CHANGES INDUCED BY TEMPERATURE IN THE MAGNETIC RECORDING MEDIUM IN AREAS WITH COAL DEPOSITS. SUPPORT FROM PORCELANITES

摘要

The case of some spectacular changes suffered by the magnetic recording medium (nur.m.), in natural conditions, is commented. The results analysed in the paper are originated in the Motru - Jil| zone, which is located in a very-known coal-generating area of the western Dacic Basin (Romania). The appearance of post-depositional thermal perturbations within the lignite - clay sequences has resulted in essential changes of the ntr.m.; "new" rocks were formed, e.g. the porcelanites. The immediate consequences are reflected by a (palaeo)geomagnetic framework modified in comparison with the initial context {i.e., prior to the intervention of the heating caused by the spontaneous autocombustion of certain lignite beds). The rock magnetic signal detected from the "baked clays", representing the "thermally-disturbed" ntr.m., is much higher in comparison with the signal sent by the "original"/"fresh" clays, representing the "initial" ntr.m. The enhancement of the initial magnetic susceptibility and of several magnetic anisotropy parameters, as well as of the natural remanent magnetisation (NRM) intensity is argued by the laboratory measurements performed on large collections of specimens sampled from both the initial ntr.m. and its subsequently transformed "fragments". The NRM direction has also been modified, from a reversed polarity (for the "fresh" clays) to a normal polarity (for the "baked" clays). Palaeomagnetic proofs that show the time of the changes produced within the ntr.m. are presented, as well. The "thermally non-affected clays" have recorded the geomagnetic polarity in Pliocene, assigned to the Gilbert Chron (the lower part of the C2Ar Subchron), whereas the "porcelanites" situated in the vicinity of the "fresh" clays have printed the palaeofield polarity in Middle - Upper Pleistocene, assigned to the Brunhes Chron. Consequently, deposits of rocks, spatially situated in an adjacent (stratigraphic) position, are temporally placed at a "distance" of about 3.5 Ma. Finally, geomagnetic consequences, applications and implications are shortly presented.