Preliminary Results from Paleomagnetic Records on Lake Sediments from South America

PRELIMINARY RESULTS FROM PALEOMAGNETIC RECORDS ON LAKESEDIMENTS FROM SOUTH AMERICA

C.S.G. GOGORZA1,1. DITOMMASO2, A.M. SINITO1, B. JACKSON3, H. NUNEZ4, K. CREER5,3,

J.F. VILAS2

Summary : The preliminary results of paleomagnetic and radiocarbon dating of latepleistocene-holocene sediments from two lakes of south-western Argentina (41 °S, 71.5°W) arepresented. The magnetic susceptibility, intensity and direction of the natural remanentmagnetisation were measured. The stability of the natural remanent magnetisation was investigatedby alternating field demagnetisation. The magnetic parameters allowed the cores within each laketo be correlated. 8I3C analysis, total organic content measurements and C14 dating were carriedout. A model of sedimentation is suggested. Using this model and the correlation, curves ofvariations of magnetic inclination and declination in time are shown.

K e y w o r d s : Paleomagnetism, secular variations, lake sediments.

1. INTRODUCTION

This paper describes the preliminary results of a collaborative, multidisciplinary investigation oflate Pleistocene - Holocene sediments from two lakes of south-western Argentina. They are locatedin the Llao Llao area (about 41 °S, 71°30'W), in Rfo Negro Province, near the border with NeuquenProvince, at 800m altitude. Fig. la shows the geographical location of the lakes from which coreswere collected, namely Escondido and Moreno.

2. GEOLOGY

The regional geological features of the Cordillera Patagdnica Septentrional have beendescribed by Gonzalez Bonorino (1979) and Gonzalez Diaz et al (1980). Several authors,particularly Feruglio (1941) and Gonzalez Bonorino (1973,1979), studied the geology andstratigraphy of the southern zone of the Nahuel Huapi Lake area, The rocks exposed in thezone of Peninsula San Pedro -Llao Llao have been assigned, in chronological order, to thefollowing geological units: the Cushamen Formation of Precambrian-Palaeozoic age(Volkheimer, 1964); the Los Machis Formation of Late Cretaceous age (Gonzalez Diaz etal., 1980); the Ventana Formation of Eocene age (Gonzalez Bonorino, 1973); and theNirihuau Formation of Oligocene age (Gonzalez Bonorino, 1973).

1 Address: UNCPBA, CONICET, Argentina (Fax: +54-293 44433;e-mail: [email protected];[email protected])

2 Address: UBA, Argentina (Fax: +54-1 7883439; e-mail: [email protected])

3 Address: University of Wisconsin, USA (e-mail: [email protected])

4 Address: IAA, Argentina (Fax: +54-1 8122039; e-mail: [email protected])5 Address: University of Edinburgh, UK (Fax: +44-131 6673628;

e-mail: [email protected])12 Studia geoph. et geod. 42 (1998), 12-29

© 1998 StudiaGeo s.r.o., Prague

Preliminary Results from Paleomagnetic Records...

Stadia geoph. et geod. 42 (1998) 13

C.S.G. Gogorza et al.

The Nahuel Huapi Group of early Cenozoic age, exposed in this area, has beensubdivided into the mainly igneous Ventana Formation of Eocene age and the mainlysedimentary Nirihuau Formation of Eocene-Oligocene age, with the formerpredominating. Granites and tonalites which have been assigned to the lower Palaeozoicare also exposed (Gonzalez-Bonorino, 1973).

The Quaternary rocks exposed in this area consist principally of coarse-grained glacialand fluvio-glacial sediments and glacio-lacustrine varves. The many moraines which areexposed mainly in the eastern part of the area are related to the lateral part of the NahuelHuapi Drift (Flint et al., 1963) which originated during the Fini-glacial stage of Caldenius(1932).

The recent deposits are predominantly fluvial, a consequence of the high annualrainfall and the hilly topography. They include important piedmonts with fanglomerates,talus and some mud-flow deposits. Medium-to fine-grained gravel and sands with ashintercalations derived from explosive volcanism occur in the intermontane zone. Finally,there are Post-Glacial lacustrine gravels around the edges of the lakes and the bottomsediments which we have studied.

3. FIELD WORK AND SAMPLING

In order to avoid the effects of turbidity currents and other sources of potential post-depositional disturbance of sediments, small lakes were deliberately chosen. Short coresup to 1.5m, and longer cores up to 6m long were taken with pneumatically operated corers(Mackereth, 1958, 1969). Several cores were collected from each lake to check thereproducibility of the results. Two short cores and nine long cores were collected fromLake Escondido and 4 long cores were obtained from a small basin of Lake Moreno. Thelong and short cores from Lake Escondido are labelled LES and ES, respectively, and thelong cores from Lake Moreno, LM. The core lengths are given in Table 1. The location ofthe coring sites within each lake are shown in Fig. 1b and 1c.

The field station base was set up in PROGEBA Institute, San Carlos de Bariloche. Allcores were cut into sections 1m long and split open. Upon opening, one half of each corewas described, and then subsampled with plastic cubic boxes of 8 cm3, sealed andweighted for paleomagnetic studies. The number of subsamples taken from each core andtheir subsequent allocation between paleomagnetic laboratories of the University ofWisconsin (UW) Milwaukee, and the University of Buenos Aires (UBA), are given inTable 1. Core LES 10 was lost when it was removed from the corer and core LES 5 wasnot studied because it appears to be completely different from the rest, and it seems tocorrespond to a sliding.

Subsampling for I4C and 813C analyses was carried out on pieces of wood, leaves andsediments. Subsampling for palynology analyses was also carried out in several cores.

14 Studia geoph. et geod. 42 (1998)

Preliminary Results from Paleomagnetic Records ...

Table 1. Data from core sampling.

Lake

MorenoLake

EscondidoLake

Core Name

LM12LM13LM14LM15

LES1LES2LES3LES4LES5LES6LES7LES8ES2ES3

Length (cm)

298.5306.5325477.5

263417.5335.5269398.53963583844341

Np,132132144206

1141811461131451601511634550

Nd

00010800010002

Paleomag Lab

AAAU

AUAAAAAAAA

Np: number of paleomagnetic samples; Nd: number of dating samples; A: Argentina; U: USA

4. MACROSCOPIC SEDIMENTOLOGICAL DESCRIPTION

The sedimentological description is the macroscopic carried out at the field, except forsome "smear slides" analysis looked through a petrographic microscope.

The sediments in both lakes are granulometrically poorly sorted, clayey-sand andsandy-clay layers are frequently found.

Clay and silt are present nearly in all the column, some thin layers (5 to 10cm)associated with coarse and very coarse sand or gravel, allow cores to be correlated takinginto account the colour and composition of the coarse components. Fine and medium, wellsorted sand is found in thin layers (1 to 5cm) which can be identified by their colour andpyroclastic composition.

The sediments have a net predominance of pyroclastic origin, epiclastic material is lessabundant. Volcanic glass, ash, tuff and pumices, dark and light in colour, are the maincomponent of the gravel, sand and silt fraction; bad rounded quartz and plagioclase arescarce.

The most frequent colours in wet sediments are very dark brown (10YR 2/2, thecoding from the MUNSELL SOIL COLOUR CHART was used) in silty-clay layers,black (10YR 2/1) at the top of many cores in clays rich in organic matter, very dark grey(10YR 3/1) in sandy-silt layers abundant in basic volcanic ash, and light grey (10YR 7/1)in coarse sand and gravel pumice fragments.

Diatoms and sponge spicules are abundant in both lakes: in fine layers the diatomsconsist of more than 10% of the sediment.

Lake Escondido has abundant wood fragments and carbon especially between depthsof 2 m and 3 m.

Studia geoph. et geod. 42 (1998) 15

C.5.G. Gogorza et al.

5. MAGNETIC MEASUREMENTS

The intensity (jn) and directions (Declination, D and Inclination, 1) of the naturalremanent magnetisation (NRM) were measured using a Digico, a Molspin and a 2-Gcryogenic magnetometer in the paleomagnetic laboratories at the UBA and UW,Milwaukee.

Magnetic susceptibility measurements were made using a Bartington susceptibilimeterin the laboratories at Universidad Nacional del Centre de la Provincia de Buenos Aires(UNCPBA) and UW, Milwaukee, and the specific susceptibility (k) was calculated.

Saturation isothermal remanent magnetisation (SIRM) and anhysteric remanentmagnetisation (ARM) were measured with a Molspin equipment in the UW, Milwaukee.

SIRM was produced in a 0.9T field created with an electromagnet. ARM was measuredusing the Earth's vertical field as the bias direct field and running the alternating fielddemagnetiser down from 100 mT to 0. SIRM and ARM were measured for one core ofeach lake.

The stability of the NRM was investigated by alternating-field (AF) demagnetisation.For one core from each lake, one sample of every four was chosen as the pilot sample.They were demagnetised successively at 5, 10, 15, 20, 30 and 35 mT peak field. Most ofthe samples showed no systematic change in the direction of their remanent magnetisationduring AF demagnetisation, few of them showed a viscous magnetisation, probablypicked up in laboratory fields, which could easily be removed by AF demagnetisation at 5or 10 mT. Median destructive field values (the AF required to reduce the remanentintensity to half its initial value) are in the range of 20-25 mT, similar to those generallyfound for other Holocene lake sediments of the area (Creer et al, 1983). This result allowsthe assumption that magnetically soft carriers are present in the samples. Examples of thisbehaviours are shown in Fig. 2a and 2b.

Taking these results into account, every sample was demagnetised at 10 mT peak field.

6. 813 C, TOTAL ORGANIC CONTENT AND I4C MEASUREMENTS

In all of 15 samples of bulk sediment from core LES2 were analysed for 813 C, Thesesamples were prepared by D. McJunkin in the Radiocarbon Laboratory, and analysed on amass spectrometer in the Chemistry Department, both at the UW, Madison.

Total organic carbon for each pollen sampling level was measured using the loss-on-ignition method (LOI) as described by Dean (1974). Approximately one-eighth of ateaspoon of sediment for each level was dried in an oven at 100°C. Each sample waspowdered in crucibles, and weighed. The crucibles were put in a muffle furnace at 500°C-550°C for one hour, they were allowed to cool there to approximately 200°C, and thenremoved and allowed to cool to room temperature. The samples were then re-weighed andthe weight difference corresponds roughly to the total organic content of the sediment. Asecond organic carbon determination was carried out for larger samples in order todetermine the amount of error associated with using a small initial sample, and to test theLOI method in general.



Fig. 2. Stereographic projections of remanence directions, relative intensity plots, and Zijdervelddiagrams for progressive AF demagnetisation of (a) sample LES8-4, showing no systematic change,and (b) sample LES8-48 showing a viscous magnetisation, easy to remove at 5 or 10 mT.



The age scale for this study is provided by 11 Accelerator Mass Spectrometer (AMS)radiocarbon dates from discrete 1 cm intervals in the cores. The fact that the AMS datingmethod requires only a small amount of elemental carbon (less than a milligram) allowsdates to be obtained from very small organic fragments or small amounts of organic richsediment.

The material used to provide the dates varies, and for the most part consisted oforganic fragments within bulk sediment. Some macrofossils were found which were largeenough to provide dates. In particular, a distinctive wood macrofossil layer was found atthe same level, in each core taken from Lake Escondido, and is clearly a continuous layerthroughout the basin. Wood fragments were identified by the UW Forest ProductLaboratory, Madison, as Nothofagus sp. A macrofossil Nothofagus leaf was also found inLES2 (identified by Dr. Maria Marta Bianchi at PROGEBA). Both the wood and the leafmacrofossils were used for 14C dates. A small clam was found living at the sediment-waterinterface of ES3. It was identified as the genus Diplodon (Castellanos, 1959; 1960) andwas determined to be five years old based on annual growth lines.

The clam, found living at the sediment-water interface of ES3, was dated to assess thepotential for ancient dead carbon within the basin. The final results indicate the basin isnot receiving dead carbon; the 14C activity of the clam measured 123.3±0.7% of the 1950pre-bomb 14C activity, consistent with the modern atmospheric 14C concentration for theregion (D. McJunkin, pers. communic.).

The number of samples taken from 14C studies are detailed in Table 1.Eight dates were obtained from LES2 and two were obtained from ESS. One

additional date comes from a wood layer in LES6. Only one date was obtained fromLM15, because the principal aim, for this lake, was to confirm the ages obtained in aprevious work (Creer et al, 1983).

Samples were sent to the Tucson laboratory as either raw samples, or as CO2 gas. Thesamples sent as CO2 gas were prepared at the UW Radiocarbon Laboratory, Milwaukee,through the standard sealed-tube method of combusting the organic material in a furnace.The combustion method uses cupric oxide as the oxidant, and a small amount of silver toscavenge sulphur that may be present in the samples. Pertinent information for eachsample, including the dates in radiocarbon years before present (RCYBP), are listed inTable 2. The date from Lake Moreno is consistent with previous results (Creer et al,1983),

The 813C record from LES2 shows a clear change at approximately 360 cm, with morenegative values above 360 cm, and less negative values below 360 cm. This change in the513C values correlates with a major pollen zone boundary, and is interpreted as a decreasein arboreal vegetation in the bottom portion of the core (Jackson, 1996.). Fluctuationsbelow 360 cm also seem to relate to distinct pollen zones.

There is a general decrease in total organic content from the top to the bottom ofLES2. Numerous layers of very small organic content are superimposed in this long-termdecrease. These layers correspond to tephra layers. Considering the rapid deposition oftephra layers, the small amount of organic is not surprising, especially if the tephras areair fall deposits, rather than ash that had fallen on the basin and subsequently washed in.



Table 2. Radiocarbon dating sample information

Tucson lab number

A A- 18634A A- 18636AA- 18630AA- 18629A A- 18635A A- 18627A A- 18637AA- 18628A A- 18631AA- 18633AA- 18632AA- 1863d

Material

sedimentsediment

woodleaf

sedimentsedimentsedimentsediment

clamsediment

woodsediment

Sent as

CO2

CO2

rawrawCO2

CO2

CO2

CO2

rawCO2

rawCO2

Core

LES2LES2LES2LES2LES2LES2LES2LES2ES3ESS

LES6LM15

Depth(cm)

1595173265330375

391.5401.5

074

233.5-237.5491.5

Date(RCYBP) ± o

840 ± 652300 ± 653570 ± 605235 ± 657950 ± 75

9565 ± 10512.050 ±95

12.250 ± 100123.3 ±0.7%*

1325 ±553640 ± 70

12165 ±115

* 14C activity of 5 years old clam (genus Diplodon) was 123% ± 0.7% of the modern atmosphericactivity of I4C.

7. PALEOMAGNETIC RESULTS

Jn, k, D, I, SIRM and ARM logs were made. Since the cores were not orientatedrelative to magnetic north, the D values for each core are centred about the averagedeclination. Fig. 3 shows the log corresponding to one core from Lake Escondido. The Jn

and k logs show the same trend; this leads to the conclusion that the variations in themagnetic mineral content must dominate the intensity of remanence. Therefore both the Jn

and k logs can be used to define tie-lines describing the lithostratigraphic correlation ofcores of the same lake. These tie-lines were consistent with the lithology. Fig. 4 shows thek logs corresponding to every core from Lake Escondido and the correlation tie-lines. Jn

and k logs also allowed correlation between ES3 and LES2; this correlation confirms therelationship suggested by visual inspection of distinctive layers of the sediment of bothcores,

One core from each lake was chosen as the "master" core, LES2 and LM15 for LakesEscondido and Moreno, respectively. The depth scales of all the cores from each lakewere adjusted to the depth scale of the respective master core using Jn and k tie-lines forcorrelation. Fig. 5 show the alignment of the major peaks and troughs along the Jn logs forEscondido, following the adjustment to a common depth scale. The results of LakeMoreno were compared with those obtained in previous studies (Creer et al, 1983;Valencio et al, 1985) and they show agreement. This is evidenced in Fig. 6, where the Jn

logs for the Moreno cores corresponding to this work (LM12, LM13, LM14 and LM15)and to the previous study (LM2, LM3, LM4 and LM5), adjusted to the master core LM15,are shown.



Fig. 6. Jn logs for Lake Moreno cores studied in this work (LM12, LM13, LM14 and LM15) andpresented by Creer et al. (1983) (LM2, LM3, LM4 and LM5), after adjusting to the depth scale ofLM15 (master core), using Jn and k tie-lines for correlation.


C.S.G, Gogorza et al.

Fig. 7. Radiocarbon ages vs. depth at Lake Escondido. Radiocarbon ages are shown with errorbars representing ± two standard deviations of the counting error reported by the Tucson AMSLaboratory. Two possible depth-age models for the past 12300 radiocarbon years before present arerepresented: two straight lines corresponding to different sedimentation rates and the bestfittingthird polynomial.

D and / logs may be used to infer the chronostratigraphy within the lake correlationand between the lake correlation of the sedimentary sequences, but this is not easybecause noise of different origins can affect these values.

However, when / and D logs after demagnetisation in 10 mT peak field for Escondidoare made, adjusting the depth scale for each individual core to the "master" core, thecharacteristic behaviour of these parameters recorded in different cores appears at thesame depth.

8. TRANSFORMATION TO A TIME SCAL

The dates listed in Table 2 indicate that the sedimentation rate of Lake Escondidoincreased in the upper part of the core, or that significant compaction has occurred lowerin the core, creating the appearance of a slower sedimentation rate. To create asedimentation model, the dates could be fitted by two straight lines, as shown in Fig.7,however, this model suggests sharp changes in the sedimentation rate, from about19.9 cm/kyr for the older section of the core, to 57.3 cm/kyr for the younger one. Sincepollen influx calculations depend on the sedimentation rate, these apparent sharp changeswould produce changes in the pollen influx which does not seem to be related to realvegetation changes.



If the hypothesis of significant compaction in the lower part of the section is assumed,a smooth polynomial can be fitted to the data. Using the computer program DEP-AGE(Maker, 1992), it was determined that a third-order polynomial provided a reasonable fitto the data from Escondido. This polynomial was based on the assumption thatapproximately 23 cm of sediment was lost from LES2. This assumption is supported bythe correlation between ES3 and LES2 mentioned above. The formula for the depth-agerelationship would then read (Jackson, 1996):

This polynomial is also shown in Fig.7.Using the polynomial, the depth of LES2 and of the rest of the cores from Escondido,

which had been adjusted to the depth scale of LES2, were transformed to radiocarbonages. Fig. 8 and 9 show the logs of / and D after demagnetisation in the 10 mT peak fieldas function of radiocarbon age. A few data, which were very different from the meanvalues were replaced by the average of the adjacent data, to avoid their masking thesignal.

9. DISCUSSION

The correlation between k logs for Lake Escondido (Fig. 4) and for Lake Morenoshows that the compaction is very similar in every core, thus the tie-lines are almostparallel.

The Jn logs for Lake Escondido (Fig. 5) show a very light trend to higher values withdepth, this trend is not present in the records of Lake Moreno (Fig. 6)

Although the best fitting polynomial was used for the time-depth transformation, thepossibility of a change in the sedimentation rate can not be completely ruled out; in afuture research, cores 10 m deep will be analysed and the new paleomagnetic andpalynological results will help to answer this question.

Another chronological problem was considered. As previously mentioned, the LakeEscondido record contains many tephra layers. These layers were depositedinstantaneously as compared to the rest of the core. There was a concern that this wouldaffect the overall sedimentation model. Therefore, a new sedimentation model wasproduced by first removing the obvious tephra layers and recalculating the depth scale forLES2. A new curve was fitted to the radiocarbon ages, and the ages of each sediment levelwere calculated from this new curve. The results indicate that the tephras were relativelyevenly spaced throughout the core, and did not significantly affect the overall depth-agerelationship.

The fact that the D and / variations from different cores may be correlated and do notcorrelate with the variations of Jn or k variations suggests that they do not reflectlithological changes, but the result of changes of the ancient geomagnetic field.

The mean values of / for Escondido fluctuate between -52° and -65° for the differentcores; the direction corresponding to the field of the axial dipole for the site is -60°. Thedifferences can be caused by a slight inclination when the corers penetrate the sediments.The amplitude of the fluctuation of D predominates over that of / for Lake Escondido.



26 Studia geoph et geod. 42 (1998)


Studia geoph et geod. 42 (1998) 27


Low values of / are observed in most of cores for ages of about 1650 RCYBP,2200 RCYBP, 2750 RCYBP, 3200 RCYBP, 4550 RCYBP, 4800 RCYBP. D logs are notso certain, but for some cores they show values apart from the mean declination for agesof about 1500 RCYBP, 2200 RCYBP and 2700 RCYBP. These ages are very similar tothe first three ages mentioned for low values of I; for the ages corresponding to olderrecords of low I, only a few cores show significant differences of the mean value of D.

In a future work the results presented in this paper will be combined with the previousresults from South Argentina for stacking in order to obtain a final master curve forpaleosecular variations, which improves the only available results at the moment. Thiscurve will allow the analysis of characteristic frequencies of these variations.

Palynological and rock magnetism researches are in progress. They will allowpaleoenvironmental conclusions to be drawn.

10. CONCLUSIONS

- The attitude of the lithological layering within each lake can be clearly defined by thecorrelation of Jn and k logs.

- The radiocarbon dating from Escondido evidences that significant compaction hasoccurred lower in the core, creating the appearance of a slower sedimentation rate. Asedimentation model is suggested, using a third-order polynomial, which provides areasonable fit to the data from Lake Escondido.

- The sedimentation model and the correlation between cores within a lake suggested bythe Jn and k logs allowed curves of / and D to be constructed as functions of time forLake Escondido. These curves show consistent results,

Acknowledgements: The authors wish to thank the Antorchas Fundation, which supported the fieldwork and the transporting of the Mackereth corer from Argentina back to the United Kingdom.They also wish to thank Universidad de Buenos Aires, Universidad Nacional del Centra de laProvincia de Buenos Aires, CONICET and the University of Wisconsin for their support.

They would like to take this opportunity to express their appreciation to the PROGEBA Institutefor the place kindly made available to carry out the sampling and for logistic support, and to Dr. BillKean for making available the facilities of the Laboratory of Paleomagnetism of the UW,Milwaukee.

The Mackereth corer and the collaboration of Mr. Alan Pike were kindly provided by theUniversity of Edinburgh.

The radiocarbon dates were provided by a special grant from the National Science Foundation tothe NSF Arizona Accelerator Mass Spectrometer Facility.

Manuscript received: 6 January 1997; Revisions accepted: 5 January 1998

References.

Caldenius C, 1932: Las glaciaciones cuaternarias en la Patagonia y Tierra del Fuego. DireccionGeneral de Minas y Geologia, Publ. 95.

Castellanos Z.A., 1959: Las especies del genero Diplodon en la Argentina. Actas del Primer Congr.Sudamer. Zool., La Plata, Argentina. 2(3), 85-94.



Castellanos Z.A., 1960: Almejas nacarfferas de la Republica Argentina del genera Diplodon (Moll.Mutelidos). Secret. Agric. de la Nacion, Publ. Misc., 421,1-40.

Creer K.M., Valencio D.A., Sinito A.M.; Tucholka P. and Vilas J.F., 1983: Geomagnetic secularvariations 0-14000 yr BP as recorded by lake sediments from Argentina. Geophys. J. Roy.astr. Soc., 74,109-222.

Dean W.E., 1974: Determination of carbonate and organic matter in calcareous sediments andsedimentary rocks by loss on ignition: comparison with other methods. J. Sedim. Petrol., 44,242-248.

Feruglio E., 1941: Nota preliminar sobre la hoja 40b, San Carlos de Bariloche. BoletinInformaciones Petroleras, 18,200,26-64.

Flint R.F. and Fidalgo F., 1963: Geologia glacial de la zona de borde entre los paralelos 39°10'y41°20' de latitud sur en la Cordillera de los Andes, Republica Argentina, Direction Nationalde Geologia y Mineria, Bol. 93.

Gonzalez Bonorino F., 1973: Geologia del drea entre San Carlos de Bariloche y Llao-Llao.Departamento Recursos Maturates y Energia, Fundacion Bariloche, Public. 16.

Gonzalez Bonorino F., 1979: Esquema de la evoluci6n geologica de la Cordillera Nordpatagonica.Asoc. Geol. Arg. Rev., 34,184-202.

Gonzalez Dfaz E and Nullo F., 1980: Cordillera Neuquina. In: Geologia Regional Argentina,Academia Nacional de Ciencias de C6rdoba, II, 1099-1147.

Jackson B.M., 1996: Paleoenvironmental Record from Lago Escondido, Rio Negro Province,Argentina. Thesis. University of Wisconsin, pp.136.

Mackereth F.J.H., 1958: A portable core sampler for lake deposits. Limnol. Oceanogr., 3,181-191.

Mackereth F.J.H., 1969: A short core sampler for sub-aqueous deposits. Limnol. Oceanogr., 14,145-151.

Maher B., 1992: Depthage conversions of pollen data. INQUA Comission for the study of theHolocene, Newsletter, 9,13-17.

Valencio D.A., Sinito A.M., Creer K.M., Mazzoni M., Alonso M.S. and Markgraf V., 1985:Palaeomagentism, sedimentology, radiocarbon age determinations and palynology of theLlao-Llao area, southwestern Argentina (lat. 41°S, long. 71°30'W): Palaeolimnologicalaspects. Quaternary of South America and Antarctic Peninsula, 3,109-147.

Volkheimer W., 1964: Estratigraffa de la zona exttraandina del Departamento de Cushamen(Chubut) entre los paralelos 42° y 42°30'y los meridianos 70° y 71°. Asoc. Geol. Arg. Rev.,19,85-107.


Preliminary Results from Paleomagnetic Records on Lake Sediments from South America

Documents

Transcript of Preliminary Results from Paleomagnetic Records on Lake Sediments from South America