Geology Reference
In-Depth Information
this example are for a sequence of Triassic age rocks in which present-day
overprinting was documented from the demagnetization data. They are
as follows:
1
If a horizon contains at least one reversed polarity sample (VGP latitude
within 45° of the reversed polarity mean VGP position), the horizon is
considered to be reversed polarity.
2
If a horizon contains at least one intermediate direction sample (VGP
latitude between 45° from the reversed polarity VGP position and 45°
from the normal polarity VGP position), the horizon is intermediate
in direction and no polarity can be assigned.
Figure 3.4
Magnetostratigraphy of Triassic carbonate rocks from the Dolomites showing the effects of present-
day normal polarity overprinting. (a) Stratigraphic column showing the lithologic interpretations, biostratigraphy,
and previous magnetostratigraphy from Kent et al. (2004). (b) VGP latitudes of the samples plotted as a function
of stratigraphic position. Note that the VGP latitude for each sample at horizon is plotted and connected with
a horizontal line. Red samples are those for which additional rock magnetic data are available. The column
immediately to the right shows the local reversal stratigraphy interpretation following the rules indicated in the
text. Black is normal polarity, white is reversed polarity. Half bars indicate a polarity interval based on only one
horizon. Gray bars are intermediate horizons in which no sample is with 45° either of normal or reversed
polarity. (c) The Triassic GPTS and the tie lines between the local magnetostratigraphy and the Triassic GPTS.
Source: Spahn, Kodama & Preto 2013. Reproduced with permission of John Wiley & Sons, Inc.
