RUI: Magnetic Stratigraphy and Tectonic Rotation of the Middle Eocene Cowlitz Formation and Equivalents, Oregon and Washington

Occidental College, Los Angeles CA

Investigators

Abstract

MAGNETIC STRATIGRAPHY AND TECTONIC ROTATION OF THE MIDDLE EOCENE COWLITZ FORMATION AND EQUIVALENTS, OREGON AND WASHINGTON Donald R. Prothero EAR 0001194 The Eocene-Oligocene transition was a critical point in Earth history, when the tropical "greenhouse" world of the Cretaceous to early Eocene was replaced by essentially modern "icehouse" conditions of the early Oligocene. This project will conduct magnetostratigraphic sampling on key exposures of marine rocks, which record the beginning of the Eocene-Oligocene transition in Washington and Oregon. Some of the most important exposures are of the upper middle Eocene Cowlitz Formation in western Washington. These strata yield the classic "Cowlitz-Coaledo" molluscan faunas, and are also important for their oil and coal deposits. Preliminary results from the type area of the Cowlitz Formation at Olequa Creek show that it yields both normal and reversed polarities. Based on a 40Ar/39Ar date of 38.9 +/- 0.01 Ma, the type section of the Cowlitz Formation probably correlates with magnetic Chron C17n-C17r (36.5-38.5 Ma). PI proposes to sample further sections in the Cowlitz Formation, and its equivalent, the Hamlet Formation of Oregon. The magnetic stratigraphy of these sections offers the potential of much higher-resolution correlation of middle Eocene to Oligocene strata with the global time scale, and with other Eocene-Oligocene strata and faunal sequences. This will allow the underexploited data base of the Pacific Northwest marine molluscs and benthic foraminifera to be compared with the growing body of knowledge about the effects of the Eocene-Oligocene climatic changes on the organisms of the rest of the world. High-precision, high-resolution chronostratigraphy of these strata will also be important to petroleum geologists who study these important reservoir rocks, and to geologists who want to better understand the timing of events in this tectonically active region. In addition, the rocks of the type section at Olequa Creek show a clockwise tectonic rotation of 97 +/- 11 degrees, consistent with other results reported from the region. By sampling a large number of Cowlitz-equivalent outcrops in both Oregon and Washington, PI hopes to improve understanding of the complex tectonic rotations that each structural block has undergone since the Eocene.