HR: 0800h
AN: GP11D-0879    [Abstracts]
TI: Anomalously Shallow Paleomagnetic Inclinations of the mid-Turonian Turbidites From Baja California Sur, Mexico
AU: * Li, Y
EM: YOL5@Lehigh.edu
AF: Dept. of Earth and Environmental Sciences, Lehigh University, 31 Williams Drive, Bethlehem, PA 18015 United States
AU: Kodama, K P
EM: KPK0@Lehigh.edu
AF: Dept. of Earth and Environmental Sciences, Lehigh University, 31 Williams Drive, Bethlehem, PA 18015 United States
AU: Smith, D P
EM: douglas\_smith@csumb.edu
AF: Earth System Science and Policy Institute, California State University Monterey Bay, 100 Campus Center, Seaside, CA 93955 United States
AB: Paleomagnetic inclinations of sedimentary rocks tend to be biased toward shallower than expected values, probably largely due to post-depositional burial compaction that is controlled by the intrinsic sedimentologic factors such as clay content, grain size, and sedimentary facies. As part of our continued efforts to quantify the sedimentalogical controls on inclination shallowing and thus to correct the shallow inclinations, we have conducted a paleomagnetic and rock magnetic study of the mid-Turonian ($\sim$92 Ma) turbidites from Arroyo San Lorenzo ($27.0\deg$N, $113.7\deg$W), Baja Californina Sur, Mexico. Fifteen block samples were collected from two 25 cm thick beds, which contain mudstone and siltstone/sandstone layers, within a deep marine sandstone-mudstone turbidite sequence. The two beds are stratigraphically $\sim$15 m apart and the mudstone layer in the stratigraphically lower bed displays faint laminations. These fine-grained beds were likely deposited from waning turbidity currents. All samples were subjected to either stepwise alternating field (af) demagnetization up to 120 mT or progressive thermal demagnetization up to $650\deg$C. Both the susceptibility and remanence anisotropies were measured to identify and correct shallow inclinations, if any. The 19 mudstone samples from the two beds show similar directions with a mean of Dec=$323.3\deg$, Inc=$29.3\deg$, k=46.4, $\alpha$$_{95}$=$5.0\deg$ and the 44 siltstone/sandstone samples from the two beds yield directions with a mean at Dec=$322.4\deg$, Inc=$47.1\deg$, k=43.8, $\alpha$$_{95}$=$3.3\deg$ in stratigraphic coordinates. The most prominent features of the paleomagnetic results are that both beds yield inclinations shallower than the expected cratonic inclination ($\sim$$57\deg$) for the sampling site and the inclinations of the mudstone samples are $\sim$$17\deg$ shallower than those of the siltstone/sandstone samples. The disparate inclinations of the mudstone and siltstone/sandstone layers suggest that these rocks carry a primary remanence but the inclinations were likely biased by depositional and/or postdepositional processes and need to be corrected prior to being used for tectonic interpretations. The magnetic susceptibility and remanence data show a predominately oblate fabric with minimum axes perpendicular to the bedding, indicating that the shallow inclinations were caused by compaction and can be corrected by their remanence anisotropies. The ongoing laboratory experiments focus on determining the {\it a} factor, the anisotropy of individual magnetic particles, for mudstones and siltstones/sandstones required by the theoretical correction model. The correction of the shallow inclinations will be discussed in the presentation.
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1518 Magnetic fabrics and anisotropy
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
DE: 1527 Paleomagnetism applied to geologic processes
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting