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Kitasu Hill and MacGregor Cone formed along the Principe Laredo Fault on British Columbia's central coast as the Wisconsinan ice sheet withdrew from the Coast Mountains. These small-volume Milbanke Sound Volcanoes (MSV) provide remarkable evidence for the intimate relationship between volcanic and glacial facies. The lavas are within-plate, differentiated (low MgO < 7%) Ocean Island Basalts, hawaiites, and mugearites that formed from ∼1% decompression melting of asthenosphere with residual garnet. Kitasu Hill, on glaciated bedrock, formed between 18 and 15 cal ka BP. Dipping, poorly stratified, admixed hyaloclastite, and glacial diamicton with large plutonic clasts and pillow breccia comprise its basal tuya platform (0–43 masl). Subaerial nested cinder cones, with smaller capping lava flows, sit atop the tuya. New marine samples show McGregor Cone formed subaerially but now sits submerged at 43–200 mbsl on an eroded moraine at the mouth of Finlayson Channel. Seismic data and cores reveal glaciomarine sediments draping the cone's lower slopes and show beach terraces. Cores contain glaciomarine diamictons, ice-rafted debris, delicate glassy air fall tephra, and shallow, sublittoral, and deeper benthic foraminifera. Dates of 14.1–11.2 cal ka BP show volcanism spanned ∼2000 years during floating ice shelf conditions. The MSV have similar proximal positions to the retreating ice sheet, display mixed volcano-glacial facies, and experienced similar unloading stresses during deglaciation. The MSV may represent deglacially triggered volcanism. The dates, geomorphic and geological evidence, constrain a local relative sea level curve for Milbanke Sound and show how ice gave way to fire. [ABSTRACT FROM AUTHOR]

Sediment cores recovered from alpine and subalpine lakes up to 250 km apart in northern British Columbia contain five previously unrecognized tephras. Two black phonolitic tephras, each 5-10 mm thick, occur within 2-4 cm of each other in basal sediments from seven lakes in the Finlay River--Dease Lake area. The upper and lower Finlay tephras are slightly older than 10 220-10 560 cal year B.P. and likely originate from two closely spaced eruptions of one or two large volcanoes in the northern Cordilleran volcanic province. The Finlay tephras occur at the transition between deglacial sediments and organic-rich postglacial mud in the lake cores and, therefore, closely delimit the termination of the Fraser Glaciation in northern British Columbia. Sediments in Bob Quinn Lake, which lies on the east edge of the northern Coast Mountains, contain two black tephras that differ in age and composition from the Finlay tephras. The lower Bob Quinn tephra is 3-4 mm thick, basaltic in composition, and is derived from an eruption in the Iskut River volcanic field about 9400 cal years ago. The upper Bob Quinn tephra is 12 mm thick, trachytic in composition, and probably 7000-8000 cal years old. A fifth tephra occurs as a cryptotephra near the top of two cores from the Finlay River area and is correlated to the east lobe of the White River tephra (ca. 1150 cal year B.P.). Although present throughout southern Yukon, the White River tephra has not previously been documented this far south in British Columbia. The tephras are valuable new isochrons for future paleoenvironmental studies in northern British Columbia. Des carottes de sediments prelevees dans des lacs alpins et subalpins, eloignes les uns des autres par des distances atteignant 250 km, et situes dans le nord de la Colombie-Britannique, contiennent cinq tephras anterieurement non identifies. Deux tephras phonolitiques noirs, chacun d'une epaisseur de 5-10 mm, se trouvent de 2 a 4 cm l'un de l'autre dans les sediments de base de sept lacs dans le secteur de la riviere Finlay--lac Dease. Les tephras Finlay inferieur et superieur datent d'un peu avant 10220-10560 annees calendaires avant le present (annees cal. BP) et proviennent probablement de deux eruptions rapprochees d'un ou de deux grands volcans dans le nord de la province volcanique de la Cordillere. Dans les carottes prelevees dans les lacs, les tephras Finlay se trouvent a la transition entre les sediments de deglaciation et la boue post-glaciaire, riche en matieres organiques; ils delimitent donc de maniere assez precise la fin de la glaciation Fraser dans le nord de la Colombie-Britannique. Les sediments dans le lac Bob Quinn, situe a la limite est de la chaine Cotiere septentrionale, contiennent deux tephras dont l'age et la composition different des tephras Finlay. Le tephra Bob Quinn inferieur, d'une epaisseur de 3-4 mm, possede une composition basaltique et il provient d'une eruption du champ volcanique de la riviere Iskut survenue il y a environ 9400 annees cal. BP. Le tephra Bob Quinn superieur, d'une epaisseur de 12 mm, a une composition trachytique et il est age d'environ 7000-8000 annees cal. BP. Un cinquieme tephra se retrouve sous forme de cryptotephra pres du sommet des deux carottes prelevees dans le secteur de la riviere Finlay; il est correle au lobe est du tephra White River (~1150 annees cal. BP). Bien qu'il soit present dans tout le sud du Yukon, le tephra White River n'a pas ete documente auparavant aussi loin dans le sud de la Colombie-Britannique. Les tephras servent de nouveaux isochrones utiles pour de futures etudes pal eoenvironnementales dans le nord de la Colombie-Britannique. [Traduit par la Redaction], Introduction The northern Cordilleran volcanic province consists of over 100 late Cenozoic volcanic centres in northwest British Columbia, southwest Yukon, and southeast Alaska (Edwards and Russell 1999, 2000). The dominant [...]