Preliminary Results from Palaeomagnetic Records on Lake Sediments from South America

GOGORZA, C.S.G.; DI TOMMASO, I.; SINITO, A.M.; JACKSON, B.; CREER, K.M.; VILAS, J.F.

Castle of Topolcianky, Slovakia

Congreso; New Trends in Geomagnetism; 1996

PRELIMINARY RESULTS FROM PALAEOMAGNETIC RECORDS ON LAKE SEDIMENTS FROM SOUTH AMERICA INTRODUCCION. In this paper we describes the preliminary results of our investigation of the Pleistocene-Holocene sediments from two lakes of south-western Argentina names Escondido Lake and Moreno Lake.   GEOLOGY. We can see in this map: The lakes are located in the Llao Llao area (about 41°S, 71°30’W). In this area we find the Nahuel Huapi Group of early Cenozoic age, it has been subdivided into the mainly igneous Ventana Formation of Eocene age and the mainly sedimentary Ñirihuau Formation of Eocene-Oligocene age. We can see granites and tonalites which have been assigned to the lower Paleozoic.   FIELD WORK AND SAMPLING. We choose small lakes to avoid the effects of turbidity currents and other sources of potential post-depositional disturbance of sediments. We took short cores up to 1.5m, and longer cores up to 6m long with pneumatically operated corers (Mackereth, 1958, 1969). We collected two short cores and seven long cores from Escondido and four long cores we obtained from Moreno. We cut all cores into sections 1m long, they were split open. We subsampled the sections with plastic cubic boxes of 8cm3, after they were sealed and weighted for palaeomagnetic studies. We carried out subsampling for 14C analyses on pieces of wood, leaves and sediments and we subsampling several cores  for palynology analyses too.   MACROSCOPIC SEDIMENTOLOGICAL DESCRIPTION. It was done a macroscopical sedimentological description at the field, except some "smear slides" analysis looked through a petrographic microscope. We could see that the sediments in both lakes are granulometrically poorly sorted, are frequently found clayey-sand and sandy-clay layers. Nearly in all the column are present clay and silt. We can use some thin layers (5 to 10cm) associated with coarse and very coarse sand or gravel to make a correlation between cores taking into account the colour and composition of the coarse components. The origin of the sediments have a net predominance pyroclastic , the epiclastic material is less abundant. Volcanic glass, ash, tuff and pumices in dark and light colour are the main component of the gravel, sand and silt fraction; bad rounded quartz and plagioclase are scarce. The most frequent colours in wet sediments are very dark brown in silty-clay layers. There black clay at the top of many cores rich in organic matter. We can find very dark grey in sandy-silt layers abundant in basic volcanic ash, light grey in coarse sand and gravel pumice fragments. It is notorious that the Lake Escondido has abundant wood fragments and carbon specially between 2m and 3m of depth.    MAGNETIC MEASUREMENTS. The measurements of the intensity (Jn) and directions (Declination, D and Inclination, I) of the natural remanent magnetisation (NRM) were maked out using a Digico, a Molspin and a 2-G cryogenic magnetometer. We are sure about the stability of the NRM because the directions typically showed no systematic change during demagnetisation. To measure the magnetic susceptibility  we used a Bartington susceptibilimeter. The measurements of Saturation isothermal remanent magnetisation  (SIRM) and anhysteric remanent magnetisation (ARM) was made up using a Molspin equipment. Was measured one core of each lake. SIRM were obtained using an electromagnet with 9000 Oe. ARM was measured using the Earth’s vertical field as bias direct field and running the alternate field demagnetizer down from maximum (1000 Oersted).   RESULTS. We can see the logs of Intensity, Susceptibility, Declination, Inclination, SIRM and ARM in  Figure one ( 1 ) corresponding to one core from Escondido and are indicated the results of the radiocarbon dating, carried out in EEUU. The Intensity  and Susceptibility logs show the same trend; this leads to conclude that variations in the magnetic mineral content must dominate the intensity of remanence. We used both, Jn and k logs to define tie-lines to describe the lithostratigraphic correlation of cores of the same lake. We find that these tie-lines were consistent with the lithology. Was choosen one core from each lake as “master” core. The depth scales of all the cores from each lake were adjusted to the depth scale of the respective master core using Jn and k tie-lines for correlation. In the Figure 2 we show the alignment of the major peaks and troughs along the Jn logs for Escondido following the adjustment to a common depth scale. We attaines similar alignments for lake Moreno. The results of this lake are consistent with those obtained in previous studies (Creer et al, 1983; Valencio et al, 1985). We will use Declination and Inclination logs to infer chronostratigraphic within lake correlation and between lake correlation of the sedimentary sequences, but this is not easy because noise of different origins can affect these values. However, when I logs for El Escondido are made adjusting the depth scale for each individual core to the “master” core, low values of I recorded  in different cores appear at the same depth. The D values for the same depth are also lower than the average D.   CONCLUSIONS. The conclusions that we arrive are the following: - Using the correlation of Jn and k logs we can attain a clear attitude of the lithological layering within each lake. -The D and I logs show consistent results when the depths are adjusted according to the Jn and k correlation tie-lines. -About the sedimentation model using the radiocarbon dating we are discussing about two possibilities: 1) sudden change of the rate of accumulation or a hiatus due to erosion or lack of deposition.It is notorious that the age of the sediment for the depth where the change would occur agree with the suggested age of the deglaciation (about 10,000 years b.p.) 2) There was a gradual change in background sedimentation and what appears to be a change in sedimentation rate may also be interpreted as increasing compaction of the sediment with depth. -These work will be completed with the palynological studies.