Dolomite and dolomitization in the Middle Devonian Winnipegosis Formation, North Dakota

Abstract

The Middle Devonian Winnipegosis Formation in North Dakota is composed of limestones, dolomitized limestones and dolostones. The Lower Winnipegosis Member was deposited basin-wide as a relatively uniform shallow marine ramp under normal, open marine conditions. The Lower Winnipegosis Member represents two distinct depositional environments: (1) a central deep basin with scattered pinnacle reefs and the coevally deposited deep-marine sediments of the Ratner Member and (2) peripheral carbonate shelf deposits. Evaporite deposits of the Prairie Formation eventually filled the basin and completely buried the Winnipegosis Formation. Six different dolomite types, five anhydrite types, dedolomitization, a late diagenetic calcite and a ferroan dolomite event were identified to have formed over three diagenetic stages. The synsedimentary diagenetic stage includes Idio-Ep dolomite and anhydrite type 1 forming in supratidal patterned dolostones. Most of the mosaic dolomite types (Idio-E, Idio-S, Xeno-S and Xeno-A) formed in the early diagenetic stage by seepage reflux and to a smaller extent by mixed-water dolomitization. The late diagenetic stage includes the formation of ISO dolomite, saddle dolomite, dedolomitization, a late diagenetic calcite and a ferroan dolomite event in a deep burial setting. Results from $\rm \delta\sp{18}O$ and $\rm \delta\sp{13}C$ analyses of dolomite in the Lower Winnipegosis Member indicate that Middle Devonian sea water was an important component of the dolomitizing fluids in the Lower Winnipegosis Member. Geochemical and petrographic data of dolomites from the Upper Winnipegosis and Ratner members indicate that dolomite has been recrystallized. A narrow range of $\rm \delta\sp{18}O\sb{dol}$ values, uniform luminescence and generally higher concentrations of Fe and Mn in rims of crystals than in cores indicate that dolomite recrystallization occurred late in the diagenetic history of the Winnipegosis Formation, after or at the same time as the ferroan dolomite event.