Lunar and Planetary Science XLVIII (2017) 1513.pdf

GEOLOGIC MAPPING AND SPECTRAL ANALYSES OF THE SOURCE REGION OF SHALBATANA VALLIS, MARS. Daniel C. Berman, Catherine M. Weitz, J. Alexis Palmero Rodriguez, and David A. Crown, Planetary Science Institute, 1700 E. Ft. Lowell Rd., Suite 106, Tucson, AZ 85719; [email protected].

Introduction: We are currently producing a craters in the region. Melting of the ice led to the 1:500,000-scale USGS geologic map of MTM collapse of the infill material, resulting in chaos quadrangles 00042 and 00047 in the Xanthe Terra material 1 [e.g., 3]. The remaining infill material was region of Mars (2.5°S - 2.5°N, 310° - 320°E) (Fig. 1). swept away during resulting flooding and outflow, The map region has been extensively modified by carving out Shalbatana Vallis, and leaving behind outflow channels and chaotic terrains and contains vast chaos material 2 and smooth floor material. highlands deformed by subsidence [e.g., 1]. The Subsequent collapse and retreat of the crater walls led overarching goal of this project is to develop an to talus deposits on the walls and landslides. Some of understanding of how regionally integrated hydrologic the blocks on the floor of chaos material 2 may also be systems have been affected by (1) impact crater from wall collapse. Dune material was the last to form. formation, (2) melting of subsurface ice leading to the Flooding in Ravi Vallis inundated highland generation of subsurface cavity space and then surfaces above the margins of its grooved floor, where subsidence, and (3) erosional and depositional flooding the floods deposited widespread smooth deposits, events of various types and ages. locally marked by small-scale streamlined landforms. Mapping results: Geologic mapping is being These smooth floors flank the lower scoured sections conducted on a THEMIS IR daytime base mosaic, with and their origin might be related to earlier floods that CTX and HiRISE images as supplements where were not topographically constrained within a channel available. Provisional unit contacts and linear features and thus spread (and thinned out) over the intercrater have been drawn and preliminary units have been plains, thereby rapidly losing velocity. This scenario defined (Fig. 1). Units include two plains units, crater indicates a possible transition from non-catastrophic units, channel/canyon units, and chaos units (see Fig. 1 floods (depositional) to catastrophic (erosional) floods. legend for complete list). The boundaries of the plains Aromantum Chaos forms the source area of both a units are similar to those of Npl1 and Npl2 in [2]. The lava flow and an outflow channel. Episodic volcanic plains units are relatively smooth to mottled and are activity in the region could have provided high marked by secondary crater chains and wrinkle ridges. geothermal conditions conducive to groundwater The ejecta blanket of Orson Welles crater has been outburst leading to catastrophic flooding [4]. mantled and subdued by aeolian materials. Talus and Light-toned deposits: Light-toned deposits are landslide units cover the crater’s retreating walls, and observed in several locations on the floors of subsided chaotic terrain dominates its floor. Many of the crater terrain (smooth floor material and chaos material 2) and large channel units are characterized by collapse and craters. CRISM analyses of these deposits show and retreat. Shalbatana and Ravi Valles are evidence for Fe/Mg-smectites (Figs. 2 and 3). In characterized by smooth and chaotic floor units. addition, Figure 2 shows light-toned Fe/Mg-smectite Mapped linear features include crater rims, deposits along the upper wall of one of the craters, volcanic and fluvial channels, grooves, grabens, ridges, indicating the deposits predate the impact. These troughs, and scarp crests. We have also mapped results are consistent with the melting of subsurface ice surface features such as dark-colored ejecta and lenses, with water interacting with and altering secondary crater chains. All craters greater than 1 km subsurface layers to form smectites, which are later in diameter have also been mapped. exposed due to either impact crater formation or Geomorphic mapping of chaotic and subsided subsidence and collapse. terrains is proceeding, along with related extensional Conclusions: Geologic mapping, combined with faults south of Orson Welles crater, showing a zone of geomorphic and spectral analyses, show that subsurface evacuation reaching to Ganges Chasma. subsurface ice in this region melted, leading to Interpretations: Orson Welles crater contains evacuation, collapse, and flooding, leaving behind chaos materials types 1 and 2, smooth floor material, cavities that have led to continuing deformation and dune material, landslide material, and deformed collapse of surface units. material, with talus material along the walls. Chaos References: [1] K.L. Tanaka et al. (2014) USGS material 2 and smooth floor material continue into the SIM-3292. [2] S. Rotto and K.L. Tanaka (1995) USGS upper reaches of Shalbatana Vallis. Mapping reveals SIM I-2441. [3] J.A.P. Rodriguez et al. (2015) Nature the sequence of events. Following impact, the crater Scientific Reports, 5, 13404. [4] D.C. Berman et al. was filled with ice-rich material similar to other filled (2016) LPSC 47, #2674. Lunar and Planetary Science XLVIII (2017) 1513.pdf

Figure 1. Preliminary geologic map of MTM Quadrangles 00042 and 00047 at 1:500,000 scale. THEMIS IR daytime base. Transverse Mercator projection.

Figure 2. Perspective view showing a merge of CRISM color from image FRT00008ebf (red is olivine index, green is 1.9 µm band depth, and blue is 2.3 µm drop off) to a Digital Terrain Model of HiRISE images PSP_006598_1780 and Figure 3. Spectra for CRISM image FRT00008ebf, ESP_033326_1780. There are light-toned deposits both consistent with Fe/Mg-smectites, with a more Fe-rich along the crater rim and on the floor. smectite likely.