Paper No. 312-6
Presentation Time: 9:00 AM-6:30 PM
HOW OLD ARE THE SP AND STRAWBERRY VOLCANIC VENTS AND FLOWS IN THE SAN FRANCISCO VOLCANIC FIELD, ARIZONA? NEW EVIDENCE FROM SOILS, OSL DATED LOESS AND HE-3 EXPOSURE DATING
The San Francisco volcanic field in northern Arizona, USA, consists of over 600 cinder cones that range in age from 6 Ma to 900 yr. The timing of the volcanic activity in the region is important for hazard assessment, however the reliability of many older-generation K-Ar results have been questioned. For example, several vents and basalt flows that were dated to be Pleistocene in age have been assumed to be Holocene based on their geomorphic character. These include SP and Strawberry craters, previously K-Ar dated to 70 +/- 4 ka and 46 +/- 46 ka respectively. Fuelling the perceived conflict between geomorphic character and calculated age, recent Ar-Ar results for SP Crater suggest an age of 72 ka and cosmogenic He-3 and Ne-21 exposure age estimates suggest an age of 43-82 ka (Fenton and Neidermann, 2014), while optically stimulated luminescence (OSL) dating of quartz xenocrysts from SP Crater flows suggest an age of 6 ka (Rittenour et al. 2014). To resolve these differences, we provide new age control for SP and Strawberry craters based on soil development, OSL dating of loess overlying basalt flows and He-3 exposure dating of the flows and crater rims.
Results from all three methods support a late Pleistocene age for SP and Strawberry craters. Soil pits from both localities show that loess deposits up to 1.3 m thick cap the basalt flows in places and contain well developed Bt and Bk horizons. OSL ages from the base of these soil sections suggest initial loess deposition began around 27-29 ka and provide minimum ages for both flows. He-3 exposure dating of samples from the rim of the cones and along the top of the basalt flows from both vents suggest SP Crater erupted 60 ka and Strawberry Crater erupted 55 ka. Results from OSL dating of quartz xenocrysts were not supported and new analysis suggest unstable luminescence properties. These new data are consistent with each other and suggest that SP and Strawberry Craters erupted 55-60 ka. The youthful geomorphic appearance of the cones and basalt flows was likely a product of armoring of the cones by lava splatter that limited erosion of the cones and arid conditions that produced patchy loess deposition and soil on the basalt flows.