Temporary Ponding of Floodwater in Athabasca Vallis, Mars

Introduction: Athabasca Vallis (Fig. 1) is the youngest known outflow channel on Mars, dated by crater counting at 2-8 Ma [1,2]. In-channel geomor-phic features indicative of flooding include streamlined forms, longitudinal lineations, and transverse dunes [1,2]. The streamlined forms have been hypothesized to be primarily depositional, i.e. composed of sediment deposited out of the floodwater behind in-channel obstacles such as impact craters or resistant knobs of bedrock. This hypothesis is based on 1) the difference in morphology between the flatter streamlined form surfaces and the rougher surface of the surrounding terrain, and 2) the strong prevalence of obstacles at the streamlined forms' upslope ends [1,2]. Examination of MOC and MOLA data during hydraulic modeling of Athabasca Vallis showed evidence consonant with this idea of the streamlined forms being depositional. This evidence suggests their formation during a period of slow flowing or temporary ponding of the floodwater in the channel. Hydraulic modeling of Athabasca Vallis: The upper reach of Athabasca Vallis was modeled using a type of step-backwater model commonly used in terrestrial engineering and paleoflood applications [e.g., 3]. For paleoflood application, the user inputs into the model representative channel cross-sections and a resistance coefficient (Manning's n), and then enters a hypothetical value for the instantaneous discharge. By adjusting this value so that the resultant modeled water height matches the geologic indications of the actual floodwater height, the user arrives at the best estimate for the paleoflood discharge. Geologic indicators of floodwater height: The geologic indicators in this study had to be identifiable in MOC images for which there are corresponding MOLA tracks, so that individual MOLA data points were available for use in the hydraulic model. Six such indicators were found, two of which are shown in Fig. 2. Fig. 2a shows the northern edge of the channel; longitudinal lineations created by fluvial flow are located to the south, and higher, uneroded terrain is visible to the north. The paleo-floodwater elevation is interpreted to be at the boundary between the lineated and unlineated terrain, at an elevation of approximately –2528 m. Fig. 2b shows a streamlined form with a minimum highest elevation of the downstream portion as –2529 m. If this downstream portion is primarily deposi-tional as hypothesized, the floodwater must have been at least as high as the top of the form. Even if the form