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

摘要

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 ( < 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.