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Detailed stratigraphy of the N2 Grande Ronde Basalt, Columbia River Basalt Group, in the central Columbia Plateau

By
R. Dale Landon
R. Dale Landon
Westinghouse Hanford Company, P.O. Box 1970, Richland, Washington 99352
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Philip E. Long
Philip E. Long
Westinghouse Hanford Company, P.O. Box 1970, Richland, Washington 99352
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Published:
January 01, 1989

Stratigraphy of individual basalt flows in the N2 magnetostratigraphic unit of the Grande Ronde Basalt (GRB) within the central Columbia Plateau has been developed using data from seven surface sections and fifteen boreholes. Twenty-one individual flows have been identified and grouped into eight flow packages. The flow correlations were developed based on chemical composition, paleomagnetic vector direction, stratigraphic position, and thickness of individual flows. A multivariate statistical procedure, discriminant analysis, was used to test the validity of using chemical composition alone to define the flow packages. Results of the test show that samples can be correctly classified within one adjacent flow package in 94 percent of the cases.

Application of discriminant analysis to chemical composition data indicates that within the Pasco Basin the upper two-thirds of the N2 GRB contains 17 individual flows, of which only 9 to 15 may be present at any one location. Seven of these flows are present throughout the portion of the Pasco Basin studied. Correlation of flows between boreholes or surface sections means that units with the same stratigraphic position and diagnostic characteristics have been identified. In most cases this means that a single flow formed from one eruption. However, some flows may not be continuous, or some correlated flows may represent eruptions separated by as much as several thousand years but which gave rise to flows with identical stratigraphic positions and similar characteristics. The greatest number of flows occurs in the southeastern part of the basin, but the thickest total section occurs in the western and northwestern parts of the basin.

The detailed flow correlations provide evidence of deformation during emplacement of the N2 GRB. Variations of the thickness of individual flows and packages of flows disclose that subsidence was greatest in the western portion of the basin and that growth of the Yakima Ridges began at least by late GRB time. This timing of deformation of the Yakima Ridges is consistent with previous interpretations.

We conclude that discriminant analysis applied to chemical composition of GRB flows provides a means of quantitatively defining correlation of flow packages. The method yields estimates of uncertainty in correlations but must be applied in the context of an appropriate stratigraphic framework. Discriminant analysis should be useful in a variety of volcanic terranes where subtle but consistent differences in composition occur between individual flows or flow packages. Successful application of the technique requires that variations in the chemistry of individual flows or packages of flows be generally less than chemical differences among flow packages and that a relatively large number of samples be available for classification.

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GSA Special Papers

Volcanism and Tectonism in the Columbia River Flood-Basalt Province

Stephen P. Reidel
Stephen P. Reidel
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Peter R. Hooper
Peter R. Hooper
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Geological Society of America
Volume
239
ISBN print:
9780813722399
Publication date:
January 01, 1989

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