FLOOD GEOMORPHOLOGY OF THE EASTERN VALLES MARINERIS REGION OF MARS

Hesperian-age aqueous activity is well preserved in the Valles Marineris region of Mars, particularly in the Eos-Capri and Aurorae Chaos basins. This integrated basin system likely formed due to collapse of highland terrain that coincided with sudden release of groundwater. Here, we attempt to quantify the aqueous history of the region by examining catastrophic outflow channels that drain the basins using high resolution imagery (CTX), terrain models (HRSC), and crater count statistics. There is abundant geomorphic evidence for turbulent flood erosion in all channels including flood grooves, knickpoints, streamlined islands, and bedrock terraces. The Eos-Capri outflow system includes Daga Valles and Columbia Valles which exhibit near complete preservation of flood surfaces. Eos Chasma, which drains eastern Eos Chaos, shows evidence for fluvial erosion on its northwestern wall only. Furthermore, the base of Eos Chasma is covered by a younger, Amazonian-age smooth fill unit. The outlet elevation for all channels that drain the Eos-Capri basin is 1000 m, suggesting a strong topographic control on flooding. Downstream of Eos Chasma a similar chaos-channel system is present named Aurorae Chaos and Aurorae Chasma, respectively. Flood surfaces at Aurorae Chasma are preserved on the southeastern wall and exhibit similar geomorphic features to Eos Chasma, as well as an extensive smooth unit that covers the channel floor. Using the morphometry of terrace intervals within all bedrock channels, we estimate a range of possible volumetric discharge values (using the Darcy-Weisbach steady flow equation) from 10⁷ to 10⁹ m³s^-1 for each flood event. The size frequency distribution of impact craters on the bedrock channel floors indicate that the outflow events post-date the chaotic basins while the smooth floor units significantly post-date the channels. The timing relationships suggest that water may have ponded in the km deep basins during the period leading up to flooding.