REPEATING WAVE-FORM INITIATED BY A 180-190-KA GEOMAGNETIC EXCURSION IN WESTERN NORTH-AMERICA - IMPLICATIONS FOR FIELD BEHAVIOR DURING POLARITY TRANSITIONS AND SUBSEQUENT SECULAR VARIATION

New paleomagnetic, lithologic, and stratigraphic data are presented fr om the sediments of Lake Chewaucan in the Summer Lake Basin, Oregon. T he new data place better age constraints on the sediments and improve the accuracy of the previously published paleomagnetic record from thi s locality. A complex, yet distinct, waveform is observed in all three components of the paleomagnetic vector. The waveform begins as the 18 0-190 ka Pringle Falls/Long Valley/Summer Lake II geomagnetic excursio n and continues for two cycles after the excursion, until the record i s interrupted by an unconformity that we correlate to the oxygen isoto pe stage 6/5e boundary. The waveform's directional morphology in virtu al geomagnetic pole (VGP) space is defined by two clockwise loops foll owed by a distinctive counterclockwise, clockwise, counterclockwise lo oping sequence. The VGP paths of the two cycles after the excursion ar e rotated 180 degrees about Earth's spin axis with respect to the VGP paths of the excursion cycle. The waveform also consists of a relative paleointensity variation which repeats during the two cycles after th e excursion. The average paleointensity of the postexcursion waveform repetitions is high relative to the extremely low values that occur du ring the excursion. This observation indicates that excursion-initiate d secular variations can occur after the field fully recovers from the low intensities which commonly typify excursions. Because of the simi larities noted previously between this excursion and full polarity tra nsitions (Tric et al., 1991), our new observations constrain models fo r a wide range of field behavior including polarity transitions, excur sions, and secular variation.