OUTCROPS OF NEOGENE ALLUVIAL FAN DEPOSITS IN THE ASU AREA, TEMPE, ARIZONA, AND THEIR APPLICATION TO TEACHING ABOUT THE MSL MISSION, MARS

The Mars Science Laboratory (Curiosity Rover) has spent the past year visiting outcrops near the bottom of Gale Crater on Mars that are presumed to represent ancient alluvial fan deposits. Arizona State University (ASU) has near-campus exposures of rocks presumed to represent alluvial fans deposited between 15 and 20 million years ago during Basin and Range normal faulting and extension. The Miocene rocks are predominantly red beds (fanglomerates) that range in facies from early, proximal (near-source) debris flows and landslides to later, distal (far from source) braided stream and sheetwash sands and muds. The sedimentary rock exposures locally contain altered silicic volcanic ash beds and flakes and at Hayden Butte are capped by quartz-bearing lava. They also contain a variety of water-related features. The source rocks for these sedimentary rocks were mainly Proterozoic meta-granite and “quartzite” (silicified metarhyolite).

The exposures used for teaching are found at three sites, all within a mile or two of ASU. The first, Hayden Butte or “A-Mountain,” overlooks ASU; it contains mainly alternating sandstones and shales representing a distal, later facies of deposition. The second, Hole in the Rock, Papago Park, is the furthest site from campus; it contains mainly fanglomerates and matrix-supported breccias representing a proximal, older facies, and also exposes Proterozoic basement source rock. The third, Curry Road, consists of a road cut and former railroad cut exposures on park land just across the Tempe Town Lake from ASU. Thanks mainly to faulting, it exposes the transition between proximal and distal facies.