COMBINED COSMOGENIC BURIAL DATING AND MAGNETOSTRATIGRAPHY OF PRE-LGM CATACLYSMIC FLOOD SEDIMENTS, PASCO BASIN, WA

Previous studies have suggested that Missoula floods or similar events have debouched across the Channeled Scablands of eastern Washington not only during the Last Glacial Maximum (LGM) but also during earlier glacials since the early Pleistocene. Our work in the Pasco Basin, WA, combines cosmogenic burial dating (²⁶Al/¹⁰Be) with magnetostratigraphy of sediments to establish more robust ages for these otherwise undatable deposits. Here we summarize data from two localities in the Pasco Basin, Cold Creek giant flood bar and Kiona Quarry. Both contain thick sections (30+ meters) of what are thought to be middle and early Pleistocene cataclysmic flood sediments and intercalated paleosols.

Magnetostratigraphy from drill cores through the Cold Creek Bar (Pluhar et al., 2005 in press) suggests two reversed (or transitional) polarity zones separated by a normal zone. This had previously been interpreted as recording the Jaramillo normal subchron (0.99-1.07 Ma). However, eight preliminary cosmogenic burial ages on weak paleosols intercalated within Cold Creek bar suggest burial during the Brunhes normal chron (0-0.78 Ma). The burial dating therefore indicates that the polarity structure may record short-lived magnetic excursion(s) during the Brunhes. If true, this may permit more accurate geochronology of pre-LGM Missoula Flood sediments by correlation with excursions during the Brunhes.

A surface exposure at Kiona Quarry, WA, also composed of cataclysmic flood sediments, yielded very different results. A pedogenically-altered gravel at this locality exhibits exponentially decreasing cosmogenic nuclide concentrations with depth, supporting its interpretation as a paleosol or paleosol remnant. Preliminary burial ages on this paleosol suggest burial during the early Pleistocene. This is consistent with the reversed polarity of the sediments directly beneath the paleosol, which indicates deposition prior to the Brunhes-Matuyama reversal at 0.78 Ma.

These results emphasize that magnetostratigraphy is best combined with supplementary independent age indicators in order to determine uniquely the proper correlation with the magnetic polarity timescale. Furthermore, previous work that assigned ages based solely on paleomagnetic polarity should be reassessed in light of these new results.