This paper describes the results of rock magnetic and petrographic studies of the Lower Massive sandstone of the Moenkopi Formation, north central Arizona, which heretofore has been considered to be a reliable magnetostratigraphic unit. The rock-magnetic studies included stepwise thermal demagnetization of samples of intraformational mudstone clasts and a deformed shale lens, both of which occur within the lower part of the sandstone, and stepwise thermal and selective destructive demagnetization (SDD) of samples of the sandstone containing black sand laminae composed largely of opaque grains. In samples both from the claystone clasts and deformed shale lens two secondary components, one of modern normal polarity and one of Triassic normal polarity, were unambiguously identified, and a third CRM component of Triassic reversed polarity was tentatively identified. There is no evidence in samples of either the claystone clasts or deformed shale lens of a remanence acquired prior to deposition or to deformation, respectively. SDD, by means of which the small-scale homogeneity of remanence within samples of the Lower Massive sandstone containing black sand laminae could be assessed, shows that each sample primarily contains a heterogeneous three-dimensional mixture of Triassic normal and reversed components. The Triassic normal direction that is characteristic of the Lower Massive sandstone is a vector resultant of almost balanced proportions of the two nearly antiparallel components, averaging about 55 to 60% normal and 45 to 40% reversed. The local variability in both direction and intensity of remanence within and between individual black sand laminae confirms that acquisition of the remanence took place over a geologic time interval that spanned at least two and probably several polarity intervals and that, on a small scale, the remanence was sporadically acquired more or less randomly, probably in response to local conditions in the interstitial physicochemical environment that affected the growth of authigenic hematite. No primary component of remanence acquired at or soon after deposition could be identified. The remanence in the black sand-bearing Lower Massive sandstone is largely carried by the black sand grains which, as determined from reflected-light petrographic studies, are primarily martite (67%) and altered ilmenite grains (20%) composed of complex intergrowths of hematite and iron-titanium minerals. These data strongly suggest that these grains, deposited initially as unaltered to only moderately altered magnetite and ilmenite, subsequently underwent oxidation during which they acquired their remanence.
