CALCULATING THE RATE OF DEPOSITION AS INTERPRETED BY LUMINESCENCE DATING FOR SELECTED LATE PLEISTOCENE GROUND-WATER DISCHARGE DEPOSITS IN SOUTHERN ARIZONA

The archeological, sedimentological and chronostratigraphic framework of ground-water discharge deposits in the San Pedro Valley of southern Arizona have been extensively studied (Haynes, 1987; 1991; 2007; Pigati, et. al, 2004). These researchers have defined a complex evolutionary history for this ground-water discharge using radiocarbon dating, faunal assemblages, and isotopic analysis of the Coro Marl, which is a white calcareous silty clay unit 1 to 2 m thick that is present in discontinuous outcrops along a 150 km stretch of the valley. The focus of this project is to refine the basal age and regional extent of the marl beyond the limits of radiocarbon dating by directly dating the sediment grain deposition using optically stimulated luminescence (OSL). One site, the Murray Springs Paleoindian site, was selected for “proof of concept” calibration between radiocarbon and OSL and samples were taken within the discharge facies. Two other localities (Cerros Negros and Horsethief Draw) were also chosen for dating purposes. At all three localities, the marl and adjacent units are largely devoid of the sand-sized grains typically used for OSL dating and, therefore, some adjustments to the OSL technique were necessary.

At Murray Springs, we collected OSL samples from the top and bottom of the marl unit, as well as from a fluvial unit positioned above the occupational Clovis layer. We also collected two samples of the Coro Marl at Horsethief Draw and four samples within similar layers at Cerros Negros which we will compare with the Murray Springs ages. Obtaining accurate OSL ages in these deposits is complicated by the fact that the Coro Marl carries a ratio of U:Th that indicates some disequilibrium, which could affect the dosimetry connected to the luminescence dating. The resolution or accommodation of this issue, as well as a comparison of the OSL ages, both internally and with other geochronologic techniques, will be addressed in the presentation. In spite of these problems, early promising ages in the range of 27,000 years have been obtained from the bottom OSL sample at Murray Springs.