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Correlation of Neogene Continental, Onshore-Marine, and Deep-Ocean Basins by Means of Tephra Layers, Pacific Coast Margin of Conterminous United States

Andrei M. Sarna-Wojcicki

Chemical "fingerprinting," petrography, and stratigraphic sequences of tephra layers are making correlations possible among Neogene basins of the northeastern Pacific margin (Pacific Coast of the conterminous United States) with greater precision than previously. Until recently, it has been difficult to make biostratigraphic temporal correlations among continental, onshore-marine, and deep-ocean basins because different fossil assemblages occur within the sediments of these three basin types. Moreover, variations in grain size, bioturbation, and other problems of signal preservation have made age determinations by magnetostratigraphy difficult or impossible in many places. Tephra layers are present in all three types of basins, however, and many of these have been dated b isotopic or other methods. Some well-dated, widespread tephra layers have been identified in all three types of basins; among these are the Wilson Grove (6.0 Ma), Alturas (4.8 Ma), Huckleberry Ridge (2.0 Ma), Bishop (0.74 Ma), and Loleta (about 0.35 Ma). About 20 major widespread, dated tephra layers, ranging in age from 6 Ma to recent, have been identified along the northeastern Pacific margin, together with several tens of tephra layers that have more limited areal distribution.

In addition to complementing existing biostratigraphic and magnetostratigraphic data within individual basins, tephra correlations make it possible to compare and reconcile provincial biostratigraphic chronologies with worldwide chronologies, such as the magnetostratigraphic and oxygen-isotope time scales. Tephra layers also provide checks on numerical datum levels derived by other geochronologic techniques. Current work is extending some tephrochronologic correlations along the northeastern Pacific margin to about 12 Ma.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.