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Formation of Lateral Patterns In Rock Properties By Dolomitization: Evidence From A Miocene Reaction Front (Bonaire, Netherlands Antilles)
Petrophysical and geochemical properties of dolomites may exhibit a nested set of lateral patterns, typically at the scales of a few meters to ∼ 20 m. This study assesses whether those lateral patterns are inherited from the limestone precursor or formed during dolomitization. Sampling was done across a preserved limestone-to-dolomite reaction front in grainstones of the Miocene Seroe Domi Formation, Bonaire, Netherlands Antilles. Post-dolomitization diagenetic overprinting in the two rock types is absent or minimal. Two ∼ 65 m lateral transects were drilled at 30-cm spacing in two separate beds. Porosity, permeability, and geochemical (δ18O, δ13C, Mn, Sr, and Na) analyses were performed on all 287 recovered samples, and 287 thin sections were point-counted for petrographic attributes.
Variography shows that dolomite abundance and porosity contain three scales of lateral variability. Much of the signal is random sample-to-sample noise as ∼ 65% of the total spatial variance occurs at 30-cm spacing. However, porosity and dolomite abundance also exhibit a short-scale correlation length of 3.7 m and a long-range oscillatory pattern (hole effect) at ∼ 16 m, both of which are also periodic. Trace elements (Mn, Sr) and permeability exhibit short-range correlation to 5.8 m, but no long-range pattern. Mole %Mg and δ13C show increasing variance with distance, and all other dolomite attributes exhibit no spatial patterns. No limestone attributes show evidence for any short-range or long-range oscillatory patterns. This indicates that the lateral patterns in dolomite abundance, porosity, permeability, Sr, and Mn contents were not inherited from the limestone precursor. Dolomitization is interpreted to be the cause of the observed spatial patterns as that is the only geologic process that has affected the dolomite but not the limestone on the other side of the front. Geochemical self-organization during the dolomitization process is suggested to be the mechanism by which the spatial patterns in the dolomites emerge. A model for that self-organizing process is proposed that involves positive feedbacks between precipitation, dissolution, textural properties, reaction rates, carbon transport, and the evolving flow field.
Relative to Mississippian, Permian, and Eocene dolomites that have been studied previously, the Seroe Domi dolomites have similar porosity variance at 30 cm spacing and hole effects with slightly greater wavelengths and much larger magnitudes. Any differences are interpreted to be related to diagenetic history. The older dolomites have been affected by burial diagenesis and/or weathering, which would likely increase sample-to-sample variability, therefore increasing short-scale randomness, decreasing the magnitude of hole effects, and shortening wavelengths of hole effects. The lateral patterning formed by dolomitization may thus not be as strongly expressed in ancient dolomites that have complex burial histories.
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