**3. Geological setting and the studied sections**

In northern Slovenia, there are four large tectonic units: the Eastern Alps, the Southern Alps, the Outer Dinarides and the south-westernmost extending of the Tertiary system of Pannonian basins [28, 29] with the Smrekovec Volcanic Complex (**Figure 1**). The most outstanding geological structure is the Periadriatic Line (PAL), a complex regional fault system which represents in palinspastic reconstructions a shear zone developed by Late Cretaceous to Paleogene subduction of the European plate below the African plate [39, 40]. In the Eocene (~45 Ma) the subduction transformed into collision although the convergence continued during the Oligocene and resulted in break off of the southeast-dipping European slab beneath the Alps that generated magmatism along the PAL [41, 42].

The related Oligocene (28-22 Ma) volcanic activity occurred in the Smrekovec Basin (**Figure 3**) that had been subsided within the Permian and Triassic clastic and carbonate successions [43]. Tertiary sedimentation began in Late Eocene in fluviatile, limnic and shallow-marine depositional environment and changed to outer neritic and bathyal during the Oligocene time [29, 43]. In a middle bathyal environment characterised by sedimentation of organic-rich clayey silts [43], simultaneous volcanic activity created a composite stratovolcano. Magmas had calc-alkaline and medium-K affinity and formed a suite ranging in composition from basaltic andesite to dacite [44–46]. Volcanic activity had entirely submarine character and after its cessation, the Upper Oligocene to Early Miocene (Egerian) sedimentation continued with fossiliferous marine clayey silt [29]. The stratovolcano hosted hydrothermal system with a deep igneous source and convective-advective flow of hydrothermal fluids (**Figure 3**).

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

As the Smrekovec Volcanic Complex is a remnant of an ancient submarine composite stratovolcano the processes of alteration of peperites described herein could

*The study area (northern Smrekovec Volcanic Complex) after [28, 29] and the sections 1 (Prese*č*nik), 2 (Javorec), 3 (Krnes) and 4 (Smrekovec G34). The Kramarica Sill is about 200 m thick and located at the base* 

The sampling was performed in the entire area of the Smrekovec Volcanic Complex, although particularly detailed study has been carried out in two sections Krnes, and Smrekovec G34 (**Figure 2**). Lithofacies was determined by field observation, and chemical, mineralogical and petrographic analysis. Over 900 thin sections

Alteration minerals were analysed by X-ray diffraction (XRD) techniques in altogether 260 samples. Zeolites and related calcium aluminosilicate minerals

be recognised in, and applied to, similar environments worldwide.

**2. Sampling and analytical techniques**

*of the section 4 along the outcrops of lower Permian limestone.*

have been inspected in detail.

**304**

**Figure 2.**

### **Figure 3.**

*A conceptual model of the Smrekovec Basin with the stratovolcano and volcanic-hydrothermal system with a deep igneous source of heat and convective-advective flow regime. Hydrothermal fluids originating from heated and chemically modified surface and marine waters ascended through fracture systems and when reached the stratovolcano edifice they outflowed laterally and downward, preferentially through high-permeability layers. High-permeability layers underwent more extensive alteration and the authigenic minerals (e.g., laumontite, prehnite) have higher temperature stability ranges than those in the adjacent underlying or overlying low-permeability layers (e.g., clinoptilolite, heulandite, analcime).*

Late Miocene tectonic activity along the Periadriatic Line dissected the stratovolcano edifice and displaced its northern sector in the south-eastern direction on a 100 km scale [29]. The remaining Smrekovec Volcanic Complex probably encompasses about one quarter of the original stratovolcano edifice [36], and scarce outcroppings of volcanic rocks occurring north of the Šoštanj fault (**Figure 1**) are the assumed displaced remnants [47]. South of the Šoštanj fault Tertiary volcanic deposits occur in the Celje Basin, and together with the Smrekovec Volcanic Complex they are united in a lithostratigraphic unit termed the Smrekovec Series [29].

The succession of lavas, shallow intrusive bodies, and autoclastic, pyroclastic, syn-eruptive resedimented volcaniclastic and mixed siliciclastic-volcaniclastic rocks is over 2500 m thick, and at least 1000 m of the overlying deposits have been eroded already. In the northwest of the complex, the oldest proximal zone lithofacies associations overlie basal fossiliferous siltstone, limestone and calcarenite. As the strata, in general, verge toward the southeast, the oldest lavas can be traced over a distance of about 2 km to the east and south although their thickness changes (**Figure 4**). Younger, medial-zone lithofacies associations occur in the east and south and their development is typically complex (**Figures 5** and **6**). The Kramarica Sill is the largest shallow intrusive body in the Smrekovec Volcanic Complex. Its emplacement was related to the formation of a new vent along the Periadriatic Line, some 6 km east of the older one located northwest of Travnik (**Figure 2**).

A detailed study of lithofacies and alteration has been carried out in two sections composed of medial-zone lithofacies associations, namely Krnes and Smrekovec

**307**

**Figure 4.**

*Submarine Stratovolcano Peperite Syn-Formational Alteration - A Case Study of the Oligocene…*

G34, attaining 400 m and 470 m, respectively (**Figures 5** and **6**). The section Krnes consists of complexly alternating pyroclastic, autoclastic and syn-eruptive resedimented volcaniclastic deposits with only two thicker lava flows attaining some 25 m and 70 m. Pyroclastic deposits are dominated by fine-grained cross-bedded and horizontally bedded tuffs related to pyroclastic density currents [36], similar to the occurrences described by [48–51]. Pyroclastic flow deposits [36] are less abundant and commonly consist of basal massive lapilli tuff and the overlaying stratified coarse- and fine-grained tuff. Syn-eruptive resedimented deposits are abundant and comprise volcaniclastic debris-flow deposits and volcaniclastic turbidity flow deposits. Hyaloclastites and resedimented hyaloclastites are subordinate in occurrence but still relatively abundant. Siliciclastic silts are very rare and occur in thin, up to some dm thick stratified units. There are nine peperite deposits occurring at

*The sections Presečnik (left) and Javorec (right) situated in the proximal zone show the change in thickness of* 

the base and along terminal parts of smaller lava flows.

*lithofacies within a distance of 1.5-2 km toward the southeast.*

*DOI: http://dx.doi.org/10.5772/intechopen.95480*

*Submarine Stratovolcano Peperite Syn-Formational Alteration - A Case Study of the Oligocene… DOI: http://dx.doi.org/10.5772/intechopen.95480*

**Figure 4.**

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

Late Miocene tectonic activity along the Periadriatic Line dissected the stratovolcano edifice and displaced its northern sector in the south-eastern direction on a 100 km scale [29]. The remaining Smrekovec Volcanic Complex probably encompasses about one quarter of the original stratovolcano edifice [36], and scarce outcroppings of volcanic rocks occurring north of the Šoštanj fault (**Figure 1**) are the assumed displaced remnants [47]. South of the Šoštanj fault Tertiary volcanic deposits occur in the Celje Basin, and together with the Smrekovec Volcanic Complex they

*A conceptual model of the Smrekovec Basin with the stratovolcano and volcanic-hydrothermal system with a deep igneous source of heat and convective-advective flow regime. Hydrothermal fluids originating from heated and chemically modified surface and marine waters ascended through fracture systems and when reached the stratovolcano edifice they outflowed laterally and downward, preferentially through high-permeability layers. High-permeability layers underwent more extensive alteration and the authigenic minerals (e.g., laumontite, prehnite) have higher temperature stability ranges than those in the adjacent underlying or overlying* 

The succession of lavas, shallow intrusive bodies, and autoclastic, pyroclastic, syn-eruptive resedimented volcaniclastic and mixed siliciclastic-volcaniclastic rocks is over 2500 m thick, and at least 1000 m of the overlying deposits have been eroded already. In the northwest of the complex, the oldest proximal zone lithofacies associations overlie basal fossiliferous siltstone, limestone and calcarenite. As the strata, in general, verge toward the southeast, the oldest lavas can be traced over a distance of about 2 km to the east and south although their thickness changes (**Figure 4**). Younger, medial-zone lithofacies associations occur in the east and south and their development is typically complex (**Figures 5** and **6**). The Kramarica Sill is the largest shallow intrusive body in the Smrekovec Volcanic Complex. Its emplacement was related to the formation of a new vent along the Periadriatic Line, some 6 km east of the older one located northwest of Travnik

A detailed study of lithofacies and alteration has been carried out in two sections composed of medial-zone lithofacies associations, namely Krnes and Smrekovec

are united in a lithostratigraphic unit termed the Smrekovec Series [29].

*low-permeability layers (e.g., clinoptilolite, heulandite, analcime).*

**306**

(**Figure 2**).

**Figure 3.**

*The sections Presečnik (left) and Javorec (right) situated in the proximal zone show the change in thickness of lithofacies within a distance of 1.5-2 km toward the southeast.*

G34, attaining 400 m and 470 m, respectively (**Figures 5** and **6**). The section Krnes consists of complexly alternating pyroclastic, autoclastic and syn-eruptive resedimented volcaniclastic deposits with only two thicker lava flows attaining some 25 m and 70 m. Pyroclastic deposits are dominated by fine-grained cross-bedded and horizontally bedded tuffs related to pyroclastic density currents [36], similar to the occurrences described by [48–51]. Pyroclastic flow deposits [36] are less abundant and commonly consist of basal massive lapilli tuff and the overlaying stratified coarse- and fine-grained tuff. Syn-eruptive resedimented deposits are abundant and comprise volcaniclastic debris-flow deposits and volcaniclastic turbidity flow deposits. Hyaloclastites and resedimented hyaloclastites are subordinate in occurrence but still relatively abundant. Siliciclastic silts are very rare and occur in thin, up to some dm thick stratified units. There are nine peperite deposits occurring at the base and along terminal parts of smaller lava flows.

**Figure 5.** *Lithofacies and the principal alteration minerals in the medial-zone section Krnes.*

The section Smrekovec G34 begins in the Kramarica Sill (**Figure 6**) and comprises three thicker lava flows attaining some 35 m, 90 m and over 50 m, respectively. Based on the occurrence of yugawaralite [37, 52] and with respect to the middle bathyal water depths, the Kramarica Sill was emplaced about 600-800 m below the then surface of stratovolcano. Pyroclastic density current deposits are far less abundant than in the section Krnes and pyroclastic flow deposits are relatively abundant only in the upper half of the section. Syn-eruptive resedimented volcaniclastic deposits are dominated by volcaniclastic debris-flow deposits. Hyaloclastites and resedimented hyaloclastites are less abundant than in the section Krnes. Altogether fifteen peperites have been recognised and most of them are related to terminal parts of lava flows.

**309**

than a few m3

**Figure 6.**

lava flows.

*Submarine Stratovolcano Peperite Syn-Formational Alteration - A Case Study of the Oligocene…*

*DOI: http://dx.doi.org/10.5772/intechopen.95480*

**4. Occurrence, texture and composition of peperites**

*Lithofacies and the principal alteration minerals in the medial-zone section Smrekovec G34.*

to several 10s m3

In the study area peperites often occur as sheet-like bodies along basal contacts of lava flows and the underlying wet sediments (**Figure 7A, B**). Sometimes they are encountered on the top of thin lava flows burrowing into a several m thick sequence of fine-grained sediments or along terminal parts of lava flows where they form irregularly shaped or lobate bodies. Peperite domains range in volume from less

mingling wet sediment was commonly fine-grained volcanic ash or siliciclastic and carbonaceous silt. More rarely peperites have been recognised in association with coarser-grained volcaniclastic deposits, and most often they occur at the base of

The most widespread type is blocky peperite whilst globular peperite and peperitic hyaloclastite are rarer in occurrence. Along the pathway of a single lava

and sometimes they can only be some cm thick. The

*Submarine Stratovolcano Peperite Syn-Formational Alteration - A Case Study of the Oligocene… DOI: http://dx.doi.org/10.5772/intechopen.95480*

**Figure 6.** *Lithofacies and the principal alteration minerals in the medial-zone section Smrekovec G34.*
