SEM/STEM observation of magnetic minerals in presumably unremagnetized Paleozoic carbonates from Indiana and Alabama

Abstract

The Silurian Wabash Formation in Indiana and the Mississippian Pride Mountain Formation in Alabama appear not to have been affected by a late Paleozoic remagnetization event. In an attempt to characterize the magnetic mineralogy in these (presumably) unremagnetized carbonates and in order to compare their magnetic mineralogy to that of remagnetized carbonates, scanning and scanning transmission microscope (SEM/STEM) observations and rock magnetic investigations were carried out.It is possible to recognize differences in magnetic mineralogy in the unremagnetized carbonate from that in remagnetized carbonates: 1. (1) iron oxides associated with iron sulfides are hematite (in this study) as a result of replacement of pyrite (instead of magnetite as was found elsewhere);2. (2) occurrences of large euhedral pure-iron oxides of secondary origin are common in the unremagnetized carbonates3. (3) a rare occurrence of fine-grained single-crystal magnetite capable of carrying a remanence in the unremagnetized carbonates is noticeable as compared to the abundance of such grains in the remagnetized carbonates. Although the abundance of the fine-grained magnetite grains in remagnetized carbonates is inferred to be a diagnostic factor to distinguish the remagnetized from the unremagnetized carbonates, this clarifies only the carriers in the remagnetized rocks and leaves the question of the carriers in unremagnetized limestones unresolved to a large extent.The lack of remagnetization is commonly attributed to a restricted amount of fluid influx into the rocks. For the Wabash and the Pride Mountain Formations this may also be true; early cementation has significantly reduced the porosity and permeability in the Wabash Formation in Indiana, whereas the presence of the impermeable Chattanooga Shale may have `protected' the Mississippian Pride Mountain Formation in Alabama.