NEOGENE DRAINAGE RATES IN THE BIGHORN BASIN FROM JAROSITE 40Ar/39Ar CHRONOMETRY; A HYDROLOGIC ANALOG FOR MERIDIANI PLANUM, MARS

The Fort Union Formation in the southern Bighorn Basin contains multiple paleosols formed on floodplain deposits interbedded with sheet sandstones representing avulsion and aggradation of meandering rivers. The paleosols formed in overbank deposits as potential acid sulfate soils that largely remained saturated due to continued aggradation through the Paleogene that buried the Fort Union Formation beneath the water table. Redox conditions prevailed until Neogene excavation and drainage of the paleosols resulted in precipitation and preservation of varying amounts of jarosite, hematite and goethite.

We propose the Paleocene Fort Union Formation as a hydrologic analog for the Burns Formation at Meridiani Planum, Mars. The sequence of deposition, burial and saturation, and eventual drainage outlined for the Fort Union Formation resembles that proposed for the ‘wetting upwards’ Burns Formation. The Burns Formation has eolian sand at the base and fluvial/lacustrine interdune sediments at the top. The section hosts evaporite and secondary mineralization/alteration products (including jarosite, hematite, and goethite) likely reflecting saturation and subsequent water table retreat during aridification that prevails today. The saturation of Burns rocks during playa surface conditions is thus analogous to the burial of the Fort Union paleosols beneath the water table. Likewise, the downward retreat of the Martian water table is analogous to the Neogene excavation and drainage of the Fort Union Formation.

Jarosite-bearing hand samples were collected along a ~25 cm vertical profile from an upper Fort Union outcrop. Jarosite was dated by the ⁴⁰Ar/³⁹Ar method. From top to bottom, systematically decreasing Neogene jarosite ⁴⁰Ar/³⁹Ar ages are interpreted to record a downward migrating jarosite precipitation front. This may reflect the retreat of the water table (i.e. drainage) as this portion of the stratigraphic section was excavated in the Neogene. Linear regression through the age-depth profile indicates a drainage rate of 3.7 cm/Ma. Jarosite recovered from future Mars sample return missions may be used to constrain the timing, duration, and rate of water drainage from the Martian surface.