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Content available remote Konkrecje hiatusowe z iłów rudonśnych Wyżyny Krakowsko-Częstochowskiej
Hiatus concretions are arly-diagenetic sedimentary bodies, which in their history of formation, underwent exhumation during sedimentation break and/or erosion of sea-floor. Then they were colonized by various encrusters and borers, before they were buried again. Within Middle Jurassic (Bajocian through Bathonian) clays, hiatus concretions occur in four localities in the Cracow-Częstochowa Upland (south and central Poland): Ogrodzieniec, Bugaj, Mokrsko and Krzyworzeka. Preliminary data about their mineralogy, organic geochemistry and palaeoecology is given. Mineralogical analyses showed, that except dominant high Mg-calcite (up to 87.10% of total carbonates), concretions possess minor amounts of such compounds as quartz, clay minerals (kaolinite and illite), pyrite, siderite or Mg-kutnahorite. However, high differences are observed between total organic carbon contents of concretions (TOC=0.46%) and surrounded clays (TOC = 2.16%), but their molecular composition is similar. Most of identified biomarkers are of terrestrial origin. In clay lithology, the hiatus concretions are the only sedimentary bodies that clearly mark the sedimentation pauses (hiatuses). In the investigated area, they form more or less continuous horizons. The concretions are calcitic, possessing various encrusting fauna such as bryozoans, oysters and oyster-like bivalves, serpulids, solitary corals and foraminifers. They often post-date the borings, belonging to such ichnogenera as Gastrochaenolites, Trypanites and Entobia. Some of the nodules show distinct transition from firmground (characterized by the presence of Glossifungites ichnocoenosis) to hardground (presence of Trypanites ichnocoenosis), pointing to the fact, that they formed at, or very close to, the sediment-water interface. Some concretions, like those from Bugaj and Mokrsko, are characterized by their high diversity of hard bottom communities. Those concretions are also irregular in shape; others, like those from Ogrodzieniec and Krzyworzeka, possess lower diversity. The concretions from Krzyworzeka are especially dominated by the borers, while the encrusters are sparse and not diverse. Those nodules that are wide and flat tend to be more bored on one surface only, while those that are more roundish, are bored on both sides more evenly. The degree of diversity is probably correlated with physical disturbance, causing the rolling and overturning of the exhumed concretions in the littoral zone. On the other hand, the overall diversity could be lowered due to destructive abrasion either of the shallowest borings, echinoid/gastropod scratch marks and shells of nestling bivalves, as well as more delicate epilithozoans.
Five lithologically different rock samples from the "Niwka" brickyard exposure, including one coal sample, were geochemically characterized as regards their mineral composition and sedimentary organic matter present in them. Various methods, including: X-ray diffraction, solvent extraction, group separation by preparative thin layer chromatography, infra-red spectroscopy for assessment of the content of different functional groups in total rock extracts and their separated polar compound fractions, and capillary gas chromatography for analysis of aliphatic hydrocarbon fractions. It was found that the "Niwka" brickyard rocks are rich in organic material present as the plant detritus (cane-break) or as compounds adsorbed on clay and carbonaceous minerals. Yields of bitumen extraction, group composition of extracts and content of various functional groups assessed by infra-red spectra seem to be related to mineral composition of the host rocks and type of kerogen present. Higher contents of clay minerals, especially chlorite, enriches both extracts and their polar compound fractions in aliphatic structures while the organic matter of both sandstones analysed contains more condensed aromatic structures and functional oxygen groups. Generally the organic matter of the host rocks was deposited in an oxic environment and belongs to type III kerogen with a dominant terrestrial biogenic source or to type II kerogen of bacterial/terrestrial origin of primary biogenic matter. Its thermal maturity stage can be estimated as end-diagenetic or early catagenetic. The highest maturity is shown by the organic matter in the basal sandstone (the S2 sandstone) and probably is caused by imput of older migrating bitumen. Biodegradation of organic matter seems to be influenced by the mineral composition of the host rock, with a high content of clay minerals giving protection.
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