Fan-shaped deposits in the northern Hellas region, Mars: Implications for the evolution of water reservoir and climate

Abstract

Hellas basin is the largest and deepest depositional sink in the southern highlands of Mars. The region to the north of the Hellas basin is characterized by a variety of fan-shaped sedimentary deposits, which record the history of aqueous activities and climate change. We newly identified 20 potential deltas, 134 alluvial fans, 42 debris flows, 137 glacier-related fans, and 121 other fan-shaped landforms including landslides. The accumulation of liquid water-related fans along the northeastern flanks of the Hellas basin is likely controlled by elevation. The focus of this study is the potential deltas and alluvial fans for which detailed geomorphic analysis was carried out. The sediment fans sit mainly within closed basins and their feeder valleys are immature, indicating a short-lived activity of water. We observed two cases of a younger fan system creating an incision through an older sediment fan, indicating a punctuated reduction in the activity of water, in which the duration of the interfluvial period was assessed. We dated the sediment fans and found that they were mainly formed in the Hesperian and Early Amazonian and underwent glacial resurfacing around 700 Myr ago. Only a few fans formed in the Late Noachian were identified. Based on our observations, we submitted snowfall as the main source of water for these sedimentary fans although, as seen in the two cases of alluvial fans, a few of them could also have been fed by small-scale glacial melt. Both water sources supported cold and dry spells. We concluded that the fans record a persistence of water/ice activity until the Middle Amazonian in Martian history.