Recent exploration has revealed extensive geological evidence for a water-rich past in the shallow subsurface of Mars. Images of in situ and loose accumulations of abundant, haematite-rich spherical balls from the Mars Exploration Rover 'Opportunity' landing site at Meridiani Planum bear a striking resemblance to diagenetic (post-depositional), haematite-cemented concretions found in the Jurassic Navajo Sandstone of southern Utah. Here we compare the spherical concretions imaged on Mars to these terrestrial concretions, and investigate the implications for analogous groundwater-related formation mechanisms. The morphology, character and distribution of Navajo haematite concretions allow us to infer host-rock properties and fluid processes necessary for similar features to develop on Mars. We conclude that the formation of such spherical haematite concretions requires the presence of a permeable host rock, groundwater flow and a chemical reaction front.

Colorful outcrop exposures of diagenetic iron oxide, clay, and carbonate cements in the Jurassic Navajo Sandstone reflect a multi-phase history of fluid-rock interactions. Characteristic mineralogical and geochemical variability occurs on microscopic and outcrop scales throughout southern Utah. We identify six common diagenetic facies and evaluate formation mechanisms, paragenetic relationships, and relative timing between alteration events. The diagenetic facies have distinctive visual characteristics, variations in mineralogy, major oxides, trace elements, and carbon and oxygen isotopes. They include red and bleached sandstone, diffuse and concretionary iron oxide precipitates, and carbonate concretions. Development of these facies requires changes in the interstitial fluid environment and an open geochemical system with basinwide fluid flow and variations in redox conditions. Spatial and temporal changes in paleohydrologic and diagenetic conditions indicate complex coupling and feedbacks between stratigraphic architecture, fluid flow, and basin evolution.

Spectacular color variations in the Lower Jurassic Navajo Sandstone reflect stratigraphic and structural control on the spatial distribution of fluid-driven alteration. Field observations and supervised classification of Landsat 7 Enhanced Thematic Mapper (ETM؉) satellite imagery show that the most extensive regional bleaching of the Navajo Sandstone occurs on eroded crests of Laramide uplifts on the Colorado Plateau in southern Utah. Alteration patterns suggest that the blind reverse faults that core the eastern monoclines associated with these uplifts were carriers for hydrocarbons and brought the buoyant fluids to the crests of monoclines and anticlines, where they bleached the sandstone in both structural and stratigraphic traps. The extent of bleaching indicates that the Navajo Sandstone (Navajo Sandstone, Aztec Sandstone, and Nugget Sandstone) may have been one of the largest hydrocarbon reservoirs known. Rapid incision and breaching of this reservoir during Tertiary uplift and erosion of the Colorado Plateau could have released enough carbon into the atmosphere to significantly contribute to global carbon fluxes and possibly influence climate.

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