EAST TENNESSEE GEOLOGICAL SOCIETY
March 2008 Meeting


Monday March 10, 2008
6:00 - 7:30 pm

Pellissippi State Technical Community College
10915 Hardin Valley Road, Knoxville
J.L. Goins Administration Building, Cafeteria Annex


MARCH PRESENTATION

RACETRACK PLAYA, DEATH VALLEY, CALIFORNIA: SHEDDING
NEW LIGHT ON AN AGE-OLD PARADOX


Geoffrey J. Gilleaudeau
Ph.D. Candidate
Department of Earth and Planetary Sciences
University of Tennessee-Knoxville

Abstract

Racetrack Playa is a desiccated mud flat located in the northwestern section of Death Valley National Park, California. The playa sits at the bottom of a hydrologically closed basin that was once the site of a Pleistocene to early Holocene perennial saline lake. The basin has undergone considerable tectonic subsidence through Tertiary time due to extension of the crust and the formation of the Basin and Range province. The basin is bound by a large normal fault (creating a half-graben), as well as the Last Chance Range to the west and the Cottonwood Mountains to the east. The modern surface of Racetrack Playa has extremely low relief, lies well above the water table, and consists of fine sand, silt and clay.

A curious aspect of the playa surface is the presence of large boulders (up to 700 lbs.) that sit at the terminations of long (>1 km) irregular tracks that have seemingly been carved in the substrate. Individual tracks appear to cross each other, take sharp (>90) turns, and curve gradually into semi-circles several meters in diameter. These tracks seem to indicate that boulders have been mechanically transported, or slid, across great distances of the playa surface. Despite decades of park visitation, no individuals have ever reported witnessing the boulders in motion. The mechanism of their transport thus remains a mystery. Early 20th century explanations for the mysterious boulder tracks ranged from the supernatural to the extraterrestrial to the absurd. More plausible hypotheses have included transport by ice, strong winds, and swelling clays. Calculations indicate, however, that wind speeds >800 mph would be necessary to move the largest boulders even across a wet clay surface. In addition, ice events are extremely rare in Death Valley, and micro-changes in topography caused by swelling clays are not likely to account for >1 km of transport. After a casual field trip to Racetrack Playa in the spring of 2007, I propose a new mechanism to account for the unusual transport of large boulders across the playa surface. This mechanism involves extreme slickening of the surface by cyanobacterial mats after precipitation events, and subsequent transport by strong winds channeled through the valley. While only a hypothesis, extreme reduction in the surface coefficient of friction by cyanobacterial mats seems to be the only plausible explanation for this bizarre phenomenon.


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