Age and correlation of a paleomagnetic episode in the western United States by ⁴⁰Ar/³⁹Ar dating and tephrochronology: The Jamaica, Blake, or a new polarity episode?

Abstract

High‐resolution paleomagnetic records from two sites near Pringle Falls, Oregon, are compared with similar records from Summer Lake, Oregon, ∼170 km to the southeast: Paoha Island, in Mono Lake, ∼660 km to the southeast and Benton Crossing, in Long Valley, approximately 700 km to the southeast, in east‐central California. The sequences at Pringle Falls contain a distinctive coarse pumice‐lapilli tephra layer which we have dated as 218±10 ka by 40Ar/39Ar step‐heating of plagioclase feldspar. Stratigraphically, this tephra is closely associated with a suite of several other tephra layers that bracket the interval studied paleomagnetically. Each tephra layer is distinguished by the unique chemical composition of its volcanic glass shards. The pumice layer dated at Pringle Falls is correlated with layers at three of the other localities. Using all the tephra layers, we can correlate the lake stratigraphic sequences and associated paleomagnetic records among the four distant localities. Additional age control is obtained from a fifth locality at Tulelake in northern California, where the stratigraphic interval of interest is bracketed between ∼171±43 and approximately 140 ka. Characteristics of the paleomagnetic records indicate virtually identical paleofield variation, particularly the geometry of a normal to normal (N‐N) geomagnetic polarity episode. The observed paleofield behavior resembles the Blake geomagnetic polarity episode, but is significantly older than the generally accepted age of the Blake episode. Either the age of the Blake episode is significantly underestimated, or the polarity episode documented here is older, perhaps the Jamaica episode, or is an as yet unreported episode. A corollary of the latter option is that paleomagnetic polarity episodes of different ages may have similar transition polar paths, a conclusion implying that a common mechanism is involved.

Global Correlation of the 223 ka Pringle Falls Event

“…The agreement in time and space is remarkable. As dis cussed in detail in Herrero-Bervero et al (1994), VGPs from the three sections in western North America are in good agreement. VGPs from volcanic rocks in New Zealand record only a fraction of the total event path and are not nearly as complete as the sedimentary records from North America.…”

“…New 40 Ar/ 39 Ar ages ( Fig. 1) from plagioclase in the "D" ash layer at Pringle Falls (Herrero-Bervero et al, 1994) indicate that the Pringle Falls event occurred at 218 ± 10 ka, significantly older than the com monly accepted 100-130? ka age for the Blake event.…”

Chronological and palaeomagnetic constraints on widespread welded ignimbrites of the Taupo volcanic zone, New Zealand

“…This unit was fortuitously emplaced during an excursion in Earth's magnetic field. An excursion of similar age is recorded in sediments in the western United States: the Pringle Falls episode (Herrero-Bervera et al 1994). Being located in the southern hemisphere, the Mamaku ignimbrite may provide evidence for this event on a global scale.…”

Paleomagnetism of Unzen volcano: A volcanic record (Senbongi excursion) of the Iceland Basin event and the Brunhes VGP distribution for Japan

“…5 along with the schematic VGP paths for the IB (Channell, 1999) and PF events (Herrero-Bervera et al, 1994). Although the meaning of an intermediate VGP location during an excursion should be treated with caution, the Senbongi VGP falls almost exactly on the VGP path for the IB event and not on the PF VGP path perhaps supporting the correlation.…”

“…Champion et al (1988) reviewed the geomagnetic events within the Brunhes chron and referred to the event at 200 ka as the Jamaica/Biwa I event. Herrero-Bervera et al (1994) produced a 40 Ar/ 39 Ar age of 218±10 ka on a tuff layer proximal to a geomagnetic event they called the Pringle Falls event that has been found in a number of lake deposits in the western USA. Among other reports of a ∼200 ka excursion is Channell et al's (1997) documentation of what they call the Iceland Basin event from detailed paleomagnetic and oxygen isotope logs of sediment APC cores retrieved off Iceland (ODP Site 983).…”