Characteristic Remanences

All samples showed evidence of normal polarity overprinting. Some of this magnetization was viscous and weak, and it was easily removed in the earliest phases of AF demagnetization. However, some samples showed strong vectors indicating normal polarity (figure 6.3A). These vectors plotted directly upon the modern inclination and declination for Porcupine Cave for most of their demagnetization trajectory. Such samples

Horizon

1993

1994

1995

Sample Site

16.17,18 19,20,21,22

23,24,25,26

Site 7 10,11,12,13

Site 8 14,15,16

Site 10 17,18,19,20

A CL

Block Sample Site 12

FIGURE 6.2 Stratigraphy and paleo-magnetic sampling in DMNH 644. The samples do not extend below unit I at approximately 110 cm.

could be interpreted as either preserved normally magnetized remanence or strong normal overprints. Sites with this kind of magnetic signature carry the designation N? on the reversal stratigraphy (figure 6.4).

A small number of samples showed normal magnetization but with a broad range of vectors. The calculated VGPs for these samples did not overlie the modern pole but were relatively close (figures 6.3B, 6.5A). They also showed a higher standard deviation than the sites interpreted to be strongly magnetized. These sites carry the designation N, although it is possible that they are also overprinted.

Roughly 25% of all samples showed good evidence of reversed polarity with a weak modern overprint. These samples showed both high declinations (180° ± 50°) and negative inclinations, thus plotting in the upper hemisphere (figures 6.3C, 6.5B). These samples commonly approached the calculated reversed polar attitude during progressive demagnetization. These sites carry the designation R.

The most common magnetic signature for samples was mixed or ambiguous. Typically, the samples yielded a range of inclinations and declinations that did not approach either calculated pole (figure 6.5C). Many samples had negative (upper hemisphere) inclinations with low declinations (0° ± 80°). Since these sites showed significant deviation from modern and ancient calculated normal poles, they probably carry a reversed signature. Because of the ambiguous quality of their remanence, these sites carry the designation R?. Samples that yielded vectors difficult to interpret were marked ??.

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