DETRITAL SANIDINE GEOCHRONOLOGY: BREAKTHROUGH METHOD FOR DATING SEDIMENTARY ROCKS

Several applications of detrital sanidine (DS) geochronology are demonstrating a tremendous potential to reveal ultra high precision maximum deposition ages (MDA) with unprecedented linkage to provenance. In several instances, detrital sanidine MDAs get younger with stratigraphic age suggesting they may closely approximate true depositional ages, providing a valuable new tool to date sedimentary deposits. DS is complementary to detrital zircon geochronology with the added advantages of a 10-100 fold increase in precision relative to U/Pb LA-ICPMS zircon ages. In many studies that utilize both methods, DS commonly provides a much younger MDA for late Mesozoic and Cenozoic rocks. Similar to concordant zircon results, plateau style age spectra of single DS grains demonstrate closed system behavior for radiogenic argon verifying accuracy of single grain fusion ages. Additionally, the K/Ca value of each DS grain is given by irradiation produced ³⁹Ar and ³⁷Ar which provides a key chemical finger print to augment age information when linking DS grains to source calderas.

DS geochronology is ongoing in many current studies of western US river terraces (e.g. Colorado, Green, Virgin, Salt, Rio Grande) as well as Cenozoic paleoriver deposits (e.g. San Juan Basin, Ogallala Group, Browns Park Fm. Santa Fe Gp, Bouse Fm.). Because sanidine-producing volcanism has been quasi-continuous throughout the western USA during the Cenozoic there is a strong likelihood that “cryptotephras” exist in most sediments that can yield MDA’s subequal to depositional ages. Additionally, volcanic activity is temporally and spatially variable thus enhancing the opportunity to distinguish provenance with unprecedented precision. Our results also demonstrate wide wind-blown dispersal of sanidine that needs to be considered when interpreting fluvial connections between source rocks and sediments. We are exploring deeper time as well with positive results stemming from DS geochronology on Triassic sediments and sanidine recognized in Cambrian strata. We continue with technique development that can ultimately utilize robotics to increase throughput and nuclear reactions on Na, Sr and Ba to enhance K/Ca data for more unique chemical fingerprinting.