Orbital Observations of a Marker Horizon at Gale Crater

A dark-toned, indurated, smooth horizon, and (or) bed (herein called "marker horizon") is exposed across much of the northwestern, southwestern, and southeastern portions of Mount Sharp in Gale crater. Compact Reconnaissance Imaging Spectrometer for Mars spectra taken from the marker horizon exhibit evidence for the presence of high calcium pyroxene and other basaltic minerals in contrast to the hydrated sulfate signatures associated with strata above and below it. Mean dips for the marker horizon and sulfate-bearing strata are 1-5 degrees with almost all azimuths radially away from the center of Mount Sharp. The marker horizon thickness varies between <1 and 3 m and its elevation changes by 1.6 km across the mound. Surface slopes along the sulfate-bearing strata above and below the marker horizon are typically 20 degrees-40 degrees compared to <5 degrees surface slopes on the marker horizon. Features observed on the marker horizon include ridges, fractures, faults, layering, oval depressions, small craters, and possible yardangs/indurated bedforms. We interpret the marker horizon to be a single unit that formed contemporaneously across Mount Sharp during the same depositional sequence that created the sulfate-bearing strata. Plausible formation mechanisms for the marker horizon include (1) emplacement of a more indurated sulfate unit, either from (a) primary deposition or (b) secondary diagenesis; (2) deposition of a more resistant sandstone unit during a brief drier period; (3) emplacement of a volcanic ash deposit laid down in the midst of the sulfate formation period; or (4) a lag deposit created during a drier period. Based on our observations, origins (1a) and (3) are the most plausible.