Back Down to Earth: Reconstructing the Timescale of a Catastrophically-formed Fan from Sedimentary Data

Abstract:

Alluvial fans and deltas on Mars, collectively termed fans, are formed by the transport and deposition of sediment by running water. Sediment transport models have been used to calculate the timescales of fan formation on Mars using orbital or rover-based imagery. These models provide a constraint on the total volume of water involved which has major implications for understanding the hydrologic history of early Mars. However, while the robustness of sediment transport models are not in doubt, the usefulness of their application to fans on Mars still requires testing using a terrestrial system of similar type, scale and possibly origin. The 1918 volcanically-generated catastrophic jökulhlaup (glacier outburst flood), southern Iceland deposited a ≤2 km3 fan over a known observed timescale of 6-8 hr. Predicted timescales using standard sediment transport models are in excellent agreement with the known timescale. This close agreement between observed and predicted timescales was arrived at only because key parameters related to the 1918 jökulhlaup are well known and tightly constrained from ground-based observations of the sedimentology and sedimentary architecture. Crucially, the latter provided independent estimates of hydraulic parameters. This work highlights the need for more detailed stratigraphic and sedimentary analysis of units on Mars (and Earth), which can provide constraints on the nature of water flow (magnitude, frequency, intermittency) and on hydraulic parameters. Such information cannot be gleaned from the final snapshot of surface geomorphology using remote imagery.