Thermal maturity of Carboniferous to Eocene Sediments of the Alpine–Dinaric Transition Zone (Slovenia)

International Journal of Coal Geology 157 (2016) 19–38

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Thermal maturity of Carboniferous to Eocene Sediments of the Alpine–Dinaric Transition Zone (Slovenia) T. Rainer a,⁎,1, R.F. Sachsenhofer a, P.F. Green b, G. Rantitsch a, U. Herlec c, M. Vrabec c a b c

Mining University Leoben, Department for Applied Geosciences and Geophysics, 8700 Leoben, Austria Geotrack International, 37 Melville Road, Brunswick West, Vic 3055, Australia University of Ljubljana, Faculty of Natural Sciences and Engineering, Department of Geology, Aškerčeva 12, 1000 Ljubljana, Slovenia

a r t i c l e

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Article history: Received 9 March 2015 Received in revised form 4 August 2015 Accepted 7 August 2015 Available online 12 August 2015 Keywords: Vitrinite reflectance Flysch Slovenian Basin Southern Alps Northern Dinarides

a b s t r a c t The Alpine–Dinaric Transition Zone in Slovenia comprises the fold and thrust belt of the Southern Alps (South Karawanken Range, Julian Alps), Slovenian Basin and the Dinarides. The Slovenian Basin located between the Julian and Dinaric carbonate platforms evolved during Middle Triassic time and remained in a deep marine setting till the Late Cretaceous. The thermal history of Carboniferous to Eocene rocks in that area was investigated using vitrinite reflectance (VR) data, apatite fission track analysis and numeric 1D basin modeling. The study shows that maturity patterns are mainly controlled by the thickness of Upper Cretaceous to Eocene flysch deposits, filling the accommodation space. Therefore the thermal overprint reaches a maximum (N4%Rr) in Triassic to Cretaceous sediments of the Slovenian Basin and decreases towards the north and south. Minor sedimentary burial of the Adriatic Carbonate Platform and the Julian Alps results in a lower thermal overprint (b 1.5%Rr). The thickness of flysch sediments was about 5 km in the area of the Sava Folds, but significantly higher in the central part of the Slovenian Basin. Heat flow during maximum burial in Eocene time was in the order of the global average (60 mW/m2). Cooling of Paleozoic and Mesozoic sediments below 110 °C occurred between Late Eocene and Early Oligocene times in different parts of the study area. Nappe stacking due to Early (Dinaric) and Late Cenozoic (Alpine) compressional tectonics did not influence the thermal maturity of the sediments. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The Alpine–Dinaric Transition Zone is situated at the intersection of the Paleogene SW-vergent Dinarides, the Neogene S-vergent Southern Alps and the N-vergent Eastern Alps (Fig. 1). Cenozoic overstep sequences associated with the Pannonian Basin System cover these geotectonic units to the east. The study area is located in the transition zone of the Southern Alps to the Dinarides and holds a stratigraphic inventory from Carboniferous to the Quaternary rocks. Due to the complex tectonic history of the region, including rifting, thrusting, lateral escape tectonics and still active fault systems, combined with the broad stratigraphic range of preserved sediments, the Alpine–Dinaric Transition Zone is a perfect region to study the effects of tectonics and sedimentary burial on the thermal alteration of organic material. ⁎ Corresponding author at: OMV Exploration & Production GmbH, Trabrennstrasse 6-8, 1020 Wien, Austria. E-mail addresses: [email protected] (T. Rainer), [email protected] (R.F. Sachsenhofer), [email protected] (G. Rantitsch), [email protected] (U. Herlec), [email protected] (M. Vrabec). 1 Present address: OMV Exploration & Production GmbH, Trabrennstrasse 6–8, 1020 Vienna, Austria.

http://dx.doi.org/10.1016/j.coal.2015.08.005 0166-5162/© 2015 Elsevier B.V. All rights reserved.

Previous thermal maturity investigations in the Dinaric and Southern Alpine realm focused on continuous vitrinite reflectance (VR) trends across the Carboniferous “Variscan discordance” (Rainer et al., 2009; Rantitsch, 1997, 2007; Rantitsch and Rainer, 2003; Rantitsch et al., 2000), on the comparison of organic matter maturation and clay mineralogy (Rainer et al., 2002), and on Upper Eocene to Miocene rocks (Sachsenhofer et al., 2001). The main aim of this paper is to document lateral and stratigraphic maturity trends in sediments ranging from Carboniferous to Paleogene in age and to assess the timing and the main geodynamic parameters controlling thermal maturity. Hence, this study may contribute to the understanding of maturity evolution and timing of hydrocarbons in similar tectonic settings elsewhere. 2. Geological setting The Alpine–Dinaric Transition Zone is subdivided into several macrotectonic units (e.g. Placer, 1999a; Fig. 1). The Eastern Alps are situated north of the dextral Periadriatic Fault (PF). The Southern Alps, including the South Karawanken Range and the Julian Alps (Julian Carbonate Platform), occur between the PF and the E–W trending deep marine Slovenian Basin. The Dinaric and Adriatic Carbonate Platforms (CP) follow towards the south.

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Fig. 1. Macrotectonic subdivision of the Alpine–Dinaric Transition Zone (modified after Placer, 1999a). BB — Belluno Basin, LB — Lombardian Basin, PB — Po Basin, ZB — Zala Basin.

The Julian Alps emerged from the Julian CP, as did the Dinarides from the Dinaric CP. To the west, south and east of Ljubljana, the Dinarides are composed of individual nappes (e.g. Hrušica Nappe, Trnovo Nappe). East of Ljubljana, the northernmost part of the Dinaric CP was intensively folded during Late Cenozoic times and the Sava Folds were formed (Placer, 1999b). The Sava Folds consist of Paleozoic and Mesozoic rocks of the Dinaric CP and Mesozoic rocks of the Slovenian Basin. 2.1. Stratigraphy The sedimentary records of the Dinaric Carbonate Platform (CP), the Slovenian Basin and the Julian CP are shown in Fig. 2. After a broad continental clastic deposition during the Carboniferous to Middle Permian times a uniform shallow carbonate platform was established throughout the study area as a consequence of a regional transgression (Buser, 1989; Ogorelec, 2011). This “Slovenian” platform (Buser, 1989) existed up to the mid-Anisian times. Due to rifting-related extensional tectonic characteristic of the southern Tethyan margin (Bertotti, 1991; Bertotti et al., 1993a,b; Stampfli et al., 1991; Winterer and Bosellini, 1981) the unique carbonate platform was disintegrated during the Middle to Late Anisian into the northern Julian CP and the southern Dinaric CP separated by the Slovenian Basin (SB; Buser, 1989).

The pre-Anisian lithostratigraphy of the unique Slovenian platform (sensu Buser, 1989) is the best evident (and sampled for this study) in the area of the Dinaric CP, where the sedimentary cycle starts with Carboniferous (Visean to Lower Westphalian) flysch deposits, overlain with an angular unconformity (“Variscan discordance”) by shallowmarine molasse sediments (Auernig Grp.; Upper Moskovian to Upper Artinskian) and Middle Permian fluviatile to lagoonal clastic sediments (Gröden Fm.; Germovšek, 1961; Skaberne, 1995). A Late Permian transgression resulted in the formation of a shallow marine Upper Permian (Žažar Fm.; Buser et al., 1986; Ramovš, 1958) to Anisian carbonate platform. Carbonate rocks with clastic layers were deposited during Scythian time (Werfen Grp.; e.g. Grad and Ogorelec, 1980). During the Ladinian some 100 m black shales, alternating with greywacke, tuff and local intercalations of thin-bedded limestone (Pseudozilian Fm.) were deposited in the SB, whereas platy shallow water limestone with chert, marl, coal and tuff layers accumulated on the basin margins (Bavec, 1999; Buser, 1989; Demšar and Dozet, 2003; Ramovš, 1994/1995). On uplifted blocks, erosion cut into Carboniferous horizons (e.g. Idrija region; Čar, 1988/89; Mlakar, 1969). Intensive volcanic activity started contemporaneously. During Carnian time thick platform carbonates accumulated on the Julian CP and the Dinaric CP (Buser, 1989, 1996). The subsequent

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Fig. 2. Generalized stratigraphic profiles and sampled horizons of the Dinaric Carbonate Platform, the Slovenian Basin and the Julian Carbonate Platform (note that the Adriatic Carbonate Platform, consisting in the study area of Cretaceous to Eocene rocks, is not shown).

evolution is described separately for each of the three areas (Julian CP, Slovenian Basin, Dinaric CP). On the Julian CP shallow-water carbonate sedimentation continued in Norian and Rhaetian times with deposition of dolostone (Hauptdolomit Fm.) and limestone (Dachstein Fm.), more than 1000 m thick (Jurkovšek et al., 1988/89; Ogorelec et al., 1984). The carbonate platform began to subside differentially in the Late Liassic. Pelagic Jurassic and Cretaceous deposits (Smuč, 2005; Smuč and Goričan, 2005) overlie transgressively the older beds. Probably because of erosion, the extension of Aptian marls is limited. Following a Turonian continental regime, Campanian flysch deposits accumulated in the Bovec Basin (see Fig. 3) and southern slope of Mt. Krn (Cousin, 1981; Kuščer et al., 1974). In the late Campanian, the Julian Alps were uplifted and emerged (Buser, 1989). Clastic rocks were deposited during the Oligocene near Lake Bohinj. In the Slovenian Basin (SB) deep marine settings persisted during the Late Triassic with deposition of dark shales, sandstones and carbonates (Carnian Amphiclina Fm., Fig. 2) and dolostones (Norian–Rhaetian Bača Fm., Buser and Ogorelec, 1987). The Jurassic succession is characterized by pelagic sedimentation of siliceous limestones and radiolarian cherts (Cousin, 1981; Rožič, 2009; Rožič and Popit, 2006). Towards the west the SB pinches out in the area of Trnovo (see Fig. 3 for location). There, shaly sequences are laterally replaced by limestone breccia with minor shale content (Buser and Debeljak, 1994/1995). In the northern part of the SB, Tithonian–Berriasian limestone is overlain by Valanginian–Hauterivian flysch deposits (Buser, 1989). The latter, representing the oldest Cretaceous flysch deposits within the SB, crop out in a tectonic window near Lake Bohinj beneath the Julian CP. In the central and the southern part of the SB these rocks are missing and Aptian–Lower Cenomanian dark flyschoid sediments, up to 500 m thick, directly overlay Doggerian or Tithonian–Berriasian limestone. The Upper Turonian to Lower Campanian platy Volče limestone, 50 to 250 m thick, overlies the flysch deposits and is followed by Maastrichtian carbonate breccia and flysch (Ogorelec et al., 1987a). The latter rocks accumulated along the deeper southern slope of the

Slovenian Basin, where pelagic sediments interfinger with turbidites from the Dinaric CP. The cumulative thickness of clastic Maastrichtian deposits in the border area between Italy and Slovenia reaches 1800 m (Tunis and Venturini, 1987). The Dinaric CP persisted from Carnian to Paleogene time and consists of a carbonate succession with an average thickness of 4000 to 5000 m (e.g. Dozet and Strohmenger, 1994/1995). Middle Carnian uplift resulted in karstification and the deposition of coal, which has been mined, e.g. near Horjul and Orle (Jelen, 1988/1989). Norian and Rhaetian strata, about 1000 m thick, are dominated by the Hauptdolomit Fm. and relative thin Dachstein limestone (Ogorelec and Rothe, 1993). The Jurassic succession is up to 1500 m thick (Buser and Debeljak, 1994/1995; Dozet and Šribar, 1997). Locally, Middle Liassic shallow-water successions contain strongly bituminous dolomite and thin coal seams (e.g. E of Kočevlje; Dozet, 1998, 1999). Near the SB, tectonic movements caused the emergence of the marginal parts of the Dinaric CP, forming dry land at the end of Middle Liassic, marked by an erosional hiatus in Fig. 3. Following tectonic events at the transition from Early to Late Cretaceous times, bituminous carbonate (Komen Fm.) was deposited in the Cenomanian–Turonian in intertidal and lagoonal environments (e.g. Cavin et al., 2000; Ogorelec et al., 1987b). Locally bauxite was formed during the Campanian. Significant movements affected the Dinaric CP in the Maastrichtian. Parts of Istria (Adriatic CP) and adjoining regions were uplifted and karstified. In the northern part, the sea again inundated the dry land with the end of the Maastrichtian and the brackish limestones of the lower part of the Liburnian Fm. were deposited. Coals were formed during the Maastrichtian–Danian (Hamrla, 1959) on the western part of the platform (Dinaric CP, Adriatic CP). Contemporaneously, the depocenter of the SB shifted southward onto the northern part of the Dinaric CP. Carbonate horsts were eroded and supplied enormous amounts of material into the nearby basin. The Maastrichtian to Paleogene flysch was deposited transgressively on the Cretaceous limestones. During the Eocene the flysch basin shifted even more to the south (Drobne et al., 2009b). Total thickness of the Maastrichtian to Middle Eocene flysch sediments by far exceeds

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Fig. 3. Thermal maturity of the Slovenian Basin, the western part of the Dinaric Carbonate Platform and the Adriatic Carbonate Platform. The cross-section is shown in Fig. 8.

4000 m (Tunis and Venturini, 1992). After a major break in sedimentation, Oligocene and Miocene sediments from the Pannonian Basin System have been deposited and are preserved in synclines of the Sava Folds (Kuščer, 1967; Placer, 1999b). In the study area the stratigraphic succession of the Adriatic CP, separated from the Dinaric CP by the External Dinaric Front (Placer, 1999a; see Fig. 1), includes Cretaceous to Paleogene sediments. In the Adriatic offshore region Upper Paleozoic and Mesozoic sediments were drilled by several wells (e.g. Cota and Barić, 1998).

2.2. Tectonics The Alpine–Dinaric Transition Zone was affected by polyphase Alpine tectonics comprising the Maastrichtian “Eoalpine phase” (S vergent thrusts), the Paleocene to Eocene “Dinaric” phase (SW vergent thrusts), the Oligocene “South-Alpine” phase (S vergent thrusts) and the “Alpine” event (S-SE vergent thrusts) from Late Miocene onward (e.g. Castellarin and Cantelli, 2000; Doglioni and Bosellini, 1987; Placer, 1999a,b; Placer and Čar, 1998; Fig. 1).

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In the western Southern Alps, N–S compression during the “Eoalpine phase” constituted S vergent overthrusts. Since Eocene flysch sediments lie discordantly on folded Maastrichtian flysch in the Natisone Valley (for location see Fig. 1; Doglioni and Bosellini, 1987), the Late Cretaceous orogenic phase possibly affected the study area as well. In the well Cargnacco-1 (Fig. 1) a break in maturity exists between Upper Cretaceous and Lower Paleogene sediments (Fantoni et al., 2002). “Eoalpine” heated sediments are also reported from Mt. Medvednica (N of Zagreb, Judik et al., 2008) and the basement of the Pannonian Basin System (Igal unit; NE of Zagreb; Árkai et al., 1991). The Dinaric CP of SW Slovenia is subdivided into several “Dinaric” units (Placer et al., 2010). These are from top to base: the Trnovo and Hrušica Nappes, and the Snežnik and Komen thrust sheets. The oldest structure appears to be the post-Middle Eocene thrusting of the Trnovo Nappe onto the Hrušica Nappe, followed by the movement of the Hrušica Nappe onto the Snežnik thrust sheet. In the Sava Folds thrusting occurred pre-Late Oligocene. During the Cenozoic “Alpine” Orogeny compression resulted in S-SE vergent thrusting and in the reactivation of Mesozoic normal faults and Dinaric thrusts as strike-slip faults. The Julian Alps were thrust onto the deposits of the SB, possibly already pre-Late Oligocene (Placer, 1999a). On the other hand, Premru (1980) and Polinski and Eisbacher (1992) argue that the nappes of the Julian and Kamnik–Savinja Alps are post-Sarmatian. In the area W of Kranj, the Julian Alps are thrusted on Miocene sediments (U. Herlec, pers. comm.). The total shortening between the Julian Alps and the SB was ~25 km (Cousin, 1981). Some SE trending dextral faults in the region are still active, e.g. the Idrija Fault (e.g. Mlakar, 1969) and the Periadriatic Fault (e.g. Vrabec et al., 2006). The W–E oriented Sava Folds east of Ljubljana are the result from N–S directed compression. Intensive folding commenced in Late Miocene or presumingly Pliocene times (Fodor et al., 1998; Placer, 1999b). Late Oligocene sediments are preserved in the cores of the synclines (Rasser and Harzhauser, 2008). To the south folding gradually fades out in the Dinaric CP. The most important thrusting of the Dinaric CP over the Adriatic CP took place between 7 and 6 Ma (Drobne et al., 2009a).

3. Samples and analytical techniques 3.1. Vitrinite reflectance (VR) Mean random VR (%Rr) was measured on c. 1000 outcrop samples and some additional samples from boreholes and former mines. The investigated samples were mainly dark shales, siltstones and marls. In the samples vitrinite is mainly present as fine detritus or as coaly particles on bedding planes. VR measurements were performed on whole rock samples prepared as polished blocks. In few cases, VR was measured using solid bitumen occurring in pores and microfractures in carbonate rocks. Mean random VR was calculated using the equation Rr = 0.618 ∗ Rb + 0.40 proposed by Jakob (1989), where Rb is reflectance of bitumen. For two samples VR was calculated using the methylphenanthrene index MPI-1 and the equation Rr = 0.60 ∗ MPI1 + 0.40 (see Radke and Welte, 1983). VR measurements were carried out using a Leica MPV-SP microscope and 50/0.85 and 125/1.30 oil immersion objectives and following standard procedures (Taylor et al., 1998). Synthetic garnet standards (0.89 and 1.69%Rr) and a wolfram-carbid standard (7.24%Rr) were used for calibration. The aim was to measure 50 particles per sample. Sampling locations, number of measurements, VR values and standard deviation for each sample can be found in Rainer (2003). Random VR can be converted to peak paleotemperatures using formulas defined by Barker and Pawlewicz (1994). In the present study the formula Tpeak = (lnRr + 1.68) / 0.0124 for burial heating is used to obtain a rough estimate of paleotemperatures.

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3.2. Apatite fission track analysis (AFTA) Apatite fission track analyses (AFTA) was carried out by Geotrack International Pty Ltd. (Australia). Determination of the chlorine (Cl) compositions was done for all individual apatite grains in which fission track ages were determined and/or lengths were measured. For rigorous thermal history interpretation the age and length data have been grouped into 0.1 wt.% Cl divisions. AFTA data in all samples have been interpreted using heating rates of 1 °C/Ma and cooling rates of 10 °C/Ma. Ages were calculated using the equation five of Hurford and Green (1983). Track length is measured following the recommendations of Laslett et al. (1982). 3.3. Numerical 1D-basin modeling Numerical basin modeling was performed using Schlumberger's Petromod 1D software. Temperatures at the sediment-water interface were calculated by an algorithm considering paleogeographic latitudes, paleoclimatic information and water depths (Wygrala, 1988). For the calculation of VR the EASY%Ro approach of Sweeney and Burnham (1990) was applied. Models were calibrated by modifying heat flow and the thickness of eroded sediments until a satisfactory fit between measured and calculated VR was obtained. 4. Results VR data from rocks with different stratigraphic age and tectonic position are compiled in Figs. 3 and 4. They show that the regional maturity pattern is dominated by a high thermal overprint of Triassic to Cretaceous sediments in the central SB west of Ljubljana (e.g. N of Cerkno) and in the Sava Folds E of Ljubljana (N 3%Rr) corresponding to peak paleo-temperatures N275 °C. Rainer et al. (2002) have shown that these rocks reach the anchizone of metamorphism. Maturity decreases near the western termination of the SB (NW Kobarid), where VR of Upper Cretaceous marls is only slightly higher than that of contemporaneous flyschoid deposits in the Bovec Basin (Julian Alps; Fig. 3). Sediments from the former northern margin of the SB are exposed N of Bohinjska Bistrica in a tectonic window beneath the Julian Alps (Fig. 3) and are sliced along the ?post-Sarmatian dextral Sava Fault north of Kamnik (Fig. 4). Near Bohinjska Bistrica VR of Lower Cretaceous rocks is relatively low (b1.5%Rr; Fig. 3), reflecting a decrease in thermal overprint towards the northern margin of the SB. The thermal overprint of Ladinian and Cretaceous sediments north of Kamnik covers a wide range (inset in Fig. 4). NE of Vransko Cretaceous black slates show significantly lower thermal overprint (b 2.5%Rr) than S of the Sava Fault, where the SB sediments are part of the Sava Folds (N3.0%Rr; Fig. 4). Maturity also decreases from the central SB to the S. West of Ljubljana (Fig. 3) the southward decrease of the VR values can be traced in both, the Trnovo and the Hrušica Nappe. In the north, the thermal overprint of the Dinaric CP reaches the intensity of the SB. East of Ljubljana (Fig. 4) the northern segments of the Sava Folds show a higher thermal overprint than the Dinaric CP to the S or the Cretaceous rocks in the SE (Fig. 4). The lowermost maturity is detected in Paleogene sediments of the Adriatic CP (Istria; Fig. 3). VR results for key stratigraphic horizons are displayed in Figs. 5 and 6 and discussed below. 4.1. Carboniferous Because no difference between pre- and post-Variscian overprint is recognized (Rainer et al., 2009), VR of all Carboniferous samples are displayed in Fig. 5a. VR within the Sava Folds (~3.5–5.1%Rr) is similar to the overprint of the “root zone” (NE part) of the Trnovo Nappe (N4.4%Rr). Significantly lower values (1.8–2.3%Rr) were detected near

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Fig. 4. Thermal maturity of the Slovenian Basin, the Sava Folds and the Dinaric Carbonate Platform east of Ljubljana. Inset shows maturity of the area north of Kamnik (Slovenian Basin).

Idrija in the “Idrija Nappe”, a part of the Trnovo Nappe (Čar, 2013; Mlakar, 1969). Samples from the Hg-mine Idrija show slightly lower values than occurrences on the Vojsko plateau (NW of Idrija) or E of Rovte (Fig. 3).

4.2. Permian The thermal overprint of Permian rocks decreases southwards (Fig. 5b). This trend is most pronounced in the Trnovo Nappe. VR of anthracitic organic matter (Gröden Fm.) in the Cu-mine Cerkno and the U-mine Žirovski vrh reaches values between 3.9 and 4.7%Rr (see also Hamrla, 1990), whereas VR of coal inclusions known in Permian beds of the Idrija Hg-mine is only 1.3%Rr. Ogorelec et al. (1996) report VR values of ~2.3%Rr N of Idrija (“Javorjev dol profile”). The southward decrease in thermal overprint is also recognizable south and east of

Ljubljana, where VR in the basement of the Dinaric CP decreases from N4.3%Rr to 2.2%Rr. 4.3. Ladinian and Carnian A maturity map for Ladinian and Carnian horizons considering data from the North Karawanken Range, the South Karawanken Range and the northern Julian Alps (Southern Alps; Rainer et al., 2002), is displayed in Fig. 5c. VR in the North Karawanken Range (Eastern Alps) is generally ~ 1.0%Rr, but locally reaches 1.6%Rr due to Oligocene magmatism (Rantitsch and Rainer, 2003). VR in the South Karawanken Range reaches 2.3%Rr in Ladinian strata and is b1.0%Rr in the northern part of the Julian Alps (Rantitsch and Rainer, 2003). Rocks with significantly higher maturity are observed in the central part of the SB. There, VR reaches 5.3%Rr in black slates of the

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Fig. 5. Thermal maturity maps for Carboniferous to Triassic rocks within the Alpine–Dinaric Transition Zone.

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Fig. 6. Thermal maturity maps for Paleogene, Cretaceous and Jurassic rocks within the Alpine–Dinaric Transition Zone.

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Pseudozilian Fm. N of Cerkno and varies between 3.6 and 5.2%Rr in the Sava Folds. Slates with very high VR (3.7–5.1%Rr) also occur 3 to 6 km N of the Sava Fault (NE of Kamnik; “halfwindow of Kranjska Reber”; Fig. 4), indicating the minimum distance of southward thrusting of the Savinja Alps onto the SB. Hinterlechner-Ravnik (1978) described Middle Triassic metamorphic rocks (greenshists) from that area with a mineral assemblage of stilpnomelan, chlorite and tremolite suggesting temperatures between 300 and 400 °C. Peak-paleotemperatures from VR point to temperatures in the order of 260 °C. Carnian sediments deposited in the northern part of the SB reach a lower maturity (e.g. N of Podbrdo and near Sela; 1.9–2.3%Rr; Fig. 3). Ladinian and Carnian strata of the Dinaric CP display a strong southward decrease in maturity. SW and SE of Ljubljana, the Raibl Grp. contains locally economic semi-anthracite and anthracite (e.g. near Horjul–Drenov Grič, Orle; Dozet, 1979; Petraschek, 1923; Jelen, 1988/1989) with VR values varying between 3.3 and 2.3%Rr. Near Oblakov vrh (Trnovo Nappe) VR varies between 2.1 and 2.6%Rr. In the vicinity, Kolar-Jurkovšek (1990) and Jurkovšek (1984) reported Langobardian black conodonts in marly limestones, indicating CAI = 5 (N300 °C). Near Idrija, Ladinian marls show 2.1%Rr, but lower maturity was detected SW of the Idrija Fault in the southern part of the Trnovo Nappe (Fig. 5c), where coal samples from the Idrija mine (e.g. Čar, 2013) show VR values between ~1.5 and 1.0%Rr. 4.4. Jurassic Although VR data from relative few samples are available, the general maturity pattern is reflected in the Jurassic samples (Fig. 6a). VR of the westernmost sample (N of Tolmin), representing Doggerian to Lower Malmian deep-water sediments of the SB (Goričan, 1983) is only ~ 1.4%Rr. However, VR reaches ~ 4%Rr at Mt. Porezen and along the northern margin of the Trojane Anticline. VR of samples from the Dinaric CP is about 2.0 to 3.0%Rr. Tmax values of Jurassic source rocks in the offshore well Rovinj-1 (417–419°C; Cota and Barić, 1998) prove a lower thermal maturity (b0.5%Rr) of the Adriatic CP. 4.5. Cretaceous VR in Upper Cretaceous rocks from the Julian Alps (Bovec Basin) is low (0.7–0.9%Rr). In contrast, the thermal overprint of coeval rocks in the central part of the SB is very high (Fig. 6b). For example, VR reaches ~ 4%Rr in Upper Cretaceous rocks near Podbrdo (Fig. 3) and varies between 3.4 and 4.4%Rr in Lower Cretaceous (Tithonian–Berriasian) sediments preserved at the northern limb of the Trojane Anticline (Fig. 4). A lower thermal overprint is observed only in sediments deposited near the former western termination of the SB (1.1%Rr) in Middle and Upper Senonian N of Kobarid (Kuščer et al., 1974) and along the former northern basin margin (0.8–1.4%Rr) in Lower Cretaceous flysch in a tectonic window N Bohinjska Bistrica (Budkovič, 1978). Comparable to the results in Ladinian sediments the area N of Kamnik is marked by mosaic-like maturity pattern (1.7–3.8%Rr; Fig. 4) resulting from dextral displacements along the Sava Fault. In outcrops, where the Cretaceous sediments rest on Ladinian strata, no difference in the thermal overprint is detected. The thermal overprint in the Dinaric CP platform reaches 3.0%Rr along its northern margin, e.g. in Maastrichtian flysch, which has been deposited in the transition zone from the platform to the SB, and decreases southwards within the Cretaceous overstep sequences resting on Triassic and Jurassic platform carbonates (Placer, 1999a). This is observed in the eastern sector, where maturity decreases gradually to about 1.2%Rr (in Senonian marls E of Metlika) and in the western sector near Most na Soči, where VR decreases within a distance of only 8 km from 2.1%Rr to 0.9%Rr. A significantly higher maturity (2.3%Rr) can be detected in Upper Cretaceous to Paleogene rocks north of Zagreb (Mt. Medvednica).

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At the SW margin of the Dinaric CP (Komen plateau) the Cretaceous sedimentary evolution is marked by carbonates, more than 1000 m thick (Jurkovšek et al., 1996). During the Upper Cretaceous five horizons with bituminous limestones and marls were deposited (Cavin et al., 2000). The thermal maturity of these sediments ranges between 0.9 and 1.2%Rr. At the SE margin of the Komen plateau (Vremski Britof area; Fig. 3) the marine neritic and littoral environment of the Upper Cretaceous times passed gradually into brackish lagoonal and fresh water environments with five coal horizons (Liburnian Fm., Kozina Bds.; Maastrichtian–Danian; Hamrla, 1959). The lower three horizons of the 200 m thick sequence are Cretaceous in age, the upper two horizons were deposited in the Paleogene. The maturity of the coal and the overlying black platy bituminous limestone in Vremski Britof is ~1.0%Rr. VR of Upper Cretaceous coal at Vinež (S of study area; 0.64%Rr) suggests that the Adriatic CP is characterized by an even lower thermal maturity. 4.6. Paleogene VR of Paleocene flysch in the Soča valley area (Trnovo Nappe) varies between 0.9 and 1.2%Rr. The presence of flysch deposits NW of Nova Gorica with an Early Eocene age (Tunis and Radrizzani, 1987) and a reflectance ranging from 0.75 to 0.83%Rr show that both, stratigraphic age and thermal overprint decrease to the SW. At the northernmost part of the Vipava syncline (e.g. NE of Nova Gorica), Eocene rocks from the SW margin of the Trnovo Nappe lie in inverse position (Čar and Gospodarič, 1988). Their VR varies between 0.7 and 0.9%Rr. The same thermal overprint was detected in the Eocene strata of the Hrušica Nappe preserved in the northern part of the Vipava syncline (0.6–0.9%Rr), in tectonic windows near Idrija (e.g. Mlakar, 1969; Pavšič and Pavlovec, 2008) (0.7–0.9%Rr), and as a small erosional remnant near Kališe (S of Logatec; Grad, 1961) (0.79%Rr) (Fig. 3). Consequently, the Eocene strata from the SW part of the Trnovo Nappe show the same thermal overprint as Eocene rocks from the Hrušica Nappe. Towards the south, Lower Eocene (Ypresian–Lutetian; Cousin, 1981) flysch sediments with low VR (0.6–0.8%Rr) are also observed in the Pivka Basin (W of Postojna; Čar and Juren, 1980) and the Reka syncline (0.6–0.9%Rr). In the west of the Pivka Basin, no difference in maturity exists between the Eocene of the Hrušica Nappe and the Komen thrust sheet (0.61%Rr: Hamrla, 1985/86, 1987). However, Eocene flysch in the southern part of the Vipava Syncline shows a higher thermal overprint (1.0–1.4%Rr). The well Šempeter-1 (E of Nova Gorica) shows an upward decrease in VR from 1.25%Rr (951 m depth) to 1.13%Rr (525 m depth). VR of an outcrop sample near the well is 1.07%Rr. According to Placer et al. (2000), the upper part of the well is located in the Hrušica Nappe, whereas the deeper part reaches the Komen thrust sheet. The thermal maturity of Eocene sediments along the “External Dinaric Front” (0.6–0.7%Rr) and in the Adriatic CP (0.5–0.6%Rr) is lower than that of the Dinaric CP. This is corroborated by VR of Lower to Middle Eocene (Ypresian–Lutetian; Hamrla, 1985/86, 1987) coals from the Šečovlje mine (0.54%Rr) and ?Paleocene and Lower Eocene (Ypresian) coal deposits in central Istria (Pičan, Labin, Raša; 0.55– 0.64%Rr; see also Hamrla, 1987). 4.7. Apatite fission track analysis (AFTA) Seven samples (#) were selected for apatite fission track (AFT) geochronology in order to determine the time at which the samples cooled below approximately 110 °C and to reconstruct the uplift path of the Alpine–Dinaric Transition Zone. The analysis was carried out on apatites from Carboniferous to Cretaceous sedimentary rocks outcropping in the Sava Folds (# 1144, 1145, 42), the Trnovo Nappe (# 1146, 1147), the Slovenian Basin (# 31001) and the Bovec Basin in the Julian Alps (# 181). The analysis results are summarized in Table 1. Sampling locations and AFT ages, completed with data from Nemes (1996), are presented in Fig. 7.

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Table 1 AFTA data, sample details and associated thermal history interpretations. Thermal history constraintsb Early tertiary ID code

Stratig.

Location

1144

Carb.

Litija Anticl.

1145

Carb.

Litija Anticl.

42

Md. Perm.

Trojane Anticl.

1146

Carb.

Trnovo Nappe

1147

Md. Perm.

Trnovo Nappe

31001

Lo. Cret.

Slov. Basin (Bohinj)

181

Up. Cret.

Bovec B. (Julian Alps)

Late tertiary

ρD

ρs

ρi

p(χ2) (%)

Fission track age

Mean track length

Max. paleo-temp.

Onset of cooling

Max. paleo-temp.

Onset of cooling

106 tracks/cm2

106 tracks/cm2

106 tracks/cm2

(No. of grains)

[Ma]

[μm]

[°C]

[Ma]

[°C]

[Ma]

0.974 (1657) 0.984 (1657) 1.016 (1657) 0.995 (1657) 1006 (1657) 1.027 (1657) 1.037 (1657)

0.558 (286) 0.626 (65) 0.408 (158) 0.485 (129) 0.299 (158) 0.709 (221) 0.284 (58)

2.527 (1296) 3.371 (350) 3.049 (1180) 3.592 (443) 1.028 (1180) 1.024 (319) 3.027 (619)

b1 (23) 54 (5) 5 (20) 31 (22) b1 (20) 6 (20) b1 (21)

40.7 ± 2.9 48.2 ± 8.3a 34.7 ± 4.8

13.41 ± 0.33 (23) 14.96 ± 0.20 (4) 12.69 ± 0.40 (15) 12.19 ± 0.54 (9) 13.35 ± 0.25 (36) 12.08 ± 0.78 (7) 14.41 ± 0.66 (4)

N105

45–27

40–90

27–0

N105

50–25

N110

45–20

40–85

15–0

N110

45–25

50–90

15–0

110–120c

70–20

50–80

35–0

b105

15–0

25.8 ± 2.3 26.0 ± 3.2a 25.5 ± 2.2 55.5 ± 5.8 60.8 ± 10.2a 133.9 ± 12.3 18.5 ± 2.6 20.0 ± 3.5a Best estimates:

85–100 N115

100–0 55–20 45–27

15–0

ps = spontaneous track density; pi = induced track density; pD = track density in glass standard external detector. Numbers in parentheses show the number of tracks counted. pD and pi measured in mica external detectors; ps measured in internal surfaces. Bold values indicate the age constraints used in data interpretation (central or pooled ages). a Central age, used where sample contains a significant spread of single grain ages (P(χ2) b5%), otherwise the pooled age is quoted. Errors quoted at 1σ. Ages calculated using dosimeter glass CN5, with a zeta of 380.4 ± 5.7 for samples. b Thermal history interpretation of AFTA data is based on an assumed heating rate of 1 °C/Ma and a cooling rate of 10 °C/Ma. Quoted ranges for paleotemperature and onset of cooling correspond to ±95% confidence limits. c The AFTA data in sample 1147 (Middle Permian; Trnovo Nappe) appear to set an upper limit of 120 °C for the maximum post depositional paleotemperature of this sample. However, this is based on only a single grain of apatite, which gives a fission track age statistically indistinguishable from the depositional age. In the context of the reported VR value of 3.89% for this sample and the AFTA data in the other samples, we consider this grain to be anomalous, and a more appropriate solution for this sample is likely to involve a maximum paleotemperature N110 °C.

AFTA data from four samples clearly require at least two discrete Cenozoic paleo-thermal episodes. Results from the other three samples provide constraints on only a single episode but in each case this is due to limitations of resolution, and the data would be consistent with two episodes. On regional geological grounds, it seems highly likely that a similar style of cooling history should be applied to all samples. Assuming that results from all samples represent the effects of the same synchronous cooling events, then combining results from all seven samples suggests that the earliest episode of cooling began some time between 45 and 27 Ma (Eocene–Oligocene), followed by a subsequent cooling event of lower magnitude some time between 15 and 0 Ma (Late Miocene). The earlier episode is defined by the fission track age data, which require paleotemperatures sufficiently high to erase all tracks. In these samples, the AFTA data provide constraints on the time at which the sample cooled to sufficiently low temperatures that tracks could be retained once more. The later cooling event, deduced from track length data, might reflect regional uplift and erosion after the deposition of the Oligocene to Miocene sediments of the Pannonian Basin System. AFTA data in all samples except for #31001 (Lower Cretaceous rocks from the tectonic window north of Bohinjska Bistrica) are consistent with maximum paleotemperatures indicated by the VR data, which are N 250 °C in samples of Late Paleozoic depositional age and ~120 to 130 °C in the Upper Cretaceous sample of the Bovec Basin. The measured AFT age of sample #31001 is statistically indistinguishable from the stratigraphic age indicating that the sample has not been heated to a sufficiently high temperature to produce observable FT age reduction. Therefore AFTA data are not consistent with the measured VR value, suggesting that the maturity of the sample might be lower than estimated. However, the sample contains a wide spread of Cl contents in the apatites (up to 1.5 wt.% Cl) suggestive of a volcanogenetic origin of the apatite. Thus, the AFTA age might indicate the formation age.

5. Discussion and interpretation 5.1. Timing of the thermal overprint and subsequent cooling The thermal maturity study revealed that Carboniferous to Upper Cretaceous rocks between the Soča valley in the west and the Sava Folds in the east suffered a high thermal overprint, reaching very lowgrade metamorphism (“anchizone”). VR of Upper Cretaceous rocks north of Cerkno and in the Sava Folds reaches 4%Rr and 2.5%Rr, respectively. In contrast, VR of Upper Oligocene coals preserved in the Sava Folds is only ~0.4%Rr (Sachsenhofer et al., 2001). This indicates a substantial gap in thermal maturity across the pre-Oligocene unconformity (location “a” in Fig. 7). In the Soča valley, VR decreases gradually from N2.5%Rr (Upper Cretaceous) in the north to 0.7–1.3%Rr (Lower Eocene) in the south (Fig. 3 and location “c” in Fig. 7). There is no break in coalification between the Upper Cretaceous and Middle Eocene rocks. Therefore it is concluded that peak paleotemperatures of the SB and the Dinaric CP were attained during Middle Eocene to Early Oligocene times. AFT data show that cooling of Paleozoic and Mesozoic rocks below 110 °C occurred between the Middle Eocene and Early Miocene times. Since there is no evidence for a large-scale magmatic pluton in the region, the cooling ages record regional uplift and exhumation.

5.2. Has nappe stacking caused the thermal overprint? Burial by thrust units can cause thermal effects, which are recognizable when VR data are integrated into a tectonostratigraphic framework. In the study area numerous nappes and overthrusts exist. Considering that the maturity of the study region was achieved in pre-Late Oligocene time, only “Dinaric” (Paleogene) thrusts could have caused the thermal overprint.

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Fig. 7. Timing constraints on the onset of cooling derived from apatite fission track data of Carboniferous to Cretaceous sediments of the Alpine–Dinaric Transition Zone. Vitrinite reflectance profiles of location “a” (southern Sava Folds) and “b” (Dolsko overthrust) are shown together with a schematic cross section displaying the Late Cretaceous to Early Eocene transgressive deposition of flyschoid sediments on the Trnovo Nappe (Dinaric CP). Note similar VR values of Trnovo Nappe with respect to overlying flysch sediments (location “c”).

5.2.1. Thrusting of the Julian Alps onto the Slovenian Basin The age of thrusting of the Julian Alps onto the deposits of the SB is still controversial. Whereas Placer (1999a) assumes a pre-Late Oligocene age, Premru (1980) and Polinski and Eisbacher (1992) postulate a Neogene (post-Sarmatian) age. The present study cannot narrow down the timing of thrusting. However, at the southern margin of the Julian Alps VR increases towards the east. Upper Cretaceous marls of the Bovec Basin show 0.8%Rr, sediments of the southern slope of Mt. Krn ~1.5%Rr. The Upper Cretaceous marls of the SB beneath the thrust front (S of the Krn) show lower values (~1.2%Rr). Additionally, Lower Cretaceous rocks of the Slovenian Basin, cropping out in a tectonic window near Bohinjska Bistrica in the area of the Julian Alps, show a low thermal maturity (VR ~ 1%Rr). As a consequence thermal maturity data indicate that the thrusting of the Julian Alps onto the Slovenian Basin has not caused the thermal overprint of the Slovenian Basin.

5.2.2. Nappe structures in the western Dinaric CP The Dinaric CP of SW Slovenia consists of several nappes (Placer et al., 2010). The Trnovo Nappe rests on the Hrušica Nappe, whereas

the Hrušica Nappe is locally thrusted onto the Snežnik thrust sheet. The oldest structure appears to be the post-Middle Eocene thrusting of the Trnovo Nappe onto the Hrušica Nappe. These main structural units are presented in a N–S cross-section from Julian Alps to the Hrušica Nappe along the Vipava valley together with VR values in Fig. 8. It shows that Triassic rocks of the Trnovo Nappe suffered a higher thermal overprint (~ 1.6%Rr) than Eocene marls of the Hrušica Nappe (b0.9%Rr) exposed in tectonic windows near Idrija. This “maturity inversion” proves the pre-tectonic thermal overprint of the region. Eocene rocks from the southwestern margin of the Trnovo Nappe and from the Hrušica Nappe exposed in tectonic windows yield similar VR values. This may indicate adjoining areas during deposition and similar depths during Paleogene sedimentary burial. Pre-tectonic thermal overprint is also recognizable within the Trnovo Nappe in the Idrija area. Fig. 9A shows the Ladinian situation of the Idrija region, when deep sedimentary basins were separated by swells (Mlakar, 1969). Due to erosion, locally Ladinian sediments are resting on Permo-Carboniferous strata. During Cenozoic time, possibly coeval with Eocene S-directed thrusting of the Trnovo Nappe, the

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Fig. 8. S–N cross-section from the Komen thrust sheet to the Julian Alps, indicating the pre-tectonic overprint of the Dinaric CP. See Fig. 3 for location of cross-section. Note the black rectangle for Fig. 9.

sediments of the northern basin were thrusted over the sediments of the southern basin (Fig. 9B). The Late Cenozoic to recent tectonic activity in the region is controlled by NW–SE striking dextral faults (e.g. Idrija Fault; Mlakar, 1969). Maturity data indicate that Ladinian rocks from the northern sedimentary basin suffered a higher thermal overprint (2.0–2.1%Rr) than the sediments from the Idrija trough (Fig. 9A). Near the northern swell Ladinian coaly marls reach 1.5–1.9%Rr. Permo-Carboniferous rocks show similar maturity (1.8–2.0%Rr). Significantly lower VR values were detected in Ladinian coals (“Skonza beds”; 0.73–1.11%Rr). The higher VR values from the northern sedimentary basin indicate a paleoposition closer to the Slovenian Basin in the north. The integration of the VR data into the recent tectonic framework approves a pre-tectonic thermal overprint (Fig. 9B). The highest maturity occurs in the upper part of the Trnovo Nappe (rocks of the former northern sedimentary basin), whereas the lower part of the Trnovo Nappe (Idrija thrust sheet) delivers lower VR values. 5.2.3. Tectonic structures in the Sava Folds Pre-Late Oligocene nappe structures affected Paleozoic and Mesozoic rocks in the southern Litija Anticline of the Sava Folds. There, the Dolsko thrust sheet, mainly composed of Triassic carbonate rocks, rests on the Žiri thrust sheet, mainly composed of Upper Paleozoic clastics. The Dolsko thrust sheet can be divided into the lower 1st and the upper 2nd part (location “b” in Fig. 7). The integration of the VR data into the tectono-stratigraphic model of Mlakar (1985/1986) shows a maturity inversion between the 1st and the 2nd part of the Dolsko thrust sheet (Fig. 7). Consequently, pre-Late Oligocene nappe piling post-dates the thermal overprint. Summarizing Section 5.2, it can be concluded that in all studied cases thrusting post-dates the thermal overprint. Consequently nappe stacking is not responsible for the observed maturity patterns in the Alpine–Dinaric transition zone. This agrees with results of Nussbaum (2000), who found that Paleogene to recent (Dinaric and Alpidic) thrusting was not able to influence the thermal maturity of Mesozoic sediments in the eastern Southern Alps. 5.3. Is there evidence for a magmatic influence on the thermal maturity? In the study area magmatic rocks occur in Permian and Middle Triassic strata. The regional extent of the rocks is limited (e.g. several m thick diabase dykes). Magmatic heat affects VR rocks over distances equal to

about two times the thickness of the intrusion (e.g. Barker and Pawlewicz, 1986). Therefore, the influence on the overall thermal maturity pattern seems negligible. Furthermore, the detected intense Late Cretaceous to Early Cenozoic thermal overprint throughout the study area resets the primary thermal architecture in the vicinity of the Permian and Triassic magmatic rocks. Late Cretaceous to Early Paleogene magmatic rocks, existing in the Internal Dinarides influenced the thermal maturity of nearby Upper Cretaceous to Lower Paleogene rocks (VR ~3.5%Rr; Pamić et al., 1992). However, time equivalent intrusives do not exist in the study area. Oligocene magmatic activity along the Periadriatic Fault influenced the heat flow evolution at the southeastern end of the Eastern Alps (Sachsenhofer, 2001) and caused elevated VR in the North Karawanken Range (Rantitsch and Rainer, 2003). There is no indication that it also affected the study area. Moreover the observed large scale, regular coalification patterns argue against an influence of fluid flow on thermal maturity.

5.4. Pre-Late Oligocene thermal overprint due to deep Late Cretaceous to Paleogene sedimentary burial A key area for the interpretation of the maturity pattern is the wider region of the Soča valley, where the geometry of the SB and the Late Cretaceous to Paleogene southward shift of the depocenter of the flysch sedimentation is well documented. VR data displayed in Figs. 3 to 6 clearly reflect an increase of thermal maturity towards the center of the SB in all stratigraphic units including Upper Cretaceous to Paleogene rocks. In order to get a rough idea about the paleo-relief and associated accommodation space for Late Cretaceous to Paleogene sediments, measured VR and derived peakpaleo-temperatures are used as vertical scale in a schematic N–S “cross-section” (Fig. 10), which roughly follows Fig. 8. Thickness of individual strata shown is defined by VR. Only stratigraphic intervals, where VR data are available are shown. The model implies that the Late Cretaceous deep marine position of the SB and the associated high accommodation space has controlled the deposition of thick Upper Cretaceous to Paleogene sediments. During the main stage of the flysch deposition (Maastrichtian–Middle Eocene), the SB was a narrow, elongated basin. Structurally it was represented by an asymmetrical graben, probably characterized by complex and differentiated bottom topography (Tunis and Venturini, 1992).

T. Rainer et al. / International Journal of Coal Geology 157 (2016) 19–38

31

Fig. 9. Thermal maturity of the Idrija region. (A) Middle Triassic setting, (B) present-day structure Placer & Car 1977.

Flysch sedimentation along the section shown in Fig. 10 commenced at the northern margin of the SB (Bohinjska Bistrica) in Lower Cretaceous (Tithonian–Berriasian) time. Low maturity levels in Berriassian strata document that only relatively thin Upper Cretaceous and Paleogene flysch was deposited in the northern part of the SB, reflecting an Aptian southward shift of the accumulation depocenter (Buser, 1989). Actually, the Aptian to Lower Campanian evolution is marked by sediments of turbiditic origin derived from the Dinaric CP to the S, deposited on the foot of the slope (~700 m thick). After the pre-flysch phase, a sudden reactivation of subsidence of the slope zone, situated to the S took place. At the Campanian–Maastrichtian transition the early stages of the Alpine Orogeny caused the destruction of the margin of the Dinaric CP. During the Maastrichtian, thick siliciclastic sequences were deposited (derived from the S and the NW) on the slope and in the basin center. Subsequently, the depocenter of the SB was shifted to the south onto the northern margin of the Dinaric

CP. The expansion of the basin is probably related with reactivated Mesozoic detachment surfaces (e.g. Tunis and Venturini, 1987, 1992). The relations between the Upper Cretaceous to Eocene overstep sequences and the Triassic to Cretaceous strata of the Trnovo Nappe (W part of the Dinaric CP) is shown in Fig. 7 (location c). VR data along the section document a continuous increase in maturity between the flyschoid sediments and the Middle Triassic to Lower Cretaceous basement. This is clear evidence that deposition of the thick flysch sediments controlled maturity patterns of the Permo-Mesozoic sediments of the Dinaric CP and the SB. Most probably the southeastern segment of the Trnovo Nappe was a high zone during the Late Cretaceous to Paleogene flysch deposition. This is indicated by the comparably low maturity of PermoCarboniferous and Triassic rocks, e.g. in the Idrija region. Local differences in thermal maturity detected in Eocene sediments of the Dinaric CP (e.g. higher VR values near the Komen plateau) and

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T. Rainer et al. / International Journal of Coal Geology 157 (2016) 19–38

Fig. 10. Schematic model of the platform-basin transition zone, displaying the paleorelief and associated accommodation space during Paleogene time. Vertical scale is VR and peak paleo temperature. Only stratigraphic intervals, where VR data available are shown.

the lower maturity of the Adriatic CP are controlled by regional thickness variations of Eocene–?Oligocene flysch deposits. 5.5. Estimation of the thickness of the Late Cretaceous–Early Cenozoic basin fill In the present study, the thickness of the eroded Upper Cretaceous to Paleogene sediment stack and the associated heat flow are estimated by numerical 1D basin modeling. Since no borehole data are available, a pseudo-well was constructed using data from the Sava Folds. The Sava Folds have been chosen, because of the range of investigated horizons

(Carboniferous to Upper Cretaceous) and the lateral continuity of the thermal overprint. VR data from location “a” in Fig. 7 have been used for calibration. The Carboniferous to Late Cretaceous burial history is compiled from Premru (1982), Aničić and Juriša (1984), Buser (1977) and Grad and Ferjančič (1974). Due to pre-Late Oligocene erosion, parts of the Upper Cretaceous succession and the whole Paleocene to Eocene sequence were eroded. Therefore, different subsidence scenarios during Late Cretaceous to Middle Eocene times were considered. Thickness information on the Upper Cretaceous to Middle Eocene flysch of the eastern Southern Alps (e.g. Friuli platform) and the Dinarides was

Fig. 11. Burial history, thermal history and heat flow model of the Sava Folds based on an assumed heat flow of 60 mW/m2 and 5000 m eroded Upper Cretaceous to Middle Eocene rocks. Note that the heat flow is only calibrated for Late Cretaceous–Early Oligocene times.

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33

Table 2 Input of the basic geological data for modeling of temperature and burial history of the Sava Folds (incl. parts of the Slovenian Basin). Name

End

Start

[Ma B.P.] Pliocene Up. Mioc. 2 Up. Mioc. 1 Erosion 4 Up. Oligoc. 2 Up. Oligoc. 1 Erosion 3 Erosion 2 Erosion 1 Lo.-Mi. Eocene Paleocene Maastricht Tur-Camp Apt-Cenom Hiatus 2 Tith-Berr Hiatus 1 Hauptdolo Raibler Pseudozil Muschelk Werfen Zazar Gröden Lo. Permian Permo-Carb

0 2 4 10 23 25 29 31 32 39 56 65 74 90 124 140 152 208 223 235 238 241 245 256 265 290

Thickness

Lithology

[m] 2 4 10 23 25 29 31 32 39 56 65 74 90 124 140 152 208 223 235 238 241 245 256 265 290 350

−9999 −9999 1000 −9999 100 400 −9999 −9999 −9999 3500 500 1000 100 200 0 300 0 500 200 500 400 200 100 300 100 200

Heat flow

Recent porosity

Water depth

SWI temp.

[%]

[m]

[°C]

[mW/m2]

0 0 100 0 100 100 0 0 0 100 100 200 200 200 0 200 0 15 45 200 45 10 45 0 10 10

13 13 13 20 19 20 21 21 23 23 22 21 23 25 26 22 21 24 27 25 27 28 26 24 23 27

55 55 55 55 55 55 60 60 60 60 60 60 55 55 55 55 55 55 55 65 55 55 55 55 55 55

Marl

34

Marl Marl

26 24

Marl Marl Marl Marl Marl

19 9 6 5 5

Marl

5

Dolomite Limestone Silt + Tuff Limestone Sand + Silt Limestone Shale + Sand Sand + Lime Sand + Silt

5 5 5 5 5 5 5 5 5

A thickness value of −9999 marks the total erosion of an earlier event. Porosities and temperatures at the sediment-water interface (SWI) are calculated by the software, based on the lithology and compaction of the sediment and on palaeolatitude and water depth at the sediment-water interface. Bold values are the amount of eroded sediments in the most likely scenario.

taken from Tunis and Venturini (1992), Cati et al. (1987a, 1987b), Buser (1986) and Cousin (1981). Flysch outcropping in the Vipava syncline is Early Lutetian in age (48 Ma). The thermal maturity of VR ~0.8%Rr, corresponding to a peak paleotemperature of ~120 °C, shows that a significant amount of post Early Lutetian sediments was once resting on top of the preserved stratigraphic succession. Due to the presence of Upper Oligocene sediments, which transgressively overlay older strata (e.g. in the Sava Folds), uplift and erosion was assumed to range from Late Eocene (39 Ma) to Early Oligocene (29 Ma) time. Apatite FT ages support the assumption, since the data suggest cooling of Late Paleozoic siliciclastics below ~ 110 °C between 45 and 27 Ma. According to Tunis and Venturini (1992), the Upper Cretaceous to Middle Eocene sediment stack preserved at the transition zone of the (western) Dinaric CP to the Slovenian Basin (e.g. Soča valley) and the adjacent Friuli CP (Natisone valley) significantly exceeded 4000 m. The model presented in Fig. 11 is calculated with a pile of 5000 m thick Upper Cretaceous to Middle Eocene sediments on top of the preserved stratigraphic succession. The model shows that, applying a sediment thickness of 5000 m, the observed VR patterns can be explained using a heat flow of 60 mW/m2 during the time of maximum burial (Middle Eocene). The basic geological input data for this model is shown in Table 2. Additional models were calculated to check the sensibility of the results, applying an Upper Cretaceous–Middle Eocene sediment thickness ranging from 2000 to 9000 m (Fig. 12). According to the calibrated 1D-modeling results and using independent thickness information (e.g. Tunis and Venturini, 1992), a thickness of eroded Upper Cretaceous–Middle Eocene flysch sediments in the range of 4000 m to 5000 m and a heat flow of 60–75 mW/m2 during maximum burial are considered most likely. The modeled amount of eroded sediments of this study fits well with the field observations of preserved Upper Cretaceous to Eocene sediments in western Slovenia and Friuli (e.g. Tunis and Venturini, 1992).

5.6. Comparison to adjacent areas within the Alpine realm The Mesozoic succession in the Southern Alps and the southern part of the Eastern Alps includes important hydrocarbon source rocks (e.g. Clayton and Koncz, 1994; Stefani and Burchell, 1993). Therefore, their thermal history has been investigated by various authors. The thermal evolution of the Southern Alps west of the present study area was investigated by several authors including Greber et al. (1997), Fantoni et al. (2002), Fantoni and Scotti (2003), Carminati et al. (2010) and Scotti and Fantoni (2013). These authors detected an early Middle Jurassic (Aalian to Bajocian; c. 170 Ma) heat flow peak varying from 85 mW/m2 in the Lombardy Basin to 105 mW/m2 in the Belluno Basin (see Fig. 1 for location). They relate the heat flow peak to Early Jurassic rifting events (Masetti et al., 2012). Even in areas with relatively thick Upper Cretaceous to Paleogene sediments (e.g. 2200 m), Paleogene burial did not change maturity of pre-Jurassic rocks (e.g. Carminati et al., 2010 cum lit.). Maturity of Upper Triassic rocks in the deepest part of the Lombardy Basin (“Iseo depocenter”) reaches 4%Rr, whereas platform regions are characterized by only 0.3 to 0.5%Rr (Anelli et al., 1996; Bernasconi and Riva, 1993; Stefani and Burchell, 1993). Further to the east (Trento Platform, Friuli Platform) the maturity of Late Triassic sediments varies between 0.7%Rr and 1.0%Rr (Balazs and Koncz, 1999; Fantoni et al., 2000, 2001; Nussbaum, 2000) and is therefore comparable to the Julian Alps. VR results from the southern part of the Friuli Platform were documented by Fantoni et al. (2002). During the Mesozoic this western extension of the Dinaric CP was intersected by the “Friuli Basin”, an internal NW–SE (“Dinaric”) trending basin (Cati et al., 1987a,b). The basin sediments are characterized by a very high thermal overprint (e.g. N 3%Rr in Jurassic shales). In the Po Basin, located south of the Southern Alps, Triassic source rocks, which were not deeply buried during Liassic time, attained their maturity only during strong Neogene to Quaternary burial and heating (Scotti and Fantoni, 2013). Similarly, thick Neogene sediments are responsible for the thermal overprint of Mesozoic rocks in Eastern Alpine

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Fig. 12. Numerical 1D-Basin modeling results for the Sava Folds/Slovenian Basin. The mentioned heat flows are calibrated for Late Cretaceous–Early Oligocene times.

T. Rainer et al. / International Journal of Coal Geology 157 (2016) 19–38

Fig. 13. Conceptual model showing factors controlling thermal maturity of the Mesozoic and Paleogene rocks in the Alpine–Dinaric Transition Zone (W of Ljubljana).

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units buried beneath sediments of the Panonnian Basin System (Clayton and Koncz, 1994). A different thermal history within the Pannonian Basin is characteristic for the Igal Unit (Fig. 5c). The Igal Unit represents a complicated narrow tectonic zone connecting the Sava Folds (Slovenia) and the Bükk Unit (Hungary). The stratigraphic and lithological evolution of the Igal Unit is very similar to that of the Sava Folds. VR of Triassic rocks ranges from 1.7 to 4.6%Rr. Lower Triassic carbonate rocks reached temperatures of ~350 °C (Árkai et al., 1991). In addition to the Eo-Alpine (Cretaceous) event, also traces of a Meso-Alpine (Early-/Middle Eocene) thermal event were demonstrated. In the Northern Karawanken Range Mesozoic rocks matured as a result of Oligocene magmatic activity and thermal perturbations related to the rapid uplift of the Tauern Window (Rantitsch, 1997, 2001; Sachsenhofer, 2001). This shows that as a result of varying sediment thicknesses and heat flow variations, the timing of hydrocarbon generation varies significantly throughout the region. 5.7. Geological-burial evolution The thermal overprint of the Alpine–Dinaric transition zone (e.g. Slovenian Basin, Sava Folds, Adriatic-/Dinaric Carbonate Platform) is closely related to the presence of thick Upper Cretaceous to Eocene flysch sediments. The sketch in Fig. 13 shows the general setting, which controlled the deposition of Triassic to Eocene sediments and their maturity. The Slovenian Basin, formed during Anisian time between the Dinaric Carbonate Platform to the south and the Julian Carbonate Platform to the north (Fig. 13a), remained in a deep marine environment till the early or middle Eocene. On the contrary, shallow marine conditions persisted on the Dinaric Carbonate Platform. The Julian Carbonate Platform drowned in the Late Triassic/Early Jurassic, but emerged during the Late Cretaceous. This created a “platform – basin – high” geometry, which controlled the architecture of the flysch sediments. In addition, the Dinaric Carbonate Platform collapsed during the Maastrichtian. Coarser grained material was deposited on the slope of the platform-basin setting and fine grained material in the central Slovenian Basin (Fig. 13b). Subsequently, the Inner Dinaric orogen in the hinterland was uplifted, followed by the deposition of thick Paleogene flysch sequences in the foredeep (Dinaric Carbonate Platform, Slovenian Basin, and locally also the Julian Alps, Fig. 13c). Our results suggest that the Maastrichtian to Middle Eocene rocks were about 5000 m thick in the Sava Folds, but significantly thicker in the central part of the Slovenian Basin. The heat flow during maximal burial was probably close to the global average (60 mW/m2). The lateral thickness variations of the flysch sediments controlled the thermal overprint of the Carboniferous to Eocene sediment stack. The Adriatic CP suffered significantly lower sedimentary burial and, therefore, attained low maturity. During Eocene to recent times a compressional regime created the present-day structural configuration (Fig. 13d). The Alpine–Dinaric transition zone was uplifted and major portions of the flysch sediments were eroded before Late Oligocene times. Thrusting of the Julian Alps, as well as local overthrusting of the Slovenian Basin onto the Dinaric Carbonate Platform and thrusting within the Dinaric Carbonate Platform had no impact on the thermal maturity of the Carboniferous to Eocene sediments. The same is true for the deposition of Oligocene and Miocene sediments related to the Pannonian Basin system. 6. Conclusions The thermal history of Carboniferous to Eocene rocks of the Alpine– Dinaric transition Zone was investigated using vitrinite reflectance (VR) data, apatite fission track analysis and numeric 1D basin modeling. The

Alpine–Dinaric Transition Zone in Slovenia comprises the fold and thrust belt of the Southern Alps (South Karawanken Range, Julian Alps), Slovenian Basin and the Dinarides. The Slovenian Basin located between the Julian and Dinaric carbonate platforms evolved during Middle Triassic time and remained in a deep marine setting till the Late Cretaceous. No coalification break between pre-Variscan and post-Variscan sediments (across the Variscan discordance) can be recognized. Consequently the post-Variscan thermal overprint reached at least the same intensity as the Variscan one. The study shows that regional maturity patterns in the Carboniferous to Eocene sediment stack are mainly controlled by the thickness of Upper Cretaceous to Eocene flysch deposits, filling the accommodation space. Therefore the thermal overprint reaches a maximum (N4%Rr) in Triassic to Cretaceous sediments of the Slovenian Basin and decreases towards the north, west and south. Minor sedimentary burial of the Adriatic Carbonate Platform and the Julian Alps results in a lower thermal overprint (b 1.5%Rr). The thickness of flysch sediments was about 5 km in the area of the Sava Folds, but significantly higher in the central part of the Slovenian Basin. Heat flow during maximum burial in Eocene time was in the order of the global average (60 mW/m2). Cooling of Paleozoic and Mesozoic sediments below 110 °C occurred between Late Eocene and Early Oligocene times in different parts of the study area. Nappe stacking due to Early (Dinaric) and Late Cenozoic (Alpine) compressional tectonics did not influence the thermal maturity of the sediments. Acknowledgments Financial support within the frame of FWF project P13309-Tec is gratefully acknowledged. The University of Ljubljana and the Geological Survey of Slovenia provided samples from former mines. We would like to sincerely thank the reviewers László Fodor and Miloš Markič for their careful and useful comments that significantly improved the revised manuscript. References Anelli, L., Mattavelli, L., Pieri, M., 1996. Structural-stratigraphic evolution of Italy and its petroleum systems. In: Ziegler, P.A., Horvath, F. (Eds.), Peri-Tethys Memoir 2: structure and prospects of Alpine Basins and Forelands. Mem. Mus. Natn. Hist. Nat. 170, pp. 455–483. Aničić, B., Juriša, M. 1984. Basic geological map of SFRJ, 1:100.000, sheet Rogatec, L33-68, Geološki zavod Beograd. Árkai, P., Lantai, C., Fórizs, I., Lelkes-Felvári, G., 1991. Diagenesis and low-temperature metamorphism in a tectonic link between the Dinarides and the Western Carpathians: the basement of the Igal (Central Hungarian) Unit. Acta Geol. Hung. 34, 81–100. Balazs, E., Koncz, I., 1999. Contribution to thermal evolution of Southern Alps and paleogeographically adjacent areas based on vitrinite reflectance data. In: Gosso, G., Jadoul, F., Sella, M., Spalla, M.I. (Eds.), Proc. 3rd Workshop on Alpine Geological Studies. Mem. Sci. Geol. 51, pp. 119–128. Barker, C.E., Pawlewicz, M.J., 1986. The correlation of vitrinite reflectance with maximum paleotemperature in humic organic matter. In: Buntebarth, G., Stegena, L. (Eds.), Paleothermics. Springer, New York, pp. 79–83. Barker, C.E., Pawlewicz, M.J., 1994. Calculation of vitrinite reflectance from thermal histories and peak temperatures. A comparison of methods. In: Mukhopadyay, P.K., Dow, W.G. (Eds.), Vitrinite reflectance as a maturity parameter — applications and limitations. ACS Symp. Ser. 570, pp. 216–229. Bavec, M., 1999. Ladinian carbonate and pyroclastic rocks between Jagršče and Želin (Slovenia). Geologija 41, 41–69. http://dx.doi.org/10.5474/geologija.1998.003 (in Slovene). Bernasconi, S., Riva, A., 1993. Organic geochemistry and depositional environment of a hydrocarbon source rock: the Middle Triassic Grenzbitumenzone Formation, Southern Alps, Italy/Switzerland. In: Spencer, A.M. (Ed.), Generation, accumulation and production of Europe's hydrocarbons III, special publication of the European association of petroleum geoscientists, 3. Springer-Verlag, Berlin, Heidelberg, pp. 179–190. Bertotti, G., 1991. Early Mesozoic extension and Alpine shortening in the western Southern Alps: the geology of the area between Lugano and Menaggio (Lombardy, Northern Italy). Sci. Geol. Mem. 43, 12–123. Bertotti, G., Siletto, G.B., Spalla, M.I., 1993a. Deformation and metamorphism associated with crustal rifting: the Permian to Liassic evolution of the Lake Lugano—Lake Como area (Southern Alps). Tectonopysics 226, 271–284. Bertotti, G., Picotti, V., Bernoulli, D., Castellarin, A., 1993b. From rifting to drifting: tectonic evolution of the South-Alpine upper crust from the Triassic to Early Cretaceous. Sediment. Geol. 86, 53–76. Budkovič, T., 1978. The stratigraphic sequence of the Bohinj Valley. Geologija 21, 239–244.

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