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EVIDENCE FOR A HOLOCENE IMPACT EVENT IN THE PARDUBICE -KUNĚTICKÁ REGION (CZECH REPUBLIC) STRENGTHENED

Authors:
EVIDENCE FOR A HOLOCENE IMPACT EVENT IN THE PARDUBICE - KUNĚTICKÁ REGION
(CZECH REPUBLIC) STRENGTHENED. M. Molnár1, P. Švanda2, P. Janíček3, K. Ventura4, K. Ernstson5.
1Resselovo nám. 76, Chrudim 537 01, Czech Republic (molnar@ego93.com), 2,3,4University of Pardubice, Czech
Republic (Pavel.Svanda@upce.cz), (Petr.Janicek@upce.cz), (Karel.Ventura@upce.cz), 5University of Würzburg,
97074 Würzburg, Germany (kernstson@ernstson.de).
Introduction: Two contributions were presented at
the past LPSCs [1, 2], that reported on a Holocene me-
teorite impact strewn field in the Czech Republic, first
proposed by geologist Z. Štaffen.
author (Z.Š).
Fig. 1. Location map.
Fig. 2. Shock effects from the Czech impact event,
photomicrographs [2]. A, B: scoria melt rock, ballen
structures and PDF in quartz. C-F:asphaltic breccia:
PDF in feldspar, multiple sets of PF in quartz, PDF and
kinkbands in quartz, silica diaplectic glass and ballen
structures, plane light and xx polarizers.
Resumption of field work and extensive mineralogi-
cal-petrographical analyses revealed widespread occur-
rences of black, green and white glasses, iron silicide
particles, glass-like carbon, pumice-like carbon matter
(chiemite, [3]), various forms of glassy scoria and
polymictic asphaltic (bituminous) breccias [2]. Meteor-
ite impact was substantiated by abundant and typical
shock metamorphism (Fig. 3). Remarkably, no associ-
ated clear impact craters could be established, what is
resumed in this paper as an important observation.
Here we report on new field work and analyses, which
shed more light on the unusual impact event in the
Czech Republic.
The new exposures - geologic setting: The new
exposures are located in the immediate vicinity of
Kunětická hora, one of the most prominent magmatic
bodies of the Bohemian Massif (Fig. 1), according to
recent investigations a phonolite. Geologically note-
worthy are contact metamorphic formations of
porcelanite and spotted spilosite, the formation of
which is generally attributed to lava flows in contact
with Cretaceous marl limestones and clay slates. Dense
black obsidian-like glasses found at the Kunětická hora
occur together with finds of polymictic melt rock sco-
ria and polymictic asphaltic breccias and chiemite. The
new outcrop was exposed by a sinkhole from which up
to 15 cm large porcelanite glass cobbles together with
a 20 cm thick layer of melt rock scoria were recovered.
Fig. 3. Investigated samples: Cut faces of a polymictic
breccia with partially bituminous matrix, a melt rock
scoria and a black glass from the sinkhole outcrop.
Results: A polymictic breccias (Fig. 3A), the
polymictic melt rock scoria (Fig. 3B) and the procel-
lainite glasses (Fig. 3C) were examined by thin-section
polarizing microscopy and by XRD (D8 ADVANCE,
Bruker AXS) and SEM (TESCAN VEGA 3 EasyProbe
with EDX analyser).
The polymictic breccia: Characteristic photomicro-
graphs of the polymictic breccia in Fig. 4 show an
enormous fragmentation of the quartz grains and cal-
cites. All the quartzes, without contact to each other,
are finely shattered throughout, without losing coher-
ence. The calcite has been completely transformed into
a "pulp" of mosaicism and relics of micro-twins. Most
spectacular are abundant quartzes with a hardly, if at
all, known fracture texture (Fig. 5). An almost pulver-
ized outer fracture zone more or less sharply encloses a
more spared core area. Fig. 6 shows that these are not
singular cases. Two features should also be empha-
sized: typical spallation fractures at the surface of the
marginal fracture zones and partial transitions into sets
of planar fractures (PFs, cleavage in quartz) (Fig.6).
Fig. 4: Polymictic breccia (Fig. 3A), photomicrograph
of extremely shattered but coherent quartz grains
"swimming" in a fine-grained matrix (left). Calcite,
completely crushed to mosaicism and relics of micro-
twins (right).
1229.pdf51st Lunar and Planetary Science Conference (2020)
Fig. 5. Quartz grain fracturing in the polymictic brec-
cia (Fig. 3A) - fracture-mechanically a mineralogical
phenomenon.
Fig. 6. More concentrically fractured quartz grains.
Their size is of the order of the grains in Fig. 4. Right:
Quartz grains with sets of strictly planar (PFs) and sub-
planar fractures.
The melt rock scoria: The glass matrix in Fig. 7 con-
tains polymictic components, including breccias-in-
breccias typical for impact. Quartz grains are rare, but
they also show the unusual fracture texture known
from Fig. 5, 6, and PFs as a shock effect.
Fig. 7. Melt rock scoria: thin section, transmitted light.
Breccia-witin-breccia texture in glass matrix, c = car-
bonaceaus, g = glass particles. mm-scale. Concentric-
ally fractured quartz grain and sets of PFs in quartz.
The porcelanite glass: The black porcelanite glass
could be confused with obsidiane and because of the
vicinity to the Kunětická hora phonolite may be asso-
ciated with it. A remarkable difference is the much
higher Mohs hardness of 7 of the glass compared to a
hardness of 5-6 for obsidian. Equally remarkable is an
equal hardness of 7 for the black impact glass Irghizite
from the Zhamanshin impact structure in Kazakhstan.
The chemistry of the glass is also roughly the same as
that of porcelanite (Fig. 8), with the very low Na and K
contents standing out against a phonolite. A further
impact indication is possibly provided by the table in
Fig. 8. While the glass shows a depletion for Si and Ca
as well as an enrichment for Fe in comparison with the
porcelanite, the glass experimentally produced from
porcelanite has similar values to the natural porcelanite
rock. A different magnetic behavior - diamagnet-
ic/paramagnetic - of the elements could have influ-
enced the formation of the glass under the effect of an
extreme impact magnetic field (pinch effect, EMP).
Fig. 8. SEM-EDX data for Kunětická hora porcelanite,
the porcelanite glass and an experimentally produced
glass from the porcelanite. Also see text.
Discussion: Apart from the descriptions here with
the great similarities to the impactites described earlier
([1,2]) with different melt rock and breccia formations
and strong shock effects, there is an apparently new, as
yet undescribed form of impact shock effects in quartz
with an enormous, more or less concentric fringe-like
fragmentation with a largely intact core (Figs. 5, 6).
The only reasonable explanation for this is an extreme-
ly short-term deformation in the form of an extreme
external thermal shock at a high confining pressure,
which has maintained the coherence of the grains. Oth-
er individual shock effects are the multiple edge spalla-
tion fractures and the multiple sets of cleavage in
quartz (planar fractures, PFs). The chemical SEM-
EDX analyses go in the same direction that the
porcelanite glass cannot be attributed to an endogenous
magmatic process (obsidian), but became an impact
glass and occurs together with other impactites. The
proximity to Kunětická hora, which may irritate geolo-
gists, is therefore purely coincidental, especially since
comparable impact phenomena are widespread and
occur at great distances.
Conclusions: The new findings on an impact event
in the Czech Republic presented here strongly support
the hypothesis, expressed earlier, that a widespread
near-ground impact airburst [4] happened, which had
enormous shock effects on the ground among which
extreme thermal shock is especially significant. In
view of the absence of impact craters, the Czech event
may be seen in relation to airburst impacts like that
which formed the Libyan Desert Glass, despite the
recently expressed doubts about its airburst origin [5].
References: [1] Molnár, M. et al. (2017) 48th
LPSC, Abstract #1920. [2] Molnár, M. et al. (2018)
49th LPSC, Abstract #1423. [3] Shumilova, T.G. et al.
2018 Acta Geologica Sinica Engl. Ed., 92, 2179-2200.
[4] Boslough, M. (2015) Airburst modeling, 55 p.,
https://www.osti.gov/servlets/purl/1328668 [5] Ca-
vosie, A.J. & Koeberl, C. (2019) Geology, 47, 609-
612.
1229.pdf51st Lunar and Planetary Science Conference (2020)
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