Origin of the size and spatial distribution of iron-oxide concretion derived from the image analysis and numerical simulation

Abstract

Spherical iron-oxide concretions are found in the Jurassic Navajo Sandstone in Utah, USA. Based on image analysis of more than a thousand concretions, we found that: (1) the spatial distribution resembles a random distribution, with deviation from the random distribution occurring as concretion clusters; (2) the spacing between concretions is much larger than the concretion sizes; (3) the width of the size distribution increases with increasing concretion size; and (4) the volume fraction of concretions is constant, irrespective of size. These characteristics can be explained by repeating cycles of CaCO₃ dissolution and precipitation, the precursor material of Fe-oxide concretions. In numerical simulations, the width of the size distribution did not depend on the input parameters, including pH and supersaturation.