LINKING ORBITAL SURFACE TEXTURES TO OUTCROP EXPRESSION AT GREENHEUGH PEDIMENT, GALE CRATER, MARS: MARS SCIENCE LABORATORY OBSERVATIONS OF THE UPPER SICCAR POINT GROUP

Significant environmental changes have occurred at Gale crater. Study of the sedimentary strata above the widespread erosional unconformity at the Mars Science Laboratory (MSL) field site provides insight into the local conditions and geologic history of the relatively recent past. To date, the Siccar Point group (SPg) is the youngest stratigraphic unit examined. Orbital-based geomorphic mapping of northwest Gale crater identified SPg in discontinuous draping exposures within a topographic depression over ~20 km². It unconformably overlies exhumed Mount Sharp strata. SPg rocks are cross-bedded aeolian sandstone through sol 2500. Greenheugh pediment (GP), the fan-shaped landform at the terminus of Gediz Valles, is the only location where SPg outcrops have been subdivided further than the upper Stimson formation. We describe the GP capping unit utilizing orbital and ground‐based images from two ascent sites.

The dominant surface texture is parallel ridges oriented E-W, ~45° askew to the northwestward GP surface gradient. Ridges extend ~100 m, have 10-15 m wavelengths, and exhibit shallow relief (<0.5 m). A secondary surface texture is NE-SW ridge segments (<5 m) that often connect upslope to the E-W ridges. In places both ridge components connect to form ridge-bound polygons.

Differences between these two ridge landforms are evident at the outcrop scale. In cross-section, the parallel ridges have thicker caprock at the apex that thins towards the lowland. Along the ridge crest are a mosaic of quasi-pyramidal rocks (<0.5 m across) that appear structureless from afar, but close-ups reveal cross-stratified rock faces. MAHLI images constrain grains to fine sand or smaller.

Ridge segment outcrops are heavily fractured, but the overall convex-hull shape is intact. They are comprised of planar, cross-stratified, medium-grained sandstone. Ridge segments consistently occur at lower elevation, apparently subjacent to the parallel ridges.

These differences suggest the uppermost GP is a composite landform with a stratigraphic architecture exposed through post-deposition modification, although the specific processes and sequence have yet to be identified. Interestingly, there are compositional similarities between alkaline-rich GP caprocks to Bradbury Rise rocks, possibly indicating common provenance.