ANATOMY OF AN ANCIENT EOLIAN SANDSTONE ON MARS: THE STIMSON FORMATION IN GALE CRATER

Since landing in 2012, the Mars Science Laboratory (MSL) Curiosity rover has traversed the plains of northern Aeolis Palus to the foothills of Aeolis Mons (informally, “Mt. Sharp”) investigating the stratigraphic record of Gale crater. Recently, the Curiosity team has been investigating the Stimson formation, a sandstone exhibiting abundant crossbedding. The contact between the Stimson and underlying Murray formation exhibits several meters relief over several hundred metres where encountered thus far. The Stimson onlaps this contact, indicating that accumulating Stimson sands unconformably buried local palaeotopography. Facies and architectural elements observed within the Stimson are interpreted to represent deposition in an eolian dune field. The Stimson is typically composed of decimeter-scale to meter-scale crossbedded sandstones, (exhibiting wind-ripple lamination and well rounded particles up to granule size). Some areas close to the basal contact include a variety of other facies, including sandstones containing Murray-formation clasts and granular sandstones. Architectural elements are visible in outcrops oriented perpendicular to the regional northwest dip. Here cross-stratified bedsets are commonly bounded by undulating surfaces with lateral extents up to 30 m. These undulating surfaces are interpreted as possible deflationary supersurfaces that formed in response to deflation or regional dune-field stabilisation. Surfaces inclined relative to the regional dip ascend between supersurfaces towards the northeast at an angle of 3-4°. These surfaces are interpreted to be dune-bounding surfaces, which are preserved when dunes climb as a result of dune-field aggradation. Aggradation of the system during the duration of the dune field’s existence possibly occurred as a response to episodic increases of sediment supply into the basin, allowing dunes to climb and preserving the basal parts of the dunes. The angle and orientation of bounding surfaces and trough crossbedding within the Stimson sandstone suggest that the dunes migrated to the northeast across a northwest dipping regional slope.