VGP Latitude

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FIGURE 6.4 VGPs and polarity stratigraphy from the Velvet Room. Error bars represent two standard deviations. Solid boxes indicate a class I site; open boxes indicate a class II site. Polarity designations are explained in the text.

Site 1, horizon C, is a class I site. Unfortunately, only low levels of demagnetization were possible. Nonetheless, the site shows strong normal signatures (low declination, high positive inclinations) that do not overlie the modern field (figure 6.6A). As such, the simplest interpretation is that this is a normally magnetized sample. Since this is the youngest site within the succession, it is most likely that this sample lies within the Brunhes polarity interval.

SITES 2 AND 3, HORIZONS C AND D

Given the high dispersion in the data, site 2 within horizon C is a class II site; site 3 within horizon D is class I site. Both of these sites contain samples with negative inclinations and high to intermediate declinations (figure 6.6B). Both sites migrate away from the modern field with increased demagnetization toward the calculated reversed VGP. Given the class I status of site 3, it is highly likely that this represents true reversed polarity. The simplest interpretation then would be that it lies within the Matuyama polarity interval. It is possible that site 2 lacks a clear signature altogether, or that it straddles the polarity flip between sites 1 and 3.

SITE 8, HORIZON G

This class I site shows the clearest signature of reversed polarity. Each sample begins with a high positive inclination but also a high declination. Through demagnetization up to

FIGURE 6.5 Zijdervelt diagrams showing the demagnetization trajectories of various samples.

(A) PC94-20, from the lower block sample. This sample points north and down throughout its demagnetization, but never overlies the modern field. Although this may reflect normal polarity, it more likely reflects a strong normal overprint.

(B) PC93-16, from site 8. This sample points south and up throughout its demagnetization, and approaches the reversed polarity VGP. Its shallow inclinations probably reflect incomplete demagnetization of a normal overprint of a reversed primary polarity. (C) PC93-13, from site 7. This sample points north and up for most of its demagnetization, and ends with a weak ambiguous signal. This site is ambiguous owing to the quality of the samples, but it may reflect incomplete demagnetization of a normally overprinted reversed polarity.

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NT Up

NT Up

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FIGURE 6.6 Detailed stereoplots from sample sites. (A) Site 1 at 100 Oe; (B) site 3 at 100 Oe; (C) site 8 at 1000 Oe; (D) block samples. All samples and means plot upper hemisphere. The data are plotted as two populations, those with strong negative polarity characteristics (n = 7) and the remainder (n = 22). In D, populations surrounded by the thick line represent low AF demagnetization, and solid squares show mean vectors; 200-Oe populations shown by shaded field, and shaded squares depict means; 700-Oe populations shown by thin line, and open squares show means.

^ Pole, Brunhes and Matuyama

• Sample vector, lower hemisphere

■ Site VGP, lower hemisphere

□ Sample vector, upper hemisphere

Q Site VGP, upper hemisphere

^ Pole, Brunhes and Matuyama

• Sample vector, lower hemisphere

■ Site VGP, lower hemisphere

□ Sample vector, upper hemisphere

Q Site VGP, upper hemisphere

FIGURE 6.6 Detailed stereoplots from sample sites. (A) Site 1 at 100 Oe; (B) site 3 at 100 Oe; (C) site 8 at 1000 Oe; (D) block samples. All samples and means plot upper hemisphere. The data are plotted as two populations, those with strong negative polarity characteristics (n = 7) and the remainder (n = 22). In D, populations surrounded by the thick line represent low AF demagnetization, and solid squares show mean vectors; 200-Oe populations shown by shaded field, and shaded squares depict means; 700-Oe populations shown by thin line, and open squares show means.

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S Down

FIGURE 6.7 Zijdervelt diagrams from the 1994 block sample. (A) Stepwise AF demagnetization of sample PC94-01 reveals removal of an overprint (possibly viscous) and a trend toward a stable reversed polarity. The final signal is sufficiently strong and robust to be interpreted confidently as reversed polarity. (B) Sample PC94-21 is similar to sample PC94-01, but shows removal of an overprint with a different vector. (C) Stepwise demagnetization of sample PC94-12 fails to yield a clear polarity. The sample vectors are always either north and up or west and up, an ambiguous result. (D) Sample PC94-16 is similar to sample PC94-12, but is even weaker at final demagnetizations.

S Down

1000 Oe, the inclination of each sample flips into the upper hemisphere to end very near the calculated reversed pole (figure 6.6C). These samples must lie within a reversed polarity interval, probably the Matuyama.

SITE 12, HORIZON I

The oriented block is an interesting site owing to both the complete demagnetization spectrum and the large number of samples from one site. In sum, it behaved very much like the entire data set. Of the 30 samples, 7 showed strong reversed polarity signatures, creating a clear class I site (figures 6.6D, 6.7A, 6.7B). One sample (figure 6.5A) showed normal magnetization trending toward the modern field. This could be caused either by primary normal magnetization or by strong normal overprinting. The remaining 22 samples showed a mixture of responses with vectors that plotted in between both calculated poles (figures 6.7C, 6.7D). The simplest interpretation is that the site lies within a reversed polarity interval, with most samples showing some degree of overprinting.

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