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Multiple constraints on the age of a Pleistocene lava dam across the Little Colorado River at Grand Falls, Arizona

Wendell Duffield¹, Nancy Riggs^,1, Darrell Kaufman¹, Duane Champion², Cassandra Fenton³, Steven Forman⁴, William McIntosh⁵, Richard Hereford⁶, Jeffery Plescia⁷ and Michael Ort⁸

¹ Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011, USA
² U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA
³ GeoForschungsZentrum Potsdam, Telegrafenberg, Haus B, D-14473 Potsdam, Germany
⁴ Department of Earth and Environmental Sciences, University of Illinois, Chicago, Illinois 60607, USA
⁵ New Mexico Bureau of Mines and Mineral Resources, 801 Leroy Place, Socorro, New Mexico 87801, USA
⁶ U.S. Geological Survey, 2255 North Gemini Drive, Flagstaff, Arizona 86001, USA
⁷ Johns Hopkins Applied Physics Lab, 11100 Johns Hopkins Drive, Laurel, Maryland 20723, USA
⁸ Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011, USA

The Grand Falls basalt lava flow in northern Arizona was emplaced in late Pleistocene time. It flowed 10 km from its vent area to the Little Colorado River, where it cascaded into and filled a 65-m-deep canyon to form the Grand Falls lava dam. Lava continued 25 km downstream and 1 km onto the far rim beyond where the canyon was filled. Subsequent fluvial sedimentation filled the reservoir behind the dam, and eventually the river established a channel along the margin of the lava flow to the site where water falls back into the preeruption canyon.

The ca. 150 ka age of the Grand Falls flow provided by whole-rock K-Ar analysis in the 1970s is inconsistent with the preservation of centimeter-scale flow-top features on the surface of the flow and the near absence of physical and chemical weathering on the flow downstream of the falls. The buried Little Colorado River channel and the present-day channel are at nearly the same elevation, indicating that very little, if any, regional downcutting has occurred since emplacement of the flow.

Newly applied dating techniques better define the age of the lava dam. Infrared- stimulated luminescence dating of silty mudstone baked by the lava yielded an age of 19.6 ± 1.2 ka. Samples from three noneroded or slightly eroded outcrops at the top of the lava flow yielded ³He cosmogenic ages of 16 ± 1 ka, 17 ± 1 ka, and 20 ± 1 ka. A mean age of 8 ± 19 ka was obtained from averaging four samples using the ⁴⁰Ar/³⁹Ar step-heating method. Finally, paleomagnetic directions in lava samples from two sites at Grand Falls and one at the vent area are nearly identical and match the curve of magnetic secular variation at ca. 15 ka, 19 ka, 23 ka, and 28 ka. We conclude that the Grand Falls flow was emplaced at ca. 20 ka.

Key Words: Grand Falls lava dam • Quaternary • ⁴⁰Ar/³⁹Ar • ³He • magnetic secular variation • infrared luminescence

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