Pula maar is a partially eroded Pliocene maar-diatreme volcano, part of the Mio-Pliocene
Bakony-Balaton Highland Volcanic Field. The surficial remnant of the maar-diatreme
volcano consists of (1) a distinct depression with a thick post-eruptive lacustrine
alginite sediment infill interbedded with coarse-grained volcaniclastic sediments,
(2) a narrow marginal zone inside the depression consisting of primary pyroclastic
rock units that are interpreted to be partly collapsed and subsided blocks of entire
sections from the tephra ring formerly surrounding the maar crater depression, and
(3) coarse-grained volcaniclastic debris-flow deposits closely associated with the
collapsed primary pyroclastic rock units in the marginal zone. The presence of coherent
lava rocks below the crater-fill units, their distribution pattern and their association
with scoriaceous beds indicate that, after the maar-diatreme-forming phreatomagmatic
explosive activity, small (100 m-scale) scoria and/or spatter cones erupted in the
maar crater. These cones are the likely source of the lava flows that partially filled
the maar crater basin. The widespread dm-to-m thick basaltic sand and/ or silt units
at the base of the post-eruptive crater-filling sedimentary succession are interpreted
to be reworked volcaniclastic material from the intra-maar scoria/spatter cones as
well as from the tephra ring. Based on comparative analyses of 53 core descriptions,
this study reveals that the original maar crater basin was larger than previously
suggested. The deep level of the maar crater is reconstructed to be a northeast-southwest
elongated depression, currently forming a c. 50-m-deep basin. Geomorphological considerations
suggest that most of the phreatomagmatic pyroclastic rocks are composed of base surge
and tephra fall deposits around the deep maar depression. These allochthonous rock
units form a 50-400 m wide zone of proximal tuff-ring sequences. The formation of
this zone is inferred to be a result of a combination of syneruptive subsidence due
to mass deficit in the rigid Triassic dolomite basement caused by the phreatomagmatic
explosions as well as post-eruptive subsidence of the crater- and diatreme-filling
successions due to diagenetic compaction. The facies in the centre of the maar lake
is a soft laminated "alginite" (mainly Botryococcus colonies, diatom frustles, calcium
carbonate crystals, clay minerals). In the section exposed in the Pula open cast mine,
a single turbiditic layer is present. This layer originated in a landslide, which
possibly could have been caused by either syn-eruptive earthquake and/or a sudden
posteruptive subsidence event of the diatreme fill. (c) 2008 Elsevier B.V. All rights
reserved.
