Age of Obsidian Butte in Imperial County, California, Through Infrared Stimulated Luminescence Dating of Potassium Feldspar from Tuffaceous Sediment

ABSTRACT

Obsidian Butte is an important lithic source for obsidian and artifacts from this location are distributed across archaeological sites in southern California and adjacent parts of northern Mexico. For this study, we used drill core material for infrared stimulated luminescence (IRSL) dating of potassium feldspar extracted from tuffaceous sediment directly underlying obsidian-bearing lava from Obsidian Butte. In addition, a core sample from lacustrine sediment below the tuffaceous sedimentary unit was dated by the same method. Stratigraphically consistent ages between 2.51 ± 0.32 kilo-annum (ka) (1 sigma uncertainty; average of two tuffaceous sediment samples) and 4.39 ± 0.49 ka (lacustrine sediment sample) were obtained. This constrains the eruption and earliest availability of the lithic resource of Obsidian Butte to 490 BCE (with uncertainty limits of ±320 years at ∼68 percent confidence, and ±640 years at ∼95 percent confidence). Since then, it would have been accessible during intermittent desiccation of Lake Cahuilla. This new date redefines obsidian from Obsidian Butte as a marker for the late prehistoric period.

Resumen

Obsidian Butte es una fuente lítica importante de obsidiana y artefactos de esta localidad están distribuidos a lo largo de sitios arqueológicos al sur de California y partes adyacentes del norte de México. Para este estudio, utilizamos material de núcleos de perforación para el fechamiento de feldespato potásico por medio de luminiscencia estimulada por infrarrojo (IRSL, por sus siglas en inglés) extraídos de sedimentos tobáceos que subyacen directamente a lava con obsidiana proveniente de Obsidian Butte. Adicionalmente, una muestra de núcleo de sedimentos lacustres ubicada por debajo de los sedimentos tobáceos fue fechada por el mismo método. Fueron obtenidas edades estratigráficamente consistentes entre 2.51 ± 0.32 kilo-annum (ka) (incertidumbre de 1 sigma; promedio de dos muestras de sedimentos tobáceos) y 4.39 ± 0.49 ka (muestra de sedimento lacustre). Esto restringe a la erupción y la disponibilidad más temprana del recurso lítico de Obsidian Butte a 490 BCE (con límites de incertidumbre de ±320 años al ∼68 percent de confianza, y ±640 años al ∼95 percent de confianza). Desde entonces, habría sido accesible durante la desecación intermitente del lago Cahuilla. Este nuevo fechamiento redefine a la obsidiana de Obsidian Butte como un marcador para el periodo prehispánico tardío.

KEYWORDS:

• Volcanism
• obsidian
• infrared stimulated luminescence
• geochronology
• late Archaic

Acknowledgments

Wilfred Elders kindly provided samples from geothermal Union Oil Company of California Well 86-2. Janet C. Harvey generated the LiDAR map of Obsidian Butte. Alejandro Cisneros provided the Spanish abstract. We also thank Wendy Teeter from the Cotsen Institute of Archaeology at UCLA for her support during planning and execution of the project. Comments on an earlier version of this article by M. Steven Shackley are appreciated, as are the constructive reviews by Scott Johnston and two anonymous reviewers.

Disclosure Statement

No potential conflict of interest was reported by the authors.