AF demagnetization of thermoremanent magnetization (TRM) and saturation isothermal remanent magnetization (IRM(s)) are used to establish diagnostic criteria for primary TRM and for secondary magnetization in igneous rocks. In applying the approach to the interpretation of natural remanent magnetization (NRM), the demagnetization of IRM(s) documents the nature of the magnetic phases in the rock, while the demagnetization of the NRM gives information about the process of magnetization. The ratio of NRM : IRM (s) measures the efficiency of the magnetization process.
The effect of diagenesis upon natural remanent magnetization (NRM) has been studied at two sites within a single bed of dolomite from the Monterey Formation in Santa Barbara County, California. Paleomagnetic direction demonstrates that the NRM is primary at one site and remagnetized at the other site. Magnetic separates from samples carrying a primary NRM yielded detrital magnetite. Separates from samples carrying secondary NRM yielded an unusual form of authigenic magnetite. Evidence suggests that the formation of the authigenic magnetite accompanied late stage dolomite diagenesis. Correlating the remagnetization direction to the paleomagnetic history of the region gives a probable age of 0.72–2.47 m.y.b.p. for both of these diagenetic events.
Stand exams conducted in western Montana over 50 years ago provide a valuable source of information on prefire suppression and preharvest condition of the region's forests. Of the early exam estimates of stand origin, 52 percent were within 20 years of estimates taken from stand exams conducted in the 1980's, and 73 percent were within 60 years. There was no significant bias toward either higher or lower age estimates. The early stand exam data can give an accurate estimate of stand age distributions over large areas.
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