NEW MAGNETOSTRATIGRAPHIC ANALYSIS INDICATES THAT ONSET OF PLEISTOCENE CATACLYSMIC FLOODING PREDATES 1.07 MA IN EAST-CENTRAL WASHINGTON

Pleistocene cataclysmic flood deposits, informally referred to as the Hanford formation, blanket the Pasco Basin in south-central Washington. A thick sequence (up to 100 m) of flood-deposited gravel, sand, and silt underlies much of the USDOE’s Hanford Site. Correlation of cataclysmic flood deposits is often difficult because of a lack of marker horizons and rapid lateral facies changes. However, paleomagnetic reversals provide an opportunity to establish time-stratigraphic units within flood deposits.

Our samples are derived from five new boreholes penetrating the giant Cold Creek flood bar. A total of 121 new paleomagnetic samples were collected in the spring of 2001 from split-spooned drill cores and analyzed for paleomagnetic inclination. This is in addition to 60 samples previously analyzed (Bjornstad et al., 2001). Paleomagnetic samples, obtained from finer-grained facies, consisted of silt to coarse-grained sand. Very detailed stepwise demagnetization experiments were performed on samples to remove magnetic overprints. We performed an average of 15 steps of alternating field (AF) demagnetization between 0 and 200 milli-Tesla (mT) or 12 steps of thermal demagnetization between room temperature and 600°C. Demagnetization revealed both reversed and normal polarities.

On the eastern end of Cold Creek bar one new and two previously-sampled boreholes span a total section of 14-82 m depth. The Bruhnes-Matuyama polarity boundary (0.78 Ma) appears to be shallower than 14 m (shallowest sample depth); the sampled sequence is dominantly reversed-polarity. One normal polarity interval between about 54 and 69 m, probably represents the Olduvai (1.77-1.95 Ma) normal subchron or less likely the Jaramillo (0.99-1.07 Ma) normal subchron. Therefore, the oldest deposits penetrated by these boreholes are at least older than 1.07 Ma and more likely older than 1.95 Ma, thus providing new constraints for the onset of ice-age flooding in the Pacific Northwest.

Samples from four new and two previous boreholes on the western portion of Cold Creek bar are mostly normal in polarity. However, two thin reversed-polarity horizons are evident in four of these boreholes. The reversed intervals may represent magnetic excursions during the Brunhes or incompletely-recorded Matuyama-aged flood deposition.