THE ACCURACY OF SEDIMENTARY PALEOMAGNETISM

The accuracy of the paleomagnetic vector in sedimentary rocks can be examined from the viewpoint of the three questions posed by this session. “Does burial compaction of sediments shallow the inclination of sedimentary rocks?” has been answered. In almost all the cases studied, it does shallow the paleomagnetic inclination of sedimentary rocks. The magnitude of the effect is on the order of 10˚-20˚. A literature survey shows that of 18 anisotropy-based corrections only two didn’t show significant shallowing and out of 22 elongation-inclination corrections only four had f (flattening) factors of 0.9 or greater, where f is the ratio of the tangents of uncorrected and corrected inclinations. These studies show that, on average, for magnetite-bearing rocks f=0.6 and for hematite-bearing rocks f=0.7.

“How much does inclination shallowing affect continental paleogeographic reconstructions?” is a remaining question and can be answered most likely in the affirmative, but the magnitude needs to be determined. Red bed results dominate the North America apparent polar wander path for time periods before (>200 Ma) synthetic reference pole paths so inclination shallowing may well have affected ancient pole path accuracy. Kent and Irving (2010) show that for even younger time periods an inclination correction shifts the North American pole path by 25˚. Bilardello and Kodama (2009) corrected the sedimentary inclinations for Carboniferous North America using magnetic anisotropy and observed an overlap of North America with Gondwana unless the sedimentary paleopoles positioning Gondwana were also corrected using the f=0.7 hematite correction.

“How much does grain-scale strain rotate the paleomagnetic vector?” is a question to come. We have yet to determine how paleomagnetism behaves in simple shear, either as a passive marker rotating into the long axis of the strain ellipse or as if the paleomagnetism were carried by small rigid particles rotating in shear. Field experiments suggest rigid particle behavior, but laboratory experiments suggest much more complicated behavior. Strain rotation of remanence could be on the order of tens of degrees. This effect will be important to understand in more detail as paleomagnetists study more ancient, multiply-deformed Precambrian rocks.