Matuyama/Brunhes Polarity Transition Near Bishop, California

Exposed Pleistocene lake sediments at three localities, separated by as much as six kilometres, near Bishop, California, contain the change from reverse to normal palaeomagnetic polarity that occurred about 0.73 million years ago-the Matuyama/Brunhes (M/B) transition. A decrease in the relative field strength is used to define the length of the transition. the width of that decrease exceeds by about one fifth the width where the palaeomagnetic directions show the switch in polarity.

Because some of the (usually) six subsamples in a measured horizon in the central part of the transition have normal polarity while others have intermediate or reverse polarity, the behaviour of the palaeomagnetic field in that region cannot be determined accurately. There is, however, a short reversal within the upper quarter of the transition at one locality that can be documented—at least 30 cm of normal polarity underlie reverse polarity at three sites spread over a distance of 150m. Taken as a whole, the palaeomagnetic data for the full transition are a warning that some rapid, short reversals within the transition that are reported in the literature might be caused by inaccurate recording of the palaeomagnetic field.

The palaeomagnetic polarity 17m beneath the M/B transition at one locality places a minimum age of about 1.0 Myr on the Glass Mountain-D ash bed that is found in east-central and southern California.