PALEODUNE DEPOSITS EXPOSED ON THE FLOOR OF MELAS CHASMA, MARS

Aeolian sediment transport and deposition have been ongoing throughout the history of Mars and may preserve clues about past environmental conditions. While rover investigations have established certain rock units were formed by aeolian processes, orbital data shows isolated landforms that meet the morphological criteria for sand dunes yet many appear eroded and inactive. Some of these occurrences are hypothesized herein to be bedforms constructed in aeolian depositional settings then subsequently preserved. This abstract describes landforms, termed “paleo-bedforms” – distinct groups of linear to crescentic, positive-relief features of a characteristic wavelength that show evidence for lithification – within Valles Marineris. For this task, we utilized High Resolution Imaging Science Experiment (HiRISE) images and derived topography.

Prime examples of candidate paleobedforms occur as dark landforms found along the floor of Melas Chasma. Here, several distinct clusters of broadly crescentic, positive-relief features are located that resemble barchan dunes, but also display superposed craters, boulders and fractures. These 133-207 m long, ~4-11 m tall features are spaced at 200-500 m wavelengths - parameters similar to many modern martian barchan dunes. Given the spatial distribution, consistent orientation and geometric parameters, these landforms are interpreted to be lithified aeolian sand dunes. It is estimated ~60-90% of the original dune morphology was preserved based on comparisons with contemporary dunes - this level of preservation is an uncommon circumstance on Earth where lee-dune faces may be fully eroded.

The presence and state of these paleodunes suggest a period of stabilization followed by cementation. A distinct, dark mantle unit that onlaps many of these areas and some paleodunes suggests a role in their burial and exhumation. Ground water upwellings following that burial might provide a pathway to lithification. The underlying Hesperian-aged layered deposits, some bearing hydrated sulfates, demonstrate it is likely some of these paleo-dune fields were deposited following that period. Other candidates higher along the walls show apparently stabilized duneforms without clear signs of cementation and may provide examples of an intermediate case. Additional geologic history scenarios will be discussed at the conference.