Paleomagnetism of the Moenkopi Formation, Colorado Plateau: Basis for long‐term model of acquisition of chemical remanent magnetism in red beds

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Abstract
Paleomagnetic studies of Moenkopi samples from several sites on the Colorado Plateau, including a complete 427‐m section at Sinbad Valley, western Colorado, reveal the following: (1) the presence of both normal and reversed Triassic components in many of the same samples at many localities, (2) the close correlation of the magnetic polarity and intensity of magnetic remanence with the lithologic characteristics in the Sinbad Valley section, (3) a streaked pattern of the remanence directions in any one member in the Sinbad Valley section similar to that expected from polar wander, (4) a lack of progressive change from the base upward, and (5) the common intermediate remanence directions of weak intensity (suggesting the presence of two or more nearly balanced anti‐parallel components) throughout the formation. The results of our investigation, combined with previously reported petrographic and paleomagnetic studies of the Moenkopi Formation, have led us to conclude that the principal remanence carried by the formation is chemical remanent magnetism (CRM) acquired diagenetically over a geologically long time interval. From the available data we have formulated the following model to explain how the CRM was acquired through natural processes acting intermittently at geologically reasonable rates. When initially deposited, the Moenkopi sediments consisted primarily of first‐cycle detritus containing a few percent of iron‐bearing accessory minerals such as biotite, hornblende, magnetite, and ilmenite. At some time after deposition, under favorable subsurface conditions, intrastratal alteration began, involving breakdown of iron‐bearing silicate and oxide minerals, partial replacement of some iron‐rich silicate grains by hematite, martitization of magnetite, growth of hematite pods and blebs in ilmenite and biotite, and precipitation of crystalline hematite in both primary and secondary voids. The authigenesis spanned at least two polarity events and probably continued over several tens of millions of years. Stratigraphic units with grossly similar lithologies underwent similar diagenetic alterations at rates and to extents that were different from other stratigraphic units having different lithologies. Because of variations in the rate and duration of alteration and variations in the length of normal versus reversed polarity events, any given lithologic unit tended to be dominated by one polarity. The model given above seems to be capable of explaining not only those magnetic and petrographic features found in the Moenkopi Formation but also those of most other red beds.