Fluvial Stratigraphy and Regional Volcanism at Hypanis Delta , Mars

Introduction: The Hypanis fan-shaped deposit in Xanthe Terra has been interpreted by many to be a late Noachian aged delta along the dichotomy boundary [15]. The putative delta was a compelling candidate landing site for the future Mars 2020 and ExoMars rover missions as it was positioned at the edge of an open basin, could have been an ideal environment for biosignature formation, concentration, and preservation, and has accessible exposures of what could be delta bottomset beds [6]. We are working to solve the key questions raised by the community about Hypanis: Is the deposit an ancient delta, as opposed to an alluvial fan or mudflow? Was the ancient topography also an open basin, suggesting a large northern sea? and How does Hypanis fit into the regional geologic timeline and history of Mars? Fluvial Timeline: Our stratigraphic study concluded that the main light-toned fan lobe was deposited first, and could have once been much larger due to identification of distal deposits over 150 km away (Fig. 1), which matched the expected morphology, thermal inertia, and albedo of the main lobe. What we interpret as an avulsion node then migrated north forming the northern lobe, which stratigraphically overlies the main lobe. Flow then migrated to the northeast braided (now expressed as inverted topography) channels, then to the east (also now inverted) channels. If Hypanis is an ancient delta, this sequence likely shows that deposition migrated basinward as water receded [2]. Geologic Context: The fluvial events mentioned above forming the deposits at the end of Hypanis Valles lie within a region characterized as transition morphology (early Hesperian transition unit eHt) [7]. The upstream catchment to the south is of late Noachian age (unit lNh), and the smooth volcanic/flood plains to the north contain large N-S trending wrinkle ridges, defining an age of Hesperian volcanism (unit eHt) [7]. Crater counts of the upstream catchment of Hypanis Valles reveal that the terrain is at least 3.6 Ga – late Noachian [89]. Thus, the fan shape deposit is of a distinct fluvial system from much earlier than catastrophic outflow and chaos terrain formation which occurred around 3.2 Ga [10]. Volcanic History: Volcanic features were mapped to the east [11] and mapping was recently updated to include the area surrounding Hypanis. This ongoing work reveals that regional episodic volcanism has persisted from before Hypanis to long after fluvial deposition and catastrophic erosion. Fig. 1. The five main parts of the Hypanis system are the main delta lobe (A), northern lobe (B), NE inverted channel (C), E inverted channel (D), and the distal deposits (E). Locations of subsequent figures shown.