INFLUENCE OF GEOLOGICAL STRUCTURE  
ON MICRO-LOCATION OF SUBMARINE KARSTIC  
SULPHUR SPRINGS NEAR IZOLA (SW SLOVENIA) 
VPLIV GEOLOŠKE STRUKTURE NA MIKROLOKACIJO 
PODMORSKIH KRAŠKIH ŽVEPLENIH IZVIROV PRI IZOLI  
(JZ SLOVENIJA)
Boštjan ROŽIČ1 & Petra ŽVAB ROŽIČ2
Abstract  UDC 546.22:551.435.85(497.4Izola)
Boštjan Rožič & Petra Žvab Rožič: Influence of Geological 
Structure on Micro-location of Submarine Karstic Sulphur 
Springs near Izola (SW Slovenia) 
In the Slovenian part of the Gulf of Trieste/Trst, submarine 
springs occur as funnel-shaped depressions in the Holocene 
sandy-silt marine sediment that forms the seafloor. These 
springs exhibit both elevated temperatures (up to 29.6 °C) and 
sulphur content. Based on their location, they are divided into 
three groups: the Izola group (three springs), the Bele skale 
group (two springs), and the Ronek group (seven springs). Pre-
vious investigations linked these springs to the Izola anticline, 
characterized by its limestone core and flysch limbs, but no de-
tailed explanation was provided. We propose that: A) sulphur 
groundwater springs from the limestone (karstic aquifer) at the 
stratigraphic boundary with the flysch and B) springs occur in 
the nearshore area, where the sedimentary cover of Quaternary 
deposits is thin enough to be penetrated by spring waters and 
washed out. According to existing data, the anticline axis is 
NW-SE directed, and the Izola group lies approximately on the 
seaward extension of the axis. Therefore, this interpretation fits 
perfectly for the Izola group, while the Ronek and Bele skale 
groups are off-axis extensions and require reinterpretation. 
In order to resolve the micro-locations of the Ronek and Bele 
skale groups, we conducted a sedimentological logging of the 
flysch deposits and detailed geological mapping. The investi-
gations revealed that: A) limestone outcrops occur only in the 
town of Izola, B) two prominent calciturbidite megabeds that 
occur in the flysch enable very detailed geological mapping, 
and C) the axis of the Izola anticline is oriented in the WNW-
Izvleček UDK 546.22:551.435.85(497.4Izola)
Boštjan Rožič & Petra Žvab Rožič: Vpliv geološke strukture na 
mikrolokacijo podmorskih kraških žveplenih izvirov pri Izoli 
(JZ Slovenija)
V slovenskem delu Tržaškega zaliva se v holocenskem pešče-
no-meljastem sedimentu, ki tvori ravno morsko dno, pojavlja-
jo podmorski izviri v obliki lijakastih kotanj. V teh izvirih so 
opazni povišane temperature (do 29,6 °C) in vsebnost žvepla. 
Glede na njihovo lokacijo jih lahko razdelimo v tri skupine: 
skupina Izola (3 izviri), skupina Bele skale (2 izvira) in sku-
pina Ronek (7 izvirov). Že predhodno so te izvire povezali z 
Izolsko antiklinalo, ki jo označujejo apnenčasto jedro in flišna 
krila, vendar natančnejše razlage niso podali. Predvidevava, da 
žveplene vode izvirajo iz apnenca (kraškega vodonosnika) na 
stratigrafski meji s flišem, in sicer v bližini obale, kjer je pokrov 
kvartarnih sedimentov dovolj tanek, da ga lahko izvirske vode 
izperejo. Glede na podatke naj bi bila os antiklinale v smeri SZ–
JV in skupina Izola bi potemtakem ležala na protimorskem po-
daljšku osi. Takšna interpretacija torej povsem ustreza skupini 
Izola, preostali dve skupini pa padeta povsem izven podaljškov 
antiklinalne osi, zaradi česar je potrebna ponovna interpreta-
cija. Da bi razložili mikrolokacije skupin Ronek in Bele skale, 
smo posneli sedimentološke značilnosti profilov flišnega za-
poredja in izvedli natančno geološko kartiranje. Raziskave so 
pokazale, da a) se izdanki apnencev pojavljajo le na območju 
mesta Izola, b) dve kalciturbiditni veleplasti, ki se pojavljata v 
flišu, omogočata zelo natančno geološko kartiranje in c) je os 
Izolske antiklinale orientirana v smeri ZSZ–VJZ. Prav ta zmer-
na rotacija antiklinalne osi v nasprotni smeri urnega kazalca 
razloži mikrolokacije vseh izvirov. Skupini Ronek in Bele skale 
ACTA CARSOLOGICA 52/1, 9-27, POSTOJNA 2023
1 Department of Geology, Faculty of Natural Sciences and Engineering, University of Ljubljana, Aškerčeva 12, 1000 Ljubljana, 
Slovenia, e-mail: bostjan.rozic@ntf.uni-lj.si
2 Department of Geology, Faculty of Natural Sciences and Engineering, University of Ljubljana, Aškerčeva 12, 1000 Ljubljana, 
Slovenia, e-mail: petra.zvab@ntf.uni-lj.si
Prejeto/Received: 7. 9. 2022
DOI: https://doi.org/10.3986/ac.v52i1.11091
BOŠTJAN ROŽIČ & PETRA ŽVAB ROŽIČ
1. INTRODUCTION
Topographic mapping of the Slovenian part of the Gulf of 
Trieste/Trst revealed a relatively flat seafloor composed of 
Holocene fine clastic marine sediments (Kolega & Poklar, 
2012; Slavec, 2012). Before early Holocene transgression, 
the area was part of a Po River floodplain characterized 
by alluvial and eolian sedimentation (Correggiari et al., 
1996; Trobec et al., 2017; Brunović et al., 2020). Post 
transgression Holocene sediments, usually several me-
ters thick, overlie Pleistocene deposits and in the near-
shore area, also a Paleogene rock basement (Ogorelec 
et al., 1991, 1997; Romeo, 2009; Trincardi et al., 2011a, 
2011b; Trobec et al., 2017, 2018, Novak et al., 2020). The 
most prominent topographic features that stand out in 
relief from the present-day sea bottom are a current-re-
lated elongated depression near Piran, a small reef near 
Cape Ronek, remains of pre-transgression river channels, 
sand dunes, and unusual funnel-shaped depressions of 
more than 10 m deep (Figure 1) (Slavec, 2012; Orlando-
Bonaca et al., 2017; Trobec et al., 2017, 2018; Novak et 
al., 2020). An initial dive into these depressions revealed 
that they are submarine springs, above which the Qua-
ternary sediment is almost completely eroded, but the 
walls as well as the bottoms of these depressions are still 
composed of fine-clastic Quaternary deposits (Žumer, 
2004, 2008). These springs show elevated temperatures 
(up to 29.6 °C) and sulphur content (Žumer, 2004, 2008; 
Faganeli et al., 2005; Žvab Rožič et al., 2021; Šušmelj et 
al., 2022). Morphological and hydrogeological character-
istics of these depressions indicate that they correspond 
to subaqueous pockmarks formed where focused fluids 
sta na južni strani, skupina Izola pa je na severni strani apnen-
častega jedra Izolske antiklinale. 
Ključne besede: žveplo, kraški izviri, Jadransko predgorje, 
Izolska antiklinala, geološko kartiranje.
ESE direction, and this moderate anti-clockwise rotation of the 
anticline axis explains the micro-locations of all springs. The 
Ronek and Bele skale groups are located on the southern side, 
and the Izola group is on the northern side of the limestone 
core of the Izola anticline.
Keywords: sulphur, karst springs, Adria foreland, Izola anti-
cline, geological mapping.
Figure 1: Digital elevation model of the southern Gulf of Trieste/Trst seafloor with major topographic features that stand out in relief. 
Submarine sulphur springs are encircled (Ronek group – green, Bele skale group – red, Izola group – green). In the lower right corner is a 
vertically exaggerated sonar-data cross section of the Izola group’s north-western spring (modified from Slavec, 2012).
ACTA CARSOLOGICA 52/1 – 202310
INFLUENCE OF GEOLOGICAL STRUCTURE ON MICRO-LOCATION OF SUBMARINE KARSTIC SULPHUR SPRINGS 
NEAR IZOLA (SW SLOVENIA) 
migrate through unconsolidated sediments toward the 
sediment surface (gas seepage, submarine groundwater 
discharge, hydrothermal vents) (King & Maclean, 1970; 
Hovland et al., 2002; Judd & Hovland, 2007; Wirth et al., 
2020). 
Based on their positions, the springs are divided 
into three groups. The first group (three springs) lies near 
the town of Izola, while the second (two springs) is locat-
ed near the Bele skale rockfall between Izola and Strun-
jan, and the third (seven springs) is near Cape Ronek at 
Strunjan (Figures 1, 3 and 9). 
In the earliest reports, the occurrence of these 
springs was linked to the geological structure, the Izola 
anticline (Žumer, 2004). Namely, the entire Slovenian 
coast is characterized by a thick sequence of Eocene 
flysch deposits that act as an aquitard. The base of the 
flysch is an aquiclude composed of transitional marl 50 
m thick that acts as a hydrogeological barrier. The only 
coastal outcropping of the underlying Eocene limestone 
is found in the core of the Izola anticline. We agree with 
early proposals that sulphur groundwaters spring from 
this limestone in the near shore area. The springs are 
therefore considered karstic in origin. This is supported 
by a historically documented (now demolished and inac-
cessible) sulphur thermal spring inside Sveti Peter Cave, 
which was used for the Izola Spa (Kramar, 2003) and 
from the 501 m-deep LIV-1/01 borehole (no longer op-
erational) drilled in the Livade part of the town of Izola 
(Benedik & Rožič; 2002; Lapanje, 2006). Both locations 
lie within the area of the Izola anticline limestone core 
(marked in Figure 9). Basic analysis of the borehole wa-
ter that was obtained during the pumping test showed 
that the groundwater is fresh and cold near the surface. 
Deeper, below the two coal layers of limestone-domi-
nated Early Paleocene Liburnia Formation (at depths of 
approximately 280m and 410m respectively) the water 
temperature rises of a 1 °C and a sulphur smell appears. 
A slight increase of Na and Cl was detected particularly 
at greater depths (Benedik & Rožič; 2002; Lapanje, 2006). 
Unfortunately, no precise geochemical data is published 
in these reports. However, interesting evidence of sul-
phur content comes from the fact that the deeper-level 
pumping test was halted by the police, because the “smell 
of rotten eggs (sulphur)” was spreading intensively across 
the entire Izola area (first author’s personal experience). 
Detailed hydrogeochemical and isotopic studies of the 
sulphuric waters from submarine springs are in the early 
stages (preliminary results in Šušmelj et al., 2022) and no 
Figure 2: The geological structure of the cliff between the bay of Simonov zaliv and Ronek cape suggested the existence of a hypothetical 
Ronek anticline (photo: M. Moškon).
ACTA CARSOLOGICA 52/1 – 2023 11
consistent results are yet available. This is mainly due to 
the water sampling problem inside the marine depres-
sions, where the mixing with seawater is a rapid process, 
and at the same time the sampling is dangerous due to 
the loose, steep walls of the depressions there. However, 
preliminary results indicate that the spring waters cor-
respond to those from the borehole and are karstic in 
origin (Šušmelj et al., 2022).
Submarine and sea-level karstic springs are com-
mon along the Adriatic coast (Bakalowicz, 2018), includ-
ing some enriched with sulphur (D'Angeli et al., 2019, 
2021; Liso & Parise, 2020). These springs represent cave 
systems that were flooded after the Holocene transgres-
sion and occur along the coastal areas, which are com-
posed entirely of carbonate rocks. In contrast, almost the 
entire Slovenian coast consists of flysch, with limestone 
outcrops limited solely to the small area of the town of 
Izola. In the sea, Holocene sediments cover a rocky base-
ment, and little is known about its continuation below 
them. Therefore, the geological and hydrogeological fac-
tors that determine the exact micro-locations (spatial 
distribution) of the springs are ambiguous and their ex-
planation is the main goal of this paper. 
According to existing data, the Izola anticline is ei-
ther a dome (Pleničar et al. 1969, 1973) or an elongated 
anticline with a NW-SE directed axis (Placer et al., 2010). 
Because the Izola group of springs lies approximately on 
the north-western (seaward) prolongation of the anti-
cline axis (extension of the dome), this interpretation fits 
for this group. At the same time, the other two groups 
are completely off-axis (or dome) extensions, and our 
knowledge of the geological structure requires adequate 
reworking. The observation of flysch beds on the cliff 
between Izola and Strunjan led to the working hypoth-
esis that predicted the existence of another anticline near 
Cape Ronek, which would run parallel to the Izola anti-
cline (Figure 2), but the limestone core would be com-
pletely covered by Quaternary deposits. The existence 
of such a structure would explain the micro-location of 
the Ronek spring group, whereas the position of the Bele 
BOŠTJAN ROŽIČ & PETRA ŽVAB ROŽIČ
Figure 3: Geological map and cross-section of investigated area – (simplified from Pleničar et al., 1973 and Placer et al., 2010) with general-
ized stratigraphic column. Note that lithology colours also mark their hydrogeological characteristics, where limestone and calciturbiditic 
beds (green) are karstic aquifers and flysch (yellow) generally acts as a groundwater barrier (aquitard, aquiclude). 
ACTA CARSOLOGICA 52/1 – 202312
skale spring group would still remain debatable (maybe 
related to buried pre-Holocene river channel). 
In order to address the problem, we conducted 
sedimentological logging of the flysch deposits along the 
coastal cliffs and made detailed geological map of the ad-
jacent mainland. We present another solution that chal-
lenges our working hypothesis and provides a minor but 
persuasive alternative of the geological structure. With 
the extrapolation of newly obtained mainland geological 
data on the sea (below Quaternary deposits), we are able 
to explain the micro-locations of all of the spring groups. 
We emphasise that the main goal of this paper is to 
provide an explanation of the spatial distribution of the 
Izola submarine springs. This will serve as the basis for 
on-going hydrogeochemical research, which will provide 
further insight into the hydrogeological characterisation 
and dynamics of the sulphur springs between Izola and 
Strunjan. 
2. GEOLOGICAL SETTING
Structurally, the investigated area is part of the External 
Dinaric Imbricated Belt, which is characterized by SW-
directed structural shortening (Placer, 2008; Placer et al., 
2010). The coastal area forms its external part known as 
the Istria-Friuli Underthrust Zone and shows minor in-
ternal tectonic deformations, like the small-scale Buzet 
Thrust Fault and Izola anticline, and was also considered 
to be an undeformed Adria Foreland (Figure 3). The area 
is confined by the Buje Fault to the SW and towards the 
NE by the Palmanova (Črni kal) Thrust Fault, with the 
latter accompanied by several smaller thrust faults (Plac-
er, 2005, 2007; Placer et al., 2010). 
The entire stratigraphic succession of the study area 
is divided into four main units. The basal unit is formed 
by Upper Cretaceous to Lower Eocene limestones of the 
Dinaric/Adriatic Carbonate Platform (Pleničar et al., 
1969, 1973). The second unit is represented by Eocene 
Flysch, which consists mainly of alternating marl and 
turbiditic sandstone beds. Calciturbidites also occur, 
mainly as thin/medium-bedded, graded calcarenites, but 
several calciturbiditic megabeds are also interstratified 
(Pavšič & Peckmann, 1996; Placer et al., 2004; Vrabec 
& Rožič, 2014). Along the southern part of the Gulf of 
Trieste/Trst the coast consists mainly of Eocene Flysch, 
while in its southern part along the Istra Peninsula, in the 
core of the Izola anticline and north of the city of Trieste/
Trst, limestones are found. 
In river valleys and especially on the sea floor, 
Cretaceous to Paleogene rocks are overlain by Pliocene 
to Quaternary deposits, which can be divided into two 
units. The lower unit (third in the overall succession) 
consists of the Pliocene-Pleistocene alluvial depos-
its (mainly silty clay, and subordinate sand and gravel) 
occasionally interrupted by marine and brackish sedi-
ments. The uppermost unit is represented by Holocene 
marine deposits that cover most of the present-day sea 
floor. These are fine-grained clastic sediments (sandy silt, 
clayey silt, silt, and silty sand) with frequent foraminifera, 
bivalve, and gastropod shells (Ogorelec et al., 1987, 1991, 
1997). The transgression is dated using the radiocarbon 
method at 8.270±50 BP to 9.160±120 BP (Ogorelec et al., 
1981; Covelli et al., 2006). The thickness of the marine 
sediment cover varies from zero (in coastal areas) to ten 
meters (Romeo, 2009; Slavec, 2012; Vrabec et al., 2013; 
Trobec et al., 2018). At the same time, the Holocene is 
still characterized on the mainland by alluvial deposits 
covering valley bottoms with brackish environments at 
river mouths (Pleničar et al., 1973), and by specific sub-
recent sedimentary environments in Sečovlje and Strun-
jan Salinas (Ogorelec et al., 1981; Kovač et al., 2018). 
3. METHODS
Geological mapping was performed on the topographic 
base map at a 1:5000 scale, combined with a shaded digi-
tal elevation model with a spatial resolution of 1 m taken 
from a lidar scan in 2014–2015 (available from ARSO – 
Environment Agency of the Republic of Slovenia). For 
the elaboration of the detailed geological map (see sup-
plement material), we used a combination of two map-
ping approaches: A) all outcrop analysis (including the 
description of lithology and structural elements) and B) 
the tracing of geological boundaries, the latter particu-
larly for the calciturbiditic megabeds within the flysch. 
The orientation of beds is graphically represented using 
Steronet11 software (for details see Allmendinger et al., 
2011; Cardozo & Allmendinger, 2013). 
Sedimentological logging was performed precisely 
bed-by-bed, and all beds thicker than 1 cm were mea-
INFLUENCE OF GEOLOGICAL STRUCTURE ON MICRO-LOCATION OF SUBMARINE KARSTIC SULPHUR SPRINGS 
NEAR IZOLA (SW SLOVENIA) 
ACTA CARSOLOGICA 52/1 – 2023 13
sured and are presented in sections (Figures 6 and 7). The 
sections were logged at different locations but could be 
compiled into two separate logs (for locations see supple-
ment material and descriptions below). 
4. RESULTS 
4.1. LITHOSTRATIGRAPHIC UNITS
The description of lithostratigraphic units is made us-
ing a combination of data from previous research (gen-
eral descriptions, biostratigraphy) and our observations 
obtained during the mapping and sedimentological re-
search. This is particularly valid for the flysch deposits 
that were logged in detail in several composite sections. 
The stratigraphically lowest outcropping succession 
is found solely in the town of Izola and belongs to the 
Alveolinid-Nummulitid Limestone Formation. This for-
mation was described in detail on the Kras (Karst) Pla-
teau by Jurkovšek et al. (2013, 2017), where it documents 
a progressive deepening of the sedimentary environ-
ment. Namely, it passes from alveolinid limestone char-
acterized by an inner/middle ramp, through nummulitid 
limestone characteristic of the middle ramp, to the outer 
ramp limestone rich in discocyclinids, orbitolitids, and 
flat forms of nummulitids (Jurkovšek et al., 2013). As evi-
dent in the large discoid foraminifers, the topmost part of 
the formation in the Izola anticline is outcropping (Fig-
BOŠTJAN ROŽIČ & PETRA ŽVAB ROŽIČ
Figure 4: a) a panoramic view towards the NW of the Izola area with encircled limestone outcrops, and their seaward extrapolation due 
to anticline structure, with locations of the submarine sulphur springs, b) limestone in the coast of the town of Izola, c) close-up of the 
limestone with large benthic foraminifera, d) the uppermost part of the transitional marl between Alveolinid-Nummulitid Limestone 
Formation and flysch deposits. 
ACTA CARSOLOGICA 52/1 – 202314
INFLUENCE OF GEOLOGICAL STRUCTURE ON MICRO-LOCATION OF SUBMARINE KARSTIC SULPHUR SPRINGS 
NEAR IZOLA (SW SLOVENIA) 
Figure 5: a) Cape Kane with duplex structure and three beds (3B) used for correlation with the Cape Ronek outcrops, b) three beds (3B) used 
for correlation with Cape Kane succession near Izola, c) flysch succession exhibits characteristics of a basin plain - distal fan sedimentary 
environment with thin, laterally continuous turbiditic beds, d) calciturbiditic bed at 88 m of the Strunjan section showing entire Bouma 
sequence (Ta-e), e-g) thickest calciturbiditic bed at 207 m of the Strunjan section below the Sveti Križ viewpoint with (e) lower graded part, 
(f) middle part with wash-out marl plasticlasts (encircled), and (g) topmost part composed of calcareous marl. 
ACTA CARSOLOGICA 52/1 – 2023 15
ures 4a-c). In the Kras Plateau, the formation is dated to 
the Ypresian stage (Ilerdian) (foraminiferal biozones SBZ 
5 to SBZ 8) (Jurkovšek et al., 2013). In Istria (including 
the Izola anticline), the formation is younger and the top-
most part is Lutecian in age (foraminiferal biozone SBZ 
14) (Pavlovec, 1985; Drobne et al., 2009). 
The limestone is overlain by an interval of transi-
tional calcareous marl several tens of meters thick, known 
as Globigerina marl (also as beds with crabs) (Figure 4d). 
This unit outcrops poorly, and its exact thickness is hard 
to determine, but based on the geological map it prob-
ably does not exceed 50 meters. The age of these beds is 
already Lutetian (nanoplankton biozone NP 14 in Pav-
lovec & Pavšič, 1986; and NP 15/16 in Pavšič & Peck-
mann, 1996).
Upwards, turbiditic sandstone beds start to occur 
within the marl, and the succession gradually changes to 
typical flysch facies (Figures 5a-c). It is composed of al-
ternating marl and thin- to medium-bedded sandstone, 
which exhibits a partial Bouma sequence. In most of the 
beds, Tb-Tc parts of the sequence are present, but rarely 
show the full sequence. Calciturbidites also occur, and 
in the mapped area form beds from 10 cm to 9 m thick 
(Figures 5d-g). The entire flysch succession is more than 
600 m thick (Pleničar et al., 1973) and is Lutetian in age 
(nanoplankton biozones NP 16; in Pavšič and Peckmann 
1996). 
4.2. DETAILED SEDIMENTOLOGY OF MAPPED 
PART OF THE FLYSCH
A detailed logging of the flysch was performed to pro-
duce a lateral tracing of the particular turbiditic beds be-
tween Izola and Cape Ronek and the position of the par-
ticularly thick calciturbiditic beds within the complete 
flysch succession. A total of 264 meters of the flysch was 
logged, and sections cover almost the entire succession 
between the transitional marl interval and the top of the 
thickest calciturbiditic bed. 
The basal part of the flysch was logged in the Si-
monov zaliv section at a thickness of 51 m (start N 
45°31'54'', E 13°38'38'', end N 45°31'60'', E 13°38'22'') 
(Figure 6). Our fieldwork observations indicate that the 
section starts just above the transitional marl interval. It 
consists of three subsections that are stratigraphically po-
sitioned one above the other, but an exact lateral stacking 
of particular beds (as in the Strunjan section) was not 
possible. Nevertheless, we consider the logged succes-
sion to be nearly continuous. The first subsection (22 m) 
starts above the beach at the Simonov zaliv bay and was 
logged towards the west up to the part of the cliff covered 
by vegetation and tuffa. The second subsection (24.5 m) 
continues on the other side of the covered cliff until the 
BOŠTJAN ROŽIČ & PETRA ŽVAB ROŽIČ
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Figure 6: The Simonov zaliv section is compiled from three subsec-
tions that represent stratigraphic continuation (Sign 3B marks the 
stratimetric position of the three beds used for correlation with the 
Strunjan section – for details see text). 
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160m
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3B
Figure 7: Strunjan section is dominated by siliciclastic turbidites (brown), whereas calciturbidites (other intense colours) are sporadic but 
can stand out. Beds at 88m (blue) and 207m (yellow) were laterally traced during detailed geological mapping. 3B marks a stratimetric 
position of 3 beds used for correlation with the Simonov zaliv section. 
ACTA CARSOLOGICA 52/1 – 2023 17
end of the cliff outcrops. The third subsection (4.5 m) was 
logged at Cape Kane.
The Simonov zaliv section begins with alternations 
of thin sandstone beds and marl (lower part of the first 
subsection). In the middle part of the section (upper part 
of the first subsection and lower part of the second sub-
section), several medium-sized beds (up to 36 cm thick) 
are present. Above, thin beds again prevail, with the ex-
ception of a 65-cm-thick bed at 35.5 m of the section 
(13.5 m of the second subsection). The top of the section 
(third subsection) is characterized by three medium-
sized sandstone beds (33, 24, and 38 cm), which contain 
large marl mud-chips (marked as 3B in Figures 5a, 6 and 
9). These clasts are washed out on the weathered surface, 
which gives it the appearance of a void. Just above these 
three beds a large-scale duplex structure (Figure 5a) of 
Cape Kane is positioned and is either tectonic or syn-
sedimentary (our preference) in origin (Vrabec & Rožič, 
2014). We note that these three beds are also repeated 
due to a minor thrust. 
The rest of the flysch was logged in the 216-meter-
thick Strunjan section (Figure 7). It is compiled from 
three subsections that represent a direct stratigraphic 
continuation of the flysch. The lower subsection (0-89 
meters of the section) was logged on the cliff east of Cape 
Ronek (start N45°32'24'' E13°36'60'', end N45°32'14'' 
E13°37'13'') (A lateral tracer to a base of the middle 
subsection was the first prominent calciturbidite mega-
bed). The middle sub-section was logged in the Mesečev 
zaliv bay from 88 to 187 meters of the section (start N 
45o32'18'' E13o36'44'', end N 45o32'15'' E 13o36'26'') (A 
lateral tracer to a base of the upper subsection was a tur-
biditic bed with a thick marl top). The upper subsection 
from 187 to 216 meters of the section was logged at the 
base of the cliff below Sveti Križ towards Cape Punta, 
and included the thickest calciturbiditc megabed (start N 
45o32'16'' E 13o36'18'', end N 45o32'13'' E 13o36'07''). The 
latter was additionally logged at the top of the cliff on the 
Sveti Križ viewpoint (Figure 5d).
The Strunjan section (Figure 7) represents the di-
rect stratigraphic continuation of the Simonov zaliv sec-
tion, which is deduced from the fact that the basal part of 
the Strunjan section (on the Cape Ronek) contains three 
medium-sized sandstone beds (50, 16, and 35 cm) with 
large, mostly washed-out marl mud-chips (marked as 3B 
in Figures 5b, 7 and 9). Based on these characteristics, the 
stacking pattern of under/overlying successions, and an 
analysis of the entire cliff outcrop we suggest that these 
three beds are the same as those from the topmost part of 
the Simonov zaliv section. We attribute the minor varia-
tions in the thickness of particular beds to the variability 
of lateral thickness. 
Above, approximately 200 m of the flysch is com-
pletely exposed. This succession shows typical distal-fan 
development with numerous, thin- to medium-bedded, 
laterally continuous siliciclastic turbidites (coloured 
brown in Figures 6 and 7). The majority of these beds are 
less than 10 cm thick, medium-sized beds are sporadic, 
and in the lower part of the section they occur at a dis-
tance marginally greater than 10 meters, whereas in the 
upper part of the section they are virtually absent (just a 
few siliciclastic turbiditic beds are 25 cm thick). 
Calciturbiditic beds start to occur in the upper part 
of the Strunjan section (intense colours in the Figure 7). 
The first bed (at the 48 m of the section) still exhibits a 
high siliciclastic content, whereas upwards these beds be-
come almost pure limestones. Two calciturbiditic (mega)
beds stand out for their thickness: the first occurs at the 
89 m of the section, showing the full Bouma sequence, 
and is 1.45m thick, with the Te part (marl top) 60 cm 
thick (Figure 5d). The thickest bed forms the top of the 
Strunjan section (Figures 5e-g). It exhibits the full Bou-
ma sequence and is 9 m thick. In the transition from the 
Ta to Tb part of the Bouma sequence, large marl mud-
chips are present. As with the siliciclastic beds, they are 
washed out and give the typical appearance of a void 
(Figure 5f). The marl top (the Te part of the Bouma se-
quence) is approximately 4 m thick. These two megabeds 
were also used as lateral tracers for detailed geological 
mapping. Between these two megabeds, five other calci-
turbiditic beds occur. These beds are thinner (10 to max. 
60 cm) and exhibit base-cut Bouma sequences (Tc-Te, 
Td-Te parts). 
4.3. GEOLOGICAL MAP
The entire mapped area shows minor structural defor-
mations. The Sveti Križ Thrust represents a major struc-
tural discontinuity that occurs in the western part of the 
mapped area and which outcrops in the Mesečev zaliv 
bay below the Sveti Križ viewpoint. The displacement is 
assessed to 80 meters and is particularly visible due to 
the movement of the upper megabed. The thrust plane 
dips towards the NW (Placer et al., 2010; Vrabec & Rožič, 
2014) and the orientation was measured at the coast to 
62°/21°. Towards Strunjan, i.e. in the footwall of the Sveti 
Križ Thrust, several folds are visible in the cliff. On the 
cliff at Cape Kane near Simonov zaliv bay a complex du-
plex structure is perfectly exposed (Figure 5a). Several 
indices, such as strongly folded beds, thick marl cover 
with practically absent sandstone beds, and partly visible 
overlying continuous sandstone beds indicate its syn-
sedimentary (slump) origin. Laterally, the rather chaotic 
folds seen at Cape Ronek could indicate the same syn-
sedimentary soft sediment deformation. Several other 
minor folds and thrusts of more univocal tectonic origin 
are seen in the cliff between Izola and Strunjan (e.g. see 
BOŠTJAN ROŽIČ & PETRA ŽVAB ROŽIČ
ACTA CARSOLOGICA 52/1 – 202318
description of the Simonov zaliv section). Some normal 
faults with only minor displacement (usually less than a 
meter of displacement) are also present in the area (see 
also Vrabec & Rožič, 2014). Placer et al. (2010) mention a 
bedding – a parallel thrust zone in the transitional marls, 
but this structure was not detected during our geological 
mapping.
The major structure of the mapped area is an Izola 
anticline (Figures 8 and 9). In the core of the anticline 
an Alveolinid - Nummulitid Limestone Formation is out-
cropping, with outcrops limited to the area of the town of 
Izola and surrounded by a rim of poorly exposed transi-
tional marl. The rest of the mapped area is composed of 
flysch. The anticline is reflected also in the outcrops of 
the two calciturbiditic beds that were traced across the 
entire area. The upper (thicker) megabed shows almost 
continuous outcrops from the Sveti Križ viewpoint to the 
south-easternmost point of the mapped area near Baredi. 
Similarly, the lower megabed can be traced in the area 
between Izola and Semedela (Koper suburbs), whereas 
it is discontinued in the Bele skale area and sinks below 
the sea, and in the area of Jagodje (Izola suburbs) due to 
the settlement.
Measurements of the orientation of beds revealed 
a uniform azimuth in the anticline SW limb between 
Mesečev zaliv bay and Šared village (grey lines in Figure 
8), which are generally oriented towards the SSW (mostly 
between 190° and 210°), and dip mostly between 15° and 
25°. Between the village of Šared and the Segadici hill the 
azimuth changes towards the south (around 180°) with 
similar dips (blue lines in Figure 8). In the eastern part 
of the mapped area between the Segadici hill and Izola 
Hospital the dip becomes almost horizontal (from 5° to 
10°), but the azimuth gradually shifts towards the east 
(orange lines in Figure 8). In the north-eastern corner of 
the mapped area, north of the Izola Hospital, the azimuth 
changes towards the NNW (around 30°) with a dip of 10° 
(green lines in Figure 8). From the compiled data we con-
clude that the Izola anticline is asymmetric, with steeper 
dips in the SW limb; it has a very poorly expressed half 
dome shape in its ESE part and the axis of the anticline is 
oriented in the WSW-ESE direction. The Izola anticline 
architecture generated from our research generally cor-
responds to previous geological maps but brings minor 
yet important modifications. The dome proposed by 
Pleničar et al. (1973) is only partly recognised in the ESE 
part of the anticline. From the town of Izola towards the 
WNW, limbs (at least the outcropping SW limb) shows 
a rather uniform dip of the beds all the way to the last 
outcrops in the Strunjan area. The axis of the anticline is 
therefore moderately rotated anticlockwise compared to 
the previously drawn NW-SE orientation (Placer et al., 
2010). 
5. DISCUSSION
Much of the Slovenian coast is marked by a thick sequence 
of Eocene flysch deposits. Only in the core of the Izola 
anticline does the underlying Eocene limestone outcrop. 
It is surrounded by a rim of transitional marl succession 
(seen in and around the town of Izola). But neither the 
dome structure (Pleničar et al., 1973) nor the NW-SE ori-
ented Izola anticline (Placer et al., 2010) can successfully 
explain the micro-locations of the Ronek and Bele skale 
spring groups. Furthermore, as revealed by our research, 
there is no additional anticline in the Cape Ronek area, 
and an azimuth as well as a dip of beds remains rather 
constant between Izola and Strunjan. The supposed Cape 
Ronek anticline is therefore only the result of a simple 
rule of V’s on the slightly curved coastline. This contra-
dicts our working hypothesis and another solution need 
to be proposed and assessed. 
INFLUENCE OF GEOLOGICAL STRUCTURE ON MICRO-LOCATION OF SUBMARINE KARSTIC SULPHUR SPRINGS 
NEAR IZOLA (SW SLOVENIA) 
Figure 8: Steronet diagram of the dips of beds in the Eocene flysch 
indicates a WNW – ESE oriented axis of the Izola anticline with 
poorly expressed dome at the ESE part of the mapped area; grey 
lines - area between Mesečev zaliv bay and village of Šared, blue – 
area between village of Šared and Segatici hill, orange lines – area 
between Segadici hill and Izola Hospital, green lines – area north 
of Izola Hospital.
ACTA CARSOLOGICA 52/1 – 2023 19
BOŠTJAN ROŽIČ & PETRA ŽVAB ROŽIČ
Figure 9: a) Schematic presentation of the geology including proposed hydrogeological conceptual model (frontal view – for better presenta-
tion, Quaternary sediments are presented in a step behind front view) of the area between Izola and Strunjan, and b) simplified geologi-
cal map, stratigraphic column and cross section of the Izola area: Izola anticline has a dome structure on the SE part and is WNS–ESE 
oriented, which explains the position of all springs – the Izola group is located at the northern margin of the anticline limestone core, and 
Ronek and Bele skale at the southern margin. 
ACTA CARSOLOGICA 52/1 – 202320
INFLUENCE OF GEOLOGICAL STRUCTURE ON MICRO-LOCATION OF SUBMARINE KARSTIC SULPHUR SPRINGS 
NEAR IZOLA (SW SLOVENIA) 
Our research points to minor yet crucial differences 
in the structure of the investigated area. Firstly, it revealed 
that the dome structure (cf. Pleničar et al. 1969, 1973) of 
the Izola anticline is present, but is very mildly expressed 
in the outcropping part, i.e. in the ESE part of the anti-
cline. Second, a more important structural alternation is 
the anticlockwise rotation of the Izola anticline axis that 
is not oriented in the NW-SE direction (cf. Placer et al., 
2010), but exhibits the WNW-ESE orientation (Figure 9). 
The SW limb of the anticline, which is well exposed on 
the surface, shows uniform azimuth (around 200°) and 
a relatively steep (up to 29°) dip of beds in a large part of 
the mapped area between the village of Šared and Strun-
jan. The opposite limb of the anticline is exposed solely 
in the NE corner of the mapped area. Measurements are 
scarce because the area is anthropogenically changed (it 
is already part of the town of Koper), but they reveal an 
approximately opposite azimuth (around 30°) and less 
dipping of beds (10°). This indicates an asymmetric an-
ticline, which is in accordance with the previously estab-
lished origin of the Izola anticline, which was interpreted 
as a thrust-propagating fold that originated with the anti-
clinal bending of the beds in the hanging wall of the Sveti 
Križ Thrust (Placer et al., 2010, Vrabec & Rožič, 2014). 
Such origin is also evident from the subsurface data from 
the Gulf of Trieste/Trst (Busetti et al., 2010a, 2010b). We 
note that the azimuth and dip of the Sveti Križ Thrust 
(62°/21°), measured at the coast in Mesečev zaliv bay, 
does not perfectly match this model (the azimuth should 
be around 20° to 30°). This can be explained either by 
the local bending of a thrust plane or, alternatively, with 
thrusting that post-date or predate folding. In the second 
scenario, the thrust plane was primarily oriented like the 
other Dinaric thrusts (dipping towards NW, i.e., 45°), 
and was rotated slightly towards the east due to younger 
folding that created the Izola anticline. A similar post-
deformation inclination was proposed also for the duplex 
structure at Cape Kane (Vrabec & Rožič, 2014). In this 
scenario, the top of the limestone core would be lowered 
westwards of the Sveti križ Thrust (as shown in Figure 
9a). However, exact geotectonic interpretation extends 
beyond the scope and focus of this paper and encourages 
further detailed structural and geophysical analysis of the 
area under discussion.
The described alteration of existing structural data 
provides a simpler but hydrogeologically consistent ex-
planation of the spatial distribution of all spring groups. 
It is based on a hydrogeological conceptual model 
(Figure 9a – frontal view) that predicts the following: 
A) water springs from the limestone aquifer (core of 
Izola anticline) at the stratigraphic boundary between 
the limestone and the overlying siliciclastic succession 
(groundwater barrier), B) springs are located in the near-
shore area, where the sedimentary cover of Quaternary 
deposits is thin enough to be washed out, and C) as is 
evident from the elevated temperatures, sulphur min-
eralization, and similarities with the LIV-1/01 borehole 
(Benedik & Rožič; 2002; Žumer, 2004, 2008; Faganeli et 
al., 2005; Žvab Rožič et al., 2021; Šušmelj et al., 2022), 
groundwater originates from well buried limestone for-
mations, below the coal-rich Liburnia Formation. We 
predict that the groundwater is recharged by a deep 
hypogenic loop from the NE (Kras Plateau) and/or SW 
(Istra Peninsula). Upward, the water penetrates the coal 
layers (internal hydrogeological barriers within the thick 
limestone sequence) at various, supposedly structurally 
governed points, and rises to the surface along karstified 
(fracture) zones. In areas where it meets the stratigraphic 
contact, it flows along it and finally springs at the margin 
of the Izola limestone core.
Combining the proposed hydrogeological concep-
tual model and a moderate anticlockwise rotation of the 
previously described (Placer et al., 2010) Izola anticline 
axis leads to the solution of micro-locations for all sub-
marine sulphur springs. Namely, if we prolong the strati-
graphic contact between the limestone and the overlying 
siliciclastic beds into the sea (below Quaternary depos-
its) parallel to the newly established axis orientation (Fig-
ure 9), we see that all spring groups fall along the margin 
of the Izola anticline limestone core. The Ronek and Bele 
skale groups are located on the southern side, and the 
Izola group on the northern side of the limestone core of 
the Izola anticline. The grouping of springs into the three 
areas can be attributed to the minor structures within the 
limestone, such as faults and associated fracture zones, 
which either act as local barriers or increase the perme-
ability of the rocks (Šušteršič et al., 2001; Šušteršič, 2006; 
Čar, 2018). This is particularly evident in the NW-SE lin-
ear distribution of the Izola group. 
Alternatively, the position of springs can be con-
nected to the variable thickness of the overlying Qua-
ternary sediments above the particular segments of the 
Izola anticline limestone core. The latter is indicated in 
the area of the Cape Ronek group. In this area, small-
scale bioconstructions (reef) exist (Falace et al., 2011; Li-
pej et al., 2016; Orlando-Bonaca et al., 2017). We propose 
that the reef formed above the topographically elevated 
area, which existed in the area of the Eocene limestone 
outcrops and would somehow represent a small-scale 
equivalent of present day Izola. During the transgression, 
such a topographic height would provide a hard substrate 
appropriate as a habitat for the Holocene reef commu-
nity. Our speculation is supported by both the position 
of this reef as well as geological data. The following is 
considered: A) three thicker sandstone beds (3B in Fig-
ures 4, 6, 7 and 9) that outcrop on the coast and form the 
ACTA CARSOLOGICA 52/1 – 2023 21
BOŠTJAN ROŽIČ & PETRA ŽVAB ROŽIČ
base of the duplex structure in Cape Kane are the same 
as the sandstone beds outcropping along the coast of 
Cape Ronek, B) the thickness of the siliciclastic succes-
sion below these three beds is the same at both locations 
(assessed to 120 m), and C) the calculated horizontal sea-
ward distance between the three beds and the limestone 
outcrops, based on the dip of beds at approximately 300 
meters. The reef and first springs of the Ronek and Bele 
skale groups are positioned at precisely such a distance 
from the coast, which proves the existence of limestone 
below the reef as well as the surrounding springs. The up-
per surface of this limestone might be elevated and the 
Quaternary sedimentary cover consequently thinner, at 
any rate enabling the groundwater to spring on each side 
of the reef area. This is already visible in the general map 
indicating the thickness of the Holocene marine sedi-
ment inside the entire Gulf of Trieste/Trst (Trobec et al., 
2018), but further, more detailed geophysical research of 
this particular area would be useful and well serve to bet-
ter frame our proposal. 
The absence of springs further to the west is ex-
plained either by thicker Quaternary deposits that dis-
able the spring waters to penetrate through them or by 
the termination of scantly buried limestone due to the 
Sveti Križ thrust plane (Figure 9a). The latter is pos-
sible, if we consider a measured dip of the thrust plane 
(62°/21°) from the Mesečev zaliv bay as characteristic for 
the entire thrust plane. 
6. CONCLUSIONS
The submarine sulphur karstic springs in the Slovenian 
part of the Gulf of Trieste/Trst near the town of Izola, 
are divided into three groups: the western Ronek group, 
the intermediate Bele skale group, and the eastern Izola 
group. Previous investigations already linked the posi-
tion of the springs to the geological structure charac-
terized by the NW-SE-oriented Izola anticline, which 
has a limestone core and flysch limbs. However, based 
on pre-existing geological data, only the position of the 
Izola group was successfully explained by such interpre-
tation. Our detailed sedimentological logging and geo-
logical mapping, and the extrapolation of the geological 
basement beneath the seafloor (below the Quaternary 
deposits) provided a structural solution that explains the 
micro-locations of all spring groups in question. 
We demonstrated that the Izola anticline has a 
slightly different orientation than previously suggested, 
namely it is oriented in the WNW-ESE direction. We pro-
pose that springs occur A) at the stratigraphic boundary 
between the limestone (aquifer) and flysch (aquiclude, 
aquitard), B) in the near-shore, where the sediment cover 
of the Quaternary deposits is thin, and C) the Izola spring 
group is located on the northern limb, and the Ronek and 
Bele skale groups on the southern limb of the Izola anti-
cline limestone core. Our research provides an explana-
tion for the spatial distribution of the submarine sulphur 
springs, moreover on-going hydrogeochemical studies 
will provide further insights regarding the specific char-
acteristics, dynamics, and sulphur origin of the springs. 
ACKNOWLEDGMENTS
This paper is written for a boy who loves the water, par-
ticularly seawater. The research was financially supported 
by the Slovenian Research Agency (research core funding 
No. P1-0195, research projects No. J1-2477 and No. J1-
1712). Our anonymous reviewers are thanked for their 
thorough review of the manuscript. We thank Jeff Bickert 
for his copy edit of the text and students of the Depart-
ment of Geology, University of Ljubljana, for their assis-
tance in the section logging. 
ACTA CARSOLOGICA 52/1 – 202322
INFLUENCE OF GEOLOGICAL STRUCTURE ON MICRO-LOCATION OF SUBMARINE KARSTIC SULPHUR SPRINGS 
NEAR IZOLA (SW SLOVENIA) 
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NEAR IZOLA (SW SLOVENIA) 
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ACTA CARSOLOGICA 52/1 – 2023 25
BOŠTJAN ROŽIČ & PETRA ŽVAB ROŽIČ
SUPPLEMENT
M05
M04
M06
M07
M08
M11 M10
M09
M01
M02
M03
21
38
23
20
14
15
25
26
22
23
14
13
18
14
10 18
28
29
32
5
15
29
21
12
25
15
25
20
6
5
5
4
10
10
22
24
20
20
18
19
24
20
24
10
18
20
22
22
2324
23
21
19
16
10
15
14
1521
15
14
22
18
20
24
21
16
19
12
5
15
13
12
5
7
10
10
10
10
10
10 10
10
10
5
5
5
5
5
5
5
5
5
25
10
8
5
8
5
10
7
10
10
5
CAPE RONEK
CAPE KANE
IZOLA
BELE SKALE
ROCK FALL
SV. KRIŽ
BAY MESEČEV ZALIV
BAY SIMONOV ZALIV
IZOLA HOSPITAL
Alveolinid - Nummulitid limestone
reefazimuth and dip of beds
thrust fault
folds
Ronek spring cluster
Bele skale spring cluster
Izola spring clusterGlobigerina marl
Flysch - lower part
First calciturbidite megabed
Flysch - middle part
Second calciturbidite megabed
Flysch - upper part
Adriatic Sea
3 correlation beds
Stratigraphic 
column
Mapped lithostratigraphic
units 
Submarine
featuresStructural elements
Stratigraphic boundaries 
100m
3B
fault
anticline
covered (extrapolated)
exposed 
500 m
26
3B
Sections
SEGADICI HILL
VILLAGE BAREDI
VILLAGE ŠARED
IZOLA - JAGODJE
Simonov zaliv
Strunjan
Lower subsection
Middle subsection
Upper subsection
Lower subsection
Middle subsection
Upper subsection
3B
3B
ACTA CARSOLOGICA 52/1 – 202326
INFLUENCE OF GEOLOGICAL STRUCTURE ON MICRO-LOCATION OF SUBMARINE KARSTIC SULPHUR SPRINGS 
NEAR IZOLA (SW SLOVENIA) 
M05
M04
M06
M07
M08
M11 M10
M09
M01
M02
M03
21
38
23
20
14
15
25
26
22
23
14
13
18
14
10 18
28
29
32
5
15
29
21
12
25
15
25
20
6
5
5
4
10
10
22
24
20
20
18
19
24
20
24
10
18
20
22
22
2324
23
21
19
16
10
15
14
1521
15
14
22
18
20
24
21
16
19
12
5
15
13
12
5
7
10
10
10
10
10
10 10
10
10
5
5
5
5
5
5
5
5
5
25
10
8
5
8
5
10
7
10
10
5
CAPE RONEK
CAPE KANE
IZOLA
BELE SKALE
ROCK FALL
SV. KRIŽ
BAY MESEČEV ZALIV
BAY SIMONOV ZALIV
IZOLA HOSPITAL
Alveolinid - Nummulitid limestone
reefazimuth and dip of beds
thrust fault
folds
Ronek spring cluster
Bele skale spring cluster
Izola spring clusterGlobigerina marl
Flysch - lower part
First calciturbidite megabed
Flysch - middle part
Second calciturbidite megabed
Flysch - upper part
Adriatic Sea
3 correlation beds
Stratigraphic 
column
Mapped lithostratigraphic
units 
Submarine
featuresStructural elements
Stratigraphic boundaries 
100m
3B
fault
anticline
covered (extrapolated)
exposed 
500 m
26
3B
Sections
SEGADICI HILL
VILLAGE BAREDI
VILLAGE ŠARED
IZOLA - JAGODJE
Simonov zaliv
Strunjan
Lower subsection
Middle subsection
Upper subsection
Lower subsection
Middle subsection
Upper subsection
3B
3B
ACTA CARSOLOGICA 52/1 – 2023 27