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Susceptibility of various tektite types to fluvial abrasion

Brachaniec Tomasz

Geological Quarterly

2019

Vol. 63, No. 1

150--161

Tektites are glass bodies, rich in silica, resulting from the impact of a large bolide into ground rocks. Similar to other impactites they are prone to erosive processes, including fluvial abrasion. This study reports the results of an experimental tumbling that aimed at estimating the potential distance that tektites from different strewn fields (moldavites, bediasites and indochinites) and Libyan Desert Glass (LDG) can withstand depending on the experimental conditions. The present study consisted of 15 cycles, in which the type of sample deposits (i.e. sand/gravel ratio) and computed transport velocity were changed, the latter being estimated at 2.5-6.5 km/h. The results clearly confirm the susceptibility of tektites to abrasion during tumbling. None of the tektites withstood the estimated distance of 12 km during the experiment, but this may have been the result of the relatively small initial size of the glasses (~1.5 g). These experiments document that LDG, despite its even smaller initial size in the experiments, can resist abrasion and fragmentation better than the tektites, thus, could probably be transported farther in a stream environment. This is most likely caused by a much higher silica content in relation to the tektites from other groups. The estimated maximum transport distances, over which moldavites, bediasites and indochinites survived in the experiments, are all very similar. The greatest weight loss for all the specimens was found after the first estimated 2 km of tumbling. This is undoubtedly caused by the irregular initial shape of the tektites and LDG. Subsequent observations recorded minor weight losses, in association with more and more rounded glass shapes. The results of the study should be treated only as a general scheme for the fluvial abrasion of tektites, due to the inability to accurately reproduce the natural fluvial environment.

New moldavites from SW Poland

Brachaniec T., Szopa K., Karwowski Ł.

Acta Geologica Polonica

2016

Vol. 66, no. 1

99--105

Four newly discovered moldavites from the East and West Gozdnica pits, SW Poland, are characterized. All specimens, including other four, reported earlier, are from Upper Miocene fluvial sediments of the Gozdnica Formation. Their weight varies between 0.529 and 1.196 g. The moldavites are bottle green in colour and have bubbles and inclusions of lechatelierite. Low degree of corrosion suggests short river transport, apparently eastward from Lusatia.