PALAEOMAGNETISM OF LITHUANIAN UPPER PLEISTOCENE SEDIMENTS

Transcription

1 GEOCHRONOMETRIA Vol. 21, pp 65-72, 22 Journal on Methods and Applications of Absolute Chronology PALAEOMAGNETISM OF LITHUANIAN UPPER PLEISTOCENE SEDIMENTS ALGIRDAS GAIGALAS 1, NIELS ABRAHAMSEN 2, VAIDOTAS KAZAKAUSKAS 3, and MONIKA MELEÐYTË 1 1 Vilnius University, Èiurlionio 21/27, 29 Vilnius, Lithuania (e-ail: Algirdas.Gaigalas@gf.vu.lt) 2 Departent of Earth Sciences, Aarhus University, Finlandsgade 8, DK-82 Aarhus N, Denark (e-ail: Abraha@geo.aau.dk) 3 Departent of Quaternary Geology, Institute of Geology, T. Ševèenkos 13, LT 26 Vilnius (e-ail: vaid.kaz@geologin.lt) Key words: PALAEOMAGNETISM, UPPER PLEISTOCNE, LITHUANIA Abstract: In the Lithuania, a palaeoagnetic study of Upper Pleistocene and Late Glacial sedients at five sections has been carried out. The palaeoagnetic study of the varved clays in the Maskauka, Didþiasalis and Girininkai sections cover the tie interval since 18, to 14, BP and Manèiagirë section covers the tie interval since 12,7 to 11,63 BP. The palaeoagnetic research of the Merkinë (= Eeian) erglacial sedients (Netiesos section) cover the tie interval since 112.±25 to 11.5±11.5 ka BP. The declination (D), inclination (I) and intensity (J) of the natural reanent agnetization of glaciolacustrine clays (Maskauka, Didþiasalis and Girininkai sections), as well as Merkinë (=Eeian) erglacial deposits (Netiesos section) and interstadial sedients (Manèiagirë section) were easured. The result of these studies is presented in diagras. The study reveals that considerable palaeoagnetic inforation can be obtained fro the varved clays. 1. INTRODUCTION Application of palaeoagnetic ethods has shown that such investigations ay be a valuable tool for stratigraphical research of Pleistocene sedients in Lithuania. Palaeoagnetic investigations of Late Pleistocene sedients in Lithuania previously have been published by M. Pevzner and A. Gaigalas (1976) and A. Gaigalas et al. (1991). Various anoalous and even negative inclinations are found in soe deposits of Late Pleistocene in Lithuania (Gaigalas et al., 1991). Anoalous agnetization of a transitional type fro noral to reverse inclination was noted in the Merkinë (=Eeian) erglacial deposits (Maksionys and Jonionys section). Reverse agnetization was noted in the Grûda (=Brandeburgian) till (22,-18, years ago) with an otherwise noral agnetization (Padubysis section). Anoalous agnetization, transiting to negative, was noted in varved clays, dated to about 13,5-13, years ago (the Kurðënai section). Negative agnetization was also noted in lacustrine deposits, aged 11,6-12,7 years ago. A palaeoagnetic study of varved clays of the Late Glacial has been carried out at Karelia in the north of Lake Ladoga and in the west and east of Lake Onega for the tie interval 16,-1,2 BP (Ekan et al., 1987; Batchutor and Zagniy, 199). The detailed curves of declination and inclination can be used for regional agnetostratigraphic studies and for correlation of sedients in adjacent regions of East European plain. A palaeoagnetic study can be used not only in stratigraphy and correlation of Late Pleistocene sedients, but in palaeogeographic reconstructions too. New inforation on fine structure of palaeoagnetic field should be analysed in a palaeocliatic aspect. The task is to establish relationships between cliatic variations and the geoagnetic field in the Pleistocene. In Lithuania glaciolacustrine deposits and varved clays are distributed widely in surface foration. They have been fored in daed periglacial basins near the argin of the retreating ice sheet. The study of varved clays can be used to obtain the secular variations and for the construction of a detailed tie-scale of the geoagnetic field in the Late Pleistocene.

2 PALAEOMAGNETISM OF LITHUANIAN UPPER PLEISTOCENE SEDIMENTS 2. RESEARCH METHODS In the present study ore than 4 saples fro five sections (Girininkai, Didžiasalis, Maskauka, Manèiagirë and Netiesos) have been agnetically investigated (Fig. 1). lination (D), inclination (I) and intensity (J) of the natural reanent agnetization (NRM) of glaciolacustrine clay (Girininkai, Didþiasalis and Maskauka sections), as well as Merkinë (=Eeian) interglacial deposits (Netiesos section) and interstadial sedients (Manèiagirë section, Fig. 2) were easured by a spinner agnetoeter (Digico Ltd.). These sections are located in different deglaciation zones of the Last Glaciation that allow us to cover a wide tie interval. Oriented, 1-inch speciens were collected in vertically cleaned profiles, using agnetic copass and a spirit level for the orientation. After perforing an initial pilot study, all speciens were AF-deagnetized in sall steps up to typical 5 or 6 T, and re-easured on the spinner after each step. By a PCA-analysis (principal coponent analysis), using Linefind in the IAPD (eractive analysis of Palaeoagnetic Data) progra (Torsvik, 1992), the ChRM (characteristic reanent agnetization) of each specien was isolated. In soe cases SIRM (saturation isotheral agnetization) experients were perfored, and also the frequency-dependant agnetic susceptibility was easured. Results are plotted in the diagras (Figs 3 and 4). The agnetic inerals vary between different environents, and hence they ay hardly be stated as a general conclusion. However, the ain ferroagnetic coponent carrying the NRM is usually fine grained agnetite, although agheite ay soeties be present. In soe cases, according to the agnetic saturation experients, the carrier of the agnetic reanence appear to be heatite (or goethite?) rather than agnetite or agheite. The palaeoagnetic study of the varved clays (Maskauka, Didþiasalis and Girininkai) covers the tie interval fro 18, to 14, BP. The sedient age has been defined by varvoetric, palynological, optically stiulated luinescence, electron spin resonance and radiocarbon analyses as well as concluded fro geological and geoorphological data. Sedientological and palaeoagnetic studies were carried out for each section. High sedientation rates of varved clays allowed us to collect oriented saples with high resolution over the tie interval and without probles of aterial liitation. The bedding of sedients used for studying the geoagnetic secular variation is in a horizontal or near horizontal position. 3. MASKAUKA SECTION The section represents the Grûda (=Branderburgian) and Þiogeliai (=Frankfurtian) glaciolacustrine varves. Magnetic declination, inclination and intensity of the sedients of Maskauka profile have been cobined in one plot (Fig. 5). Both angles of agnetic declination and inclination appear to show systeatic trends, but with high scatter. On the diagra (Fig. 5), each dot indicates the AF-cleaned ChRM (characteristic reanent agnetization) of one oriented specien. The declination ay show a systeatic trend, but the scatter is quite high (Fig. 5). The westerly values between 4.7 and 5.1 above the river level are situated just above the Grûda sedient (4.3 to 4.65 ). Ðiauliai Klaipëda Panevëþys Neunas Didþiasalis Kaunas Girininkai Neris 25 5 k VILNIUS Maskauka Netiesos Manèiagirë Merkys Fig. 1. Location of the Maskauka, Didþiasalis, Manèiagirë and Netiesos sections of Late Pleistocene deposits. 66

3 Gaigalas et al Maskauka Upper Pleistocene ( ~ 18, - 16,5 years BP) Didþiasalis Upper Pleistocene ( ~ 14, - 8, BP) Girininkai Upper Pleistocene ( ~ 15, - 14, BP) Manèiagirë Late Glacial (12,7-11,63 BP) River level Netiesos (lower part) Merkinë (=Eeian) erglacial ( ~ 125, - 15, BP) River level River level Fig. 2. Palaeoagneticaly investigated sections: Maskauka, Didþiasalis, Manèiagirë and Netiesos. 1 sand, 2 silt, 3 varved clay, 4 hoogeneous clay, 5 clay disturbed by cryogenic process, 6 till, 7 linic tufa, 8 sapropelite, 9 gyttja, 1 peat, 11 deluviu. 67

4 PALAEOMAGNETISM OF LITHUANIAN UPPER PLEISTOCENE SEDIMENTS Þiogeliai (=Frankfurtian) varves cover the interval between 5.1 and 7.4 (sandy and silty lake sedients). The top part between 7.45 and 8 consists of reddish disturbed clay. A certain pattern in the inclination ay be present, but the scatter is quite high (Fig. 5). Most values are ore shallow than the expected geocentrical axical dipole (GAD) value of the site, Io=7.2. The shallow inclination ay indicate a sedient copaction. The low values between 4.7 and 5.1 are situated just above the Grûda sedient (4.3 to 4.65 ). The top part between 7.45 and 8 consists of reddish disturbed clay. The sall axiu in the intensity between 4.7 and 5.1 is situated just above the Grûda sedient (4.3 to 4.65 ; Fig. 5). Stereogra of the AF-cleaned direction of individual speciens, with their individual α 95 confidence circles is given in Fig. 3. The ean direction of glaciolacustrine varved clay, sand and silt of Grûda Þiogeliai age is (D,I)=(7. W, 55.7 ), N=52, α 95 = DIDÞIASALIS SECTION In the Didþiasalis section of varved clays both agnetic declination and inclination show a long-wave variation (Fig. 6), the declination with ore westerly values between 6 and 3.5 depth, and a short-wave variation at the top of the profile between 2.5 and. On the diagras, each dot indicates the AF-cleaned ChRM of one oriented specien. The swings in declination between 3 and 2.5 are likely to be due to soe kind of sedient disturbance, the intensity being quite high (Fig. 6). Soothed declination varies around between ±3 (Fig. 6). Except the top, the inclination is systeatic low as copared to the geocentrical axical dipole (GAD) value of Io = 7.9, which ay indicate soe copaction of the sedient (Fig. 6). The variation as well as the local scatter in the intensity values are likely to indicate lithological variations in the clay and silt proportions of the thin-lainated varved clays (Fig. 6).The ean direction of glaciolacustrine clay of Didþiasalis section is (D,I)=(6.5 E, 55. ), N=58, α 95 =5.7 (Fig. 3). Maskauka North Didþiasalis North North Netiesos North Fig. 3. Stereographic plot of AF-cleaned ChRM directions of the Maskauka, Didþiasalis, Manèiagirë and Netiesos profiles. 68

5 Gaigalas et al. SAMPLE: GI 6 N 1. AXIS= 5 A/M LINES : T N 2 7, U p G i A X IS = 1 A /M L IN ES : T 2 7, U p G i 3 N 1. A X IS = 1 A /M L IN ES : T Fig. 4. Stereographic, intensity decay and orthogonal plots of AF-cleaned ChRM-directions of 3 pilot speciens (saples No. 6, 17 and 3) fro the Girininkai profile. 69

6 PALAEOMAGNETISM OF LITHUANIAN UPPER PLEISTOCENE SEDIMENTS Height above river ( ) Maskauka Magnetic Secular Variation, & (3-pt oving average) superposed upon a short-wave variation at the lower half of the profile. The general trend in declination is a change fro westerly declination at the lower part towards easterly declination at the top (Fig. 8). The swings in declination between 1.15 and 1.35 are likely to be due to soe kind of sedient disturbance, the intensity being quite high. The upper part of the profile ay be disturbed due to roots and due to low intensity of ore sandy sedient on the top. As entioned above, systeatic short wave pattern appears between.4 and 1.3 in inclination fro around 7 at.8 to around 8 at 2 (Fig. 8). The GAD value of the site is Io=7.1. The NRM intensity of black gyttja is systeatically higher than of the sandy intervals at 1 and above 2 (Fig. 8). Mean direction of the Manèiagirë profile (Alleröd) is (D, I)=(3. W, 77.8 ), N=44, α 95 =2.7 (Fig. 3). -5 Didþiasalis Magnetic Secular Variation D ec, & (3-pt oving average) Fig. 5. Magnetic inclination (), declination () and intensity () of deposits of Maskauka profile. Full lines are 3-point oving averages. 5. GIRININKAI SECTION Judging fro 6 pilot speciens fro the Girininkai profile, the agnetic direction is fairly stable (Fig. 7). However, agnetic declination is rather scatter, and is hardly diagnostic for the geoagnetic secular variation, expected variation of which would be close to true North, i.e. ±2. lination is also soewhat strange, being rather shallow around 3 ±3, whereas the expected inclination for an average geoagnetic field in Lithuania would be around 7 ±1. This shallow inclination ay indicate copaction of the sedient. The palaeoagnetic investigation of the interstadial sedients covers the tie interval fro 11,63±12 to 12,7±8 years ago in the Manèiagirë section (Gaigalas, 1994). Depth below soil () MANÈIAGIRË SECTION The Manèiagirë section spans the Alleröd interstadial over the Ûla River. AF-cleaned declination and inclination both show a systeatic trend with low values and low scatter between.4 and 1.3 above the river level, 8 Fig. 6. Magnetic inclination (), declination () and intensity () of deposits of Didþiasalis profile. Full lines are 3-point oving averages. 7

7 Gaigalas et al NETIESOS SECTION The palaeoagnetic research of the Merkinë (=Eeian) erglacial sedients covers the tie interval since 112±25 ka to 11.5±11.5 ka years BP in the Netiesos section. The Merkinë deposits are represented by gyttja and peat with silt in the botto part of the section. The geoagnetic Blake event, soe ties recorded in Eeian deposits (e.g. Abrahasen, 1995), is not found in the Netiesos profile (Fig. 9). lination (AF-cleaned) is ostly East of North, but the scatter is quite high, due to sandy character of sedients (Fig. 9). In inclination, a certain pattern ay be seen in the 3-point oving average (full line), but the scatter is rather high due to sandy character of sedients (Fig. 9). There has been no copaction of the sedients, as the average inclination is close to the GAD value of the site, Io=7.2 o. The NRM intensity of the profile decreases upwards, the sedients becoing ore sandy Girininkai M agnetic S ecular Variation D ec, & towards the top (Fig. 9). Stereogra of the AF-clean direction of individual speciens. The ean direction is (D,I)=(19.4 o E, 71.1 o ), N=66, α 95 =3.2 o, k=3.1 (Fig. 3). 8. CONCLUDING REMARKS Our data appear to be too scattered to define any significant excursion of the geoagnetic pole in the tie interval of 18,-14, BP (Maskauka, Didþiasalis and Girininkai) and in 11,6-12,7 BP (Manèiagirë interstadial), as well as in the tie of the Last interglacial (112±25 ka to 11.5 ± 11.5 ka BP). However, soe agnetic inforation can be obtained fro the varved clays of the Late Pleistocene tie. The variation as well as the local scatter in the intensity values are likely to indicate lithological variations in the clay and silt proportions of the thin lainated varved clays. The swings in declination are likely to be due to soe kind of sedient disturbance (sedient copaction due to glacial activity), the intensity being quite high. We do not find the Gothenburg excursion in the tie interval of 18,-11, BP. The VGP (Virtual geoagnetic pole) do not fall below 6 N. The geoagnetic Blake event, soe ties recorded in Eeian deposits is not found in the Netiesos profile of the Merkinë erglacial Manèiagirë M ag netic S ecular Variation, & (3-pt ov ing average) Depth below soil ( ) Heig ht abov e riv er ( ) Fig. 7. Magnetic inclination (), declination () and intensity () of deposits of Girininkai profile. Fig. 8. Magnetic inclination (), declination () and intensity () of deposits of Manèiagirë profile. Full lines are 3-point oving averages. 71

8 PALAEOMAGNETISM OF LITHUANIAN UPPER PLEISTOCENE SEDIMENTS Height abov e riv er ( ) Netiesos (low er part) M ag netic S ecular Variation D ec, & (3-pt oving average) Fig. 9. Magnetic inclination (), declination () and intensity () of deposits of Netiesos profile. Full lines are 3-point oving averages. ACKNOWLEGEDGEMENTS Financial support for the study was provided by the Lithuanian State Science and Studies Foundation under grant No. T-522 of 21 year. REFERENCES Abrahasen N., 1995: Palaeoagnetis of Quaternary sedients fro Anholt, Denark. The onset of the Blake Event and Ee. Bull. Geol. Soc. Denark 42: Bakhutov V.G. and Zagniy G.F., 199: Secular variation of the geoagnetic field: data fro the varved clays of Soviet Karelia. In: Physics of the Earth and Planetary eriors 63. Elsevier Science Publishers B. V., Asterda: Ekan I., Bakhutov V. and Zagny G., 1987: Stratification and correlation of varved clays in ters of the fine palaeostructures of the Earth s agnetic field. In: Methods for the investigation of lake deposits: palaeoecological and palaeocliatological aspects. Vilnius: Gaigalas A., 1994: On palaeogeography of Late Pleistocene in the Lithuania. Acta Universitatis Nicolai Copernici. Geografia XXVI. Nauki Mateatyczno-Przyrodnicze Zeszyt 92: Gaigalas A., Pevzner M. and Meleðytë M., 1991: Palaeoagnetization of the Late Quaternary deposits of the South Baltic region. In: Geochronological and isotope-geocheical research into Quaternary Geology and Archaeology. Vilnius University Press: 27-4 (in Russian). Pevzner M. and Gaigalas A., 1976: Reanent Magnetization. In: The buried paleo-incisions of sub-quaternary rocks surface of the South-East Baltic region. Mokslas Publishers, Vilnius: Torsvik T., 1992: IAPD eractive analysis of Palaeoagnetic Data. In: Manual. NGU, N-72 Trondhei, Norway: