HR: 1340h
AN: U33A-0009    [Abstracts]
TI: Complex Ice-flow Directions in the Gateway to the Cordillera Blanca, Peru
AU: * Smith, J A
EM: smithj5@union.edu
AF: Union College, Geology Dept., F.W. Olin Center, Schenectady, NY 12308-3107 United States
AU: Rodbell, D T
EM: rodbelld@union.edu
AF: Union College, Geology Dept., F.W. Olin Center, Schenectady, NY 12308-3107 United States
AU: Ramage, J M
EM: ramage@lehigh.edu
AF: Lehigh University, Earth and Environmental Sciences 31 Williams Hall, Bethlehem, PA 18015 United States
AB: Exceptional circumstances are required to preserve long glacial sequences that can be dated with cosmogenic isotopes. Necessary conditions include a small last glacial maximum (LGM; marine isotope stage 3 or 2) event relative to pre-LGM glaciations, low boulder-erosion rates, and space for ice to spread out and form moraines that are not superimposed. These conditions have been met in the Junin region of the Peruvian Andes (Smith et al., Journal of Quaternary Science, in press), where pre-LGM piedmont glaciers flowed onto the high-altitude Junin Plain repeatedly over the past half million years, producing a sequence of moraines. We believe we have found another setting in which old moraines lie on a high-altitude plateau in the Peruvian Andes. The Nevado Jeulla Rajo massif (S 10° 00', W 77° 16') lies at the southern end of the Cordillera Blanca and the Callejon de Huayllas valley in the central Peruvian Andes. Laguna Conococha and the Conococha Plain (~4050 masl) border the western side of the massif, which has peak altitudes of ~5600 meters above sea level (masl) and currently hosts a number of small glaciers. Several sets of large lateral moraines extend onto the Conococha Plain from the Jeullesh Valley. Multiple smaller end moraines lie upvalley, closer to the active ice margin. The largest pair of lateral moraines cross-cuts another pair from Jeullesh Valley and a pair from Quenua Ragra Valley to the east. Field observations of boulder weathering (mainly granodiorite and quartzite boulders) and moraine morphology suggest that these large lateral moraines all predate the LGM. We have sampled boulders on the three pairs of large lateral moraines and on the lowermost end moraine loop in Jeullesh Valley for surface exposure dating with cosmogenic isotopes. We hypothesize that the sampled end moraine marks the LGM ice limit in the Jeullesh Valley.
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
DE: 1625 Geomorphology and weathering (0790, 1824, 1825, 1826, 1886)
DE: 1637 Regional climate change
DE: 4926 Glacial
DE: 9360 South America
SC: Union [U]
MN: Fall Meeting 2005