POSSIBLE CONTROLS ON ELEMENT TRENDS
There are a number of possible sedimentary and/or geochemical influences which may control element concentration trends within geologic formations. The noted trend in the Prairie du Chien associated with PC1 would appear to be most strongly influenced by Rare Earth Elements and iron (Table 3). As discussed earlier, the concentration of REE's in a sediment corresponds to the solid load (terrigenous sediments) carried by the tributary streams (McLennan, 1989). This leads to the possibility that a decrease in REE concentration might reflect an increasing distance from the sediment source in the depositional basin. In their study of Illinois cherts, Luedtke and Meyers (1984) suggested that this might be the source of the element concentration trends shown by their data. Unfortunately, however, the observed trend in the Prairie du Chien is an increase in concentration to the southeast, whereas the theorized location of the Ordovician shoreline and sediment source is in the opposite direction, to the north or northwest (Austin, 1972). Also, the hypothesis of distance from sediment source does not explain why there is also an observed increase to the southeast in elements associated with oxidation/reduction conditions (Fe and Mn) and highly soluble salts (Na and Br).
A better hypothesis must account for a more overall increase in element abundances. A likely culprit may be found in the silicification process which created the chert nodules through replacement of carbonate with silica. During this replacement process, the silica acts as a dilutant for nearly all other elements, due to the extreme amounts of SiO2 necessary for the process to produce the >95% pure quartz chert nodules. Therefore the absolute abundance of trace elements in the chert may be controlled by the amount of SiO2 in the silicification process (Murray, 1994).
It seems quite possible, perhaps likely, that the observed trend of overall increase of trace element concentrations toward the southeast is a result of more extreme silicification and dilution towards the northwest. This is, therefore, probably a diagenetic imprint we see on the geochemistry, not one due to primary sedimentary or post-diagenetic alteration influences. The nature of what condition or process might have encouraged enhanced silicification in the northwest can only be guessed at.
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Intro and Background Fieldwork Sample Prep Data Analysis PCA Correspondence Analysis Stepwise DA
Discriminant Analysis More PCA Element Trends Conclusions Bibliography Appendix A: Part 1 Part 2 Part 3 Part 4 Part 5 Appendix B Appendix C