GeolCarp_Vol69_No6_528

GEOLOGICA CARPATHICA

, DECEMBER 2018, 69, 6, 528–544

doi: 10.1515/geoca-2018-0031

www.geologicacarpathica.com

Miocene paleogeography and biostratigraphy

of the Slovenj Gradec Basin: a marine corridor between

the Mediterranean and Central Paratethys

KRISTINA IVANČIČ



, MIRKA TRAJANOVA

, STJEPAN ĆORIĆ

BOŠTJAN ROŽIČ

and ANDREJ ŠMUC

Geological Survey of Slovenia, Dimičeva ulica 14, 1000 Ljubljana, Slovenia;



kristina.ivancic@geo-zs.si

Geologische Bundesanstalt, Neulinggasse 38, 1030 Vienna, Austria

University of Ljubljana, Faculty of Natural Sciences and Engineering, Department of Geology, Privoz 11, 1000 Ljubljana, Slovenia

(Manuscript received June 15, 2018; accepted in revised form November 28, 2018)

Abstract: The Miocene evolution of the area transitional from the Eastern Alps to the Pannonian Basin System was

studied through the paleogeographic evolution of the Slovenj Gradec Basin in northern Slovenia. It is based on mapping,

section logging, nannoplankton biostratigraphy, and petrography. The results are correlated with the lithological column

of the borehole MD-1/05. The evolution of the basin is connected with the development of the Pannonian Basin System,

and the global 3

order cycles, which influenced the connection with the Mediterranean Sea. Sedimentation started in

the Karpatian in a fluvial to lacustrine environment and terminated at the end of the Early Badenian. During this period,

three transgression–regression cycles were recorded. The first transgression occurred in the Karpatian and corresponds to

the TB 2.2. cycle. The sediments reflect proximity of the hinterland. After a short break in sedimentation, the Early

Badenian deposition followed. It marks the second transgression into the SGB, the first Badenian, correlated with

the TB 2.3 cycle. There are signs of a transitional environment, which evolved to marine in advanced stages. At the high-

stand system tract, the sea flooded the entire Slovenj Gradec Basin. Subsequent reduced quantity and diversity of

the microfossils marks the onset of the second regression stage. It is followed by the third transgression, the second in

the Badenian, correlated with the TB 2.4 cycle. The late Early Badenian deposition continued in the lower-energy, though

occasionally still turbulent environment. Silty sediments with upward increasing content of organic matter indicate

shallowing of the basin, until its final diminishing. Layers of fresh-water coal already bear witness to the existence of

restricted swamps. After the Early Badenian, the area of the Slovenj Gradec Basin became dry land, exposed to erosion.

Keywords: Slovenj Gradec Basin, Central Paratethys, Pannonian Basin System, Miocene, biostratigraphy, paleogeography,

sequence stratigraphy.

Introduction

The Central Paratethys represents a large Oligocene to Miocene

paleogeographic unit. It formed due to the Tethys closure, and

continental collision of the European plate and Adriatic micro

plate, and the consequent rise of the Alpine, Dinaric, Karpa-

tian and Pontian mountain chains (Royden 1988; Báldi 1989;

Rögl 1998; Rasser et al. 2008; Kováč et al. 2017b). The subse-

quent (Ottnangian and later) compressional regime and move-

ments of the crustal plates, resulted in reduction of the Central

Paratethys to a smaller area, known as the Pannonian Basin

System (PBS) (Horváth & Royden 1981; Royden 1988;

Horváth 1993, 1995; Rögl 1998; Kováč et al. 1998). The base-

ment of the PBS structurally consists of two major crustal

blocks: ALCAPA (northern part), and Tisza–Dacia (southern

part) separated by the WSW–ENE trending Mid-Hungarian

fault zone (Csontos et al. 1992; Tari et al. 1993; Csontos &

Nagymarosy 1998; Lorinczi & Houseman 2010) (Fig. 1).

In the course of the PBS’s evolution, these two blocks

underwent a complex process of rotation and extension

(e.g., Lorinczi & Houseman 2010), which left a significant

imprint on the internal structure and morphology of the PBS.

The basement of the south-western margin of the PBS is, how-

ever, formed by the Southern Alps and Dinaric units (Schmid

et al. 2008).

Marginal parts of the western Central Paratethys with

Southalpine / Dinaric basement are cropping out in the sedi-

mentary successions of eastern Slovenia, including the area

east of Celje, and the wider Laško and Krško area, whereas

the basins of north-western Slovenia, including the herein inves-

tigated Slovenj Gradec Basin (SGB), have ALCAPA basement

(Mioč & Žnidarčič 2001; Hasenhüttl et al. 2001; Rižnar et al.

2002; Otoničar & Cimerman 2006; Schmid et al. 2008; Vrabec

et al. 2009; Poljak et al. 2016; Ivančič et al. 2018). The sedi-

ments were formed in two main basins: the Mura–Zala, and

the Styrian Basins (Mioč & Žnidarčič 1989; Stingl 1994; Ebner

& Sachsenhofer 1995; Piller et al. 2004, 2007; Hohenegger et

al. 2009; Vrabec et al. 2009; Fodor et al. 2011). In this context,

the SGB represents one of the marginal Central Paratethyan

subbasins (Ivančič et al. 2018). It is related mostly to the Mura–

Zala Basin, but similarly to the Lavanttal subbasin, it also

shares characteristics with the Styrian Basin.

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The aim of the present paper is to elucidate the Miocene

sedimentary evolution of the SGB and its connection with

the surrounding basins (Lavanttal, Mura–Zala, Styrian, and

North Croatian Basins). An effort was made to identify the role

of the global sea-level changes in the SGB sedimentary forma-

tions, and to put the paleogeographic evolution of the SGB

into the wider context of the western part of the Central

Paratethys and PBS. The proposed model of the SGB evolu-

tion is based on previously published data (Ivančič et al.

2018), a model of the Pohorje tectonic block (Trajanova 2013),

and the results of new investigations. The latter comprises

a record of an additional section, petrography, revaluation of

the results from the borehole MD-1/05 (Ćorić et al. 2011), and

nannoplankton based biostratigraphy and paleoecology.

Geological setting

The SGB is situated in northern Slovenia (Fig. 2A). It is

surrounded by tectonic units of the Eastern and Southern Alps,

toward which the contacts are normal or reversely reactivated

faults, and it belongs to the PBS by origin (Trajanova 2011,

2013). The basin is underlain by Old-Paleozoic formations,

Mesozoic carbonate rock (Mioč & Žnidarčič 1976; Mioč 1978),

and Oligocene tonalite (Ivančič et al. 2018), and filled with

the Middle Miocene typical molasse sedimentary succession

named Ivnik beds. They consist of alternating beds of conglo-

merates, sandstones, siltstones, marlstones, and claystones,

and are overlain by Pliocene–Quaternary clastic sediments

(Mioč 1978; Mioč & Žnidarčič 1983; Ivančič et al. 2018).

Broadly south–north compression and tectonic activity along

two major tectonic structures, the Periadriatic and Labot

faults, are responsible for later deformation of the SGB.

The studied area is characterized by two synclines, and one

Fig. 1. Major tectonic units of the Carpathian–Pannonian Basin,

slightly modified after Lorinczi & Houseman (2010); LF — Labot

fault, SGB — Slovenj Gradec Basin, PFZ — Periadriatic fault zone.

Fig. 2. A — Simplified geological map of the Eastern Alps in Slovenia with the Miocene SGB

(modified after Buser 2009; Hinterlechner-Ravnik & Trajanova 2009; Kralj et al. 2018); rectangle

of the mapped area and locations of samples with nannoplankton are inserted,

B — geological map of the area with marked sections: GV — Grad Vodriž, V — Vodriž,

JV — Juvanov vrh, PL — Plešivec, ČP — Črni potok, VE — Velunja, and GA — Gaberke.

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anticline (Fig. 2B), crosscut by several local faults trending

prevailingly NW–SE and SW–NE.

Sampling and methods

The investigated area was mapped in the 1:5000 scale.

Based on the map, six individual sections were located and

recorded in the 1:100 scale (Ivančič et al. 2018), and one addi-

tional section (Gaberke, GA) in the south-west (Figs. 2B, 3).

From the successions, 25 additional standard thin sections

were made for sedimentological and petrographic analysis

and 70 previously made samples were re-evaluated. The litho-

logical column of the borehole MD-1/05 was interpreted on

the basis of the drilling cuttings and petrography, and biostra-

tigraphy determined on the basis of nannoplankton assem-

blages (Ćorić et al. 2011). From the borehole, 93 samples were

investigated when the borehole was drilled in 2005. Samples

from 36 m to 852 m contain rich calcareous nannofossil

assemblages and were quantitatively analysed (at least 300

specimens were counted). Because of low nannofossil content

in sediments from 600 m to 852 m only presence / absence

investigations were performed. Changes in abundances are

expressed in percentages and plotted in Figure 4. Apart from

the borehole, twenty-one samples were semi-quantitatively

examined for nannoplankton (Fig. 2A) from the SGB, of

which only four were acquired from the marly to silty layers

in the sections. All smear slides were prepared following

the standard procedure described by Perch-Nielsen (1985).

The slides were investigated under microscope Leica DMLP

with 1000 × magnification (crossed and parallel pola rizers).

For the biostratigraphic definition, the standard zonation of

Martini (1971) was applied. The Mediterranean Neogene

Nannoplankton (MNN) zonation (Fornaciari et al. 1996) was

used for correlation with the Mediterranean region.

Results

Lithological column of the borehole MD-1/05

The 1260 m deep borehole MD-1/05 was drilled on

the southern margin of the Pliocene-Quaternary fill of the SGB

in the central part of the Miocene basin, north-east of

the investigated sections (Fig. 2A). It represents the thickest

continuous cross-section through the Miocene sediments in

the SGB.

The sedimentary succession starts with mainly carbonate

conglomerate, and conglomeratic breccia, containing big

dolomitic blocks. Upward, thin layers of sandstone, siltstone,

and marlstone are interlayered in conglomerates (Fig. 5).

Frequently, coal fragments can be found, and some fragments

of hardened bituminous material. Carbonate rocks prevail

over quartz, phyllite, chert, slates, and quartzite pebbles in

the conglomerate. Limestone cutting with nummulitidae was

found in the conglomerate. In the lower part of the borehole,

coarse- to very coarse-grained sediments predominate, con-

taining big limestone blocks in the lower 340 metres. Drilling

rock chippings testify of rock blocks up to 1 m in size.

The borehole section between 450 and 920 m consists of

alternating fine- to coarse-grained sediments. Most frequent

are conglomerates with variable content of sandy matrix and

again interlayered with sandstone, siltstone, and marlstone

(Fig. 5). In the conglomerate, carbonate (dolomite and lime-

stone) and quartz pebbles prevail, though some intervals are

rich in siliciclastic, mostly metamorphic rocks. An interruption

in sediment continuity at the depths between 586 and 576 m is

indicative. Practically no rock chippings were recovered by

drilling up to 576 m. This about 10 m thick interval is charac-

terized by alternating lighter and dark silty/clayey layers,

which are enriched with organic matter. Above this layer, con-

glomerate was deposited, and further upwards corallinacean

(lithothamnium) limestone (from 525 m to 550 m) with thin

interlayers of mudstone.

From 525 m to 550 m marly limestone is found. Frequent

fossil fragments in the chippings point to minor occurrences of

corallinacean (lithothamnium) limestone.

In the upper 450 m of the borehole, fine-grained sediments

prevail. They are represented mostly by siltstone, which is

exceptionally interrupted by thin layers of conglomerate

(Fig. 5) and by dark marly intervals, rich in organic, often bitu-

minous matter. The conglomerate consists of pebbles of

quartz, gneiss, and mica schist. Dark marly intervals occur

between 270 and 160 m depth. The column ends with about

40 m of dark, clayey to silty interval with upwardly increasing

content of organic matter until a coal layer about 3 m thick

was reached at the depth of 23–25 m. Biostratigraphic ages are

reliably documented only from 38 m to 658 m, and with some

doubt from 658 m to 830 m. They range from Ottnangian(?),

reliably documented from Karpatian to Badenian (Langhian–

lowermost Serravallian).

The uppermost 38 m of the column belongs to Pliocene–

Quaternary sediments, which overlay unconformably the Mio-

cene succession.

General characteristics of the sections

The position of the sections is shown in the geological map

of the area in Figure 2B.

The total length of the investigated SGB sections is 656 m.

A common characteristic of all the sections is frequent alterna-

tion of conglomerate and sandstone, interlayered with beds of

siltstone, marly siltstone (section VE, GA, PL and GV), and

marlstone (GA, ČP and V sections) of varying thicknesses.

Conglomerate is coarse- to fine-grained, with the thickness of

beds reaching up to 7 m. Clasts mainly belong to quartzite,

carbonate and metamorphic rocks, while igneous rocks are

rare, except in the immediate proximity of the Eisenkappel/

Železna Kapla igneous belt. Sandstone is fine- to coarse-

grained with beds up to 10 m thick. The beds are graded,

locally laminated, cross-bedded, and contain relatively fre-

quent plant remains. In the composition of sandstone, lithic

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grains prevail, belonging mostly to metamorphic and carbo-

nate rocks, while fragments of igneous rocks are rare. Common

constituents are mono and polycrystalline quartz, and frag-

ments of monocrystalline phyllosilicates, while feldspars,

accessory minerals, and allochemical components are rare.

The latter belong to glauconite, red algae (Fig. 6A), bryozoan,

benthic and planktic foraminifera (Fig. 6B), brachiopods, and

echinoderm plates. Thicknesses of the siltstone and marly silt-

stone beds range up to 30.8 m, and of marlstone up to 4.8 m

(Ivančič et al. 2018).

North-west of the Velunja section, an additional section has

been recorded in the vicinity of Gaberke (GA). It comprises

a 101 m thick succession of alternating fine- to coarse-grained

bedded polymict conglomerate, medium- to coarse-grained

sandstone, silty marlstone and silty claystone. The most fre-

quent clasts are carbonates, quartz of metamorphic origin, and

tonalite. In the upper part of the section, a layer with predomi-

nating tonalite pebbles (Fig. 6C) occurs. The conglomerate is

grain supported, and in places normally graded. Conglomerate

and sandstone are cross bedded (Fig. 6D), and form dunes

(Fig. 6E), and ripples (Fig. 6F). Sandstones are laminated in

places. The gastropod Terebralia lignitarium lignitarium

(Eichwald, 1830) (Fig. 6G), oyster shells, and plant remains

were found in the upper part of the section. Allochemical

components of glauconite and red algae were found in

the coarse-grained sandstone and fine-grained conglomerate,

in the uppermost part of the section.

Calcareous nannofossils

Biostratigraphy

Nannoplankton associations are the base for the chronostra-

ti graphic definition of the SGB sedimentary fill. They were

deter mined in the sections PL and ČP, borehole MD-1/05 (Fig. 7),

and in the central and south-eastern parts of the SGB (Fig. 2A).

Fig. 4. Changes in abundances of calcareous nannofossils, recorded in the borehole MD-1/05: A — Sphenolithus heteromorphus,

B — Helicosphaera ampliaperta, C — Coccolithus pelagicus, D — Helicosphaera carteri, E — Reticulofenestra spp., F — Cyclicargolithus

floridanus.

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in Hay et al. 1967) Bukry, 1971, Helicosphaera carteri

(Wallich 1877) Kamptner, 1954, Helicosphaera walbers

dorfensis Muller, 1974,

Pontosphaera multipora (Kamptner

1948 ex Deflandre in Deflandre & Fert 1954) Roth, 1970,

reticulofenestrids (Reticulofenestra gelida (Geitzenauer

1872) Backman, 1978, Reticulofenestra haqii Backman,

1978, Reticulofenestra minuta Roth, 1970, Reticulofenestra

pseudoumbilicus (Gartner 1967) Gartner, 1969), Spheno

lithus heteromorphus Deflandre, 1953, Sphenolithus mori

formis (Bronnimann & Stradner 1960) Bramlette &

Wilcoxon, 1967 etc. Rare discoasters are represented by

Discoaster adamanteus Bramlette & Wilcoxon (1967),

Discoaster deflandrei Bramlette & Riedel, 1954, Discoaster

musicus Stradner, 1959 and Discoaster variabilis Martini &

Bramlette, 1963.

The stratigraphic attribution of this part of the borehole into

NN5 is based on the absence of Helicosphaera ampliaperta

Bramlette & Wilcoxon, 1967 and the presence of Spheno

lithus heteromorphus Deflandre 1953 (Fig. 4A, B). The last

occurrence of Helicosphaera waltrans Theodoridis, 1984

was observed in a sample from 228 m (Fig. 8). This event is

dated to 14.357 Ma (Anthonissen & Ogg 2012). The boun-

dary NN4/NN5 is defined by the last occurrence of

H. ampliaperta and dated by 14.91 Ma (Anthonissen & Ogg

2012). An increase in percentages of S. heteromorphus

(Fig. 4A) allows the correlation of this unit with Medi ter ra-

nean Zone MNN5a (Sphenolithus heteromorphus–Helico

sphaera walbersdorfensis Interval Subzone; Fornaciari et

al. 1996).

Rare reworking (max. 3.68 % in sample 218 m) from

the Paleogene (Reticulofenestra bisecta (Hay 1966) Roth,

1970, Toweius sp., Zygrhablithus bijugatus (Deflandre 1954)

Deflandre, 1959 etc.), and Cretaceous (Prediscosphaera

cretacea (Arkhangelsky 1912) Gartner, 1968, Retecapsa

crenulata (Bramlette & Martini 1964) Grün, 1975,

Watznaueria britannica (Stradner 1963) Reinhardt, 1964,

Watznaueria fossacincta (Black 1971) Bown, 1989 etc.)

could be identified in the lower part of this interval, but they

do not exceed 5 % of the total fossil assemblage.

• 270 m – 570 m Early Badenian, NN4 (Helicosphaera ampli

aperta Zone, Martini, 1971); species rich nannoplankton

assemblages contain Helicosphaera ampliaperta, and Sphe

no lithus heteromorphus accompanied by high percentages

of C. pelagicus, Cy. floridanus, P. multipora, reticulo-

fenestrids (R. pseudoumbilicus, R. gelida, R. minuta etc.),

helicoliths (H. carteri, Helicosphaera scissura Miller, 1981,

H. walbersdorfensis), and discoasters (D. deflandrei, D. musi

cus, D. variabilis). The uppermost part 288 m – 520 m of this

unit is characterized by the decrease in the content of

S. heteromorphus, and therefore can be correlated with

the MNN4b Zone (Sphenolithus heteromorphus Absence

Interval (Paracme)) defined for the Mediterranean region

(Fornaciari et al. 1996).

• 570 m – 868 m Ottnangian–Karpatian, NN4 (Helicosphaera

ampliaperta Zone, Martini 1971); rare lower Miocene nanno-

fossils dominated by H. ampliaperta, and S. heteromorphus.

The following species also occur: C. pelagicus, C. florida

nus, H. carteri, P. multipora, R. gelida, R. haqii, R. minuta,

and R. pseudoumbilicus. Based on higher amounts of

S. heteromorphus (Fig. 4A), this unit can be correlated with

Zone MNN4a (Helicosphaera ampliaperta–Sphenolithus

heteromorphus Interval Zone; Fornaciari et al. 1996).

• 860 m –1260 m Lower Miocene(?); 18 samples from this

part were barren for calcareous nannofossils, and no biostra-

tigraphic attribution was possible.

Samples from the locality Plešivec contain assemblage with

common Coccolithus pelagicus accompanied by Braarudo

sphaera bigelowii (Gran & Braarud, 1935) Deflandre, 1947,

Coccolithus miopelagicus Bukry, 1971, Cyclicargolithus

floridanus, Discoaster musicus, Helicosphaera carteri,

Reticulofenestra pseudoumbilicus, and Sphenolithus hetero

morphus. This association allows attribution to NN5 and can

be correlated with the uppermost part of the borehole MD-1/05

(Fig. 5).

Sedimentary successions in the SGB and their significance

Sedimentary successions, and particularly the mapped area,

are chronostratigraphically divided into four units: Lower

Miocene (Ottnangian–Karpatian), Karpatian, Early Badenian,

and late Early Badenian.

Lower Miocene (Ottnangian –Karpatian)

The occurrence of Lower Miocene sediments in the SGB is

uncertain. In the interval between 860 m and 1260 m of

the bore hole MD-1/05, samples contained no calcareous

nanno fossils, so biostratigraphic attribution is not possible.

As reconstructed from the rock chippings, unsorted rock mate-

rial similar to breccia or conglomerate with angular pebbles

represents a proximal, high energy terrestrial environment.

The succession is more or less continuous up to the 868 m

depth, with an interruption at around 1026 m, where a distinct

layer of coal occurs. Reduced water energy is marked by depo-

sition of silty sands in the upper part of the Ottnangian–

Karpatian succession.

Karpatian

The oldest sediments in the SGB, unambiguously confirmed

by nannoplankton, belong to the Karpatian. Sediments are found

in the lower part of the VE section, below the fault (Figs. 3, 5),

and in the borehole MD-1/05 (Figs. 4, 5). The initial succes-

sion is characterized by a pile of about 40 m thick basal con-

glomerate and conglomeratic breccia, with a thin interlayer of

sandstone that overlies the tonalite basement in the VE sec-

tion. Presence of minor fine-grained lithologies and absence of

marine biota indicate that the sediments were deposited in

a high-energy terrestrial environment, most probably as rock-

fall breccia, and alluvial fan deposits. These sediments are

conformably overlain by at least 30 metres of conglomerate

containing abundant glauconite grains, fragments of red algae,

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and bryozoans, which indicate sedimentation in a shallow-

marine environment (Fig. 3). The transition from terrestrial to

marine environment is gradual, marked by the absence of

coarse-grained conglomeratic breccia.

Early Badenian

Due to vegetation cover, no direct exposure of the Karpatian/

Badenian transition was found in the investigated area, but it is

clearly marked in the borehole MD-1/5 at the depth of around

576 m by a change of lithology.

The succession continues with the Early Badenian sedi-

ments, recorded in the upper part of the VE section (above

the fault), ČP, GA, JV, GV, and V sections and in the borehole

MD-1/05 (Figs. 5, 9B, C). Initial Early Badenian sedimenta-

tion is characterized by alternation of fine- to coarse-grained

layers of common thickness up to 60 m, deposited in the ter-

restrial environment documented in the GA, VE and GV sec-

tions (Fig. 3). Terrestrial deposits are overlain by up to 16 m of

prevailingly fine-grained sediments, including silty marlstone

and marlstone (VE, and GA), or by up to 20 m of conglome-

rate (GV), deposited in the transitional environment. The sedi-

ments reflect the onset of the first Badenian transgression in

the SGB (Fig. 9B). A lagoonal environment was formed,

determined by the occurrence of the gastropod Terebralia

lignitarium lignitarium (Eichwald 1830) (Fig. 6G), found in

the GA and ČP sections. The environment with deposition of

marlstone appears to be similar in the V section, though no

gastropods were found there. In the GV section, an accretio-

nary Gilbert-type delta environment developed (Fig. 6H).

Sedimentation continued in the marine environment, deter-

mined in the GA, ČP, JV, V, and GV sections, and in the bore-

hole MD-1/05 (Fig. 9C). In the GA section, cross lamination

(Fig. 6D), dunes (Fig. 6E), and ripples (Fig. 6F) represent

shoreface deposits. A shallow marine environment is indicated

by allochemical grains, mostly glauconite, and bryozoa.

In the upper part of the GA section, a layer with predominating

tonalite pebbles occurs. It is correlated with a similar layer in

the ČP section (marked by the same level in Fig. 5), and

documents the same sediment provenance and contempo-

raneous sedimentation. In the ČP, and JV sections, the maxi-

mum transgression is marked by the highest variety and

Fig. 7. Calcareous nannofossils found in the borehole MD-1/05 with depth indicated: 1–3 — Helicosphaera ampliaperta Bramlette and

Wilcoxon, 1967; 510 m; 4 — Helicosphaera carteri (Wallich 1877) Kamptner, 1954; 510 m; 5 — Pontosphaera multipora (Kamptner 1948 ex

Deflandre in Deflandre & Fert 1954) Roth, 1970; 510 m; 6 — Coccolithus pelagicus (Wallich 1877) Schiller, 1930; 510 m; 7 — Cyclicargolithus

floridanus (Roth & Hay, in Hay et al. 1967) Bukry, 1971; 510m; 8, 9 — Sphenolithus heteromorphus Deflandre 1953; 230 m;

10 — Reticulofenestra pseudoumbilicus (Gartner 1967) Gartner, 1969; 230m; 11 — Reticulofenestra minuta Roth, 1970; 510 m;

12 — Coronosphaera mediterranea (Lohmann 1902) Gaarder, in Gaarder & Heimdal, 1977; 230 m.

Fig. 8. Samples containing stratigraphically significant form of

Helicosphaera Waltrans.

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quantity of allochemical components, which is reflected in the

V section as well (Figs. 3, 5). Presence of planktic forami-

nifera found in the ČP, and JV sections points to an offshore

transition environment. A lot of plant remains, similar to sea

grass define proximity of the coast in the V, and part of

the ČP sections. In the V section, marine and terrestrial envi-

ronment are alternating (Fig. 3) (Ivančič et al. 2018). Generally,

in the lower part of the ČP, JV, and V sections, conglomeratic

deposits of the first Badenian transgression prevail, while in

the upper part of these sections, sandstone layers of the fol-

lowing regression stage prevail (Fig. 3).

Late Early Badenian

The youngest sediments in the SGB belong to the upper-

most part of the Early Badenian, assigned here as late Early

Badenian. They were found in the PL section and in the upper

part of the borehole MD-1/05. Deposition of finer-grained

sediments (siltstones) prevailed, containing glauconite, ben-

thic, planktic foraminifera, calcareous nannofossils, and a lot

of plant remains similar to sea grass, found in the PL section.

Siltstones are practically without thick layers of conglomerate.

Only one characteristic conglomeratic layer occurs (Fig. 5) in

the section and the borehole. It has been used as a marker of

simultaneity. Sedimentation took place in a shallow marine

environment with low water energy. Dark, silty to marly inter-

vals in the borehole MD-1/05 between 270 and 160 m depth

are rich in organic and bituminous matter. Above, the succes-

sion continues with lighter intervals. The uppermost part ends

with a coal layer, and dark clays, which mark the end of sedi-

mentation in the SGB (Fig. 5).

Discussion

The sedimentary succession of the SGB presented here

enables more detailed correlation of the SGB with the sur-

rounding basins (Fig. 10). The correlation is based on the 3

order sequences after Haq et al. (1988), and Hardenbol et al.

(1998). However, the standard geological time scale does not

take into account the regional and local geodynamics of pro-

cesses and the changing of marine gateways (Kováč et al.

2018). Consequently, our sequences could not necessary coin-

cide precisely to the global sequences but may slightly deviate

from them. Therefore the 3

order sequence boundaries of

the SGB are also correlated with the sequences of the Central

Paratethys after Hohenegger et al. (2014) (Fig. 10).

Paleoecology based on changes in calcareous nannofossils

assemblages

Nannofossils are a good tool for the reconstruction of paleo-

environments in the water column, and for the correlations

between individual basins. Changes in calcareous nanno-

plankton assemblages usually reflect oscillations in depth,

temperature and salinity of marine water, and nutrient supply.

The investigated sediments from the lowermost part of

the borehole MD-1/05 lack calcareous nannofossils (860 m to

1260 m). The rest, which contain nannofossils, are dominated

by C. pelagicus, helicoliths, and small reticulofenestrids.

For the reconstruction of paleoconditions in the SGB, we used

oscillations in percentages of these species.

Coccolithus pelagicus is well known as an r-strategist,

which is abundant in cold and nutrient rich waters (Okada &

McIntyre 1979; Winter et al. 1994). The average content of

C. pelagicus (Fig. 4C) in the lower part of the borehole (from

200 m – 590 m) is about 45 %, and increasing in the upper part

(40 m – 220 m), reaching maximum (78 %) in the sample from

the depth of 50 m. High percentages of C. pelagicus (50–76 %)

indicate high nutrient input (usually caused by upwelling con-

ditions) and eutrophic conditions in the water column during

NN4 and lower NN5. Increasing content of C. pelagicus in

the upper part of NN5 (max. 76 % in sample form 50 m)

accompanied by high percentages of helicoliths characterize

shallowing of the sea.

Helicoliths are common in shallow, near continental envi-

ronments and indicate an upwelling regime (Perch-Nielsen

1985). Helicosphaera carteri (stratigraphic range from NN1

until extant) is the most common species among helicoliths in

the borehole. The distribution pattern of this cosmopolitan

species (Fig. 4D) is very similar to C. pelagicus, signifying

shallowing of the sea, as recorded in the upper part of the bore-

hole (NN5).

Small reticulofenestrids (Reticulofenestra haqii and R. minuta)

generally dominate assemblages along continental margins

(Haq 1980). They were used for the paleoecological interpre-

tation of Lower/Middle Miocene sediments in the Austrian

Alpine–Carpathian Foredeep (Molasse Basin; Ćorić & Rögl

2004) and Middle Miocene sediments from the Vienna Basin

(Ćorić & Hohenegger 2008). Higher percentages of small

reticulofenestrids indicate warmer, significantly stratified

water columns (reduced eutrophic conditions) in contrast to

assemblages with the dominance of C. pelagicus. The dis-

tribution pattern of small reticulofenestrids in the borehole

(Fig. 4E) has the opposite trend of C. pelagicus. This confirms

more stable conditions during the NN4/lower NN5 and

the shallowing trend in the upper NN5.

Cyclicargolithus floridanus (Fig. 4F) occurs in all investi-

gated samples from the borehole. Shcherbinina (2010) consi-

dered that Cyclicargolithus genera point to eurytopic conditions

and usually adapts to a large spectrum of paleoenvironmental

conditions. Auer et al. (2014) interpreted the increased amount

of C. floridanus as a result of reduced upwelling conditions.

Melinte-Dobrinescu & Brustur (2008) investigated Oligocene/

Miocene calcareous nannofossils from the Eastern Carpathians

(Romania) and concluded that higher percentages of C. flori

danus indicate warmer and stable climate conditions.

This cosmopolitan species participates in the whole nanno-

plankton assemblages from 3.01 % to 32.00 % (mean value

8.21 %) in the lower part of the borehole (140 m – 590 m).

In the upper part (38 m – 130 m) the mean value decreased

to 3.89 % (min. 2.67 %; max. 5.67 %). A slight increase in

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the lower part of the borehole (NN4, lower NN5) points to

relatively stable paleoconditions, whereas the decrease in

the upper part (upper NN5) indicates a shift to a more unsta-

ble, more turbulent environment in the upper part.

Discoasters belong to the K-strategists group, and are gene-

rally common in oligotrophic, warm, and deep oceanic water,

and point to stable paleoenvironments (Aubry 1992; Lohmann

& Carlson 1981; Young 1998). Low percentages of these forms,

which do not exceed 1 %, point to a eutrophic sedimentation

milieu close to the coast.

Helicosphaera waltrans, present in the upper part of the bore-

hole, is usually correlated with a warm surface water layer and

a change from estuarine to anti-estuarine circulation (Holcová

et al. 2018), which could point to the beginning of the isolation

of the SGB.

Depositional sequences and correlation

Sedimentation in the SGB is the temporal and partly facial

equivalent of the surrounding basins. Its initial Early Miocene

evolution reflects the syn-rift phase of the PBS evolution

(Royden 1988; Tari et al. 1992).

Lower Miocene (Ottnangian–Karpatian?)

Initial sedimentation in the SGB is represented by breccia

and conglomerates with big limestone blocks. These terrestrial

deposits, considered to accumulate along steep slopes as talus

deposits, most probably represent rock-fall breccia, partly

redeposited, and mixed with alluvial fan sediments. They rep-

resent nonmarine (limnic/fluvial) environment. Their age is

uncertain due to the lack of faunal evidence; therefore,

the Ottnangian to Karpatian age is presumed. An important

phenomenon for correlation is the occurrence of the coal layer

at the depth of around 1026 m in the MD-1/05 borehole. In

similar limno-fluvial coarse-grained sediments lignite remains

and seams are found also in the Ottnangian of the Styrian

Basin (Hohenegger et al. 2009). The coal bearing layers were

found in other parts of the Central Paratethys as well (Vass et

al. 1979, 1999; Sachsenhofer 1996; Kováč et al. 2017b). Muddy

breccia and conglomerates were also found in the Mura–Zala

Basin (Fodor et al. 2011), but in the shallow marine environ-

ment. In North Croatia, rock-fall breccia and conglomerate are

equally interpreted as Lower Miocene talus and alluvial fan

deposits, deposited in a continental sedimentary environment

(Pavelić & Kovačić 1999, 2018).

Karpatian

The deposition of coarse-grained sediment continued in

the marginal parts of the SGB in the Karpatian. The high-

energy terrestrial deposits are similar to those from the Lower

Miocene. Comparable limnic-fluvial Karpatian sediments

are known from the Lavanttal Basin as Margarethen Gravel

(Beck-Mannagetta 1952; Reischenbacher et al. 2007).

The Karpatian events were marked by the establishment of

the east–west trending sea-way from the Central Paratethys to

the Mediterranean via Slovenia, following the contours of

Fig. 10. Stratigraphic time scale and correlation of investigated sections in the SGB with the global 3

order sequences, and with formations

of the surrounding basins (Lav. — Lavanttal; M–Z — Mura–Zala; N.C. — North Croatian) for Ottnangian, Karpatian and Badenian in

the Central Paratethys; lithostr.u. — lithostratigraphic unit.

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the Mid-Hungarian Line (Sant et al. 2017). According to Jelen

et al. (2008), the Karpatian sediments in Slovenia represent

the infill of the accommodation space, established due to

E–W to NE–SW directed back-arc extension and rifting.

Some authors attribute the initial rift phase of the PBS to

the Karpatian (17.5–16.5 Ma) opening of pull-apart basins at

the margins of the Pannonian basin, and pure extension to

the Badenian (16.5–14 Ma) (Royden et al. 1983; Horváth

1993; Csontos 1995; Huismans et al. 2001). In the model of

Trajanova (2013), the origin of the SGB is presumably of pre

Karpatian age, developed initially along local faults related to

the E–W opening of the Labot fault; its initiation is not consi-

dered to be a pull-apart basin, but as an Early Miocene rifting

related extensional basin developed on the passive margin,

like the Pohorje intrusion. Despite differences in the interpre-

tations, it is clear that transgression reached several newly

established basins, including the SGB, already in the Kar-

patian. This stage is correlated to the sea-level cycle TB 2.2 of

Haq et al. (1988).

The Karpatian transgression to the SGB followed, which

affected the Mura–Zala (Fodor et al. 2011) and the Styrian

Basins, where marine fine-grained sediments predominate

(Hohenegger et al. 2009), pointing broadly to the eastward

deepening of the sea. According to Rögl et al. (2007), this

transgression influenced mostly the western part of the PBS.

According to the provenance analyses of the Karpatian sedi-

ments, significant amounts of the rock material was derived

from the lithic recycled orogen, corresponding to the Austro-

alpine units (Ivančič et al. 2018). The latter formed a hilly area

in the Egerian (Frisch et al. 1998), and mostly remained dry

land throughout the SGB filling up. At that time, the Pohorje

tectonic block is considered to occur north of its present posi-

tion and granodiorite intrusion was still not exposed to erosion

(Trajanova 2013). Strong inflow is documented from the uplif-

ting Northern Karavanks. Based on the Eisenkappel tonalite

pebbles, individual sediment influx pulses are recorded from

the Periadriatic fault zone. Abundant clasts of carbonate rocks

indicate that the Southern Alps were partly unroofed, which

enabled sediment delivery also from the south (Fig. 9A).

The Karpatian transgression established a new connection

with the Mediterranean basin via the Trans-Tethyan-Trench-

Corridor (Bistricic & Jenko 1985; Rögl 1998). According to

the paleogeographic reconstruction of the Central Paratethys

(Kováč et al. 2017a), the corridor ran south of the SGB

(Fig. 9).

Karpatian/Early Badenian boundary

Contact of the Karpatian and Early Badenian sediments can-

not be traced on the surface in the SGB; however, it has

charac teristics of a short break in sedimentation in the bore-

hole MD-1/05 (Fig. 5). The regression stage is characterized

by deposition of conglomerate, marlstone, and sandstone.

Localized equivalents of this succession occur in the western-

most part of the basin at localities Leše and Holmec, where

the abandoned coal mines are located (Mioč & Žnidarčič

1983). Coal layers originated in a fresh water environment

(Gostiša et al. 1984), which marks the most notable turnover

in the sedi mentation environment.

Continuous sedimentation from Karpatian to Lower Bade-

nian has equally not been found in the Central Paratethys yet

(Harzhauser & Piller 2007). Hence, we consider the Karpatian–

Early Badenian contact in the SGB as discontinuity (Fig. 5).

A very prominent and well known erosional discontinuity is

exposed south of Leibnitz, in the old brickyard of Wagna

(Spezzaferri et al. 2002, 2004; Gross et al. 2007; Rögl et al.

2007). Sea-level drop at the Karpatian/Badenian boundary is

recorded in the entire Central Paratethys (Rögl et al. 2002),

and is considered a consequence of global events and regional

tectonics, which caused the uplift of separate crustal blocks in

the PBS (Horváth 1993; Pavelić 2005). The regression stage in

the SGB (Fig. 10) could be related to the Bur5/Lan1 of

Hardenbol et al. (1998). On the other hand, in semi-enclosed

basin it is important to take into consideration the local tec-

tonic processes, therefore the boundary cannot coincide with

the global sequence boundary (Kováč et al. 2018) and is most

probably closely related to the boundary after Hohenegger et

al. (2014), which is positioned at 16.303 Ma (Fig. 10).

Early Badenian

The base of the Badenian flooding is characterized by depo-

sition of sand and gravel in most of the Central European

basins; the sediments frequently contain admixtures of

reworked fossils (Sant et al. 2017). The first Badenian trans-

gression stage was relatively short and indicates interplay of

the tectonic uplift and eustatic sea-level rise (Pavelić 2005).

In the SGB, this stage corresponds to the NN4 Zone, and cor-

relates well with the first Badenian transgression of the Central

Paratethys. There is evidence of rapid deepening from

the shal low water to offshore environment. In the north-

western part of the SGB, coal layers were covered with brac-

kish and marine sediments (Gostiša et al. 1984). Corallinacean

(lithothamnium) limestone and calcareous nannofossils show

evidence of stable paleoconditions in the central part of the SGB.

On the margin, increasing quantity and variety of allochemical

components evidence transgression until the highstand system

tract (HST) (Figs. 5, 9C), correlated with the global 3

order

cycle TB 2.3 (Fig. 10). It is suggested that advanced rifting

and extension widened lowland area along the Labot fault

(Trajanova 2013) therefore enabling ingression of the sea into

the Lavanttal Basin from the south (Fig. 9C), and formation of

a marine embayment. Local more frequent alternation of

marine and non-marine environment (V section) is presumably

a reflection of inflows into the shallow sea and of near-shore

paleomorphology.

The Early Badenian transgression in the SGB is temporal

and partly facies equivalent to the transgression recorded in

the surrounding basins of the PBS: Mura–Zala, Styrian,

Lavanttal, North Croatian (e.g., Reischenbacher et al. 2007;

Ćorić et al. 2009; Hohenegger et al. 2009; Fodor et al. 2011;

Pavelić & Kovačić 2018). Similar evolution is found in

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the Lower Badenian Mühldorf formation of the Lavanttal

Basin, expressed by deltaic-estuarine offshore transition,

shoreface, and lagoon (Reischenbacher et al. 2007). Gilbert-

type fan deltas, determined in the GV section of the SGB, also

formed in the North Croatian Basin, and offer a proof for

the existence of coastal area during the earliest Badenian

(Pavelić 2005). A lagoonal environment existed at the same

time in the SGB. The Terebralia lignitarium lignitarium, and

oyster shells were found, but they are not indicative for bio-

stratigraphic subdivision. Similar fauna was determined on

the northern margin of the Oberpullendorf Basin (north-west

of the Styrian Basin) within the Middle Badenian (Harzhauser

et al. 2013).

Regression after the HST is correlated to the expansion of

the East Antarctic ice sheet (Flower & Kennett 1993; Shevenell

et al. 2004), and corresponds to the Lan2/Ser1 sequence

boundary (Piller et al. 2007) (Fig. 10). This event is recorded

as deposition of sandstone above conglomerates in the SGB

sedimentary succession and as reduced diversity and quantity

of allochemical components.

The main delivery of the sediment to the SGB was from

the north-west, and west (Ivančič et al. 2018). Tonalite peb-

bles, up to 50 cm in size, testify that the Periadriatic fault

zone was its proximal hinterland, which delivered much more

sediment to the SGB in the Early Badenian than in the Kar-

patian. This reflects relatively rapid uplift and exhumation of

the Eisenkappel igneous belt. It was not possible to identify

sediment delivery from the Southern Alps. The sediment pro-

ve nance and environmental characteristics in the Early Bade-

nian are a sign of a direct connection of the SGB with

the Lavanttal basin (Reischenbacher et al. 2007). The first

Badenian transgression probably formed an embayment in the

direction of the Klagenfurt Basin as well (Fig. 9C).

Late Early Badenian

This period is characterized by the third transgression in

the SGB, the second in the Badenian, correlated with the cycle

TB 2.4. It is defined by the occurrence of H. waltrans, which

is significant for the base of this transgression (Holcová et al.

2018). At that time, the main extensional phase in the PBS was

still in progress (Royden et al. 1982). Sea ingression into

the SGB and sea level oscillations are marked by the occur-

rence of dark sediments indicating changed conditions with

abundant flora, which caused frequent changes in the near-

bottom oxygenation. Sedimentation took place in more quiet

conditions, as reflected in deposition of fine-grained sediments.

The calcareous nannoplankton assemblages argue for an unsta-

ble environment, more turbulent paleoconditions, and shal-

lowing of the sea water. Their age is correlated with the NN5

Zone. Co-occurrence of the NN5 Zone in the Lavanttal Basin

and SGB points to the existence of the sea connection between

the two basins, suggesting a northward trending embayment.

Contemporary sedimentation is recorded in the Styrian

(Hohenegger et al. 2009), Northern Croatian (Ćorić et al. 2009;

Brlek et al. 2016; Pavelić & Kovačić 2018), Mura–Zala (Fodor

et al. 2011) and Lavanttal basins (Reischenbacher et al. 2007).

Marine sedimentation continued into the Middle Badenian in

all the stated basins, but ceased in their shallower peripheral

parts, in the SGB, and in the Lavanttal Basin, after the late

Early Badenian. Deposition of clayey material with coal layer

continued without break on the top of the NN5 zone in

the MD-1/05 borehole, which marks the end of the Miocene

sedimentation in the SGB. After the break, sedimentation in

the SGB continued in a fluvial regime in the Plio-Quaternary.

This late Early Badenian regression could be correlated to

a sea-level drop in the upper part of the Upper Langhian

(sequence boundary 2 (SB2), Fig. 10) (Strauss et al. 2006).

SB2 is correlated with the first Antarctic cooling step at

14.2 My (Shevenell et al. 2004). This event is not expressed

globally and could be confined to the Central Paratethys only

(Rögl et al. 2007).

Post Early Badenian

Absence of the Middle Badenian and younger sediments in

the SGB was the result of several contemporaneous events.

Apart from the sea-level drop, additional reasons for the ces-

sation of sedimentation could be interrupted connection of

the SGB with the surrounding basins around the end of

the Lower Badenian. It was presumably caused by the uplift

and exhumation of the Pohorje tectonic block, and its oblique

shift and rotation along the Labot fault (Trajanova 2011,

2013). Uplift of the Pohorje tectonic block, and its synchro-

nous counter-clockwise rotation (Márton et al. 2006) gradually

cut the connection of the SGB with the Lavanttal basin to

the north, and with Mura-Zala and Styrian Basins to the north-

east and south-east (Trajanova 2011, 2013) (Fig. 9D). Erosion

of the Miocene sediments started and led to thickness reduc-

tion and the absence of Karpatian and Lower Badenian sedi-

ments on the fold hinges. A synchronous erosional event can

also be traced in the shallower parts of the North Croatian

Basin (Avanić 1997; Pavelić 2005).

Conclusions

The evolution of the SGB is correlated to the evolution of

the PBS. The sedimentary successions record three trans-

gression-regression cycles, which generally correspond to

the global 3

order sequences.

• The evolution of the SGB started in the Lower Miocene, in

the terrestrial environment of the Ottnangian/Karpatian with

deposition of talus and alluvial fan sediments.

• The first transgression of the cycle TB 2.2 followed, cor-

related to the NN4 Zone.

• Regression stage at the Karpatian/Badenian boundary is

correlated to the Bur5/Lan1. Sedimentation took place in a

ter restrial environment.

• The second transgression in the SGB occurred in the Early

Badenian initiated in a transitional environment with simul-

taneous deposition of lagoonal and deltaic sediments.

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Transgressions reflected in gradual increase in variety and

quantity of marine microfossils until the HST, when the entire

area of the SGB was flooded. This transgression is correlated

to the TB 2.3 cycle. The calcareous nannoplankton assem-

blages indicate stable paleoconditions of the water column.

• The following regression is correlated to the Lan2/Ser1

sequence boundary.

• The second Badenian transgression is correlated to the glo-

bal sea-level rise and corresponds to the TB 2.4 trans-

gression cycle. The calcareous nannoplankton assemblages

demonstrate unstable paleoconditions with shallowing trend

of the sea water, and turbulent environment. Assemblages

with domination of C. pelagicus are a sign of shallow,

nutrient rich paleoenvironment.

• The diminishing of the basin is marked by deposition of

clayey, coal bearing sediments, and is correlated to the SB2.

• Based on investigations of calcareous nannofossils, the Medi-

terranean zones MNN4a, MNN4b and MNN5a were iden-

tified. The connection with the Mediterranean region was

established during NN4 (MNN4a), and probably interrupted

in regression stages between the Karpatian/Early Badenian

and NN4 Early Badenian/NN5 Early Badenian, and finally

interrupted in the upper NN5 (MNN5a).

• No younger sediments were recorded in the basin, which

indicates cessation of sedimentation in the late Early

Badenian and subsequent erosion prior to the onset of

the Pliocene–Quaternary fluvial sedimentation.

Acknowledgements: The present study was funded by

the Slovenian Research Agency (ARRS) in the framework of

the Young Researchers programme and the research pro-

grammes P1-0020 (Groundwaters and Geochemistry) and

P-0025 (Mineral Resources). Thanks to Mathias Harzhauser

(NHM, Vienna) for determining molluscs collected during

the mapping work. We would like to thank Mladen Štumergar

for preparation of samples for petrographic and geochemical

analysis. The authors also owe gratitude to Dragomir Skaberne,

Jernej Jež, Blaž Milanič, Manja Žebre, and Matevž Novak for

their generous help and support.

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