M.S. (Master of Science)
Department of Geology
Conodonts||Geology, Stratigraphic--Ordovician||Geology--Middle West
Conodont assemblages from 8 sections of the Decorah Subgroup in the upper Midwest were examined to (a) establish the geographic distribution of species, (b) determine species groupings, and (c) test assemblage response to environmental changes represented by a regional lithologic gradient. Quantitative assemblage determinations were made on 76 samples (avg. # elements = 434). The lithologic transition was monitored with percent carbonate measurements. Factor analysis of the assemblage data revealed 3 independent factors which accounted for 92% of the total assemblage variance. Factor 1 shows an inverse relationship between ̲Pa̲n̲d̲e̲r̲o̲d̲u̲s̲ + D̲r̲e̲p̲a̲n̲o̲i̲s̲t̲o̲d̲u̲s̲ s̲u̲b̲e̲r̲e̲c̲t̲u̲s̲ and P̲h̲r̲a̲g̲m̲o̲d̲u̲s̲ u̲n̲d̲a̲t̲u̲s̲. Factor 2 shows a positive relationship among P̲l̲e̲c̲t̲o̲d̲i̲n̲a̲, O̲u̲l̲o̲d̲u̲s̲ s̲e̲r̲r̲a̲t̲u̲s̲, and P̲r̲a̲v̲o̲g̲n̲a̲t̲h̲u̲s̲ i̲d̲o̲n̲e̲a̲. Factor 3 shows a positive relationship among B̲e̲l̲o̲d̲i̲n̲a̲ c̲o̲m̲p̲r̲e̲s̲s̲a̲, D̲i̲s̲t̲a̲c̲o̲d̲u̲s̲ v̲a̲r̲i̲a̲b̲i̲l̲i̲s̲, and O̲i̲s̲t̲o̲d̲u̲s̲ p̲s̲e̲u̲d̲o̲a̲b̲u̲n̲d̲a̲n̲s̲. Factor 1 is independent of lithology and does not show a simple geographic pattern of species dominance. Factor loadings change considerably over short geographic distances, making water depth explanations for P̲a̲n̲d̲e̲r̲o̲d̲u̲s̲ + D̲r̲e̲p̲a̲n̲o̲i̲s̲t̲o̲d̲u̲s̲ and P̲h̲r̲a̲g̲m̲o̲d̲u̲s̲ patterns of distribution unlikely. There is an overall increase in Ekragmodus dominance upsection. Factor 2 shows a strong northwest to southeast trend perhaps best related to lithologic changes. In the upper Decorah the Plectodina assemblage is closely related to the shale sections in Minnesota. The assemblage is absent from the carbonate units of Wisconsin, Iowa, and Illinois. Factor 3 shows an inverse relationship to shaly lithologies, but is positively related only to certain carbonate units. The environmental response summarized in factor 3 is not related to water depth and is only weakly related to the lithologic transition.
Frederick, Daniel L., "Regional variations of conodont assemblages in the Decorah subgroup (Middle Ordovician) of the upper Midwest" (1987). Graduate Research Theses & Dissertations. 4684.
Northern Illinois University
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