ABSTRACT

Most current models of dolomitization are incapable of explaining the genesis of dolostones that lack evidence of supratidal origin and are not associated with evaporites. These models require sea water evaporation and a high Mg²⁺/Ca² ratio in solution as essential factors for dolomitization. Calculations show that mixing meteoric ground waters with up to 30% sea water causes undersaturation with respect to calcite, whereas dolomite saturation increases continuously. Therefore, in the range of approximately 5 to 30% sea water, calcite can be replaced by dolomite. A new term "Dorag" (Persian for mixed blood or hybrid) is used for this model of dolomitization.

The Dorag model satisfactorily explains the origin of dolostones along positive elements or epicontinental shelves; it is based on the effect of ionic strength on the solubility of carbonate minerals, and does not require a Mg²⁺/Ca²⁺ ratio greater than one, a ratio obtained at calcite-dolomite equilibrium.

Dolomitic facies of the Middle Ordovician Series in southwestern Wisconsin and adjacent states is an example of dolostones associated with positive elements. Lithologic and paleontologic evidence, as well as primary structures, within the Mifflin Member (Platteville Formation) suggest the existence of a shallow and open-marine environment over the Wisconsin Arch at the time of deposition of the Member, whereas deeper water conditions prevailed farther from the Arch. Oxygen and carbon isotopic analyses of the Mifflin Member and chemical analyses for Sr and Na in the limestones and dolostones indicate exchange of the Mifflin carbonates with meteoric water. Because (a) evaporites, algal mats, mud cracks and other evidences of restricted environments are not found in the Mifflin Member, ( ) isotopic and chemical data suggest exchange of the Mifflin carbonates with meteoric water, and (c) the dolomite facies of this member is only associated with structural highs, the dolomitization of the Mifflin Member is best explained by mixing of sea water and ground water in the phreatic zone. Assuming Dorag dolomitization has been operative, three major lateral shifts in the dolostone-limestone boundary in the Champlainian Series can be shown to represent episodes of transgression-regression during the Middle Ordovician.

Conclusions can be drawn concerning (a) paleo-sea level fluctuation, (b) paleobathymetry, and (c) configuration and position of the paleo-ground water lens.