Eruptive and depositional history of a Pliocene tuff ring that developed in a fluvio-lacustrine...

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Journal of Volcanology and Geotherm

Eruptive and depositional history of a Pliocene tuff ring that

developed in a fluvio-lacustrine basin: Kissomlyo volcano

(western Hungary)

Ulrike Martina,*, Karoly Nemethb,1

aInstitut fur Geologie, Universitaet Wurzburg, Pleicherwall 1, D-97070 Wurzburg, GermanybDepartment of Mapping, Volcanology Projects, Geological Institute of Hungary, (Eotvos University,

Department of Regional Geology), 14. Stefania ut, Budapest, H-1148, Hungary

Received 22 December 2003; accepted 25 April 2005

Abstract

Kissomlyo volcano is a Pliocene erosion remnant of an alkaline basaltic tuff ring, belonging to the Little Hungarian Plain

Volcanic Field. Late Miocene shallow subaqueous, fluvio-lacustrine sand and mud units underlie sub-horizontally bedded lapilli

tuff and tuff beds with an erosional contact. The pyroclastic units, a sequence up to ~20 m thick, constitute a semi-circular

mound with gentle (b58) inward-dipping beds. Sedimentary features and field relationships indicate that the pyroclastic units

were formed in a terrestrial setting. Phreatomagmatic explosions occurred at a shallow depth, producing a large amount of

juvenile ash and lapilli, which were transported and deposited predominantly by pyroclastic density currents, subordinate fallout

and reworked by gravity currents. The tuff ring is overlain by a 5 m thick sequence of cross- and parallel laminated siltstone and

mudstone deposited in a lake inferred to have developed in a crater. The textural and structural differences between the

lacustrine units beneath and above the tuff ring sequences suggest that they did not belong to the same lacustrine environment.

The post-tuff ring lacustrine sequence is invaded by basanite pillow lava. The lava shows a basal peperitic margin partially

destroying the original structure of the lacustrine beds due to fluidisation. The time gap between the tuff ring formation and the

emplacement of the lava flow is estimated to be in the order of thousands of years.

D 2005 Elsevier B.V. All rights reserved.

Keywords: phreatomagmatic; tuff ring; crater lake; volcanic glass; lacustrine; Pannonian Basin; Hungary

0377-0273/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.jvolgeores.2005.04.019

* Corresponding author. Tel.: +49 931 31 6019; fax: +49 931 31

2378.

E-mail addresses: [email protected] (U. Martin),

[email protected] (K. Nemeth).1 Present address: Institute of Natural Resources, Department of

Soil and Earth Sciences, Massey University, Palmerston North, New

Zealand.

1. Introduction

In shallow water, small-volume volcanic eruptions

form cones, rings, or mounds consisting of bedded

pyroclastic deposits that are composed by fallout,

density currents and/or downslope remobilization of

al Research 147 (2005) 342–356

U. Martin, K. Nemeth / Journal of Volcanology and Geothermal Research 147 (2005) 342–356 343

tephra (Fisher and Schmincke, 1984; Sohn and

Chough, 1989; Sohn and Chough, 1992). The forma-

tion of monogenetic volcanic fields are often related

to phreatomagmatism associated with groundwater

sources, where seasonal climatic changes as well as

the availability of surface and groundwater play an

important role in the evolution of volcanic landforms

(Carn, 2000; Nemeth et al., 2001). A great variety of

volcanic landforms can develop depending on the

status of the hydrological environment during erup-

tions (White, 1991). Study of volcanic fields is also

useful in paleoenvironmental reconstruction of a re-

gion where monogenetic volcanoes erupt (Kokelaar,

1986; Sohn, 1996; White, 1996; White and Houghton,

2000). Systematic volcanological and sedimentologi-

cal studies can reveal changes in depositional envir-

onments before, during and after an eruption (White,

2001; Sohn et al., 2003). Studies that attempt to

characterise the overall paleo-geomorphological his-

tory of intracontinental volcanic fields are generally

rare (White, 1989, 1990; Godchaux et al., 1992; Carn,

2000).

Kissomlyo, the focus of this paper, is an erosional

remnant of a tuff ring in the Little Hungarian Plain

Volcanic Field (LHPVF), western Hungary. At Kis-

somlyo, pyroclastic rocks are sandwiched between

lacustrine units. The lacustrine unit above the tuff

ring sequence was later invaded by lava that has a

different composition from the underlying succession

suggesting different lacustrine environments. We use

the Kissomlyo volcanic remnant to demonstrate the

eruption mechanism of a tuff ring erupted in an

ephermal lacustrine environment post-dating the clos-

ing stages of the previously widespread Pannonian

Lake system. In addition, we demonstrate that Kis-

somlyo volcano is an example of rejuvenation of

basaltic volcanism in the same location (same vent)

in a short (thousand years-scale) period of time, and

highlights the complexity of small-volume intraconti-

nental volcanism.

2. Geological setting

Kissomlyo volcano is part of the Mio/Pliocene

alkaline basaltic Little Hungarian Plain Volcanic

Field (LHPVF), and is located in the western part of

the Pannonian Basin (Fig. 1). The Pannonian Basin is

considered to be a back-arc basin with a subduction-

related Neogene, calk–alkaline volcanic chain along

its northern and eastern margins (Horvath, 1993; Pecs-

kay et al., 1995). During the Miocene, extensional

tectonic events behind the subduction zone resulted

in lithospheric thinning and asthenospheric uprise

(Stegena et al., 1975; Csontos et al., 1991). From

Miocene to Pliocene times, extensive alkaline basaltic

volcanism characterised this region including the

LHPVF, and the Bakony–Balaton Highland Volcanic

Field (BBHVF) (Embey-Isztin et al., 1989, 1993;

Pecskay et al., 1995). The LHPVF consists of eroded

remnants of scoria cones, tuff rings and maars (Jugo-

vics, 1915, 1916, 1968; Jambor et al., 1981; Nemeth

and Martin, 1999; Martin and Nemeth, 2004). The

basaltic volcanoes are located near the major tectonic

lines, such as the Raba detachment fault and perpen-

dicular strike-slip faults (Jugovics, 1915, 1916; Var-

rok, 1953; Tari et al., 1992; Tari, 1994).

The underlying basement of the LHPVF consists

of Paleozoic to Mesozoic metamorphic rocks (gneiss,

schist etc.) covered by thick (up to 3000 m) Miocene

siliciclastic sediments (Magyar et al., 1999). Shortly

before volcanism started during the Neogene, a large

lake, the Pannonian Lake, occupied the Pannonian

Basin (Kazmer, 1990; Gulyas, 2001). Siliciclastic

sediments of the Lower Miocene units are deep-

water deposits of the Pannonian Lake. During the

Upper Miocene, the Pannonian Basin was charac-

terised by prograding deltas that developed from NW

to SE, which led to the diminishing of the deep sub-

basins of the Pannonian Lake first in the LHPVF

area (Juhasz, 1994; Vakarcs et al., 1994; Magyar et

al., 1999). Shallow lacustrine sandstones, mudstones,

and marls of the brackish Pannonian Lake are wide-

spread in the western part of the Pannonian Basin

and form the immediate underlying rocks of many

Mio/Pliocene alkaline basaltic volcanic rocks, includ-

ing Kissomlyo. Recent studies, based on comparative

drill core analyses, seismic surveys and paleontolog-

ical studies, showed that the extensive lacustrine

sedimentation in tens of metres of the Pannonian

Lake ceased in the area of the Kissomlyo volcano

about 9 Ma ago (Muller and Magyar, 1992; Vakarcs

et al., 1994; Muller, 1998; Magyar et al., 1999;

Gulyas, 2001).

Pre-volcanic sedimentary rocks that crop out at

Kissomlyo consist dominantly of sandstone, siltstone

Fig. 1. Location of the study area in the Pannonian Basin (Little Hungarian Plain) and localities mentioned in the text. Circle on (a) around

Kissomlyo volcano shows an area of 10 km-radius where no other volcanic rocks are known. Contour intervals on (b) are 5 m. AAV indicatescross section on Fig. 2. Arrows marked by 1 and 2 correspond to stratigraphic logs shown on Fig. 3. Italic numbers next to the bedding symbols

represent dip values. Bold and italic numbers represent steep dipping blocks inferred to have been rotated recently. LHPVF — Little Hungarian

Plain Volcanic Field, BBHVF — Bakony–Balaton Highland Volcanic Field.

U. Martin, K. Nemeth / Journal of Volcanology and Geothermal Research 147 (2005) 342–356344


and mudstone, with marly and/or pebble interbeds

deposited in a shallow sublacustrine to fluviolacus-

trine environment (Jambor, 1989; Bence et al.,

1999). These pre-volcanic sediments commonly are

very fine-grained and distinguished by their creamy

colour (Varrok, 1953; Magyar et al., 1999). Individ-

ual siliciclastic beds immediately underlying the

volcanic rocks are structureless to weakly bedded

and/or cross-stratified. The contacts between pre-

volcanic and pyroclastic beds seen in cores indicate

an erosional contact.

At least 5 m of siliciclastic deposits overlie the

pyroclastic units of Kissomlyo volcano, and were

invaded by subsequent, partly intrusive lava with a

peperitic contact. Two K/Ar radiogenic ages from

Kissomlyo volcano, 3.94F0.2 Ma (Balogh et al.,

1982) and 5.42F0.24 Ma (Balogh et al., 1986),

indicate that the volcanism post-dates the Pannonian

Lake (8–9 Ma). The existence of subsequent lacus-

trine siliciclastic sediments above the pyroclastic units

highlights the difficulty in distinguishing between

pre-, syn-, and post-volcanic sedimentary cycles in

an intracontinental setting as well as the paleogeo-

graphical importance in regard to reconstructing the

ever-changing fluviolacustrine system over the Plio-

cene in this region.

3. Kissomlyo volcano

3.1. Erosion remnant morphology

Kissomlyo is a small, flat volcanic remnant consist-

ing of mainly pyroclastic rocks and sporadic lava. The

pyroclastic rocks form a semi-circular mound that has

been preserved by an up to 5 m thick cap of lava in its

centre (Fig. 1b). The highest point of Kissomlyo is

~220 m above sea level (absl.) in the central part of the

remnant. The contact between the pre-volcanic silici-

clastic and the pyroclastic rocks is not exposed but is

inferred to be between 195–200 m on the basis of

differences in GPS elevation measurements between

locations of pyroclastic and siliciclastic units (Fig. 1).

Also GPS measurements indicate that the remnant sits

on the base level of the Little Hungarian Plain at 140–

150 m absl. The pre-volcanic siliciclastic units are still

exposed in small communal sand pits near the north-

eastern limit of the village of Kissomlyo on the flank of

the hill about 160 m absl., and have laterally persistent

sub-horizontal bedding (Fig. 1b). Similar sub-horizon-

tal bedding of the pre-volcanic silicilcastic units have

also been reported from wells drilled to search for

groundwater.

Bedding of the pyroclastic rocks is sub-horizontal

to gentle dipping (10–208) in the marginal zones of the

pyroclastic mound (Fig. 1b). Steep dipping (25–608)toward the centre of the preserved pyroclastic units has

also been reported (Jugovics, 1915) on the SW and W

sides of the mound, where large (10 m-scale) tilted

blocks are located about 30 m above the floor of the

Little Hungarian Plain. The pyroclastic rocks have

gentle hillward dipping orientation, inferred to be the

primary dip angle on the basis of the shape of bed-

forms and distrution and geometry of bedding sags

caused by ballistic volcanic bombs. Changes in dip

direction relate to position within the tuff ring (inner or

outer rim), distance from the crater, and/or stratigraph-

ic height. The 608 dip is interpreted as a block inferred

to be a result of recent block rotation in the southwest-

ern margin of Kissomlyo hill (Fig. 1b). Metre-sized

blocks with steep bedding are inferred to have been

tilted by recent landslides on the basis of their fracture

pattern and their common location near the steep top of

the hill. However, differential compaction of loading

by the pre-volcanic sequences cannot be ruled out.

This style of compaction maybe more prominent in

the inner tuff ring zone, where the conduit-filling

pyroclasts resettle during diagenesis creating post-vol-

canic subsidence and dip orientation changes similar to

those of maar volcanoes (Lorenz, 2003).

The preserved pyroclastic sequence is at least 20 m

thick and dip radially (b158) towards the centre of themound (Fig. 3) forming an exposure collar on the

southern side of the mound hundred metres in length

by a few metres in thickness. The pyroclastic units

exhibit a relatively monotonous alternation of poorly

sorted, coarse, fine bedded lapilli tuffs and tuffs (Figs.

3 and 4). There are no characteristic stratigraphic

discontinuities marked by sedimentary features such

as bedding, composition, and colour or grain textures

in the pyroclastic units.

3.2. Pyroclastic units

The exposed pyroclastic sequence can be subdi-

vided into two major lithofacies: thinly bedded lapilli

Fig. 2. Reconstruction of the Kissomlyœ volcano. Legend: a pre-volcanic siliciclastic units, b shallow standing water, c tuff ring sequence, d

post-tuff ring lacustrine unit, e post-tuff ring lava. 1) Pre-volcanic landscape with swamp and/or open surface water-filled valleys. 2)

Phreatomagmatic volcanism initiated subaerial conditions with abundant shallow ground and surface water from well-localized shallow (m-

scale) standing water bodies. Post-eruptive crater lake sedimentation accumulated few meter thick post-tuff ring lacustrine sediments. 3) After a

few thousand years of quiet period accompanied with lacustrine sedimentation, volcanism rejuvenated, producing lava flows emplaced into

water-rich environment. 4) Present scenario after post-volcanic erosion.


tuff and tuff (P1) and massive lapilli tuff (P2). These

lithofacies have been distinguished on the basis of

bedding, grain size, componentry and the ratio between

juvenile and accidental clasts. P1 contains a large vol-

ume (over 50 vol.% by visual estimates) of fine-

grained, accidental lithic fragments or accidental lith-

ic-derived mineral-phases. P1 beds are thinly bedded,

often cross-bedded lapilli tuff and tuff beds. In com-

Fig. 3. Idealised, stratigraphic sections from the south (1) and west (2) of Kissomlyo. Finer-grained beds in both Log 1 and Log 2 are primary

pyroclastic surge beds (P1 lithofacies) in contrast to coarser grained beds, which are phreatomagmatic fall beds that are commonly reworked (P2

lithofacies).


Fig. 4. (a) Planar-bedded pyroclastic units of the tuff ring (pyx) overlain by lacustrine sequences (lc) admixed with and covered by subsequent

lavas with pillowed texture (pl). The lavas have peperitic contacts with the lacustrine units (lc). (b) A close up view of the contact between

pillowed lava (pl) and lacustrine sediments (lc). Note the relatively sharp and flat contact between pyroclastic (pyx) and lacustrine units (lc).


parison, P2 consists of coarser-grained, rounded juve-

nile lapilli-bearing, calcite-cemented, thickly bedded,

massive to weakly stratified lapilli tuffs. P1 and P2 are

randomly intercalated and there is no preferential dis-

tribution pattern of the beds of P1 and P2 in the pyro-

clastic sequence.

Single P1 beds are often composite layers and are

normally to reverse graded (Fig. 5a). They occasion-

ally show low-angle cross-stratification, antidunes or

undulating bedforms (especially in the fine-grained

tuffs and lapilli tuffs; Fig. 5a and b). Beds are gener-

ally a few cm thick, but 15–20 cm thick, massive,

fine-grained tuff beds are also present in the lower part

of the succession. Sorting of the pyroclastic beds of

P1 is poor to moderate irrespective of their average

grain size (Fig. 4c). Tuffs and fine lapilli tuffs often

have randomly distributed accretionary lapilli as well

as armoured lapilli. In general, beds are laterally

continuous for up to 10 metres, and the entire section

forms a layer-cake structure.

Tuffs and lapilli tuffs are mainly comprised of

moderate to highly vesicular subrounded to blocky

sideromelane glass shards (Fig. 5c and d), glassy

volcanic lithics, tachylite, microcrystalline to apha-

nitic basaltic and pre-volcanic lithic clasts as well as

armoured lapilli. Larger clasts are predominantly

rounded to subrounded, dense, often radially frac-

tured sandstone fragments (up to 1 m in diameter)

(Fig. 5b) and/or flat, fluidally shaped and plastically

deformed mud fragments (up to 20 cm in length).

Deep-seated crystalline or other exotic accidental

lithic clasts are rare. The sandstone lithic fragments

often form impact sags on the underlying beds. There

is, however, no systematic correlation between the size

of lithics and the depth of impact sags. In comparison,

the flattened mud chunks do not overlie impact sags.

Deep bedding sags are bed-specific and commonly

appear below coarse lapilli tuff beds (Fig. 5b). In

contrast, beds with no or very shallow bed sags occur

below fine-grained layers (Fig. 5a), even though blocks

Fig. 5. (a) Alternating coarse (c)- and fine (f)-grained tuff and lapilli tuff (contact is marked by white dots) with accidental lithic lapilli derived

from Pannonian sedimentary units (arrows). Coarse lapilli tuff bears more evidence of immediate syn-eruptive remobilisation of pyroclasts (P2)

in comparison to the finer-grained lapilli tuff and tuff beds with textures more characteristics for primary origin (P1). Note the planar bedding.

(b) Subrounded accidental lithic block with impact sag derived from the Pannonian fluviolacustrine unit (Ps) from the western outcrops of

Kissomlyo pyroclastic succession. Note the coarse, scoriaceous juvenile lapilli accumulation around the lithic clast (white arrows). Dashed black

line helps to identify the impact crater this block caused. (c) Photomicrograph (plane-parallel light) of a hand specimen from a lapilli tuff bed of

the middle section of the pyroclastic sequence (Log 1-P1). Note the marly lapilli (m) derived from Pannonian units as well as the white fine

clasts of broken quartz derived from the pre-volcanic Pannonian units. Arrows point towards blocky to semi-rounded, slightly micro-vesicular

glassy pyroclasts. (d) Photomicrograph (plane-parallel light) from a lapilli tuff bed (Log 1-P2), exhibiting features such as a rounded

sideromelane glass (s), semi-rounded vesicular tachylite (black clasts-t) and calcite cement indicating slight reworking. Note the limit of

blocky sideromelane glass shard (dotted line) in a semi-rounded matrix (thick line).



commonly reach diameters of up to 50 cm. In the lower

part of the succession, there are small clasts with deep

impact sags. Impact sags are more commonly associ-

ated with dense basalt and/or cauliflower lapilli and

bombs (up to 15 cm in diameter). The transportation

direction determined from impact sags shows a radially

outward direction from the centre of the erosional

remnant. Cauliflower bombs are characteristic in

every part of the preserved tuff ring sequences, com-

monly cored with olivine megacrysts. The proportion

of accidental lithic clasts or mineral phases derived

from accidental lithic clasts is high and varies between

10 and 60 vol.% (visual estimates) (Fig. 5c).

Coarse-grained lapilli tuff beds of P2 appear to

exhibit more pronounced inverse or inverse-to-normal

grading, but the exact grading is difficult to determine

in many cases due to diffuse bedding. Most of these

beds have characteristic separation of a lower lapilli-

rich and an upper fines-enriched layer (Fig. 5a). This

grading is enhanced by a colour difference, being

greyish in the lower part of the lapilli tuff and yel-

lowish tan in the upper fine-grained part. This gives a

prominent appearance of bed couplets in certain out-

crops (Fig. 4a). Cross-lamination occurs within fines-

enriched, muscovite-bearing (b5 cm thick) beds that

have diffuse contacts into coarser-grained, lapilli tuff

beds. Lapilli tuff beds consist of juvenile glass shards

and small amount of matrix but are often strongly

cemented by micritic as well as spathic calcite. The

lapilli are semi-rounded to well-rounded, having

abraded outer rims. The vesicularity and microlite

content of the glass shards strongly vary.

3.3. Interpretation

The presence of sideromelane glass shards (Fig.

5c and d), cauliflower lapilli and bombs in the P1

and the presence of characteristic (often bed-specific)

impact sags as well as the large volume of accidental

lithic and accidental lithic-derived mineral phases

suggest a phreatomagmatic origin (Heiken and Woh-

letz, 1986). The moderate to high vesicularity (Fig.

5d) of the volcanic glass and/or glassy pyroclasts

suggests magma/water interaction during near-sur-

face vesiculation of magma. The general poor sort-

ing, variable individual bed thickness over the entire

section, high energy bedforms (antidunes), and the

presence of accretionary lapilli and/or mud clots are

more characteristic of normal base surge deposition

(e.g. gas-supported pyroclastic density current) than

eruption-fed, water-supported pyroclastic density cur-

rents (e.g. White, 2000). Pyroclastic beds of P1 are

very rich in accidental lithics and mineral phases

derived from the country rock and they are inferred

to have explosively disrupted and excavated the

surrounding country rock. Accidental lithic clasts

were then transported either ballistically or within

base surges. Variable shapes from subangular to

angular reflect variable degrees of grain abrasion in

P1 beds, which is more consistent with a base surge

origin rather than a water supported origin. The

presence of accretionary lapilli in P1 beds also indi-

cates a subaerial eruption plume as the presence of

free water does not favour formation of accretionary

lapilli. Although accretionary lapilli are common in

phreatomagmatic deposits associated with subaerial

tuff rings and maars, relatively small amount have

been identified at Kissomlyo, indicating that there

may have been excess (e.g. free) water in the erup-

tion cloud (Schumacher and Schmincke, 1991,

1995). This means that the transporting base surges

were charged with pore water (e.g. water derived

from water saturated slurry). The presence of accre-

tionary lapilli is not as widespread and systematic in

the pyroclastic beds at Kissomlyo as it is in other

e.g. maar-forming tephra rings elsewhere in western

Hungary (Martin and Nemeth, 2004) and therefore it

is inferred that some explosions were too water-rich

to form accretinary lapilli.

However, the fact that the number of impact sags

associated with large clasts are small indicates that the

energies of the impacts were suppressed by either 1)

high-density pyroclastic density current activity, and/

or 2) the presence of water or sediment-laden water

(slurry) in the depositional environment reducing the

impact energy of larger bombs. In contrast, the deep

sags, caused by small clasts, may have developed

upon impact into a low particle concentration base

surge cloud. The bed-specific distribution of impact

sags in the exposed pyroclastic succession suggests

occasional phases of clearing of the volcanic conduits

(e.g. after wall rock collapsed into the conduit). The

lack of systematic change in the depth of impact sags

and/or their fashion (e.g. fractured versus plastically

deformed) suggests that their eruptive environment

did not change significantly through time.


The large amount of accidental lithics and/or min-

eral phases derived from the pre-volcanic sedimentary

rock units suggests near-surface phreatomagmatic

fragmentation of uprising melt. Soft, unconsolidated

mud is interpreted to have been from the uppermost

part of the pre-volcanic surface. Consolidated, hard

sandstone fragments have been derived from the dee-

per (possible Lower Miocene) zones of the pre-volca-

nic rock units. The dominant proportion of

unconsolidated country rock fragments and mineral

phases derived from uppermost pre-volcanic silici-

clastic sediments (Upper Miocene units) indicates

that the explosion locus more or less stayed in the

uppermost pre-volcanic rock units.

Low-angle, cross-bedding and antidune structures

are both indicative of traction sedimentation of dilute

pyroclastic density currents. The small proportion of

large (N20 cm) accidental lithic bombs to the total

number of accidental lithic fragments in the pyro-

clastic succession indicates that the disrupted pre-

volcanic material was predominantly loose, or semi-

consolidated, which facilitated an easier breakage

during eruption. The presence of siliciclastic lithic

and/or siliciclastic-derived mineral phases all suggest

that the volcanic eruption occurred in a soft rock

environment (Lorenz, 2002); thus the pre-volcanic

Pannonian (Upper Miocene/Pliocene) sediments

must have been still unconsolidated in the time of

volcanism at Kissomlyo.

In contrast, the P2 lapilli tuff beds that are randomly

distributed among P1 beds are coarser grained, thicker

and only diffusely bedded. The common presence of

inverse grading, structureless texture comprising abun-

dant abraded lapilli of predominantly juvenile origin,

and tabular, undulating or lensoid beds that are inter-

bedded with primary pyroclastic density current depos-

its of P2 are inferred to be phreatomagmatic fall beds

formed directly from the eruption plume ormodified by

lateral transport that underwent some degree of syn-

depositional reworking (Belousov and Belousova,

2001). The interbedded nature of P2 beds with primary

pyroclastic density current deposits of P1 beds sug-

gests only minimal remobilisation of P2 beds. The

random distribution of primary and reworked tephra

beds suggest a deposition environment where particles

were free to remobilise right after initial settling.

The most favourable environment to generate al-

ternating succession of deposits of P1 and P2 is an

eruption that occurred in a water-laden environment,

e.g., very shallow (a few metres) water. The existence

of surface water is supported by the common presence

of large amount of unconsolidated mud in the tuff and

lapilli tuff beds. The erosional contact between pyro-

clastic rocks and basal deposits originating from the

Pannonian Lake indicates that the very shallow lakes

were well localized. The cross lamination in fines-

enriched, muscovite-rich (b5 cm thick) pyroclastic

beds with diffuse contacts to coarser grained lapilli

tuff beds are interpreted to be the direct results of

shallow water waves generated by the sudden dis-

placement of the shallow water mass due to the

phreatomagmatic explosion. They indicate a non-uni-

form transportation direction, which may be a reflec-

tion of repeated outward and inward wave movement

that followed the individual explosions. Base surges

build a mound like a pyroclastic apron. The soft host

sediment environment allowed the formation of a

wide bowl-shaped depression in and around which

the pyroclastic deposits have accumulated. The pres-

ence of two major types of lithofacies in the pyroclas-

tic sequence suggests an immediate reworking of

freshly deposited tephra randomly forming alternating

reworked and primary beds. The lack of intercalated

beds from suspension settling in the pyroclastic units

could be the result of eruption through a very shallow

lake where the explosive eruptions were able to dis-

place the lake water easily and quickly reach pure

subaerial conditions where base surges transported

horizontally the pyroclasts. In purely subaerial condi-

tions, reworking in inter-eruptive periods usually pro-

duces more characteristic erosional features in the

tephra beds such as steep sided small channels (e.g.

Buchel and Lorenz, 1993). The complete lack of such

morphological features in the beds of the pyroclastic

succession suggests that the textural characteristics

related to reworking are direct results from immediate

reworking effected the entire bed planes on the freshly

deposited tephra.

4. Post-tuff ring lacustrine sequence

4.1. Description

A well-bedded ~5 m thick, laminated, cross-lami-

nated, fine-grained, yellow to grey siltstone unit over-


lies the pyroclastic units (Fig. 6a and b). Its sedimen-

tary structures are preserved only in its lower ~70 cm.

The siliciclastic unit is only exposed in the southwest-

ern side of the volcanic erosional remnant below the

lava flow unit. Its exact extent cannot be determined;

however, the presence of sandy patches between co-

lumnar joints of capping lava units in the whole area

as well as baked siltstone xenoliths in the lava suggest

that this sedimentary unit was relatively extensive.

There are low-angle (5–108) cross-laminated packets

with no preferred orientation pattern in the preserved

siliciclastic units. Above this, the sedimentary struc-

tures are truncated by invading lava (Fig. 4a and b).

Fig. 6. (a) Photomicrograph of the post-pyroclastic lacustrine suc-

cession (plane-parallel light). Note the slightly inverse-to-normal

grading of fine alternating dark and light laminae as well as the

imbrication of platy clasts such as muscovite. (b) Close up overview

of the microlaminated post-pyroclastic beds. Note the slight undu-

lation of bed thickness, and cross lamination suggestive of currents

or wave action in the lake.

This post-volcanic siliciclastic unit differs from the

pre-volcanic quartzofeldspathic units in being richer

in oriented muscovites, clay minerals, and being lam-

inated with 0.2–0.4 mm laminae (Fig. 6b). Individual

laminae are laterally continuous for over several

metres. Dark and light coloured laminae form a dis-

tinct rhythmic structure (Fig. 6b). Locally there are

inverse-to-normal-graded fine sandstone interbeds

(Fig. 6a). There is no conclusively identifiable organic

material, fossils or pollen. However, lenses of scoria-

ceous, strongly altered basaltoid lapilli have been

identified in the lowermost 30 cm zone of the post-

pyroclastic units. In addition, the topmost scoriaceous

lapilli layer of the pyroclastic sequence is infiltrated

by fine mud. The transition zone extends up to 5 cm in

thickness, but otherwise the contact between the py-

roclastic units and the overlying siliclastic unit is

sharp. Contact with the overlying lava is discordant

and irregular with brecciated zones of coherent lava

and fluidal, highly vesicular detached lava fragments

(Fig. 4a and b).

4.2. Interpretation of the post-tuff ring lacustrine

sequence

The normal grading, good sorting and the pres-

ence of laminae and micro-laminae with aligned,

platy muscovite flakes indicate predominate suspen-

sion settling. Deposition is inferred to have occurred

in a water mass with limited water movement

recorded by the non-uniform cross-laminated parts.

Water movement is interpreted to be a result of wave

action as more persistent unidirectional cross-lamina-

tion would be expected from unidirectional inflow

water. The sediments deposited from a crater lake.

Crater lakes may also develop in constructional vol-

canic edifices such as tuff cones, e.g. NW China

(Feng and Whitford-Stark, 1986); however, signifi-

cant constructional edifices are unlikely in case the

of Kissomlyo due to its flat mound-shaped remnant

and more less horizontal bedding of pyroclastic units.

The large amount of muscovite is inferred to have

been derived from nearby Pannonian pre-volcanic

sediment ridges as well as from the still-unconsoli-

dated tephra ring. The lack of clear evidence of an

inflowing water course into the post-tuff ring lacus-

trine system suggests that the muscovite may have

been transported by either wind action or sediment


gravity flows from the crater rim and then deposited

by suspension settling. The presence of interbedded,

coarser-grained inverse-to-normally graded laminae

or thin beds suggests traction at transport. The pres-

ence of volcanic grains in the lowermost sequence of

the post-pyroclastic units indicates that the pyroclas-

tic remnant was still unconsolidated by the time a

lake developed. However, lack of volcanic detritus in

the post-volcanic lacustrine beds in higher strati-

graphic positions indicates that the source area was

dominantly non-volcanic.

There is no evidence of any major slumping or

collapse events in the pyroclastic mound suggesting

that no significant destructive events took place prior

to the development of the upper lacustrine siliciclastic

units and/or the original landform was too flat to allow

significant mass redistribution into the subsequently

developed crater. This allows reconstruction of 1) a

relatively low-lying tuff ring in which no, or just

insignificant, detritus was transported, 2) significant

erosion of the volcanic succession prior to the post-

tuff ring lacustrine sedimentation or 3) the preserved

part of the volcanic remnant is a distal part of a

volcano. The bedding of the preserved pyroclastic

succession is much too low an angle to reconstruct a

significantly elevated volcano. Lack of volcanic de-

tritus in the upper lacustrine sediments suggests that

no significant erosion of the original tuff ring has

occurred. Therefore it is inferred that either the orig-

inal tuff ring morphology was inappropriate to give

enough source material to build up the post-pyroclas-

tic lacustrine deposits, or that the basaltic tephra lith-

ified fairly rapidly, so that loose tephra was not

available. The well-localized post-tuff ring lacustrine

units suggest that it is deposited in a crater. Also, the

very limited amount of volcanic detritus in the post-

tuff ring lacustrine units indicates their deposition in a

closed crater lake.

Lack of organic material, such as vertebrate, insect,

tree leaf or fruit fossils, in the post-tuff ring deposits

indicates unsuitable conditions for life in the lake.

Post-volcanic crater lakes with similar ages to Kis-

somlyo volcano have been described in the vicinity of

Kissomlyo (Fischer and Hably, 1991; Kvacek et al.,

1994; Hably and Kvacek, 1998). However, these vol-

canic crater lake deposits are rich in fossils and often

record evidences of mesophytic forests in the Pliocene

around crater lakes (Hably and Kvacek, 1998). Rich

fossil accumulations in Pliocene crater lakes have also

been described (Ognjanova-Rumenova and Vass,

1998) in other parts of the Pannonian Basin. Paleo-

botanical evidences support the reconstruction of a

dry and hot climate in the area of LHPVF (Hably

and Kvacek, 1998). The surroundings of the craters

must have been humid but the climate, in general, was

presumably quite dry (Hably and Kvacek, 1998). The

general lack of pollen in the post-volcanic lacustrine

sequences at Kissomlyo suggests either low vegeta-

tion cover around Kissomlyo and/or relatively short

deposition time of the accumulation of this unit.

Therefore it is inferred that large open surface sand

ridges may have existed after the eruption of Kissom-

lyo giving substantial source material of wind-blown

dust, which was able to deposit in the volcanic de-

pression of Kissomlyo. Perhaps the lack of pollen may

also be related to the insufficient amount of outcrops

to identify their presence. The lacustrine sedimenta-

tion in the crater lake of Kissomlyo, based on the

presence of a ~5 m thick crater lake lacustrine unit, is

inferred to have taken place over a period of a few

thousand years.

5. Subsequent effusive activity

5.1. Description

A lava flow was subsequently emplaced onto the

post-tuff ring lacustrine units and produced mega

pillows, pillow breccias and peperitic margins along

lava and/or dyke margins (Martin et al., 2002). The

preserved lava flow outcrops sporadically and forms a

semi-circular distribution in map view, with one major

rosette-like columnar jointed part, currently forming

the highest topographic relief on the volcanic remnant.

The thickness of the exposed coherent lava flow

changes largely from a few metres to tens of metres

towards the centre of the erosional remnant based on

sporadic outcrops. Globular peperite occurs beneath

the lava flow and (Fig. 4a and b) formed by pillow-

shaped lobes up to 50 cm in length that penetrated into

the wet sediment. Unconsolidated sediments penetrate

into the lava suggesting incorporation during flowage.

Fluidally shaped clasts, but also some blocky ones, in

a wide size-range (centimetre to metre scale) are

dispersed along the margin of the lava flow (Martin


et al., 2002). Locally there are also disconnected

pillows with near-spherical bulbous shapes, which

are detached from the main lava flow and supported

by the fluidised host sediment (Fig. 4a and b). Orig-

inally laminated lacustrine sediment became homog-

enized due to intense fluidisation by the intruding

magmatic bodies.

New high precision Ar–Ar incremental step heat-

ing measurements give a date of 4.63F0.03 Ma age

for the Kissomlyo lava (Wijbrans et al., in. press). The

closest volcanic remnant (~10 km) that includes a

large volume of lava is Sag-hegy, having an Ar–Ar

age of 5.48F0.03 Ma, significantly different from the

age of Kissomlyo lavas.

5.2. Interpretation

The well-localized, semi-circular distribution of the

lava at Kissomlyo indicates that its movement was

controlled by a semi-circular barrier (e.g. crater rim)

and therefore it has been accumulated in the volcanic

crater. It is unlikely that the origin of the lava is from a

source other than the Kissomlyo volcano as the clos-

est nearby volcano (Sag-hegy) is located at a range of

10 km distance from Kissomlyo, and there are no

preserved lava remnants between these two localities

(Fig. 1a). In addition, the new Ar–Ar ages have been

confirmed that Sag-hegy is significantly older than

Kissomlyo (5.48F0.03 Ma, Wijbrans et al., press).

The changes of the estimated thickness of the lava at

Kissomlyo also point to a more proximal origin.

6. Discussion: Mio/Pliocene intracontinental

volcanism and the Pannonian Lake

Late Miocene sedimentation from a large, exten-

sive lake (Pannonian Lake) is inferred to have fin-

ished around nine million years ago in the area of

LHPVF, and the youngest known sediments depos-

ited in this lake system are interpreted to be six

million years old in southwestern Hungary (Magyar

et al., 1999). This conclusion is generally accepted

among researchers in the region. However, the exis-

tence of large but shallow (a few metres) short-lived

lakes after the complete disappearance of the Panno-

nian Lake is still under debate. Intraplate volcanism

in western Hungary started significantly later (8–2.3

Ma with dominant ages centred around 4 Ma) than

the interpreted finishing stage of Pannonian sedimen-

tation in West Hungary (Balogh et al., 1982, 1986;

Balogh, 1995).

Pyroclastic rocks of Kissomlyo are rich in acciden-

tal lithic, or lithic-derived mineral fragments, all in-

dicative of a near-surface phreatomagmatic interaction

in a water-rich environment. Kissomlyo eruption thus

is inferred to have taken place in a soft, unconsolidat-

ed and water-rich, slurry-like lacustrine sediment,

leading to excavation and incorporation of these

loose sediments with the volcanic ejecta. Primary

beds (e.g. P1) of Kissomlyo are characteristic of a

subaerial formation. However, eruption may have

occurred in periodic presence of very shallow water

during the course of the eruption.

7. Conclusion

The pyroclastic deposits of Kissomlyo record a

unique sedimentary sequence among Neogene intra-

continental volcanoes in the western Pannonain

Basin, exhibiting features characteristic of tuff ring-

associated base surge deposition influenced by water

surplus in the depositional environment. The 20 m

thick pyroclastic beds are overlain by a ~5 m thick

lacustrine unit of sand and silt. These sand and silt

were invaded by lava extrusions and an intrusion that

formed peperite that records the wet and unconsoli-

dated state of the sand and silt during the lava

emplacement. The textural differences between un-

derlying and overlying siliciclastic units suggest that

the post-volcanic lacustrine beds developed in a well

localized basin (e.g. crater lake), similarly to other

post-volcanic deposits reported from the LHPVF

(Jambor and Solti, 1976; Hably and Kvacek,

1998). In this study we gave an example of an

eroded tuff ring that is inferred to have developed

in subaerial conditions where very shallow lakes

may provide substantial surface water to fuel phrea-

tomagmatism. In this study it has been highlighted

that such environments are very common in fluvio-

lacustrine basins where base-level drop and rise may

occur in a very short time scale. Such change may

influence the eruptive environment where monoge-

netic volcanoes are forming very diverse pyroclastic

successions.


Acknowledgments

Partial financial support from the DAAD within the

DAAD German–Hungarian Academic Exchange Pro-

gram to UM and KN, the Hungarian Science Foun-

dation (OTKA F 043346) and the Magyary Zoltan

Postdoctoral Fellowship grant to KN is acknowl-

edged. Review of an earlier version of the manuscript

by James White (Otago University, New Zealand) is

also acknowledged. DFG travel grant to UM as well

as an AGU travel grant to KN to attend the Chapman

Conference on Subaqueous Explosive Volcanism

(Dunedin, New Zealand, 2001) is greatly appreciated.

Constructive reviews by journal reviewers Sharon

Allen and Young Kwan Sohn helped to strengthen

the manuscript.

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