40Ar/39Ar and K-Ar chronology of Pleistocene glaciations in Patagonia

Abstract

During the Pleistocene, east of Lago Buenos Aires, Argentina, at 46.5 °S, at least 19 terminal moraines were deposited as piedmont glaciers from the Patagonian ice cap advanced onto the semi-arid high plains adjacent to the southern Andes. Exceptional preservation of these deposits offers a rare opportunity to document ice-cap fluctuations during the last 1.2 m.y. 4 0 Ar/ 3 9 Ar incremental-heating and unspiked K-Ar experiments on four basaltic lava flows interbedded with the moraines provide a chronologic framework for the entire glacial sequence. The 4 0 Ar/ 3 9 Ar isochron ages of three lavas that overlie till 90 km east of the Cordillera at Lago Buenos Aires, and another 120 km from the Andes along Rio Gallegos at 51.8 °S that underlies till, strongly suggest that the ice cap reached its greatest eastward extent ca. 1100 ka. At least six moraines were deposited within the 256 k.y. period bracketed by basaltic eruptions at 1016 ′ 10 ka and 760 ′ 14 ka. Similarly, six younger, more proximal moraines were deposited during an ∼651 k.y. period bracketed by an underlying 760 ′ 14 ka basalt and the 109 ′ 3 ka Cerro Volcan basalt flow that buried all six moraines. Coupled with in situ cosmogenic surface exposure ages of moraine boulders, the 109 ka age of Cerro Volcan implies that moraines deposited during the penultimate local glaciation correspond to marine oxygen isotope stage 6. Further westward toward Lago Buenos Aires, six additional moraines younger than the Cerro Volcan basalt flow occur. Surface exposure dating of boulders on these moraines, combined with the 1 4 C age of overlying varved lacustrine sediment, indicates deposition during the Last Glacial Maximum (LGM, 23-16 ka). Although Antarctic dust records signal an important Patagonian glaciation at 60-40 ka, moraines corresponding to marine oxygen isotope stage 4 are not preserved at Lago Buenos Aires; apparently, these were overrun and obliterated by the younger ice advance at 23 ka. Notwithstanding, the overall pattern of glaciation in Patagonia is one of diminishing eastward extent of ice during successive glacial advances over the past 1 m.y. We hypothesize that tectonically driven uplift of the Patagonian Andes, which began in the Pliocene, yet continued into the Quaternary, in part due to subduction of the Chile rise spreading center during the past 2 m.y., maximized the ice accumulation area and ice extent by 1.1 Ma. Subsequent deep glacial erosion has reduced the accumulation area, resulting in less extensive glaciers over time.