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

JORGE RABASSA, JUAN FEDERICO PONCE, ANDREA CORONATO, ARI IGLESIAS, ANALÍA ARTABE, ADOLFINA SAVORETTI, MARGARITA OSTERRIETH, MARILÉN FERNÁNDEZ

Rio Cuarto

Congreso; V Congreso Argentino de Cuaternario y Geomorfología; 2012

Asociación Geológica 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 restudied by us for their stratigraphical relationships and their paleontological content in mummified plant macrofossils and insect remains, pollen and spores, phytoliths and diatoms, presently under study. This fossiliferous locality is found in the lowlands of the Atlantic coast of Tierra del Fuego, at only a few meters above present sea level, and forming part of mid Holocene, marine paleocliffs. These layers have a total thickness of approximately 1 meter, unconformably lying over Oligocene-Miocene sediments of the Cabo Peñas Formation. The sedimentological section shows marine Tertiary beds at the base, 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. Cryoturbations have been found that affect directly the Tertiary bedrock, which indicate the existence of a tundra environment, with no ice cover but with permafrost conditions at the times of the peat accumulation. 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 rich organic sediments from oxidation and erosion. According to our regional knowledge of the distribution of glacial units, it is likely that these sediments are older than the Last Glaciation and were formed sometime during Middle Pleistocene times. 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 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 mummified mosses have been found, which occur with excellent conservation in three dimensions, with cells and tissues preserved. The material 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 so far. 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 corresponds to an unknown species, neither for Tierra del Fuego nor in any of the Sub-Antarctic islands within the Class Bryopsida. The presence of a new fossil moss of Middle Pleistocene age could indicate that plant extinction had happened in the southern Hemisphere within the bryophyte flora since the Middle Pleistocene.