The Marhřgda Bed occurring at base of the Adventdalen Group in Sassenfjorden, Spitsbergen contains common ankeritereplaced belemnite skeletons. Petrographic, major element geochemical, and stable carbon and oxygen isotopic data indicate that the ankerite originated in a catagenic environment associated with thermal degradation of kerogenan d hydrocarbongen erationinthe sequence. It formed at maximum temperature of 150°C under burial of approx. 2 000 m, most probably during Paleogene filling and subsidence of the Central Spitsbergen Basin. Dissolution of biogenic calcite and precipitation of ankerite reflect extensive heat flow through the Adventdalen Group sequence related to the Cretaceous and Paleogene magmatic and orogenic activity in Svalbard.