USGS-GD-Scientific Capabilities - TEPHROCHRONOLOGY Technique

 

TEPHROCHRONOLOGY

Technique


 

Tephra layers (volcanic ash beds and tuffs) of Quaternary and late Neogene age are found throughout the western U.S. These layers provide opportunities for dating geologic events or determining rates of geologic processes, including volcanic eruptions and eruption frequencies, volcanic provenance, fault displacements, earthquake recurrence intervals, ages of depositional events, and ages of other surface and subsurface events and processes. This technique exists in Menlo Park.

The Tephrochronology Laboratory provides stratigraphic correlation and numerical age control to:

  • studies of faults, earthquake recurrence, and neotectonics;
  • studies of volcanic hazards, eruption recurrence, and eruptive sources of tephra;
  • studies of global change, particularly correlation and dating of climate-proxy data among depositional basins, and between marine and continental basins;
  • topical and regional geologic mapping studies.

Tephra layers are collected at critical locations where age control is required. The physical characteristics of tephra components are described (mineralogy, glass shard morphology, and the presence and nature of other components), and the volcanic glass is separated from the tephra and analyzed by one or more chemical techniques to determine a compositional ìfingerprintî. This fingerprint is compared by computer matching programs with our database of over 4500 analyses of previously-analyzed samples, and the best matches are identified and form a pool of candidates for correlative samples. These are evaluated in terms of petrographic, stratigraphic, and chronologic criteria to select the best matches. These procedures allow us to correlate tephra layers at critical sites to other localities where the same tephra layers are present, and often to sites where the age of the layers has been previously determined by one or more numerical dating methods (e.g., 14C, 40Ar/39Ar). New tephra layers are analyzed and dates at those sites where the best materials for dating can be obtained. The results of our tephrochronologic research are combined with other chronostratigraphic data (e.g., magnetostratigraphy, oxygen isotope chronostratigraphy, stratigraphic sequence information) to develop a four-dimensional spatial and temporal chronostratigraphic framework for late Neogene sediments and rocks in the western U.S. and the Pacific margin.

A typical small study requiring some tephrochronologic support involves collection, examination, analysis, and evaluation of results of about 10 samples, costing about $9 K and about 0.13 FTE. A large study may involve as many as 100 samples, $90 K in cost, and 1.3 FTE. The latter type of study is usually spread over a longer time period, generally involving more than one fiscal year, because field, analytical, and evaluation work tend to be iterative, as the study develops, and tephra data need to be integrated with other chronostratigraphic information and analysis. The Tephrochronology Lab usually conducts several large and small studies during the course of a year, with a productivity of between 250 and 350 samples per year, depending on budget and staff support.

 


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| Geochronology, Geochemistry, and Tracer Studies  |
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URL http://geology.cr.usgs.gov/capabilities/gronemtrac/geochron/teph/tech.html