Hydrated mineral stratigraphy in Ius Chasma, Valles Marineris

Introduction: Ius Chasma is a linear trough in western Valles Marineris, formed by horst and graben structures and mass wasting [e.g. 1-6]. Sulfates and other hydrated minerals are found in thin floor deposits, which differs from the thick sulfate-bearing Interior Layered Deposits (ILDs) found in the basinal chasmata, like Capri and Melas. Kieserite (MgSO4 · H2O), a polyhydrated sulfate (PHS), hydrated silica, Fe/Mg phyllosilicate, and a hydrated silicate (possibly consistent with an acid-leached phyllosilicate) are found in light-toned units within Ius Chasma. The close association of phyllosilicate, sulfate, and hydrated silica are ideal to assess relationships among aqueous processes in the canyon system. Furthermore, this tests the hypothesis that global regimes of alkaline then acidic groundwater chemistry affected crustal alteration [e.g. 7]. Using high-resolution data from MRO, we identify morphologic units with CTX and HiRISE imagery and their aqueous mineralogy from spectra in targeted CRISM observations. From these observations we define stratigraphic and superposition relationships. We find mineralogic evidence for alkaline and acidic alteration, and while the relative timing of all alteration events is uncertain, we suggest that regional groundwater chemistry was more complex than previously thought. Geologic setting: The region consists of a megabrecciated basement [11] overlain in the Late Noachian by sequences of horizontally layered lava flows at least 8 km thick [4, 12, 13]. Ius Chasma clearly shows the strong tectonic control that opened Valles Marineris in the preferred orientations of its sapping channels and faults [3]. The central ridge, Geryon Montes, is interpreted as a horst between the two graben of north and south Ius [1]. Ius Chasma may have been separated from Melas Chasma during its earliest stage [1]. The water debouched in large outflows channels at the eastern end of Valles Marineris did not leave fluvial features in Ius Chasma, either because the bulk of the water was sourced eastward of Ius, or because later deposition and landslides obscured those features. Datasets: CRISM, a visible-near infrared hyperspectral imager on Mars Reconnaissance Orbiter (MRO), has 544 wavelengths from 0.362-3.92 μm and takes targeted observations at up to 18m/pixel [8]. The MRO HiRISE camera is capable of acquiring co-aligned imagery with CRISM and can resolve details down to ~30 cm/pixel [9]. The MRO CTX (Context) camera is also co-aligned and acquires imagery at ~6 m/pixel [10]. Spectral Results: Hydrated sulfates have an absorption near 2.4 μm due to H2O and OH combinations and sulfate bending overtones [14]. Kieserite is identified by characteristic vibrational absorption features at 1.6 and 2.1