EVIDENCE FOR GEOCHEMICAL SELF-ORGANIZATION AT THE DECAMETER SCALE DURING DOLOMITIZATION

There has been negligible analysis of the attributes of dolomites in the lateral dimension at the decameter scale. To fill this void, we sampled, at a 1-ft spacing, 433 ft and 495 ft lateral transect through a dolowackstone and dolograinstone of the Mississippian Madison Formation. Porosity and permeability were determined on all samples. Trace and major element chemistries were quantified for purified dolomite powders of the dolowackestone. The results show periodic oscillations in all attributes except Ca and Mg. Variogram analyses define three correlation structures: 1) A near-random component that is shorter than the 1-ft sampling interval and accounts for ~50-60% of the variance in a variable. 2) A short-range structure on the order of 10-25 ft that is reflected in increasing variance at separation distances of < 25 ft on the variogram. 3) Low-magnitude, long-range oscillatory features at scales of 25-30 ft for porosity, Mn, and Fe, 45 ft for log permeability, and 70 ft for Sr and Na. The long-range oscillatory features account for ~10-20% of variance in an attribute. The fact that both chemical and physical properties exhibit oscillatory structures indicates that the origin of the pattern is not an inherited depositional characteristic. Rather, we suggest that the oscillatory patterns are the product of geochemical self-organization during dolomitization. Self-organization is the ability of a system to select one pattern from all possible patterns and amplify it into a well-ordered observable structure. Limestones always contain textural heterogeneities that can be selected during fluid flow, and then enlarged by nonlinear feedback loops during dolomitization. If the reaction front is planar, the product might be a series of wave fronts. More likely is that the reaction front fingers and meanders. This in turn can lead to reaction-induced flow diversion, the re-focusing of the flow past the reacted mass, and formation of a 3D spot pattern in dolomite attributes. In either case, a single lateral transect will yield the oscillatory structures observed. With numerical modeling, it maybe possible to use the oscillatory structures to define paleo fluid-flow vectors at the scale of decameters, the migration rate of the dolomitizing front, and the thickness of flow units within the dolomitizing system.