Chronology of deposition and alteration in the Mawrth Vallis region, Mars

Abstract

The Mawrth Vallis area displays some of the largest phyllosilicate-rich outcrops of Mars, on Noachian highlands. The Mawrth Vallis region is located just at the dichotomy boundary between the Noachian highlands and the younger, northern lowlands. A large, thick, layered clay-rich unit is present throughout the inter-crater plateaus. Clay-rich layers have also been identified in parts of the Mawrth Vallis and Oyama crater floors. The age of the alteration and its relationships with other processes such as fluvial activity is fundamental for estimating the timing of aqueous activity and habitability in this region, and on Mars. We have investigated the relative stratigraphy and ages of the regional plateau, of key surfaces of the inter-crater plateau, of Oyama crater's floor and of Chryse Planitia deposits in Mawrth Vallis' mouth to constrain the age of the clay unit and its alteration. According to the cratering model results, the main layered unit may have started forming prior to ∼4.0Ga ago, was largely deposited by ∼3.9Ga ago, and suffered erosion and redeposition up to ∼3.8Ga ago, as indicated by the latest age of the deposits on the floor of Oyama crater. Surface aqueous alteration stopped no later than 3.7–3.6Ga ago, corresponding to the age of the dark, non-altered material capping the region, and of the dark deposits in Mawrth Vallis' mouth. This work provides useful boundaries for constraining the time period of surface or shallow sub-surface water activity in this region. This preserved window into early phases of aqueous activity on Mars gives us a unique opportunity to study an aqueous environment of exobiological interest in the early solar system.