Low- to high-amplitude oscillations and secular variation in a 1.2 km late Miocene inclination record

PHYSICS OFTHE EARTH ANDPLANETARY INTERIORS

ELSEVIER

Physics of the Earth and Planetary Interiors 90 (1995) 37-53

Low- to high-amplitude oscillations and secular variation in a 1.2 km late Miocene inclination record Mikl6s Lantos

a,*

Donald P. Elston

b

a Hungarian Geological Institute, Stefdnia fit 14, P.O. Box 106, H-1442, Budapest, Hungary b US Geological Survey, Flagstaff, A Z 86001, USA Received 23 May 1994; revision accepted 9 January 1995

Abstract

A Miocene section of 2 km thickness was continuously cored near Szombathely, NW Hungary. A detailed magnetostratigraphic study has been integrated with results of lithologic, sedimentologic, and paleontologic studies. Progressive alternating field (a.f.) and thermal demagnetization, and rock magnetic and mineralogic studies indicate that the natural remanent magnetization (NRM) resides in magnetite and the strata contain only minor secondary magnetizations. Major polarity zones have been defined by inclinations and correlated with the geomagnetic polarity time scale for the interval 10-9 Ma. Inclinations for samples collected at 51 m intervals display fine-scale oscillations representing secular variation with period times of 400-700 years. Fluctuating amplitudes of oscillation range from low (less than 10° peak-to-peak) to high (more than 40°), reflecting apparently varying stabilities of the geomagnetic field. Additionally, amplitudes of oscillation progress from low to high, and return to low, forming oscillation cycles with a periodicity of 6.2 +_ 1.8 kyr. The oscillations and boundaries of oscillation cycles are generally unrelated to lithology and stratigraphy. Many oscillation cycles appear to be incomplete and 'interrupted' by high-level oscillations. Some incomplete cycles appear to arise from brief interruptions to the depositional record, whereas other interruptions may irregularly arise from a separate component of the geomagnetic field.

1. Introduction

During the 1980s, the Hungarian Geological Institute drilled a series of continuously cored stratigraphic test holes of 1.2-2 km depth. The cores were studied intensively for stratigraphy, sedimentology, paleontology, and magnetostratigraphy. The object was to solve long-standing problems concerning the subsurface correlation and timing of deposition of late Miocene and Pliocene ('Pannonian') strata in the Pannonian * Corresponding author.

basin. Subsurface study of the thick, deeply buried Pannonian sedimentary section was intense because Hungary had need for its hydrocarbon resources (Dank, 1987). Detailed magnetostratigraphic studies were begun in 1982 and carried out cooperatively between the Hungarian Geological Institute and US Geological Survey (USGS). Paleomagnetic measurements initially were m a d e at Flagstaff (Arizona), and later mostly at Budapest. Paleomagnetic results have been related to details of the stratigraphy, paleontology, and sedimentology (Elston et al., 1990, 1994). Abundant seismic

0031-9201/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 0 3 1 - 9 2 0 1 ( 9 5 ) 0 3 0 1 7 - 4

38

M. Lantos, D.P. Elston /Physics of the Earth and Planetary Interiors 90 (1995) 37-53

stratigraphic profiles also were developed, and some passed through locations of the stratigraphic test holes (Pogficsfis et al., 1992). Polarity time lines in the core sections were applied to the seismic profiles, and served to control temporally marker horizons in the subsurface traced across the Pannonian basin (Lantos et al., 1992; Pogficsfis et al., 1992). Large intervals of dominantly normal and reverse polarity were encountered in two core holes of 1.2-2 km depth drilled in the Great Hungarian Plain (Tiszapalkonya and Kaskantyfi; Fig. 1), and the zonations were correlated with the polarity time scale for the late Miocene (Elston et al., 1990, 1994). Abundant narrow reversals and intervals of mixed polarity separating the polarity intervals could only be partly correlated between drill holes. Such fine-scale polarity structures are not shown in the polarity time scale. The possibility seemed to be emerging from the Hungarian cores that field behavior was more complex than has been deciphered from the ocean floor polarity record. Another deep core hole drilled in northwest Hungary, Szombathely (47.2°N, 16.5°E; Fig. 1), penetrated a section that displayed an oscillating

,~t

inclination record and contained multiple finescale reversals. Correlation of the coarse polarity intervals with the polarity time scale indicated that the Szombathely area had subsided rapidly to receive about a kilometer of fine-grained clastic sediments in less than ~1 m.y. of time (Elston et al., 1994, Fig. 9). This study describes characteristics of the oscillating inclination record, and presents an interpretation of field behavior.

2. Geology 2.1. Pannonian Basin

The Pannonian Basin is a complex depression bordered by the Carpathian Mountains, Alps, and Dinarides (Fig. 1). The basin subsided following Oligocene and early Miocene deformation, and subsidence continued during the middle and late Miocene, Pliocene, and Pleistocene. The Carpathian Mountains correspondingly were orogenically uplifted during the early and middle Miocene. The mountains developed as a result of compression that arose partly from encroachment on the south of the African plate on the Eurasian

ca,~ERN cARPATHIANs

C4#,o

%,

Fig. 1. Map showing limits of Pannonian deposits (stippled areas enclosed by continuous lines) with respect to the boundary of Hungary, outlines of Great Hungarian Plain (GHP) and Little Hungarian Plain (LHP), and locations of selected deep drill holes, o, Core holes having geomagnetic polarity time lines: T--Tiszapalkonya-1; K--Kaskantyfi-2; SZ--Szombathely-2. Crosses, Drill holes having K - A r isotopic dates: N--Nagykozgtr-2; K1-Kecel-1; K2--Kecel-2; K3--Kiskunhalas-3.

M. Lantos, D.P. Elston / Physics of the Earth and Planetary Interiors 90 (1995) 37-53

plate, and partly from interaction on the east of the Eurasian plate with the Anatolian plate (e.g. Hgtmor, 1984). The Pannonian Basin subsided during the late Miocene and Pliocene, and received from 0.2 to 7 km of dominantly clastic sediments. The deposits initially accumulated in deep water, in a semi-enclosed to closed inland sea. As the basin became shallower, the depositional environment changed from brackish water to lacustrine, and then to fluvial (B6rczi and Phillips, 1985). Hungarian geDepth m 0

STAGE Pliocene? and -- Pleistocene

ologists apply the term Pannonian 1:o deposits that accumulated following the end of deposition of Sarmatian strata (at approximate][y 12 Ma), and before the onset of Pleistocene deposition at about 2.5 Ma.

2.2. Stratigraphy In the Szombathely core section, basal strata of the Lower Pannonian consist of conglomerates and turbidites of mixed marine and lacustrine DEPOSITIONAL ENVIRONMENT

LITHOLOGY

Early late Miocene

500

Upper

Flood plain

Clay, silt, sand, lignite

Pannonian

Deltaic

/

1000

distributary channe~

Sandstone, / siltstone

\ Delta front siltstone

Lower 1500

Pannonian

Early late Miocene Sarmatian

39

Brackish water

Mudstone, clay

Basin-floor

Sandstone,

/siltstone

ffTurbidite \ Basal conglomerate

~

e

Badenian 2000

Fig. 2. Stages, depositional environments, and lithology in Szombathely core section (after Phillips et al. (1992)).

40

M. Lantos, D.P. Elston / Physics of the Earth and Planetary Interiors 90 (1995) 37-53

origin (Phillips et al., 1992). Above the turbidites (Fig. 2), Lower Pannonian strata consist of deposits that accumulated in deep (600-800 m), quiet, near-anoxic and brackish-water conditions. These deposits consist of very fine grained mudstones, marls, and silty clays which contain authigenic pyrite, mollusks, ostracods, and fish scales. The basal Upper Pannonian unit consists of gray, fine-grained cross-bedded sand, sandstone, siltstone, and clayey marl. These deposits represent the initial deposits of a progradational deltaic sequence (delta front), now deeply buried in the basin (Mattick et al., 1985, 1994). Overlying Upper Pannonian strata consist of a cyclical series of gray, partly cross-bedded layers of sand, silt, clay, and lignite. These strata accumulated under swampy fluvial and shallow-water (1-20 m deep) lacustrine conditions, and contain a brackish- to fresh-water fauna consisting of mollusks and ostracods. Pyrite is rare in the entire Upper Pannonian. Analysis of the stratigraphy and sedimentary structures by Phillips et al. (1992) indicated that the Pannonian section of 1.8 km thickness in Szombathely accumulated rapidly, during a single, nearly continuous episode of deposition. In the Upper Pannonian, Phillips et al. recognized only a few diastems that represented minor erosion and hiatuses. The top of the Pannonian section in Szombathely lies close to the present surface of the Little Hungarian Plain. At the surface, about 2 m of Holocene alluvium overlies about 20 m of Pliocene(?) and Pleistocene sand and gravel.

2.3. Micromineralogical study The character and distribution of heavy minerals and rock-forming minerals across the Szombathely section have been studied by L. Ravaszn6 Baranyai (unpublished data, 1991). Thirty-six intervals in the Lower and Upper Pannonian, ranging from 1 to 5 m in thickness, were studied. A variety of heavy minerals and rock-forming minerals were identified. The grain size studied ranged from 100 to 200/xm. Magnetite-ilmenite ranged from common to abundant, and was observed in nearly all sampled intervals. Pyrite was

irregularly present and considerably less abundant than magnetite. Not surprisingly, limonite was abundant near the surface, and was reported from only two horizons in the fluvial Upper Pannonian (depths of 424-425 m and 837-843 m), one associated with a root zone. Only a very few root zones showed evidence for the local formation of limonite. Except for these zones, the Lower and Upper Pannonian strata exhibited rather uniform gray colors representing unoxidized sediments. The fine-grained Upper Pannonian clastic sediments are first cycle (not reworked) sediments and accumulated mainly in an aggrading fluvial environment. Potassic feldspar and muscovite, ranging from common to abundant, were observed across the Szombathely section in the micromineralogical study of L. Ravaszn6 Baranyai (unpublished data, 1991). Garnet is abundant, and is the dominant detrital heavy mineral. Sources for the garnet and other clastics included metamorphic rocks in the Eastern Alps found 20-50 km away. Similar metamorphic rocks were encountered in the Szombathely core below 2085 m. Very fine grained detrital magnetite, associated with the coarser-grained magnetite reported by Ravaszn6 Baranyai, therefore would be expected to be an important carrier of the stable magnetization in the Szombathely sediments. The Szombathely strata accumulated and were buried rapidly, and have remained undisturbed and unexposed since burial. The sediments also remained wet. No evidence was observed in the cores that would suggest the movement of connate or ground water. On geologic grounds, the strata escaped secondary magnetizations impressed on sections exposed by erosion. Modifications attributable to weathering and near-surface alterations were observed only near the top of the cored section. Pleistocene clay at a depth of 2 m exhibited strong limonitic alteration. Underlying strata, to 11.5 m, also were yellow, and goethite and kaolinite were identified by I. Viczifin (personal communication, 1993) from strata at 32.6 m. These observations indicate pronounced weathering of the uppermost part of the Szombathely section; secondary magnetizations were recognized in this part. Strata below 35 m

M. Lantos, D.P. Elston /Physics of the Earth and Planetary Interiors 90 (1995) 37-53

cores ranged from 10 to 4.5 cm. Only a few intervals of poor core recovery were encountered; core recovery was nearly 100%. Sampling for paleomagnetism had priority over sampling for all other studies. Samples were collected from the central parts of the cores, away from surfaces that had been in contact with the core barrel. Each 'string' of core was scribed longitudinally as it was extracted from the core barrel. This procedure provided arbitrary reference directions for orienting samples in declination from each core-run. Many cores were extracted and scribed as unbroken cylinders. Cores were sliced transversely at each sample horizon. This procedure was less likely to produce an accidentally inverted sample and spurious polarity reversal than the sampling

exhibited no evidence for weathering or post-depositional alteration-visually, mineralogically, and paleomagnetically. The Szombathely strata are much farther removed physically from the effects of weathering than exposed strata commonly studied for paleomagnetism.

3. Paleomagnetic studies

3.1. Sampling The Szombathely-2 drill hole was continuously cored from the surface to a depth of 2.1 kin, mainly employing a core barrel of 5.9 m length and subordinately of 9 m length. Diameter of the

27M

up

I

vv

w

2.5

m T ~

5O

Z

471.6 m

3O

40 50 I 60

N up

NRM

Up E

1752.4 rn

2O

374.8 rn

41

4£ 50 6O

15

8C b

N

up

15-& 20

\~

1001.5m

10~k~"30 40~ 50

i12

W

N up

569.9 m ~q~

175,9 W "1 I l l l l l l l

up

[I

r,T

W

2.5~40

e w

5~

f

15oo~iit° NRM~-

Fig. 3. Orthogonal diagrams showing representative demagnetization behavior of pilot samples from the Szombathely core section. (a)-(e) show stepwise a.f. demagnetization; (f) shows progressive thermal demagnetization. +, Vertical plane; e, horizontal plane.

42

M. Lantos, D.P. Elston / Physics of the Earth and Planetary Interiors 90 (1995) 37-53

of a longitudinally cut face. Paleomagnetic sampies were collected at ~1 m intervals immediately following removal of core from the core barrel. Slightly more than 3400 oriented samples were collected. The samples were trimmed into cubic shapes with a brass knife or diamond saw, then immediately placed in plastic boxes, sealed, and stored in a refrigerator to inhibit desiccation.

3.2. Laboratory procedures The samples from Szombathely were processed mainly in Budapest, at the joint laboratory of the E6tv6s Lorfind Geophysical Institute and Hungarian Geological Institute. Laboratory measurements employed a two-axis CCL cryogenic magnetometer (Cryogenic Consultants Ltd., London). Following measurement of the NRM (natural remanent magnetization), 85 pilot samples representing different lithologies and depths, and exhibiting normal and reverse polarity, were selected for progressive a.f. demagnetization. These pilot samples were demagnetized in alternating fields in a one-component Schonstedt demagnetizer (Schonstedt Instrument Co., Reston, VA) having a maximum peak field of 100 mT. Sixteen pilot samples from the lower, more consolidated part of the section were selected for thermal demagnetization, and progressively demagnetized in a Schonstedt thermal demagnetizer. A second group of pilot samples then was investigated. Second-stage stepwise a.f. demagnetization was carried out at the Hungarian and USGS laboratories, supplemented by rock magnetic studies at Flagstaff. First, 106 samples were progressively demagnetized in alternating fields to 60 mT and higher. Then, 217 samples previously partially demagnetized at Budapest were processed at Flagstaff. When remeasured at Flagstaff, most samples exhibited nearly the same intensities and directions, indicating stable magnetizations that did not change either with transport or subsequent storage in shielded space.

3.3. Demagnetization 3.3.1. Alternating fieM demagnetization Representative demagnetization behavior of the pilot samples is depicted in orthogonal de-

magnetization diagrams (Figs. 3(a)-3(e)). Most samples exhibited two components of magnetization (Figs. 3(a)-3(c)), and relatively soft secondary magnetizations that disappeared at 10-20 mT (Figs. 3(b)-3(c)). In other cases, the secondary magnetizations were removed by 25-30 mT (Fig. 3(a)). A very few sandstone samples displayed no definite stable inclinations (Fig. 3(d)). Maximal deviations in inclination in these samples were approximately 25°. A few samples exhibited disturbed demagnetization behavior above 30-40 mT (Fig. 3(e)), where a third component of magnetization, a gyroremanence, probably was acquired during demagnetization. Most pilot samples displayed no changes in polarity with demagnetization. About 20% of the sample inclinations changed polarity. Some samples displayed stable directions to above 40 mT. However, evaluation of demagnetization data from first- and second-stage analysis of pilot samples indicated that 30 mT was the highest demagnetization step that could be used without exceeding the stability range of a majority of samples. A two-step demagnetization procedure then was carried out on the 3000 remaining samples--at 20 and 30 mT for the interval 13001800 m, and at 15 and 25 mT for the interval 23-1300 m. Paralleling demagnetization characteristics of pilot samples, 80% of the inclinations following demagnetization were closely similar to inclinations observed before demagnetization. The majority of inclinations thus exhibited no hint of different polarities near the threshold level of stability. As for the pilot samples, minor secondary magnetizations were removed at low to moderate demagnetization levels.

3.3.2. Thermal demagnetization A lack of cementation precluded thermal demagnetization of the Upper Pannonian sediments. Stepwise thermal demagnetization in the range of 150-500°C therefore was carried out on 16 pilot samples from the Lower Pannonian. Stable directions of magnetization were revealed from 150 to 300°C (Fig. 3(f)). Changes in direction occurring in the range of 300-400°C probably were caused by the alteration of pyrite to magnetite.

M. Lantos, D.P. Elston / Physics of the Earth and Planetary Interiors 90 (1995) 37-53

Following demagnetization in an a.f. field of 20 mT, an additional 108 physically coherent samples suitable for heating were selected from two intervals, and demagnetized at 250°C. Paleomagnetic directions remained unchanged with respect to directions from adjacent and intervening samples demagnetized only by a.f. treatment. Differences in inclination between the thermal and a.f. demagnetization averaged - 3 . 2 °.

3.4 Rock magnetic study Low-field susceptibility (X) was measured before and after the samples were investigated for characteristics of isothermal and anhysteretic remanent magnetization (IRM and ARM) at Flagstaff. Susceptibility was measured with a Sapphire Instruments (Kingsville, Ont., Canada) SI-2 Magnetic susceptibility and anisotropy instrument. Remanence was measured with an ScT two-axis cryogenic magnetometer (Superconducting Technology Inc., Mountain View, CA). Stepwise demagnetization of natural, isothermal, and anhysteretic remanent magnetization was carried out in a tumbling degausser. Anhysteretic magnetizations were imparted with a locally made instrument. The ARMs in our experiments were imparted on samples in a decaying approximately 120 mT a.f. having a 0.05 mT bias field, promptly followed by stepwise a.f. demagnetization. IRMs then were imparted on the same samples in steps of 0.01, 0.03, 0.05, 0.1, 0.4, and 0.7 Tesla in 1 rain exposures, followed by slow decay of the d.c. field, and then by stepwise a.f. demagnetization. A representative saturation IRM (sIRM) acquisition curve is shown in Fig. 4.

3.5 Low-field susceptibility The susceptibilities of all other Szombathely samples then were measured at Budapest, also employing an SI-2 magnetic susceptibility and anisotropy instrument. Lower Pannonian mudstone strata characteristically exhibited higher (10-4-10 -3 vol. SI) and more uniform susceptibilities than were found in coarser-grained Upper Pannonian strata. Intensities of magnetization in the Lower Pannonian strata correlated mainly

43

140 120

~

100

i

~0 8 0 6o

i

~

•

•

0.3

0.4

•

40 20

0

0.1

0.2

0,5

0.6

0.7

Applied field (Tesla) Fig. 4. Representative I R M saturation curve normalized with respect to NRM. Sample is from 1652.2 m.

with susceptibility, indicating that: intensity changes reflected changes in concentration of the magnetic mineral(s) and not changes :in intensity of the ambient field. Large fluctuations in susceptibility did not allow similar relationships to be identified in Upper Pannonian strata. Susceptibilities from the fluvial Upper Pannonian strata ranged from 10 -s to 2 ;< 10 - 4 vol. SI. These susceptibilities are close to the level of the paramagnetic signal in clay (e.g. Rochette, 1987). Changes in susceptibility in Upper Pannonian strata therefore may reflect differences in clay content.

3. 6. Carrier of stable magnetization IRM acquisition curves for paleomagnetic samples collected across the Szombathely section showed that saturation is reached in 0.2 T (Fig. 4). This rapid acquisition is characteristic of magnetite (Fe304) , and also titanomagnetite (Fe3_ xTixO4) , maghemite (yFe203) , and pyrrhotite (FeSI+ x) (e.g. Lowrie, 1990, Table 1). The presence of pyrite precludes the use of tJhermal demagnetization diagrams to identify magnetite as the primary magnetic mineral because, above Table 1 D e p t h and time intervals, and accumulation rates, across upper 1500 m of Szombathely core section, Hungary D e p t h interval

Time interval

Accumulation rate

(m)

(Ma)

m kyr-1

years m 1

117.2-1232.2 1232.2-1402.4 1402.4-1494.0

9.149-9.428 9.428-9.491 9.491-9.592

4.00 2.70 0.91

250 370 1103

M. Lantos, D.P. Elston / Physics of the Earth and Planetary Interiors 90 (1995) 37-53

44

-4

2 O'

,

"- 6

/

I

Z ARM 10

4

,

[

6

i

I

SI

8

i

I

10 r

I

"T

0

8

/200 gm

\

~#m

.

10 Fig. 5. Plot of ARM susceptibility vs. bulk low field susceptibility for selected samples. Lines are the expected relationship between xARM and X for different grain sizes of magnetite based on the model of King et al. (1982, 1983). Numbers above the lines are the expected grain size of magnetite.

rill, 1980), but these conditions were not met in the Pannonian depositional environment. Greigite (Fe3S 4) also does not appear to be an important carrier of magnetization. As shown in Fig. 4, about 30% of the sIRM was acquired at 0.05 T. This is slightly below the range of 35-50% acquired by Fe-Ti-bearing samples, and distinctly above the approximately 20% sIRM acquisition by greigite-bearing samples (Reynolds et al., 1994, Fig. 11 and p. 505). Moreover, intensities of magnetization in the Upper Pannonian strata (10 - 4 10 -3 A m - t ; see Fig. 8) are at least an order of magnitude lower than the high intensities that characterize greigite-bearing sediments (Reynolds et al., 1994, Fig. 9). The foregoing observations indicate that magnetite is the principal carrier of stable magnetiza-

1"00 ~ - - - . ~ R M 0.80

a. 1380.4 m

o8o

300°C, secondary magnetite forms from decomposition of pyrite. Stability of magnetization in the range 15-30 mT is consistent with magnetite as the carrier of magnetization. Grains of detrital magnetite-ilmenite were identified in the micromineralogical study of L. Ravaszn6 Baranyai (unpublished data, 1991). Hematite was not observed in any study. The iron hydroxide geothite (FeOOH) was looked for but not found (I. Viczian, personal communication, 1993). Dispersed pyrite is common in the Lower Pannonian, and is of synsedimentary and early diagenetic origin (H~imor, 1989). Pyrite is rare in the dominantly fluviatile Upper Pannonian, and restricted to local occurrences of organic matter. Pyrite was identified by Ravaszn6 Baranyai at several horizons in the Szombathely section. XRay diffraction analysis by I. Viczi~n (personal communication, 1993) also revealed pyrite in Lower Pannonian sediments, whereas maghemite and pyrrhotite were looked for but not observed. Pyrrhotite in the entire Pannonian was not identified at all under the scanning electron microscope (T. Hftmor, personal communication, 1992). Magnetic pyrrhotite may form under conditions of very high pH and low pE (Henshaw and Mer-

0.40 0.200.00-0

"~-----~~-------------~'~ 20 40 60 80 AF demagnetization (mT)

1.00 I ~ L . . _ : : ~ 0,80

-

b. 1398.1m

~ o.6o-T

~.-~"~Rm

-~ 0.40 0.20

NRM ~ " ~

-

. "~' ~

'----.. ~ n -~

o

0

20 40 60 AF demagnetization (mT)

1.006~=~

+~--'~r~

c. 1471,2 m

o8Oooo, o

0.40-

NRM " ~ I L . ' ~ . ~ _

0.200.00 0

~

" -.

-

~ -

1.00~*~_~_~_

o ~-

' o.4o~

~

:

20 40 60 AF demagnetization (roT)

IR~r""~"~ ~

NR

80

d. 1652.2 m

0.80- ~"'-.-'~-~

0 60 -

ao

~

0.20 0.00

ARM M

~ ~

~ . - ~ ~ :

0

20

40 60 AF demagnetization (mT)

80

-" 100

Fig. 6. Normalized a.f. demagnetization behavior of NRM, ARM, and IRM for selected samples.

M. Lantos, D.P. Elston / Physics of the Earth and Planetary Interiors 90 (1995) 37-53

method should be used only to determJine approximate grain size, and then only for particles in the range of 1-15/~m. In Fig. 5, xARM is plotted vs. X for samples from Szombathely. Data points describe a linear array that falls in the field lying between 5 and 20 tzm, with grain sizes lying near 10/xm. Magnetite particles in this size range are considered to be pseudo-single domain (PSD), suitable for retaining a stable primary remanence (Johnson et al., 1975; King et al., 1983). Our rock magnetic study therefore indicates thai: a substantial proportion of the magnetite grains in Szombathely are in the PSD size range, and are capable of becoming aligned with the ambient field during deposition. Johnson et al. (1975) employed the LowrieFuller test (Lowrie and Fuller, 1971), and re-

tion in the Szombathely sediments, and the general lack of iron hydroxides indicates a lack of alteration of the magnetite. Additionally, maghemite, pyrrhotite, and greigite are not important, particularly in the Upper Pannonian. Greigite conceivably could be present in Lower Pannonian sediments below 1042 m. However, if present, greigite has not affected the stable inclination record in those sediments (see Figs. 8(h) and 8(i), below).

3.6.1. Grain size and stability of magnetite The ratio of anhysteretic susceptibility (xARM) to low field susceptibility has been used to detect variations in relative grain size of magnetite in sedimentary sequences (King et al., 1982, 1983). As stressed by King et al. (1982), this Polarity Time Ma 8

45

Szombathely

Scale m 0

Pliocene? and • Pleistocene u M

/ /

i

/

/

/

1

i

t

f

/

I

i

i

Upper 5(

M

Pannonian M M

101

"~" " ' ~ " ~ .~..

d

10 "~-

"~-

M

"~" -~.

15C

Lower

M M

Pannonian

M u

M

Sarmatian

11 20G~

Fig. 7. Correlation of major polarity zones in Szombathely core section with geomagnetic time scale of Cande and Kent (1992). Polarity zonation for the drill core section is highly generalized. Black--normal polarity, white--reversed polarity; black and white with M--mixed polarity; u--unconformity; d--major diastem (minor hiatus); nc--no control. Correlation modified from Elston et al. (1994).

M. Lantos, D.P. Elston / Physics of the Earth and Planetary Interiors 90 (1995) 37-53

46

ported that the test permitted multidomain (MD) particles to be discriminated from single domain (SD) particles. ARM in SD particles exhibits more resistance to a.f. demagnetization than saturation IRM (slRM). This behavior is characteristic for grain sizes below about 100 ~ m (Heider et al., 1992). In diagrams from Szombathely, the ARMs mostly displayed higher relative stability (Figs. 6(b)-6(d)). The higher values indicate that of less than particles of 100 /xm are common. In one diagram, the ARM displayed a slightly smaller value with respect to the IRM (Fig. 6(a)), suggesting the presence of MD particles. The ARMs and IRMs are more stable than the NRMs (Fig. 6). Median destructive fields (MDFs) are about 20-30 mT for the NRM, 30-40 mT for the IRM, and near 40 mT for the ARM.

{a)

m

Stratigraphy

280

290

Inclination .90 °

270

DISTRIBUTARY CHANNEL

+90 °

Intensity

(A/m)

lO 5 10 -4 lO-3 10 2 p !

These MDFs indicate a generally high stability of remanence.

4. Correlations and ages A large interval of normal polarity occurs in the lower part of the Lower Pannonian section below about 1500 m (Fig. 7). Above this, intervals of reverse, normal, and mixed polarity are found to a depth of about 1150 m. Above 1150 m, a long interval of reverse polarity extends to near the present surface. A similar, overly thick interval of reverse polarity was encountered in a well located about 15 km west of Szombathely. Although the base of the reverse polarity interval was not encountered in the second well, similarities in

(b) lO +1

m

IncLination -90 °

POND LIMESTONE 380

ROOTS 390 -

300

Stratigraphy

+90 °

Intensity (A/m) 10-5 10 4 10-3 10 2 10 1

370 - ~

¶

MUDSTONE

DISTRIBUTARY CHANNEL

400 DISTRtBUTARY CHANNEL

310

410

329

DISTRIBUTARY CHANNEL

.~.

420 DISTRIBUTARY CHANNEL ACCBETIONARY BANK

330 DISTRIBUTARY CHANNEL 340

430

440

1

~ ACCRETIONARY BANK DISTRIBUTARY CHANNEL

ACCRETIONARY BANK DISTRIBUTARY CHANNEL

350

i

} ~

a

45O LIGNITE

360

DISTRIBUTARY CHANNEL

460CLAYSTONE TO SANDSTONE, LIGNITE

DISTRIBUTARY CHANNEL 370

470

4

Fig. 8. Oscillation levels and cycles shown with respect to inclination and intensity records, and to the stratigraphy, for the Szombathely core section from 370 to 1170 m. Stratigraphic section after Phillips et al. (1992). White areas--structureless mud; black--lignite; distributary channels consist of interbedded clay mud, silt, and sand. a, b, c are oscillation levels. Oscillation cycles are numbered to right of oscillation levels.

M. Lantos, D.P. Elston / Physics of the Earth and Planetary Interiors 90 (1995) 37-53

stratigraphy and polarity between the drill core sections indicated a stratigraphically reproducible interval of reverse polarity. Magnetostratigraphic correlations among Hungarian core sections (Elston et al., 1990, 1994) show a large normal polarity interval in Lower Pannonian strata. In the shallower parts of the basin, this interval overlies a regional unconformity that separates Pannonian strata from underlying Sarmatian strata. In drill cores in the Great Hungarian Plain, the first appearance of the Congeria banatica molluskan ecozone coincides broadly with chron C5n (Elston et al., 1990, 1994). Additionally, Limnocardium praeponticum occurs only in the lowermost part of the Lower Pannonian, between 11.5 and 10 Ma (Jfimbor et al., 1987). The L. praeponticum and C. banatica zones also are present in the lower part of the Szombathely section. Because of their presence, the long interval of normal polarity below 1500 m

(C)

m

Stratigraphy

Inclination -90 o

+90 °

Intensity

(A/m)

10 -s 10 4 10 3 10 2 104

47O

47

is correlated with chron C5n on both paleomagnetic and paleontologic grounds (Fig. 7). Fossils in stratigraphically higher parts of the Szombathely section are indicative mainly of paleoenvironment. Seismic stratigraphic profiles cross the Pannonian Basin. They connect magnetostratigraphic test holes that intersect late Miocene and Pliocene strata and also bore holes that contain K-At isotopic age dates derived from included volcanic rocks (Fig. 1). Ages for the seismic horizons determined from magnetostratigraphy and K-Ar dates have been extended by tracing seismic horizons across the basin (Pogacsfis et al. 1992, 1994). A seismic horizon that corresponds to the base of a normal polarity interval at 8.2 Ma in the polarity time scale of Berggren et al. (1985) was observed to terminate near the surface in the Szombathely area (Gy. Pog~csas, personal communication, 1993). This horizon now corresponds to an

(d)

m

Intensity

Inclination

Stratigraphy .90 °

10 ~5 1 0 4

+90 °

57Q

580

480

590

490

(

ROOTS

600

500

hn

:¥

OVERBANK DISTRIBUTARY CHANNEL 610

5~0

520

rl

DISTRIBUTARY CHANNEL D]STRIBUTARY CHANNEL

62O

'~_ 630

53O DISTNIBUTARY CHANNEL

640

54O .e,

CLAYSTONE

650

550

ff b

je 660

560 D]STRIBUTARY CHANNEL

c

b c

570 -

670

Fig. 8 (continued).

L

(A/m)

lO-3 10-2

10 1

M. Lantos, D.P. Elston /Physics of the Earth and Planetary Interiors 90 (1995) 37-53

48

age of 8.86 Ma in the polarity time scale of Cande and Kent (1992). Reverse polarity deposits in the Szombathely section immediately below the unconformity at 23.6 m (Fig. 7) thus have been assigned to the reverse polarity interval that directly underlies chron C4An. The underlying relatively small normal polarity zone that has its base at 117 m therefore has been correlated with chron C4Ar.ln, having an age of 9.15 Ma at the base. These assignments in the upper part of the Szombathely section are in accord with the assignment of underlying polarity zones anchored to the long interval of normal polarity of chron C5n (marine magnetic anomaly 5). Average rates of accumulation for differing intervals in Szombathely then were calculated (Table 1). These estimates are average minimum rates of accumulation and d o not take into account the effects of compaction.

(e)

Stratigraphy Inclination m -900 +90° 670-~ DISTRIBUTARY

~1

O,ANNEL

Intensity (A/m) 10 5 10,4 10-3 10-2 10 1

5. Inclination record in Szombathely Only the inclination record for the Upper Pannonian is discussed here. Reference declination lines were obtained by longitudinally scribing the cores during extraction from the core barrels, and by applying these directions to samples from the individual core-runs. However, no declination data are reported here because many cores tended to twist in the core barrel during drilling, particularly in the Upper Pannonian, and controlled corrections were not possible. 5.1. Oscillations

The inclination record is characterized throughout by fine-scale oscillations. Amplitudes of oscillation range from low to very high (from less than 10° to more than 120°). Repeated pro-

(f)

m 770

Stratigraphy

Inclination -90o +90°

c b

780

11

680 " ~

I 790

c

690-~

700

DISTRLBU~RY CHANNEL

b c

710-i--

MARSH: CLAYSTONETO ~ SANDSTONE, DOLOMITE

I

.~.

DISTRIBUTARY CHANNEL

800

POND DOLOMITE

810

DISTRIBUTARY CHANNEL

,J

b

820

,

830

=

730-

,

b 10 740-i

SILTSTONE, SANDSTONE

750-

760"

DISTRIBUTARY CHANNEL LIGNITE

77O

i i.

ACCRETIONARY BANK LIGNITE ROOTS

840

850

MAJOR DISTRIBUTARY CHANNEL

860

FLOODPLAIN OVERBANK DiSTRIBLITARY CHANNEL

870

Fig. 8 (continued).

Intensity (A/m) 10-5 10-4 10 3 10"2 10-1

M. Lantos, D.P. Elston / Physics of the Earth and Planetary Interiors 90 (1995) 37-53

gressions in oscillation level occur across the section. A stable base level, Oscillation Level a, is characterized by low (less than 10 ° peak-to-peak) amplitude fluctuations in inclination. In the succeeding level, Oscillation Level b, higher-amplitude oscillations mostly range from 10 ° to 40 ° peak-to-peak. Still higher amplitudes of oscillation, represented by Oscillation Level c, are greater than 40 ° peak-to-peak, and many exceed 120 ° peak-to-peak to form intervals of mixed polarity. With respect to the geocentric axial dipole, an inclination of 65 ° is expected for the latitude of Hungary. However, a large number of overly steep reverse polarity inclinations occur mainly in Oscillation Levels b and c. Oversteepened reverse and normal polarity inclinations also occur directly before a n d / o r after abrupt polarity switches. Because time intervals that separate the accumulation of individual samples are very brief, we (g)

m 870

_90°

t

5.2. Oscillation cycles Some apparently systematic progressions in amplitudes of oscillation have allowed us to identify oscillation cycles. Amplitudes of oscillation progressing from low to high are fi~llowed by progressive returns to lower amplitudes (Fig. 8). Such systematic progressions are defined here as oscillation cycles. A typical oscillation cycle is shown in Fig. 8(g), between a depth of 935 and 905 m. (h)

m 970 -

Stratigraphy

Inclination -90 o ~

+90 o

a c

Y

NEL

PROGRADATIGN

980 -

~

~--a~ b

990 c

SILTSTONE

b

900

1000•

SHALLOW LAKE QMESTONE, CLAYSTONE

910

920

PROGRADATION

~ ~

~

LIGNITE

c

b

1010-

c

4

1020-

1030-

b I

a

940

1040-

--

LIGNITE FLOOD P~IN TO MARSH

~ 1050-

r

CHANNEL

~

b

3

I 960-

970 -

I~

b ~a

1 1

f

b

L

2

~

i

930-

950-

Intensity ( A / m ) 10 s 10 .4 10 3 10 2 10 -1

i b

OVERBANK ACCRETIONARY BANK DISTRIBUTARY

~

interpret the fine-scale oscillations to represent secular variation. Periodicity of oscillation is somewhat variable. Employing Table 1, three to six samples that constitute individual oscillations correspond to period-times of 380--750 years. Similar high-amplitude fluctuations considered to represent secular variation have been reported by other workers (Opdyke et al., 1972; Creer, 1974; Lund et al., 1988; Levi and Karlin, 1989).

Intensity ( A / m ) 10 .5 104 10-3 10-2 I0-1

+90 °

DOLOMITE, >i POND CLAYSTONE

860

890

Inclination

Stratigraphy

49

1060SHALLOW LAKE 1070

Fig. 8

(continued).

~

b

1

50 (i)

M. Lantos, D.P. Elston / Physics of the Earth and Planetary Interiors 90 (1995) 37-53 Stratigraphy

rn

Inclination -90 °

1070-

Intensity +90 °

MUDSTONE

(A/m)

10 s 10.4 10 .3 10 2 10 1 -

-

1~-

lOSO~

TURB,D,TE

Table 2 Distribution of Oscillation Levels a, b, and c with respect to lithology and depositional environments in Lower Pannonian strata (1042-1809 m) Oscillation Level a

lo9o-

1100

STRUCTURELESS

~UDSTO,E

~

.~ 111o

92 8 100

b 90 7 3 100

c 92 8 100

~

112o

. . - I " ~ T, '

~-~ 1130

Mudstone, clay (%) Sandstone (%) Turbidite (%) Total (%)

for

the Szombathely

section,

a true

spectrum

cannot be obtained.

-

6. Discussion 1140

11~0-

.~."

116o

~ .~--"-

~

1170-

Fig. 8 (continued).

Progressively evolving oscillation cycles are most clearly recognizable in Oscillation Level b and lower Level c. Fifteen cycles are identified in Figs. 8(a)'8(h). Periodicity of the oscillation cycles can be estimated only by employing correlations with the polarity time scale (Fig. 7). Lengths of the individual cycles were measured and then transformed into time based on the average rates of accumulation shown in Table 1. The resulting average periodicity is 6.2 _+ 1.8 kyr. It is not clear if the cycles are strictly periodic or only pseudoperiodic. A p p a r e n t variabilities in periodicity can arise from differing rates of accumulation on a fine scale rather than differing periodicities of oscillation cycles. W e note that spectral analysis will not help determine periodicity of the oscillation cycles because adequate time control is lacking. For computing a spectrum, the data must be interpolated to equally spaced time intervals. Because such intervals of time cannot be directly determined

The oscillating inclination record in Szombathely might be criticized as being 'noisy', with the noise having resulted either from lithologic variations or else from secondary magnetizations that somehow has become impressed across the section. To check for lithologic control, the stratigraphic distributions of Oscillation Levels a, b, and c were compared with the distribution of differing lithologies and depositional environments. The distributions of amplitudes of inclination do not show any systematic correlations with changes in stratigraphy a n d / o r lithology (Tables 2 and 3). Moreover, the symmetric progressions of amplitudes in the oscillation cycles clearly cannot be related to the irregularly deposited, asymmetric mixtures of different sedimentary types. The lack of correspondences indicates that the

Table 3 Distributionof Oscillation Levels a, b, and c with respect to lithologyand depositional environments in Upper Pannonian strata (146-1042 m); distributary channels consist of interbedded clay, mud, silt, and sand Oscillation Level a b c Clay, silt, mudstone (%) 64 64 56 Sand, sandstone (%) 15 13 15 Distributarychannels (%) 18 16 26 Lignite(%) 3 7 3 Total (%) 100 100 100

M. Lantos, D.P. Elston / Physics of the Earth and Planetary Interiors 90 (1995) 37-53

oscillations and oscillation cycles developed independently of depositional events. Some minor geologic noise is suspected where a few one-sample reversals corresponded to relatively coarse lithology. However, in the main, the Szombathely record displays repetitive progressive changes in amplitudes of inclination that cannot be related to changes in lithology and stratigraphy. The paleomagnetic record in the Upper Pannonian strata is contained in fine-grained, rapidly deposited fluvial strata. The strata accumulated with only minor breaks in deposition, never have been exposed to weathering, and exhibit no evidence for alteration related to fluid movement. Therefore, a relatively complete and unmodified record of magnetization acquired during deposition is to be expected. One restricted fold test from a core provided clear evidence for the rapid acquisition and preservation of an original magnetization. In Lower Pannonian strata encountered in another deep drill core, the sediments were abruptly slump-folded as a single unit, and then promptly buried. Paleomagnetic analysis has shown that a stable original remanence was acquired before slumping, and in less than 500 years (H~mor and Lantos, 1994). The analysis also demonstrated unambiguously that the strata have not been subsequently remagnetized by burial or any other process since the time of folding. Oversteep inclinations, observed in both normal and reverse polarity intervals, cannot arise from overprinting on the basis of any known mechanism. Overprinting produces shallower, not steeper, inclinations than expected. Some oscillations in inclination might be postulated to have been produced by secondary magnetizations imposed during more recent normal polarity chrons. However, to produce the observed systematic progressions in amplitudes of oscillation would require the regularly varying development of secondary minerals by an unknown geological process. Such inferred distributions of secondary minerals also should be related to changes in lithology. As seen in Fig. 8, no correlations exist between oscillation cycles and lithology. We consider the inclination record in Szombathely to be unique and to reflect field behavior closely.

51

Oscillation Levels a, b, and c are considered to represent stable, moderately stable to moderately unstable, and unstable fields, respectively. Stratigraphic intervals characterized by stable field behavior display repetitions of a- and b-level oscillations (Fig. 8). Moderately stable fields are characterized by dominantly b-level oscillations, and contain subordinate a- and c-level oscillations. Unstable fields are characterized almost entirely by repetitions of Oscillation Levels b and c. Cycles peak at Oscillation Levels b and c. Oscillation cycles are not recognized in stable fields (e.g. 1160-1046 m, Fig. 8). In short to long stratigraphic intervals, a number of cycles appear to be incompletely developed because they are interrupted by c-level oscillations. The interruptions display three different patterns. Two produce asymmetric sawtooth patterns. First, highlevel oscillations appear abruptly and are followed by declining amplitudes of oscillation (e.g. 765-758 m). Such high-level oscillations can have their bases at diastems, which is indicative of loss of geologic and paleomagnetic record. Second, progressively increasing amplitudes of oscillation are followed by relatively abrupt decreases in amplitude that do not appear related to any break in deposition (e.g. 621-609 m). Third, clevel oscillations can appear and end abruptly, and interrupt a-level and b-level oscillations (e.g. at 1150 m and 580 m). All such 'interruptions' appear to occur independently of the secular variation record. The source for the interruptions is not known, but they presumably arise from a separate, unrelated component of the geomagnetic field. In unstable fields in the Szombathely record, amplitudes of oscillation increase and some period-times appear to decrease. A highly rapid field change has been documented for the middle Miocene Steens Mountain reversal, recorded in basalt that erupted and cooled during a transition (Coe and Pr6vot, 1989). Coe and Pr6vot estimated that the ambient field direction changed at a rate of 3° day-~ during the transition. This rate is two orders of magnitude greater than the most rapid transitions that can be estimated from the Szombathely record. Even a 3° year --~ rate of

52

M. Lantos, D.P. Elston / Physics of the Earth and Planetary Interiors 90 (1995) 37-53

change allows the highly oscillating intervals in Szombathely to be considered valid. The oscillating inclination record in Szombathely accords with a dynamo model. Hagee and Olson (1991) examined long-term behavior of the magnetic field on theoretical grounds by employing calculations of nonlinear, axisymmetric, kinematic dynamos. They showed periodic, increasing and decreasing amplitudes of oscillation in their Figs. 7 and 9, and in their Fig. 4 showed an abrupt change occurring with decreasing oscillations. An oscillating model would explain excursions, brief reversals, and aborted reversals. Such features have been shown in several previous studies. In the Szombathely record, such features are part of a continuously oscillating record.

Acknowledgments We acknowledge the careful laboratory work of Istvfin Szalay and Vincent Yazzie, and the geologic expertise and excellent stratigraphic support of A. J~mbor. This work was supported in part by the Geological Framework and Synthesis Program of the US Geological Survey. This publication is based on work sponsored by the Hungarian-US Science and Technology Joint Fund in cooperation with the US Geological Survey, Department of the Interior, and the Hungarian Geological Institute, under Project 030/90. Initial measurement of samples from the Szombathely well was funded by the Hungarian Foundation for Scientific Research (OTKA), Project 208/11. We thank Richard J. Blakely and Edward A. Mankinen for their helpful reviews of an earlier manuscript. We also thank two anonymous reviewers, whose comments and criticisms led to an improved interpretation of the data.

References Brrczi, I. and Phillips, R. L., 1985. Processes and depositional environments within Neogene deltaic-lacustrine sediments, Pannonian Basin, southeast Hungary. Geophys. Trans. Ertvrs Lor~ind Geophys. Inst., 31: 55-74. Berggren, W.A., Kent, D.V., Flynn, J.J. and van Couvering, J.A., 1985. Cenozoic geochronology. Geol. Soc. Am. Bull., 96: 1407-1418.

Cande, S.C. and Kent, D.V., 1992. A new geomagnetic polarity time scale for the Late Cretaceous and Cenozoic. J. Geophys. Res., 97: 13917-13951. Coe, R.S. and Pr~vot, M., 1989. Evidence suggesting extremely rapid field variation during a geomagnetic reversal. Earth Planet. Sci. Lett., 92: 292-298. Creer, K.M., 1974. Geomagnetic variations for the interval 7000-25000 BP as recorded in a core of sediment from Station 1474 of the Black Sea cruise of "Atlantis II". Earth Planet. Sci. Lett., 23: 34-42. Dank, V., 1987. The role of Neogene deposits among the mineral resources in Hungary. In: Proc. 8th Congress, Regional Committee on Mediterranean Neogene Stratigraphy, Budapest, 15-22 September 1985. Annals, Vol. 70, Hungarian Geological Institute, Budapest, pp. 9-17. Elston, D.P., Lantos, M. and H~mor, T., 1990. Az Alfrld Pannrniai (s.1.) krpzrdmrnyeinek magnetosztratigrfififija. Annual Rep. 1988, Hungarian Geological Institute, Budapest, pp. 109-134. Elston, D.P., Lantos, M. and H~mor, T., 1994. High resolution polarity records and the stratigraphic and magnetostratigraphic correlation of late Miocene and Pliocene (Pannonian, s.1.) deposits of Hungary. In: P.G. Teleki, R.E. Mattick and J. Krkai (Editors), Basin Analysis in Petroleum Exploration. Kluwer Academic, Dordrecht, pp. 111-142. Hagee, V.L. and Olson, P., 1991. Dynamo models with permanent dipole fields and secular variation. J. Geophys. Res. 96: 11673-11687. Hfimor, G., 1984. Paleogeographic reconstruction of Neogene plate movements in the Paratethyan realm. Acta Geol. Hung., 27: 5-21. H~mor, T., 1989. Geochemical and sedimentological indicators of anoxicity of mollasic sedimentation. 28th International Geological Congress, Washington, DC, Abstracts, Vol. 2, pp. 21-22. H~imor, T. and Lantos, M., 1994. An evaluation of slump fold formation using palaeomagnetic techniques. Sediment. Geol., 90: 233-240. Heider, F., Dunlop, D.J. and Soffel, H.C., 1992. Low-temperature and alternating field demagnetization of saturation remanence and tbermoremanence in magnetite grains (0.037/xm to 5 mm). J. Geophys. Res., 97: 9371-9381. Henshaw, P.C. and Merrill, R.T., 1980. Magnetic and chemical changes in marine sediments. Rev. Geophys. Space Phys., 18: 483-504. J~mbor, A., Bal~zs, E., Balogh, K., Brrczi, I., Brna, J., Horvftth, F., Gajdos, I., Geiger, J., Hajrs, M,, Kordos, L., Korecz, A,, Korecz-Laky, I., Korp~s-Hrdi, M., Krv~ry, J., Mrsz~ros, L., Nagy, E., N~meth, G., Nusszer, A., Pap, S, Pogfics~s, Gy., Rrvfsz, I., Rumpler, J., Siitr-Szentai, M., Szalay, A., Szentgyrrgyi, K., Sz~les, M. and Vrlgyi, L., 1987. General characteristics of Pannonian s:l. deposits in Hungary. In: Proc. 8th Congress, Regional Committee on Mediterranean Neogene Stratigraphy, Budapest, 15-22 September 1985. Annals, Vol. 70, Hungarian Geological Institute, Budapest, pp. 155-167.

M. Lantos, D.P. Elston / Physics of the Earth and Planetary Interiors 90 (1995) 37-53 Johnson, H.P., Lowrie, W. and Kent, D.V., 1975. Stability of anhysteretic remanent magnetization in fine and coarse magnetite and maghemite particles. Geophys. J. R. Astron. Soc., 41: 1-10. King, J.W., Banerjee, S.K., Marvin, J. and (gzdemir, O., 1982. A comparison of different magnetic methods for determining the relative grain size of magnetite in natural materials: some results from lake sediments. Earth Planet. Sci. Lett. 59: 404-419. King, J.W., Banerjee, S.K. and Marvin, J., 1983. A new rock-magnetic approach to selecting sediments for geomagnetic paleointensity studies: application to paleointensity for the last 4000 years. J. Geophys. Res., 88: 5911-5921. Lantos, M., H~mor, T. and Pog~cs~s, Gy., 1992. Magneto- and seismostratigraphic correlations of Pannonian s.1. (late Miocene and Pliocene) deposits in Hungary. Paleontol. Evol., 24-25: 35-46. Levi, S. and Karlin, R., 1989. A sixty thousand year paleomagnetic record from Gulf of California sediments: secular variation, late Quaternary excursions and geomagnetic implications. Earth Planet. Sci. Lett., 92: 219-233. Lowrie, W., 1990. Identification of ferromagnetic minerals in a rock by coercivity and unblocking temperature properties. Geophys. Res. Lett., 17: 159-162. Lowrie, W. and Fuller, M., 1971. On the alternating field demagnetization characteristics of multidomain thermoremanent magnetization in magnetite. J. Geophys. Res., 76: 6339-6349. Lund, S.P., Liddicoat, J.C., Lajoie, K.R., Henyey, T.L. and Robinson, S.W., 1988. Paleomagnetic evidence for longterm (104 year) memory and periodic behavior in the Earth's core dynamo process. Geophys. Res. Lett., 15: 1101-1104. Mattick, R.E., Rumpler, J. and Phillips, R,L., 1985. Seismic

53

stratigraphy of the Pannonian Basin in southeastern Hungary. Geophys. Trans. EStvSs Lor~nd Geophys. Ins., 31: 13-54. Mattick, R.E., Rumpler, J., Ujfalusy, A., Szanyi, B. and Nagy, I., 1994. Sequence stratigraphy of the B6k6s basin. In: P.G. Teleki, R.E. Mattick and J. K6kai (Editors), Basin Analysis in Petroleum Exploration. Kluwer Academic, Dordrecht, pp. 39-65. Opdyke, N.D., Ninkovich, D., Lowrie, W. and Hayes, J.D., 1972. The palaeomagnetism of two Aegean deep-sea cores. Earth Planet. Sci. Lett., 14: 145-159. Phillips, R.L., J~mbor, A. and R6v6sz, I., 1992. Depositional environments and facies in continuous core from the Szombathely-II well (0-2150 m), Kisalf61d Basin, western Hungary. US Geol. Surv. Open File Rep., 92-250: 14. Pog~cs~s, Gy., Szab6, A. and Szalay, J., 1992. Chronostratigraphic relations of the progradational delta sequence of the Great Hungarian Plain. Acta Geol. Hung., 35: 311-327. Pog~cs~s, Gy., Mattick, R.E., Elston, D.P., H~mor, T., J~mbor, A., Lakatos, L., Simon, E., Vakarcs, G., V~irkonyi, L. and V~rnai, P., 1994. Correlation of seismo- and magnetostratigraphy in southeastern Hungary. In: P.G. Teleki, R.E. Mattick and J. K6kai (Editors), Basin Analysis in Petroleum Exploration. Kluwer Academic, D,ordrecht, pp. 143-160. Reynolds, R.L., Tuttle, M.L., Rice, C.A., Fishman, N.S., Karachewski, J.A. and Sherman, D.M., 1994. Magnetization and geochemistry of greigite-bearing; Cretaceous strata, North Slope Basin, Alaska. Am. J. Sci., 294: 485528. Rochette, P., 1987. Magnetic susceptibility of the rock matrix related to magnetic fabric studies. J. Struct. Geol., 9: 1015-1020.