Relict periglacial features in Tierra del Fuego and Santa Cruz, Argentina

RABASSA, J.; BOCKHEIM, J.; CORONATO, A.; ERCOLANO, B.; PONCE, J. F.; BUJALESKY, G.

Santiago, Chile

Congreso; GEOSUR 2007; 2007

 Dpto. de Geología, FCFM, Universidad de Chile; Proyecto ARTG-04, Conecciones Antártica - Patagonia, PBCT-CONICYT; Dto. de Ciencias Ecológicas, Pontifica Universidad Católica de Chile

Two generations of ice-wedge casts and one generation of sand wedges have been identified in Tierra del Fuego (53º S; 67º W), whereas at least three generations of sand wedges and two generations of ice-wedge casts have been surveyed in Southern Santa Cruz (51º-52º S; 68º-71º W). Two ice-wedge cast systems found in Tierra del Fuego are intruding Tertiary marine rocks (Estancia María Behety, Río Grande) and Last Interglacial (ca. 125 ka B.P., Marine Isotope Stage [MIS] 5e) coastal marine deposits (Cabo Peñas), respectively. Sand wedges intrude Tertiary marine rocks and Quaternary glaciofluvial gravels, being infilled by aeolian sands dating from the Last Glaciation (i.e., ca. 100-20 ka B.P., MIS 4-2). The latter wedges are frequent, short and cone shaped, and regularly spaced (Cabo Ewan). Fossil sand-wedge relics and ice-wedge casts are ubiquitous in southern Santa Cruz. At 6 sites we conducted detailed investigations of stratigraphy, soil genesis, and wedge frequency and characteristics. The features are dominantly relic sand wedges and funnels with an average length, maximum width, minimum width, and L/W of 78, 39, 3.8, and 2.9, respectively. The host materials are predominantly fine-textured (silt loam, silty clay loam, clay loam) till and the infillings are aeolian sand (sand-wedge relics) or silt (ice-wedge casts). The observed soils are primarily Calciargidic Argixerolls that bear a legacy of climate change. In Southern Santa Cruz, at least three sand wedge formation episodes have been identified: (a) an event which took place during one intermediate stadial of the Great Patagonian Glaciation (GPG, 1.1-1.0 Ma; MIS 30-34), since they are intruding a lower till of the Sierras de los Frailes Drift (GPG, Route 3, Estancia Tres de Enero, and Route 40, Estancia Bella Vista); the first ones are uncomformably covered by a younger till of the same drift; (b) an event younger than the GPG drift but older than a Last Interglacial (ca. 125 ka B.P.; MIS 5e) soil (Route 3) and (c) an event during the Last Glaciation, in which the wedges are intruding several older drift units and also the Last Interglacial soil (Chimen Aike quarries). Two ice wedge cast formation episodes have been identified: (a) intruding the Cabo Vírgenes Drift (Early-Middle Pleistocene, ca. 0.8-0.5 Ma), which is possibly of the same age of the (b) sand wedge event, whereas there are other sand wedges penetrating Tertiary marine rocks (Monte León), although no firm correlation can be yet established, and (b) intruding alluvial gravels of a glaciofluvial cone, probably of the Punta Delgada Drift (Middle Pleistocene, ca. 05-0.3? Ma). The existence of other wedge formation periods cannot be ruled out, particularly those corresponding to the extended period between the GPG and Last Interglacial times. Spacing of the wedges has been estimated between 3.5-6.6 m. From a palaeoenvironmental point of view, these wedges indicate the presence of at least three, and perhaps even five events of continuous permafrost during the Pleistocene. Older events may have taken place as well but we have no means of dating these events yet. A key finding is that the Sierra de los Frailes Drift is in fact a multiple-stage glaciation, that is, including more than one major glacial stage. Sand wedges are interpreted as very strong palaeoclimate signals, since they are known to have formed under regional mean annual temperatures (MAT) of ca. -4 / -8 ºC. These conditions would have forced a MAT lowering of ≥ 8-10 / 12-14 ºC, according to the geographical location of the different sites studied. This amount of MAT lowering is higher than previously estimated based on pollen analysis and other proxy elements and it would be indicating very harsh climate conditions in southernmost South America during Pleistocene glacial episodes.