Most ancient Podolia’s algae as trigger for the formation of phosphorites

Abstract

The phosphorites are rather widely represented in platform carbonate and quartz-glauconite litho-tectonic complexes. They are also present in Kalyus Beds of Nagoryany Formation in Podolian Middle Dnister area.These beds are folded by homogeneous, dark-grey to black, thin-bedded mudstones. Characteristic feature of the beds is the occurrence of phosphorite concretions of different sizes. Dispersed phosphate mineralization (4–30 %) is also present in mudstones. Kalyus Beds have two levels enriched by remains of Vendotaenian algae. Lower level is located in the bottom surface and upper level is located near top surface. It is the most ancient and numerous imprints of algae in rocks of Upper Vendian. According to the general appearance of thalli, the nature of sporangia and the type of metabolism, they are referred to as brown algae, which were adopting a benthic lifestyle. And they include an assemblage of microphytofossils too. The black color, the presence of globular pyrite, the bitumen interlayers and the value of the protoxid module – 1,32–1,83 indicate the conditions for sedimentation recovery. Algal textures say that sedimentation occurred in the euphotic zone, that is, at the depth of light penetration. This is confirmed by the ecology of modern brown algae, which live from the low-water line to a depth of 20–30 m.In the Late Vendian, there was no terrestrial vegetation, therefore sloping substrates eroded much faster and the land was a vast plain almost at the level of the water’s edge. The coastline (in the modern sense) didn’t exist, it constantly migrated and this led to the fact that in the “coastal zone” formed numerous gulfs, overgrown with algae. The main source of phosphorus was the weathering crust of the subsilicic tuffogenic rocks with a high content of Р2О5. Then, phosphoric compounds fell into the sedimentation basin and assimilated by cyanobacteria and algae in the form of polyphosphoric acids. Cyanobacterial communities had multidirectional vectors of their life activity, so ones created various biochemical barriers. Acidic medium was for dissolving apatite and/or francolite, and then alkaline one, which was necessary for phosphorus precipitation. In addition, phosphorus could be precipitated as a result of seasonal fluctuations in temperature, which led to departure of the chemical equilibrium. Also algal films and microbiofilm could hold phosphorus-containing pelitic particles on their surfaces.Land vegetation cover absent, therefore the coastline constantly migrated so the primary structure of algal mats and biofilms were periodically disturbed. They were rolling, sticking, etc. took place. As a result, thrombolytics (nonlayered clot structures) were formed. Further lithification takes place under reducing conditions at the bottom and at the top of the sediment. If the concentration of phosphorus is high in sludge waters, phosphorite concretions can form from thrombolites then.