IMPACT CRATERS AS STRATIGRAPHIC MARKERS: CONSTRAINTS ON THE GEOLOGIC EVOLUTION OF DEUTERONILUS MENSAE, MARS

With the availability of new high-resolution datasets for Mars, ages of individual impact craters can be estimated from counts of small craters superposed on their ejecta blankets and used as local stratigraphic markers. Along the dichotomy boundary in the Deuteronilus Mensae region, ages derived for impact crater deposits are being used to provide important temporal constraints on the formation of the highland-lowland boundary, development of fretted terrain, and emplacement of lobate debris aprons. Deuteronilus Mensae exhibits a variety of geologically young, ice-related features, including ice-cemented mantling deposits, lobate debris aprons, concentric crater fill, and lineated valley fill. Preliminary results using Mars Reconnaissance Orbiter Context Camera (~5 m/pixel) images for ejecta from Cerulli crater (112 km diameter; 22.0°N, 32.2°E) in Arabia Terra and an unnamed crater (22.3 km diameter, 23.4°N, 42.1°E) on the lowland plains indicate that populations of both superposed and partially buried/modified craters are evident. The size-frequency distribution for fresh craters in the size range ~750 m - 4 km superposed on part of the Cerulli ejecta blanket shows a stable Hesperian surface with depletion of craters at smaller sizes. The ejecta blanket of the unnamed crater north of the dichotomy boundary is partially covered by a lobate debris apron extending from an adjacent mesa. The size-frequency distribution for this crater ejecta is consistent with an impact event in the Late Hesperian to Early Amazonian and provides a distinct lower constraint on the emplacement of the debris apron. The size-frequency distribution for the debris apron surface suggests a Middle Amazonian age, consistent with the observed stratigraphy but significantly older than deposits associated with recent obliquity-driven Martian climate change. Depletion of small craters (<250 m) on debris apron surfaces is more extensive than on ejecta and crater floor deposits, consistent with their interpretation as ice-rich flow features.