Origin of hematite and its associated remanence in the Copper Harbor Conglomerate (Keweenawan), upper Michigan


  • R. Douglas Elmore,

  • Rob Van der Voo


To study further the origin remanence in red beds, the volcanogenic Copper Harbor Conglomerate has been investigated using both paleomagnetic and petrographic techniques. Paleomagnetic results suggest the presence of two magnetic components, both possessing a westerly and shallow or normal Keweenawan direction. One component (in the vicinity of declination 294°, inclination −7.6) comprises approximately 30% of the total natural remanent magnetization, is removed by alternating field demagnetization following chemical demagnetization, and shows a consistent inclination error that is controlled in sense and amount by the dip of the cross beds in which the samples were taken. The low coercivities of this component suggest that the magnetization resides in magnetite. This and the inclination error due to the bedding indicate that the component is a detrital remanent magnetization.

The predominant component (declination 286.5°, inclination 0.0°, based on six sites) is removed by chemical demagnetization and shows no consistent relationship between inclination and dip of the cross beds, which suggests that it is a postdepositional magnetization. This component resides in hematite that is removed during the chemical demagnetization procedure. Petrographic studies show that hematite is present authigenically as pigment and as partly to wholly martitized grains. Martitization may have occurred before or after deposition; petrographic results are at present inconclusive. A pole position calculated from this postdepositional component (168.3°E, 11.2°N) is similar to poles from the overlying Nonesuch Shale and Freda Sandstone, which suggests that the remanence was acquired before the end of Freda deposition.