Paleomagnetic study of glaciaric sediments from northern Tierra del Fuego Island.

ANA MARIA WALTHER; JORGE RABASSA; ANDRÉA CORONATO.; ALEJANDRO TASSONE; HORACIO LIPPAI; JUAN FRANCISCO VILAS

19-20 de noviembre. Santiago. Chile

Simposio; International Geological Congress on the Southern Hemisfere (Geosur 2).; 2007

GEOSUR

Pioneering works on Fuegian glacigenic sediments date from the latest XIX century to middle XX century (Nordenskjöld, 1898; Bonarelli, 1917; Caldenius, 1932; Feruglio, 1950). During the last twenty years there has been an increasing development with various approaches and methodologies focused on the sedimentary record related to the several glaciations of the island of Tierra del Fuego (Meglioli, 1992; Rabassa et al. 1990, 1992, 2000, 2005; Rabassa and Claperton 1990; Claperton 1993; Claperton et al., 1995; Coronato 1990, 1993, 1994, 1995; Rabassa and Coronato 2002; Coronato et al., 2004; Kaplan et al., 2007, among others). At least six glaciations affected northern Tierra del Fuego (Meglioli 1992). The oldest one, of Late Pliocene age (2.5-1.86 Ma, Coronato et al., 1999) is represented by scattered erratic boulders (Río Grande drift). The Pampa Beta glaciation of Pleistocene age (1.1-1.0 Ma) is represented by the moraines of northern Magellan Strait (Sierra de los Frailes drift, Meglioli, 1992). The Río Cullen glaciation in Early-Middle Pleistocene (>0.36-<1.07 Ma) produced well defined moraine units (Cabo Vírgenes drift; Meglioli, 1992) in the Magellan Strait area and on both margins of the Bahía Inútil-Bahía San Sebastián depression (Río Cullen drift; Meglioli, 1992; Bujalesky et al., 2001). The San Sebastián glaciation also deposited moraines to the north and within the San Sebastián bay area (Punta Delgada and Sierra de San Sebastián drifts, respectivelly; Meglioli, 1992). In the area between the two bays the frontal moraines of the Middle Pleistocene Lagunas Secas glaciation are located. This glaciation is equivalent to the Primera Angostura drift in the Magellan Strait (Meglioli, 1992). The Late Pleistocene Bahía Inútil glaciation corresponds to the Segunda Angostura drift of the Magellan Strait (Meglioli, 1992) and deposited frontal moraines in the Bahía Inútil area (Bujalesky et al., 2001). Recently INGEODAV and CADIC have begun a multidisplinary study including the first paleomagnetic surveys on the glacigenic sedimentary record of northern Tierra del Fuego Island with the aim of obtaining accurate age data of these deposits. Research was initiated on the Middle Pleistocene Río Cullen and Sierra San Sebastián drifts –Bahía San Sebastián- which show outcrops with good lateral continuity. A preliminary sampling was carried out on these outcrops; the obtained cores were processed applying the specific routine at INGEODAV. Thirteen sites were sampled, seven of them in the sediments of Río Cullen drift and the remaining 6 in the Sierra San Sebastián drift. Three specimens were obtained from each sampling site. The laboratory processing included the measurement of natural remanent magnetism (NRM), the coherence in the MRN directions among different specimens of each site was verified. In order to analyze the MRN stability, demagnetization stages by high temperatures (100 to 600 ºC) were applied and the residual remanent magnetism (RRM) was measured at each stage. 8 to 10 demagnetization stages were measured for each sample. The magnetization directions of different samples from a single site show a strong internal coherence. The characteristic remanent magnetization (MRC) was obtained from each specimen. The vectorial analysis of magnetic components was performed with the Zijderlveld orthogonal graphs. The MRC displays a better clustering than the MRN when represented on stereographic projection which indicates that the demagnetization treatments have been effective to eliminate secondary remanent magnetizations. The paleomagnetic analysis reveals that the studied cores bear more than one magnetic component. Two blocking temperature ranges have been recognized in the Río Cullen drift: a low (300-400ºC) and a high (500-580ºC) Curie temperature rank. The mineral carriers of magnetic remanence with low blocking temperatures are Ti-rich titanomagnetites and those with high Curie temperatures are magnetite and Ti-poor titanomagnetite. The studied samples of the San Sebastián drift, in turn, show a low stability secondary magnetic component and a high-stability unidirectional magnetic component. Two blocking temperature ranges have been also observed in the San Sebastián drift: a low (200-350ºC) and a high (450-580ºC) Curie temperature range. The magnetic carrier in both drifts should be a titanomagnetite l.s. since the blocking temperatures span between 400 and 580 ºC. The virtual geomagnetic pole (VGP) was computed with the MRC declination and dip (Valencio, 1980) and GPS data for each sampling site. These data allowed the determination of MRC polarity which is normal for all the studied samples. The age of these glacigenic deposits has been considered as <1.01 Ma. This is the age of Pampa de Beta Drift or the “Large Patagonic Glaciation” (LPG) which is older than the glacigenic events resulting in the deposits sampled for this work. Therefore, the only possible correlation of the obtained data with the table of reversions of the Earth´s magnetic field is with the Brunhes chron (<0.78 Ma). The suggested magnetic age for the studied deposits is Middle or Late Pleistocene, younger than the Brunhes/Matuyama boundary (<0.78 Ma; Singer and Pringle, 1996) and older than the Lagunas Secas Glaciation (equivalent to the Illinois Glaciation of North America, marine oxygen isotope stages 6-8, ca. 0.125-0.25 Ma). Taking in consideration these relations, the studied drifts have a Middle Pleistocene age within the 0.78-0.24 Ma time span (Early to Middle-Middle Pleistocene). These ages agree with recent age determinations by cosmogenic isotopes on erratic boulders in the moraines of Río Cullen Drift (Punta Sinaí) which yielded a minimum exposure age of 0.174 ± 0.0004 Ma (Kaplan et al., 2007).