Middle Pleistocene glaciofluvial deposits in Eastern Tierra del Fuego, southernmost South America, preserving rare peat mosses

RABASSA, J; PONCE, J. F.; CORONATO, A.M; IGLESIAS, ARI; ARTABE, A.; SAVORETTI, ADOLFINA; FERNÁNDEZ, MARILÉN; OSTERRIETH, M.

Río Cuarto

Congreso; VI Congreso Argentino de Geomorfología y Cuaternario; 2012

Asociación Argentina de Geomorfología y Cuaternario

Middle Pleistocene glaciofluvial deposits in Eastern Tierra del Fuego, southernmost South America, preserving rare peat mosses Jorge Rabassa1,2, Juan Federico Ponce1, Andrea Coronato1,2, Ari Iglesias3,4, Analía Artabe3,4, Adolfina Savoretti4, Marilén Fernández1, Margarita Osterrieth5 1Laboratorio de Geomorfología y Geología del Cuaternario, Centro Austral de Investigaciones Científicas (CADIC-CONICET). Bernardo Houssay 200, Ushuaia (9410), Argentina; 2Universidad Nacional de la Patagonia-San Juan Bosco, Sede Ushuaia and Universidad Nacional de Tierra del Fuego; 3División Paleobotánica, Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata (UNLP-CONICET). Paseo del Bosque s/n, La Plata (1900), Argentina;4Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata (UNLP), La Plata, Argentina;5Universidad Nacional de Mar del Plata, Facultad de Ciencias Naturales, Mar del Plata, Argentina Fossil peat layers, almost transformed into lignified coal layers due to compression by glaciofluvial sediments, were originally described by Auer (1956) in Cape Viamonte, Eastern Tierra del Fuego. These beds are presently being re-studied for their unique stratigraphical relationships and their paleontological content, including mummified plant assemblages (mosses, pollen, spores, phytoliths, diatoms, and insects. This fossiliferous locality is found in the lowlands of the Atlantic coast of Tierra del Fuego, at only few meters above present sea level, and forming part of mid Holocene, marine paleocliffs. The geological section starts with Oligocene-Miocene marine sediments of the Cabo Peñas Formation, unconformably overlain by a glaciolacustrine sequence that includes the fossil peat layers and coarse glaciofluvial units which are interpreted as proglacial drift generated by a prograding lowland glacier. The Tertiary bedrock has been affected by cryoturbation, indicating permafrost conditions and the existence of a tundra environment at the times of the peat accumulation. From a paleoecological and paleoenvironmental points of view, the position of the peat layers in relation with the glaciofluvial/glaciolacustrine deposits may be indicating their formation in proglacial environments, during a temporary stand-still or recession of the ice but during a glacial period, when mean annual temperatures in Tierra del Fuego may have been close or below freezing point, and not during an interglacial period as Auer (1956) stated. The fossiliferous strata have a total thickness of approximately 1 meter, and the compression of the peat layers is interpreted as being of syn-sedimentary origin. When the peat was still water saturated, the glaciofluvial sediments accumulating above forced high pressure with consequent dehydration and compaction. Thus, the peatland was still active when the glacier was advancing and therefore, the age of the peat might be considered as essentially coeval with the genesis of the glaciofluvial deposits. The peat would have been formed in a proglacial lacustrine environment during the onset of a glacial event. The large size of the pebbles and cobbles of the overlying drift suggest that the lacustrine sequence may have been deposited not too far from a lowland glacier, and their rapid burial by the proglacial drift would have preserved the organic-rich sediments from oxidation and erosion. No terminal moraines have been recognized so far along the littoral zone where this glaciofluvial/glaciolacustrine sequence occurs. The gravels overlying the upper clayey silty layers could be interpreted as the remnant of a large glaciofluvial terrace formed by meltwater coming from the ice lobes of central Tierra del Fuego. There are several geological evidences that the Last Glacial Maximum does not affect this eastern area. Based the aforementioned observations, the age of the Viamonte beds is estimated to be older than the last interglacial (Marine Isotopic Stage 5e; ca. 125,000 years old), thus probably pertaining to the Penultimate Glaciation, Marine Isotopic Stages 6 to 8, or older, that is, sometime in the Middle Pleistocene (125-780 ka ago). In one of the fossiliferous beds, excellent mummified mosses with cells and tissues preserved have been found. The moss assemblage is monospecific with both gametophytic and sporophytic life forms. More than ten sporophytic capsule and setae in close association to gametophytic shoots with attached leaves and rhizoids have been recovered. The fossil moss presents a unique suite of characters: ramified gametophytes (pleurocarpous), excerted capsule with well-developed setae and arthrodontous peristome (big tooth with horizontal thickenings). This plant belongs to a new species within Bryopsida that is unknown for Tierra del Fuego and the Sub-Antarctic islands. The lack of this species or any related taxa in the extant moss flora could indicate that some extinction had happened in the southern Hemisphere within the bryophyte group since the Middle Pleistocene. Key words: Proglacial and periglacial environments, Middle Pleistocene, Tierra del Fuego, Southernmost Patagonia, fossil Bryophyte, peat moss, pollen, phytholiths, diatoms, fossil insects.