Gypsum, opal, and fluvial channels within a trough of Noctis Labyrinthus, Mars: Implications for aqueous activity during the Late Hesperian to Amazonian

We investigate in detail the morphology, mineralogy, and stratigraphy of light-toned deposits within one trough of Noctis Labyrinthus, centered at -6.8 degrees N, 261.1 degrees E. CRISM spectra taken from light-toned layered deposits in the northern portion of the trough exhibit absorptions around 1.41, 1.92 and 2.21 gm, consistent with mixtures of opal and Al-clays that are exposed beneath younger lava flows and between high-standing mesas of chaotic terrain. In the southern portion of the trough, opal occurs as a patchy surficial deposit along the southwestern lower wall. Gypsum appears to be present in the southern portion of the trough where spectra show triplet absorptions at 1.44, 1.48, and 1.54 gm, and additional absorptions at 120, 1.74, 1.95, 2.22, 2.27, and 2.49 mu m. The gypsum-bearing materials consist of one to several beds that typically fill low-lying regions, including valleys. A bright mound on the trough floor exhibits spectral features at 1.43, 1.92 and 2.43 mu m, characteristic of polyhydrated sulfates. The bright mound appears distinct in morphology from chaotic terrain, and along its base are exposures of gypsum-bearing materials. Fluvial channels in the southwestern portion of the trough incise surface slopes at 4-6 degrees and lack obvious sources. The channels display first and second order tributaries arranged in a parallel pattern and may have formed by localized surface discharge from melting snow and/or ice. Both opal and gypsum occur in close proximity in the southwestern region of the trough, but gypsum is found alone in the southeast and opal in association with Al-clays is found to the northwest. We do not believe gypsum and opal formed coevally because they are not always found together, they require different aqueous conditions (i.e., opaline deposits require high silica availability while the gypsum deposits require high Ca availability in solution), and they appear at stratigraphically distinct levels. Although we identified evidence for fluvial landforms within the trough, cross-cutting relations indicate their incision post-dates deposition of the opal and pre-dates deposition of the gypsum. Hence, several periods of aqueous activity and alteration likely occurred within the trough from the Late Hesperian into the Amazonian that reflect favorable localized conditions within the Noctis Labyrinthus region and may be contemporaneous with late aqueous activity occurring elsewhere on Mars. (C) 2013 Elsevier Ltd. All rights reserved.