39 A New Three-Dimensional Look at the Geology, Geophysics, and Hydrology of the Santa Clara Valley, California: A Showcase of Urban Earth Science (Posters)

Sunday, 1 May 2005
9:00 AM - 5:00 PM

THE PALEOMAGNETIC RECORD AS MEANS OF CORRELATING SEDIMENTARY SEQUENCES ENCOUNTERED IN RESEARCH WELLS OF THE SANTA CLARA VALLEY, CALIFORNIA

Paper number: 39-11
Program: THEME SESSIONS
Time: 9:00 AM – 5:00 PM

Authors

Abstract

Paleomagnetic study of samples from six partially-cored research wells in the alluvial sediment of Santa Clara Valley provides a means of correlation within the basin. The pattern of reversals of the Earth's magnetic field during the past few m.y. is well established and ages of the reversal boundaries are often used to provide accurate time lines for geologic correlation. The 300-m nominal depth of the wells limits the expected age range of much of the sediment encountered to be younger than the last major reversal (the 780-ka Brunhes/Matuyama). In addition to seeking that boundary, therefore, we also address comparisons with changes that occur on time scales of 105 years or less, the paleosecular variation (PSV) of the geomagnetic field. Sedimentation in the valley was not continuous, and in only one well was more than about 20% of the depth cored, so we cannot identify long-term, continuous PSV trends but must rely entirely on brief periods when magnetic inclinations extended outside of the “normal” range.

Geomagnetic excursions generally range from about 500 years to perhaps 3-5 thousand years in duration and rarely record a complete polarity reversal. Most of the numerous excursions that have been proposed for the Brunhes Normal Polarity Chron worldwide are probably only regionally significant, making the total present in any given area limited. Despite intermittent sampling of the sedimentary record in the Santa Clara Valley, we have thus far encountered the Brunhes/Matuyama boundary in three wells and identified at least five younger stratigraphic intervals that record anomalous paleomagnetic inclinations. An age of 28,090 ± 330 radiocarbon years B.P. (calibrated age ~32.8 ka) obtained from the youngest of these intervals establishes its correlation to the Mono Lake excursion. Using estimates of sedimentation rates, we tentatively correlate other anomalous inclinations to excursions that have been reported for the western USA.