A magnetostratigraphic and secular variation study of Level Mountain, northern British Columbia

Abstract

Summary. Palaeomagnetic results are reported from 50 flows erupted by Level Mountain, which is a composite volcano in northern British Columbia. Fourteen magnetic polarity zones are represented, and these allow a magneto- stratigraphy to be established which indicates that eruptive activity began about 6.5 Myr ago and continued up to very recent times with several hiatuses of up to 1 Myr or more. Most of the flows yield acceptable normal or reversed palaeopoles, but two of them record the Gauss/Matuyama polarity transition, and a further four yield a tight group of divergent directions, which are here interpreted as a geomagnetic excursion near the base of the Gauss Epoch. This observation lends support to an earlier suggestion (Bingham & Stone) that the geomagnetic field was abnormally disturbed about 3.5 Myr ago, at least locally in north-western North America. The overall average palaeopole obtained after filtering out flows with high internal variance and those yielding divergent virtual geomagnetic poles (VGPs) lies at 256E, 85N (N= 38, A,, = 7.2). A more representative palaeopole is obtained by combining the results with those of Souther & Symons from Mount Edziza, a closely related contemporaneous volcano some 100 km away. This pole lies at 292E, 87N (N= 92, Ag5 = 4.1 ), and again includes the geographic pole within its 95 per cent confidence limits. How- ever, the mean itself is not far-sided as Wilson's model predicts, neither is it left-handed as the analysis by Merrill & McElhinny predicts. The slight near- sidedness observed agrees with the global compilation of Wilson & McElhinny who show that the north-western region of North America is the only area, out of 15 regions worldwide, to yield a near-sided pole. The dispersion of the VGPs (angular standard deviation = 22) is a little higher than would be expected from global summaries and secular variation models. This was also found to be the case in data from Alaska (Doell & Cox). The angular dispersion, the occurrence of geomagnetic excursions, and the