|
Diagenetic hematite concretions are common in the eolian Jurassic Navajo Sandstone in southern Utah (and some correlative units in Arizona and Nevada). The zones of alteration formed by structurally and stratigraphically influenced subsurface groundwater flow and localized iron oxide precipitation within porous sedimentary rocks. In many geologic systems on Earth, iron is a sensitive fluid flow indicator1. Mobilization and precipitation of iron oxides and sulfides requires specific variations in fluid chemistry. Precipitation of iron oxides in discrete concretionary zones further requires specific host rock characteristics. These characteristic color variations and zones of mineralization in the Jurassic Navajo Sandstone occur in a variety of cementation patterns with structural and stratigraphic relationships that have been well documented. Iron for the concretions is likely sourced internally from hematite grain coatings. Near surface, meteoric waters and processes of weathering commonly distribute disseminated iron films that impart a pink to orange-red color to the sandstone early in the depositional or burial history. The disseminated iron oxides are commonly mobilized and removed by reducing fluids, leaving the sandstone white. When these fluids mix with oxidizing groundwater in the Utah example, concentrated hematite precipitates, typically in the form of spherical balls. Many other concretion geometries commonly occur where anisotropy and preferential fluid flow pathways exist. Some of these shapes include pipes, sheets, bulbs, angular bricks, and repetitive bands. The differing geometries appear to be primarily a function of permeability barriers and pathways. Both sandstone coloration and the presence of hematite concretions (+/- other iron oxide minerals) record evidence of past fluid flow and reactions in subsurface sedimentary rocks. These are products of low-temperature, near-surface, hydrologic, chemical diagenetic reactions. Biomediation can also enhance the diagenetic precipitation of cements.
In addition to elucidating a complex history of fluid flow in Utah subsurface, analysis of these concretions can help us to better understand the recently discovered hematite concretions on Mars. The NASA Mars Exploration Rover (MER), Opportunity has discovered spherical nodules in Meridiani Planum, that have been identified to be predominately hematite in composition5,6. These Mars concretions bear a remarkable resemblance to hematite-cemented concretions in sandstones of southern Utah. Hematite is one of few minerals currently found on Mars that can be genetically linked directly to water-related processes7. Although the general process of chemical precipitation has been proposed, diagenetic concretionary precipitation, or ferruginization, has been previously overlooked as a potential formation mechanism. This terrestrial analog in Utah has important implications for biomediated precipitation and for subsurface and potentially atmospheric chemical conditions on Mars.
|
