Further studies of carbonate remagnetization in the northern Appalachian Basin

Abstract

Paleomagnetic and rock magnetic studies of sedimentary carbonates in the northern Appalachian Basin demonstrate that the late Paleozoic remagnetization in this area is probably related to authigenesis of single domain magnetite. Fifteen sites within Middle Devonian carbonates were sampled along a transect extending from Albany, New York, to the Algonquin Arch in southeastern Ontario. Paleomagnetic studies show that all sites except the two westernmost sites on the Algonquin Arch were remagnetized during Alleghenian time. The very weakly magnetic unremagnetized sites retain a dual‐polarity magnetization of probable Devonian age. In the remagnetized sites, both the size of the Kiaman components and magnetite concentration, estimated from low‐field susceptibility (X), are highest in the central part of the transect and lower on either side. X ray powder diffraction analyses demonstrate that K‐feldspar is also most abundant in the middle of the transect, with lower concentrations to the west and east. We suggest that formation of authigenic magnetite and K‐feldspar may be related in this setting. Sites with the largest Kiaman components have the highest unblocking temperatures and the largest ratios of anhysteretic remanent magnetization (ARM) to bulk susceptibility (ARM/X), indicating that these sites have the greatest amounts of single‐domain magnetite. Several remagnetized sites exhibit northerly, shallow magnetic components which may be Kiaman “normal” directions. We conclude that remagnetization along the New York‐Ontario transect is a chemical remanent magnetization resulting primarily from authigenesis of single‐domain magnetite. We present a model in which pulses of heated fluids triggered magnetite formation. Scanning electron microscope observation of magnetic extracts from 14 sites show that most of the resolvable magnetite grains are multidomain grains of irregular shape; many sites also contain a small percentage of magnetic spheres and/or euhedral magnetite particles. A majority of the magnetite grains contain Fe as the only cation, but many also contain minor amounts of other cations including various combinations of Cr, Ni, Ti, Zn, Mn, Si, Al, Ca, and Mg. Single domain magnetite is not resolvable with our observational techniques, but its presence is inferred from rock magnetic tests. Much of this magnetite may be of authigenic origin.