SEDIMENTARY HISTORY AND MASS-FLOW STRUCTURES OF CHRYSE-PLANITIA AND ACIDALIA-PLANITIA, MARS

Geologic mapping and crater counting in Chryse and Acidalia Planitiae (CAP) reveal five major sedimentary deposits of Hesperian to Early Ama zonian age, including (1) a mass flow deposited during the Early Hespe rian near Deuteronilus Mensae (northeast of the map region) that may h ave resulted from the carving of Kasei Valles, >3000 km southwest of t he exposed part of the deposit; (2) knobby plains material consisting of channel (likely from Simud and Tiu Valles and possibly Ares and Sha lbatana Valles) and mass-wasting deposits in central and eastern CAP; (3) material largely from Maja and Ares Valles emplaced in at least we stern apd southern CAP (outcrops in southern Chryse Planitia developed thermokarst); (4) a thin mass flow covering much of southern Chryse P lanitia that emanated from Simud and Tiu Valles; and (5) a thick, exte nsive (perhaps >3500 km across) mass flow deposit in central and north ern CAP derived from accumulation and backflow of the preceding thin m ass flow or perhaps melting of polar deposits. Other possible deposits may not be recognizable owing to burial by younger materials or a lac k of morphologic signature. Various associated landforms appear to be consistent with the mass flow interpretations, including lobate and li near scarps along deposit edges, fractures related to desiccation of t hick sediments, troughs, and ridges near the edges of the deposit indi cative of secondary mass movement and deformation, pitted domes and fi ssure-fed flows possibly formed by sedimentary (mud) eruptions, and lo ngitudinal channel grooves perhaps formed by roller vortices. No convi ncing evidence for paleoshorelines or stagnant ice sheets is found in CAP. These findings suggest that mass flow and hyperconcentrated flood ing may have been the predominant processes of outflow-channel dissect ion in CAP. Elsewhere in the northern plains, similar landforms are pr evalent. The mass flow interpretation does not require either multiple episodes of extraordinarily high water-discharge rates achieved by fr eeing huge volumes of water from the crust, repetitive recycling of im mense volumes of water into highland aquifers at the heads of Chryse c hannels, or profound climate change. Mars Pathfinder will most likely land on and inspect the surface of the thin mass flow that originated from the canyons of Simud and Tiu Valles.