◄ Carnets Geol. 18 (10) ►
Outline:
[1. Introduction]
[2. Localities and their geological setting]
[3. Methodology]
[4. Results]
[5. Discussion]
[6. Summary]
and ... [Bibliographic references]
Department of Geochemistry, Mineralogy and
Petrography; Faculty of Earth Science; University of Silesia; Będzińska
Str. 60, PL- 41-200 Sosnowiec (Poland)
Published online in final form (pdf) on July 27, 2018
DOI 10.4267/2042/68186
[Editor: Bruno Granier;
language editor: Stephen Eager]
Moldavites as ejecta glasses are fragile and transient: they are quickly abraded in fluvial conditions, this was confirmed by tumbling experiments. In the present study, multiple tumbling experiments were conducted to simulate the hydrogeological conditions of deposition of moldavites found in several different gravel pits. These experiments threw new light on the evolution of tektites during reworking. It appears that the original glass shape and mass as well as environmental conditions such as river velocity and the type of sediment with which they are associated are all important variables. However, the experiment did not simulate other significant variables, such as the variability of environmental energy. With given advantageous conditions, moldavite glasses could probably have withstood dozen kilometers of reworking, but this assertion is not sufficient to constrain the distance to their supply areas.
• moldavite;
• tektite;
• fluvial abrasion;
• redeposition;
• reworking
Brachaniec T. (2018).- Variations in fluvial reworking of Polish moldavites induced by hydrogeological change.- Carnets Geol., Madrid, vol. 18, no. 10, p. 225-232.
Variations dans le remaniement fluviatile de moldavites polonaises induites par modification hydrogéologique.- Les moldavites, comme tout verre d'impact, sont fragiles et éphémères : Elles sont facilement réduites à l'état de fines particules lors de leur transport hydraulique comme le confirment des études expérimentales d'écoulement gravitaire. Dans cette nouvelle étude, différents cycles de transport sont considérés afin de reconstituer les conditions hydrogéologiques caractéristiques de chaque gravière où ont été récoltées ces moldavites. Ces expériences apportent des informations nouvelles sur l'évolution des tectites au cours de leur remaniement. La forme originale et le poids initial du verre d'impact, mais aussi les conditions environnementales (telles que la vitesse d'écoulement de l'eau et la nature du sédiment encaissant), constitueraient les variables les plus importantes. Notre expérimentation n'aura cependant considéré qu'un nombre limité de paramètres significatifs, tels que la variation de l'énergie du milieu. Dans des conditions favorables, les moldavites ont pu parcourir plusieurs douzaines de kilomètres lors de leur remobilisation. Cette conclusion n'est toutefois guère satisfaisante car il nous est impossible de déterminer précisément la distance totale parcourue depuis le gisement originel.
• moldavite ;
• tectite ;
• érosion fluviatile ;
• resédimentation ;
• remaniement
Moldavites are usually bottle-green glass ejecta with the characteristic homogeneous structure, ejected by the Ries event as a by-product of the melting of quartz sands, clay minerals and carbonates (Rodovská et al., 2016; Skála et al., 2016; Žák et al., 2016). Tektite hardness falls between 5 and 6 on the Mohs scale (Simmons and Ahsian, 2007), making them susceptible to abrasion during transport with gravel. In most central European locatilities, moldavite glasses are reworked and redeposited in the Miocene rivers (Bouška, 1964, 1988; Žebera, 1972; Lange, 1996; Bouška et al., 1999; Trnka and Houzar, 2002; Buchner and Schmieder, 2009). This explains why the age of moldavite-bearing deposits differs so drastically from the age of these tektites' formation. The preliminary results of the experimental tumbling of moldavites confirmed their high susceptibility to fluvial abrasion (Brachaniec, 2018). However, the generally accepted river-flow velocities in SW Poland and sedimentary deposition variability did not allow for the precise determination of the relationship between river flow velocity, sedimentary type, and distance of fluvial transport of tektites at hand (Brachaniec, 2018). An important factor determining the distance of glass transport in river environments seems to be the amount of gravel: in the case of moldavite-bearing sediments of SW Poland, the percentage of gravel in these deposits increases from the West to the South-East (fieldwork observations). In connection with this, supplementary tumbling cycles were carried out: according to the respective tektite-bearing deposits, proper hydrogeological conditions were adopted to check how it affects the distance of glass reworking respectively.
All Polish moldavites were found
in SW Poland (Fig. 1 
). Moldavite-bearing sediments are the fluvial sandy gravel
of theLate Miocene Gozdnica Formation and the Pleistocene fluvial terraces of
the Lusatian Neisse (see more details in Brachaniec, 2017; Brachaniec
et
al., 2014, 2015,
2016; Szopa et
al., 2017).
|
Figure 1:
Schematic map of towns and modern river systems in SW Poland (modified after Adamska,
2013; Adynkiewicz-Piragas and Lejcuś,
2013) with respective
location of moldavite-bearing pits (red stars). |
Tumbling experiments on moldavites were conducted at the Faculty of Earth Sciences of the University of Silesia, using a rotating barrel LPM-20 (Glass GmbH & Co. KG Spezialmaschinen). Its radius was 15 cm and height 40 cm. According to this dimensions volume of the barrel was estimated to 0.028 m3. In each cycle one moldavite was used. The experiment ended after complete destruction of tumbled moldavites (i.e., no tektite was found in the barrel).
Sediment
samples to test this experiment were taken from every pit where Polish
moldavites were found (Fig. 1 ). In
everypit with moldavites (Fig. 1 ), a 100 kg sample of bulk sediments was taken
back to the laboratory facility. Subsequently the percentage composition
of sand and gravel was measured by sieving. In addition, the percentage content
in the gravel fraction was determined for the following size classes: diameter
up to ca 3 cm, diameter ranging from 3 to 8 cm, and larger clasts (more than 8
cm in diameter). On this basis, a 5 kg sediment sample (built based on
percentage composition of 100 kg sample) for each cycle was elaborated (Table
1), and then put into the tumbling barrel filled
with 10 l of water.
Table 1: Methodology involved in experimental tumbling of moldavites.
| Cycle | Pit(s) - River | Age of Sediments | River velocity (km/h) | Sediment sample | Observation | Moldavite specimen weight/dimensions - length x width x height | |||
| Sand (kg) | Gravel (kg) | ||||||||
| up to 3 cm | from 3 to 8 cm | more than 8 cm | |||||||
| Cycle no. 1 | Gozdnica and Lasów pits -Lusatian Neisse | Late Miocene / Pleistocene | 3.8 | 2.1 - 42% | 1.0 - 20% | 1.4 - 28% | 0.5 - 10% | every 30 min (~1.9 km of transport) | 1.372 g/20x17x15 mm |
| Cycle no. 2 | Bielany pit -Cicha Woda | Late Miocene | 2.2 | 1.2 - 24% | 1.3 - 26% | 1.6 - 32% | 0.9 - 18% | every 50 min (~1.8 km of transport) | 1.615 g/32x16x14 mm |
| Cycle no. 3 | Nowa Wieś Kącka, Mielęcin and North Stanisław pits - Bystrzyca | Late Miocene | 2.2 | 0.3 - 6% | 1.7 - 34% | 1.9 - 38% | 1.1 - 22% | every 50 min (~1.8 km of transport) | 1.493 g/25x19x15 mm |
From the high compositional similarity of both the Late Miocene deposits from the Gozdnica pit and the Pleistocene sediments from the Lasów pit, and from their location within the alluvial accumulation area of the Lusatian Neisse, they were included in a single cycle (cycle no. 1). In cycle no. 2 was a deposit sample from the Bielany pit that is located next to the River Cicha Woda. As for cycle no. 3,the Nowa Wieś Kącka, North Stanisław and Mielęcin pits were lumped together due to the same sediment and their location near the Bystrzyca river. The same situation was in the cycle no. 1 for Gozdnica and Lasów pits.
In this study, the yearly average value of river flow was chosen independently for river in the moldavite distribution area (Haładyj-Waszak, 1975, 1978, 1980). The rotation speed of the barrel was then adjusted accordingly in each cycle (Table 1).
Keeping in mind the lower tumbling speeds used formerly (Brachaniec, 2018), during these new cycles tumbling was stopped after ca. 2 km of transport. This corresponds to different time intervals pending on the river-flow velocity (Table 1). Once removed from the barrel, tektites were then sieved by hand on 5 mm mesh, and their state of preservation and dimensions being recorded. After each tumbling step, they were put back in the barrel for the next tumbling step.
For reliable comparisons (results of each cycle) moldavites of similar weight were chosen. Their dimensions are shown in Tables 1-4.
During this cycle six abrasion
stages were observed (Fig. 2.A , see also
Table 2). Originally, the moldavite
specimen weighed 1.372 g and its dimensions were 20x17x15 mm. After 30 minutes
of tumbling (ca. 1.9 km of transport) this tektite lost about 44% of its
preliminary weight. It was sub-angular with low-sphericity in shape. The glass
surface became smoother and matte.The glass edges
blurred and became noticeably blunt. In the next stage (ca. 3.8 km of
transport) the moldavite became more rounded with an additional weight loss
of about 23%. The glass surface became matt with abrasion signs. It was
sub-angular with a high-sphericity shape. Observation
after a distance of 5.7 km recorded a further weight loss of ca 15%. From this
stage, the moldavite became sub-rounded with high sphericity. During the last
two stages the weight loss reached 10 and 7% respectively. The glass shape
remains generally unaffected. It was a characteristic small, rounded with
high-sphericity, grain with smooth surface. Between 9.5 and 11.4 km of transport,
the moldavite was totally destroyed.
During this cycle four abrasion
stages were observed (Fig. 2.B , see also
Table 3). Originally, the moldavite
weighed 1.615 g and its dimensions were 32x16x14 mm. After 1.8 km of transport,
a weight loss of about 57% was recorded. The shape of the tektite became
sub-rounded with low sphericity. Subsequent observations documented a reduced
weight loss of moldavite. After 3.6 km oftransport, the tektite became rounded
with low-sphericity, with surface abrasion signs. During the last step observation
(after 5.4 km of transport) moldavite was well-rounded with low-sphericity. It
clearly showed surface abrasion signs. Between
this distance and 7.2 km, the remaining moldavite speck was completely
destroyed.
During this cycle only one
abrasion stage was characterized (Fig. 2.C , see also
Table 4). Originally
moldavite weighted 1.493 g and its dimensions were 25x19x15 mm. After 1.8 km of
transport, the moldavite lost almost 76% of its weight, and became smoother and
rounded with low-sphericity shape. Glass was completely destroyed between 1.8 to
3.6 km of reworking.
|
Figure
2:
Diagram showing the
respective stages of moldavites abrasion during three cycles (see also Fig. 3 |
Table 2: Progressive steps of moldavite abrasion during cycle no. 1. Comments and explanations in text (see also
Figs. 2
- 3 ).
| Distance (km) | Weight (g) | Dimensions - length x width x height (mm) | Weight loss (g) | Weight loss from initial weight (%) |
| Primary moldavite | 1.372 | 20x17x15 | 0.000 | 0.000 |
| 0-1.9 | 0.764 | 12x10x8 | 0.608 | 44.376 |
| 1.9-3.8 | 0.584 | 10x9x7 | 0.180 | 23.622 |
| 3.8-5.7 | 0.495 | 9x9x6 | 0.089 | 15.394 |
| 5.7-7.6 | 0.445 | 7x7x5 | 0.050 | 10.118 |
| 7.6-9.5 | 0.413 | 6x6x5 | 0.032 | 7.377 |
Table 3: Progressive steps of moldavite abrasion during cycle no. 2. Comments and explanations in text (see also
Figs. 2
- 3 ).
| Distance (km) | Weight (g) | Dimensions - length x width x height (mm) | Weight loss (g) | Weight loss from initial weight (%) |
| Primary moldavite | 1.615 | 32x16x14 | 0.000 | 0.000 |
| 0-1.8 | 0.689 | 15x8x8 | 0.926 | 57.347 |
| 1.8-3.6 | 0.52 | 10x7x6 | 0.169 | 24.566 |
| 3.6-5.4 | 0.426 | 7x5x5 | 0.094 | 18.241 |
Table 4: Progressive steps of moldavite abrasion during
cycle no. 3. Comments and explanations in text (see also Figs.
2
- 3 ).
| Distance (km) | Weight (g) | Dimensions - length x width x height (mm) | Weight loss (g) | Weight loss from initial weight (%) |
| Primary moldavite | 1.493 | 25x19x15 | 0.000 | 0.000 |
| 0-1.8 | 0.362 | 6x5x4 | 1.131 | 75.783 |
Initial results of experimental reworking of moldavites showed that their abrasion progresses very quickly (mainly during the early phase of cycles) and their transport is only possible on relatively short distances (Brachaniec, 2018). For the purpose of experiment tumbling cycles carried out by Brachaniec (2018) are provided as "cycles no. 0" (see also Table 5) since they constitute preliminary results. Undoubtedly, results of cycles no. 0 evidence that the initial size and shape of moldavites are influential on the total reworking distance, despite the same sedimentary type and river velocity used in the cycles. Nevertheless, these new results provide more accurate data on the suspected original hydrogeological conditions. Based on the results of the three cycles, four main steps of fluvial abrasion of moldavites are distinguished:
From these results it was noticed that both the type of sediment and
the river velocity are equally important to state the rate of fluvial abrasion (Table
5). In the case of the Lusatian Neisse (cycle no. 1), moldavite survived longer reworking, in contrast to cycles 2 and 3,
with river velocity lower by 42% and a higher proportion of gravel fraction. In
addition, the same applies in cycles 2 and 3, with similar river velocity but a
different amount of gravel (in cycle 3 there was a higher proportion of gravel).
However these values remain approximate but are nonetheless highly indicative.
Comparing accurately such results between all cycles requires having available
three identical moldavites, of similar shape and weight. In the results of cycle
no. 1, the sedimentary type and initial size of moldavite were very similar to
those of cycles no. 0 (Table 5). Nevertheless, due to the much lower speed of tumbling (by
about 65%) in the current study, 6 stages were recorded and moldavite
withstood up to 11.4 km of transport, more than the 7.2 km of cycles no. 0, all this
indicates the importance of the speed of tumbling. It is
also worth noticing that the currently used tektite had a lower initial weight
of 0.3 g. Interestingly, the results obtained by Brachaniec
(2018) and
these from cycle no. 2 are similar despite the difference in the river velocity
and the sedimentary type. The weight of the tektites in this case was
similar, so it does not have an influence on the final results. In the case of
cycle no. 2 the same
reworking distance is probably caused by a combination of a lower river velocity
and a higher gravel amount. The gravel amount plays a major influence on the
glass reworking distance, as shown in cycle no. 3. The
significant increase of gravel fraction in the host sediments highly contributed
to the rapid destruction of the tektite. Of
special interest also is the percentage of weight loss during stages no. 1. The first episode of reworking seems to be the most
important, because of the highest weight loss, and a clear change in the tektite
shape. In cycle no. 1,
the weight loss was approx. 44%. In the previous results from cycles no. 0, this value even reached 64%. This difference is
mainly due to the tumbling speed, since the deposits at hand were practically
identical. So a huge weight loss is induced by the primal, irregular glass shape
making it much more vulnerable
to abrasion in contrast to regular shapes. The
percentage rate of weight loss decreases stepwise with the increased rounding of
the glass (Fig. 3 ). In stages 2 of cycles 0 and 1 the weight loss is noticeably
different (ca. 5%). However, it most likely results from quite significant
differences in the initial shape of tektite. In stage 1 of cycle no. 2, the weight loss is 57%. This value is comparable to those of cycles no. 0. The same rate of
fluvial abrasion contributes evenly to the same reworking distance (7.2 km).
When the tektite gets fully rounded, the rate of erosion is subsequently
strongly reduced. It
is interesting to note that moldavite was completely destroyed after stage 3 in
cycle no. 2, when the mass loss decreases
with the transport distance (Fig. 3 ). More stages should havebeen expected, as
in the case of cycle no. 1, in which the
weight decrease is slighter. Nevertheless, the lack of stage 4 in cycle no. 2 indicates an uneven rate of fluvial abrasion, probably due to the
destruction of large gravel.
Table 5: Reference summary showing the distance of moldavite reworking depending on the hydrogeological conditions adopted in experimental tumbling.
| River velocity (km/h) | Deposits - sand/gravel (kg) | Moldavite weight (g) | Total distance of transport (km) | |
| Brachaniec (2018) (cycles no. 0) | 10.8 | 2/3 | 1.642 | to 7.2 |
| 10.8 | 2/3 | 1.497 | to 5.4 | |
| this study (cycle no. 1) | 3.8 | 2.1/2.9 | 1.372 | to 11.4 |
| this study (cycle no. 2) | 2.2 | 1.2/3.8 | 1.615 | to 7.2 |
| this study (cycle no. 3) | 2.2 | 0.4/4.6 | 1.493 | to 3.6 |
|
Figure
3:
Diagram showing the relationship between weight loss (%) and reworking
distance of tumbled moldavites during each cycle and stage. General type of
glass shape is also shown. m.d. - moldavite destruction. See also Fig.
2 |
The present results
clearly indicate that the most favourable environments delivering redeposited
moldavites are rivers closer to the German border, in sediments with a
significantly larger quantity of sand. Nevertheless it should be noted that the
experimental conditions only theoretically reflect genuine environmental
conditions. The velocity of rivers depends on the area they are running through,
the climate and the geomorphology of the ground surface. It should also be kept
in mind that the velocity of rivers varies along its many sections. Numerous
bends and river meanders as low-energy environments could contribute to the
settlement and to the good preservation of the tektite glass. Unfortunately,
such conditions cannot be faithfully reproduced in laboratory conditions. This
makes the term 'supply areas' of Polish tektites debatable. According to Szopa
et al. (2017) and Brachaniec
(2018) moldavites from the Gozdnica
and Lasów pits could successfully resist several tens of kilometers of transport
(assuming their large initial size) and may most likely come from the Zittau
area (Fig. 1 ). Results of presented cycles tend to support such an assertion.
Tektites found in the Lusatian Neisse sediments could have theoretically
withstood longer reworking than those from the Strzegom region, due to the
higher sand fraction in the river sediments. A large amount of gravel in the
river deposits undoubtedly limited the distance of tektite reworking in pits
located between Wrocław and Strzegom (Fig. 1 ). Even assuming their large
initial size, it seems impossible they could survive more than a few dozen
kilometers of transport. As Brachaniec (2018) already suggested, they may
most probably originate from the Strzegom Hills area. Despite the large amount
of gravel in these deposits, this region, according to Grocholski
(1977)
and Kural (1979), was likely facilitating longer reworking of tektites
due to the numerous depressions. It should also be kept in mind that
the intense destruction of tektites in the Strzegom region is supported by the
fact that the sharp-edged tektites found in the North Stanisław pit are
most likely shards, separated from the main glass mass. According to Trnka
and Houzar (2002), fragmentation of tektite glass is a final result of
abrasion.
The cardinal question is the origin of the moldavites within the identified potential supply areas. The recent find of an entire moldavite in the North Stanisław pit within Middle Miocene mud (Brachaniec, nearing completion) most likely indicates that these tektites may have been ejected at a distance over 500 km from the Ries structure, as the numerical simulations of Stöffler et al. (2002) and Artemieva et al. (2013) have shown.
The present study reveals how difficult it is to determine the relationship between the weight loss of tektite glass, the type of sediment and the river velocity. Nevertheless both of these factors seem to play an important role in moldavite fluvial abrasion. However the experimental tumbling results at hand should be treated theoretically. Actually the moldavite transport does not only depend on the river velocity and the type of sediment, but also on many environmental factors that can not be reproduced in laboratory conditions. Various shapes of glass and mass, variable environmental energy over different reworking distances, and local sedimentary changes do contribute to the respective differential stages of fluvial abrasion, and to the duration of resistance to its reworking. Undoubtedly, given the experimental results, must come to the conclusion that abrasion of impact glass happens quickly, and that the tektites could not be transported over considerable distances. Theoretically, assuming favourable environmental conditions (mainly high amount of sand in deposits) and a large initial size, moldavite could endure tens of kilometers of reworking transport, making identification of their supply areas highly speculative.
The author would like to thank two anonymous reviewers who provided valuable advice and constructive comments on the manuscript as well as to Bruno Granier for careful editorial handling. The photographic assistance of Wojciech Krawczyński is gratefully acknowledged. The author also wishes to thank Mariusz Salamon for providing the tumbling barrel, Janusz Badura and Joanna Czekaj for consultation about hydrogeological issues and Bruno Ferré for ironing up the English version.
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