GEOLINKS International Conference 2019, Book 1
PALEONTOLOGICAL SUBSTRATE OF BEEKITE RINGS (ON THE EXAMPLE OF THE COLLECTIONS OF THE PHANEROZOIC INVERTEBRATES IN THE MINING UNIVERSITY, ST. PETERSBURG)
Assoc. Prof. Maria Tsinkoburova, Viktor Slastnikov, Timur Khalimov
Nowadays there is plenty of published mineralogical and geochemical literature, in which scientists are trying to explain the mechanism of the appearance of such a peculiar form of chalcedony as Beekite rings (special forms of silica, resulted by the secondary silicification of many fossils). From the paleontological point of view, Beekite occurs on various Phanerozoic invertebrates, mainly on Paleozoic brachiopods with a calcite skeleton. At the same time, it was repeatedly noticed that the partial silicification, typical in the formation of Beekite, is due to both environmental and biological factors. The authors discovered samples of the brachiopod of the suborder Syntrophiidina (genus Porambonites Pander, the Ordovician sediments of Baltoscandia) and some samples of the bivalves of the order of Dysodonta (specie Pycnodonte simile (Pusch), the Danian sediments of the Crimean Mountains), containing Beekite rings, in the collections of the Mining University. The representatives of the genus Porambonites Pander are characterized by their thin sculpture and numerous pits located between the costae. The collection shows different stages of the development of Beekite rings on brachiopod shells. The initial stage is characterized by the appearance of small rings (0,5 mm) on the shell's periphery; the ephebic stage is distinguished by the development of Beekite all over the shell. The appearance of Beekite rings also varies from poorly developed, almost unexpressed on the relief of the shell to embossed rings with the siliceous crust and bubbly formations with a very small diameter (0.1 mm). Specie Pycnodonte simile is characterized by thin concentric growth lines, special closely spaced near the external part of the left valve. Distribution of the initial centers of crystallization of aggregates was being controlled by concentric sculpture, clearly expressed in shell's micro-relief, and by features of valves microstructure. Taphonomic features of this species, such as the burial of separated valves contributing to the development of aggregates on both internal and external valves, favored the active siliconization. According to the previous investigations,the appearance of the rings and the intensity of their development depends on the amount of silica in solutions at this place and the character of the dissolution of carbonate minerals. By the authors’ opinion uneven character of the distribution of Beekite rings on surface of shells of invertebrate is due to the features of the shells, i.s. more peculiar for specific minimal taxa. Flat concentric-zonal aggregates
indicate the growth of Beekite rings in conditions of the sufficiently limited space for the crystallization of silica. These conditions were possible both in the still non-litification sediment at the early stage of fossilization, and in the conditions of hypergenesis. Volcanic activity, bentonite interlayers, siliceous organisms were considered as a possible source of ones according to the previous investigations. The last two factors can be considered as presumably initiating the formation of the described structures in the situation with the Ordovician deposits of Baltoskandia. This is confirmed by both the appearance of multiple bentonite interlayers in the Ordovician deposits of the east of Baltoskandia and the formation of so-called sponge horizons. For the formation of Beekite rings on specimens of Bivalves from the Paleocene sediments of the Crimean Mountains, the biogenic factor was probably decisive.
Baltoscandia, Ordovician sediments, the Crimean Mountains, Paleocene sediments, Beekite rings, silicification of fossils