Terraced Cutbanks and Longitudinal Bars in Gully Channels on Mars: Evidence for Multiple Episodes of Fluvial Transport

Introduction: First identified by Malin and Edgett [1], gullies are among the youngest and most enigmatic geomorphic features on Mars. Originally hypothesized to be the result of confined aquifer breakouts, they have subsequently been interpreted to be the result of dry mass flows (e. shown persuasively that the global distribution of gullies is preferential. The greatest concentrations of gullies are observed in the mid-latitudes (~30-50°) of both hemispheres where gullies exhibit a poleward orientation preference (>80% in the southern hemisphere survey of Dickson et al. [11]) and are found at a variety of elevations (-5177 to 3089 m). Finally, gullies are found exclusively on steep slopes, typically 20-30° [11,12]. The Mars Orbiter Camera with a typical resolution of 1.5-m/pixel [13,14] allowed for the definition of gully alcove source regions, channel transport areas, and depositional aprons, but did not allow detailed geo-morphic studies of these components. The High Resolution Imaging Science Experiment (HiRISE) [15] now enables detailed sedimentary geomorphic investigation (e.g., [16,17]); we use these new data to analyze in-channel sedimentary features, which can provide evidence regarding the style of transport in gully systems. HiRISE Observations: Drawing on the survey results outlined above, we focus on the 30° to 45°S latitude band where gullies are abundant. Approximately 175 HiRISE observations of gullies in the survey area are contained in the first two PDS releases examined. Channel Morphology: The channel portion of a gully system is a region of transport in which sediment from the alcove source region passes through prior to deposition in the gully fan. As such, sedimentary structures in the channels (Fig. 1) can provide information on the prevailing style of transport and markers of significant events that are not recorded in the (eroded) alcoves and may not be visible in the fans. The presence of terraced cutbanks (terraces) and longitudinal bars (Fig. 1) suggest multiple distinct episodes of flu-vial transport in these channels. The low preservation potential of these high-slope, steep-sided, non-lithified features suggest that they are very latest Amazonian in age. Consideration of these structures in comparison to terrestrial analogs for proposed gully formation mechanisms suggests a discrimination between the proposed hypotheses.