Ammonite stratigraphy and organic matter of the Pałuki Fm. (Upper Kimmeridgian–Lower Tithonian) from the central-eastern part of the Łódź Synclinorium (Central Poland)

Abstract

The ammonite biostratigraphy as well as the organic matter content, its type and maturity of the Paluki Formation, belonging to the fine, siliciclastic Kimmeridge Clay type facies, were investigated in five deep boreholes from the central-eastern part of the Łodź Synclinorium in Central Poland. The studied deposits are assigned to the Eudoxus and Autissiodorensis zones of the Upper Kimmeridgian as well as the Klimovi, Sokolovi, Pseudoscythica and Puschi (=Tenuicostata) zones of the Lower Tithonian (“Lower Volgian”). The Paluki Formation shows in its lower and middle parts average TOC concentrations of ca. 2.5 wt.% and prominent, restricted increases in organic matter content, which are found in the mid-Eudoxus Zone, the lowermost part of the Autisiodorensis Zone, and at the Sokolovi–Pseudoscythica zone boundary. These stratigraphical intervals correlate well with rich in organic matter levels present in the Kimmeridge Clay Formation of NW Europe. The periodic expansions of Submediterranean and Subboreal-Boreal ammonites corresponded mostly to the transgressive phases, often correlated with a higher content of organic matter. The development of special morphologies of ammonites, such as the small-sized, nectopelagic forms of Nannocardioceras in the Late Kimmeridgian, has also been related to the deposition of shales rich in organic matter during the transgression maxima. The organic matter present in the Paluki Formation mostly consists of Type II kerogen and is immature or early mature with respect to hydrocarbon generation, which is in agreement with previously published data. Evaluation of the new and published geochemical, lithological and structural data from the Paluki Formation in the central-eastern part of the Łodź Synclinorium shows that these deposits could not have been a considerable source of crude oil or gas.