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Abstract

Lower Carboniferous and Upper Jurassic sedimentary rocks from wells of the southwestern part of the Lower Saxony Basin (LSB) show locally anomalous maturity reaching 4.5% VRr. The reason for this high maturity is discussed controversy. It may either be related to the intrusions of supposed Late Cretaceous igneous plutons in the area of Bramsche and Vlotho or is explained by models involving deep burial during Early Cretaceous times and uplift during Late Cretaceous/Tertiary times. A fault-slip analysis covering outcrops within the LSB and along the Osning Lineament (OL) has been performed and showed that the area was affected by a stress state with a horizontal N-S- to NE-SW-directed maximum compression (σ1) and a low stress ratio (R=(σ2-σ3)/(σ1-σ3)) which induced reverse and strike-slip faulting. The regional paleostress field can be related to the Late Cretaceous-Cenozoic intraplate compressional deformation that affected the Alpine foreland as a result of the Alpine orogeny Other stress fields estimated in the study area are much less prevalent. Tectonic activity induced the migration of fluids and let to precipitation of quartz and calcite on veins, fissures and slickensides. Calcite mostly precipitated from local formation waters that were in close equilibrium with the host rocks rather than being derived from a large scale circulating hydrothermal system. Fluid inclusions studies in hydrothermal minerals from wells and outcrops in the study area yield migration of differently composed fluids and gases during stages of burial and uplift. In the vicinity of supposed intrusions the migration of hot fluids (>200°C) is recorded in quartz mineralization that is hosted by Upper Carboniferous to Cretaceous sediments. The salinity of the studied fluid inclusions is highly variable suggesting that different fluid reservoirs were drained. However, fluid entrapment often occurred along with the entrapment of gas mixtures with variable contents of CH4, CO2, and locally N2. These gas inclusions formed at pressure conditions close to hydrostatic most probably during basin inversion.