Identifying independent parameters to bracket the oil window in sedimentary basins is of first importance for petroleum exploration. We investigate the magnetic assemblage of Cretaceous claystones collected from the western volcanic margin of Greenland. These claystones experienced low-grade burial (5 km). To assess the maturity of organic matter, we measured the vitrinite reflectance (R_o) and the temperature at maximum release of hydrocarbons (T_max) that occurs during Rock-Eval pyrolysis. These data (0.37% R_o 2%, 422C T_max 604C) show that most claystones are immature to mature with respect to the oil window. We suggest that magnetic minerals can be an indication of burial temperature. The magnetic assemblage of the same samples is elucidated by low-temperature (300 K) and high-temperature (300 K) properties of isothermal remanent magnetization. The magnetic assemblage of immature claystones (lower R_o) resides in goethite, greigite, and some magnetite. The magnetic assemblage of immature to early mature claystones (higher R_o) resides in stoichiometric magnetite and fine-grained pyrrhotite. By comparing these results with those from claystones from the Paris Basin (France) and the Jura Mountains (Switzerland), we suggest the existence of similar mechanisms for the formation of magnetic minerals during burial. If confirmed by further studies, the magnetic assemblage of claystones may be diagnostic of oil windows.