Magnetic Stratigraphy and Tectonic Rotation of the Oligocene Pysht Formation (Type Juanian Stage), Clallam County, Washington

C. Burns¹, D. R. Prothero², and E. Nesbitt^1
1Geology, Burke Memorial Museum, Univ. Washington, Seattle, WA
²Geology, Occidental College, Los Angeles, CA

The Pysht Formation (upper Twin River Group) consists of about 2000 m of deep-water sandstones and mudstones between Pillar Point and Mudrock Creek on the southern shore of the Straits of Juan de Fuca. It was the basis for Durham’s (1944) Echinophoria apta molluscan zone, and for Addicott’s (1976) “Juanian Stage.” This stage was originally considered early Miocene, but others have correlated it with the late Oligocene, or thought that it straddled the Oligocene-Miocene boundary. Paleomagnetic samples spanning the entire Pysht Formation showed a stable remanence held mainly in magnetite, which passed a reversal test. The Pysht Formation has undergone a tectonic rotation of 49 +/–6 degrees, consistent with similar rotations obtained from the overlying Clallam Formation, or from the Blakeley Formation near Seattle, but not with earlier data which showed scattered directions. We correlate the Pysht Formation with Chrons C11r-C6Cn3n (23.7–30.5 Ma), making the Juanian Stage late early Oligocene to latest Oligocene in age.

AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California