46th Lunar and Planetary Science Conference (2015) 2538.pdf AQUEOUS DEPOSITS RELATED TO FORMATION OF HALE CRATER IN SOUTHERN MARGARITIFER TERRA, MARS. J. A. Grant1, and S. A. Wilson1, Smithsonian Institution, NASM CEPS, 6th at Independence SW, Washington, DC, 20560 ([email protected]). Introduction: Hale crater (35.70S, 323.60E) is a 137 km-diameter impact structure that formed near the Amazonian-Hesperian boundary  or in the early-tomiddle Amazonian . The formation of Hale destroyed the probable head of the Uzboi-Ladon-Morava meso-scale outflow channel and blocked the southern end of the Uzboi Vallis segment of the system. During and after the impact event that formed Hale, fluidized debris flows and channels sourced near the crater rim incised or modified the ejecta deposit . Mapping reveals these water-driven flows  extend considerably further north than previously mapped, reaching the floor of Uzboi Vallis just south of Nirgal Vallis (Fig. 1) [3-5]. Fig. 1. Southern Margaritifer Terra north of Hale crater (centered on 30.50S, 323.50E) shows the extent of Hale flow deposits (light teal) beyond the Hale ejecta (yellow along bottom right side) . Cratered uplands and plains are brown, valleys are purple, and lacustrine and ponded water deposits are green. North towards top. Expression of Late Flow Deposits: The deposits associated with the Hale impact vary in expression from relatively thin veneers along newly incised segments and within pre-existing valleys and topographic lows (Fig. 2) to thicker lobes where materials accumulated behind and below topographic constrictions (Fig. 3). Incised reaches and streamlined forms are sometimes present downstream of constrictions that appear to have slowed and partially blocked the flows [4, 5]. In other locations, close to two crater diameters north of Hale, the material ponds and shallowly embays local relief in a large crater. The material deposited by the flows is darker-toned than bounding surfaces and is mostly smooth and featureless at scales of 10’s to 100’s of meters. The deposits embay Hale secondary craters and appear to generally thin northward (distally). Fig. 2. Hale deposits (Ahs) within pre-existing valleys and topographic lows in the Noachian Highlands and Terra units (Nh and NHt). North towards top. Fig. 3. Hale deposits (Ahs) accumulating in and below a topographic constriction in Nh. North towards top. Thicker flow lobes have distinct margins and capping ridges are typically oriented perpendicular to the flow direction . Where exposed at flow fronts, crosssections show no obvious layering and deposits are often crossed by fractures (Fig. 4). Despite an overall morphology that sometimes appears broadly similar to volcanic flows, the local erosion of the digitate, distal 46th Lunar and Planetary Science Conference (2015) margins produces isolated remnants by what appears to be deflation due to the absence of bounding erosional deposits (Fig. 4). This implies a significant finegrained component: there are few boulders or other coarse fragments observed with the exception of the tops of some thicker flow lobes. Fig. 4. Digitate ends of Hale deposit in Fig. 3. Erosion and absence of associated deposits implies deflation of constituent fines. North towards top. Timing Relative to Other Regional Events: One of the last, regional events in southern Margaritifer Terra prior to the formation of Hale crater was the occurrence of a 4000 km2 lake within Uzboi Vallis . Hale flow deposits superpose the Uzboi lake deposits and in the southern reaches of Uzboi extend across a degraded crater where a series of meter-scale bedforms occur . These may represent primary depositional structures or later eolian reworking of the Hale deposits . If the former, the bedforms imply water depths greater than the scale of the structures was present locally over a crater diameter from Hale. The Hale deposits were the last water-driven activity in the region. Origin of the Deposits: Because most deposits occur within topographic lows and some can be traced back to the channels emerging from the Hale ejecta, their origin appears related. There are more isolated occurrences where the connection to Hale is not obvious (Fig. 1). The apparent absence of layering in flow fronts points to their emplacement during a single event Nonetheless, pulses of flow cannot be ruled out. Emplacement likely occurred shortly after the Hale impact based on the embaying relation between the deposits and Hale secondary craters. The water-driven flows north of Hale occurred near the Amazonian–Hesperian boundary or even later when 2538.pdf climate was likely cold and characterized by mostly below freezing temperatures , thereby implying rapid freezing would be expected. Almost half the distance traversed by the flows, however, was across warm Hale ejecta that would have slowed or even precluded freezing . Additional water sourced from the Hale ejecta may have contributed to discharge. Once beyond the ejecta, freezing at the surface of the flows likely created an insulating layer of ice that enabled the underlying water-sediment mix to continue downslope . Freezing may also have occurred at the bottom of the flows, but constrictions and steeper grades likely created higher velocities and the local incision observed. Slower moving, locally thicker accumulations (Fig. 3) could have developed a thicker, more rigid ice crust that fractured to create observed capping boulders. Continued cooling and further contraction caused additional fracturing. Once halted, the water in the flows could sublime away, leaving behind the thin deposits observed in many locales. Gradients along the depressions traversed by the flows are often relatively low and some reaches include local relief the flows would have had to overcome. Two relatively long reaches trending SSW-NNE just west of Bond crater (Fig. 1) have average gradients of slightly less than 10 m/km. Coupled with the long runout, this suggests flow viscosities and velocities were relatively low, characterized by transport rates that may not have exceeded several meters/second. If correct, these velocities are broadly consistent with the inference that the flows carried mostly fine-materials [11, 12], perhaps aided by freezing at the bed that limited entrainment of large fragments from all but the locally steepest reaches. Nevertheless, such velocities would enable the flows to reach two crater diameters from Hale in a day or two, thereby implying they could relate to very short-lived conditions and events. References:  Cabrol et al., 2001, Icarus, 154, 98–112, doi:10. 1006/icar.2001.6661.  Jones et al. (2011), Icarus, doi:10.1016/j.icarus.2010.10.014.  Wilson et al., (2013), Abs. Ann. Mtg. Planet. Geol. Mappers, Washington, DC.  Wilson et al. (2013), LPSC Abst. 2710.  Wilson et al., (2013), Abs. Ann. Mtg. Planet. Geol. Mappers, Flagstaff, AZ.  Wilson et al. (2015) LPSC Abst. 2492.  Grant et al. (2011), Icarus, doi:10.1016/ j.icarus.2010.11.024.  Carr (2006), The Surface of Mars, 307 pp., Cambridge Univ. Press, Cambridge, UK.  Mangold (2012), PSS, doi:10.1016/j.pss.2011.12.009.  Gregg and Greeley (1993), JGR, 98, 10,873-10,882.  Burr et al. (2006), Icarus, doi: 10.1016/j.icarus.2005.11.012.  Baker et al. (1988), Flood Geomorphology, 503 pp., Wiley Press, NY.
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