Paleomagnetism of the Devonian Onondaga Limestone Revisited
Ninety-six oriented samples were collected from 13 sites in the Onondaga limestone of New York for paleomagnetic study, including rock magnetism and magnetic susceptibility anisotropy. The sites were located in a small penecontemporaneous fold and a larger fold of imprecisely known origin, as well as flatlying beds. The natural remanent magnetizations (NRM) give directions (declination 195.1°, inclination 84.4°, and α_95 = 6.9° for N = 13 sites) that are similar to those originally measured for the Onondaga limestone by Graham (1956). Alternating field and thermal demagnetization analyses show that the NRM is a composite of a large viscous component along the present geomagnetic field and a smaller, more stable component of magnetization with a shallow and southerly direction. The site mean directions after 300 Oe of magnetic cleaning, but without correction for bedding tilt, give a formation mean of declination 168.7°, inclination 13.6°, and α_95 = 8.3° for N = 9 sites (the magnetizations at four sites could not be adequately resolved and are omitted). Although the stable magnetization is shown to be of postfolding origin, essentially the same direction is obtained at sites with flat-lying bedding as at the sites with folded bedding prior to tilt correction. Rock magnetic investigations indicate that some form of magnetite is the important magnetic mineral in the limestone. The similarity in magnetic fabric between a small penecontemporaneous fold and a larger fold suggests acquisition of magnetic properties while the sediment was still unconsolidated. The postfolding origin of stable remanence is therefore interpreted as a form of postdepositional detrital remanent magnetism. The paleomagnetic pole position derived from the formation mean direction is located at latitude 39.7°N, longitude 120.7°E (dp = 4.3°, dm = 8.5°) and agrees reasonably well with other Devonian poles from cratonic North America, although a Permian age for the magnetizations of the Onondaga cannot be entirely discounted.
- JB084iB07p03576.pdf application/pdf 1.05 MB Download File
Also Published In
- Journal of Geophysical Research