A Lower Miocene pyroclastic-fall deposit from the Bukk Foreland Volcanic Area, Northern Hungary: Clues for an eastward-located source

摘要

Detailed investigation of a Lower Miocene Plinian pyroclastic sequence that crops out in the Bukk Foreland Volcanic Area (BFVA) in Northern Hungary is presented here. The studied eruptive products are part of a ca. 50 metres thick pyroclastic succession comprising of a basal ignimbrite that is covered by stratified pyroclastic unit including a topmost ignimbrite (Mango ignimbrite unit, part of the Lower Pyroclastic Complex). The investigated pyroclastic unit is part of the Mango ignimbrite unit, and consists of a pyroclastic fallout deposit, a ground-surge deposit, and an ignimbrite, all indicating a complete Plinian eruption phase. This pyroclastic succession has been identified in three locations, which crops out along a similar to 20 km long, SW-NE transect in the BFVA (two in the western, and one in the eastern part). The pyroclastic rocks in these sites are correlated well on the basis of the lithologically and texturally similar layers and their identical field volcanological properties. The correlation is also supported by the paleomagnetic signature of the two ignimbrites (upper ignimbrite - declination: 275-302 degrees, lower ignimbrite with overprint magnetization - declination: 320-334 degrees). The paleomagnetic directions of the stratigraphically upper ignimbrite suggest that this sequence belongs to the oldest known pyroclastic rock assemblages of the BFVA (Lower Pyroclastic Complex, deposited between 18.5 and 21 Ma according to previously published K/Ar dating results in good agreement with paleomagnetic measurements). Based on proximal-to-distal variations in the grain size of the pyroclastic fallout deposit (with maximal thickness is 71 cm), a potential source region to the east (or northeast, or southeast) of the BFVA has been inferred in a relatively close distance (similar to 5-15 km). The (north)eastward-located source region is also supported by comparison of the characteristics of the studied fallout deposit with the spatial distribution of selected Plinian fallout tephra from worldwide examples using their digitalized isopach maps.