DISCRIMINATING BETWEEN CAUSES OF CRATER AND BASIN INFILLING IN THE MARTIAN HIGHLANDS

Hundreds of craters in the Martian highlands are deeply filled, with relatively flat floors and no central peak [1-2]. In addition, late-Noachian and some mid-Noachian surfaces in the Martian highlands are typically associated with topographic lows (basins) [2]. The association with topographic lows suggests either fluvial and/or volcanism as the dominant resurfacing process [2], and the process likely varies from location to the next. For many of these infilled basins and craters, portions of the fill material are relatively free of unconsolidated sediment (assessed from thermal inertia and high resolution imagery) [2-3], allowing detailed assessment of composition and morphology of ancient fill materials. In this work, we document and interpret the spectral, thermophysical, and morphological properties of basin/crater fill materials in an effort to constrain the dominant and/or last fill process. This will allow us to characterize the degree of spatial, topographic, or geologic control on basin/crater fill type, which will greatly improve our understanding of how resurfacing processes may have varied as a function of space, time, or context. Our preliminary work shows that some solidified basin/crater fill materials are spectrally distinct from surrounding, higher-standing terrain, and exhibit embayment relationships or terraced margins, suggesting a volcanic origin. In other areas, crater fill materials lack spectral distinction from surroundings, and exhibit clear layering in exposed scarps, suggesting lithified sedimentary fill. One such example is Gringauz crater, located just west of Holden crater and southwest of Ladon basin. High-thermal inertia materials there are spectrally indistinguishable from the regional regolith and exhibit coarse layering. Interestingly, the fill materials also lack evidence of hydration in VNIR data. This example demonstrates that recognition of lithified sedimentary materials need not rely solely on spectral detections of hydrated minerals or fine-scale layering; enabling recognition of similar materials in this and future studies will help to place important constraints on the Martian sedimentary record. [1] Edwards et al., 2014, Icarus, 149-166 [2] Irwin et al., 2013, JGR-Planets, 118, 278-291 [3] Rogers and Nazarian, 2013, JGR-Planets, 118, 1-20.