Remanent Magnetization of a 50 m Core from the Moenkopi Formation, Western Colorado

The NRM of 1117 specimens cut from a core taken from a 49·7 m inclined bore in the Lower Triassic Moenkopi Formation revealed one normal and two reversed magnetic zones. Samples were oriented using the ellipticity of the bedding plane sections cut by the core. The NRM directions of samples from the core are better clustered than those previously measured in surface samples from the same stratigraphic interval. This suggests that much of the scatter observed in the surface samples was due to surface effects, such as weathering. The estimate of the concentration parameter, K, increases with decreasing grain size. Storage tests showed that specimens with low intensities (lower than 0·2 × 10−5 emu cm−3) are generally the most unstable. This is considered to be due to larger viscous moments in these specimens, possibly associated with superparamagnetic components. Thermal demagnetization resulted in a change in the mean field direction for each polarity zone of less than 5° accompanied by a slight decrease in the dispersion.