Eocene-Oligocene Boundary Problems, West Coast, North America: ABSTRACT

Abstract

Correlation of the international Eocene-Oligocene boundary with the provincial biostratigraphic framework of the northeast Pacific margin has been and continues to be controversial. The controversy centers about historical nomenclature and correlations, and current correlations based on planktonic fossil group. The Geological Society of America's C. E. Weaver Committee published the first interdisciplinary correlation chart for the Cenozoic rocks of the western United States in 1944. The committee placed the Eocene-Oligocene boundary at the base of the Keasey Molluscan Stage and Refugian Benthic Foraminiferal Stage. These correlations were based on faunal similarity between Pacific Coast and European assemblages. The most useful provincial boundaries of Late Eocene to Oligocene age are the Narizian-Refugian and Refugian-Zemorrian Benthic Foraminiferal Stage boundaries. Reevaluation of the Refugian Stage has recently been completed. The stage boundaries have been correlated to the international geologic time scale using planktonic microfossils. Rigorous stratigraphic superposition of planktonic floras and faunas from continuously deposited foraminiferal-rich rocks results in correlation of the Narizian-Refugian Benthic Foraminiferal Stage boundary with the calcareous nannofossil zonal boundary NP18/NP19, and with planktonic foraminiferal Zone P15, in California, Oregon, and Washington. Thus, the Narizian-Refugian boundary represents a provincial chronostratigraphic datum of early late Eocene age. Planktonic assemblages are rare in samples from above and below the Refugian-Zemorrian Benthic Foraminiferal Stage boundary. In California this boundary is commonly at an unconformity or without superposition of diagnostic faunas. In southwestern Washington the Refugian-Zemorrian boundary occurs in continuously deposited and foraminiferally rich sections. Nannofloras assigned to Zone NP22 and planktonic foraminifera assigned to the interval of Zones P17 to P20 occur in strata containing foraminifera assigned to the Zemorrian Benthic Foraminiferal Stage. Nannofloras assigned to the intervals of Zones NP20 and NP21, and Zones NP19 to NP21, and planktonic foraminifera assigned to the intervals of Zones P16 and P17, and Zones P15 to P17 occur in strata containing foraminifera assigned to he Refugian Benthic Foraminiferal Stage. Therefore, the Refugian-Zemorrian boundary occurs within planktonic Zones NP21 and P17, roughly correlative with the international Eocene-Oligocene boundary. The above correlations result in the reassignment of much of the 1944 Weaver chart's Oligocene to the Eocene: the Keasey and Lincoln Molluscan Stages and the Refugian Benthic Foraminiferal Stage are now recognized as late Eocene in age. Radiometric calibration of the provincial boundaries is not yet possible. Whole rock potassium-argon and fission track dates are available but both have very large error bars or lack adequate biostratigraphic control to be useful. Fossiliferous stratigraphic sections have rocks with sufficient remanent magnetism for magnetostratigraphic studies but to date only reconnaissance data are available. End_of_Article - Last_Page 413------------