Eruptive and depositional history of a Pliocene tuff ring that developed in a fluvio-lacustrine basin: Kissomlyó volcano (western Hungary)

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Abstract

Kissomlyó 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 (< 5°) 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.

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 tephra (Fisher and Schmincke, 1984, Sohn and Chough, 1989, Sohn and Chough, 1992). The formation 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, Németh et al., 2001). A great variety of volcanic landforms can develop depending on the status of the hydrological environment during eruptions (White, 1991). Study of volcanic fields is also useful in paleoenvironmental reconstruction of a region where monogenetic volcanoes erupt (Kokelaar, 1986, Sohn, 1996, White, 1996, White and Houghton, 2000). Systematic volcanological and sedimentological studies can reveal changes in depositional environments before, during and after an eruption (White, 2001, Sohn et al., 2003). Studies that attempt to characterise the overall paleo-geomorphological history of intracontinental volcanic fields are generally rare (White, 1989, White, 1990, Godchaux et al., 1992, Carn, 2000).

Kissomlyó, the focus of this paper, is an erosional remnant of a tuff ring in the Little Hungarian Plain Volcanic Field (LHPVF), western Hungary. At Kissomlyó, 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 Kissomlyó volcanic remnant to demonstrate the eruption mechanism of a tuff ring erupted in an ephermal lacustrine environment post-dating the closing stages of the previously widespread Pannonian Lake system. In addition, we demonstrate that Kissomlyó 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 intracontinental volcanism.

Section snippets

Geological setting

Kissomlyó 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 (Horváth, 1993, Pécskay et al., 1995). During the Miocene, extensional tectonic events behind the subduction zone resulted in lithospheric thinning and

Erosion remnant morphology

Kissomlyó is a small, flat volcanic remnant consisting 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 Kissomlyó is ∼220 m above sea level (absl.) in the central part of the remnant. The contact between the pre-volcanic siliciclastic and the pyroclastic rocks is not exposed but is inferred to be between 195–200 m on the basis of differences in

Description

A well-bedded ∼5 m thick, laminated, cross-laminated, fine-grained, yellow to grey siltstone unit overlies the pyroclastic units (Fig. 6a and b). Its sedimentary structures are preserved only in its lower ∼70 cm. The siliciclastic unit is only exposed in the southwestern side of the volcanic erosional remnant below the lava flow unit. Its exact extent cannot be determined; however, the presence of sandy patches between columnar joints of capping lava units in the whole area as well as baked

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

Discussion: Mio/Pliocene intracontinental volcanism and the Pannonian Lake

Late Miocene sedimentation from a large, extensive lake (Pannonian Lake) is inferred to have finished around nine million years ago in the area of LHPVF, and the youngest known sediments deposited 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 existence of large but shallow (a few metres) short-lived lakes after the complete disappearance of the

Conclusion

The pyroclastic deposits of Kissomlyó record a unique sedimentary sequence among Neogene intracontinental 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

Acknowledgments

Partial financial support from the DAAD within the DAAD German–Hungarian Academic Exchange Program to UM and KN, the Hungarian Science Foundation (OTKA F 043346) and the Magyary Zoltán Postdoctoral Fellowship grant to KN is acknowledged. 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

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    Present address: Institute of Natural Resources, Department of Soil and Earth Sciences, Massey University, Palmerston North, New Zealand.

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