RED ROCK CONCRETIONS: GROUNDWATER RECORDS, SCIENCE RESOURCE, AND ANALOGS TO MARS

Concretions comprise an important record of diagenetic history that may not be readily apparent from the more abundant host rock. Numerous examples of concretions occur in many Colorado Plateau formations. Porous eolian, fluvial, or shoreline sandstones can contain concretions that form when iron-saturated reducing waters mix with oxygenated ground water to precipitate iron oxide as a cement, or from reduced iron (e.g. pyrite) concretions that later oxidize upon interaction with oxygenated groundwater. Concretions are typically spherical (like marbles) where the host rock is homogeneous and isotropic to fluid flow.

The expressions of variable iron oxide concretion geometries reflect the effects of iron and oxidant mobility with the influence of fluid conduits, diffusive mass transfer, and time. Abundant “marble”-sized concretions (a few mms to several cms) comprise an Earth analog for newly discovered small hematite concretions (“blueberries”) on Mars. The presence of hematite concretions implies saturated groundwater conditions, with variations in geochemistry over time. Variables include fluid composition, supply of Fe²⁺, supply and identity of the oxidant, and reactions with host rock mineralogy. The Grand Staircase-Escalante National Monument in particular has some of the most remarkable accumulations of abundant marbles as well as other variable concretion forms exposed in the Jurassic Navajo Sandstone. This example forms a unique and valuable geologic resource to be considered in park science, management, and outreach in addition to the terrestrial analog for planetary geology.