Environmental Transitions Recorded by Fluvial Fan Stratigraphy at Dingo Gap and Moonlight Valley, Gale Crater, Mars

The Mars Science Laboratory (MSL) Curiosity rover investigated a number of sedimentary outcrops across Aeolis Palus. Observations of conglomerates, cross-bedded sandstones, and mudstones suggest that fluvial and lacustrine environments were common. Outcrops that expose cross-sections are particularly helpful for identifying stratigraphic relationships and changes through time. In the vicinity of an outcrop informally named Dingo Gap, the drive strategy shifted from driving on the topographic high areas to the topographic low areas, which resulted in a unique vantage point to observe stratigraphy in cross-section. During Sols 513-541, Curiosity investigated 4 m of stratigraphy exposed at Dingo Gap and Moonlight Valley. The valley walls reveal five distinct sedimentary facies, including 1) a fine-grained evenly laminated facies interpreted as upper plane bed stratification, 2) ripple and dune cross-stratified sandstone facies, interpreted to represent the southward migration of subaqueous bedforms, 3) fully preserved bedform sandstone facies, formed during high rates of deposition, 4) planar-bedded granule-rich sandstone facies, interpreted as sheet flood deposits, and 5) a weakly stratified, poorly-sorted conglomerate facies, interpreted to represent rapid deposition from a high-energy fluvial flow. The conglomerate facies is unlike other conglomerates observed thus far in the mission, on the basis of both texture and chemistry. Analysis of conglomerates reveal that they occur as distinct channel bodies, incised into cross-stratified sandstones. This coarsening upward signature is interpreted to record a prograding fan succession. Channel bodies appear to be time-equivalent, which suggests a major change in the system, likely associated with a drop in base level. The unique viewing geometry offered by Dingo Gap and Moonlight Valley makes it possible to observe these environmental changes.