Marine transgression(s) to evaporite basin: The case of middle Miocene (Badenian) gypsum in the Central Paratethys, SE Poland

The middle Miocene Badenian evaporite basin of the Carpathian Foredeep Basin was a saline lake, separated by a barrier from the sea and supplied with seawater seeping through the barrier or overflowing it occasionally in the form of short-lived marine transgressions. Such transgressions could leave behind marine microfossils in marly clay intercalations. One of them (2.3 m thick) occurs in the uppermost part of the sulphate sequence, in the unit ‘o’, in the Babczyn 2 borehole section. It contains marine palynomorphs (dinoflagellate cysts) and foraminiferal assemblages indicating a marine environment. The low-diversity benthic foraminiferal assemblages are dominated by opportunistic, shallow infaunally living species, preferring muddy or clayey substrate for thriving, brackish to normal marine salinity, and inner shelf environment. Dinoflagellate cyst assemblages, although taxonomically impoverished, consist of marine species; euryhaline forms that tolerate increased salinity are missing. Relatively common microfossils found in clay intercalations within gypsum have important palaeogeographical implications: they strongly suggest that there existed an additional inflow channel supplying the Polish Carpathian Basin from the south during the evaporite deposition and afterwards.


Introduction
The Paratethys was an epicontinental sea that developed as a relic of the Tethys. It existed between early Oligocene and late middle Miocene times and constituted a system of marine basins extending from the Alpine-Carpathian region to the modern Aral Sea during Oligocene to Miocene times. They were periodically connected with the Mediterranean, Indo-Pacific and Atlantic oceans (Rögl 1998). The Paratethys is divided into three segments: Western, Central and Eastern, and the regional time scale, based mostly on endemic fossil assemblages, is applied for particular parts of the Paratethys (Fig. 1). Water circulation in the Paratethys was strongly controlled by two shallow and narrow gateways: the Slovenian (Trans-Tethyan Trench) corridor (Rögl 1998;Kováč et al. 2017) and the Bârlad Strait in Romania (Palcu et al. 2015) connecting the Central and Eastern Paratethys. Both gateways were located in tectonically active regions (Simon et al. 2019) and controlled the recorded palaeoenvironmental changes in these basins, although the exact mechanisms are still poorly understood (Palcu et al. 2017). At 13.8 Ma, the global climate cooled, triggering a glaciation, which led to a global sea-level fall of~50-70 m (John et al. 2011;Simon et al. 2019), which drastically reduced the Trans-Tethyan Strait to a very shallow depth and disconnected the two Paratethyan basins (Fig. 2). The Badenian Salinity Crises ended before 13.32 ± 0.07 Ma (de Leeuw et al. 2018). A transgression that re-installed normal marine conditions in the Carpathian Foredeep Basin resulted from reconnection of the basin with the Mediterranean and Eastern Paratethys, primarily by tectonic modification of the interconnecting gateways, i.e. tectonic lowering of the barriers between the Central Paratethys and the neighbouring basins (de Leeuw et al. 2018).
The system of middle Miocene Badenian evaporite basins of the Central Paratethys (Fig. 2), which started to form at some 13.81 Ma, gained occasional connections with the Mediterranean marine reservoir through the Slovenian (Trans-Tethyan Trench) corridor or with the Eastern Paratethys (low-salinity water reservoir - Palcu et al. 2017) through the Bârlad Strait. Barred evaporite basins are sourced mostly by seepage (Kendall 2010;Warren 2016). When overflow happens, then depending on the volume of seawater inflow and the overflow duration, the brines of barred basin may be eventually replaced by seawater. One eminent connection of barred evaporite basin of the Carpathian Foredeep Basin with an open-sea reservoir was previously recognized in the middle part of the gypsum sequence of southern Poland where a marly clay intercalation sandwiched between the gypsum contains benthic and planktonic foraminifera indicating a short-lived inflow of marine water (Peryt 2013a ; Fig. 2). The foraminifer-bearing clay intercalation in southern Poland occurs in the middle of the upper part of the gypsum sequence, i.e. at the same stratigraphic position as other foraminifer-bearing clay intercalations in the gypsum sequence, which were recorded by Odrzywolska-Bieńkowa (1975) in SE Poland (cf. Pawłowski et al. 1985;Kos in Kasprzyk 1989;Kubica 1992). In this paper, we report the foraminiferal and dinoflagellate cysts assemblages from the clay intercalation of the topmost part of the gypsum sequence in the Babczyn 2 borehole (SE Poland; Fig. 2). These assemblages indicate marine invasion into the Central Paratethys prior to the termination of the evaporite stage that occurred 13.36 Ma when the Central Paratethys returned to open-marine conditions, and the Eastern Paratethys remained more brackish (Simon et al. 2019).

Geological setting
Badenian sulphate deposits occur in most of the northern and northeastern Carpathian Foredeep Basin from Moravia in Czech Republic in the west through southern Poland and western Ukraine to the Ukrainian-Romanian borderland and Moldova (Peryt 2006(Peryt , 2013b. Sulphate deposits in southeastern Poland consist of a sequence of lithofacies grouped into 18 lithostratigraphic units (from 'a' to 'r'; Kasprzyk 1993) or seven lithosomes (from 'A' to 'G'; Kubica 1992Kubica , 1994Bąbel 2005). In SE Poland, in the Lubaczów area located close to the Polish-Ukrainian border (Fig. 2), the primary gypsum is partly or completely replaced by anhydrite and secondary gypsum (Kasprzyk 1995). In the Cieszanów 1 borehole, units 'a'-'o' have a total thickness of 44.8 m (Kasprzyk 1995), and in the Babczyn 2 borehole the gypsum sequence (32 m thick, depth 409-441 m) consists of units 'a'-'l' and 'm'-'o' (the upper part of the sequence is shown in Fig.  2, after Peryt and Kasprzyk 1992). Unit 'm' is composed  and Hilgen et al. 2012, updated by Palcu et al. 2015 and the occurrence of microfossils-bearing clay intercalation (arrowed) in the Babczyn 2 borehole section. Numbers right of the borehole section indicate the depth (in metres); allochth. g. u. = Autochthonous gypsum unit of laminated and bedded microcrystalline gypsum that contains layers and irregular bodies of gypsiferous, occasionally brecciated, microbial-peloidal carbonate, while unit 'n' consists of thinly laminated gypsum with three beds of carbonate-gypsum breccias in the middle part. Unit 'o' is nodular gypsum intercalated by a 2.3 m layer of marly clays (depth: 413-415.3 m) rich in plant remains and carbonate lithoclasts (Pawłowska and Kubica: Karta otworu wiertniczego Babczyn 2, unpublished) as well as crushed mollusc (Modiolus?) shell fragments (Śliwiński et al. 2012). Interpretation by Peryt and Kasprzyk (1992) assumed that, following a sea-level drop after the deposition of unit 'n', very shallow subaqueous and subaerial conditions prevailed during deposition of  Peryt and Jasionowski 1994) and Babczyn 2 borehole (middle and upper parts, after Peryt and Kasprzyk 1992; lower part is not shown); black arrows show the intercalations of microfossils-bearing clays described by Peryt (2013a) and this paper, respectively; alabastrine gypsumalabastrine gypsum originated due to recrystallization of finegrained gypsum precipitated in association with microbial mats (cf. Orti Cabo et al. 1984) and then transported during storms (cf. Kwiatkowski 1972); sabre gypsumgypsum type composed of curved gypsum crystals resembling sabres which grew upwards and simultaneously curved laterally, thus most crystals are inclined and curved in the same direction (Bąbel 1986 unit 'o' (Peryt and Kasprzyk 1992). Accordingly, this marly clay is younger than unit 'ł' in which Peryt (2013a), in the Borków quarry section (Fig. 2), recorded previously the benthic foraminifer assemblages composed of pioneer, opportunistic, r-selected species dominated by elphidiids.
The samples for the palynological study were processed following the procedure including 38% hydrochloric acid (HCl) dissolution, 40% hydrofluoric acid (HF) dissolution, heavy liquid (ZnCl 2 + HCl; density 2.0 g/cm 3 ) separation, and sieving at 15 μm on a nylon mesh in ultrasounds for 10-15 s. No oxidation in nitric acid (HNO 3 , fuming) was applied. The quantity of each rock sample processed was 10 g. Two slides from each sample were made using glycerine jelly as a mounting medium. Dinoflagellate cysts were qualitatively determined up to 300 specimens using a Zeiss Axiolab microscope; the remaining specimens were scanned for their taxonomy only. Palynofacies for each sample were calculated based on 500 particles. The unprocessed rock samples, palynological residues and slides are stored in the collection of the Institute of Geological Sciences, Polish Academy of Sciences, Research Centre in Kraków.

Results
Low-diversity foraminiferal assemblages are recorded in the interval studied (Table 1; Figs. 3, 4 and 5). Their preservation is poor to moderate. Foraminiferal tests are mostly recrystallized and often broken. The benthic foraminiferal assemblages consist entirely of calcareous species of very small and tiny tests. They are dominated by 3 taxa, i.e. Nonion parvus, Haynesina depressula and Ammonia spp. (mainly A. parkinsoniana; also common transitional forms from parkinsoniana to tepida and A. tepida). They form 60% to 80% of the assemblages (Fig. 6). Elphidium spp. is also a common and persistent contributor to the assemblages (6%-20%). Minor components are Rosalina obtusa and hauerinids (Quinqueloculina sp., Triloculina sp., Pseudotriloculina consobrina, and Sigmoilinita tenuis) forming up to 5% of the assemblages. Nonion parvus and Ammonia spp. fluctuate significantly throughout the interval. Relative abundance of Ammonia spp. varies between 6% and 62%; Nonion parvus accounts for 9% to 64%. Foraminiferal assemblages from samples 2 and 4 represent near-monospecific faunas (Hayward 2014). In sample 2, Ammonia spp. makes 62% of the assemblage; other components do not exceed 10% each. In sample 4, Nonion parvus makes up 64% of the assemblage, while the contribution of Ammonia spp. drops to 6% (Fig. 6).
The 2.3-m-thick layer of marly clays yielded high amounts of palynological organic matter composed chiefly of terrestrial elementsmainly dark brown and black phytoclasts and pollen grains, the proportions of which oscillate between 80% and 90%. Marine elements that are present throughout the whole intervaldinoflagellate cysts, other algae and subordinate foraminifera organic linings, are up to a few per cent. Amorphous organic matter (AOM) is also present in the whole interval; except for sample 3, where it does not exceed 10% (Fig. 6).
Dinoflagellate cysts are well to moderately preserved, being all light-coloured (Table 1; Figs. 7,8 and 9). Their assemblages are dominated by three taxa: Spiniferites (mainly S. ramosus), Operculodinium and Systematophora placacantha (Table 2). Proportions of these taxa fluctuate slightly, but generally Spiniferites is most common in all these samples; Systematophora placacantha is most frequent in basal samples, and becomes rarer in the upper part of the interval. They are accompanied by subordinate Lingulodinium machaerophorum, Pentadinium laticinctum, Labyrinthodinium truncatum, Melitasphaeridium choanophorum and Pyxidiniopsis. A characteristic feature of the dinoflagellate cyst assemblages is almost complete lack of peridinioids. There are no Palaeocystodinium (known from older Badenian strata of the Carpathian Foredeep). Rare specimens of Lejeunecysta may be reworked.

Interpretation
The presence of marine palynomorphs (dinoflagellate cysts) and foraminifera suggests a marine environment during deposition of the interval in question. Interestingly, the material contains very rare Leiosphaeridia, a prasinophycean genus (see e.g. Guy-Ohlson 1996) that commonly occurs in the Miocene of the Carpathian Foredeep, accumulated in stress environments associated with increased salinity (e.g. Gedl 2016). Moreover, the dinoflagellate cyst assemblages lack Polysphaeridium, another indicator of increased (e.g. Bradford and Wall 1984) or decreased salinity (Edwards and Andrle 1992).
Nonionids, represented in the material by Nonion parvus and Haynesina depressula, are infaunal organisms, widespread in marginal marine environments, common in sediments with highly variable mud and total organic carbon (TOC) contents (Murray 1968(Murray , 2006Alve and Murray 1999;Vaniček et al. 2000). Haynesina tolerates lower salinity (0-30‰), while Nonion prefers normal marine salinity (Murray 1991). Ammonia, an opportunistic genus widespread in marginal marine environments worldwide (Walton and Sloan 1990;Murray 1991Murray , 2006Buzas-Stephens et al. 2003), is common in sediments with highly variable mud and TOC contents and able to tolerate low oxygen for several days (Moodley and Hess 1992;Murray 2006). The genus is represented, in the material, by A. parkinsoniana and A. tepida with common transitional forms. Their environmental requirements are quite well-recognized, which enables to interpret environmental changes. The two speciescommon to abundant in the foraminiferal assemblagesare well adapted to salinitystressed conditions (Fiorini 2004).
Assemblages dominated by the two species are characteristic of environments strongly influenced by fluvial waters. A. tepida is a typical species of river-dominated shelves, benefited from a noticeable input of riverine phytodetritus to the sediment and the most tolerant for high organic matter supply (Debeney et al. 2009). The significant changes in relative abundance in the Ammonia spp. group throughout the interval may indicate changes in salinity and oxygenation of bottom waters.
The benthic foraminiferal assemblages in the interval are dominated by a few infaunal species preferring muddy or clayey substrate for living, brackish to normal marine salinity, and inner shelf depths. Planktonic foraminifera recorded in the material are indicators of shallow and cool water (Bicchi et al. 2003). However, their scarcity in the material studied may indicate that they did not live there but they were transported by waves from deeper part of the sea. Shallow, cool water with slightly lowered oxygenation at the bottom and a slightly hyposaline to normal salinity marine environment is thus concluded for the foraminiferal assemblage from the marly intercalation in the Badenian gypsum.
No indications of salinity fluctuations have been recorded throughout the interval. Although the predominance of terrestrial elements in the composition of dinoflagellate cysts points to an intense influx from neighbouring land, pollen grains could be airborne. Higher proportions of cuticles (samples from the upper part of succession), especially large-sized, can be associated with source proximity and/or increased intensity of terrestrial input. However, freshwater algae occur in none of these samples. Lack of peridinioids commonly associated with nutrient-rich estuarian or upwelling environments additionally excludes freshwater input and points rather at oligotrophic conditions in the water column (photic zone). Table 2 Distribution of dinoflagellate cysts and acritarchs (asterisked are reworked species). x -Appearance of a taxon beyond the count. Total count includes taxa presumed to be in situ only

Discussion
As the Badenian evaporite basin was located in a depression in which the brine top level occurred below the contemporaneous sea level, it could be subject to rapid flooding when the sea level rose or when the physical barrier blocking this basin from the Tethys/Paratethys reservoir was temporarily removed (Peryt 2006). The connection of the Carpathian Foredeep Basin with the marine reservoir was short-lived. After such environmental change, benthic foraminifers started to colonize a new, previously defaunated niche, and the pattern of benthic foraminiferal colonization is similar in each case to that related to the reflooding terminating the Badenian Salinity Crisis (cf. Szczechura 2000; Peryt and Peryt 2009;Gedl and Peryt 2011). This refers in particular to the poorly diversified, small and strongly abraded foraminifera occurring just above the gypsum in the Jamnica S-119 borehole section (located some 100 km WNW of the Babczyn 2 borehole) and described by Szczechura (2000). She concluded that a dominance of infaunal elements in the assemblage, as well as the state of preservation of the tests and their low diversity, indicates oxygen-deficient conditions and/or insufficient CaCO 3 saturation. Peryt et al. (2004) recorded an impoverished foraminiferal assemblage dominated by Nonion and Haynesina from siliciclastic deposits, overlying gypsum deposits, of the Ptashkino section (East Crimea, Ukraine). They interpreted this assemblage as typical for a shallow water marsh or lagoon environment with a decreased salinity. Thus, it seems that the foraminiferal assemblage of the Babczyn 2 borehole represents a typical late Badenian pioneer, colonizing assemblage. Palynofacies and dinocysts from Jamnica S-119 borehole (Gedl 1999) and Babczyn 2 borehole also show similarities. In Jamnica S-119 borehole, continental elements dominate the assemblages coming from the lower part, and marine elements (dinocysts) start to be more common above it. The dinocysts assemblages are also similar in both sectionsthey are characterized by lack of Polysphaeridium. Leiosphaeridia appears in Jamnica S-119 borehole in the sample from the depth interval of 257-258 m, but its more common occurrence was noted in the sample from the depth of 255-256 m (Gedl 1999), which is a counterpart of samples from the upper part of the Babczyn 2 borehole clay intercalation in gypsum.
The marly clay intercalation of the Babczyn 2 borehole intercalation, containing crushed mollusc (Modiolus?) shell fragments, was previously interpreted by Śliwiński et al. (2012) as karst cavity infilling. The middle Miocene karstification of Badenian gypsum deposits of the Carpathian Foredeep occurred in two phases: soon after gypsum deposition and then after deposition of the lower series of the Pecten clays with Neobulimina longa (see Osmólski 1976, with references therein). However, there is no proof that the marly clays in the uppermost part of the gypsum sequence of the Babczyn 2 borehole represent karst cavity infilling. The composition of the micropalaeontological assemblages differs both from those recorded in the strata overlying gypsum deposits at Shchyrets (Peryt et al. 2014) and from the assemblages of the Pecten beds in the Babczyn 2 borehole.
It remains enigmatic, however, why no such intercalations of marine clays have been reported from elsewhere. One possible explanation is that the upper parts of the gypsum sequence have been eroded prior to the onset of deposition of the Late Badenian Ratyn Limestone and/or marine clays. In the final stages of the Badenian Salinity Crisis, gypsum beds that previously accumulated along the basin margins became then exposed, leading to reworking of clastic gypsum into the deeper basin (Kasprzyk 1993;Peryt 1996Peryt , 2000de Leeuw et al. 2018). As the result, the marine facies of the Ratyn Limestone in western Ukraine overlies various gypsum units (even the equivalent of unit 'e' in southern Polandsee Peryt 1996: Fig. 4). Consequently, it is probable that the intercalations of marine clays escaped from the erosion in few cases only (e.g., clays deposited in local depressions). In addition, the Pecten Beds bear intercalations of clastic gypsum and this indicates a partial erosion of the marginal gypsum before and during deposition of these strata (Śliwiński et al. 2012).

Implications
The marine salina model is successfully applied for the Fore-Carpathian Basin (Peryt 2001(Peryt , 2006(Peryt , 2013bBąbel 2004Bąbel , 2007Cendón et al. 2004). It assumes that the Badenian evaporite basin was located in a depression in which the brine top level occurred below the contemporaneous sea level, and it was supplied with seawater seeping through a barrier that separated the basin from the sea or overflowing it occasionally in the form of short-lived marine transgressions. Consequently, it was in fact a saline lake supplied with marine water (Bąbel 2007) as far as the Polish and Ukrainian parts of the basin are concerned. Sedimentary history of the Badenian gypsum suggests two major salinity rises (and hence two saline cyclothems were recognized in the Carpathian Foredeep Basinsee Bąbel 1999b, with references therein) and several marine influxes causing partial reworking and/or dissolution of pre-existing gypsum (Kasprzyk 1999, with discussion therein).
As discussed by de Leeuw et al. (2018), global sealevel generally remained low during the Badenian Salinity Crisis although some of the modelled short-term global highstands may have temporarily increased the connectivity of the Central Paratethys with the global ocean, leading to a decrease in its salinity as indicated by the short-lived return of normal marine foraminifera in the Borków section (Peryt 2013a). Accordingly, during eustatic sea-level rise or tectonically triggered modification of the barrier between the open sea and the Carpathian Foredeep Basin, new marine water could enter this depression and bring with it a temporary pulse of marine fauna (Peryt 2001). However, this not necessarily led to the origin of microfauna-bearing strata, as described by Peryt (2013a) and in this paper, or to thin discontinuous lenticular limestone characterized by Peryt (2001), which is equivalent to unit 'h' in southern Poland (Kasprzyk 1993) and the marker bed 2 in western Ukraine (Peryt 2001). The unit clearly indicates considerable brine dilution and is mostly developed as clastic, laminated and redeposited gypsum forming an intercalation within a sabre gypsum unit. In western Ukraine the intercalation is passing shoreward into thin lenticular marine limestone indicating that this brine-dilution was not triggered by a climatically controlled influx of meteoric waters which have earlier caused the basinwide changes in salinity in most of the Carpathian Foredeep Basin, making possible the successful application of the event-stratigraphy methods (Bąbel 1999a(Bąbel , 1999b(Bąbel , 2005Bąbel and Bogucki 2007;Peryt 2001Peryt , 2006. On the other hand, given the size of the Carpathian Foredeep Basin that is the largest European foredeep basin (see Oszczypko et al. 2006), no wonder that there are differences in the development of the gypsum facies between the western and eastern parts of the foredeep (Peryt et al. 1997;Peryt 2013b;Wysocka et al. 2016). In the westernmost part of the Badenian evaporite basin, in Moravia, clay intercalations and large pore structures between the selenite crystals occur (Peryt et al. 1997). The majority of the Moravian gypsum sequence is interpreted as originated in deeper, density-stratified waters, with a seawater layer present, as evidenced by the occurrence of an assemblage of Serravalian planktonic foraminifers in many parts of the gypsum sequence (Peryt et al. 1997 : Fig. 7). The water circulation pattern in the Badenian evaporite basin of the Central Paratethys was quite complex: antiestuarine brine circulation is assumed for the Early and mid-Badenian (Báldi 2006), including the time interval of evaporite deposition (Bąbel and Becker 2006). In the Late Badenian, a reversal of circulation to estuarine type occurred (Báldi 2006). Relatively common microfossils found in clay intercalations within gypsum along the northern Carpathian Foredeep, in particular in Moravia, strongly suggest that there existed, as compared to the current reconstruction, an additional inflow channel. This channel was supplying the Polish Carpathian Basin from the south during the evaporite deposition and probably in the 300 kyr following, during the generally low global sea-level (de Leeuw et al. 2018). 8 Conclusions 1) Short-lived temporary connections between the Central Paratethys and the Mediterranean existed during the middle Miocene Badenian Salinity Crisis. 2) This was facilitated by the palaeogeographical context, in particular by the location of the Badenian evaporite basin below the contemporaneous sea-level, and by the geotectonical context. 3) One of such connections left behind marine microfossils in a marly clay intercalation in the uppermost part of the gypsum sequence in SE Poland. 4) Both foraminiferal and dinoflagellate assemblages indicate brackish to normal marine salinity at inner shelf depths; however, dinoflagellate cyst assemblages that tolerate increased salinity are missing. 5) Marine palynomorphs (dinoflagellate cysts) and foraminiferal assemblages indicate a marine depositional environment of marly clay intercalation within gypsum sequence. 6) At the same time they strongly suggest that there existed additional, compared to the assumed so far, channels for marine organisms migration.
Abbreviations AOM: Amorphous organic matter; TOC: Total organic carbon