Palaeoflow reconstruction from fan delta morphology on Mars

Kurzfassung

Alluvial fans and deltas on Mars record past hydrological conditions. Until now these conditions have been inferred from the morphology of the feeder channels and the deposits from images and digital terrain models (DTMs), and from calculations of the bulk fluxes of water and sediment based on the dimensions of upstream channels. Neither method can distinguish between dilute (river-like) flows and dense (sediment-laden) flows, however, while the formation time scales for these two sediment transport modes differ by orders of magnitude. The objective of this paper is to compare DTM data quantitatively with a morphological model to infer sediment transport mode and formative duration. We present a quantitative morphological model for fan and delta formation that assumes as little as possible. The model calculates the growth of a sedimentary body in a crater lake, represented by a low-gradient, subaerial cone on top of a high-gradient, subaqueous cone. The volume of the cone is constrained by the influx of sediment while the elevation of the break in slope, that is, the shoreline, is constrained by the influx of water. The water and sediment fluxes were calculated with physics-based predictors based on the feeder channel. Small-scale morphology, such as crater wall irregularities, concavity of the fan surface and channel avulsion, is ignored. The model produces alluvial fans, stair-stepped fan deltas and Gilbert fan deltas as well as hitherto unidentified crater wall drapes. The parameters that determine which morphology emerges are the supply of sediment and water to the basin, the size of the basin and the duration of the flow. A direct comparison between the cone model and HRSC DTM data for five deltas and an alluvial fan demonstrates that single-event dilute flows of short duration (days to years) have created all of the deposits. Two Gilbert fan deltas were formed in overspilling crater lakes from long low-gradient upstream channels. One alluvial fan was formed in a similar manner except that the damaged crater did not lead to ponding. Three stair-stepped deltas were formed from short high-gradient upstream channels that only partially filled the crater lakes.