Terrigenous organic matter and formation of siderite in the Bathonian Ore-Bearing Clay Formation at Gnaszyn, Poland – a petrochemical study

The Bathonian Ore-Bearing Clay Formation, outcropping in the Gnaszyn open-pit mine at Częstochowa (Poland), includes several horizons of abundant iron carbonate concretions . cores concretionary bodies commonly contain organic matter (OM), dominated by fragments wood. These particles usually display well-preserved primary structures and occur rarely as more deformed and/or completely degraded fragments. Their original are frequently replaced or filled with secondary mineralization, mostly represented pyrite. maceral composition OM wood is huminite subordinate inertinite resinite. Vitrinite reflectance analyses revealed values lower than 0.45%. total carbon content (TOC) displayed variable results between 2% 18%. Rock-Eval low amounts hydrogen (< 45 mg HC/g TOC) relatively high oxygen (up to 136 CO 2 /g TOC). Analysed samples contained small quantities free hydrocarbons (S1 peak < 0.26 rock) well hydrocarbons, generated during pyrolysis (S2 7.05 rock). features characteristic for immature type IV kerogen terrigenous origin. However, frequent occurrence siderite affecting parameters may indicate that belonged III. According previous authors, origin was delivered well-oxygenated water palaeo-basin area. present data intensive biodegradation this shallow burial depleted supply within sediment, driving pore into dys- anoxic conditions. activity microorganisms reducing sulphates became dominant reaction, introducing Fe 2+ HCO 3 - ions system. Negative δ 13 C cortex analysed bicarbonate consumed precipitation supplied microbial activity. microenvironments developed sediment acted nucleation sites precipitation.