Regional patterns of magnetite authigenesis in the Appalachian Basin: Implications for the mechanism of Late Paleozoic remagnetization

We report paleomagnetic and rock magnetic data from sedimentary carbonates of the Appalachian Basin that have implications for the mechanism of widespread late Paleozoic remagnetization in this rock type. Sampling was accomplished mostly along two across-basin transects, one in Devonian carbonates of New York State and the other in Ordovician and Mississippian carbonates of Tennessee and Alabama. Paleomagnetic investigations of the New York Devonian carbonates along a transect from Albany to Buffalo indicate that these rocks were completely remagnetized in Alleghenian time. Rock magnetic studies show that magnetite concentration varies in a regular fashion along the transect with a local maximum near Syracuse and decreasing gradually to the east and west. This pattern shows a striking correlation with the degree of diagenetic alteration of clay minerals in a Devonian bentonite horizon which took place during Alleghenian time. We conclude that most of the magnetite present in these carbonates is of authigenic origin and that clay mineral alteration and magnetite authigenesis were coeval, late Paleozoic events that were controlled by the same diagenetic factors. Thermoviscous remagnetization processes cannot be ruled out in this setting in light of the observed degree of thermal maturity. However, our results suggest that chemical factors played a critical role in remagnetization since they allow us to infer that magnetite authigenesis and remagnetization are about the same age. We propose a geochemical model for magnetite authigenesis wherein the iron is derived from detrital smectites during diagenetic illitization, which is triggered by the introduction of potassium-rich brines. Results from the Tennessee transect from Nashville to Chattanooga show a very different pattern. Paleomagnetic studies indicate that late Paleozoic remagnetization has affected Ordovician carbonates of the Nashville Dome. Paleozoic carbonates in the overthrust belt near Chattanooga also carry the late Paleozoic remagnetization. However, Mississippian carbonates between the dome and the overthrust belt are very weakly magnetized and show no evidence for late Paleozoic remagnetization. Samples show only a present field magnetization or, at one locality in northwestern Alabama, a dual-polarity magnetization of probable Mississippian age. Rock magnetic studies indicate that higher concentrations of magnetite are present in the remagnetized rocks of the Nashville Dome and the overthrust belt than in the nonremagnetized Mississippian carbonates. We therefore conclude that magnetite authigenesis and remagnetization are related events in this setting and that the remagnetization must be due to chemical processes. The presence of a remagnetization “shadow” in the younger rocks now exposed between the fold-thrust belt and the flexural arch is attributed to westward flow of magnetite-forming aqueous fluids through deep, sub-Carboniferous aquifers during Alleghenian time. Upward migration of the fluids through the Mississippian carbonates may have been prevented by the Devono-Mississippian Chattanooga Shale, an impermeable stratum that is present throughout the region.