well as spores and pollen.In addition, the palynological data was complemented with previously published palynology from 21 core and SWC samples from Stenlille-2 (Dybkjaer 1991).Stratigraphic range charts and summary charts were produced in Stratabugs 2.1.1.for each well.On the range charts (Supplementary Figures 1-9), microalgal and acritarch percentages are displayed as percentages of the total palynoflora, while spore-pollen abundances are calculated as percentages of the spore-pollen flora only; on the summary charts all palynological groups are calculated as percentages of the total palynoflora (Supplementary Figures 10-18).The spore-pollen zonation was established with the aid of the CONISS (stratigraphically constrained cluster analysis) method included in Stratabugs but was primarily based on the plotted data.The occurrence and abundance of key taxa, i.e. taxa with well-established stratigraphic ranges, were noted in each well in order to facilitate correlation between the wells.The wells are correlated along a west-to-east transect (Figure 1B), based on the spore-pollen zonation, the stratigraphic ranges of key taxa and sequence stratigraphic surfaces, and this is illustrated in Supplementary Figure 19.

Correlation of the Stenlille wells
The subdivision of the Gassum Formation-Lower Fjerritslev Formation into SQs and correlation of these between the Stenlille wells was based on interpretations of vertical GR motifs and sedimentologic interpretations of cores supplemented with biostratigraphic data.Figure 2 shows a log panel of the herein studied wells along the transect in Figure 1B.The log panel allows pinpointing of palyno-events within the Stenlille succession and determining whether these are dependent on depositional facies changes between the wells.

Selection of time scales
The estimation of the duration of the Rhaetian to Sinemurian succession relied on age estimates of chronostratigraphic boundaries from Ogg et al. (2020), and known or estimated ages of palynological events and negative bulk organic CIEs.There are two suggested ages for the base of the Rhaetian: the long Rhaetian with the Norian-Rhaetian boundary at 209.5 Ma, and the short Rhaetian with the base of the Rhaetian at 205.7 Ma (Ogg et al. 2020).In accordance with Ogg et al. (2020), the short Rhaetian is used as standard herein.

Palynological trends, events and sequence stratigraphy
The changes in overall palynological composition (marine versus terrestrial) and correlation of core, SWC and CU samples are shown in Supplementary Figures 10-19; they indicate that below SB5 in the succession the marine influence is low or absent, while the abundances of dinoflagellate cysts increase markedly at or above SB5.The most striking palynological changes during the Rhaetian to Sinemurian were the virtual disappearance of dinoflagellate cysts and a subsequent marked decline of pollen in favour of spores in the crisis interval just prior to the TJB (Supplementary Figures 10-19).Although dinoflagellate cysts again increase in abundance around and after the TJB, they remain less abundant compared to before the crisis.

The spore-pollen zonation
There are marked abundance changes in the spore-pollen record of Stenlille that allow the succession to be divided into six local spore-pollen assemblage zones across the TJB (Figure 3).Because marker taxa are often rare and sporadically occurring, the zones are named after the two to three most prominent taxa within each interval; in some cases the third nominate taxa is a marker species for the lower boundary of the zone.The zones are presented in stratigraphically ascending order below and their main characteristics are summarized in Table 1.Any marker taxa that can be used for identifying the lower or upper boundaries of the zones are mentioned under each zone, and first and last occurrences (abbreviated FO and LO, respectively), first and last common occurrences (FCO and LCO, respectively, referring to common abundance of a taxon), as well as first and last consistent occurrences (Fcon and Lcon, respectively, referring to consistency in occurrence) of important taxa are noted under each zone.Intervals in the Stenlille wells assigned to a zone are listed under each specific zone, in meters below Kelly Bushing.For Stenlille-4 the depths refer to unshifted core depths.The new zones are correlated with the previous palynozonations for the Danish Basin by Lund (1977) and Dybkjaer (1991) (Figure 3).The stratigraphic ranges of key spore-pollen taxa within the investigated interval of the Stenlille succession are shown in Figure 4 and selected palynomorphs are illustrated on Plates 1-7.

Perinopollenites-Ricciisporites-Rhaetipollis (PRRh)
Assemblage Zone (new) Definition.The lower boundary of this zone is defined by the FO of Rhaetipollis germanicus, which together with Ovalipollis ovalis is consistently present.In this zone Perinopollenites elatoides is generally the most abundant taxon, together with common Ricciisporites tuberculatus.Spores, usually Calamospora tener, Punctatisporites spp., and/or Deltoidospora spp.are also common.Cheirolepidiacean pollen may be common.Rare and sporadic occurrences of pollen taxa typical of Norian palynofloras, e.g.Enzonalasporites vigens, E. ignacii, Rimaesporites aquilonalis, Klausipollenites gouldii, Duplicisporites granulatus and Camerosporites secatus, are present within the zone.The LO of Enzonalasporites spp.occurs close to the boundary to the overlying zone, but the event can be difficult to pinpoint as the taxon is very rare.Generally, assemblages of this zone lack Limbosporites lundbladiae, the FO of which usually occurs at the lower boundary of the succeeding zone.However, in Stenlille-15 L. lundbladiae also occurs in core samples from the topmost part of the PRRH Zone.It also occurs in some of the CU samples from this zone, but because these often contain caved material, it is uncertain whether these occurrences are in situ or not.Besides L. lundbladiae, the boundary of the overlying zone is marked by the incoming of several mid-Rhaetian taxa, i.e.Kraeuselisporites reissingeri, Triancoraesporites ancorae and Zebrasporites laevigatus.
Lithostratigraphy.The zone encompasses the lower part of the Gassum Formation, possibly straddling into underlying units.
Correlation to sequence stratigraphy.The PRRh Zone covers an interval from within SQ1 to the lower part of SQ4 (below TS 4) (Figures 3 and 4).
Correlation and age.The PRRh Zone roughly correlates with the Classopollis Enzonalasporites Zone (Lund 1977   Zebrasporites laevigatus.The zone is generally characterised by a dominance of Perinopollenites elatoides and abundant Classopollis.Ricciisporites tuberculatus is common to abundant, and Granuloperculatipollis rudis is generally present but usually never common.Enzonalasporites spp.and Praecirculina scurrilis appear to have their LOs close to the lower boundary of this zone.In addition, the LOs of Duplicisporites granulatus and Camerosporites secatus are in this zone; however, they are all rare and occur only sporadically.Vitreisporites bjuvensis and V. pallidus are more consistent and common in the upper part of the PCL Zone, from around MFS5, than in the lower part or in the underlying zones. Reference section.Stenlille-1, 1578.50-1518.74m (SQ4-SQ6) for the upper part of the zone (Supplementary Figure 10), and Stenlille-19, 1643.90-1643.70m (SQ4) for the basal part of the zone.In Stenlille-1 the zone most likely extends down to 1588.5 m, which corresponds to the level for the FO of L. lundbladiae in Stenlille-19 (Supplementary Figure 18).However, it should be noted that in Stenlille-15 the boundary to the underlying zone (PRRh) is less clear, as L. lundbladiae also occurs in CU samples in that zone.
Lithostratigraphy.The PCL Zone is found in the middle part of the Gassum Formation.
Lithostratigraphy.The GCP Zone is found in the upper part of the Gassum Formation and within the black claystones of the lowermost Fjerritslev Formation.Lindstr€ om, van de Schootbrugge et al. ( 2017) extended the zone down to 1523.46 m in Stenlille-1.However, with addition of new palynological data from the middle part of the Gassum Formation, cluster analysis (CONISS) suggests that assemblages from 1518.74 m to 1523.46 m should belong to the previous zone.In Stenlille-1, the upper boundary of the zone appears to coincide with the base of the 'grey siltstone interval'.In Stenlille-4 the zone instead appears to extend into the lowermost 0.45 m of the 'grey siltstone interval'; however, the lowermost part of that interval contains numerous clay clasts, which were probably derived from the unit below.
Correlation to sequence stratigraphy.The GCP Zone covers an interval extending from the upper part of SB6 to SB8.It thus includes both MSF6 and MFS7 (Figures 3-4).
Organic C-isotope stratigraphy, Stenlille-1.The organic Cisotope record for the GCP Zone in Stenlille-1 can be divided into two phases (Figure 1C).Phase 1 ranges from the base of the zone, where the d 13

Polypodiisporite-Ricciisporites-Deltoidospora (PRD)
Assemblage Zone (new) Definition.The lower boundary of this zone is marked by a dramatic increase in Polypodiisporites polymicroforatus and Deltoidospora spp.(predominantly D. toralis).Ricciisporites tuberculatus is also a characteristic component, as both tetrads and various large fragments.The PRD Zone is markedly different to the previous GCP assemblage.Within the zone monosulcate pollen decrease in abundance and conifer pollen virtually disappear.The palynomorphs of this interval are often darker in colour than in the underlying and the succeeding intervals.Ovalipollis ovalis has its Lcon near the base Comments.The palynoassemblages of the PRD Zone (and to some extent also the succeeding CCM Zone) are (besides the characteristics of the spore-pollen flora) easily distinguished by their high abundances of reworked palynomorphs (acritarchs and microalgae, chitinozoans, spores and pollen) of Lower Palaeozoic, Carboniferous and Middle-Late Triassic ages.Sphaeromorphs of various sizes and preservation are often abundant and are also considered reworked.Some of the sphaeromorphs are reticulate indicating that they are of prasinophycean affinity.In addition, the in situ spore-pollen assemblages often contain specimens that are darker in colour than in the zones preceding the PRD Zone and succeeding the CCM Zone.This was already noted in the 1970s as a conspicuous feature of the Ober-rhaet palynofloras of the North German Basin by Lund (see Lund 2003).The PRD Zone and the high abundances of reworked palynomorphs were also recognised in the Rhaetian of Scania (Lindstr€ om, Erlstr€ om et al. 2017). Reworking.

3.2.5.
Calamospora-Conbaculatisporites-Monosulcites (CCM) Assemblage Zone (new) Definition.This zone comprises only three samples and is characterised by a dominance of sphenopsid spores assigned to Calamospora tener and initially also of the cupressacean pollen taxon Perinopollenites elatoides.A successive decrease in P. elatoides is accompanied by an increase in ground fern spores Conbaculatisporites spp.and monosulcate pollen.Within this zone are the Lcons of Limbosporites lundbladiae, Lunatisporites rhaeticus and Semiretisporites gothae (Figure 5).Rare occurrences of these taxa higher in the succession may be due to reworking.The assemblages contain a relatively high proportion of unidentified thin-walled, granulate sporomorphs that may be of either plant or algal origin.The lowermost assemblage also contains abundant Botryococcus braunii and rare marine dinoflagellate cysts assigned to Dapcodinium priscum.
decrease in Polypodiisporites polymicroforatus and a marked increase in Perinopollenites elatoides.The zone is characterised by a massive dominance of Perinopollenites elatoides, together with abundant Deltoidospora spp.There is an increase in Classopollis spp.compared to the two previous zones.Spores assigned to Stereisporites spp.are common to abundant.Within this zone is the Lcon of Triancoraesporites triancorae (Figure 5).Higher occurrences of this taxon may be reworked.The first regular occurrence of Ischyosporites variegatus is in the middle of this zone.The occurrence of Naiadita sp.just above or at MFS 8 in Stenlille-1 and -4, respectively, is noteworthy.
Correlation to sequence stratigraphy.The PDS Zone covers a narrow interval within the SQ8 which includes MFS8 (Figures 3-4).Correlation and age.This zone does not correspond to any previously described zone from the Danish Basin but may in part correlate with the topmost upper Rhaetic assemblages of Lund (1977Lund ( , 2003)).Based on correlations by Lindstr€ om, van de Schootbrugge et al. (2017) this zone straddles the TJB, which is here taken to coincide with MFS8, and thus encompasses the uppermost Rhaetian and earliest Hettangian.

3.2.7.
Deltoidospora-Perinopollenites-Pinuspollenites (DPPi) Assemblage Zone (new) Definition.The base of this zone is defined by a marked increase in abundance of Pinuspollenites minimus compared to the previous zone.The zone is dominated by Deltoidospora spp., while Perinopollenites elatoides and Pinuspollenites minimus are abundant.Stereisporites spp.and Trachysporites spp.are common constituents.In the middle part of the interval there is an increase in the abundance of Aratrisporites minimus.

Lithostratigraphy. Fjerritslev Formation.
Correlation to sequence stratigraphy.The DPPi Zone covers the uppermost part of SQ8 and the lowermost part of SQ9, and thus includes SB9 (Figures 3-4).
Organic C-isotope stratigraphy.The trend with positive Cisotope values continues during the PPi Zone, with values of À24.30‰ to À24.80‰ in all samples except for the uppermost two, where there are again slightly more negative values of À25.61‰ and À25.70‰.The more negative trend continues in the succeeding core section from Stenlille-2, from 1491.78 to 1476.08 m, with values fluctuating between 3 Plate 5. Selected non-saccate pollen and other palynomorphs.The scale bar in Figure 3 is 20 lm and applies to all photographs.
À25.70‰ and À27.10‰.The latter value marks the top-Tilmanni CIE at 1486.37 m in Stenlille-2 (Figure 1C).Values continue to lie between À26.0‰ and À26.7‰ in the two succeeding core sections from Stenlille-1.In the uppermost core section, from Stenlille-1, the d 13 C values have increased slightly to between À25.9‰ and À26.3‰.3.2.9.Deltoidospora-Perinopollenites-Classopollis (DPC) Assemblage Zone (new) Definition.The base of this zone is marked by a decrease in Pinuspollenites minimus, while Deltoidospora spp.again becomes abundant to dominant together with Perinopollenites elatoides.The zone is also characterised by an increased abundance in Classopollis spp.compared to the previous zone, at least in Stenlille-1 and -5, while Stenlille-2 instead shows a slight decrease in the abundance of Classopollis.The reason for this difference is not clear.

The dinoflagellate cyst zonation
In general, the aquatic palynological record of the Rhaetianlower Sinemurian succession in the Stenlille area is dominated by marine microalgae, primarily dinoflagellate cysts, but acritarchs, prasinophytes and freshwater algae are also present.The succession is divided into three zones following the zonations established for the UK by Woollam and Riding (1983) and for the Danish Basin by Poulsen and Riding (2003); namely, in ascending order, the Rhaetogonyaulax rhaetica Zone, the Dapcodinium priscum Zone, and the Liasidium variabile Zone.Here, however, the two lowest zones are subdivided into three local (to Stenlille) and informal subzones each, based on the abundances of the respective nominate taxa.

Rhaetogonyaulax rhaetica Zone
Definition.The base of the zone is marked by the FO of Dapcodinium priscum.The zone is generally characterised by the presence to abundant occurrence of R. rhaetica in assemblages from marine to coastal environments.The lower boundary of this zone is not known in the Stenlille area but is suggested to coincide with the FO of D. priscum.Rhaetogonyaulax rhaetica is first registered with certainty from the lowermost parts of the Gassum Formation above SB2.The top of the zone coincides approximately with the base of the PRD Zone, as R. rhaetica only occurs sporadically above this level.The zone can be subdivided into three informal subzones, described below.
Rhaetogonyaulax rhaetica subzone (a), from below the base of the Gassum Formation to just above SB3 (Figures 3-4).Dinoflagellate cysts, including D. priscum and R. rhaetica, occur sporadically.Acritarchs and other marine microalgae are occasionally present, such as Cymatiosphaera sp., Micrhystridium spp., Veryhachium spp.and Halosphaeropsis liassicus.The enigmatic acritarch Celyphus stenlillensis may be present.Because the R. rhaetica subzone (a) only covers CU samples, there is a possibility that the dinoflagellate cyst occurrences may be due to cavinghence the distinction between subzones (a) and (b).In addition, L. scaniense was not recorded in subzone (a).
Rhaetogonyaulax rhaetica subzone (b), from above SB3 to between TS5 and MFS5 (Figures 3-5), is characterised by rare occurrences of the nominate taxon and other dinoflagellate cysts including Dapcodinium priscum and Lunnomidinium scaniense.Occasional samples may contain common dinoflagellate cysts, but they are never consistently common.Acritarchs and other marine microalgae are occasionally present, such as Cymatiosphaera sp., Micrhystridium spp., Veryhachium spp.and Halosphaeropsis liassicus.
Correlation to sequence stratigraphy.Subzone (a): from within SQ1 to just above SB3.Subzone (b): from just above SB3 to between TS5 and MFS5.Subzone (c): encompasses an interval from between TS5 and MFS5 to just above SB8 (Figures 3-4).

Dapcodinium priscum Zone
Definition.The lower boundary of the zone occurs just above the virtual disappearance of dinoflagellate cysts at the top of the previous zone.Apart from the lowermost subzone, which is almost devoid of dinoflagellate cysts, the zone is characterised by the presence of the nominate taxon, while Rhaetogonyaulax rhaetica is absent or only occurs rarely and sporadically.The top of the zone is marked by the FO of Liasidium variabile.The zone can be divided into three subzones: Dapcodinium priscum subzone (a) covers the interval from just above SB 8 to MFS 8 and is defined by a virtual absence of dinoflagellate cysts.Dapcodinium priscum subzone (b) encompasses the interval from MFS 8 to about halfway between MFS 10 and SB 11.It is characterised by common to abundant occurrences of D. priscum.The LO of R. rhaetica is registered in the lowermost part of this subzone, close to but above MFS 8.In the lower part of the subzone, the acritarch Leiofusa jurassica has its FO, a little higher above MFS8.Correlation to sequence stratigraphy.From between MFS 10 and SB11 to just below MFS11 (Figures 3-4).

Age and duration of the Stenlille spore-pollen zones
The Gassum-Fjerritslev Formation boundary in the Danish Basin is highly diachronous as the boundary is of latest Rhaetian age in the Stenlille area and in similar deep and distal parts of the basin, whereas the boundary is of early Sinemurian age along the basin margin in northern Jylland and Sjaelland (Michelsen 1975;Bertelsen 1978;Nielsen 2003).
The herein proposed correlation to the absolute time scale allows an estimation of the duration of the defined palynological zones and of the time interval for deposition of the Gassum Formation in the Stenlille area.The estimates are probably also valid for similar distal parts of the basin, where deposition of Gassum sand stopped with the formation of the mudstone-dominated transgressive system tracts, TS7 to MFS7, of SQ7.Ideally, the time duration of biozones should be estimated in areas characterised by continuous and homogeneous sedimentation without breaks.The heterogeneous composition of the Gassum Formation with thick sandstone intervals alternating with mudstones and siltstones, makes time estimations difficult.It is likely that the sandstones were deposited as fill of fluvial and estuarine river channels, as coastal barrier islands and deltas and formed rapidly within few thousand years as is exemplified by well-described and probably analogous Holocene-Recent proximal marine succession (e.g.Fruergaard et al. 2018).Therefore, when the genesis of the sandstones in the Gassum Formation is considered, it is evident that this part of the Rhaetian succession contains a number of hiatuses at the erosional SBs and transgressive marine surfaces.Hence, although the sediment thickness cannot be directly translated to time, the duration of the biozones and the duration of the entire Gassum Formation may be estimated by combining chronostratigraphic ages, age estimates of the onset of the end-Triassic event, and estimated ages of biostratigraphic events and CIEs.The onset of the end-Triassic crisis at 201.51 Ma and the 201.35  According to K€ urschner and Herngreen (2010) and Heunisch and Wierer (2021), the FAD of Limbosporites lundbladiae coincided with the last appearance datums (LADs) of the pollen taxa Enzonalasporites vigens, E. densus, E. ignacii, and Duplicisporites spp.However, in the Stenlille succession pollen grains assigned to Enzonalasporites spp.and Duplicisporites spp.are generally very rare and occur only sporadically in the lower part of the Gassum Formation.In the Stenlille-1 record the LO of E. vigens occurs in a CU sample at 1616 m, while the FO of L. lundbladiae is registered in a core sample at 1542.29 m.The earliest occurrence of L. lundbladiae in core samples from Stenlille-15 and Stenlille-19 occurs below but close to TS4, at a level that equates to around 1588.5 m in Stenlille-1.In Stenlille-4, the LO of E. vigens is in a CU sample from 1593 m, i.e. just below TS4 in that well, similar to the FO of L. lundbladiae.This level is here transferred to the Stenlille-1 record as the FO of L. lundbladiae and LO of E. vigens.
It is difficult to relate this level directly to the Boreal and Tethyan ammonoid and conodont zonations (Ogg et al. 2020).However, the FO of L. lundbladiae in the northern Tethys area is from the so called 'Carpathian facies' of the K€ ossen Formation (Morbey and Neves 1974), which corresponds to the Misikella hernsteini-posthernsteini to lower Misikella rhaetica conodont zones (Galbrun et al. 2020).Hence, it possibly correlates with the lower boundary of the Tethyan Vandaites sturzenbaumi Ammonoid Zone (Figures 3-4), which has an estimated age of 204.4 Ma (Ogg et al. 2020).The boundary between the Vandaites sturzenbaumi Zone and the succeeding Choristoceras marshi Zone has an estimated age of 202.20 Ma (Ogg et al. 2020).This level is difficult to define in the Stenlille succession due to the lack of ammonoids or conodonts.However, a marked increase in the abundance of Dapcodinium priscum is registered between TS 5 and MFS5, and this is here tentatively correlated with the base of the C. marshi Zone.The top of the Marshi Zone is placed at the top of the Gassum Formation, which coincides with the negative CIE referred to as the Marshi CIE.This corresponds to the onset of the marine mass extinction, which is estimated at 201.51 Ma (Wotzlaw et al. 2014).The base of the Jurassic, i.e.where the ammonite Psiloceras spelae marks the base of the Psiloceras tilmanni Zone, is estimated at 201. 36   The spore/pollen record of the Rhaetian to Sinemurian succession in the marine Stenlille succession compares well with previously published palynological records from Denmark and southernmost Sweden (Lund 1977 ).This suggests that the parent plants grew in coastal settings, possibly on coastal islands, rather than inland, in accordance with previous interpretations that cheirolepidiaceans grew in coastal settings under seasonally dry to semiarid conditions (Batten et al. 1994;Abbink et al. 2004).The cupressacean/taxodiacean conifer pollen Perinopollenites elatoides is an abundant constituent in the Gassum Formation and the succeeding black claystones.It is also abundant in the more terrestrial coal-bearing successions from Scania (Lund 1977; Lindstr€ om and Erlstr€ om 2006).Perinopollenites elatoides is known to be particularly abundant in lagoonal assemblages (Mussard et al. 1997), and it seems likely that the parent plants thrived in wet lowland environments.The spore-pollen zonation of the Stenlille wells can be compared with the zonation established by Lund (1977), which encompassed both Scanian material and material from the North German Basin, in particular the cored succession of the Rødby-1 well situated at northern rim of the North German Basin.Following criteria used by Lund (1977), the co-occurrence of Rhaetipollis germanicus and Limbosporites lundbladiae suggests that the Perinopollenites-Classopollis-Limbosporites (PCL) Zone and the Granuloperculatipollis-Classopollis-Perinopollenites (GCP) Zone both correlate to the Rhaetipollis Limbosporites Zone by Lund (1977), while the older Perinopollenites-Ricciisporites-Rhaetipollis (PRRh) Zone correlates with Lund's (1977) Corollina Enzonalasporites Zone (herein referred to as the Classopollis Enzonalasporites Zone).The latter zone occurs in the Postera Schichten or Unter Rh€ at, while the Rhaetipollis Limbosporites Zone encompasses the Contorta Schichten or Mittel Rh€ at of the North German Basin (Lund 2003).According to Lund (2003), the transition between the Classopollis-Enzonalasporites Zone and the succeeding Rhaetipollis-Limbosporites Zone was gradual, with some taxa that were common in Unter-Rh€ at, such as Enzonalasporites, lingering on into the Mittel Rh€ at.However, Enzonalasporites only occurs sporadically in the lowermost zone in the Stenlille succession and disappears at almost the same level where L. lundbladiae and Triancoraesporites ancorae have their FOs.Other taxa typical of Norian assemblages in the North German Basin, e.g.Duplicisporites granulatus and Camerosporites secatus, also occur sporadically within the succeeding PCL Zone.
The PCL and GCP zones can be correlated with Lund's (1977) Rhaetipollis-Limbosporites Zone, the lower boundary of which he considered marked by the FO of L. lundbladiae, based on the co-occurrence of the nominate taxa.According to Lund (1977) the Rhaetipollis-Limbosporites Zone is characterised by abundant Ricciisporites tuberculatus, but that taxon rarely exceeds 10% within the zone.Within the PCL and GCP zones at Stenlille, R. tuberculatus often exceeds 10%, but then it should be noted that the counts herein include tetrads, single grains (a few, very rare) and larger fragments of R. tuberculatus.Lund (1977) did not record R. germanicus with certainty above the Rhaetipollis-Limbosporites Zone.At Stenlille, R. germanicus is rare and only occurs sporadically above the GCP Zone.The GCP Zone differs from the Rhaetipollis-Limbosporites Zone in its relatively high abundance of Granuloperculatipollis rudis, a taxon which Lund (1977) only found in low abundances within the Rhaetipollis-Limbosporites Zone.Granuloperculatipollis rudis is also only found in low abundances in the possibly time-equivalent Ricciisporites-Perinopollenites (RPe) Zone from the more proximal settings of the H€ ollviken-2 core and the FFC-1 well in SW Scania (Lindstr€ om, Erlstr€ om et al. 2017).This is not surprising as cheirolepidiacean conifer pollen are relatively rare in those two records (Lindstr€ om, Erlstr€ om et al. 2017).
The Ricciisporites-Polypodiisporites-Deltoidospora (RPD) Zone can be correlated with the Ricciisporites-Polypodiisporites Zone of Lund (1977) Lund (1977) considered it to be one of the the characteristic taxa of the zone.
The presence of abundant reworked palynomorphs of primarily Ordovician-Silurian, Carboniferous, and Middle to Late Triassic age in the grey siltstone beds indicates increased weathering and soil erosion on land.In the PRD Zone of Stenlille sphaeromorphs of various sizes, maturity and preservation are particularly common and have been interpreted as reworked from Scania to the east, as the differences in preservation and maturity can be explained by contact metamorphism from Permian intrusions in the Scanian Lower Palaeozoic (Olsson-Borell and Ahlberg 2003).The increased reworking in the grey siltstone beds and the PRD Zone (and to some extent also within the CCM Zone) is widely recognised in equivalent stratigraphic intervals across NW Europe and interpreted as a consequence of landscape destruction by deforestation, increased storminess and wildfire frequency, and increased seismic activity (Lindstr€ om et al. 2015; van de Schootbrugge et al. 2020).Assemblages from the PRD Zone generally contain a higher proportion of darker palynomorphs, both reworked and probably in situ, as well as a generally high abundance of inertinite (see Lindstr€ om 2021), and this allows easy recognition of the zone during drilling campaigns.
The CCM Zone at Stenlille cannot be correlated with certainty to any other assemblages within the Danish Basin, and this may be due to the very short duration of the zone which requires very close sampling to be identified.It possibly correlates in part to the RD/CaPe Zone of Lindstr€ om, Erlstr€ om et al. ( 2017) and Lindstr€ om, van de Schootbrugge et al. (2017), which also is marked by an increased abundance of Calamospora tener.Lund (1977) recognised the presence of palynofloras transitional between the Ricciisporites-Polypodiisporites Zone and the succeeding Pinuspollenites-Trachysporites Zone, which he referred to as the 'topmost upper Rhaetic' (Figure 3).This most likely corresponds to the PSD Zone at Stenlille, although the composition of assemblages from that zone are not identical to those described from Rødby-1 and Scania by Lund (1977).The succeeding DPPi and PPi zones at Stenlille can be correlated with the Pinuspollenites-Trachysporites Zone (Lund 1977), although in the Stenlille wells ornamented trilete spores such as Trachysporites asper and T. fuscus are less abundant than Deltoidospora spp.The identification of two ammonites from Rødby-1, Psiloceras cf.tilmanni and ?Nevadaphyllities sp., just prior to the top-Tilmanni CIE, corroborates the correlation (Lindstr€ om, van de Schootbrugge et al. 2017).
Palynological records from various wells and sites in the North German Basin largely follow the zonation established by Lund (1977Lund ( , 2003)), and the palynofloras there generally follow the same trends as at Stenlille (Heunisch et al. 2010;Barth et al. 2018;Gravendyck et al. 2020;Bos et al. 2023).However, in North German Basin records, in contrast to the Danish Basin records, cheirolepidiacean pollen are often more abundant than Perinopollenites elatoides, in particular after the equivalents of the PRD Zone.Recently, Gravendyck et al. (2020) published an informal palynological zonation for the Bonenburg section in NW Germany, which is supplemented by an organic C-isotope record (Schobben et al. 2019) and can be correlated to the Stenlille record.The Bonenburg record commences with an assemblage from the uppermost Postera beds that Gravendyck et al. (2020) assigned to the informal Classopollis-Enzonalasporites assemblage zone, which is dominated by Classopollis spp.and Geopollis zwolinskai but also contains rare occurrences of Enzonalasporites spp.This assemblage was correlated and named after the Classopollis Enzonalasporites Zone of Lund (1977) (Figure 3).
Assemblages from the succeeding Contorta Beds were assigned to the Rhaetipollis-Limbosporites (RL) zone, which is dominated by Ricciisporites tuberculatus and Ovalipollis ovalis, with abundant Classopollis spp.The lower boundary of the zone is marked by the appearance of Limbosporites lundbladiae, and Rhaetipollis germanicus is consistently present (Gravendyck et al. 2020).This zone was named after and correlated with the Rhaetipollis-Limbosporites Zone of Lund (1977).The RL zone can be correlated with the GCP Zone of Stenlille (Figure 3) but differs primarily in that Gravendyck et al. (2020)  ).The differences in spore-pollen abundances between these two north German localities may be related to their palaeogeographical position, and possibly also to differences in depositional setting, in that Mariental was closer to the Bohemian Massif and the Fennoscandian High, while Bonenburg was located closer to the Rhenish Massif (Barth et 3).In Bonenburg, the Triletes Beds are succeeded by the Psilonotenton Formation, and assemblages from this are assigned to the Pinuspollenites-Kraeuselisporites (PiK) zone of Gravendyck et al. (2020).The PiK zone is characterised by the incoming of common Pinuspollenites minimus and consistent occurrences of Kraeuselisporites reissingerii (Figure 3).This FO of Sciadopityspollenites thiergartii is registered at the base of the zone (Gravendyck et al. 2020).The PiK zone most likely corresponds to the DPPi Zone at Stenlille, as it, compared to Mariental, lacks aberrant Deltoidospora-type spores.However, this correlation suggests that there is a hiatus between the RP zone and the PiK zone at Bonenburg, or that this part of the section is highly condensed, similar to the closely located Schandelah-1 succession (see discussion in Lindstr€ om, van

Duration of the defined biozones
The duration of the defined palynomorph zones, the time of the identified sequence stratigraphic surfaces and the depositional time for the entire Gassum Formation were estimated based on the selected short Rhaetian scenario (Ogg et al. 2020).If the base of the Gassum Formation is taken to coincide with, or lie close to, the base of the Rhaetian, this implies that the 144 m thick formation in the Stenlille area represents a time span of c. 4.2 My.As stressed above, the individual sandstone units and also some of the associated mudstones and siltstones were most likely formed within a very brief geological time, thus highlighting that most of the 4.2 My is hidden at the SBs and TSs.Likewise, less significant internal discontinuity surfaces between the various facies and beds also represent breaks in the deposition.It is worth noting that some of the biozone boundaries occur at sequence stratigraphic surfaces.Hence, gradual changes in the palynoflora may have been masked by the hiatuses.On the other hand, the Rhaetian terrestrial vegetation appears to have been remarkably stable, with only slight abundance changes of major plant groups during most of the Rhaetian.
With these reservations in mind, the time interval represented by the biozones may be estimated as shown in Figure 6.The 64 m thick PRRh Zone in the lower part of the Gassum Formation, i.e. from below SB2 to below TS4, represents a time span of 1.34 My.The succeeding PCR Zone, covering TS4 to TS6, represents 2.52 My, while the overlying GCP Zone covering the uppermost Gassum was formed during a time span of 400 ky.The uppermost part of the GCP Zone, from the top of the Gassum Formation to SB8, is here estimated to have formed over 30 ky.In the age model, the top of the succeeding PRD Zone, which encompasses the majority of the grey siltstone beds of Lindstr€ om et al. (2012,2015) and Lindstr€ om, van de Schootbrugge et al. (2017), is located c. 1.25 m below MFS8, i.e. the TJB, which is dated as 201.36 Ma.Thus, the PRD Zone is estimated to correspond to $115 ky.The thin CCM Zone that overlies the PRD Zone was probably formed rapidly, probably in <5 ky, as its duration cannot be resolved from the age model.The succeeding PDS and DPPi zone probably encompass c. 10 ky each.The PPi

Conclusions
The Danish Basin contains a thick Triassic-Jurassic succession that has been the target of various exploration activities for decades.The distribution, stratigraphy, age, and depositional environment of the succession are therefore well documented.In particular, the Stenlille structure in the eastern part of the basin provides unique possibilities for thorough scientific studies as a very comprehensive geophysical and geological dataset has been acquired, because the Rhaetian reservoir sandstones of the Gassum Formation are here used for seasonal storage of natural gas sealed by the uppermost Rhaetian-Lower Jurassic Fjerritslev Formation.The numerous well sections with high-quality well logs and cores across the TJB have made the succession one of the key localities in NW Europe and it has, as such, become important for research on the ETME.Recently, the structure was selected for a detailed investigation, with current studies including seismic mapping, age dating, and construction of a robust sequence stratigraphic framework to evaluate the potential for permanent CO 2 storage.The present palynological study is the first comprehensive, high-resolution biostratigraphic study of the Gassum Formation and lower Fjerritslev Formation that includes data from multiple wells.The many intensively cored wells situated within a small area provide the unique possibility for a detailed study based on close sampling of a continuous succession.The 19 wells, placed both at the top of the domed structure and close to the structure, document the vertical succession as well as lateral changes in lithology.The cored intervals are $60 m in most of the wells and provide good data on both vertical and lateral variations in lithology and depositional environments, that is seldom found in natural outcrops where scree, vegetation or other obstacles hinder the construction of a complete succession.By integrating the palynological records from nine of the 19 wells it has been possible to establish a refined biostratigraphic zonation consisting of five Rhaetian zones and five Hettangian to early Sinemurian ones, based on the abundances and stratigraphic ranges of spore and pollen taxa.The spore-pollen zones are correlated with dinoflagellate cyst zones and events in the same succession and are further tied to the sequence stratigraphic scheme developed at Stenlille and tied into the basin-wide sequence stratigraphic framework based on the widely spaced well sections representing the major part of the basin.The combination of overall palynological composition, spore-pollen and dinoflagellate cyst events will enable the recognition and distinction of the specific sandstone units within the Gassum Formation and the interbedded units of mudstones and heteroliths in most of the Danish Basin.The palynological record of the Stenlille wells suggest that the base of the Gassum Formation approximately corresponds to the Norian-Rhaetian boundary, and palynological and d 13 C org events have allowed the establishment of a composite age model for the Stenlille Rhaetian-lower Sinemurian succession.Thus, it is for the first time possible to assess the duration of the individual palynozones.The overall palynology shows that although marine influences occurred already in the lower part of the Gassum Formation, i.e. in the early Rhaetian, the Danish Basin did not become fully marine until the beginning of the late Rhaetian, c. 202 Ma.The terrestrial vegetation was remarkably stable during the first 4 million years of the Rhaetian, i.e. during the deposition of most of the Gassum Formation.Changes related to the ETME are evident in the uppermost part of the Gassum Formation, where several key taxa decline markedly in numbers or disappear.The top of the Gassum Formation is associated with the Marshi CIE, a negative excursion in d 13 C org that marks the onset of the marine mass extinction, previously estimated at 201.51 Ma.Major changes in both the marine and terrestrial palynological records occur over the c. 150 ky long extinction phase in the late Rhaetian.The impact of the end-Triassic crisis on the marine phytoplankton record is effectively demonstrated by the overall palynological composition.From having been highly abundant to dominant in the late Rhaetian, marine phytoplankton virtually disappeared during the deposition of the event beds (i.e. the grey siltstone beds).Although some of the dinoflagellate cyst taxa returned after the mass extinction, their diversity and abundances remained reduced during the Hettangian and Sinemurian.In contrast, the recovery of the terrestrial vegetation appears to have been more rapid, with the three palynozones representing three successive stages in the recovery which together lasted less than 25 ky.

(
2003) with additional interpretations and integration of sedimentological core data, petrophysical log data (mainly gamma-ray motifs), three-dimensional seismic data, palynological data and provenance data (Lindstr€ om et al. 2015, 2019; Lindstr€ om 2016, 2021; Lindstr€ om, Erlstr€ om et al. 2017; Lindstr€ om, van de Schootbrugge et al. 2017; Olivarius et al. 2022).The numbering of sequences and their associated surfaces follows the sequence stratigraphic nomenclature given in Nielsen (2003).A sequence (SQ) is considered to be composed of four systems tracts sensu Hunt and Tucker (1993) and Catuneanu et al. (2009).The lowstand systems tract is characterised by a progradational to aggradational stacking pattern and topped by the transgressive surface (TS); the transgressive systems tract is characterised by a retrogradational stacking pattern and topped by the maximum flooding surface (MFS); the highstand systems tract is characterised by an aggradational to progradational stacking pattern, and the falling stage systems tract is characterised by a down-stepping progradational stacking pattern (exemplified by Nielsen and Hamberg 2000).Each SQ is based by a sequence boundary (SB) formed at the time of maximum fall in relative sea level.In the Stenlille area the Gassum Formation to lowermost Fjerritslev Formation is subdivided into 11 SQs.The basal part of the Gassum Formation, which encompasses Sequence 1 (partially) to the middle of Sequence 3 (SQ1-SQ3) has not been cored in the Stenlille wells, and lithological interpretation therefore rests on petrophysical logs and CU samples (Figure

Figure 2 .
Figure 2. Geophysical log correlation of the investigated Stenlille wells along the transect shown in Figure 1B.MD ¼ measured depth.

Figure 3 .
Figure 3. Correlation of spore-pollen zonations from the Danish Basin and the North German Basin mentioned in the text.
3.2.2.Perinopollenites-Classopollis-Limbosporites (PCL)Assemblage Zone (new) Definition.The base of this zone is marked by the FO of Limbosporites lundbladiae, and several other taxa appear to have their FOs close to the base of this zone, e.g.Kraeuselisporites reissingeri, Triancoraesporites ancorae and

3 Plate 1 .
Selected spores.The scale bar in Figure1is 20 lm and applies to all photographs.

3 Plate 2 .
Selected spores.The scale bar in Figure 1 is 20 lm and applies to all photographs. of the PRD Zone, and Granulatisporites rudis and Rhaetipollis germanicus have their Lcons in the lower part and Cingulizonates rhaeticus in the upper part of this zone (Figure 5).In the uppermost part of the zone, the pollen taxon Sciadopityspollenites thiergartii (previously Cerebropollenites thiergartii but recently transferred to Sciadopityspollenites by Gravendyck et al. 2023)the accessory marker for the TJB (Hillebrandt et al. 2013)is registered for the first time.The zone contains an abundance of reworked palynomorphs of Ordovician-Silurian, Carboniferous, and Middle-early Late Triassic age.The top of the zone is marked by a distinct decrease in Ricciisporites tuberculatus.Correlation and age.This zone correlates with theRicciisporites-Polypodiisporites Zone ofLund (1977), which is assigned a late Rhaetian age.The FO of Sciadopityspollenites thiergartii in the uppermost part of this zone indicates a latest Rhaetian age, as in the GSSP section at Kuhjoch in Austria the FO of Sciadopityspollenites thiergartii occurs 3 m below the TJB as marked by the FO of Psiloceras spelae(Hillebrandt et al. 2013).The zone is thus correlated with the unnamed interval between the Choristoceras marshi and Psiloceras tilmanni Boreal ammonite zones(Ogg et al. 2020).

3 Plate 3 .
Selected monosulcate and polyplicate pollen.The scale bar in Figure10is 20 lm and applies to all photographs.
Ma age of the TJB are constrained by U/Pb isotope ages from detrital zircons in ash beds in Peru and Nevada (Schoene et al. 2010; Wotzlaw et al. 2014) and are used as anchor points for the time estimations based on the correlation by Lindstr€ om, Erlstr€ om et al. (2017) and Lindstr€ om, van de Schootbrugge et al. (2017).The base of the Psiloceras planorbis Zone is set at 201.3 Ma according to Gradstein et al. (2020), and this is also used as an anchor point, based on the correlation of the composite Stenlille C-isotope curve with that of St. Audrie's Bay (Lindstr€ om et al. 2012; Lindstr€ om, Erlstr€ om et al. 2017; Lindstr€ om, van de Schootbrugge et al. 2017) where the top-Tilmanni or main negative CIE is identified.The LCO and LO of Dapcodinium priscum, the FO of Sciadopityspollenites macroverrucosus, and the occurrence of Liasidium variabile help to pinpoint the Hettangian-Sinemurian and early-late Sinemurian boundaries (Hesselbo, Ogg et al. 2020).In central parts of the Danish Basin, the base of the Gassum Formation is presumed to correspond more or less to the base of the Rhaetian (Nielsen 2003), and in the Stenlille-1 well it is located at 1651 m.The presence of rare D. priscum as well as Rhaetipollis germanicus in a CU sample at 1652 m may corroborate this, as D. priscum has its known first appearance datum (FAD) close to the base of the Rhaetian (Mangerud et al. 2019), and R. germanicus also has its FAD at the base of the Rhaetian (K€ urschner and Herngreen 2010).The base of the Rhaetian is placed at 205.74 Ma (Ogg et al. 2020).
Ma (Wotzlaw et al. 2014; Hesselbo, Hudson et al. 2020).Due to the lack of ammonites in the investigated Stenlille succession, the boundary is constrained by the FO of the accessory marker for the base of the Jurassic, Sciadopityspollenites thiergartii, and the Spelae CIE in the organic C-isotope record.Sciadopityspollenites thiergartii first appears c. 3 m below the FO of P. spelae in the GSSP locality at Kuhjoch in Austria (Hillebrandt et al. 2013), at the base of the Spelae CIE sensu Lindstr€ om, van de Schootbrugge et al. (2017).At Stenlille it also has its FO at the base or just below the Spelae CIE.

1 .
Comparison with previous spore-pollen zonations from the Danish Basin and the North German Basin

4. 3 .
Relation of the defined biozones to the Triassic-Jurassic extinction event Lindstr€ om (2016) divided the then informal Stenlille palynozonation from the GCP to the PPi zones into four palynofloral phases: the GCP Zone was considered to represent the pre-extinction flora, the PRD and CCM zones the extinction flora, the PDS Zone the recovery and the DPPi and PPi zones the post-recovery flora.In this study, it is possible to corroborate that the terrestrial ecosystem was remarkably stable during most of the Rhaetian, suggesting that the pre-extinction flora includes the PRRh, the PCL, and the lower part of the GCP zone.Profound changes in the palynoflora started in the upper part of the GCP Zone (i.e.uppermost part of the Gassum Formation) at a level that coincides with the Marshi CIE and the onset of the marine mass extinction.The latter was estimated to 201.564 ± 0.015 Ma byBlackburn et al. (2013).U/Pb isotopic ages of zircons from an ash layer that occurs just above the LO of the ammonoid Choristoceras crickmayi in the Pucara Basin (Peru) suggests that the onset of the mass extinction occurred close but prior to 201.51 ± 0.15 Ma (Schoene at al. 2010;Wotzlaw et al. 2014).With the TJB estimated at 201.36 ± 0.17 Ma and marking the top of the marine extinction interval(Wotzlaw et al. 2014), the marine extinction phase thus lasted $150 ky.At Stenlille the extinction or crisis flora encompasses the upper part of the GCP Zone, which includes MFS7 as well as the succeeding PRD Zone, an interval estimated to have had a duration of $145 ky following the age model herein.Lindstr€ om (2021) suggested that the increase in Calamospora tener and freshwater algae in the CCM Zone indicated increased input of fresh water at that time.This could of course be related to climate, but also to an on-going transgression which would possibly increase humidity and raise the groundwater table in coastal areas.The CCM, PSD and DPPi zones may represent the initial recovery and at least partial re-establishment of the coastal mires.The age model thus suggests that the initial recovery phase of the terrestrial ecosystem in the Danish Basin, i.e. the CCM and PSD zones, was quite fast, lasting less than 15 ky.The later part of the recovery phase encompassed the establishment and proliferation of the parent plants of Pinuspollenites minimus during the DPPi Zone, which lasted another 10 ky.The CCM, PSD and DPPi zones may together represent different successional stages of the recovery stage, which then lasted for <25 ky.The succeeding spore-pollen record at Stenlille suggests that the vegetation was quite stable during the remaining part of the Hettangian, from the PPi Zone to the base of the Sinemurian, with only slight changes in the overall composition of the flora.

Table 1 .
; Barth et al. 2018; Gravendyck et al. 2020) and is assigned an early Rhaetian age.It is here assumed to roughly correlate with the Paracochloceras suessi Boreal Ammonoid Zone (Ogg et al. 2020).Summary of the characteristics of the herein defined spore-pollen zones.FO ¼ first occurrence.FCO ¼ first common occurrence.Fcon ¼ first consistent occurrence.LO ¼ last occurrence.LCO ¼ last common occurrence.
Limbosporites lundbladiae, Ovalipollis ovalis, Lunatisporites rhaeticus and Rhaetipollis germanicus, are consistently present to common within this zone.Lycopodiacidites rhaeticus and Cryptopalynites pseudomassulae (previously Cerebropollenites pseudomassulae but recently transferred to Cryptopalynites by Gravendyck et al. 2023) appear to have their LOs within this interval.The LCO of Granuloperculatipolles rudis and the Lcons of Rhaetipollis germanicus and Corollina zwolinskai are also in this zone.
The small dinoflagellate cyst Valvaeodinium hymenosynypha (Morbey) Lindstr€ om 2023 has its LO just above SB9 (Lindstr€ om 2023).The upper boundary of the subzone is located between MFS10 and SB11 and marked by the LCO of D. priscum.Dapcodinium priscum subzone (c) is characterised of rare to low occurrences of D. priscum in its lower part, but it is absent in the uppermost part of the subzone, prior to MFS 11.The upper boundary of the subzone and the top of the zone is marked by the FO of Liasidium variabile.
The FO of the acritarch Leiofusa jurassica in the lower part of subzone (b) is interesting.In St. Audrie's Bay in the UK, this taxon is first registered in the upper part of the P. tilmanni Zone (van de Schootbrugge et al. 2007).
; Guy-Ohlson 1981; Dybkjaer 1988, 1991; Koppelhus and Batten 1996; Lindstr€ om and Erlstr€ om 2006; Larsson 2009; Lindstr€ om, Erlstr€ om et al. 2017).Differences in abundance of various plant groups between sites are most likely attributable to variations in depositional setting such as distance to vegetated land areas, position relative to fluvial input, prevailing wind directions, etc. Cheirolepidacean pollen (Classopollis) are much more abundant at Stenlille than at the Scanian localities (Lindstr€ om and Erlstr€ om 2006; Lindstr€ om, Erlstr€ om et al. 2017 based on the abundance of Polypodiisporites polymicroforatus.Previous publications have shown that the common to abundant occurrence of P. polymicroforatus is typical of the so-called event beds of the ETME, such as the Boserup beds in Scania and the grey siltstone beds at Stenlille (Lindstr€ om et al. 2012; Lindstr€ om 2016, 2021; Lindstr€ om, Erlstr€ om et al. 2017; Lindstr€ om, van de Schootbrugge et al. 2017).Some taxa, e.g.Semiretisporis gothae, occur more consistently throughout the PRD Zone, further indicating correlation with the Ricciisporites-Polypodiisporites Zone as recorded only very few Perinopollenites elatoides in the upper part of their RL zone at Bonenburg.The RL zone is generally associated with d 13 C org values around À27.0‰, with a few negative peaks reaching below À28.0‰ (Schobben et al. 2019).Age-equivalent assemblages from the cored Mariental-1 borehole $155 km to the NE are dominated by Classopollis and Ricciisporites together with consistently common Ovalipollis, but also abundant to common Perinopollenites in the Contorta Schichten of the Exter Formation (Heunisch et al. 2010; Barth et al. 2018).These assemblages from Mariental-1 were assigned to the Rhaetipollis Limbosporites Zone of Lund (1977) (Heunisch et al. 2010), and are associated with d 13 C org values around À26.0‰ (van de Schootbrugge et al. 2013 (Heunisch et al. 2010;Barth et al. 2018;Gravendyck et al. 2020;Bos et al. 2023lille, there is an increase in Deltoidospora-type spores and a decrease in Classopollis and R. tuberculatus in the uppermost sample of the Rhaetipollis Limbosporites Zone in Mariental-1(Heunisch et al. 2010;Barth et al. 2018).In Schandelah-1 the decrease in Classopollis in the uppermost RLi zone, equivalent to the Rhaetipollis Limbosporites Zone, is also evident(Bos et al. 2023).The Triletes Beds in Bonenburg and Mariental-1, as well as in Schandelah-1, are assigned to equivalents of the Ricciisporites-Polypodiisporites Zone ofLund (1977)(Heunisch et al. 2010;Barth et al. 2018;Gravendyck et al. 2020;Bos et al. 2023) and can be correlated with the PRD Zone of Stenlille (Figure3).At all four localities this interval is associated with more positive d13C org values (Lindstr€ om et al. 2012; van de Schootbrugge et al. 2013; Lindstr€ om, Erlstr€ om et al. 2017; Lindstr€ om, van de Schootbrugge et al. 2017; Schobben et al. 2019; Bos et al. 2023) compared to the previous zone.Again, there appear to be some local differences in the palynofloras.As discussed by Lindstr€ om, van de Schootbrugge et al. (2017), R. tuberculatus declines markedly in abundance in the lower part of the Ricciisporites-Polypodiisporites Zone in Mariental-1, while it is abundant to dominant in the PRD Zone in Stenlille.In the Bonenburg record, R. tuberculatus is abundant to dominant within the RP zone of Gravendyck et al. (2020), and this is also the case in Schandelah-1 (Bos et al. 2023).In all three records P. polymicroforatus is an abundant to dominant constituent of the zone, together with laevigate triangular trilete spores assigned to Deltoidospora or Concavisporites.Similar to the grey siltstone beds of the Stenlille succession, the Triletes Beds were deposited during a regressive event, and in Mariental-1 there is a sharp decrease in Polypodiisporites polymicroforatus at the top of the Triletes Beds, which may represent an SB (Lindstr€ om, van de Schootbrugge et al. 2017).The organic C-isotope values turn more negative at the top of the Triletes Beds, but even more so in the lower Lias a1 beds (van de Schootbrugge et al. 2013).The lowermost 1.5 m of the succeeding Lias a1 beds, originally called the Transition Zone by Heunisch et al. (2010), was assigned to the Deltoidospora-Concavisporites (DC) Zone by Barth et al. (2018).The DC Zone is characterised by high abundances of laevigate trilete spores assigned primarily to the nominate genera, and also contains abundant aberrant spores (Barth et al. 2018).Correlation suggests that the DC Zone of Mariental-1 corresponds to the PDS Zone in the Stenlille record, but the former appears to be condensed in comparison to the latter or, alternatively, part of the DZ Zone is missing.The DC Zone in Mariental-1 is succeeded by assemblages assigned to the Pinuspollenites-Trachysporites Zone of Lund (1977) (Heunisch et al. 2010; Barth et al. 2018), which in its lowermost part is characterised by a dominance of Deltoidospora, Classopollis spp., Perinopollenites elatoides and Pinuspollenites minimus.This most likely corresponds to the DPPi Zone in the Stenlille record (Figure de Schootbrugge et al. 2017; van de Schootbrugge et al. 2019; Bos et al. 2023).