DETRITAL SANIDINE GEOCHRONOLOGY: LINKING VOLCANIC SOURCES TO SEDIMENTARY SINKS WITH HIGH AGE PRECISION, COMPOSITIONAL FINGERPRINTS AND IMPROVED MAXIMUM DEPOSITION AGES

Multiple new applications that use high-precision detrital sanidine (DS) geochronology are revealing unprecedented resolution to determine the provenance and timing of deposition of sedimentary units. Choosing sanidine rather than the entire K-feldspar population concentrates the younger populations of feldspars thus increasing the likelihood that maximum deposition ages (MDA) approximate true depositional ages. DS is complementary to detrital zircon geochronology with the added advantage of a 10-100 fold increase in precision relative to U/Pb LA-ICPMS zircon ages. Similar to concordant zircon results, age spectra of single DS grains that yield plateaus 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 providing a key chemical fingerprint to determine provenance.

DS geochronology is ongoing in many western US landscape evolution studies. DS river terrace data (e.g. Colorado, Green, Virgin, Salt, Rio Grande) help constrain river birth and evolution. DS is also powerful for chrono- and magneto-stratigraphy (e.g. San Juan Basin, Ogallala Group, Browns Park Fm., Santa Fe Gp, Bouse Fm.). Because of the abundant sanidine-bearing eruptions throughout the western USA during the Cenozoic there is a strong likelihood that detrital sanidine exist that can yield MDA’s subequal to depositional ages. This is being tested using stratigraphic and geomorphic relative ages, for example using strath-to strath comparisons in well preserved terrace flights. Additionally, volcanic activity is temporally and spatially variable thus enhancing the opportunity to distinguish provenance with unprecedented precision. Our results also demonstrate that widespread wind-blown dispersal of sanidine (and likely zircon) needs to be considered when interpreting fluvial connections between source rocks and sediments. Accurate linkage of DS to source is currently somewhat limited by inadequate dating of major volcanic centers and compilation of existing data. We continue with technique development that utilizes nuclear reactions on Na, Sr and Ba that produce Ne, Kr and Xe, respectively to enhance K/Ca data for more unique chemical fingerprinting.