HR: 11:35h
AN: V12B-06 [Abstracts]
TI: Pleistocene Paleoclimatic Features at Mount Mazama Volcano and the Crater Lake Region, Oregon, Dated by
Ar Geochronology
AU: * Bacon, C R
EM: cbacon@usgs.gov
AF: USGS, 345 Middlefield Rd, Menlo Park, CA 94025
United States
AU: Lanphere, M A
EM: alder@usgs.gov
AF: USGS, 345 Middlefield Rd, Menlo Park, CA 94025
United States
AB:
Fifteen examples of the interplay between glacial ice and magma or volcanic rock in the Crater Lake region have been dated by
K-Ar or 40Ar/39Ar. Ages of glacial features compare well with times of extensive ice presence implied by marine
oxygen isotope stages (MIS; Bassinot et al. 1994 EPSL 126:91-108). Prior to the 7.7-ka caldera-forming eruption, the summit
of Mount Mazama was 3700 m asl. Moraines of the last glacial maximum (LGM) reached as low as 1400 m. Late MIS 10 glaciation
is recorded at 1900-2000 m: in the SE caldera wall, glaciated andesite (340±6, 341±8 ka; 1σ) is overlain by
intracanyon dacite (306±5 ka); in the E wall, similar glaciated andesite underlies till capped by dacite (336±6 ka);
on the SW lakeshore, glaciated dacite (351±12 ka) is overlain by andesite (302±10 ka). Early MIS 8 is represented
by: andesite at Arant Point (297±12 ka) and a contiguous tuya that indicates >60 m of ice at 2000 m; the subglacial
276±11-ka dacite of Munson Ridge that implies >170 m of ice at 2040 m. In late MIS 8 or in MIS 7.4, voluminous
andesite of Applegate Peak (7 ages 269±12 to 211±16 ka) chilled against thick ice along the E edge of Sun Creek
valley at 1850 m and higher. Andesite of Garfield Peak (224±9 ka; 2100-2400 m) flowed over andesite (269±12 ka)
glaciated during MIS 8.0 or 7.4. North of Castle Creek at 1740 m dacite (216±4 ka) lies on andesite (258±7 ka)
glaciated in MIS 7.4 or 8.0. Andesite of Roundtop (159±13 ka) extending 3 km NE of the caldera rim is an MIS 6
ice-bounded lava flow. At Pumice Point, polygonal jointing and breccia occur in thick andesite (117±3 ka) that rests on
glaciated (1900 m) mafic andesite (122±20 ka); the andesite is overlain by subaerial dacite (116±5 ka). Although the
mafic andesite could have been glaciated in MIS 6, ice-contact/meltwater chilling of the overlying andesite probably dates
from MIS 5.4. Below Llao Rock, andesite (70±4 ka) caps sediments deposited on dacite (116±9 ka) during MIS 5.4-4.
Andesite (87±15 ka) from a subaerial cone (base 1900 m) ESE of Mazama flowed into ice-free Scott Creek during MIS
5.1-5.3. Andesite of Grotto Cove forms two thick ice-bounded lava flows (68±13 and 74±10 ka; MIS 4) that extend 5 km
NE of the caldera. A paleovalley in the W caldera wall buried by dacitic pyroclastic-flow deposits (50±3 ka) was
glaciated during MIS 4 in andesite lava (70±4 ka). Preclimactic rhyodacites (27 ka) erupted in late MIS 3 when ice was
present and were further glaciated in MIS 2: the Redcloud Cliff flow is encased in vitrophyre columns where ice was >160 m
thick at 2200 m and at the base of its caldera-facing cliff where it banked against ice 5 km E of Mazama's summit; Grouse
Hill dome rises 200 m above its pancake-shaped, ice-bounded lava flow whose height suggests >100 m of syn-eruption ice at
2000 m 7 km N of the summit. The andesite tuya coeval with adjacent glaciated rhyodacite of Bear Bluff (24±3 ka) formed
during MIS 2 when ice thickness was >100 m at 1900 m 12 km S of the Mazama summit. Sharp Peak domes (18±4 ka) were
extruded 7-9 km NE of Mazama's summit; glaciated outcrops on 1826-m Sharp Peak indicate LGM ice >170 m thick after dome
emplacement. Ice-contact structures have not been found in units that erupted during MIS 1, 5.1, 5.3, 5.5, 7 (except 7.4),
9, or 11, which presumably would have been ice free, save perhaps the highest elevations. The correlations with MIS
constrain the number of times ice advanced down the major canyons of the S flank of Mazama since ca. 270-210 ka eruption of
andesite of Applegate Peak to as many as seven (MIS 8, 7.4, 6, 5.4, 5.2, 3-4, and 2).
DE: 1105 Quaternary geochronology
DE: 1115 Radioisotope geochronology
DE: 1616 Climate variability (1635, 3305, 3309, 4215, 4513)
DE: 8408 Volcano/climate interactions (1605, 3309)
DE: 8425 Effusive volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005