RED ROCK DIAGENESIS: FROM EARTH TO MARS

Red to white sandstones of the Colorado Plateau show a history of diagenesis and iron-oxide mobility on local millimeter to regional kilometer scales on Earth, and provide an important possible analog to the new discovery of hematite concretions on Mars. In permeable Jurassic Navajo Sandstone of southern Utah, coloration from iron oxides (e.g., hematite, goethite) is an index to (1) the character of the fluids which caused both precipitation (oxidizing) and bleaching (reduction), and (2) the fluid pathways. Most sandstones were likely originally red early in the diagenetic history from small amounts of disseminated hematite. Bleaching by reducing solutions later mobilized and removed iron on small scales (e.g., along eolian laminae, deformation bands, and lithologic contacts), to regional formation scales over tens of kilometers. Flow paths are controlled permeability, lithology, sedimentology, stratigraphy, and structure. When reduced waters carrying the iron meet and mix with oxygenated ground water, iron precipitates in a variety of cemented concretions. Spherical concretions are typically formed through a self-organizing process.

Hematite is one of few minerals found on Mars that can be linked directly to water-related processes. This terrestrial analog of iron concretions requires groundwater mechanisms to produce the distributed spherical morphology and hematite composition, with later weathering to accumulate the aprons of loose spheres. Based on this analog, hematite concretions on Mars similarly require permeable host rock, groundwater flow, and a reaction front, although the chemistry, iron source and mobility, and host rock compositions of the system may differ considerably. The potential role of biomediation in the precipitation of some terrestrial hematite concretions could also hold important clues in the search for life on Mars.

A solid foundation of sedimentary petrology can link “color-coded” diagenesis to geochemistry, hydrology, basinal fluid flow, hydrocarbon exploration, and the future frontiers of planetary geology.