FLUCTUATIONS OF THE SOUTH PATAGONIAN ICE-FIELD DURING THE LAST GLACIATION AND THE HOLOCENE

Mapping along a transect from the southeastern margin of the South Patagonian Ice-field in Torres del Paine National Park (Chile) to the limits of fresh moraines of the last glacial cycle indentified eight glacier advances. The four younger ones have been dated by dendrochronology tephrochronology and radiocarbon dating. Although the bases of 10 m deep bogs were samples, close limiting radiocarbon dates were not obtained because bog formation in this rain-shadow area appears not to have commenced until ca. 12000 yr ago. The outermost Little Ice Age moraine formed during the seventeenth century and three inner ones were deposited around ad 1805, 1845 and after 1890. Densel, vegetated older moraines contiguous with Little Ice Age deposits are possibly of late Holocene age. Tephra from the eruption of Reclus volcano at ca. 11880 yr BP was incorporated by 1 readvance that deposited large multiple moraines 10-16 km from the modern ice-front; the oldest basal pear found inside the moraine has been dated to ca. 9200 yr BP. These bracketing dales indicate that some eastern outlet glaciers of the ice-field advanced at a time when some western tidewater outlet glaciers terminated inside their modern limits. This questions the view of J. H. Mercer and other that Patagonian glaciers did not readvance during the late-glacial interval. A stadial event also occurred when the glaciers were some 18-20 km from their modern positions and is closely dated to ca. 11880 yr BP because Reclus pumice flushed down-glacier forms thick upper beds in outwash deltas deposited in proglacial lakes. The four older moraines pre-date the late-glacial eruption of Reclus but are not dated closely. Comparison of their spatial extent with well-dated moraines in the Chilean Lakes Region suggests that they may mark advances culminating at ca. 14 000 yr BP, ca. 20 000 yr BP and earlier.