Formation of bed‐scale spatial patterns in dolomite abundance during early dolomitization: Part I. Mechanisms and feedbacks revealed by reaction–transport modelling

Dolomites of varied ages exhibit metre‐scale nested patterns of lateral periodic variation in permeability and porosity and, by inference, dolomite abundance as most examples are 100% dolomite. Two‐dimensional reaction–transport modelling simulations of bed‐scale dolomitization were used to assess whether those patterns in dolomite abundance could form during near‐surface replacement dolomitization. Simulations used a 2 m high and 18 m long model domain, a low‐Mg calcite grainstone precursor and an evaporated Mississippian seawater brine (430 parts per thousand salinity) as the dolomitizing fluid. The domain was initially populated with random variations in porosity and/or grain size. Results reveal that spatial patterns in dolomite abundance emerge when there is as little as 1% dolomite formed, with similarities between the modelled patterns and outcrop‐documented patterns. The nested patterns include a near‐random component that constitutes ≤40% of the total variance, short‐range correlation ranging from 1·5 to 3·3 m and a longer‐range periodic trend with a wavelength up to 6·5 m. The emergence of pattern in dolomite abundance is the result of an autogenic self‐organizing phenomenon. It is triggered by variation in initial calcite reactive surface area that occurs due to the random heterogeneities in initial porosity and/or grain sizes. The pattern develops due to a combination of kinetic disequilibrium reactions (dolomite precipitation and calcite dissolution) and positive feedbacks between dolomite growth, calcite dissolution and fluid flow. Flow is around loci of higher dolomite, lower porosity and higher reactive surface areas, but through loci of lower dolomite, higher porosity and lower reactive surface areas. The resulting less porous/more dolomite and more porous/less dolomite structures at the metre‐scale arise from those localized interactions. This self‐organizing mechanism for pattern formation constitutes a new model for geochemical self‐organization during dolomitization and is the only self‐organization model that is proven applicable to the formation of metre‐scale patterns during early, near‐surface dolomitization.