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Using Fan Facies Models to Predict the Location of a Mudstone in Gale Crater, Mars


A geomorphically defined alluvial fan extends from Peace Vallis on the NW wall of Gale Crater, Mars into the Mars Science Laboratory (MSL) Curiosity rover landing ellipse. Prior to landing, >30 MSL team members each mapped 1-4 1.4x1.4km “quads” from the ellipse and surrounding areas, using georeferenced HiRISE images and DEMs acquired from orbit. The map was compiled into a single GIS project with ongoing remapping of units. Map relationships suggest that bedded rocks east of the landing site are likely associated with the Peace Vallis alluvial fan, which led to the decision to send Curiosity east. Curiosity's Mastcam color images were used to refine local map relationships and predict alluvial facies distributions as these beds came into view. Bedded rocks were mapped as two units prior to landing, but were divided into 4 members of the Yellowknife Bay formation. The lowest member (Sheepbed) consists of fractured bedrock with an upper recessive mudstone unit. It is overlain by regionally continuous, decimeter-thick sandstone beds of the Gillespie Lake member. The overlying Point Lake member is of unknown origin, and the upper Shaler member contains cross bedded fluvial sandstones. The Sheepbed and Gillespie Lake members consist of fine-grained and medium-coarse grained alluvial facies, as predicted by map relationships. The Sheepbed member extends laterally at least 60 meters and forms a recessive interval below the Gillespie Lake member with several cm-scale interbeds. Tracing of the distinctive erosional profile of the Sheepbed-Gillespie Lake contact in orbital images shows that the upper recessive part of the Sheepbed member likely covers a minimum area of ∼4 km2. A vertical reference section across the Sheepbed-Gillespie Lake contact yields a thickness of >150 cm for the recessive interval. Grain sizes were determined from MAHLI images, which showed that most grains are >60 μm in size, demonstrating that the recessive interval is a mudstone. Its very fine grain size, regional distribution, and significant lateral extent of interbeds all suggest deposition of the primary grains due to settling from suspension. Thus, mapping relationships, regional context, and local sedimentologic constraints all point to a distal alluvial fan or proximal lacustrine setting for the Sheepbed member.