An overfilled lacustrine system and progradational delta in Jezero crater, Mars Implications for Noachian climate

The presence of valley networks and open-basin lakes in the late Noachian is cited as evidence for overland flow of liquid water and thus a climate on early Mars that might have supported precipitation and runoff. Outstanding questions center on the nature of such a climate, its duration and variability, and its cause. Open basin lakes, their interior morphology, and their associated channels provide evidence to address these questions. We synthesize the extensive knowledge of terrestrial open basin lakes, deltaic environments, and fluvial systems to assess these questions with evidence from Jezero crater, a 45 km diameter open basin lake and its 15,000 km catchment area, 645-km long drainage network, interior sedimentary facies, and 50 km long outlet channel system. We document the presence of extensive scroll bars and epsilon cross-bedding, both indicative of meandering distributary channels that are not observed on alluvial fans but are typical of fluvial-deltaic depositional environments. A fluvial-deltaic environment is further supported by the post-formational erosion of the deltaic complex: the present-day ‘‘delta front’’ is actually an erosional escarpment truncating delta plain features with the clay-rich prodelta environment, predicted from facies models to make up the outer third of the complex, having been largely removed by eolian erosion. The extensive development via lateral accretion of scroll bars and epsilon cross-bedding, and the reconstructed sedimentary architecture suggest a stable baselevel, in contrast to an environment of constantly rising and falling baselevel related to variable input and evaporation that would favor incision during lowstands. The development of the outlet channel is interpreted to have provided baselevel control in the Jezero openbasin lake. The maturity of the outlet channel, in contrast to the catastrophically scoured landscapes typical of dam-breach channels, favors a consistent overfilled hydrology for the paleolacustrine environment. Sediment transport modeling studies of other valley network and related deposits on Mars have suggested durations in the decades to centuries range. We review meander migration rates in terrestrial fluvial environments to provide a comparison for considering the temporal stability implied by the evolution of scroll bars; values of 20–40 years are not uncommon for the structures and migration implied by observations in Jezero. Taking sediment accumulation rates from a variety of terrestrial fluvial-lacustrine environments in conjunction with our estimates of the sedimentary basinfill thickness suggest timescales of the order of 10–10 years, far longer than implied by some sediment transport models, but still a short period of time geologically. The presence of significant residual accommodation space (space available for potential sediment accumulation) in Jezero indicates that sediment transport into the lake terminated before the basin was completely filled. Climate conditions sufficient for sustained overland flow of water in the valley networks are required to fill Jezero crater, to cause its breaching in a non-catastrophic manner, and to form the significant fluvial-deltaic environment of laterally migrating fluvial channels and scroll bars formed with an apparently stable baselevel. The lack of late-stage channel downcutting suggests that the conditions producing overland flow of water into the basin may have ended abruptly. Our estimates of the duration of fluvial activity (of order 10–10 years) suggest longer times than previously suggested (years to centuries) by sediment transport models, but generally relatively short durations from a geologic perspective. & 2012 Elsevier Ltd. All rights reserved. ll rights reserved. þ401 863 397. [email protected] (S.C. Schon). S.C. Schon et al. / Planetary and Space Science 67 (2012) 28–45 29