Palaeoflow reconstruction from fan delta morphology on Mars Maarten G. Kleinhans a, ⁎, Hester E. van de Kasteele b , Ernst Hauber c a Faculty of Geosciences, Department of Physical Geography, Utrecht University, PO-Box 80115, 3508 TC Utrecht, The Netherlands b De Ontdekkingsreis, Public Primary School, PO-Box 23, 3970 AA Driebergen, The Netherlands c DLR-Institut für Planetenforschung, Rutherfordstrasse 2, D-12489 Berlin-Adlershof, Germany abstract article info Article history: Accepted 11 November 2009 Available online 9 December 2009 Keywords: fan delta alluvial fan sediment transport model 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. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Many alluvial fans and fan deltas have been identified on Mars over the past decade as the image resolution increased (Grin and Cabrol, 1997; Ori et al., 2000; Cabrol and Grin, 2001; Malin and Edgett, 2003; Moore and Howard, 2005; Fasset and Head, 2005; Irwin et al., 2005; Mangold and Ansan, 2006; Di Achille et al., 2006b; Weitz et al., 2006, and references therein). Deltas and fans form when a water– sediment mixture enters a sudden expansion. Thus, they record past hydrological conditions in various manners. The most obvious is their total volume, which must have taken a certain amount of time to be deposited, with the reasonable assumption that a delta or fan is a perfect sediment trap. The formation time can be calculated as the ratio of the volume and a volumetric sediment transport rate. But this ignores the information recorded in morphology, which is considered quantitatively in this paper. To calculate sediment transport rate, modelling of the flow is required. Input parameters are derived from geometry and gradient of the feeder channel upstream of the fan or delta. In the flow modelling, the friction is a rather crucial problem that can be overcome by careful study of bar height and terrace height in the data to infer water depth, as well as general hydraulic roughness predictors that include gravitational acceleration, surface particle size and the potential for bed states such as ripples, dunes or antidunes (Wilson et al., 2004; Kleinhans, 2005). An order of magnitude estimate of the transport rate itself can be made with sediment transport predictors that have been nondimensionalised carefully (including gravitational accelera- tion, of course) (Komar, 1979; Kleinhans, 2005). Such analyses yielded the surprising result that the fan deltas of Mars formed in very short periods of the order of days to decades (Kleinhans, 2005; Kraal et al., 2008; Hauber et al., 2008). By implication, flowing water Earth and Planetary Science Letters 294 (2010) 378–392 ⁎ Corresponding author. E-mail addresses: m.kleinhans@geo.uu.nl (M.G. Kleinhans), hester@de-ontdekkingsreis.nl (H.E. van de Kasteele), ernst.hauber@dlr.de (E. Hauber). URL: http://www.geog.uu.nl/fg/mkleinhans (M.G. Kleinhans). 0012-821X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2009.11.025 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl