Depositional environment of the Owadow-Brzezinki conservation Lagerstatte (uppermost Jurassic, central Poland): evidence from microfacies analysis, microfossils and geochemical proxies

摘要

The Owadow-Brzezinki quarry is one of the most important paleontological sites in Poland, known for its exceptionally well-preserved Upper Jurassic (Middle Volgian = uppermost Lower Tithonian) fossils. Carbonate deposits of the section record a transition from an offshore to coastal and lagoonal settings and have been studied based on microfacies, micropalaeontological, isotope and chemical proxies. The obtained data point to normal marine conditions during the deposition of the older part of the quarry section in an offshore setting and a gradual transition into lagoonal environment characterized by high-amplitude variations in seawater salinity and oxygenation level of bottom waters, both of which resulted in considerable changes in benthic fauna assemblages or in the total lack of the fauna at some intervals. Above-mentioned conditions during the deposition of the middle part of the quarry section have probably allowed the preservation of diversified fauna with soft tissues. The microfacies and chemical data indicate that dysoxic/anoxic episodes may have occurred not only during the deposition of known fauna-rich beds but also during the deposition of poorly studied, so far, younger part of the section. The uppermost part of the carbonates exposed in the Owadow-Brzezinki quarry originated during the re-appearance of normal marine chemistry mostly in the intertidal-subtidal settings. The depositional conditions of the Owadow-Brzezinki site are nontypical, among famous conservation Lagerstatten, owing to the rapid fluctuations in the oxygenation and salinity of bottom waters.