Abstract
A major debate in Mars science is the nature of the early Mars climate, and the availability of precipitation and runoff. Observations of relict erosional valley networks have been proposed as evidence for extensive surface run‐off around the Noachian‐Hesperian boundary. However, these valley networks only provide a time‐integrated record of landscape evolution and thus the timing, relative timescales and intensity of aqueous activity required to erode the valleys remain unknown. Here, we investigate an ancient fluvial sedimentary system in western Arabia Terra, now preserved in positive relief. This ridge, ‘Aram Dorsum’, is flat‐topped, branching, ~ 85 km long, and particularly well‐preserved. We show that Aram Dorsum was an aggradational alluvial system and that the existing ridge was once a large river channel‐belt set in extensive flood plains, many of which are still preserved. Smaller, palaeochannel‐belts feed the main system; their setting and network pattern suggest a distributed source of water. The alluvial succession is up to 60 m thick, suggesting a formation time of 105 to 107 years by analogy to Earth. Our observations are consistent with Aram Dorsum having formed by long‐lived flows of water, sourced both locally, and regionally as part of a wider alluvial system in Arabia Terra. This suggests frequent or seasonal precipitation as the source of water. Correlating our observations with previous regional‐scale mapping shows that Aram Dorsum formed in the mid‐Noachian, making it one of the oldest fluvial systems described on Mars and indicating climatic conditions that sustained surface river flows on early Mars.
Highlights
Numerous Noachian‐aged terrains on Mars are dissected by systems of valleys incised into bedrock and are interpreted to have formed by fluvial erosion (e.g., Carr, 1987; Craddock & Howard, 2002; Hynek Beach & Hoke, 2010; Hynek & Phillips, 2001; Irwin & Howard, 2002)
We show that Aram Dorsum was an aggradational alluvial system and that the existing ridge was once a large river channel belt set in extensive flood plains, many of which are still preserved
We infer that the Aram Dorsum ridge does not record a single fluvial channel fill deposit but rather represents several channel fill deposits that have become vertically stacked during channel belt aggradation
Summary
Numerous Noachian‐aged terrains on Mars are dissected by systems of valleys incised into bedrock and are interpreted to have formed by fluvial erosion (e.g., Carr, 1987; Craddock & Howard, 2002; Hynek Beach & Hoke, 2010; Hynek & Phillips, 2001; Irwin & Howard, 2002). Williams deposits, interpreted as both alluvial and delta fans (e.g., Di Achille & Hynek, 2010; Fawdon et al, 2018; Goudge et al, 2017; Kraal et al, 2008; Malin & Edgett, 2003; Moore et al, 2003; Moore & Howard, 2005; Rice et al, 2013) Morphometric analysis of these fluvial systems suggests that the majority probably formed during the late Noachian (Hoke et al, 2011) or late Noachian to early Hesperian (e.g., Fassett & Head, 2008b; Howard et al, 2005; Irwin, Craddock, & Howard, 2005), in an arid to semiarid climate (e.g., Ramirez & Craddock, 2018), possibly supported by a multibar CO2 atmosphere (e.g., Jakosky et al, 2017). Davis et al (2019, 2016) presented evidence for an extensive network of Noachian age depositional fluvial channel belts, exposed in inverted relief, within erosional windows in the Arabia Terra region.
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