SPHEROIDAL IRON (OXYHYDR)OXIDE CONCRETION FORMATION: NEW INSIGHTS FROM FIELD STUDY

Two primary models for iron (oxyhydr)oxide concretion formation have emerged based on the stunning examples within the Jurassic Navajo Sandstone in the Spencer Flat area of Grand Staircase Escalante National Monument in southern Utah, USA. The first model originally developed by Chan and others employs two fluids where iron is mobilized as Fe²⁺ and then reprecipitated when the iron-bearing fluid meets and mixes with an oxidizing fluid in a near stagnant water table. The second model, developed by Yoshida and others, hypothesizes that concretions are initially precipitated as calcium carbonate, that is dissolved by an acidic Fe³⁺-bearing fluid. The input of carbonate into the fluid buffers the fluid enough to precipitate iron oxide in the same morphology as the original calcium carbonate precursor.

This research tests these models using extensive field observations performed throughout the region and extending south. Field observations include the presence of calcium carbonate and iron (oxyhydr)oxide concretions interspersed within the same stratigraphic horizons. Most of the morphologies are very different between the two mineralogies and iron (oxyhydr)oxide concretions have a diverse range of internal morphologies not observed in the calcium carbonate examples. Calcium carbonate concretions south of Spencer Flat are clustered along horizons suggesting early diagenetic precipitation at the top of a water table. This type of clustering is not observed in the iron (oxyhydr)oxide precipitates.

Field comparison of differing concretion mineralogies show the variability of fluid chemistries throughout geologic time and illumine the complexities of the diagenetic history of this porous and permeable unit. The Yoshida model does not appear to be supported by field relations in the larger region, but the Chan model does remain a viable explanation for the formation of the iron (oxyhydr)oxide concretions. Additionally, precipitation rates are likely a large factor in the formation of the two different mineralogies as calcium carbonate precipitates faster than iron (oxyhydr)oxide. In this region, the calcium carbonate concretions likely precipitated much earlier and more rapidly than the iron (oxyhydr)oxide concretions. In contrast, the iron concretions suggest near stagnant fluid conditions for a considerable amount of time to develop the complex diversity of interior morphologies observed within the region.