Orbital observations of the marker bed at gale crater

A dark-toned, indurated, smooth bed (here termed “marker bed”) is exposed across much of the northwestern, southwestern, and southeastern portion of Aeolis Mons in Gale crater on Mars. CRISM spectra of the marker bed exhibit evidence for the presence of high calcium clinopyroxene and other basaltic minerals, in contrast to the hydrated sulfate signatures that dominate the spectra of strata above and below the marker bed. Mean dips for the marker bed and strata above and below it are 2–5°, with almost all bedding azimuths oriented radially away from the center of Aeolis Mons. The thickness of the marker bed varies between <1 m to 3 m across the mound. The lowest elevation of the bed is in the northwest at –3835 m and the highest elevation in the southwest at –2240 m. While the surface slopes along the sulfate-bearing strata above and below the marker bed are typically 20–40°, the marker bed has surface slopes <5°, which is why it appears as a flat ledge that is a favorable surface to collect aeolian sand, airfall dust, and talus debris shedded from upslope. Features observed on the marker bed include ridges, fractures, grabens, layering, oval depressions, numerous small craters, and possible yardangs. We favor the interpretation that the marker bed represents a single unit deposited contemporaneously across Aeolis Mons during the same depositional sequence that created the sulfate-bearing strata. The most plausible formation mechanisms for the marker bed include the following processes: (1) emplacement of a more indurated sulfate unit relative to the less indurated sulfates above/below it (either from primary deposition or secondary alteration); (2) deposition of a more resistant sandstone unit during a brief drier period within Gale crater; (3) emplacement of a volcanic ash deposit laid down in the midst of the sulfate formation period; or (4) a lag deposit formed during aeolian deflation. The Curiosity rover is expected to traverse across ~150 m of the marker bed during the extended mission that will enable detailed in situ measurements to evaluate which, if any, of these hypotheses for its origins is supported.