G.S. ISKYUL (VSEGEI)

Phosphatized discontinuity surfaces in the limestone of the Kunda Regional Stage (Middle Ordovician) in northwest Russia

Phosphatized discontinuity surfaces are widely distributed in Lower – Middle Ordovician condensed carbonate sections of Baltoscandia. However, it is difficult to diagnose them and therefore they are poorly studied as compared to other impregnated surfaces. Morphological typification and genetic interpretation of the phosphatized discontinuities are given using material from the Endoceras limestone of the Kunda Regional Stage. The distribution of the discontinuity surfaces on the paleofacies profile is shown. The discontinuity surfaces with fine, usually eroded impregnation and phosphate-filled shells which are relics of full-eroded surfaces are common in the most shallow-water and high-energy lithofacies (bioclastic grain- and packstones, proximal wackestones). In the deeper-water wackestones, the discontinuity surfaces with deeper phosphate impregnation, varying from hardground (strongly phosphatized, not bioturbated) to softground (weakly phosphatized, bioturbated) are common.
Keywords: Ordovician, Kunda Regional Stage, discontinuity surfaces, bioturbation, hardground, softground.