Early diagenetic conditions during formation of the Callovian (Middle Jurassic) carbonate concretions from Łuków (eastern Poland): evidence from organic geochemistry, pyrite framboid diameters and petrographic study
The Callovian (Middle Jurassic) fossiliferous carbonate concretions from the clay-pit at Łuków, eastern Poland, have been analysed using organic geochemistry, petrography and pyrite framboid size distribution in order to retrieve information about early diagenetic conditions
during clay sedimentation. Additionally, a fossil wood sample was also analysed. The carbonate concretions are formed almost entirely by fine-grained, pure calcite. Other main minerals occurring in the concretions are quartz, feldspars and bright mica with features of muscovite. The clay fraction
obtained from the concretions are mainly composed of quartz, but also contains pyrite, muscovite, mid-order kaolinite and chlorite with an admixture of a mix-layered structure. Small admixtures of siderite, ankerite, rutile and feldspars were also detected. The most plausible source of the
terrigenous phases occurring in the concretions investigated is the Fennoscandian Shield, where sodic magmatic and metamorphic rocks are common. In the fossil wood, the main mineral phase is calcite, which fills the wood-cells. Fluoroapatite also occurs, but is generally confined to the margins
of the cells. The other phase detected is pyrite, which may also fill the cell interiors and replace the cell walls. The Callovian organic matter (OM) from Łuków is immature. The vitrinite reflectance (Rr) of the analysed concretion samples ranges from 0.45 to 0.54
%, and is higher than from the fossil wood (Rr = 0.33 %). Moreover, the concretion samples contain hopanoids with biological configurations, such as ββ-hopanes and hop-17(21)-enes and characterized by low values of ββ/(αβ + βα)-hopane and
31S/(S + R) homohopane biomarker parameters. The occurrence of acyclic ketones with odd over even carbon number predominance in the concretions studied, suggests intensive microbial degradation of OM. The pyrite framboid size distribution does not indicate euxinic or anoxic conditions in the
water column and the sea-floor. Although small-sized framboids (< 5 μm), which may form during euxinic conditions in the water column, occur in all samples investigated, the mean sizes of framboids in all samples are well above 6 μm and large framboids (up to 29 μm) are also present.
Their size distribution points to the lower dysoxic conditions at most, because during such conditions the mean values of framboid sizes begins to increase and larger framboids (> 10 μm) are present. The oxic to suboxic depositional conditions during OM sedimentation are also confirmed
by low concentrations or absence of C33-C35 homohopanes, moderate Pr/Ph values and no organic compounds characteristic of anoxia and water column stratification.
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