Pacific Section AAPG Convention

Datapages, Inc.Print this page

Looking For Normal In The Lodo Gulch: Magnetostratigraphy Across The Paleocene-Eocene Thermal Maximum


A paleomagnetic study on Ocean Drill Program (ODP) core section 13H6 from Walvis Ridge, Southeastern Atlantic Ocean conducted by Lee and Kodama (2009), reported a brief normal subchron spanning the Paleocene-Eocene Thermal Maximum (PETM) within Geomagnetic Polarity Timescale Chron C24r. Since such a feature would be of tremendous utility in synchronizing stratigraphic records, our study attempts to test the existence of this brief normal subchron. The Lodo Formation type section, located in western Fresno County, contains the PETM. Paleocene- to Eocene-age Lodo Formation was deposited on a continental margin where sedimentation rates were much higher, 16.8 cm ka-1 (John et al., 2008) than those for the abyssal ocean, ∼1 cm ka-1 (Stap et al., 2009). Our dense sampling of the PETM consisted of thirty-one samples from a 5.16-meter section. Samples were subdivided into specimens and subjected to either thermal demagnetization up to 580 degrees C in fifteen steps in a nitrogen atmosphere oven or alternating field (AF) demagnetization and measured using a SQUID cryogenic magnetometer housed in a mu-metal shielded room (Kirschvink et al., 2008). Twenty-eight of the thermally demagnetized samples produced good results while the AF demagnetized results were mostly problematic. Remanence carriers were likely magnetite and maghemite which oxidizes to hematite at high temperatures with single-domain magnetite hosting the ChRM. The site mean ChRM of Ds=158.0 degrees Is=-64.8 degrees n=28 and alpha 95=11.6 degrees produces a paleomagnetic pole indistinguishable from that of Bess and Courtillot (2000) for North America at this time. This indicates no inclination shallowing, no poleward transport, no vertical axis rotation, and higher-than-expected angular dispersion (s=32.7 degrees). This study shows no evidence for a normal polarity during the PETM.