Preliminary oxygen isotope analysis of a mixed conodont assemblage from the Type Chappel Limestone (Tournaisian Stage, Mississippian subsystem), San Saba County, Central Texas




Sheaffer, Kristine Noel

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The Type Chappel Limestone is a condensed unit with conodont taxa from multiple biozones ranging in age from Ordovician to Lower Carboniferous. It is characterized by disarticulated crinoid fragments, and ranges from a few inches to several feet thick within and adjacent to sinkholes in the underlying Ellenburger Group. Previous conodont ecological models indicate that various genera inhabit different portions of the water column, and as such, amalgamation of the elements from different taxa can homogenize δ18OPO4 variability. Conodonts from the Type Chappel Limestone were collected in order to gather samples sizes large enough for mono-specific oxygen isotope analyses. Stable isotopes of phosphate were used to test whether A.) conodonts maintain their isotopic composition reflective of their paleoenvironment (δ 18O of seawater and temperature) during the life of the conodont or B.) the isotopic composition of the conodonts were reset or rehomogenized to reflect the deposition of the Chappel Limestone. Results from five different conodont species with affinities towards various faunal zones as described by Hass (1959) within the Type Chappel Limestone exhibit a slight variation in δ18O PO4 values, ranging from 21.18 to 23.74‰ V-SMOW. This indicates that current oxygen isotopic methods are robust and are not influenced by diagenesis.


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Conodont, Stable oxygen isotopes, Type chappel limestone, Geochemistry