Ancient Martian Lakestands and Fluvial Processes in Iani Chaos: Geology of Light-Toned Layered Deposits and their Relationship to Ares Vallis Outflow Channels

English Abstract:

Iani Chaos lies at the head of the Ares Vallis outflow channels. Mapping of Ares Vallis reveals multiple episodes of erosion, probably linked to discharge events from the Iani Chaos aquifer. We present the first detailed geomorphological map of the region. Among five chaos units, distinguished by varying levels of modification (by erosion and fracturing), Light-toned Layered Deposits (LLD) have been mapped and described in Iani Chaos. LLD are characterized by a light-toned albedo, high thermal inertia and a pervasively fractured morphology. The LLD both fill the basins made by the collapsed chaotic terrains and are partially modified by the chaos formation. LLD also overlap chaos mounds or are themselves eroded into mounds after deposition. These stratigraphic relationships suggest that LLD deposition occurred episodically in the Iani region and throughout the history of the development of the chaos. Water seems to have had an active role in LLD deposition and erosion. The composition and morphologies of the LLD are consistent with deposition in an evaporitic environment, entailing stable water on surface. At the same time, we observed potential fluvial features (i.e., channels, streamlined islands, terraces, grooved surfaces) on the surface of the LLD. These landforms mark a fluvial system that can be traced from central Iani northward. Using topographic data, we compared the elevation of the LLD and channel units and find that their altitudes have been controlled by the Ares Vallis outflow events. This demonstrates that 1) a fluvial system within Iani was linked to the Ares Vallis channels system, characterized by five episodes of outflow at least, 2) the LLD are coeval with the Ares Vallis outflow channels. On the basis of our analysis, we propose a formation model of Iani Chaos, given by a cyclic alternation of two stages with time: 1) Fracturing and tectonic subsidence of the bedrock and outflows. Fluvial erosion of LLD; 2) Evaporitic deposition of LLD.