Abstract
The Sierra Nevada has been repeatedly glaciated throughout the Quaternary. Full glacial periods saw glaciers grow out of the mountains into the valleys to the east of the range. Morphostratigraphic relations of glacial moraine complexes at several canyons in the east-central Sierra Nevada show that on several occasions younger glaciers chose paths different from previous glaciers when not confined within the bedrock walls. This glacial avulsion may be caused by several exogenic and endogenic mechanisms: glacial surging, changes in ice volume, change in glacier sediment load, gradual shifting of ice flow, lateral moraine collapse/erosion, basin infilling/morainic accumulation or tectonic movement. The tectonic activity within the eastern Sierra Nevada is probably the principal mechanism initiating glacial avulsion. Morphostratigraphic and age relations of moraine complexes determine where and when the avulsion events took place, but do not satisfactorily explain the mechanisms which caused the avulsion.
Moraine relations at Pine Creek suggest two types of glacial avulsion: (1) Interglacial fluvial erosion cut several gaps through the right lateral Early Rovana moraine (rock varnish catio-ratio dated (RVCRD) at ~180 ka (early marine 18O stage 6)). The Late Rovana glacier (RVCRD at ~140 ka (late marine 18O stage 6)) chose to flow through a water gap which was wide and deep enough to accommodate the ice. (2) An early Tioga glacier (~21-24 ka?) or a Tahoe recessional glacier deposited a low moraine or series of low moraines. The next ice advance rode on top of the low deposits, raising its base to near the level of the confining lateral moraines. Then the glacier could follow one of two courses: the steep outer side of the confining left lateral moraine or its previous channel. Melt water and small lobes of ice flowed over the lateral moraine for a period of time, but did not fully change the path of the glacier. The ice temporarily retreated, then flowed down the previous channel during its next advance (Tioga maximum, radiocarbon dated at 19.5 ka).
Glacial avulsion appears to be polygenic. Avulsion events identified in moraine complexes may be valuable indicators of climatic change (i.e., stadial/interstadial events) within glacial periods and/or tectonic events. The time of avulsion can be determined using surface exposure dating techniques, allowing comparison to other paleoclimatic and paleohydrological data to extract a clearer signal as to the trigger of the avulsion event.