PETROLOGICAL EVIDENCE OF EARLY DOLOMITIZATION FOLLOWED BY NODULAR GYPSUM-ANHYDRITE IN THE LUCAS FORMATION, MICHIGAN BASIN, USA

It is well accepted that most Phanerozoic dolomites are replacive, meaning that they form when a calcite or aragonite [CaCO₃] precursor is replaced by dolomite [CaMg(CO₃)₂] via a dissolution-precipitation reaction in an Mg-bearing fluid. However, the timing, conditions, settings, and mechanisms associated with this diagenetic process are strongly debated, resulting in many proposed dolomitization models. The common field association between evaporite minerals and dolomite has led many researchers to propose a model in which dolomitization is driven by evaporative fluids. That is, the evaporative model predicts that gypsum and dolomite co-precipitate in evaporative fluids. This model has received recent criticism because certain predictions have not been observed.

In the current study, newly acquired petrographical (thin section, scanning electron microscopy), mineralogical (powder x-ray diffraction), bulk elemental (x-ray fluorescence), and micro-scale elemental (SEM-energy dispersive spectroscopy) data are integrated to evaluate the dolomite-evaporite association in the Lucas Formation of the Michigan Basin (Brown Snowplow #1-5, Alpena Co., Michigan). Core observations and petrographical analyses show that the studied interval (2117.5-2118.5 ft) is characterized by nodular-anhydrite crystals (10-20 µm) and small dolomite rhombs (<10 µm). Dolomite crystals occur as inclusions within the anhydrite nodules, suggesting that the dolomite formed relatively early, prior to gypsum/anhydrite precipitation. Detailed mineralogical and geochemical data further support this interpretation. To further constrain the chemistry of the dolomitizing fluids, conventional isotope analyses are planned.