J. Flahaut

Extensive surface pedogenic alteration of the Martian Noachian crust suggested by plateau phyllosilicates around Valles Marineris

[1]xa0Thousands of phyllosilicate-rich outcrops, mainly iron or magnesium-rich are exposed on Noachian terrains in the Martian southern highlands. We analyzed 90 CRISM observations and more than a hundred HiRISE images located on the plateaus surrounding Valles Marineris. We mapped an extensive Al- and Fe/Mg-phyllosilicate-rich formation covering at least ∼197,000 km2, for which we introduce the name “Plateau Phyllosilicates.” Tens of meters in thickness, this light-toned formation crops out at various elevations on top of the Noachian units Npl1 and Npl2, as flat exposures on plateaus and along scarps such as valley walls, chasma walls, pit walls and impact crater rims. The Fe/Mg-phyllosilicate-rich lower member of the formation is composed of Fe/Mg-smectites (nontronite, saponite) and vermiculite. The Al-phyllosilicate-rich upper member of the formation contains Al-smectites (montmorillonite, beidellite) and locally kaolinite and/or halloysite. We suggest that the Plateau Phyllosilicates were mainly formed by pedogenesis related to the weathering of the Noachian bedrock by percolation of meteoric water or melted snow under a temperate and subarid climate during the Noachian Epoch in an alkaline to neutral environment. Kaolinite and/or halloysite may have formed in areas of more intense drainage at the surface under slightly acidic environments during the Noachian and Hesperian Epochs. Fluvial activity and deuteric alteration may have locally contributed to the genesis of phyllosilicates. This study suggests that the alteration of the Noachian basement of the plateaus surrounding Valles Marineris was widespread during the Noachian Epoch, and was still active during the Hesperian Epoch even though the water availability was limited.

Dikes of distinct composition intruded into Noachian‐aged crust exposed in the walls of Valles Marineris

[1]xa0Valles Marineris represents the deepest natural incision in the Martian upper crust. Previous studies showed that the upper parts of the walls were made of finely layered probable basalts in most of the chasmata, while the base of the stratigraphy reveals primary Noachian crustal blocks. Exposures of pristine Noachian bedrock are rare on Mars, and mostly observed outside of their geological context. The occurrence of well-preserved and extended outcrops of pristine material in this giant rift could bring valuable information on the early processes that took place at the surface of Mars. Analyses of high resolution data over the best exposures of lower walls in Coprates Chasma, central Valles Marineris, revealed the presence of multiple magmatic intrusions interpreted as dikes. These dikes intrude an old, massive, fractured bedrock interpreted as being preserved ancient Noachian crust. Their composition, determined using CRISM data, and distribution, limited to this ancient crust at the bottom walls, indicate that they might have formed early in the rift formation, and therefore represent exceptionally well-preserved outcrops of the early history of Mars.

Interior layered deposits within a perched basin, southern Coprates Chasma, Mars: Evidence for their formation, alteration, and erosion

A basinA¢Â�Â�like area containing three interior layer deposits (ILDs) on the southern nmargin of Coprates Chasma was studied. We interpret the area as an ancestral basin and ndemonstrate that ILD deposition postdates the formation of the current wall rock slopes. nThe geometry of the ILD and the wall rock spurs form a catchment area between each nILD and the plateau to the south. Erosional remnants of extensive ash or dust layers ndeposited on the plateau south of Valles Marineris also crop out on the southern plateau of nCoprates Chasma. A mass balance calculation suggests that the volume of each ILD is ncompatible with the volume of the ash or dust that would have been deposited within each ncatchment area. We propose that the ILDs likely formed by episodically washing such naerially deposited material down from chasma walls. Rifting of the IusA¢Â�Â�MelasA¢Â�Â�Coprates ngraben opened the enclosed basin and removed any standing water. Faults within the nILDs are compatible with this chasm opening. Sulfates are associated with the ILDs and nlightA¢Â�Â�toned material on the basin floor. We suggest that they result from water alteration of npreexisting deposits, though the timing of that alteration may predate or postdate the nbreaching of the basin. Scours within one ILD are similar to terrestrial glacial scours. nDuring a period of high obliquity ice would accumulate in this region; hence we argue the nscours are Martian glacial scours. A late deposited layer marks the end of the active nlocal geological history between 100 My and 1 Gy.

Stratigraphy and mineralogy of Candor Mensa, West Candor Chasma, Mars: Insights into the geologic history of Valles Marineris

Candor Mensa, an interior layered deposit (ILD) in Valles Marineris, Mars, consists of two nstratigraphically distinct units, the lower of which comprises the bulk of the mensa. This lower unit is napproximately 5 km thick and composed of parallel layers, 4 to 14 m in thickness and associated with nmonohydrated sulfates. The lower unit is disconformably overlain by an upper unit composed of thinner n(< 3 m) layers with diagnostic polyhydrated sulfate signatures. The original extent of proto-Candor Mensa nand its lower unit included neighboring Baetis Mensa. We suggest that the source material for both units nis airborne dust or ash but that the depositional environment for the units differs. First, the lower unit was ndeposited during the subsidence of an enclosed water-filled basin. This basin/lake could have been frozen nperiodically, with freeze-thaw episodes possibly linked to Martian obliquity cycles. Erosion, including the npotential action of glaciers, was able to remove large volumes of material out of the basin during the ntectonism that produced the current geometry of Valles Marineris. Deposition of the upper unit postdates nthis event and took place in the absence of standing water at high elevation. Groundwater or snowmelt nmay have provided the water required for sulfate formation and deposit induration. We conclude that the nmajor break in sedimentation recorded by this ILD deposit coincides with linking of ancestral basins into nthe current geometry of Valles Marineris chasmata and that it was possible to form hydrated minerals after nthis event.