Sedimentological, geochemical and magnetic susceptibility studies of the Aalenian-Lower Bajocian (Middle Jurassic) black shales from the central part of Polish Basin – preliminary report

Sedimentation of the Middle Jurassic deposits in the Polish Basin took place in a shallow storm-dominated epicontinental sea. Three complexes of the offshore fine-grained deposits separated by transition - middle shoreface... [ view full abstract ]

Sedimentation of the Middle Jurassic deposits in the Polish Basin took place in a shallow storm-dominated epicontinental sea. Three complexes of the offshore fine-grained deposits separated by transition - middle shoreface heteroliths and sandstones are observed in the succesion. The oldest complex, Upper Aalenian - Lower Bajocian in age, is about 150-300 m thick. It is located at a depth of less than 1200 m and can be studied only by analyzing the cores material. The main objective of the study is to analyze the relationship between the sedimentological features, amount of organic matter, the characteristics of the major and trace elements occurrence and magnetic susceptibility in order to examine the possibility of more precise recognition of this fine-grained complex and it’s inside correlation between boreholes.
Sedimentological studies of the Aalenian – Lower Bajocian complex show that there are mostly black colored noncalcareous shales, with no sedimentary structures, sometimes with pyrite, marly-sideritic or sideritic concretions and almost totally lack of bioturbations. Less frequently muddy claystones and mudstones with lenticular lamination or bedding are observed. In this part of profile rare Planolites, Schaubcylindrichnus and Chondrites trace fossils are present. Benthic macrofauna is represented by single bivalves usually in form of cast and benthonic foraminifera. Previous studies show that this complex is typified by the predomination of benthic agglutinated foraminifera (75%) over calcareous ones (25%). Agglutinated foraminifera are dominated by epifaunal morphogroups. Few geochemical analysis point that the TOC content ranges from 0.9 to 12.7% and it is mostly terrestrial organic matter (vitrinite) of low maturity.
For the purposes of the present study geochemical analysis of organic matter, major and trace elements was performed in conjunction with studies of magnetic susceptibility.To identify the type and maturity of organic matter and to detect petroleum potential in investigated sediments Rock Eval TOC pyrolysis analyses of 28 samples was done. Geochemical analysis of main and trace elements for 6 samples of these which showed the lowest and highest values of TOC was done. For most elements XRF analyses was performed. For V, Ni, Cu, Mo, U, Th and REE elements ICP-MS method was used. Continuous profile of magnetic susceptibility for 145 m succession of shales was made.