HR: 1340h
AN: V13D-0573    [Abstracts]
TI: Argon Geochronology of Lavas at Mt. Veniaminof Volcano Dates Pleistocene Climatic Events on the Alaska Peninsula
AU: * Calvert, A T
EM: acalvert@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, MS-910, Menlo Park, CA 94025 United States
AU: Bacon, C R
EM: cbacon@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, MS-910, Menlo Park, CA 94025 United States
AU: Sisson, T W
EM: tsisson@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, MS-910, Menlo Park, CA 94025 United States
AB: Mt. Veniaminof is a 350 km³, 2500 m-high tholeiitic Aleutian arc volcano with an 8 km-diameter ice-filled caldera. Terminal moraines and drift indicate that ice tongues reached the Bering Sea coastal plain during the last glacial maximum (LGM). Presently, terminations of major valley glaciers range from ~250 to ~1000 m asl. Many Veniaminof lava flows erupted throughout its >250 kyr history have palagonitic breccias and/or polygonal chill-jointing. Ice-free conditions are difficult to document, but several candidate lavas flowed unimpeded into glacier-eroded valleys. Nearly 30 ice-diagnostic deposits have been Ar dated (±1σ) in a detailed study of Veniaminof's eruptive history. We compare results to the marine oxygen isotope stages (MIS) 2-8 of Bassinot et al. (1994, EPSL, v. 126, p. 91-108). Unlike techniques applied to recessional features of glaciers, dating ice-contact lava flows also yields ice thickness estimates at time of eruption. Lavas with ages near MIS 8, 6, 4 and 2 commonly have ice-marginal textures at <500 m asl. For example, the 259±18 ka age of basalt with palagonitic breccias at 2650 m on the ENE flank matches MIS 8.2-8.4. Multiple features record MIS 6.2: on the W, 150±3 ka basalt ponded against Cone Glacier ice >180 m thick at 1000 m; to the SW, 142±7 ka dacite banked against >400 m of ice at 1000 m; and in the broad Chignik valley, subglacial andesite at 150 m correlates with 147±3 ka ice-bounded lava on the edifice. During MIS 4.2 to 4.0, chilled andesite mantles the N Cone Glacier valley wall (63±10 ka, 1580 m; 63±3 ka, 1430 m) suggesting ice 350-500 m thick; similarly, 56±1 and 54±2 ka dacite encountered >100m thick ice below 1000m on the SE flank; and subaerial 53±3 ka basaltic andesite on the SE flank caps a coeval lava delta, implying 240 m of impounded water. In late MIS 3 or early MIS 2, thick dacite at the head of Fog Glacier chilled against >150 m of intracaldera ice at 2000 m, demonstrating thick ice within the caldera by 33±6 ka. Chill-jointed andesite (31±5 ka, 900 m, >30 m ice) also overrode previously glaciated 48±2 ka andesite on the NW flank; and 28±4 ka andesite flowed into ponded water at 460 m on the WNW flank. Two units likely erupted during the LGM: a basalt W of Fog Glacier valley at 1100 m (18±18 ka), and an undated tuya indicating 240 m of ice at 700 m on the S flank. In contrast, interglacial conditions are suggested at several locations: subaerial basalt in the center of a broad valley on the WSW at 150 m, correlated with 116±7 ka (MIS 5.5) basalt at 900 m; subaerial basaltic andesite (NW, 82±3 ka, MIS 5.1) at 640 m in Fog Creek; 79±1 ka dacite that has ice-chilled margins at 1280 m on the E rim of a glacial valley but not at 750 m; and subaerial 59±2 ka (MIS 4.1) basaltic andesite overlain by chill-jointed basaltic andesite at 400 m on the SE flank. Lavas from MIS 3 indicate ice was present at upper elevations on some portions of the edifice: chill-jointed andesite on the SW flank (41±6 ka, 640 m, 240 m ice); basaltic andesite interstratified with palagonitic tuff S of Cone Glacier (39±10 ka, 1220 m, 900 m ice); and andesite from a NNW shield that chilled against a valley glacier (36±7 ka, 670 m, ?60 m ice). Well preserved levees on a MIS 3 lava substantiates a lack of thick ice on the NW flank at 100-400 m. Our results show that Ar geochronology and geologic mapping of a large volcano that frequently produced extensive lava flows provide quantitative local ice thicknesses and a high latitude climatic history consistent with the marine record.
DE: 1105 Quaternary geochronology
DE: 8408 Volcano/climate interactions (1605, 3309)
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005