Abstract
Abstract. Valleys with theater-shaped heads can form due to the seepage of groundwater and as a result of knickpoint (waterfall) erosion generated by overland flow. This ambiguity in the mechanism of formation hampers the interpretation of such valleys on Mars, particularly since there is limited knowledge of material properties. Moreover, the hydrological implications of a groundwater or surface water origin are important for our understanding of the evolution of surface features on Mars, and a quantification of valley morphologies at the landscape scale may provide diagnostic insights on the formative hydrological conditions. However, flow patterns and the resulting landscapes produced by different sources of groundwater are poorly understood. We aim to improve the understanding of the formation of entire valley landscapes through seepage processes from different groundwater sources that will provide a framework of landscape metrics for the interpretation of such systems. We study groundwater seepage from a distant source of groundwater and from infiltration of local precipitation in a series of sandbox experiments and combine our results with previous experiments and observations of the Martian surface. Key results are that groundwater flow piracy acts on valleys fed by a distant groundwater source and results in a sparsely dissected landscape of many small and a few large valleys. In contrast, valleys fed by a local groundwater source, i.e., nearby infiltration, result in a densely dissected landscape. In addition, valleys fed by a distant groundwater source grow towards that source, while valleys with a local source grow in a broad range of directions and have a strong tendency to bifurcate, particularly on flatter surfaces. We consider these results with respect to two Martian cases: Louros Valles shows properties of seepage by a local source of groundwater and Nirgal Vallis shows evidence of a distant source, which we interpret as groundwater flow from Tharsis.
Highlights
Valleys with theater-shaped heads exist in the landscapes of Earth and Mars
We consider these results with respect to two Martian cases: Louros Valles shows properties of seepage by a local source of groundwater and Nirgal Vallis shows evidence of a distant source, which we interpret as groundwater flow from Tharsis
We studied groundwater seepage processes and subsequent valley formation using a series of large sandbox experiments
Summary
Valleys with theater-shaped heads exist in the landscapes of Earth and Mars. On Mars, examples of such valleys are Louros Valles (Fig. 1a) and Nirgal Vallis (Harrison and Grimm, 2005). Much smaller examples that are similar in shape are valleys that emerge in eroding riverbanks (Fig. 1c) or those on the beach that develop during a receding tide (Higgins, 1982; Otvos, 1999; Fox and Wilson, 2010; Hagerty, 1991). Such theater-headed valleys can form by the seepage of groundwater in erodible sediment (e.g., Howard and McLane, 1988).
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