Mechanisms of Late Paleozoic Remagnetization in Some Sedimentary Rocks of the Appalachian Basin.

Gang Lu, Louisiana State University and Agricultural & Mechanical College


Paleomagnetic, rock magnetic, geochemical and petrographic investigations of some Paleozoic sedimentary rocks indicate that the Late Paleozoic regional diagenetic events associated with orogenic fluid migration gave rise to magnetite authigenesis, hematite authigenesis and related chemical remagnetization. Good correlations between K/Al ratio and the content of diagenetic magnetite, K-feldspar, quartz, Fe-rich chlorite and degree of illitization in the Onondaga Formation along a Rochester-Syracuse transect in the Northern Appalachian Basin suggest a link between remagnetization residing in fine-grained magnetite and potassic metasomatism. The mechanisms of magnetite authigenesis can be expressed by two hypothetical reactions which are related to the breakdown of Fe-bearing smectite in the presence of K. Thermodynamic calculations indicate that these reactions are favored by increasing activity ratio ($\rm aK\sp+/aH\sp+$), relatively reducing conditions, increasing pH and precipitation of quartz. On the Nashville Dome, the Ordovician and the Lower/Middle Mississippian strata at most of the sites are remagnetized. A Devonian magnetization appears to be preserved at two sites. Patterns of magnetizations can be explained by a stratigraphic control on aquifer geometry for the remagnetizing fluids and magnetite grain size effects. Remagnetization resides principally in single domain and pseudosingle domain magnetite. The Late Paleozoic remagnetization on the Jessamine Dome has been overwhelmed by hematite authigenesis and attendant post-Late Paleozoic overprint related to faulting along the Kentucky River Fault Zone. A comparative study of magnetic anisotropies indicates that ARM anisotropy appears to primarily reflect a pre-deformation fabric with minimum axes near vertical and a weak lineation. Conversely, IRM anisotropy has minimum axes near horizontal and oriented NW-SE, which is compatible with a tectonic fabric due to Alleghanian deformation. A well defined remagnetization residing in hematite in the Middle Silurian Rose Hill Formation from the Valley and Ridge was apparently acquired during Alleghanian defomation. The authigenic hematite in multiple generations is texturally related to the diagenesis of berthierine, Fe-rich chlorite, Fe-rich illite and detrital Fe-Mg silicates. Iron for the hematite authigenesis was probably supplied by detrital Fe-Mg silicates, although an additional, external source is needed. Syn-folding remagnetization is also detected in the Valley and Ridge carbonate rocks.