Conodont color alteration and microdolomite composition - implications to the Muschelkalk limestones (Upper Triassic) overlying the Upper Cretaceous intrusive body of the Vlotho Massif (Weserbergland, Northwest Germany)

Abstract

The conodont color alteration index (CAI) and the microdolomite composition (MC) were used to evaluate the thermal diagenesis in the Upper Muschelkalk (Middle Triassic) limestones overlying the Cretaceous intrusive body of the Vlotho Massif in the southern part of the Lower Saxony Basin in Northwest Germany. The validity of these two methods is discussed. The CAI exhibits a greater accuracy at high levels of organic metamorphism, especially above temperatures of 180°C, whereas the microdolomite composition is an excellent indicator for differentiating low-temperature diagenesis. The good positive correlation between % Rm and CAI can probably be attributed to the fact that in both cases organic matter changes during diagenesis. The maximum palaeotemperature range of 300-345°C ascertained by the CAI value of 5 in sediments directly over the Vlotho Massif, lies higher than the temperatures estimated with the help of other geothermometers.