Early Tertiary Foraminiferal and Nannofossil Zonation of California: Overview and Recommendation

A. A. Almgren, M. V. Filewicz, H. L. Heitman

Since 1939, age dating and correlation of Paleogene strata in California have been accomplished primarily using the benthic foraminiferal zones of Laiming and Mallory. Their zones and stages are defined by assemblages of benthic foraminifers in Opellian Zones at designated type sections or "typical" sections. Laiming's zonation, used extensively in the petroleum industry, relied on surface sections and subsurface control. Mallory's later work defined zones based largely on surface sections, in particular the section at Media Agua Creek. Since Mallory's work was published, however, many workers have noted that the zonation is fraught with certain problems in correlation.

In 1980, Poore pointed out that Mallory's stages needed revision. Poore's study of calcareous nannofossils and planktonic foraminifera found with the benthic foraminifera indicated that Mallory's stage assignments in his surface section were of different age relative to the planktonic zonation standard. Some of the problems of age assignment and miscorrelation inherent in Mallory's zonation are reviewed herein.

This paper proposes a multidisciplinary approach to establish age control by using a modification of Laiming's benthic zonation and Okada and Bukry's calcareous nannofossil zonation. This interrelated biostratigraphic zonation permits reliable age assignments and interbasin correlations.

We recommend using a modified Laiming zonation rather than a revision of Mallory's zonation. We would retain Laiming's zonal nomenclature to avoid confusion. Age and correlation of Laiming's benthic zones are not radically altered; revision of Mallory's stages would require redesignation of type sections as well as redefinition of species ranges. We believe that the continued use of Mallory's stages, with dual definitions, would lead to further confusion.

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