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首页> 外文期刊>Planetary and space science >An overfilled lacustrine system and progradational delta in Jezero crater, Mars: Implications for Noachian climate
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An overfilled lacustrine system and progradational delta in Jezero crater, Mars: Implications for Noachian climate

机译：火星Jezero陨石坑中湖水系统和过度发育三角洲的过度充填：对诺亚式气候的影响

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摘要

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~2 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 open-basin 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 basin-fill thickness suggest timescales of the order of 10~6-10~7 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 106-107 years) suggest longer times than previously suggested (years to centuries) by sediment transport models, but generally relatively short durations from a geologic perspective.
机译：Noachian晚期山谷网络和流域湖泊的存在被认为是液态水陆上流动的证据，因此火星早期的气候可能支持了降水和径流。悬而未决的问题集中在这种气候的性质，持续时间和变异性及其成因上。开放盆地湖泊，其内部形态及其相关通道为解决这些问题提供了证据。我们综合了陆上开放盆地湖泊，三角洲环境和河流系统的广泛知识，以评估这些问题，并从Jezero火山口，直径45 km的开放盆地湖泊及其15,000 km〜2集水区，-645 km长的排水网络得到了证据，内部沉积相和-50 km长的出水通道系统。我们记录了广泛的滚动条和epsilon交叉床上用品的存在，两者都表明在冲积扇上没有观察到蜿蜒的分布通道，但它们是河流三角洲沉积环境的典型特征。三角洲复合体的形成后侵蚀进一步支持了河流三角洲环境：当今的“三角洲前缘”实际上是一个侵蚀陡峭的悬崖，它通过富含粘土的三角洲环境截断了三角洲平原特征，这是根据相模型预测的。在复杂的外部三分之一处，已被风蚀侵蚀大部分除去。与滚动输入和蒸发相关的不断变化的基础水平不断上升和下降的环境（在低地势时有利于切口）相比，通过滚动条的横向积聚和ε交叉垫层的广泛发展以及重构的沉积构造表明基础水平是稳定的。出口通道的发展被解释为在杰泽罗开放流域湖泊中提供了基础控制。与大坝冲刷河道典型的灾难性冲刷景观相反，出口河道的成熟度有利于古古柏碱环境持续一致的水文学。其他火星上的河谷网络和相关沉积物的泥沙运移模型研究表明，其持续时间在数十年至数百年的范围内。我们回顾了陆地河流环境中的曲折迁移率，为考虑滚动条演变所隐含的时间稳定性提供了比较。对于Jezero观测所隐含的结构和迁移，20-40年的值并不罕见。结合各种陆地河流-湖泊环境的沉积物沉积速率，以及我们对沉积盆地填充厚度的估计，我们得出的时间尺度大约为10〜6-10〜7年，远大于某些沉积物传输模型所暗示的时间尺度，但在地质上仍是很短的时间。杰泽罗（Jerzero）中存在大量剩余的容纳空间（可用于潜在的沉积物蓄积的空间），表明沉积物向湖泊的输送在盆地完全充满之前就终止了。需要足够的气候条件来填充山谷网络中持续的陆上水流，以填充Jezero火山口，以非灾难性的方式破坏该火山口，并形成一个重要的河流-三角洲环境，从而横向移动河流河道和基本稳定的基本水平。缺乏后期河道缩减的情况表明，导致陆上水流向流域的条件可能已经突然结束。我们对河流活动持续时间（106-107年左右）的估计表明，沉积物运移模型的时间比以前建议的时间长（数年至几个世纪），但从地质学角度来看，持续时间通常相对较短。

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