Magnetic stratigraphy works in the Devonian

Using alternations of magnetic field intensity, and the patterns that they show over time, has been a standard method in stratigraphy for times back to about 200 Ma ago.  There is no sea floor older than that, and although reversals are known widely from earlier times, there is no continuity that allows its use.  Moreover, reversals are too widely spaced in time to allow for more than calibrating stratigraphic sequences.  Much finer stratigraphic resolution comes from direct and rapid measurement of the intensity of magnetization that can be induced in sediments from their content of various magnetic minerals.  The Ocean Drilling Programme and studies of loess sections in China have long established such magnetic susceptibility logging as a correlative tool.  Empirically, it works, and the loess studies suggested that variations relate to changes in global climate.  Its usefulness in marine sediments is now seen to relate to the production of massive amounts of very fine-grained magnetic minerals in tropical soil formation during warm-humid episodes.  Being so fine, the particles reach the most distant ocean basins after soil erosion.  Susceptibility seems to vary with global changes in the amount of continental erosion.

Detailed correlation between widely separated marine stratigraphic sequences of all ages is notoriously difficult.  Consequently, rapid methods based on magnetic susceptibility, which can produce near-continuous logs, have useful potential.  A team of Us, Spanish and Moroccan geologists has demonstrated its use in definitive correlation between Lower Devonian rocks found in Spain, Morocco and Bolivia (Ellwood, B.B. et al.  2001.  Global correlation using magnestic susceptibility data from Lower Devonian rocks.  Geology, v.  29, p. 583-586).

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