Magneto Stratigraphy of the Coaledo Formation, Coos Bay, Oregon.

Abstract

The Coaledo Formation, exposed in Coos Bay on the southern coast of Oregon, consists of 6000 feet of deltaic sediments, which filled the forearc basin in the Oregon Coast Ranges. This unit has yielded middle Eocene molluscs of the "Cowlitz-Coaledo fauna," one of the oldest and best studied molluscan faunas of Oregon. Over 100 magnetic samples were collected and analyzed using both alternating field and thermal demagnetization. They yield at a stable remamence held mainly in magnetite which passed a reversal test and showed a tectonic rotation of 95 +/- 6 degrees. The magnetic polarity pattern best correlates with Chrons C18r - C20r (40.0 - 44.0 Ma), which is significantly older than the magnetic pattern of the Cowlitz Formation (36.5 - 38.3 Ma).