A study in Ordovician K-Bentonites of North America and Europe


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ISBN :
978.625.7049.29.0
Yayınevi :
Berikan Yayınevi
Basım Yeri / Yılı :
ANKARA / 2020
Sayfa Sayısı :
164
Ebat :
14 cm X 21 cm
Fiyatı :
Sorunuz..
The Ordovician marine successions in eastern North America and northwestern Europe contain numerous widespread ash beds now altered to potassium rich clay beds called K-bentonites. Biostratigraphical, geochemical, isotopic and paleogeographic data suggest that the Millbrig K-bentonite, a widespread isochron and marker bed in North America is the same as the Kinnekulle K-bentonite in Baltoscandia (Huff et al. 1992). Regional biostratigraphic studies and discriminant function analysis of whole rock trace element data were important tools in establishing this correlation.
Trace element geochemistry shows that the ash for both beds was derived from a felsic calc-alkalic magmatic source characteristic of volcanism in a destructive plate margin setting. Also within the resolution of current graphic correlation and biostratigraphic techniques it is understood that the Millbrig and Kinnekulle beds occur in the same chronostratigraphic position.
The Millbrig-Kinnekulle beds and the associated Deicke K-bentonite in North America define narrow time-equivalent sequences över wide areas, making possible the sedimentologic, paleontologic and geochemical study of the contained interval. In addition, they permit the high-resolution evaluation of regional tectonic events on extinction rates among various taxa.
Haynes et al. (1995) suggested that the utility of the discriminant function analysis of whole rock compositions, which are obtained by neutron activation analysis for the correlation of K-bentonite beds, is valid for stratigraphic correlation on a local scale but not for large scale regional or global correlation. They maintained that uncertainty results from the variable mobility of majör and certain trace elements during diagenesis, and may result in regional shifts in bulk composition. Haynes et al. (1995) studied the compositions of volcanically generated biotite phenocrysts that survived diagenesis and concluded that they are more reliable correlation tools than bulk rock composition even though long distance correlation based on phenocryst compositions can be stili suspect for various reasons. Biotite which is one of the principal ferromagnesian silicate phenocrystic minerals in calc-alkalic tephras produced by large volume volcanic eruptions was investigated, and Haynes et al. (1995) found that there is a compositional difference between Kinnekulle and Millbrig biotites. Specifıcally, Millbrig biotites contain more MgO and less TiO2 than Kinnekulle biotites. So although Millbrig and Kinnekulle K-bentonites are coeval within the limits of isotopic ages, biozone stratigraphy, tectonomagmatic setting and whole rock chemistry according to Huff et al. (1992), Haynes et al. (1995) fbund that there is significant compositional differences in biotite phenocrysts suggesting they represent separate eruptive events.
However, Haynes et al. (1995) failed to evaluate within-bed variation in biotite composition because they used data from only one Millbrig site in North America and one Kinnekulle site in Baltoscandia. It is proposed here to test the conclusions of Haynes et al. (1995) by establishing within-bed compositional variability for biotite in Millbrig and Kinnekulle beds and then applying statistical tests of significance.



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