NEW 40AR/39AR DATA FROM PLEISTOCENE BASALTS ON THE EAST SNAKE RIVER PLAIN, IDAHO, AND THEIR IMPLICATIONS FOR THE SUBSURFACE STRUCTURE OF THE BIG LOST TROUGH

New age data were obtained for 2 subsurface basalt samples collected at the Idaho National Engineering and Environmental Laboratory. The sample from well CH-2-2A from a depth of 103.6 m yielded a plateau age of 0.71 ± 0.06 Ma on a groundmass concentrate from a reverse-polarity basalt flow (depth 97-120 m, inclination=-74.9, Bestland et al., 2002). The sample from well C1A from a depth of 364.2 m yielded a plateau age of 0.82 ± 0.16 Ma on a groundmass concentrate of a normal-polarity basalt flow. The age spectra for the CH-2-2A sample was flat and for the C1A sample was very-slightly discordant.

In CH-2-2A, a negative polarity (inclination=-73.9, Bestland et al., 2002) basalt flow at a depth of 90.9 m was 39Ar/40Ar dated at 0.58 ± 0.15 Ma by Hughes et al. (2002). The simplest explanation for these two CH-2-2A dates in reverse-polarity basalt is that the later may belong to the Big Lost SubChron of Champion et al. (1988) and the former to the very top of the Matuyama Chronozone. The new age date for C1A is more problematic. C1A reverse polarity intervals are at depths of 121-173 m, 213-326 m and below ~449 m (Champion, 2002). If the C1A age date at 364.2 m depth is correct, then the normal polarity basalts from 362-425 m can not belong to the Olduvai SubChron as previously thought (e.g. Blair, 2002) but belong either to an unidentified paleomagnetic excursion, the younger Jaramillo SubChron or at the bottom of the Bruhnes at ~780 ka. The C1A sediment interval at ~425-449 m, which has previously defined the shallowing southern lip of the subsurface depression of the Big Lost Trough (Blair, 2002) on the basis that these sediments belonged to the Olduvai SubChron, is likely much younger than this. This pushes the Olduvai SubChron much deeper in C1A and suggests that the Big Lost Trough may actually be a southward plunging subsurface syncline instead.