The stratigraphy of Meridiani Planum, Mars, and implications for the layered depositsʹ origin

Large exposures of water-altered layered deposits have recently been identified on the surface of Mars. The source materials, formation, and aqueous alteration history are presently poorly understood. The Mars Exploration Rover Opportunity has examined a tiny fraction of large-scale layered deposits in Meridiani Planum, and many questions remain about the origin and history of these widespread materials. Here we present the first detailed stratigraphic study of sulfate-bearing layers throughout the region. We used altimetry data to examine the three-dimensional disposition of twenty-two distinct stratigraphic horizons along their exposures. Our results show that most of these benchmark horizons: (1) are planar and coherent over at least a 100-km scale, and (2) have dip azimuth and magnitudes that are similar to the underlying regional slope, which was emplaced by 3.70 Ga. Nearby ancient river valleys, that appear to have been formed by precipitation-fed surface runoff, originally were incised at ~ 3.74 Ga and then reactivated near the Noachian–Hesperian boundary (3.70 Ga). Mapping relations with these valleys suggest that the Meridiani layers formed near and after this time and without significant volumetric contributions of material excavated from the valleys. Thermal infrared data and erosional expressions imply that significant physical compositional differences exist within the stratigraphy, and these likely reflect a changing paleodepositional environment and/or chemical alteration histories. Any hypothesis for the origin of these regional-scale materials must be consistent with all these observations. We conclude that the previously stated hypotheses of aeolian deposition cemented by a fluxing groundwater table and sulfur-rich volcanic processes are both viable possibilities, while other hypotheses are not supported by these observations.