USES OF GEOCHEMICAL AND GEOCHRONOLOGICAL DATABASES DEVELOPED IN THE U.S. GEOLOGICAL SURVEY TEPHROCHRONOLOGY LABORATORY, MENLO PARK, CA

U.S. Geological Survey Tephrochronology Laboratory researchers in Menlo Park, CA have compiled a large geochemical database composed of the normalized major-element chemistry data of approximately 7,000 tephra or volcanic ash samples, mostly from the western conterminous United States. The chemical composition of volcanic glass shards from an unidentified tephra sample is determined using electron or ion microprobe and statistically compared to other analytical results in the database. Next, a ranked list of up to 100 “best matches” is generated using a clustering algorithm. Then the chemical fingerprints of the glass shards in the samples are used to identify and stratigraphically correlate tephra layers.

The tephra identifications, relative ages, and correlations are further supported by a second, relational database containing field notes, locality data, petrographic, mineralogic, and chronologic data for each sample. Previously investigated tephra sample correlations are also in this database. Moreover, ⁴⁰Ar-³⁹Ar, K/Ar, INAA, ICP-MS, and other accessory data in the above databases may be used as evidence to constrain geochronologic conclusions. In the near future, we hope to integrate our geochemical and relational databases.

The Tephrochronology Laboratory databases are primarily used to help determine the ages of regional lithostratigraphic units containing geochemically identifiable tephra. However, these data sets also can be used to: 1) help map the areal distribution of tephra layers, especially in conjunction with GIS software; 2) provide age control for recurrence interval calculations of, for example, volcanic eruptions, flood events, etc.; 3) reconstruct climatic history, e.g., constrain pluvial highstands; 4) determine clay sources in Native American pottery. The tephra databases also have wide applications in environmental studies, paleontology, hydrologic modeling, and archeology, among other disciplines.