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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 920

Last Page: 920

Title: Eocene Eustatic Versus Tectonic Changes on Pacific Margin--Comparison Between San Diego, California, and Coos Bay, Oregon: ABSTRACT

Author(s): R. H. Dott, Jr., M. Chan

Article Type: Meeting abstract


Distinction of eustatic from tectonic causes of sea level change in environments like the Pacific margin is difficult, but the worldwide Vail sea level curve provides a useful standard for comparison. It has been applied with apparent success to the Paleogene of southern California. At San Diego, early Eocene regression was followed by middle Eocene transgression and partial filling of a submarine canyon. At the end of middle Eocene time, regression allowed a gravelly fan-delta to debouch into the canyon. Transgression followed in late Eocene time, and then latest Eocene regression resulted in deposition of nonmarine sediments. These three sea level changes fit the Vail curve so well that eustatic changes seem indicated.

Around Coos Bay, Oregon, middle Eocene turbidites (Flournoy-Tyee) with bathyal foraminifera are overlain by siltstones with evidence for deposition in shallowing-upward neritic depths (Elkton). This prograding shelf sequence is punctuated by channels filled with siltstone or massive sandstone. Shelf deposits are overlain unconformably by a coarse, sandy, coal-bearing facies (Coaledo), which represents progradation by a delta complex across the former narrow shelf. Both here and at San Diego, deltas remained active during late Eocene transgression. Regression at the end of middle Eocene time here and in Washington correlates so closely with San Diego and the Vail curve as to suggest a eustatic fall as its cause. In latest Eocene time at Coos Bay, however, abrupt transgression with depo ition of muds (Bastendorf) at lower bathyal depths occurred while widespread regression characterized southern California. Seemingly in Oregon, local tectonic subsidence masked the latest Eocene eustatic fall.

Preliminary comparisons between widely scattered synchronous localities suggest that the Vail curve offers promise for ultimately distinguishing the elusive causes of transgression and regression even in tectonically mobile regions.

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Copyright 1997 American Association of Petroleum Geologists