Ammonite fauna from uppermost Bajocian (Middle Jurassic) calcitic concretions from the Polish Jura—biogeographical and taphonomical implications

Abstract

Uppermost Bajocian (Parkinsoni Zone, Bomfordi Subzone) calcitic concretions from the epicratonic basin in South-Central Poland (Polish Jura) contain numerous, well-preserved ammonites unique for this area. Characteristic of the assemblages is the presence of small, juvenile specimens. Associated with dominant parkinsoniids ( Parkinsonia ( Parkinsonia) aff. dorni Arkell) are lytoceratids [ Nannolytoceras tripartitum (Raspail)], lissoceratids [( Lissoceras ( Lissoceras) oolithicum (d'Orbigny) and Lissoceras ( Microlissoceras) solitarium Zatoń and Marynowski nov. sp.)], leptosphinctids ( Vermisphinctes sp.) and strigoceratids [( Strigoceras ( Strigoceras) sp. juv. gr. strigifer/ pseudostrigifer)], as well as fragments of phylloceratids ( Phylloceras sp.). Parkinsoniids (the most numerous) are considered here as host fauna. Other ammonites, especially phylloceratids and nannolytoceratids, are supposed migrants from the Mediterranean area into the shallow epicratonic Polish Basin during the Late Bajocian sea-level rise. N. tripartitum, which has a broad geographical range probably drifted as a plankton-feeder in the upper water column and, together with phylloceratids, are considered to be unsuccessful immigrants which did not adapt to the new environment. It seems that the whole fauna, inclusive of plant remains, was accumulated by bottom currents in randomly situated depressions on the sea-floor, where carcasses became ideal nucleation sites for concretion genesis. Early diagenesis, which played an important role, resulted in well-preserved ammonite assemblages close to the original biological assemblage. Most of described ammonites (nannolytoceratids, lissoceratids, strigoceratids and leptosphinctids) are reported from the Upper Bajocian epicratonic deposits of Poland for the first time. This makes the concretions a true “taphonomic window” into latest Bajocian census biocenosis. The organic geochemistry results suggest predominance of terrestrial organic matter in both concretions and surrounding clays, with well oxygenated conditions in the sedimentary basin. The large concentrations of Δ 13(17)diasterenes in the analysed carbonate concretions indicates that these compounds must have been formed by contact with clay minerals before formation of the carbonate concretions. The processes described may have followed early stage of diagenesis, but certainly under tens of cm to even a few meters of burial. The high negative δC 13 values in the inner zone of the concretions suggest that concretion growth began in the sulphate reduction zone and may have continued in the methanogenesis zone.