ROCK-MAGNETIC STUDIES ON PALEOLAKE SEDIMENTS FROM TAJAMAR FORMATION María A. Irurzun, Nicolás Larcher, Gustavo González Bonorino, Claudia S. G. Gogorza Abstract In this paper we conducted a detailed study of rock magnetism to characterize sediments obtained from three cores located in an outcrop in Guachipas (25º 31'S 65° 30.5 'W), Tajamar Formation. Guachipas sediments are located in the river drainage of Guachipas River, which drains Meso-Cenozoic outcrops that bordering the southern end of the Lerma valley. Measurements of magnetic susceptibility (specific,  and volumetric, k), declination, inclination and intensity of the natural remanent magnetization (D, I and NRM), anhisteretic remanent magnetization (ARM), isothermal remanent magnetization (IRM), reaching saturation (SIRM) were performed. A reverse field was applied to determine the coercivity of remanence (BCR). Associated parameters were calculated. The stability of the NRM was analyzed by alternating field demagnetization. The studied samples showed a viscous remanent magnetization (VRM) removed with alternating fields of about 10-15mT., Inclination vary around -20° and declination around 1°.The magnetic properties have shown highly variable values, showing changes in both grain size and concentration of magnetic minerals. It was found that the main carrier of remanence is magnetite with the presence of hematite in different percentages. San Agustín Guachipas Campo aplicado [mT] RM*100 NRM N O E S O, arriba S, S Prof: 85cm D: 5º I: -21º MAD: 4.4º Tajamar Formation (Gallardo et al., 1996; Gallardo and Georgieff, 1999) is constituted by pelitic and sandy sediments deposited in a lake that occupied the eastern and southern Lerma Valley. The laminated pelites are of lacustrine origin and timely crop along the Lerma valley from Guachipas to San Agustín (25º S 65º 26'O). Malamud et al. (1996) conducted the only magnetic study published on the Tajamar Formation. By thermal demagnetization of NRM from Rumical samples (24º 58'S 65º 23'W) determined that Tajamar Formation was deposited in a period of normal polarity (Brunhes) and the main carrier of magnetization is hematite. These authors dated volcanic ash layers of La Viña whose contact with Tajamar Formation is through an erosional unconformity. The ages obtained were 100 ± 40 years, 5.13 ± 0.25 and 3.92 ± 0.19 x 103 years. The lack of magnetic records becomes necessary to study the outcrops of this formation. Alternating-field demagnetization results. Over 90% of the samples exhibits this behavior. There is a VRM that is removed between 0 and 10-15mT. These values ​​were eliminated for the subsequent vector analysis. c d b a Using principal component analysis determined the values ​​of D and I studied for three cores. The inclination has a minimum around 72 cm and changes ranging from -34° to 13° between 200 and 280 cm. Presents noticeable changes around the mean (-20° ± 6°), a value significantly higher than -43°, corresponding to the value of I on the GAD model for the latitude of the studied site. The declination shows small variations around the mean value (1° ± 0.6°), which coincides with the value of 0º, according to this model. The main magnetic carrier is magnetite with presence of hematite in different proportions (a). The percentages of high coercitividad magnetic minerals (d) were determined using the method of Chaparro et al. (2004). The magnetically hard mineral percentages range from 8 to 45%. In one sample was found 60% of hematite-like mineral. Interparamétricos ratios (c) show little variation in magnetic grain sizes and are between 4 and 8 m. Concentration of magnetite minerals varies from 0.006 to 0.03% (b). F factor values indicate no presence of SP minerals (c). In 136 of the 158 levels, %softIRM values ​ greater than 50% were observed, confirming a predominance of magnetite type mineral in the cores. References Aguilera, N., 1988. Tectónica de la comarca del Valle de Lerma - Provincias de Salta y Jujuy. Informe Final, Beca de Iniciación, CONICET. Inédito, Salta Chaparro, M.A.E., Sinito, A.M. 2004. An alternative experimental method to discriminate magnetic phases using IRM acquisition curves and magnetic demagnetisation by alternating field. Rev. Bras. Geof., 22 (1): 17-32. Gallardo, E. F., Aguilera, N. G., Davies, D. A. y Alonso, R. N., 1996. Estratigrafía del Cuaternario del Valle de Lerma. Provincia de Salta. Argentina. XII Congreso Geológico de Bolivia. Actas II, 483-493. Gallardo, E. F. y Georgieff, S. M., 1999. Estratigrafía y paleogeografía del Cuaternario del Valle de Lerma, Salta. Relatorio, XIV. Malamud, B. D., Jordan, T. E., Alonso, R. A., Gallardo, E. F., González, R. E., Kelley, S. A., 1996. Pleistocene Lake Lerma, Salta Province, NW Argentina. En: XIII Congreso Geológico Argentino y III Congreso de Exploración de Hidrocarburos. Asociación Geológica Argentina, Buenos Aires, 103–114. c logs obtained for the three cores used. Some of the correlation tie lines used to adjust the data to a common depth are shown. The curves obtained show a high degree of similarity, which allowed a good correlation between levels of sediment. For samples between two tie lines, linear regression was performed. Conclusion Through c logs, an excellent correlation between the different levels could be performed, allowing to adjust all cores to a common depth. Magnetic demagnetization curves showed a stable behaviour after 10 to 15 mT AF demagnetization. The inclination ranges from -30º and -10º while the declinination is centered at 1°. The magnetic properties showed wide variations in the samples from Guachipas, the major changes are in the concentration of magnetic minerals.  The main magnetic carrier is magnetite with presence of hematite in different proportions. Only 13% of the samples had a predominance of hematite as the main carrier.The magnetic grain size shows small variations and is centered around 8 mm (pseudo-single domain).