Master of Science (MS)
Geology and Geophysics
The dolomitization and diagenetic history of Ordovician carbonates of southern Wisconsin has been studied for over a century. Previous studies attributed dolomitization to various single or multiple diagenetic factors and environments. The goal of the study was to resolve arguments regarding dolomitization models, including Badiozamani’s often cited but recently questioned mixing zone model, using LA-ICP-MS focusing on REE to determine the nature of dolomitizing fluids. Analysis revealed that particulate material incorporated into the dolomite affected the geochemical results of many of the samples. Integrating geochemical data with petrographic evidence for diagenetic history, the studied Decorah Formation dolomites were assigned to two realms: shallow burial and hydrothermal. Shallow burial dolomites exhibit three distinct REE patterns. Dolomite from the middle portion of the Guttenberg formed inside a trilobite fossil maintained a seawater-like REE pattern, while dolomite in lime mud inside this fossil and dolomite in micrite from another sample in this interval exhibit LREE enrichment consistent with early burial. Carimona, Specht’s Ferry, and Lower Guttenberg dolomites are often burrow associated and exhibit MREE enrichment associated with Fe-oxide desorption in anoxic porewaters. The proximity of these dolomites to samples to K-bentonite beds is interpreted as having been the result of Mg leaching from the volcanic ash during alteration. Extensively dolomitized samples in the upper Guttenberg and Ion Member exhibit evidence of hydrothermal dolomitization. The relation of these heavily dolomitized samples to interbedded limestones provides evidence for a recently proposed hydrothermal dolomitization model invoking pressure solution of calcite and precipitation of dolomite. These early burial and hydrothermal depositional models are consistent with models proposed for overlying and underlying Ordovician dolomites. This study revealed no evidence of extensive dolomitization due to Badiozamani’s mixing zone model. Due to the location of this outcrop relative to the Wisconsin Arch, this study cannot directly disprove that dolomitization in eastern Wisconsin is the result of Badiozamani’s Dorag dolomitization. Despite this, Luczaj’s argument that the Dorag model should not have been widely applied to dolomites of the southern Wisconsin area seems appropriate.
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Callen, John Michael, "In Situ Geochemistry of Middle Ordovician Dolomites of the Upper Mississippi Valley: Evaluation of the Dorag Model and New Implications for Dolomitizing Fluids" (2016). LSU Master's Theses. 1941.