Dune Forms and Ages and Associated Oblate Depressions in the Chasma Boreale Region of Planum

Introduction: THEMIS and MOC images acquired over the Chasma Boreale contain key geologic and stratigraphic information relating to aeolian deposition and the Polar Layered Deposits (PLD). Preliminary analyses of THEMIS VIS and MOC images show a clear progression of morphologic change of dark albedo dune deposits along the chasma axis. Geologic Relations: The source for the dunes appears to be moderately dark material, relatively high in sand-sized grains of the Olympia Undae unit as redefined by [1]. This unit overlies the thick layers of the Early Amozonian Rupes Tenuis unit and underlies the bright, finely layered PLD [1]. MOC images show the dark material being eroded from chasma wall layers that exhibit varying thickness. Here, small individual dunes with simple morphologies of domes and barchans are common. Farther down-chasma, where depositional processes have concentrated a larger volume of dark material, the dune morphology changes to reflect the effects of multiple wind directions and the availability of increased sediment [2]. Barchans with elongated horns and longitudinal dune forms begin to dominate. Even farther along the chasma axis, barchanoid-and transverse-ridge dunes are being exhumed from beneath the younger PLD, suggesting an pre-PLD age for the dune field. However, minor dune migration and emplacement and erosion of younger PLD's may have occurred multiple times in the chasma after initial emplacement of the dunes.Temperatures derived from THEMIS IR images suggest the presence of water-ice during the early spring in the chasma, surviving for several sols past the last sublimation period for CO 2 in the area [3]. Previous work [4], suggests that many Martian dunes exhibit over-steepened slip faces and cornices of aeolian material bound by subsurface water ice. Given the stratigraphic and geographic location of the older dune field in Chasma Boreale (Fig. 2d), it is likely the same process is at work on these dunes; i.e. paleo-dunes from a drier warmer climatic period have been frozen and buried beneath PLD and are now being exhumed. Dune form: The dune forms seen in the four MOC images of the Chasma Boreale region (Fig. 2), progress from simple to more complex in form and smaller to larger in size as one travels down-chasma (Table 1). This change reflects the