Magnetic stratigraphy and tectonic rotation of the middle Eocene Coaledo formation, Southwestern Oregon

Abstract

The Coaledo Formation in Coos Bay on the southern coast of Oregon consists of over 2000 m of deltaic sediments that filled a forearc basin during the middle Eocene. This unit yields molluscs of the 'Cowlitz-Coaledo fauna', one of the oldest and best-studied faunas in the Pacific Northwest, but its precise age has been controversial. Over 100 magnetic samples spanning the formation were analysed using both thermal and alternating-field demagnetization. They yielded a stable remanence held mainly in magnetite, which passed a reversal test and showed a tectonic rotation of 105° ± 5°. This is greater than any other tectonic rotation so far reported from the region. It is consistent with the previously observed trends that the tectonic rotations in the Coast Ranges of Oregon and Washington increase with increasing age of the rock unit, and increase towards the southwest. The magnetic polarity pattern in the Coaledo Formation best correlates with Chrons C18r-C20r (40.0-44.0 Ma), which is significantly older than the magnetic pattern in the Cowlitz Formation (36.5-38.3 Ma). This suggests that the 'Cowlitz-Coaledo fauna' is very long-lived (at least 36.5-44.0 Ma), spanning most of the middle Eocene.