Multiple working hypotheses for the formation of compositional stratigraphy on Mars: Insights from the Mawrth Vallis region

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• 作者：Joseph R. Michalski ; P.B. Niles ; J. Cuadros ; A.M. Baldridge
• 关键词：Mars ; Infrared observations ; Mineralogy ; Geological processes
• 刊名：Icarus
• 出版年：2013
• 出版时间：September-October, 2013
• 年：2013
• 卷：226
• 期：1
• 页码：816-840
• 全文大小：22655 K

文摘

A unique aspect of martian geology is the presence of similar compositional stratigraphy observed in many locations throughout the surface. Where the Al-rich and Fe/Mg-rich clay minerals occur together, aluminous clays typically overly layered ferromagnesian clays, possibly indicating that precipitation-driven leaching occurred in a warmer, wetter climate. Sulfates generally occurring stratigraphically above clays could be relics of a global environmental shift between clay-forming and sulfate forming epochs. These compositional relationships speak to an important aspect of the martian geosystem that is yet poorly understood. We synthesized recent ideas to produce several working hypotheses for the formation of compositional stratigraphy on Mars and tested the hypotheses on well-exposed sulfate- and clay-bearing rocks found in the Mawrth Vallis region. In the Mawrth Vallis area, interpretations of compositional stratigraphy are strongly constrained by the fact that the sulfates and clays occur within a friable unit (probably loessite or tephra) that was deposited unconformably onto cratered terrain of fundamentally different character. Within the friable unit, the presence of aluminous clays over ferromagnesian clays might represent evidence for leaching associated with rainfall, but the presence of montmorillonite, beidellite, and sulfates argue against intense leaching as a dominant process. We suggest that ice/snow-mediated chemical weathering of dust could produce a deposit consistent with the observations through hydrolysis reactions facilitated by acidic, briny solutions within the icy dust deposit. Slow downward transport of Mg²⁺ and possibly Fe²⁺ in these solutions could potentially have produced the crude compositional stratigraphy, but regional scale leaching of basaltic bedrock by rainfall is unlikely to explain the observations. Because both clays and sulfates are found within draping, friable sedimentary deposits that occur at a range of elevations, formation of the alteration minerals by upwelling groundwater is implausible. The locations where compositional stratigraphy is observed may not be relics of a global environmental change, but rather they cold represent the locations where significant deposits of snow/ice and dust/ash once existed.