Abstract
Microdolomite inclusions in crinoids of the Middle Triassic “Trochitenkalk” stage from north and central Germany were formed during diagenesis in a closed system and normally consist of 52–55 mole % CaCO3 (48-45 % MgCO3). In areas influenced by thermal activity these calcium microdolomites are tempered to stoichiometric dolomites. Thus, the microdolomite composition can be used as a new criterion for assessing the degree of diagenesis in sedimentary rocks. As an effect of the Upper Cretaceous intrusive bodies of the Bramsche-Vlotho-Solling Massifs, which influenced the overlying sediments for 1–10 million years, a microdolomite composition of d(104)=2.889 A (~51 mole % CaCO3) is associated with an illite crystallinity of 350 (Hbrel; <2μm). The composition of the microdolomites is more sensitive to short term heating (for example: ascending thermal waters near the faults of the Hessen grabens) than is illite crystallinity.
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