Variations in fluvial reworking of Polish moldavites induced by hydrogeological change

Abstract

V Va ar ri ia at ti io on ns s i in n f fl lu uv vi ia al l r re ew wo or rk ki in ng g o of f P Po ol li is sh h m mo ol ld da av vi it te es s i in nd du uc ce ed d b by y h hy yd dr ro og ge eo ol lo og gi ic ca al l c ch ha an ng ge e Tomasz BRACHANIEC 1 Abstract: 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.Résumé : 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.

Modavite finds in Middle Miocene (Langhian Stage) deposits of southwestern Poland

“…510 km from the Ries crater (BRACHANIEC, 2017), but fluvial transport made it challenging to reliably estimate the original ex-tent of the dispersal area of all redeposited tektites. Generally, tektites could survive hundreds of km by fluvial transport (HURTIG, 2019), not as previously thought, a dozen or so at most (VAM-BEROVÁ and ŠEVČÍK, 1990;BRACHANIEC, 2018aBRACHANIEC, , 2018bBRACHANIEC, , 2019aBRACHANIEC, , 2019b. The discovery of moldavites in the Middle Miocene deposits in SW Poland is consistent with modeling (STÖFFLER et al, 2002;ARTEMIEVA et al, 2013).…”

A scheme for moldavite fluvial abrasion based on observations from a natural river stream

“…From this point (1.5 km) to point B, the river runs along with gentle arches and straight sections (from 1.5 to 6 km). The average velocity of the Pilica River was measured at 2.2 km h −1 , which corresponds to the average velocities of moldavite‐bearing rivers from SW Poland (Brachaniec 2018b). The average width of the Pilica River in the study stretch was 5 m. Its average depth was 0.7 m in straight sections and 1 m in arches.…”

“…1). Initial results of experimental barrel observations (Brachaniec 2018a(Brachaniec , 2018b(Brachaniec , 2019a(Brachaniec , 2019b and compiled literature data ( Zebera 1972;Bou ska et al 1999) indicate that moldavites display low resistance to water abrasion, and subsequently can only withstand reworking over a short distance (up to a dozen km). However, such a process is so poorly known and so complex that such an issue requires baseline analyses and detailed research aimed at re-creating the process of water transport of impact glass.…”

“…1). Initial results of experimental barrel observations (Brachaniec 2018a, 2018b, 2019a, 2019b) and compiled literature data (Žebera 1972; Bouška et al. 1999) indicate that moldavites display low resistance to water abrasion, and subsequently can only withstand reworking over a short distance (up to a dozen km).…”

“…Schematic map of the river systems flowing through SW Poland with respective location of moldavite‐bearing pits (modified after Brachaniec 2018b). (Color figure can be viewed at wileyonlinelibrary.com)…”