New Late Pennsylvanian Paleomagnetic Results from Parana Basin (Southern Brazil): a Possible Record of the Persistent Non-Dipolar Component During Kiaman Times?

The validity of the Giant Gaussian Process (GGP) models based on the last 5 Ma has been already confirmed for Cenozoic and Proterozoic times, but it is still an open question for the Paleozoic. During the Permo-Carboniferous Reversed Superchron (Kiaman) the behavior of the geomagnetic field was significantly different from the recent field, and therefore it is not expected that the distribution of the magnetic remanent directions follows the same distribution as predicted by the models for the recent magnetic field. In this way, a paleomagnetic work was carried out on a Late Pennsylvanian section of glacial rhythmites from southern Brazil (the Mafra Formation, Paraná Basin). The section corresponds to a regular repetition of pairs of sandstone/silt and thin dark lamina of silt/clay with thickness varying from some millimeters up to five centimeters. Oriented blocks were collected from 124 consecutive layers (except for some gaps on the upper part of the sequence) through a 5m wall. The time span was evaluated using a cyclostratigraphic analysis based on the magnetic susceptibility and the anhysteretic remanent magnetization (ARM) data. Potential Milankovitch signal was recognized and allowed the conclusion that the deposition of the sampled sequence comprehended at least 800ky. The layers are completely flat and displayed a very stable magnetization carried by single-domain magnetite after thermal demagnetization. The ARM anisotropy indicated a negligible shallowing correction of f=0.97. The mean magnetization direction for the Mafra sequence is: Dec=144.2°, Inc=69.5° (N=111, k=134, α₉₅=1.2°, E=1.76, Ei=1.32, Eu=2.45 , Paleolat=53.2°S). The distribution of the magnetization directions is more elongated than predicted by the recent models. An analysis of this high elongation (positive inclination) result along with those from the literature showing low elongation (negative inclinations) indicated that it could be related to the long-term of the non-dipolar contributions. Moreover, this result is an alert that the GGP model may be not suitable for correcting the sedimentary data from Carboniferous time.