Dolomitization of the Madison Group, Wyoming and Utah Overthrust Belt

Abstract

Two stages of pretectonic dolomitization are recorded in the upper Madison Group of the Wyoming-Utah thrust belt. Marine to hypersaline dolomitization occurred during deposition of upper Madison carbonates and evaporites in peritidal and sabkha settings. This timing is supported by the common association of fine-grained, nonluminescent dolomite with restricted, shallow-water facies, and the relatively enriched isotopic composition of the dolomite (^dgr18O = +2.0 ^pmil; ^dgr13C = +4.0 ^pmil). A second stage of dolomitization began during exposure of the Madison shelf and continued through deposition of the overlying Amsden Formation. Regressions and transgressions during this time caused migration of marine-meteoric mixing zones across the Madison shelf and widespread dolomitization of all depositional facies within the Madison Group. The second stage of dolomitization is volumetrically dominant, has a meteoric isotopic composition (^dgr18O = -2.0 ^pmil; ^dgr13C = +2.0 to +7.0 ^pmil), and in most locations has masked the isotopic record of the first-stage, synsedimentary dolomite. In addition to replacive dolomite, dolomite and calcite cements were also formed during this diagenetic stage. These three components exhibit isotopic variations concomitant with stratigraphic position that reflect spatial gradients in ground water chemistry.