CATASTROPHIC FLOOD SEDIMENTS IN CHRYSE BASIN, MARS, AND QUINCY BASIN,WASHINGTON - APPLICATION OF SANDAR FACIES MODEL

Viking visible and thermal infrared observations and terrestrial catas trophic flood deposits provide clues to identify the outflow channel s ediments that went into Chryse Basin on Mars. On Earth, sandar (outwas h plains formed by coalescence of many jokulhlaup floods) are describe d in terms of three laterally adjacent facies: proximal, midfan, and d istal. The Missoula Flood sediments deposited in Quincy Basin, Washing ton, comprise a miniature analog of Chryse Basin. The terminology and general characteristics of the sandar facies model are applied to Quin cy Basin, although the depositional environment and clast sizes are so mewhat different (higher-energy flood, larger clasts, subaqueous rathe r than subaerial deposition). For example, the Ephrata Fan (a deposit of boulders, cobbles, and pebbles) forms the midfan facies analog; a d ownfan sandy deposit (reworked into a dune field) comprises the distal facies analog. In Chryse Basin the midfan is defined by a heterogeneo us rocky (0-25%); intermediate-albedo (0.21-0.26), intermediate therma l inertia (260-460 J m(-2) s(-0.5) K-1) surface, while the distal faci es has a low albedo (0.14-0.16) and higher thermal inertia (340-700 J m(-2) s(-0.5) K-1). The Chryse midfan unit has rocks and windblown dus t exposed at the surface. The sand of the distal facies in Chryse/Acid alia is reworked by the wind, as in Quincy Basin. The Viking 1 and Mar s Pathfinder landing sites are located on the midfan unit. Observation s that can be made at the Mars Pathfinder site might help in reevaluat ing whether or not Viking 1 landed on outflow channel sediments.