Abstract
The Grad area, north-eastern Slovenia is characterised by Upper Pliocene volcanic activity, which produced minor lava flows and scoria deposits. In the late-stage of volcanic activity, hydrovolcanism predominated and it produced pyroclastic surge deposits rich in accretionary lapilli. This alkali basaltic (basanitic) volcanism occurred on the surface of alluvial fan, and consequently, its products were easily reworked by fluvial currents. Some accretionary lapilli and armoured lapilli survived redeposition and can be encountered in mixed fluvial-volcaniclastic sediments. Armoured mud balls developed under normal sedimentary processes involving sliding of unconsolidated material under gravitation.
Highlights
Accretionary lapilli are spherical aggregates of ash
Late Pliocene fluvial-volcaniclastic complex at Grad, northeastern Slovenia, comprises the remains of deposits, which consist of tuffs rich in accretionary lapilli; armoured lapilli are encountered, but they are subordinate in occurrence
Armoured mud balls overlie scoria flow deposits and they were produced by a massflow. The aim of this contribution is to emphasise the importance of accretionary lapilli and pyroclastic surge deposits of the Grad complex as they prove the existence of hydrovolcanic stage of volcanic activity
Summary
Accretionary lapilli are spherical aggregates of ash. They are not commonly encountered in Tertiary volcaniclastics of Slovenia. Late Pliocene fluvial-volcaniclastic complex at Grad, northeastern Slovenia, comprises the remains of deposits, which consist of tuffs rich in accretionary lapilli; armoured lapilli are encountered, but they are subordinate in occurrence. Armoured mud balls overlie scoria flow deposits and they were produced by a massflow The aim of this contribution is to emphasise the importance of accretionary lapilli and pyroclastic surge deposits of the Grad complex as they prove the existence of hydrovolcanic stage of volcanic activity. Alkali basaltic volcanism at Grad occurred on the surface of alluvial fan In the beginning, this volcanic activity produced smaller lava flows, and possibly, a small cinder cone, too. The surface of alluvial fan is very dynamic environment, and volcaniclastic deposits produced by effusive and explosive volcanic activity were immediately resedimented by fluvial currents. Today, redistributed and texturally mixed fluvial-volcaniclastic sediments vastly predominate in the Grad complex, and they practically bear the only information about the existence of the primary volcanic and volcaniclastic rocks in the area
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