**1. Introduction**

Peperite is a volcaniclastic rock related to *in situ* disintegration of magma intruding and mingling with the host sediment that is unconsolidated or poorly consolidated, and typically wet [1]. Peperite commonly occurs along the contacts between intrusions and wet sediments and at the base of lava flows overriding or indenting wet sediments [2–4]. The composition and texture of magmas involved in the formation of peperites may range from basaltic to rhyolitic and aphanitic to porphyritic, respectively, and the mingling sediments may have rather diverse texture, grain size and composition. As the availability of wet unconsolidated sediments is

a prerequisite for the development of peperites they have been commonly encountered in submarine environments with contemporaneous volcanic activity and sedimentation, such as volcanic arcs and back-arc basins [5–9].

The formation of peperite is a complex process and depends, in general, on the magma and host sediment properties, their mass ratio and total volume of pore water heated during their contact and mingling. For magma, the most relevant properties are composition, the content of volatiles and rheology, and for the host sediment that is texture and water-saturation. An important stage in the process of peperite formation is magma disintegration that can be brittle or ductile and attained by quenching, hydromagmatic explosions, surface tension effects, mechanical stress related to the movement of magma and density contrast to the sediment, and magma-sediment shearing. The contact of magma and wet sediment causes heating and expansion of pore waters, and the resulting disruption of coherence and sometimes fluidisation and shear liquefaction of the host sediment facilitate dispersion of clasts derived from magma away from the site of formation. The intricate processes of intermixing finally result in the formation of peperite [4, 10–14].

Two textural types of peperite have been recognised on the basis of shape of clasts derived from magma. Blocky peperite consists of sharply angular, blocky or platy clasts while in globular or fluidal peperite lensoidal, lobate, ameboid or bulbous clasts occur [5]. The term peperitic hyaloclastite refers to a peperitic rock in which magma fragmentation is largely the result of quenching, mechanical stress, or pore-water steam explosions [4, 12].

Several detailed studies of peperite occurrence and formation have been carried out in the system of Pannonian basins, in particular, in the Tokaj Mountains [15] and Western Hungary at Hajagos-hegy, Kissomlyó and Ság-hegy [9, 16, 17]. Subaqueous Miocene rhyolitic dome-cryptodome complex outcropping at Pálháza, the Tokaj Mountains, is surrounded by a carapace of hyaloclastites, hyaloclastite breccia, and globular and blocky peperite. Closely packed peperite zone with jigsaw-fit juvenile clasts formed next to a rhyodacitic body, and toward the boundary with the host sediment, a transition into the clast-rotated and clast dispersed zones of peperite has been recognised [17]. In the volcanic conduits, vents and crater lakes of phreatomagmatic volcanoes in Mio/Pliocene volcanic fields of Western Hungary globular and blocky peperite occur together regardless of the grain-size and texture of host sediment [15]. The study supports conclusions that the formation of different peperite textures depends on several factors, e.g., break-down of vapour films at the magma/wet-sediment interface, viscosity of magma and/or magma flux rate, a change in temperature, microlite crystallinity and gas content of magma, thermal properties of the host sediment and steam explosions [18–22].

The alteration of peperite is common and may begin contemporaneously to its formation owing to the release of deuteric magmatic fluids and volatiles, transfer of heat from magma or lava to the host sediment and heating of pore-waters therein. Large magma intrusions can cause contact metamorphism along the margins and initiate or modify fluid circulation on a several-kilometre-scale that may last a long period of time after the peperite formed [23]. Lavas undergo more rapid cooling, and effective circulation of heated pore-waters can be attained only locally along the contacts with the wet sediment until thermal gradients exist. Most often, the formation of secondary minerals such as carbonates, Fe-oxides and silica along the contacts of juvenile clasts has been reported [23–27].

The Oligocene Smrekovec Volcanic Complex (**Figures 1** and **2**) located in the south-westernmost extending of the Tertiary system of Pannonian basins, is a remnant of a submarine stratovolcano. Prior to erosion, and tectonic dissection and displacement along the Periadriatic Line, the stratovolcano extended in an area

**303**

of over 1000 km2

*OD – Outer Dinarides.*

**Figure 1.**

Carpathian-Pannonian region [33–35].

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

. Similar large submarine volcanoes have been encountered in

modern and ancient environments worldwide (e.g., [30–32]), and also, within the

*Simplified geological map of northern Slovenia after [28, 29] with the study area (framed) in the Smrekovec volcanic complex (SVC). PAL -Periadriatic Line; LF – Lavanttal (Labot) fault; SF – Smrekovec fault; ŠF – Šoštanj fault; DF- Donat fault; SFZ – Sava fault zone; SA – Southern Alps; EA – Eastern Alps;* 

The stratovolcano is composed of a succession of lavas and shallow intrusive

Peperites are the most abundant in medial-zone lithofacies associations. The mingling lavas range in composition from andesitic to rhyodacitic and the host sediments are mixed siliciclastic-volcaniclastic silts and calcareous muds or volcaniclastic deposits of various texture and grain size, i.e., fine- and coarse-grained tuffs, lapilli tuffs, volcaniclastic breccias [36]. Blocky and fluidal peperite and peperitic hyaloclastite are common in occurrence although their formation has not been

The stratovolcano-hosted hydrothermal system with convective-advective flow regime developed, and as a result, alteration minerals formed, the most widespread assemblage being laumontite, chlorite, ordered mixed layer chloritesmectite, quartz and albite. Despite of complex alteration that affected lithofacies associations, peperites very often contain authigenic minerals with typically higher temperature stability ranges than those in the adjacent underlying or overlying volcaniclastic deposits [37, 38]. Their formation must have occurred contemporaneously to the development of the host rock itself owing to thermal gradients originating from the parent lava or shallow intrusive body and geochemical gradients

bodies, and autoclastic, pyroclastic, resedimented volcaniclastic and mixed siliciclastic-volcaniclastic deposits. Lithofacies associations change from proximal, medial and distal zones over a distance of 0-2 km, 2-5 km and 5-20 km, respectively. The proximal zone is dominated by lavas and autoclastic deposits, and in the medial-zone pyroclastic and syn-eruptive resedimented volcaniclastic deposits become abundant. The distal zone is dominated by fine-grained pyroclastic, syn-

eruptive resedimented volcaniclastic and siliciclastic deposits.

related to the texture, grain size or porosity of the host sediment.

related local circulation of heated pore waters and deuteric fluids.

*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 1.**

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

sedimentation, such as volcanic arcs and back-arc basins [5–9].

peperite [4, 10–14].

or pore-water steam explosions [4, 12].

a prerequisite for the development of peperites they have been commonly encountered in submarine environments with contemporaneous volcanic activity and

The formation of peperite is a complex process and depends, in general, on the magma and host sediment properties, their mass ratio and total volume of pore water heated during their contact and mingling. For magma, the most relevant properties are composition, the content of volatiles and rheology, and for the host sediment that is texture and water-saturation. An important stage in the process of peperite formation is magma disintegration that can be brittle or ductile and attained by quenching, hydromagmatic explosions, surface tension effects, mechanical stress related to the movement of magma and density contrast to the sediment, and magma-sediment shearing. The contact of magma and wet sediment causes heating and expansion of pore waters, and the resulting disruption of coherence and sometimes fluidisation and shear liquefaction of the host sediment facilitate dispersion of clasts derived from magma away from the site of formation. The intricate processes of intermixing finally result in the formation of

Two textural types of peperite have been recognised on the basis of shape of clasts derived from magma. Blocky peperite consists of sharply angular, blocky or platy clasts while in globular or fluidal peperite lensoidal, lobate, ameboid or bulbous clasts occur [5]. The term peperitic hyaloclastite refers to a peperitic rock in which magma fragmentation is largely the result of quenching, mechanical stress,

Several detailed studies of peperite occurrence and formation have been carried out in the system of Pannonian basins, in particular, in the Tokaj Mountains [15] and Western Hungary at Hajagos-hegy, Kissomlyó and Ság-hegy [9, 16, 17]. Subaqueous Miocene rhyolitic dome-cryptodome complex outcropping at Pálháza, the Tokaj Mountains, is surrounded by a carapace of hyaloclastites, hyaloclastite breccia, and globular and blocky peperite. Closely packed peperite zone with jigsaw-fit juvenile clasts formed next to a rhyodacitic body, and toward the boundary with the host sediment, a transition into the clast-rotated and clast dispersed zones of peperite has been recognised [17]. In the volcanic conduits, vents and crater lakes of phreatomagmatic volcanoes in Mio/Pliocene volcanic fields of Western Hungary globular and blocky peperite occur together regardless of the grain-size and texture of host sediment [15]. The study supports conclusions that the formation of different peperite textures depends on several factors, e.g., break-down of vapour films at the magma/wet-sediment interface, viscosity of magma and/or magma flux rate, a change in temperature, microlite crystallinity and gas content of magma, thermal

The alteration of peperite is common and may begin contemporaneously to its formation owing to the release of deuteric magmatic fluids and volatiles, transfer of heat from magma or lava to the host sediment and heating of pore-waters therein. Large magma intrusions can cause contact metamorphism along the margins and initiate or modify fluid circulation on a several-kilometre-scale that may last a long period of time after the peperite formed [23]. Lavas undergo more rapid cooling, and effective circulation of heated pore-waters can be attained only locally along the contacts with the wet sediment until thermal gradients exist. Most often, the formation of secondary minerals such as carbonates, Fe-oxides and silica along the

The Oligocene Smrekovec Volcanic Complex (**Figures 1** and **2**) located in the south-westernmost extending of the Tertiary system of Pannonian basins, is a remnant of a submarine stratovolcano. Prior to erosion, and tectonic dissection and displacement along the Periadriatic Line, the stratovolcano extended in an area

properties of the host sediment and steam explosions [18–22].

contacts of juvenile clasts has been reported [23–27].

**302**

*Simplified geological map of northern Slovenia after [28, 29] with the study area (framed) in the Smrekovec volcanic complex (SVC). PAL -Periadriatic Line; LF – Lavanttal (Labot) fault; SF – Smrekovec fault; ŠF – Šoštanj fault; DF- Donat fault; SFZ – Sava fault zone; SA – Southern Alps; EA – Eastern Alps; OD – Outer Dinarides.*

of over 1000 km2 . Similar large submarine volcanoes have been encountered in modern and ancient environments worldwide (e.g., [30–32]), and also, within the Carpathian-Pannonian region [33–35].

The stratovolcano is composed of a succession of lavas and shallow intrusive bodies, and autoclastic, pyroclastic, resedimented volcaniclastic and mixed siliciclastic-volcaniclastic deposits. Lithofacies associations change from proximal, medial and distal zones over a distance of 0-2 km, 2-5 km and 5-20 km, respectively. The proximal zone is dominated by lavas and autoclastic deposits, and in the medial-zone pyroclastic and syn-eruptive resedimented volcaniclastic deposits become abundant. The distal zone is dominated by fine-grained pyroclastic, syneruptive resedimented volcaniclastic and siliciclastic deposits.

Peperites are the most abundant in medial-zone lithofacies associations. The mingling lavas range in composition from andesitic to rhyodacitic and the host sediments are mixed siliciclastic-volcaniclastic silts and calcareous muds or volcaniclastic deposits of various texture and grain size, i.e., fine- and coarse-grained tuffs, lapilli tuffs, volcaniclastic breccias [36]. Blocky and fluidal peperite and peperitic hyaloclastite are common in occurrence although their formation has not been related to the texture, grain size or porosity of the host sediment.

The stratovolcano-hosted hydrothermal system with convective-advective flow regime developed, and as a result, alteration minerals formed, the most widespread assemblage being laumontite, chlorite, ordered mixed layer chloritesmectite, quartz and albite. Despite of complex alteration that affected lithofacies associations, peperites very often contain authigenic minerals with typically higher temperature stability ranges than those in the adjacent underlying or overlying volcaniclastic deposits [37, 38]. Their formation must have occurred contemporaneously to the development of the host rock itself owing to thermal gradients originating from the parent lava or shallow intrusive body and geochemical gradients related local circulation of heated pore waters and deuteric fluids.

### **Figure 2.**

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

As the Smrekovec Volcanic Complex is a remnant of an ancient submarine composite stratovolcano the processes of alteration of peperites described herein could be recognised in, and applied to, similar environments worldwide.
