Borehole Magnetostratigraphy of Sediments in a U.S. Geological Survey Multiple-Completion Well, San Diego County, California

Abstract:

Magnetostratigraphy is a useful method in providing geochronologic information for stratigraphic sequences. In general, this dating method requires paleomagnetic samples to be collected from outcrops or drill cores, and the magnetic polarities of the samples to be determined in the laboratory. However, suitable exposures or cores are not always accessible for sampling, especially during investigations of deep-basin stratigraphy. One alternative to collecting discrete rock samples is to use a downhole magnetometer to log the magnetic properties of sediments in a borehole. Downhole measurements are often used to determine the magnetostratigraphy of sediments in deep sea drilling projects, where fine-grained sediments contain reliable paleomagnetic signals. We test whether this methodology can resolve the magnetic polarity of coarse-grained estuarine, fluvial, and marine sediments in a U.S. Geological Survey (USGS) multiple-depth water well in San Diego County, California, by correlating borehole magnetic data with discrete paleomagnetic samples from sediment cores collected in the same well. The well is 363 meters deep and penetrates artificial and Quaternary deposits, overlying Eocene and Cretaceous sedimentary formations. We use a BVM-03 borehole probe (use of this product does constitute endorsement by the USGS), with a three-component vector magnetometer (0.1 nT sensitivity) and a susceptibility sensor, to continuously record the in situ total magnetic induction and susceptibility of the surrounding sediment. Post-processing of these magnetic data produces a continuous magnetic polarity record of the borehole sediment, and preliminary results suggest the presence of multiple magnetic polarity reversals. Successful determination of magnetic polarity in the well will allow researchers to establish more precise ages for sedimentary formations in the San Diego area, and will support the use of borehole magnetometer systems in coarse-grained, fluvial environments.