Abstract

Calcitization of dolomite in sedimentary carbonate sequences is often interpreted as a near-surface process reflecting either an erosional unconformity within a sequence or late, postburial weathering. In the Madison Group of the Western Overthrust Belt there is evidence of widespread dedolomitization. Petrographic, isotopic, and trace element analyses of Madison dedolomite indicate that multiple phases of calcitization have occurred in several different diagenetic environments. Isotopic trends in replacement calcite suggest progressive burial interrupted by tectonic thrusting, fracturing, and uplift. Burial dedolomite in the Madison Limestone is characterized by depleted oxygen (-8.0 delta 18O per thousand PDB) and carbon (-8.6 to 17.5 delta 13 C per thousand PDB) isotopic compositions relative to other replacive calcite phases. Introduction of depleted carbon into the system via hydrocarbon alteration is suggested by the depleted carbon isotopic composition of burial replacive calcite. Trace element data also indicate diverse origins for the various dedolomite phases. The burial-related dedolomite has low iron and manganese (Fe = 200 ppm; Mn = 30 ppm) and moderate strontium (Sr = 350 ppm) while other replacive components have higher iron and manganese and trace levels of strontium.

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