LOESSOID DEPOSITS IN ARGENTINA AND ON MARS

Loess covers Mars: from a thin veneer to more than 3km thick. Loess on Earth is defined as aeolian 20-50-micron dust, such as the thick deposits of China and Argentina. While often associated with glacial activity, the thick (200m-300m) Argentine deposits are derived from wind-blown silts (volcanic shards/minerals) that were fluvially transported from the Andes [Zárate, 2003]. The greater range of grain sizes (as well as numerous unconformities and paleosols), however, require calling these loessoid deposits. High cliffs expose generally flat-lying layered deposits containing resistant carbonate layers or nodules. Hence, loess or loessoid deposits need not implicate glacial processes or the nature of the source materials.

On Mars, the present polar-layered deposits should be considered loess (airfall of both dust and ice). Thick equatorial sequences in Arabia, Memnonia, and Medusa Fossae also resemble loessoid deposits: 1km-3km thick unconformable deposits covering the Noachian cratered highlands [Schultz and Lutz, 1988; Thomson et al. 2011]. Associated features closely resemble those with the polar-layered terrains: pedestal craters, unaffected exhumed terrains (craters), sinuous ridges, and relict deposits (such as crater-filled deposits in Nicholson, Henry, and Gale craters). Such similarities prompted the hypothesis that these antipodal deposits resulted from polar wandering during the Late Noachian [Schultz and Lutz, 1988]. The thickness, long duration, and presence of large pedestal craters within these deposits preclude an origin by brief periods of extreme obliquity. Since 1988, three separate paleomagnetic studies placed paleo-poles at nearly the same locations, while radar sounding found evidence for deeply buried ice beneath the Memnonia deposits [Watters et al., 2007]. Timing requires that ejecta deposits (glasses, breccias, blocks)- and even meteorites - would occur throughout the layers, but post-deposition fluvial transport resulted in antecedent drainage, mudstones, and concentration of poorly transported clastics. Such processes overprint the initial depositional sequence and complicate deciphering its origin as a purely loess (Grotzinger and Milliken, 2011). Hence, this process remains relevant, especially with the ongoing exploration by Curiosity Rover.