Date of Defense

4-3-2024

Date of Graduation

4-2024

Department

Geosciences

First Advisor

Stephen Kaczmarek

Second Advisor

Peter Voice

Abstract

It is thought that most Phanerozoic dolomites form via the replacement of precursor calcium carbonate [CaCO3] with dolomite [CaMg(CO3)2] through a dissolution-precipitation reaction with a Mg-bearing fluid. Although this may be widely accepted, the timing, conditions, settings, and mechanisms associated with the dolomitization process are still strongly debated, resulting in the proposal of various dolomitization models. The association between evaporite minerals and dolomite in the rock record is common and has led many to propose a model genetically associating evaporative fluids and dolomitization. The evaporative model posits as marine fluids evaporate and become more concentrated, gypsum [CaSO4·2H2O] precipitates, increasing the Mg:Ca ratio of the fluids. Dolomitization is favored as these dense Mg-rich brines infiltrate and flow downward through the underlying carbonates. Although this model is theoretically robust, it has been criticized, as evidence of downward refluxing brines is scarce.

In the current study, newly acquired petrographical (thin section, scanning electron microscopy), mineralogical (powder x-ray diffraction), bulk elemental (x-ray fluorescence), micro-scale elemental (SEM-energy dispersive spectroscopy), and conventional stable isotope data are used to evaluate test the evaporative reflux model in the evaporite associated dolomites of the informal “Massive Anhydrite” unit in the Iutzi Member of the Lucas Formation in the Michigan Basin. The studied interval (2117.5-2118.5 ft) in the Brown Snowplow #1-5, Alpena Co., Michigan is characterized by nodular-anhydrite crystals (10-20 micrometers) and small dolomite rhombohedra (<10 micrometers), which occur between anhydrite nodules and as inclusions within anhydrite nodules. Microcrystalline dolomites are isotopically light (δ18O = -5.88 ‰ VPDB) and formed prior to the precipitation of the gypsum/anhydrite characterized in the “Massive Anhydrite” unit. Collectively, these observations are inconsistent with the evaporative reflux model for dolomitization.

Access Setting

Honors Thesis-Open Access

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