Miocene Slănic Tuff, Eastern Carpathians, Romania, in the Context of Badenian Salinity Crisis

Abstract

New geochronological investigations for the Slănic Formation, correlated with previous bio- and lithostratigaphical information, allow for a better succession of events for the Middle Miocene, including the absolute age of the Badenian salinity crisis in the bend sector of the Eastern Carpathians. Within the green Slănic Tuff, white tuff layers were in evidence. The main element distribution of the white and green tuffs indicates a dacitic composition, with higher SiO₂ content for the white tuff. The white tuff has a distinct mineralogical composition with quartz, plagioclase, biotite and clinoptilolite. From such a tuff layer a biotite concentrate gives a ⁴⁰Ar/³⁹Ar age of 13.7 ± 0.2 Ma. As above these tuff layers discrete levels of gypsum occur, the age documents the beginning of the restrictive circulation and formation of evaporites in this sector of Carpathians during Badenian times.

1. Introduction

Miocene tuffs of calc-alkaline composition are widespread in the Carpathians, Pannonian and Eastern Alpine realm. Their occurrences are described in outcrops as well as in the subsurface. The presence of tuffs may offer important criteria for stratigraphic correlations allowing the determination of the absolute age of deposits and associated climatic and environmental changes. For example, Miocene tuffs have been described for the Transylvania Basin [1,2,3], the Getic basin [4], the Maramureş Basin [5,6], the Gutâi Mts. [7], the Apuseni Mts. [8], the Abrămuț Basin (part of the eastern Pannonian basin) [9], the Beregovo and northern Trans-Tisza regions [10] and the Styrian Basin [11,12].

In contrast, in the outer Carpathian arc, the occurrences of Badenian tuffs are scarce in comparison with the inner Carpathian areas. For example, investigations have been done for the Slănic Formation in the foreland of the Eastern Carpathians [13,14].

In this study, we integrated previous bio- and lithostratigraphical information with new mineralogical, geochemical and geochronological data for the Slănic Formation, which is situated in the foreland of the Eastern Carpathians. The aim is to correlate the position of this tuff with other Miocene occurrences situated in the Carpathian area in order to get a better regional geochronological image regarding the position of tuffs with respect to the Badenian salinity event. Finally, the Carpathian events are correlated with some of the worldwide-established biostratigraphical events.

2. Geological Setting

The Slănic tuff deposits are part of Mid Miocene (Badenian—mid Sarmatian) post-tectonic succession (mollase deposits) of the Tarcău Nappe belonging to the Outer Flysch Zone of the Eastern Carpathians known as the Moldavides [15] (Figure 1A). The Badenian succession overlies the Burdigalian Doftana Formation [16] and is represented by marls, green tuffs, evaporites and shales [17,18,19] (Figure 2).

The tuff deposits are exposed at the Green Stone Hill (Muntele Piatra Verde) situated in the northern flank of Slănic syncline (Figure 1B), which is well known for the diapiric salt located in the central part. There, the salt is around 500 m in thickness and is actively mined just below the Slănic locality. The Green Stone Hill outcrop section has been earlier described by [23,24,25]. The outcrop consists of (a) Slănic Formation with Globigerina Marls followed by the Slănic green tuff. Within the Slănic tuff discrete layers of gypsum and white tuffs rich in biotite occur (Figure 2). The tuffs are covered by the (b) Evaporitic Formation comprising salt breccia, massive halite and gypsum as continuous layers or lenses. The evaporites are covered by the Radiolarian Shales Formation, which contains rich radiolarian assemblages with Coenosphaera, Dictyocoryne, Halicapsa, Rhopalodictyum, Sethocapsa and Spongodiscus [26]. The succession ends with the Spirialis Marl Formation. The stratigraphic age of this is indicated by the Globorotalia bykovae and Orbulina suturalis (Mid Badenian) and Globoturborotalia druryi and Globorotalia transsylvanica and Velapetina (Late Badenian) Foraminera Zones [27,28].

The Globigerina Marls are characterized by an association of warm water planktonic foraminifera (Praeorbulina glomerosa, P. circularis, Orbulina suturalis, Globigerinoides triloba, G. sacculifer, Paragloborotalia mayeri, Globorotalia bykovae etc.). This forams association indicates the presences of the Globorotalia bykovae and Orbulina suturalis Planktonic Foraminifera Zone (PFZ) [27,28,32].

Just below where the tuff deposits started, within a reddish marls intercalation, the first appearance of the tropical warm species Globoturborotalia druryi was recorded, together with the first occurrence of the endemic species Globorotalia transsylvanica [27,28]. The first appearance of G. druryi in Paratethys marks the base of the Wielician substage [17,27,28,32]. G. transsylvanica is an endemic species in the Paratethys and a common Wielician species. Moreover, these two forams are used to define Globoturborotalia druryi and Globorotalia transsylvanica PFZ [27,28]. The nanofloral assemblages observed in the lower part of the Slănic Formation have been assigned to the NN5 Calcareous Nanoplankton Zone (CNZ) of [34], based on the co-occurrence of Sphenolithus heteromorphus, Helicosphaera walbersdorfensis, H. waltrans and H. wallichii. Based on the calcareous nanofossil association, the upper part of Slănic Formation, where the dominant lithology is represented by tuffs and was assigned by [25] to the NN6. Up to now, for this outcrop, the LO of Sphenolithus heteromorphus is not precisely determined [25] in respect to the foraminifer zonation, which was determined by [17,27,28,32]. Considering also the diachronous occurrence of Sphenolithus heteromorphus in the Atlantic and Mediterranean realms, the dated sample is positioned in Figure 2 to the foraminifer zonation of [32]. The uppermost part of the Slănic Formation is rich in foraminifera, the warm water planktonic foraminifera being replaced with a colder water assemblage (Globigerina bulloides, G. concinna, Globigerinita uvula etc.). Unlike the lower part of the Slănic Formation almost devoid of benthonic forminifera, from this interval a rich typical Wielician type association with Pseudotriplasia minuta, Uvigerina orbignyana and other rather deep water species (Glandulina laevigata, Sphaeroidina bulloides, Melonis pompilioides etc.) was recorded [27]. For the Evaporite Formation was not described any foraminifera or calcareous nanoplankton associations.

Based on the planktonic foraminifera association, the Radiolarian Shales Formation (containing G. transsylvanica, G. druryi, G. bykovae, P. mayeri, G. triloba, G. bulloides, P. glomerosa, P. sicana, O. suturalis, Globoturborotalita woodi, Globigerina cf. tarchanensi) is assigned to Globoturborotalia druryi & Globorotalia transsylvanica PFZ. The calcareous nanofossil association indicated the presence of NN6 [25] fact also supported by the absolute age of a tuff intercalation dated by [33] at 13.45 ± 0.06 Ma (Figure 2).

The last formation outcropping in Green Stone Hill is the Spirialis Formation. The foraminifera association was described contain different species of Velapertina genus (e.g., V. indigena, V. luczkowskae) and indicated by the presence of Kossovian substage (upper part of Late Badenian, Velapertina Foraminifera Range Zone after [27,28]) (Figure 2). The calcareous nanoplankton associations indicate for this interval the NN6 [13].

3. Material and Methods

3.1. XRD Investigations

Qualitative and quantitative mineralogical analysis was performed on two samples collected from the Green Stone Hill outcrop. X-ray diffraction patterns were measured on a Bruker AXS D8 diffractometer equipped with a one-dimensional strip-detector (lynx-eye) using a Cu-kα (40 kV, 40 mA, sample rotation). The measuring conditions were: step-size 0.02°, 2 sec/step (equivalent to 304 sec/steps with a conventional point-detector). Mineralogical compositions are given in Figure 3.

3.2. Major Elements

Two rock samples, one from the green and the other from the white tuff layers, were crushed and afterwards milled in a McCrone micronizing mill. Adsorbed water was removed at 110 °C and the loss of ignition was determined in platinum crucible at 950 °C. Half gram of the sample was mixed with 2.5 g lithium tetraborate and 1 g ammonium nitrate as oxidizer. This mixture was thoroughly homogenized in an agate mortar and molten at 1100 °C. The resulting melt was cast in a moult with a flat bottom and solidified under a cold air stream. A fragment of this glass disc was embedded in epoxy resin and the surface was polished afterwards with diamond abrasives down to 1 micron. Quantitative analytics were performed on a Jeol 6610 LV scanning electron microscope. Mg, Al, Si, K, Ca, Ti and Fe were measured with an Oxford 50 mm² energy dispersive detector (carbon coating, 20 kV, 20 nA). In order to minimize beam damage, as well as to increase the measurements accuracy, the beam was set on a rectangle of 90 × 120 µm. Mineral standards were used for internal calibration. Data reduction was performed with the PAP routine [35] implemented in the INCA software of Oxford Instruments. Sodium was separately determined with an Oxford wavelength dispersive system. The main element distribution is given in

Table 1. Major element distribution for the investigated tuff samples.

	SiO2	TiO2	Al2O3	FeO	CaO	MgO	Na2O	K2O	Loss of Ignition	Total
Green tuff
1	76.25	0.6	12.68	1.41	4.43	0.2	1.96	0.86	5.4	103.79
2	75.7	0.45	12.68	1.31	4.43	0.15	1.86	0.91	5.4	102.89
3	74.59	0.55	12.42	1.41	4.58	0.15	2.52	0.86	5.4	102.48
4	72.48	0.5	12.22	1.51	4.68	0.2	2.72	0.86	5.4	100.57
5	70.07	0.4	11.87	1.41	4.58	0.2	3.27	0.91	5.4	98.11
6	71.12	0.4	11.77	1.26	4.48	0.2	2.77	0.86	5.4	98.26
7	70.37	0.45	11.82	1.41	4.58	0.15	2.26	0.91	5.4	97.35
8	70.17	0.5	11.82	1.36	4.33	0.05	2.16	0.86	5.4	96.65
Average	72.59	0.48	12.16	1.39	4.51	0.16	2.44	0.88	5.4	100.01
St. Dev.	2.57	0.07	0.39	0.08	0.11	0.05	0.47	0.03	0.00	2.78
White tuff
1	76.27	0.13	10.84	0.94	4.21	0.67	0.87	2.21	11.47	107.61
2	68.04	0.13	9.7	0.87	4.01	0.6	0.87	2.14	11.47	97.83
3	69.64	0.2	10.3	1.07	4.82	0.8	0.67	2.21	11.47	101.18
4	68.91	0.27	9.83	0.8	4.01	0.6	0.87	2.01	11.47	98.77
Average	70.72	0.18	10.17	0.92	4.26	0.67	0.82	2.14	11.47	101.35
St. Dev.	3.76	0.07	0.52	0.12	0.38	0.09	0.10	0.09	0.00	4.41

.

3.3. ⁴⁰Ar/³⁹Ar Dating

Biotite was concentrated from a white tuff layer. The tuff was sieved, washed and the biotites were picked up manually in the Mineralogy Department, Universalmuseum Joanneum. Biotite crystals are all of same size, with perfect crystallographic shapes and show no sign of alteration. Dating of the biotite concentrate was than performed at Mass Spectrometry Laboratory, UMCS, Lublin, Poland. For this purpose, the fraction size of 0.2–0.5 mm was selected. An aliquot of about 50 mg was wrapped in an Al-foil package and marked. On both ends of package with a total length of about 30 mm aliquots of standard MMhb-1 were placed. This procedure allows obtaining information about the value of flux coefficient of irradiation and about the homogeneity of neutron beam during irradiation. The package was placed in a quartz ampoule evacuated and then sealed. The package was irradiated in the Ewa nuclear reactor at the National Nuclear Center in Świerk, Poland. The sample package was inserted into one of the channels in the reactor and irradiated for 100 h with a homogenous beam of fast neutrons. The selected channel was covered with cobalt in order to reduce the intensity of undesired thermal neutrons.

After removing the ampoule from the reactor channel, several weeks have elapsed to reduce the harmful radiation emitted by short-living radioactive isotopes generated during irradiation. Then sample and standards were taken out from the ampoule and placed in a high vacuum extraction and purification line. The gases from the sample and standards were released using step-wise technique. The expected ages of samples were relatively low and thus concentration of radiogenic ⁴⁰Ar isotope was also low, therefore it was decided to extract gases in two heating steps. The time of the gases extraction in each step was about 20 min. Extracted gases were purified from non-noble gases using a getter pump. Then isotopes of argon were analyzed on a modified (with new electronics) MS-10 mass spectrometer.

Four isotopes of argon were measured: ⁴⁰Ar of both radiogenic and atmosphere origin, ³⁶Ar used to calculate the percent of radiogenic ⁴⁰Ar in total ⁴⁰Ar, ³⁹Ar generated in nuclear reactor from ³⁹K and ³⁷Ar which was used for ³⁹Ar correction. This correction is necessary because part of ³⁹Ar is the product of neutron irradiation of ³⁹Ca isotope (

Table 2. Results of radiometric ⁴⁰Ar/³⁹Ar dating on biotite crystals from the Slănic tuff.

Temp. [°C]	40Ar/39Ar	37Ar/39Ar	36Ar/39Ar	Time after Irradiation [day]	% of Total 39Ar	% of 40Ar *	Age [Ma]	Av. Age [Ma]
1050	3.411	0.001	0.00195	166.7	35.1	83.1	13.9 ± 0.3	13.7 ± 0.2
1200	3.353	0.001	0.00196	167.5	41.4	82.8	13.6 ± 0.2

* Fluence = 0.002725.

).

The age in each step was calculated based on the formula: t = (1/λ)ln(JR + 1), where λ is a decay constant of ⁴⁰K, J is the flux coefficient, which is a parameter associated with the irradiation process, determined from measurements of argon isotopes concentration in MMhb-1 standard [36] irradiated together with the samples and R is the ⁴⁰Ar/³⁹Ar ratio.

4. Discussions and Conclusions

4.1. Mineralogical Composition

XRD investigation of the green Slănic tuff indicates that the main mineralogical component is clinoptilolite (zeolite) followed by quartz and plagioclase (Figure 3A). For this type of tuff there is no crystalline phase, which may be used for radiometric dating. Within the green tuff interval, we found discrete layers of a much coarser, white tuff, with mineralogy consisting of quartz, plagioclase, biotite and clinoptilolite-heulandite (Figure 3B). From a white tuff layer (Figure 2) a biotite concentrate was prepared and dated using the ⁴⁰Ar/³⁹Ar technique. The determined mineralogical composition of the green tuff is consistent with the data of [14] and [4]. According to [14] the Si:Al ratio of the clinoptilolite-heulandite framework typology zeolite from Slanic Prahova is >4 and therefore clinoptilolite [37]. The occurrence of clinoptilolite indicates similarities with the tuffs from the Getic basin [4], Transylvania basin [2] or Coştui-Maramureş basin [6]. For the Abrămuț basin, that is located in eastern part of Pannonian basin, the main zeolite determined in Badenian tuff layers was analcime, indicating that the tuffs were heated during burial process at the temperature over 60 °C to 80 °C, but less than the temperature requested in order to cross the analcime-albite boundary [9]. For this study, the absence of albite in XRD spectra analysis confirmed that the tuff samples are in the analcime zone. According to [38], the analcime to albite transition take place at 123 °C. In contrast to the Abrămuț basin, in the Transylvanian basin, the burial temperature for tuffs was estimated at 80° ± 10 °C [2,39] that is in the estimated range for clinoptilolite formation and stability [38,40]. For the present investigated case, the presence of clinoptilolite in Slănic tuff indicates that during the burial process the temperature was not crossing the analcime zone.

4.2. Major Element Composition

Previous major element compositions of tuffs are given in Figure 4. The present investigated samples indicate similar alkali contents (Table 1), the white tuff showing a higher SiO₂ content in comparison to the green tuff (Figure 4). The white tuff is at the limit between the dacitic and the rhyolitic fields, representing the most SiO₂ enriched tuff, which has been put in evidence for the external Carpathians area.

4.3. ⁴⁰Ar/³⁹Ar Dating

Regionally after the Mid Badenian climatic optimum, evaporitic formation and lithological changes occur related to restricted circulation for which both cooling and associated drop of sea-level were responsible in the Alpine-Carpathian realm [11,44,45].

⁴⁰Ar/³⁹Ar dating of biotite concentrate from the white tuff gives an age of 13.7 ± 0.2 Ma (Table 2), the dated tuff being situated above the FO of G. druryi and G. transsilvanica [27,28] (Figure 2), which marked the base of Wielician substage. Hohenegger et al. (2014) positioned the base of this substage at 13.82 Ma corresponding to the Langhian/Serravallian boundary [30]. From this level upward discrete gypsum layers occurs within the green tuffs, thus the age may be considered indicating for this sector the onset of the restrictive circulation. The regional distribution of Badenian evaporites in the Carpathian realm is shown in Figure 5. For the northern Carpathian sector, the beginning of the restricted circulation in Paratethys area occurred about 14 Ma ago, with asynchronous evaporites formation, gypsum occurrence being older toward the Carpathian chain and younger toward the platform [44].

In Figure 6, the biostratigraphical markers—the position of dated tuffs from the extra and inter-Carpathian realm—are shown along with the new determined age of 13.7 ± 0.2 Ma on a biotite concentrate from the white tuff level occurring at Slănic. As in the present case, at the Green Stone Hill, salt breccia and massive gypsum occurs above the dated tuff level indicating 13.7 ± 0.2 Ma (Figure 2), the main evaporitic episode is younger than for example the lower tuff level WT-1 dated for Wieliczka [46] with 13.8 ± 0.06 Ma. Also, salt accumulation started in the Carpathians with about 0.4 Ma latter than the main Badenian cooling event for which, in the Parathetys, an age of 14.2 Ma was estimated [11].