Konzentrat-Lagerstätte-type carbonate concretions from the uppermost Bajocian (Middle Jurassic) of the Częstochowa area, South-Cental Poland

Early-diagenetic calcitic concretions of latest Bajocian (parkinsoni Zone, bomfordi Subzone) age in the Polish Jura contain an extremely rich and well-preserved macrofauna. In addition to bivalves and brachiopods, there are numerous ammonites of the (sub)genera Parkinsonia, Nannolytoceras, Lissoceras (Lissoceras), L. (Microlissoceras), as well as rare representatives of Vermisphinctes, Strigoceras (Strigoceras) and Phylloceras. Carapaces of primitive prosopid crabs are found associated. Such a diverse and well-preserved fauna (previously unknown from Bajocian epicratonic deposits of the Polish Jura), is typical of Konzentrat-Lagerstätte-type deposits. The random distribution of the concretions in exposure and the small size of the fauna may suggest that all remains were concentrated by bottom-current action in small depressions on an uneven sea-floor. The presence of numerous small-sized ammonites may be regarded as reflecting post-spawning assemblages which sufferedmass-mortality. Although cause of their death remains unclear, storm events are a possibility. Such depressions acted as natural “traps” as well as constituted a size-limiting factor for the accumulating fauna. The fact that all elements are well preserved indicates brief post-mortem transport and rapid burial, followed by early diagenesis that protected them from compaction. The absence of such fossil-rich concretions higher in the Middle Jurassic sequence clearly shows that our current assessment of biotic diversity may be an underestimate. The carbonate concretions contain a predominantly terrestrial type of organic matter (OM). Samples analysed are extremely immature, but early diagenetic transformations of OM occurred prior to concretion formation.