Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)
Accretionary lapilli in melaphyres of the Krkonoše Piedmont Basin
Abstract
Two types of accretionary lapilli were observed in pyroclastic sequence exposed in the abandoned quarry Hvězda near Nová Paka. The first type is characteristic by size corresponding to peas and overlies brecciated surface of lava flow, where it fills joints and pockets. The accretionary lapilli have coarser core and fine-grained rim and they appear to be sedimented from an ash-cloud. Fragments of this type lapilli were observed in the overlying base-surge deposits. The second type is larger in diameter reaching 2-3 cm and it is often deformed – to the form of sponge biscuits. These larger lapilli are associated with asymmetric bomb-sag structures in unconsolidated pyroclastic deposits. The geometry of impact prints argues for a ballistic transport of these features. Hence, the second type accretionary lapilli must have been formed during phreatomagmatic eruption in the conduit.
