GREEN RIVER FORMATION EVAPORITES AS A TEXTURAL ANALOG TO MERIDIANI PLANUM SEDIMENTS

From a petrographic perspective, sedimentary strata at the Opportunity landing site and at Endurance Crater have a close analog in the Eocene Green River Formation that occurs in portions of Colorado, Utah, and Wyoming. Green River Formation evaporites were deposited in a playa lake complex with episodically dry lake basins. Outcrops of evaporitic facies (e.g. Wilkins Peak Member) in the Green River Formation contain abundant crystals and crystal molds of alkaline-earth carbonates (Shortite, Trona) and Halite that grew in surficial sediments. The observed rock textures closely resemble sedimentary rocks from Meridiani that have elongate dissolution vugs after evaporite minerals.

Unlike the Meridiani sediments that appear to have been deposited under acidic conditions, evaporites in terrestrial playa lake and dune-interdune settings are usually characterized by alkaline waters and precipitation of alkaline-earth carbonates. This difference, however, has no fundamental bearing on the physical processes that operate in the environment. Playa flats can be dominated by crusts of dolomite and alkaline-earth carbonates (Green River Formation) or by sulfate crusts (White Sands, Meridiani). In both cases desiccation leads to fragmentation of these crusts, providing detritus that can be transported during floods or by winds. In the Green River Formation this resulted in flat pebble conglomerates, carbonate sands, and calcareous mudstones. In the case of Meridiani primarily impure evaporitic sandstones were produced that show evidence of transport by wind as well as water. Thus, even though diagenetic environments differed significantly, examining Meridiani sediments from a Green River perspective still allows us to ask pertinent questions with regard to sedimentary features, stratigraphic relationships, and processes that we might encounter with continued study of these strata.