OSL DATING OF WIND-BLOWN SANDS AND SILTS OVERLYING THE SP AND STRAWBERRY FLOWS, NORTHERN ARIZONA: INITIAL RESULTS FROM THE SAN FRANCISCO VOLCANIC FIELD REU PROGRAM

SP crater and Strawberry crater are two basaltic cinder cones in the San Francisco Volcanic Field in northern Arizona, USA. There is considerable uncertainty in the age of these geomorphically young cinder cones and basalt flows, impacting hazard assessments for the region. For example, optically stimulated luminescence (OSL) ages from xenocrysts in the SP crater basalt flow suggest an age of 6 ka (Rittenour et al. 2012), while whole rock K-Ar and newer Ar-Ar and He cosmogenic ages suggest an age of 70 ka (Baski, 1974; Fenton and Niedermann, 2013). Previous age control for the Strawberry crater basalt flow is limited to an imprecise whole-rock K-Ar age of 46 +/- 46 ka (Damon and Leventhal, 1974).

OSL samples were collected from wind-blown sands and silts (loess) overlying the SP crater and Strawberry crater basalt flows in order to (1) provide minimum age estimates for their eruptions, (2) improve age control for the Strawberry crater basalt flow and (3) to help to resolve the conflicting age estimates for SP crater. Basal loess samples from a soil pit on the largest lobe of the Strawberry crater flow produced an age of 12 ka, while basal samples from two soil pits on the SP crater flow produced ages of 7 ka and 33 ka. These preliminary results provide minimum age constraint for the Strawberry and SP Crater flows. While the OSL results presented here from the Strawberry crater flow suggests an age of > 12ka, a lower loess and soil horizon was found below a layer of cinder. OSL results from this lower layer are expect to be older and better constrain the age of the flow. OSL results from loess overlying the SP crater flow are consistent with K-Ar, Ar-Ar, and cosmogenic results along with soil development indices, suggesting that the xenocrysts OSL ages may be inaccurate.

OSL dating of loess overlying volcanic flows has been found to be a useful technique to constrain the age of eruptions in the San Francisco Volcanic Field. These ages results, along with others, can be used to assess future volcanic hazards in the region.