Anisotropies of partial anhysteretic remanence and susceptibility in compacted black shales: Grainsize‐ and composition‐dependent magnetic fabric

We have quantified the magnetic fabric of the highly‐compacted Heebner Shale (Pennsylvanian) of Kansas in terms of the anisotropies of partial anhysteretic remanent magnetization (pARM) and low‐field susceptibility. Hysteresis loops exhibit almost purely paramagnetic behavior, and the ratio of high‐field linear susceptibility to low‐field susceptibility is very close to one; the susceptibility anisotropy therefore reflects the fabric of the paramagnetic matrix, while the remanent anisotropies are due to the preferred orientations of ferrimagnetic grains. Both the paramagnetic and ferrimagnetic fabrics are dominated by a horizontal foliation, with a weak northeasterly lineation, but the pARM anisotropies are generally much larger than the susceptibility anisotropy. The degree of foliation measured with pARM's decreases markedly with increasing coercivities, indicating stronger foliation in the coarser grainsize fraction. In contrast, the pARM mean lineations are grainsize‐independent, suggesting a post‐compactional origin, related to tectonic compression. The susceptibility lineation appears to have a slightly different azimuth, and may therefore be due to paleocurrents.