Marine magnetic anomalies are a critical observation used to investigate seafloor spreading and the transition between oceanic and continental crust at passive margins. However, pronounced post-accretion sedimentation disturbs the thermal state of the crust and therefore alters its remanent magnetization. To study the link between sedimentation and magnetization of the oceanic crust, we built a series of thermomagnetic forward models coupled with different sedimentation histories. We test our approach against observations from the early Cretaceous southern South Atlantic Ocean. Our simulations suggest that, depending on the thickness of post-accretion sediments, the remanent magnetization of the extrusive basalts is partly or completely removed. Therefore, the typical long-wavelength sea surface marine magnetic anomalies observed above oceanic crust covered by a thick sedimentary pile is almost entirely generated by the magnetization of the deeper crustal layers.