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Organic matter preservation and associated conditions during deposition, important in the context of fossil fuel exploration, are commonly determined by advanced geochemical analyses. However, the relation between organic matter preservation and magnetic mineral composition remains poorly constrained. The aim of the studies was to check the potential of magnetic mineral differentiation between facies containing various amounts of organic matter as a factor to better understand the processes which influence water chemistry at the bottom of sedimentary basins, and thus to better understand factors controlling the preservation of organic matter. To determine the composition and the properties of magnetic minerals, detailed low-temperature measurements of Saturation Isothermal Remanent Magnetization and hysteresis loops were performed on two types of rocks, Silurian shales from the Baltic Basin (northern Poland). The analysed shale facies are characterized by similar thermal evolution, but different amounts of organic matter: the Pelplin Formation, containing a modest content of organic matter, in which we also examined early diagenetic carbon concretions; and the Jantar Formation, which represents an organic-rich 'sweet spot' layer. In both facies, the results indicate the presence of multi- or pseudo-single domain magnetite, which is interpreted as detrital in origin. However, the main observation gained from this study is the relation between magnetic mineral assemblage in the studied shales and the amount of organic matter: in the rocks with modest amounts of organic matter we observed hematite, while in organic-rich layers hematite was absent. Hematite (mostly single-domain grains) preserved in the Pelplin Formation suggests that stable oxygen-rich conditions were present at the bottom of the sedimentary basin continuously during deposition, concretion cementation and compaction. In turn, its absence in the Jantar Formation suggests that during sedimentation and early diagenesis more anoxic conditions appeared. Generally, findings show that the presence of hematite is related to the significantly lower amount of organic matter in sedimentary rocks. Thus, presence of this mineral may be a useful indicator of organic matter preservation. [ABSTRACT FROM AUTHOR]

In this paper, we present results of the 3-D tomographic modelling of the crustal structure and Moho topography applied to data recorded in SE Poland during the CELEBRATION 2000 seismic experiment. The target area covers ca. 500 km × 500 km and represents a complex geological setting from old Precambrian platform (East European Craton, EEC), through the crustal blocks (terranes) that form the Trans-European Suture Zone, to the young Alpine orogen—the Carpathians. We test two different inversion strategies using two different algorithms: (i) coupled inversion of and arrivals to constrain both the crustal velocities and the Moho depths; (ii) decoupled inversion of arrivals only using previously obtained smooth 3-D crustal velocity model. The coupled inversion of and arrivals results in a much smoother crustal velocity field than the one previously obtained by inversion of first arrivals only. Also, the obtained Moho structure is much smoother than the Moho map compiled from the existing 2-D models. Decoupled inversion of the reflections provides Moho structure comparable in resolution to the compiled map. Synthetic tests indicate that with our data set we are able to resolve larger than 100-km-size Moho structures. The modelled Moho is shallowest in the area of the Upper Silesian Block ( ca. 32 km), then it is deepening by ca. 10 km over 100-km-wide zone along the margin of the EEC and finally it reaches up to 48 km depth in the area of the EEC. Our favoured strategy for modelling large refraction/wide-angle reflection data set consists of derivation of a smooth crustal velocity model by a first-arrival tomography and then complementing this model by a decoupled inversion of reflections in order to constrain minimum-structure Moho topography. [ABSTRACT FROM AUTHOR]