Reconstructing the Catastrophic Flood History of Eastern Valles Marineris, Mars

Abstract:

The km-deep chaotic terrains of the eastern Valles Marineris region have long been recognized as depositional sinks for eolian and air fall material. Furthermore, many have suggested that groundwater influx into these basins may have contributed to the development of lacustrine environments, basin-floor fluvial systems, and basin-marginal catastrophic outflow channels. However, the lacustrine hypothesis has been challenged by suggestions that the proposed fluvial-lacustrine landforms may have formed by non-aqueous mechanisms (e.g. volcanic) or processes that require low volumes of liquid water (e.g. glacial). In this work, we describe the basin-marginal landscape of eastern Valles Marineris at the junction between Eos Chaos and Aurorae Chaos basins. Using new high resolution topography data (HRSC DTMs), imagery (CTX), and impact crater chronology we identify and describe four catastrophic outflow channels that occur along a 500 km arc that traces the eastern margin of Eos Chaos. The fluvial origin of these channels is evidenced by (1) occurrence of longitudinal grooves that cross-cut Noachian-age (3.8 Ga) highland terrain, (2) multiple bedrock terraces that occur at consistent elevations across each channel, (3) streamlined landforms, (4) inner channels headed by cataracts, and (5) crater size frequency distributions that indicate resurfacing of highland craters (Figure 1). The terraces provide evidence for progressive incision from individual flood events with discharges of ~10⁶ – 10⁸ m³ s^-1. The topographically highest terrace in each channel initiates at the same elevation of ~1000 m along the entire basin margin. The lowest terraces and grooved terrains require up to 5 km of total incision, down to a base level of ~4100 m that is set by the downstream floor of Aurorae Chaos. The observations not only require a regional topographic control on flood initiation in all channels, but the existence of a base level that suggests the chaotic terrains pre-date the flood events. This is confirmed by the older crater retention age of the basin floors (3.5 Ga) relative to the outflow channels (3.0 Ga). These combined observations point to the past occurrence of an upstream lake within Eos Chaos, spillover of that lake along its eastern margin, and incision of outflow channels, integrating Eos Chaos with Aurorae Chaos.