Detrital magnetization in red beds of the Moenkopi Formation (Triassic), Gray Mountain, Arizona

Abstract

Specularite, apparently formed and in part altered to red hematite before deposition, carries much of the stable magnetization in sandstone, siltstone, and claystone of the Moenkopi Formation (Lower Triassic) at Gray Mountain, northern Arizona. High‐temperature exsolution textures seen in polished section indicate that some, if not much, of the specularite is detrital. Progressive thermal and chemical demagnetization analyses indicate that the most stable magnetization resides in specularite. Last, a systematic depositional inclination ‘error’ is seen in both normally and reversely polarized sandstone. Part of its cause was the deposition along fine bedding laminations of subtabular grains of specularite carrying a strong direction of magnetization in the plane of tabularity. Magnetic inclinations are significantly shallower in sandstone than in siltstone, and the amount and sense of inclination error are controlled in detail by the dip of the horizontal and cross laminations. The foregoing leads to the conclusion that the stable magnetization is a detrital magnetization acquired during the course of deposition and that the Moenkopi Formation thus is suitable for magnetostratigraphy. Neither the formation of specularite nor the acquisition of the stable magnetization appears to be the consequence of prolonged and complicated postdepositional chemical alteration, a process invoked by some workers for formation and magnetization of ancient red beds. Because specularite is common in other ancient red bed sequences, their stable magnetizations also may be partly or largely detrital.