Abstract. Observations permitted by the newly acquired Mars Observer Laser Altimeter data have revealed a system of gigantic valleys northwest of the huge Martian shield volcano, Arsia Mons, in the western hemisphere of Mars (northwestern slope valleys (NSVs)). These features, which generally correspond spatially to gravity lows, are obscured by veneers of materials including volcanic lava flows, air fall deposits, and eolian materials. Geologic investigations of the Tharsis region suggest that the system of gigantic valleys predates the construction of Arsia Mons and its extensive associated lava flows of mainly late Hesperian and Amazonian age and coincides stratigraphically with the early development of the outflow channels that debouch into Chryse Planitia. Similar to the previously identified outflow channels, which issued tremendous volumes of water into topographic lows such as Chryse Planitia, the NSVs potentially represent flooding of immense magnitude and, as such, a source of water for a northern plains ocean.

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Holocene debris fl ows do not occur uniformly on the Colorado Plateau province of North America. Debris fl ows occur in specifi c areas of the plateau, resulting in general from the combination of steep topography, intense convective precipitation, abundant poorly sorted material not stabilized by vegetation, and the exposure of certain fi ne-grained bedrock units in cliffs or in colluvium beneath those cliffs. In Grand and Cataract Canyons, fi ne-grained bedrock that produces debris fl ows contains primarily single-layer claysnotably illite and kaolinite-and has low multilayer clay content. This clay-mineral suite also occurs in the colluvium that produces debris fl ows as well as in debris-fl ow deposits, although unconsolidated deposits have less illite than the source bedrock. We investigate the relation between the clay mineralogy and major-cation chemistry of fi ne-grained bedrock units and the occurrence of debris fl ows on the entire Colorado Plateau. We determined that 85 mapped fi ne-grained bedrock units potentially could produce debris fl ows, and we analyzed clay mineralogy and majorcation concentration of 52 of the most widely distributed units, particularly those exposed in steep topography. Fine-grained bedrock units that produce debris fl ows contained an average of 71% kaolinite and illite and 5% montmorillonite and have a higher concentration of potassium and magnesium than nonproducing units, which have an average of 51% montmorillonite and a higher concentration of sodium. We used multivariate statistics to discriminate fi ne-grained bedrock units with the potential to produce debris fl ows, and we used digital-elevation models and mapped distribution of debrisfl ow producing units to derive a map that predicts potential occurrence of Holocene debris fl ows on the Colorado Plateau.