Using Paleomagnetic Remanence and Magnetic Susceptibility Data for the Differentiation, Relative Correlation and Absolute Dating of Quaternary Sediments

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USING PALEOMAGNETIC REMANENCE AND MAGNETIC SUSCEPTIBILITY DATA FOR THE DIFFERENTIATION, RELATIVE CORRELATION AND ABSOLUTE DATING OF QUATERNARY SEDIMENTS RENE W BARENDREGT

ABSTRACT Paleomagnetism is used in Quaternary stratigraphic studies as a tool for correlation and relative age dating of equivalent strata or f o r the absolute dating of deposits. The method is based on the dekection of changes in the earth's magnetic field and especially changes of nolarity that are recorded by ferromagnetic sediments at the time of deposition. Dating by paleomagnetic characterization and geomagnetic polarity ,n.lstory is a relatively new technique. The large-scale features of the earth's magnetic field character have been well worked out for the past 5 nlllion years or so. The detailed small-scale features for this neriod are still being discovered and defined through analysis of terrestrial sediments. Because of the much greater sedimentation rate on i a ~ d ,these are more likely to show short-lived events and record the excursions which ultimately will become useful correlative tools. Fine-grained sediments, lava flows, and baked pottery are the media most frequently used. Because reversals have occurred repeatedly in the past their identification within incomplete sedimentary records is only possible through comparison with other stratigraphic o r radiorletric data collected for similar or related sedimentary sequences. Continuously deposited marine or terrestrial sediments which show a nigh sedimentation rate provide isochrons which can be used for worldwide correlation. The recent flourishing of research activity into the secular variation of the earth's non-dipole field promises to greatly refine and embellish the geomagnetic timetable for the Quaternary. The only practical way of demonstrating the validity of interpreted Tagneto-stratigraphy is to show that results are reproducible in widely separated sections with different lithology and sedimentation rates. Ir Canada where Pleistocene deposits are largely glacial in origin, and were thus episodic, one must be aware that these deposits may only have recorded the earth's magnetic field in short time intervals. Possible subsequent alteration of these sediments by the processes outlined in this paper, must be borne in mind. Great Lake sediments and other glacial lake sediments provide excellent opportunity for magnetostratigraphic correlation and dating. INTRODUCTION Dating of Pleistocene sediments beyond the range of radiocarbon dating as well as the dating of sediments which contain no dateable carbon material has always been a problem. The new and the perfected absolute methods (potassium-argon, fission track, amino acid racemization and accelerated carbon-14) require media which are often lacking in Pleistocene sediments.

A new method of dating based upor, the paleomagnetic characterization

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of s e d i m e n t s a n d r o c k s o f f e r s g r e a t p r o m i s e for t h e p a r t i a l a l l e v i a t i o n of t h i s p r o b l e m . I n t h e p a s t t e n y e a r s c o n s i d e r a b l e u s e h a s b e e n made of t h i s t e c h n i q u e i n Canada, U.S.A., J a p a n , The N e t h e r l a n d s , B r i t a i n , The U . S . S . R . , and e l s e w h e r e . I t s m a j o r v a l u e i n P l e i s t o c e n e s t r a t i g r a p h i c work w i l l come i n t h e n e a r f u t u r e , when a more d e t a i l e d r e c o r d o f t h e s e c u l a r v a r i a t i o n of t h e n o n - d i p o l e f i e l d becomes a v a i l a b l e .

A wide r a n g e of m a t e r i a l s has b e e n employed w i t h t h e m e t h o d . Silt t o f i n e s a n d - s i z e d s e d i m e n t s work b e s t , b u t i n g e n e r a l any f i n e - g r a i n e d s e d i m e n t c a n p o t e n t i a l l y b e a n a l y z e d . Marine a s w e l l a s t e r r e s t r i a l s e d i m e n t s have been u s e d . I n a d d i t i o n , l a v a flows s e r v e as e x c e l l e n t r e c o r d e r s o f t h e e a r t h ‘ s p a s t m a g n e t i c f i e l d and can b e sampled w i t h r e l a t i v e e a s e . Baked p o t t e r y a n d o t h e r o b j e c t s can a l s o b e u s e d i f t h e c l a y o f which t h e y a r e made t o o k on a t h e r m a l r e m a n e n t m a g n e t i z a t i o n d u r i n g t h e t i m e of l a s t b a k i n g . Archaeomagnetic s t u d i e s p r o v i d e i n f o r m a t i o n a b o u t s e c u l a r c h a n g e s o f t e n more d e t a i l e d t h a n t h a t which h a s b e e n o b t a i n e d from l a k e s e d i m e n t s , b u t a r e l i m i t e d by t h e i n t e r m i t t e n t r i s e and f a l l of p a s t c i v i l i z a t i o n s . G E O M A G N E T I C DYNAMO THEORY

I t i s w i d e l y a c c e p t e d t h a t t h e m a g n e t i c f i e l d o f t h e e a r t h and s u n a r e p r o d u c e d by dynamos. I n t h e c a s e of t h e e a r t h t h e dynamo a c t i o n i s t h o u g h t t o be p r o d u c e d b y m o t i o n s i n t h e e l e c t r i c a l l y c o n d u c t i n g f l u i d c o r e ( F i g u r e 1 ) . E d w a r d B u l l a r d ( 1 9 7 2 ) s t a t e s t h a t t h e r e i s no d i r e c t d e m o n s t r a t i o n of t h e e x i s t e n c e of t h e s e dynamos; t h e r e a s o n s f o r bel i e v i n g i n them a r e t h e a b s e n c e of any o t h e r s a t i s f a c t o r y t h e o r y and r a t h e r vague q u a l i t a t i v e a r g u m e n t s a b o u t t h e i r p o s s i b l e p r o p e r t i e s . The m a g n e t i c f i e l d for which t h e t h e o r y a t t e m p t s t o a c c o u n t i s a somewhat i d e a l i z e d v e r s i o n of t h e a c t u a l f i e l d and i s d e s c r i b e d by B u l l a r d as a f i e l d t h a t i s a p p r o x i m a t e l y (i20%) a d i p o l e f i e l d , t h a t c h a n g e s by a l a r g e f r a c t i o n o f i t s e l f i n a few h u n d r e d y e a r s , t h a t has r e p e a t e d l y a n d r a t h e r a c c u r a t e l y r e v e r s e d i t s d i p o l e component, and t h a t t h e nond i p o l e p a r t ( F i g u r e s 2 and 3 ) and i t s r a t e of change have l e n g t h s c a l e s of a f e w t h o u s a n d k i l o m e t e r s , a r e q u i t e complex, have a t e n d e n c y t o d r i f t w e s t w a r d , and a r e n o t c o r r e l a t e d w i t h g e o l o g y or geography ( e x c e p t f o r a s y s t e m a t i c t e n d e n c y t o s m a l l r a t e s of change o v e r t h e P a c i f i c ) . I n t h e dynamo t h e o r y t h e t o p o g r a p h y of t h e n o n - d i p o l e f i e l d i s p r o b a b l y associated with eddies i n the convection p a t t e r n s i n the f l u i d core. B u l l a r d c o n c l u d e s t h a t t h e r e a r e n o t many ways i n which a c o m p l i c a t e d changing, r e v e r s i n g f i e l d can b e produced i n a s p h e r e of molten i r o n shut i n a r i g i d container. The o n l y p l a u s i b l e one i s t o assume t h a t t h e r e a r e m o t i o n s i n t h e m a t e r i a l and t h a t t h e s e a c t a s a s e l f - e x c i t i n g dynamo. He s t a t e s t h a t i n a bounded, s t a t i o n a r y e l e c t r i c a l l y c o n d u c t i n g b o d y , any s y s t e m of e l e c t r i c c u r r e n t s w i l l d e c a y e x p o n e n t i a l l y w i t h i t s own t i m e c o n s t a n t . The t i m e c o n s t a n t i s p r o p o r t i o n a l t o t h e e l e c t r i c a l c o n d u c t i v i t y and t h e c h a r a c t e r i s t i c l e n g t h r e p r e s e n t i n g t h e d i s t a n c e i n which t h e f i e l d c h a n g e s by a n a p p r e c i a b l e p a r t o f i t s e l f . B u l l a r d h a s c a l c u l a t e d t h i s t i m e c o n s t a n t t o b e 1 5 , 0 0 0 y e a r s for t h e e a r t h ( a s s u m i n g a r a d i u s o f 3500 km and a c o n d u c t i v i t y of a b o u t 3x1050hm-’m’’). As this p e r i o d i s v e r y s h o r t from a g e o l o g i c a l p o i n t o f v i e w , i t i s e s s e n t i a l t o m a i n t a i n t h e f i e l d , which i n a s e l f - e x c i t i n g dynamo i s done by t h e c u r r e n t s p r o d u c e d by e l e c t r o m a g n e t i c i n d u c t i o n r e s u l t i n g from t h e movement of t h e c o n d u c t i n g m a t e r i a l t h r o u g h t h e f i e l d . B u l l a r d g o e s on t o s t a t e t h a t i t i s f a r from o b v i o u s w h e t h e r a g i v e n m o t i o n can m a i n t a i n a f i e l d , or w h e t h e r a n y m o t i o n c a n . He l i s t s t h r e e non-dynamo t h e o r e m s t h a t p r o h i b i t p a r t i c u l a r t y p e s o f m o t i o n or f i e l d i n a s p h e r e from a c t i n g a s dynamos a n d d i s c u s s e s t h e work o f H e r z e n b e r g ( 1 9 5 8 ) , B a c k u s (1958), C h i l d r e s s (1969) and R o b e r t s ( 1 9 7 0 ) . He s t a t e s t h a t R o b e r t s h a s shown t h a t a l m o s t a l l m o t i o n s s p a t i a l l y p e r i o d i c i n t h r e e d i m e n s i o n s a c t as dynamos. Of t h r e e - d i m e n s i o n a l m o t i o n s s p a t i a l l y p e r i o d i c i n two d i m e n s i o n s , h e has shown t h a t a b o u t h a l f a c t as dynamos. It i s a l m o s t c e r t a i n t h a t t h e r e i s m o t i o n i n t h e c o r e . W i t h o u t i t t h e r e c a n b e no dynamo, and i t seems i m p o s s i b l e t o a c c o u n t f o r t h e s h o r t t i m e s c a l e of t h e m a g n e t i c v a r i a t i o n s . If t h e r e i s m o t i o n , t h e r e must b e f o r c e s t o m a i n t a i n i t . The c o r e i s w e l l p r o t e c t e d from e x t e r n a l i n f l u e n c e s , s o no g r e a t v a r i e t y of f o r c e s can p l a u s i b l y b e s u p p o s e d t o produce t h e motion. E a r l i e r workers suggested thermal convection

103

Figure 1

F a r a d a y D i s k Dynamo g e n e r a t e s e l e c t r i c c u r r e n t s ( s h o r t a r r o w s ) when a c o p p e r d i s k i s t u r n e d t h r o u g h t h e m a g n e t i c l i n e s of f o r c e of a b a r magnet o r through a c o i l of w i r e ( a s shown). In a selfs u s t a i n i n g dynamo t h e g e n e r a t e d e l e c t r i c c u r r e n t s s e r v e t o r e i n f o r c e t h e magnetic f i e l d of t h e c o i l s o t h a t no e x t e r n a l s u p p l y o f m a g n e t i s m i s n e e d e d beyond t h a t which o r i g i n a l l y s e r v e d t o t r i g g e r t h e dynamo. The m e t a l l i c l i q u i d i n t h e c o r e of t h e e a r t h i s b e l i e v e d t o f l o w i n s u c h a way a s t o a c t a s a m e c h a n i c a l dynamo i n g e n e r a t i n g m a g n e t i c f i e l d s of t h e e a r t h . (From C a r r i g a n a n d G u b b i n s , 1 9 7 9 ) .

r e s u l t i n g from r a d i o a c t i v e h e a t i n g . W h e t h e r , i n f a c t , t h e r a d i o a c t i v i t y o f t h e c o r e i s h i g h enough and w h e t h e r t n e p r o c e s s e s f o r removing h e a t from t h e o u t s i d e o f t h e c o r e a r e s u f f i c i e n t l y e f f e c t i v e t o p r o d u c e t h e t e m p e r a t u r e g r a d i e n t n e e d e d t o i n i t i a t e c o n v e c t i o r i s unknown. The main requirement i s t h a t t h e t e m p e r a t u r e g r a d i e n t e x c e e d t h e a d i a b a t i c ; t h e heat flow r e q u i r e d f o r t h i s g r e a t l y e x c e e d s t h e e n e r g y a b s o r b e d by t h e dynamo ( B k l l a r d , 1972). The q u e s t i o n o f r e v e r s a l s o f t h e e a r t h ' s m a g n e t i c f i e l d i s a c h a l lenge t o any t h e o r y o f t h e o r i g i n o f t h e e a r t h ' s f i e l d . A t w o - d i s k dynamo c o n n e c t e d t o g e t h e r s o t h a t e a c h d i s k f e e d s c u r r e n t t o t h e c o i l o f $he o t h e r has S e e n d e v i s e d t a d e m o n s t r a t e t h a t r e v e r s a l s c r n b e p r c duced. Although t h e d i s k dynamo i s f a r from a n y t h i n g t h a t c a n b e i m agined t o e x i s t i n t h e e a r t h ' s c o r e , t h e r e i s a c e r t a i n s i m i l a r i t y i n t h e s t r u c t u r e o f t h e e q u a t i o n s t h a t c o n t r o l i t and t h e e q u a t i o n s cons t r u c t e d f o r magneto-hydrodynamic dynamo t h e o r i e s . I f t h e e q u a t i o n s give u r j t a b l e s o l u t i o n s t h a t f l i p from one d i r e c t i o n t o t h e o p p o s i t e o n e , t h e n a r e v e r s a l h a s no s p e c i f i c " c a u s e " , i t i s s i m p l y a c o n s e q u e n c e o f t h e u n f o l d i n g i n t i m e of t h e s o l u t i o n o f t h e e q u a t i o n . It i s not c e r t a i n t h a t t h i s i s t h e r e a l s t a t e o f a f f a i r s ; i t might be t h a t t h e r e v e r s a l s were a r e s u l t o f d i s t 1 x b a n c e oI" t h e R o t i o n b y s o n e c a k a s t r o c h i c e v e n t , s u c h a s t h e f a l l of a v e r y l a r g e m e t e o r i t e . It i s a l s o p o s s i b l e t h a t t h e c a u s e i s s t a t i s t i c a l and i s a s s o c i a t e d w i t h random f l u c t u a t i o n s i n t h e e l e c t r i c c u r r e n t s and t h e m o t i o n s or t h e f o r c e s . I t i s known t h a t random e m f ' s c a n c a u s e r e v e r s a l s i n a disk-dynamo ( B u l l a r d , 1955). Cox ( 1 9 6 8 ) has s u g g e s t e d t h a t random f l u c t u a t i o n s i n the n o n - d i p o l e p a r t o f t h e f i e l d may p r e c i p i t a t e a r e v e r s a l . He h a s developed a p a r t i c u l a r model i n some d e t a i l and h a s shown t h a t i t l e a d s t o a Poisson d i s t r i b u t i o n o f t i m e i n t e r v a l s between r e v e r s a l s .

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Figure 2

Map o f n o n - d i p o l e f i e l d , v e r t i c a l c o m p o n e n t , f o r epoch 1945, Contours labelled i n milligauss. P o s i t i o n s a r e shown of t h e r a d i a l d i p o l e s w i t h which, together with a central dipole, Alldredge & Hurwitz (1964) r e p r e s e n t e d the 1945 f i e l d : symbols 0 r e p r e s e n t p o s i t i o n s of d i p o l e s p o i n t i n g down w h i l e symbols W r e p r e s e n t p o s i t i o n s of d i p o l e s pointing up. A l l the r a d i a l dipoles are located a t .025 Earth r a d i i . The a r r o w s i n d i c a t e w h e t h e r the d i p o l e s d r i f t e d eastwards o r westwards between epochs 1945 and 1955. (From Creer, 1 9 7 7 ) .

C o m p a r i s o n w i t h t h e o b s e r v e d d i s t r i b 1 J t l o n :her?. l e a d s t c a n e s t b a t i c r . o f t h e p a r m e t e r s o f ‘,he m o d e l . L n e v i t a b l y t h e m o d e l i s b a s e d o n i n t u i t i o n s a b o u t t h e dynamo p r o c e s s r a t h e r t h a n o n a n y f u n d a m e n t a l t h e o r y ; i t i s x o n e t h e l e s s s u g g e s t i v e , and i t seems p o s s i b l e , t h a t t h e r e v e r s a l s o f t h e f i e l d a r e a s s o c i a t e d w i t h c h a n g e s i n t h e complex n o n - d i p o l e f i e l d ( B u l l a r d , 1 9 7 2 ) . An e x c e l l e r i t d i s c u s s i o n o f t h e s o u r c e o f t h e e a r t h ’ s m a g n e t i c f i e l d i s f o u n d i n C a r r i p a n 3rd J u b b i n s ( 1 9 7 9 ) . PALEOMASNETIC REMANENCE A N 3 SUSCEPTIBILITY ANALYSIS Yhe e a r t h ’ s m a g n e t i c f i e l d h a s two s t a b l e s t a t e s : i n i t s n o r m a l s t a t e t h e f i e l d i s b e l i e v e d t o wobble s e v e r a l t e n s of d e g r e e s a b o u t a d i p o l e d i r e c t i o n which, over t h e e a r t h ‘ s s u r f a c e , i s d i r e c t e d toward t h e north. I n i t s r e v e r s e d s t a t e , i t wobbles about a s o u t h o o l a r d i r e c t i o n . T r a n s i t i o n from one s t a t e t o a n o t h e r t a k e s a n a v e r a g e o f 1 0 , 0 0 0 y e a r s . A r e c o r d o f t h e s e c h a n g e s i s l e f t b e h i n d ir! t h e f o r m o f w h a t i s t e r m e d a remanent m a g n e t i z a t i o n of s e d i m e n t s and r o c k s . I n t h e c a s e o f v o l c a n i c r o c k s , t h i s remanence, c a l l e d t h e r m o r e m a r e n t .nag-etism (‘TRV) i s very s t r o n g . Because of t h e s h a r p n e s s o f t h e t r a n s i t i o n zones, t h e y p r o v i d e v e r y u s e f u l s t r a t i g r a p h i c markers which are e a s i l y + , r a c e a b l e and mappable i n t h e f i e l d .

Two d i f f e r e n t t y p e s o f g e o m a g n e t i c p o l a r i t y t i n e w i t s c a n b e d i s t i n g u i s h e d on t h e b a s i s of t h e i r d u r a t i o n . The 1 o r . g e r o f t h e two i s t e r m e d a g e o m a g n e t i c p o l a r i t y e p o c h a n d is d e f i n e d a s a t i m e i n t e r v a l d u r i n g w h i c h t h e e a r t h ‘ s f i e l d was e n t i r e l y o r p r e d o m i n a n t l y o f o n e

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Secular v a r i a t i o n of t h e geomagnetic f i e l d d i r e c t i o n A l l values have beenreduced n e a r London a n d P a r i s . t o t h e p r e s e n t o b s e r v a t o r i e s a t H a r t l a n d (50°59.7'N, 350'31.0'E) and Chambon-la-ForGt ( 4 8 " 0 1 . 4 ' N , 2'15.6' E). The s o l i d c u r v e s a r e t a k e n p r i m a r i l y f r o m c o n tinuous observatory records, whereas t h e dashed c u r v e s a r e b a s e d upon h i s t o r i c a l v a l u e s g i v e n by Gaibar-Puertas (1953). For t h e d a t e s i n parenthes i s , t h e v a l u e s o f t h e d e c l i n a t i o n were o b t a i n e d by extrapolation. (From S k i l e s , 1 9 7 0 ) .

polarity to that of the epoch. The time epochs and events, constructed by Cox e t cther workers (see Figure 4) is based on paleomagnetic measurements from about 60 Hawaii, Alaska, Europe and Africa.

scale for geomagnetic polarity a2. (1964) and modified by the potassium-argon dates and lava flows from California,

The lines of force in the earth's magnetic field are directed toward the magnetic poles and the angle o f any point between true North end the direction o f the field is called the declination. The lines of force are also directed (except at the equator) toward or away from the center of the earth, and the angle above o r below the horizontal is called the inclination (see Figure 5). It is along these lines of force that 'memory elements' have been oriented. The memory elements are rragnetic domains (local small zones within the ferromagnetic material that have a large uniform spontaneous magnetization within the various iron and titanium grains).

A 'good' sample from the sedimentary record should be fine-grained, strongly magnetized, free from secondary mineralization or weathering and unlikely to have a history of lightndng strikes. Fine-grained samples are essential because minute grains of ferromagnetic material in the sediment must be oriented by the earth's ambient magnetic field rather than by the geologic agent that transported and deposited the minerals at the sampling site. Magnetization resulting from these oriented mineral grains is called detrital remanent magnetization (DRM) and conveys a record of the earth's magnetic field at the time of deposition. This DRM may be destroyed or weakened through weathering, secondary mineralization, or lightning strikes. These secondary magnetizations may be an unstable part of the original remanent magnetization and are referred to as a viscous remanent magnetization (VRM). The unwanked secondary magnetizations must be removed to isolate any existent primary magnetization which recorded the ancient earth's field.

106 Figure 4 Revised Late Cenozoic p o l a r i t y t i m e scale reconciled t o the new K - A r c o n s t a n t s . Each short horizontal l i n e ind i c a t e s a potassium-argon a g e and m a g n e t i c p o l a r i t y determined f o r one volcanic cooling unit. Stippled pattern indicates periods of normal p o l a r i t y with coarse stippled pattern indicating events whose l i m i t s a r e p o o r l y defined. Arrows i n d i c a t e possible brief polarity events. The t i m e s c a l e is based on v o l c a n i c c o o l i n g u n i t s and deepsea c o r e s . (From Mankinnen and Dalrymple,

1979).

I f paleomagnetism i s t o provide a valuable d a t i n g t o o l , t h e magnetic r e c o r d must b e s t a b l e a n d o f a s i n g l e component.

It has b e e n shown b y A s and Z i j d e r v e l d ( 1 9 5 8 ) and one c a n n o t s i m p l y d i v i d e s a m p l e s i n t o m a g n e t i c a l l y s t a b l e and u n s t a b l e o n e s , b u t t h a t e a c h rock usually contains several n a t u r a l magnetizations of d i f f e r e n t s t a b i l i t y a n d o f t e n d i f f e r e n t d i r e c t i o n s . The t o t a l n a t u r a l r e m a n e n t m a g n e t i z a t i o n (NRM) o b t a i n e d by s i m p l y m e a s u r i n g a s a m p l e i s t h e

107 NP

Figure 5

C a l c u l a t i o n of p a l e o m a g n e t i c p o l e f r o m a v e r a g e The sampling d i r e c t i o n s o f m a g n e t i z a t i o n ( D m , I*). s i t e S h a s l a t i t u d e and l o n g i t u d e ( A , @ ) and t h e (From McElhinny, pole P has coordinates ( A ' , @ ' ) .

1973).

resultant of these different magnetizations. If a stable magnetization remains after 'quality' cleaning it is inferred to be primary. However, a primary magnetization need not necessarily be stable if, for example, it was recorded by soft multi-domain particles. Various field and laboratory tests have been devised to determine stability of magnetization and are outlined in all textbooks of paleomagnetism. The most commonly used method involves alternating field (a.f.) demagnetization and is best described by Zijderveld (1975). If a sample is placed in a magnetic field of a certain intensity, a part of its remanent magentization will become unblocked from its fixed position and aligned in this ambient field. Upon intensifying this ambient field, more and more of the remanence will be unblocked and redirected. When the applied field is alternating and is slowly decreased from its peak value the unblocked grains are randomized (eliminated) leaving behind the more stable remanence fractions. The sample can be demagnetized stepwise with increasingly stronger alternating magnetic fields by which fractions with higher and higher unblocking fields are randomized. Complete measurement after each treatment gives the magnetization remaining af'ter each demagnetization step. By vector subtraction the direction and intensity components are determined. The strength of the alternating field entirely eliminating the secondary magnetization lies at the point where the resultant vector stops changing direction. This is an important value since it is just with an alternating magnetic field of that strength, that the sample is said to be magnetically 'cleaned' at which time the remaining vector is assumed to be the DRM. It must be pointed out however that vector subtraction will only yield the direction and intensity of the subtracted vector when a single component is removed. Components of remanencemay have overlapping coercivity spectra so that two or more components may be removed simultaneously in the same proportion. Furthermore, 'cleaning' may in some cases leave a stable secondary remanence as for example in sediments where the smallest grains ( i . e . of magnetite) were finally fixed in position sometime after the main DRM. These smallest

108

g r a i n s may b e t h e most s t a b l e component t h r o u g h b e i n g s i n g l e dornained. Chemical o v e r p r i n t s u c h a s d e u t e r i c o x i d a t i o n may s i m i l a r l y l e a v e a s t a b l e s e c o n d a r y remanence. To b e t t e r u n d e r s t a n d t h e means b y w h i c h s e d i m e n t s b e c o m e m a g n e t i z e d

i t i s necessary t o i d e n t i f y t h e magnetic minerals responsible f o r t h e NRM ( s e e L b v l i e e t a;., 1 9 7 1 ) . If f o r e x a m p l e , t h e p r i n c i p l e m a g n e t i c m i n e r a l i s m a g n e t i t e and t h e d e p o s i t s a r e n o t s u b s t a n t i a l l y w e a t h e r e d , t h e i r remanence i s most p r o b a b l y a d e t r i t a l r e m a n e n t m a g n e t i z a t i o n ( D R M ) a n d d a t e s f r o m t h e t i m e of d e p o s i t i o n of t h e s e d i m e n t . I n a d d i t i o n t o m a g n e t i c remanence c h a r a c t e r i s t i c s , s e d i m e n t s ex-

h i b i t m a g n e t i c s u s c e p t i b i l i t y c h a r a c t e r i s t i c s which can b e used i n t h e i r

d i f f e r e n t i a t i o n . M a g n e t i c s u s c e p t i b i l i t y i s a m e a s u r e of t h e d e g r e e t o w h i c h a s u b s t a n c e i s a t t r a c t e d t o a m a g n e t , t h a t i s , t h e r a t i o of t h e i n t e n s i t y of m a g n e t i z a t i o n t o t h e m a g n e t i c f i e l d s t r e n g t h i n a m a g n e t i c c i r c u i t . I n a m i n e r a l g r a i n t h i s s u s c e p t i b i l i t y may b e i s o t r o p i c or a n i s o t r o p i c d e p e n d i n g on t h e s h a p e ‘ a n d c r y s t a l l o g r a p h y of t h e g r a i n . I n g e n e r a l t h e maximum s u s c e p t i b i l i t y o f a n i y r e g u l a r - s h a p e d g r a i n l i e s a l o n g i t s g r e a t e s t d i m e n s i o n and minimum s u s c e p t i b i l i t y a c r o s s i t . C e r t a i n e x c e p t i o n s may e x i s t however. The o r i e n t a t i o n o f any p a r t i c l e w h i c h h a s a m a g n e t i c a n i s o t r o p y may b e a f f e c t e d by t h e m a g n e t i c f i e l d of t h e e a r t h d u r i n g d e p o s i t i o n . Also, t h e magnetic p a r t i c l e s of a s e d i m e n t may h a v e a d i f f e r e n t s i z e or s h a p e d i s t r i b u t i o n from t h a t o f t h e non-magnetic f r a c t i o n and s o may b e a f f e c t e d d i f f e r e n t l y b y hydrodynamic f o r c e z . M a g n e t i t e , which i s i s o t o p i c , has no s t r o n g c r y s t a l l o g r a p h i c a n i s o t r o p y of s u s c e p t i b i l i t y . Any a n i s o t r o p y i n m a g n e t i t e i s due t o t h e d e v i a t i o n i n s h a p e o f t h e g r a i n from a s p h e r e . T h e r e f o r e , u n l e s s most m a g n e t i t e g r a i n s i n a s e d i m e n t a r e e q u a n t , t h e s u s c e p t i b i l i t y anisotropy w i l l r e f l e c t t h e i r alignment. I n a r o c k or s e d i m e n t however, t h e e f f e c t s a r e o b s e r v e d o n l y i f e i t h e r t h e c r y s t a l l o g r a p h i c or s h a p e a x e s o f many g r a i n s a r e a l i g n e d . A p r i m a r y f a b r i c i s a s s o c i a t e d w i t h c o n d i t i o n s a t t h e t i m e o f t h e d e p o s i t i o n and i s g e n e r a l l y c h a r a c t e r i z e d by minimum s u s c e p t i b i l i t y a l i g n e d a b o u t t h e normal t o t h e bedding plane. Thus a s t r o n g a n i s o t r o p y i n m a g n e t i c s u s c e p t i b i l i t y t e n d s t o r o t a t e t h e d i r e c t i o n of m a g n e t i z a t i o n i n t o t h e p l a n e of m a x i mum s u s c e p t i b i l i t y ( U y e d a e t u Z . , 1 9 6 3 ) . G r a v e n o r e t aZ. ( 1 9 7 3 ) and B a r e n d r e g t e t a Z . ( 1 9 7 6 ) h a v e shown t h a t b u l k m a g n e t i c s u s c e p t i b i l i t y v a l u e s of g l a c i a l s e d i m e n t s can be u s e d a s a r a p i d a n d a c c u r a t e d i a g n o s t i c t e c h n i q u e for t h e g e o l o g i s t t o : (1) i d e n t i f y t h e s o u r c e a r e a of s e d i m e n t s and ( 2 ) d i f f e r e n t i a t e dep o s i t s . G r a v e n o r and S t u p a v s k y ( 1 9 7 4 ) a l s o showed t h a t t h e r e i s a s i g n i f i c a n t c o r r e l a t i o n b e t w e e n t h e amount o f m a g n e t i t e a n d t h e amount o f heavy m i n e r a l s i n a t i l l , t h u s a l l o w i n g m a g n e t i c s u s c e p t i b i l i t y m e a s u r e m e n t s t o r e p l a c e t h e t e d i o u s t a s k of heavy m i n e r a l s e p a r a t i o n , i d e n t i f i c a t i o n a n d c o u n t i n g which i s o f t e n r e q u i r e d for t i l l d i f f e r entiation. ABSOLUTE A G E D A T I N G I n t h e p a s t , g e o c n r o n o l o g i c a l d e t e r m i n a t i o n s u s i n g paleomagnetism h a v e b e e n c a r r i e d o u t a l m o s t e n t i r e l y on d e e p - s e a s e d i m e n t s and l a v a f l o w s . A l t h o u g h some e v i d e n c e for r e v e r s a l s o f t h e e a r t h ’ s f i e l d was r e c o g n i z e d l o n g a g o , i t was n o t u n t i l 1963 t h a t a t t e m p t s were made t o d e f i n e t h e g e o m a g n e t i c p o l a r i t y h i s t o r y . R e v e r s a l s a r e g l o b a l phenomenon and t h u s p r o v i d e u s e f u l marker h o r i z o n s for t h e s t r a t i g r a p h e r . S i n c e s e v e r a l r e v e r s a l s h a v e occu”rred i n t h e l a s t 2 m . y . s o m e o t h e r d a t i n g t o o l or s t r a t i g r a p h i c c o n t r o l must i d e n t i f y t h e r e v e r s a l b e f o r e a n a b s o l u t e c o r r e l a t i o n can b e made. The p o l a r i t y t i m e s c a l e and ass o c i a t e d n o m e n c l a t u r e was f i r s t o u t l i n e d i n d e t a i l b y Cox ( 1 9 6 9 ) a n d h a s b e e n r e f i n e d b y a sub-commission of t h e I n t e r n a t i o n a l Commission on S t r a t i g r a p h y ( I n t e r n a t i o n a l Union o f G e o l o g i c a l S c i e n c e s ) . A new v e r s i o n of t h e t i m e s c a l e ( F i g u r e 4) has b e e n b a s e d on more c o m p l e t e K - A r d a t a (Mankinen and D a l r y m p l e , 1 9 7 9 ) . The b e s t r a d i o m e t r i c a g e i n t h e s c a l e o b t a i n e d by m u l t i p l e K.-Ar d a t e s of ashes and l a v a s , f i x e s t h e b a s e o f t h e O l d u v a i e v e n t a t 1 . 8 t .1 m.y. BP ( C u r t i s a n d H a y , 1 9 7 2 ) . O t h e r d a t e s marking b o u n d a r i e s between normal and r e v e r s e epochs a r e

109

much less accurate. Opdyke e t aZ. (1977) have shown from work in Anza Borrego State Park in California that faunal changes from Blancan to Irvingtonianland mammal ages can be accurately dated using the magnetic record of sediments in which the bones occur. Johnson e t a%. (1975) have shown from work in California, Texas and Kansas, that the oldest Irvingtonian based on the occurrence of Lepus(hare) and other small mammals occurs within the Matuyama reversed polarity epoch in the region o f theOlduvai event, while Blancan faunas occur in the Gauss normal p,olarity epochand range into the lower Matuyama epoch. This correlation has also been found in Saskatchewan, Canada by Foster and Stalker (1976). Nany more fossil ages will undoubtedly become fixed with continued correlation between the paleomagnetic and fossil records in terrestrial sediments (Barendregt and Stalker, 1978). CORRELATION AND RELATIVE AGE DATING In addition to absolute dating, paleomagnetic studies can be used to correlate equivalent horizons and $0 relative age-date horizons provided they show one or more o f the following: (1) a secular variation (oscillations of the local magnetic vector originating principally from the non-dipole field' which can be recognized over a relatively large area); (2) comparable total intensity oscillations of the NFM; and (3) similar magnetic susceptibility characteristics (Gravenor and Stupavsky, 1974; Barendregt e t a t . , 1976). In these cases magnetostratigraphy affords a convenient and simple means of correlating Pleistocene deposits o f either terrestrial or marine origin (see Figure 6). One of the more exciting prospects emerging f r o m such chronologic correlations is calibration of evolution rates and directions of dispersal in restricted mammalian species (Opdyke e t a%., 1977; Lindsay e t a l . , 1976; and Johnson e t a%., 1975). The secular variation record whose basic period generally varies from 4 t o l o 4 years (McElhinny and Merrill, 1975) has been shown to be o f some use for regional correlations by Stober and Thompson, 1977 (see Figure 6) as well as by Turner and Thompson (1979) and Vitorello and van der Voo (1977). 'Turner and Thompson (1979) have shown that secular variation is not caused by wobbling of the main geomagnetic dipole as suggested by Kawai and Hirooka (1967) but rather results dominantly from more localized non-dipole changes involving both westward and eastward drift (see also Denham, 1974). They arrive at this conclusion because their 0-7000 year BP secular variation record f r o m Loch Lomond sediments in Scotland does not compare with Japanese archaeomagnetic records or with North American sediment data f r o m this same period. The cause of long or short period oscillations of the geomagnetic secular variation record remains unknown. The oscillations may be due to the main dipole field wobble, westward or eastward drift of non-dipole sources, fluctuations in intensity o f stationary non-dipole sources, or combinations of each of these. Xowever, any one o f these mechanisms would produce different secular changes in widely separated localities. It is likely that secular variation generally must be due to both non-dipole and dipole changes. The matter is still under intensive discussion in the literature. Verosub (1979) has shown that the geomagnetic pole can make large shifts of 10" or more in less that 200 years. This implies that a sudden change in the paleomagnetic directions as recorded by sediments 'The n o n - d i p o l e f i e l d i s t h e r e s u l t of l o c a l c o m p l e x i t i e s i n t h e m a g n e t i c f i e l d , h a v i n g an a v e r a g e v a l u e o f a b o u t 5 % o f t h e m a i n f i e l d a t t h e e a r t h ' s s u r f a c e , a n d s h o w i n g some e i g h t r e g i o n s , o f c o n t i n e n t a l dimensions d i s p l a y i n g p o s i t i v e o r n e g a t i v e v a l u e s w i t h a n a m p l i t u d e of around .15 O e . The n o n - d i p o l e f i e l d h a s a w e s t e r l y d r i f t o f some . 2 t o .3 d e g r e e s o f l o n g i t u d e p e r y e a r a n d i s t h o u g h t t o o r i g i n a t e f r o m r e g i o n s n e a r t h e core-mantle boundary, where l o c a l c e n t e r s of f l u i d m o t i o n may d i s t o r t t h e m a i n t o r o i d a l f i e l d l o c a l l y . ( F i g u r e s 2 and 3)

110

-

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Paleomagnetic r e l a t i v e d e c l i n a t i o n logs f o r a c o r e from Lake Windermere ( E n g l a n d ) , V u o k o n j a r v i ( F i n l a n d ) c o r e 2 (NRM a n d p a r t i a l l y d e m a g n e t i z e d a t 200 Oe) a n d V u o k o n j a r v i c o r e 1 ( 2 0 0 Oe d e m a g . ) Windermere d a t a i s from Mackereth (1971). ( F i g u r e i s f r o m S t o b e r and Thompson, 1 9 7 7 ) .

may n o t b e i n t e r p r e t e d a s e v i d e n c e f o r t h e o c c u r r e n c e o f a d i a s t e m ( m i n o r d e p o s i t i o n a l b r e a k ) . Many s t u d i e s h a v e shown n o m a g n e t i c anoma l i e s i n l o c a t i o n s a n d s e d i m e n t s where t h e y w o u l d b e e x p e c t e d o n t h e b a s i s o f o t h e r c o r r e l a t i o n s . Though g a p s i n t h e r e c o r d may b e p r e s e n t a n d d a t i n g e r r o r s a r e p o s s i b l e , i t i s n o t u n l i k e l y t h a t some a p p a r e n t p o l a r i t y e x c u r s i o n s ' a r e only very l o c a l i z e d geomagnetic o c c u r r e n c e s o r a r e t h e r e s u l t o f n e c h a n i c a l d i s t u r b a n c e o f s t r a t a . Much more d a t a are needed b e f o r e c o n c l u s i o n s about u s i n g s e c u l a r f i e l d o s c i l l a t i o n s f o r s t r a t i g r a p h i c c o r r e l a t i o n can b e made.

'The I n t e r n a l commission on s t r a t i g r a p h y of t h e I n t e r n a t i o n a l Union of G e o l o g i c a l S c i e n c e s d e f i n e s p o l a r i t y e x c u r s i o n as "a s e q u e n c e of v i r t u a l g e o m a g n e t i c p o l e s w h i c h may r e a c h i n t e r m e d i a t e l a t i t u d e s a n d w h i c h may e x t e n d b e y o n d 1 3 5 " o f l a t i t u d e f r o m t h e p o l e s , f o r a s h o r t i n t e r v a l of t i m e , before returning t o t h e o r i g i n a l p o l a r i t y " (Watkins, 1976)

111

Correlation on the basis of intensity fluctuations presents yet another relative dating tool. Cox (1968) constructed a curve showing the variation in the dipole moment (inte-nsity) of the geomagnetic field. The composite curve obtained by Opdyke e t aZ. (1972) from deep-seacores is similar. One must of course be confident that intensity changes are Hot related to lithologic or mineralogical changes. Correlation on the basis of intensity requires correction to a common datum such as the equator, since intensity varies according to latitude. It was found ‘;hat a gradual drop in sample intensity often corresponds with shallowing of inclination and declination swings. Kean e t aZ. (1979) have <-howrithat bog and lake cores from Cedarburg bog and Lake Michigan have lntensity records which afford good correlation having similar long and s h o r t wavelength features, and both records are correlatable with r’ecords from Lake St. Croix described by Lund and Banerjee (1979). The ratio between natural remanent magnetization (NRM) intensity :?rd initi.ai susceptibility (x) sometimes referred to as the modified Koenigsberger ratio or Q ratio which in deep-sea sediments often re”lects changes in intensity of the geomagnetic field (Harrison, 1966; Lpdyke, 1972; Creer, 1974) has been used by others (Thompson, 1975; ievi and Banerjee, 1976) to correlate cores from lake sediments. Thompson concludes that the Q ra.tio of sediments does not correlate between lakes or in certain cases even within lakes and states that in c,pparently uniform lake sediments and possibly also in deep sea cores lit is important to have shown that NRM intensity and (x) (initial suszeptibility) are due to the same magnetic minerals before using normalized intensities as indicators of ancient field intensities. Levi and aarerjee state that before u s i ~ ga particular sediment core for relative Faleointensity determinations, the sediment’s remanence properties must be established, because only those sections with similar remanence pronerties can be compared for relative paleointensities. They found that :homogeneity can be established from similarities in the NRM properties cf the sediment and from similarities of properties of laboratory inG-uced remanences such as ARM (anhysteretic remanent magnetization) and I R M (isothermal remanent magnetization). On the other hand they found that magnetic susceptibility and saturation magnetization are not favored as normalizing parameters, because they are likely to activate ‘1 disproportionately large fraction of the superparamagnetic and multidomain particles which are relatively less important as stable NRM carriers. The non-dipole portion of the geomagnetic field is presently o b :le~vedt o be drifting westward about . 2 ” per year (Bullard e t aZ., 2959). Drift in a westward direction is in accord with magnetohydrodynamic models in which convective and Coriolis forces control the dominant processes in the fluid core, with electromagnetic coupling occurring between the core and mantle (Denham 1974). However, some features of the field show other than westward motion, as for example at Sitka, Alaska, where the three principle orthogonal field components are all drifting eastward at the present time (Skiles, 1970). Eastward drift has also occurred in the past as can be seen from the paleomagnetic record. These non-dipole magnetic fluctuations may cause the local field vector to move about in a looping fashion, with periods on “ne order of hundreds to thousands o f years. These vector loops may also provide a means of sediment correlation and thus can be considered as a relative dating tool. Working with a suggestion of Runcorn (1959), Skiles (1970) discussed the method for inferring the drift direction using paleomagnetic data from a single site and showed that the study zf vector loops could be valuable for gaining insight into the drift of the ancient geomagnetic field. The procedure is to view the unitvectors of magnetic direction along their axis from negative toward positive, and note whether they tend to trace a clockwise or a counterclockwise loop as time advances (Figure 7). In a normally polarized -r.ain field, such loops were shown to correspond to westward or eastward drift respectively, independent of the source polarity or the latitude of the source and observation site (Denham, 1974). Skiles also notes ‘,hat if westward drift persists through a field reversal as the theories of Bullard e t al. (1950), Hide (1966) and Skiles (1969) indicate, then during a reversal we should look for a counterclockwise

112

a)

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Figure 7 Equal-area stereographic p r o j e c t i o n of paleom a g n e t i c d i r e c t i o n s a t two s i t e s a r o u n d Mono L a k e f r o m Denham ( 1 9 7 4 ) . Each r e c o r d exhibits counterclockwise r o t a t i o n of t h e magnetic v e c t o r a l t h o u g h d e t a i l s of t h e two r e c o r d s a r e d i f f e r ent. The t r i a n g l e r e p r e s e n t s the present field direction. t h e arrow d e n o t e s t h e posi t i o n of a d i s t i n c t a s h l a y e r used i n c o r r e l a t i n g between t h e two s i t e s . Arrows i n dicate counterclockwise l o o p i n g motion as t i m e advances. (From V e r o s u b , 1977).

60"

I

,90"E

r o t a t i o n o f t h e f i e l d v e c t o r a t a l l o b s e r v a t o r - 3 s ( S k i l e s , 1 9 7 0 ) . Acc o r d i n g to H i d e (1966) s l o w e a s t w a r d m o t i o n s on t h e c o r e a r e t h e o r e t i c a l l y p o s s i b l e , s o l o n g as t h e o v e r a l l a v e r a g e d r i f t i s m a i n t a i n e d t o wards t h e west. I n t u i t i o n s u g g e s t s t h a t t h e l i f e - s p a n o f a n eastward d r i f t i n g s o u r c e i s i n v e r s e l y r e l a t e d t o i t s r a t e o f d r i f t . Motions c o n t r a r y t o t h e o v e r a l l w e s t w a r d d r i f t may h a v e a low p r o b a b i l i t y o f t r a v e r s i n g g r e a t d i s t a n c e s b e f o r e dying o u t . Thus, it i s q u i t e p o s s i b l e t h a t t h e l i f e - s p a n s o f t h e l o o p a n d t h e p e r t u r b i n g s o u r c e were s i m i l a r (Denham, 1 9 7 4 ) . A s more d e t a i l e d s e d i m e n t a r y d a t a become a v a i l a b l e , d i s t i n c t e v e n t s o f p a l e o - s e c u l a r v a r i a t i o n , s u c h as t h e Mono L a k e c o u n t e r c l o c k w i s e l o o p i n g d e s c r i b e d by Denham ( 1 9 7 4 ) may p r o v e u s e f u l f o r Quaternary s t r a t i g r a p h i c c o r r e l a t i o n over r e l a t i v e l y l a r g e areas. C r e e r e t al. (1972) f o u n d a l o n g - p e r i o d c o u n t e r c l o c k w i s e l o o p i n g o f V . G . P . ' s a b o u t t h e g e o g r a p h i c p o l e f o r t h e i r Lake W i n d e r m e r e d a t a . T h i s data f u r t h e r m o r e s u g g e s t s a h a l f - p e r i o d of a b o u t 1 1 , 0 0 0 y e a r s and t h e y s t a t e t h a t i t i s tempting t o a s s o c i a t e t h i s w i t h t h e p e r i o d of p r e c e s s i o n o f t h e e q u i n o x e s , or some 2 5 , 8 0 0 y e a r s . (Due t o t h e g r a v i t a t i o n a l a t t r a c t i o n of t h e moon a n d t h e s u n on t h e e a r t h ' s e q u a t o r i a l b u l g e , t h e e a r t h ' s s p i n a x i s d e s c r i b e s a cone around t h e c e l e s t i a l p o l e w i t h a p e r i o d of a b o u t 25,800 y e a r s . A s one f a c e s n o r t h t h e p r e c e s s i o n a l m o t i o n i s c o u n t e r c l o c k w i s e , i.e. i n t h e o p p o s i t e s e n s e to t h e r o t a t i o n C r e e r e t aZ., 1 9 7 2 ) . Abrahamsen a n d Readman ( 1 9 8 0 ) a l s o d e s c r i b e a V . G . P . w h i c h moves f i r s t i n a c l o c k w i s e and t h e n i n a c o u n t e r c l o c k w i s e d i r e c t i o n as time a d v a n c e s which t h e y i n t e r p r e t as i n d i c a t i n g e i t h e r a r a t h e r sudden c h a n g e i n t h e s e n s e o f d r i f t o f t h e n o n - d i p o l e f i e l d f r o m westward t o e a s t w a r d , or a f a i r l y r a p i d g r o w t h or d e c a y o f l o c a l d i p o l e c o n f i g u r ations i n the outer core. They u s e t h i s v e c t o r i a l r o t a t i o n p a t t e r n as a p o s s i b l e d a t i n g t o o l and s u g g e s t t h a t t h e Older Yoldia c l a y s n e a r N o r r e L y n g b y , Denmark, may h a v e a n a g e o f 2 4 , 0 0 0 y r s . B P , s i m i l a r t o t h e Mono Lake s e d i m e n t s c o n t a i n i n g s i m i l a r V . G . P . l o o p i n g p a t t e r n s .

113

PROBLEKS AEID L I N I T A T I O N S I N PALEOMAGNETIC ANALYSIS There are a number of possible reasons why the natural remanent magnetization of sediments should not always faithfully reproduce the directions of the ambient magnetic field in which they were deposited. These are outlined by van Montfrans (1971), Verosub (1975), Verosub and Banei,jee (1977) and are summarized here. The reasons include the following: (1) Inclinations may be systematically low due to the preferential alignment o f elongated, tabular or flat shaped magnetic particles. Inclination error may also be due to compaction after deposition. (2) The presence of currents during the deposicion of the sediment, may result in systematically deviating directions of magnetization. ( 3 ) The presence of magnetic material that is too coarse to be aligned by the ambient field during or shortly after deposition and thus fails to record the directions of the magnetic field. (4) Bioturbation after deposition of the sediment results in randomly distributed directions of magnetization or a re-alignment of particles in a field direction different from the one present during original deposition. (5) Collapsing of sediment on a small scale during decalcification. (6) The presence of cryoturbatic structures resulting in randomly distributed directions in these structures. (7) Deformation of sediments as a result of glacial pushing (over-riding), turbidity currents, slumping, liquifaction or some other process which moves sedime'nts away from their original depositional orientation spoils the record. (8) Possible self-reversal, as a result of Dost-secondary alterations ir, the ionic ordering of the crystal framework of the magnetic constituents of the sediment (Irving, 1964). (9) Formation of new Tagnetic minerals during a tlme when the polarity of the geomagnetic field was different or opposite to the period of original deposition. (10) Instability of the natural remanent magnetization, allowing for viscous components of magnetization to set in. (li) Small inaccuracies in field sampling and nreoaration of specimens in the laboratory ( i . e . drying). (12) Errors and misorientations, made during field sampling r,r labelling of samples. Corinq operations may result in breaking and twisting of' the sediments which may then be interpreted as an excursion. These errors can usually be traced. For the above reasons, some sample data must be discarded because +he analytical results Show: (1) a remanent magnetization which makes too large an angle with the direction which would result from the field of an axial geocentric dipole, taking into account secular variation;

(2)

the sediments were not stably magnetized; and

(3)

the present of a stable secondary magnetization.

The glacial sediments of the Quaternary represent special problems in magnetostratigraphic correlation. Just as glaciers picked up, transported and deposited bedrock blocks (Stalker, 1976) they may well have picked up, transported and deposited blocks of frozen glacial sediments and placed them in anomalous stratigraphic positions. A l s o , over-riding of glacial sediments by renewed glaciation may have induced stress deformation (Stupavsky e t al., 1979) and given rise to structures which will yield 'apparent' magnetic excursions, such as those described by Verosub (1975). Other mechanisms of deformation and disturbance of glacial (lake) sediments include seismic activity, turbidity flows, density currents, and periglacial activity such as cryoturbation and congelifluction. Glacial sediments often lack continuous fine-grained sediments or contain large hiatuses or were subject to erosion, all of which represent interruptions in their paleomagnetic record. Glacialtills also represent special problems (Barendregt e t a Z . , Time stratigraphic lines within tills are often n o t horizontal due to different rates of erosion and/or deposition. Stupavsky e t aZ. (1979) in a discuesion of the Meadowcliffe till show a lack of correlation of the remanence characteristics over short interprofile separations leading them to conclude that the till deposition was non-uniform h-ith rapid and variable rates of deposition from point to point within the till. In a study of the Seminary till Gravenor e t a Z . (1979) and

1977).

114 Symons e t aZ. ( 1 9 8 3 ) h a v e p r o p o s e d t h a t t h e t h i x o t r o p i c e f f e c t r e p o r t e d b y Garzes ( 1 9 7 7 ) f o r a d o b e b r i c k s , i s a p r o b a b l e c a ~ l s efor s c n e r e v e r s e d d i r e c t i o r - s . According t o them, t h e l a c k o f c o r r e l a t i o n i n remanenee d i r e c t i o n s b e t w e e n t w o p r o f i l e s .7 m a p a r t a n d t h e l a r g e w i t h i n - c o r e v a r i a t i o n of 1 3 " between specimen p a i r s : s u g g e s t t h e p o s s i b i l i t y o f r e m a n e n c e r e s e t t i n g c a u s e d by t h e hammer b l o w s o r t h e p l a s t i c t u b e s a . n p l e r which t h e y u s e d .

I t h a s b e e n s u g g e s t e d . t h a t t i l l s e x n i b i i ; a m a g n e t i c r e m a n e n c e ',hat i s l a r g e l y t h e r e s u l t o f t h e d i r e c t i o n o f i c e movement,' a n d n o t d u e t o t h e p r e s e n c e o f a l a y e r o f water a t t h e i c e / s e d i m e n t i n t e r f a c e which w o u l d h a v e a l l o w e d t h e m a g n e t i c g r a i n s t o become a l i g n e d i n t h e e a r t h l s a m b i e n t f i e l d . The f o r m e r h a s b e e n r u l e d o u t by a n i s o t r o p y o f m a g r e t i c s u s c e p t i b i l i t y measurements c a r r i e d o u t by Gravenor and Stupavsky ( 1 9 7 4 ) who h a v e shown t h e r e m a n e n c e a n d t h e a n i s o t r o p y o f m a g n e t i c s u s c e p t i b i l i t y of t i l l s t o have d i s t i n c t l y differer,t d i r e c t i o n s . I n c l i n a t i o n e r r o r s a r e a l s o commonly r e p o r t e d f o r t i l l s . L a b o r a t o r y e x p e r i m e n t s c o n d u c t e d by V e r o s u b e t aZ. ( 1 3 7 9 ) showed t h e p a r t i a l r e a l i g n m e n t o f a r t i f i c i a l s e d i m e n t s l , x m i e s on s t i r r i n g by ! s h e a r induced l i q u i f a c t i o n ' t o give a post-depositional DRY. I n t h e same s t u d y , i n c l i n a t i o n e r r o y s o f up t o 30" were p r o d g e e d i n t h i n - t i l l s l u r r i e s w h i l e i n t h i c k - t i l l s l u r r i e s n o s i g n i f i c a n t e r r o r s w e r e n o t e d . The a b o v e m e n t i o n e d v a r i a b i l i t y as w e l l a s t h e s h o c k - i n d u c e d t h i x o t r o p i c r e s e t t i n g a s a r e s u l t o f s a m p l i n g p r o c e d u r e s may e x p l a i r . t h e f a r - s i d e d V . G . P . ( V i r t u a l G e o m a g n e t i c P o l e ) p o s i t i o n s for t i l l s s a m p l e ? , i n O n t a r i o (Symons e t aZ., 1 9 8 0 ) a n d i n A l b e r t a ( B a r e n d r e g t e t aZ., 1 9 7 7 ) . I n c l i n a t i o n e r r o r due t o t h e p r e f e r e n t i a l a l i g n m e n t of i r r e g u l a r - s h a p e d m a g n e t i c p a r t i c l e s n a y a;so e x p l a i n some o f t h e f a r - s i d e d V.G.?. positions. A r e c u r r i n g problem f a c i n g t h e s t u d e n t of Q u a t e r n a r y paleomagnetism t o d a y , i s t h e l a r g e number o f r e p o r t e d e x c u r s i o n s w h i c h c a n n o t b e prop e r l y c o r r e l a t e d or a r e b a s e d o n m e a s u r e m e n t s o f d o u b t f u l a c c u r a c y . C o n c l u s i o n s a r e s o m e t i m e s d r a w n f r o m Lncorr-Dlete d a t a (Opdyke , 1 9 7 6 ) or a r e b a s e d o n i n s u f f i c i e n t s a m p l i r , g s i t e s ( K u k l a a n d Nakagawa, 1 9 7 7 ) . For t h e n o v i c e t h e f o l l o w i n g g u i d e l i n e m i g h t w e l l b e u s e f u l : "a n e g a t i v e i n c l i n a t i o n d o t h n o t a r e v e r s a l m-ake".

I n s e v e r a l p a r t s of t h e w o r l d , t e r r e s t r i a l paleorr.agr.etlc r e c o r d s show e x c u r s i o n s d u r i n g t h e S r u n h e s r o r m a l p o l a r i t y e p o c h . T h e s e e x c u r s i o n s a r e - r e p o r t e d i n g r e a t numbers f o r s e d i m e n t s which a r e between 8,000 a n d 2 0 , 0 0 0 y e a r s i n a p e . However, i n d e e p - s e a r e c o r d s a n d t h e Aegean S e a , t h e r e i s n o e v i d e n c e f o r a r e v e r s a l or e x c u r s i o n o f t h e f i e l d d u r i n g t h i s t i m e (Opdyke e t aZ., 1 9 7 2 ) . T r u e r e v e r s e d m a g n e t i z a t i o n s i n s e d i m e n t s a r e c a u s e d by a r e v e r s a l of t h e main d i p o l e f i e l d and s h o u l d be s e e n world-wide. Excursions of t h e f i e l d ( d e v i a t i o n s of t h e d i r e c t i o n of t h e f i e l d g r e a t e r t h a n t h e n o r m a l s e c u l a r v a r i a t i o n o f a f e w t e n s o f d e g r e e s ) codld also b e c a u s e d b y t h e m a i r d i p o l e f i e l d t i l t i n g a t a l a r g e a n g l e t o t h e r o t a t i o n a x i s and t h e n r e t u r n i n g t o t h e same o r i e n t a t i o n a s b e f o r e . S u c h e x c u r s i o n s s h o u l d a l s o b e s e e n w o r l d w i d e . Kowever, s o u r c e s of t h e n o n - d i p o l e f i e l d c o u l d a l s o p r o d u c e a n e x c u r s i o n t h a t c o u l d c h a n g e t h e d i r e c t i o n o f t h e f i e l d a t o n e p o i n t by 1 8 0 0 , r e s u l t i n g i n a n a p p a r e n t r e v e r s a l a t t h a t p o i n t . Such c o n d i t i o n s would be i n d i s t i n g u i s h a b l e from a r e v e r s a l o f t h e d i p o l e f i e l d , u n l e s s o b s e r v a t i o n s a t e x a c t l y t h e same t i m e i n t e r v a l f r o m a n o t h e r p a r t o f t h e e a r t h do n o t show a r e v e r s a l . I n d e t e r m i n i n g t h e z o n e o f d i s t u r b a n c e o f a d i p o l e f i e l d c a u s e d by ( i . e . one n o t i n v o l v i n g t h e main d i p o l e f i e l d ) a t one p o i n t i n t h e e a r t h l s s u r f a c e , H a r r i s o n and Ramirez ( 1 9 7 5 ) developed a model which p l a c e d a v e r t i c a l d i p o l e a t t h e s u r f a c e o f t h e e a r t h ' s c o r e whose m a g n e t i c f i e l d i s o p p o s i t e t o t h a t o f t h e main d i p o l a r f i e l d . They show t h a t t h e a r e a l c o v e r a g e o f t h e d i s t u r b e d m a g n e t i c f i e l d a r o u n d s u c h a p s e u d o - r e v e r s a l c a n b e q u i t e s m a l l , s u c h t h a t o b s e r v a t i o n s made o n l y a f e w t h o u s a n d km away w o u l d show n o a n o m a l o u s d i r e c t i o n . If t h i s m o d e l i s a p p l i c a b l e t h e r e a r e n o c o n t r a d i c t o r y o b s e r v a t i o n s o f t h e exi s t e n c e o f a r e v e r s a l a t Laschamp a n d o t h e r l o c a t i o n s h a v i n g s i m i l a r a g e , p r o v i d e d %hey a r e p s e a d o - r e v e r s a l s c a u s e d by n o n - d i p o l e f i e l d

a pseudo-reversal

115

s o u r c e s . H a r r i s o n and Ramirez, f u r t h e r show t h a t i f t h e v e r t i c a l d i p o l e model f o r t h e n o n - d i p o l e f i e l d i s c o r r e c t , t h e n p s e u d o - r e v e r s a l s s h o u l d be much l e s s common a t low l a L i t u d e s t h a r , a t h i g h l a t i t u d e s . Geomagnetic e x c u r s i o n s d u r i n g t h e Brunhes e p o c h a r e more and more b e i n g c o n s i d e r e d as m a g n e t o s t r a t i g r a p h i c m a r k e r s . These e x c u r s i o n s a r e a p p a r e n t l y s h a r p movements o f t h e V i r t u a l Geomagnetic P o l e ( V G P ) t o w a r d s or beyond t h e e q u a t o r f o l l o w e d by a r e t u r n t o t h e s t a b l e p o s i t i o n , a l l w i t h i n t h e t i m e s p a n o f a few t h o u s a n d y e a r s . Table 1 provides a l i s t of some w e l l documented e x c u r s i o n s . A u s e f u l d i s c u s s i o n o f some o f t h e s e g e o m a g n e t i c e x c u r s i o n s i s p r o v i d e d by Verosub and B a n e r j e e (1977) who view w i t h c a u t i o n t h e p a l e o m a p e t i c e v i d e n c e p r o v i d e d by t h e v a r i o u s authors. T a b l e 1.

Name

Paleomagnetic P o l a r i t y Excursions and Proposed Ages D u r i n g B r u n h e s Epoch

a

Proposed age ( y r s . BP)

Researchers

Laschamp e v e n t

Bonhommet a n d B a b k i n s ( 1 9 6 7 ) Bonhommet a n d Z a h r i n g e r ( 1 9 6 9 ) H a l l , Y o r k a n d Bonhommet ( 1 9 7 9 ) Heller (1980)

8,000-20,000

Mono L a k e Excursion

Denham a n d Cox Denham ( 1 9 7 4 )

2 4 ,0 0 0

Gothenburg Event

M o r n e r e t at. ( 1 9 7 1 ) Morner and L a n s e r (1974) Morner (1977)

12,350-12,400

Blake Event

Smith and F o s t e r (1969) Denham e t at. ( 1 9 7 6 ) Denham e t al. ( 1 9 7 7 )

1 0 5 , 000

L a k e Mungo E v e n t

B a r b e t t i and McElhinny

L a k e Biwa Excursions

N a k a j i m a e t at. ( 1 9 7 3 ) Y a s k a w a e t al. ( 1 9 7 3 )

18,000 and 104,000-117,000

Erieau Excursion

C r e e r e t at. ( 1 9 7 6 )

7,600-14,000

Lake Michigan Excursions

V i t o r e l l o a n d v a n d e r V oo

Gulf of Mexico Excursions

Freed and Healy (1974) C l a r k and Kennett (1973)

15,000-18,000 30,000-33,000

Mott and F o s t e r

1 2 ,5 0 0

Maple H u r s t Lake and Basswood Road Lake E x c u r s i o n

(1971)

(1976)

(1977)

(1973)

2 9 ,5 0 0

7,500 and 13,000

Meadowcliffe Excursion

s t u p a v s k y e t at. ( 1 9 7 9 )

3 0 , 500

P o r t Dover Excursion

Morner

13,300

Maelifell Event

P e i r c e and C l a r k (1978)

Rubjerg Excursion

Abrahamsen and Knudsen

(1979)

23 ,000-40

N o r r e Lyngby Excursion

A b r a h a m s e n and Readman

(1980)

23,000-40,000

Kipp E x c u r s i o n

Barendregt and S t a l k e r ( i n p r e s s ) 24,000(?)

(1976)

and

40,000 (?)

a L o c a t i o n names a r e u s e d o n l y t o i d e n t i f y t h e r e p a r t e d a r e n o t n e c e s s a r i l y e s t a b l i s h e d names.

,0 0 0

e x c u r s i o n s and

116

The G o t h e n b u r g e v e n t ( l a b e l J e d a s a m a g n e t i c ' f l i p ' i n t h e l i t e r a t u r e ) i s e s p e c i a l l y problematic since t h e authors suggest t h a t the e v e n t l a s t e d o n l y some t e n s o f y e a r s w h i l e t h e i n c l i n a t i o n s w i t c h o n l y t o o k a few y e a r s ( M o r n e r , 1 9 7 7 ) . T h i s would make i t t h e most r a p i d m a g n e t i c p o l a r i t y change known a t p r e s e n t . The mechanism f o r s u c h a n abr?.ormally r a p i d ' f l i p ' i s d i f f i c u l t t o i m a g i n e . R e c e n t l y Thompson and 3 e r g l u n d ( 1 9 7 6 ) r e p o r t e d f i n d i n g no e v i d e n c e f o r t h e Gothenburg e v e n t i r . a d e t a i l e d s t u d y of c o r e s from Sweden. They a t t r i b u t e p r e v i o u s r e p o r t s o f anomalous d i r e c t i o n s as a n example o f t h e ' r e i n f o r c e m e n t syndrome' ( W a t k i n s , 1 9 7 2 and W a t k i n s , 1 9 7 6 ) . From c o n t i n u o u s r e c o r d s of t h e g e o m a g n e t i c f i e l d for t h e p e r i o d 0 - 1 6 , 0 0 0 y e a r s BP o b t a i n e d from t h e s e d i m e n t s o f two p o s t g l a c i a l l a k e s i n M i n n e s o t a and a r e - e x a m i n a t i o n o f t h e p r i m a r y d a t a r e l a t e d to t h e Gothenburg e v e n t a n d E r i e a u e x c u r s i o n , B a n e r j e e and Lund ( 1 9 7 9 ) c o n c l u d e t h a t b o t h e x c u r s i o n s m i g h t i n f a c t be a r t i f a c t s o f t h e l i t h o l o g y ( s e e a l s o B a n e r j e e e t al., 1 9 7 9 ) . T h u s i t becomes o b v i o u s t h a t p a l e o m a g n e t i c r e c o r d s a s r e p o r t e d i n t h e l i t e r a t u r e must b e c r i t i c a l l y w e i g h e d . Even from d e e p - s e a c o r e s t a k e r , o v e r t h e p a s t 2 0 y e a r s , t h e r e i s e v i d e n c e f o r ' s t a b l e ' and ' u n s t a b l e ' c o r e s . A c c o r d i n g t o Verosub and a a n e r j e e ( 1 9 7 7 ) : "A s t a b l e c o r e i s one w h i c h g i v e s a c l e a n p a l e o m a g n e t i c r e c o r d w i t h s i m p l e f e a t u r e s t h a t are generally consistent with the accepted r e v e r s a l seq u e n c e . An u n s t a b l e c o r e on t h e o t h e r hand g i v e s a complex p a l e o m a g n e t i c r e c o r d w i t h many a p p a r e n t c h a n g e s i n p a l e o m a g n e t i c d i r e c t i o n . These cannot b e c o r r e l a t e d w i t h t h o s e i n o t h e r c o r e s . " H a r r i s o n (1974) c o n c l u d e s t h a t many of t h e d e e p - s e a s e d i m e n t c o r e s show s h o r t - p e r i o d e v e n t s which c a n n o t b e c o r r e l a t e d w i t h t h e p r e s e n t l y known t i m e s c a l e of r e v e r s a l s a n d t h e r e f o r e s h o r t - p e r i o d e v e n t s a r e n o t r e l i a b l e s t r a t i graphic markers. O t h e r c o r e s i n which t h e r e i s a c o r r e l a t i o n between d i r e c t i o n a n d / o r i n t e n s i t y o f m a g n e t i z a t i o n , and c l i m a t i c i n d i c a t o r s , do n o t seem t o h a v e a c c u r a t e l y r e c o r d e d D a r a m e t e r s of t h e E a r t h ' s magnetic f i e l d . I t i s hoped t h a t t h r o u g h d e t a i l e d work on t e r r e s t r i a l s e d i m e n t s , t h e mechanism whereby s e d i m e n t s become m a g n e t i z e d w i l l b e b e t t e r u n d e r s t o o d and some of t h e d i s c r e p a n c i e s s o l v e d .

On l a n d , N o r t h American s t u d i e s o f l a k e s e d i m e n t s h a v e o f t e n b e e n made on s i n g l e s a m p l e s p e r h o r i z o n , I 4 C d a t e s a r e few and f r e q u e n t l y have l a r g e e r r o r l i m i t s . Verosub and B a n e r j e e ( 1 9 7 7 ) o b s e r v e t h a t : "Most d i s t u r b i n g o f a l l i s t h e f a c t t h a t t h e o b s e r v e d e x c u r s i o n s a r e f r e q u e n t l y o b s e r v e d n e a r l i t h o l o g i c b o u n d a r i e s r e f l e c t i n g a change from g l a c i a l t o p o s t g l a c i a l times w i t h concomitant r a p i d f l u c t u a t i o n s expected i n t h e depositional conditions i n the lakes." H a r r i s o n and Ramirez ( 1 9 7 5 ) h a v e shown t h a t t h e s m a l l e s t l a t e r a l d i s t a n c e o v e r which a g e o m a g n e t i c f l u c t u a t i o n would b e o b s e r v e d i s a r o u n d 1 0 0 0 km. For t h i s r e a s o n , a n e x c u r s i o n s h o u l d b e r e c o r d e d i n n e a r b y a r e a s i n s e d i m e n t s o f s i m i l a r a g e and as Verosub and B a n e r j e e (1977) c l e a r l y p o i n t out: "Failure t o d e t e c t a paleomagnetic excursion i n a n a d j a c e n t l a k e or o c e a n b a s i n i s a n i m p o r t a n t r e s u l t which s h o u l d n o t go u n r e p o r t e d b e c a u s e i t i s u n i n t e r e s t i n g . " U s i n g a s i m i l a r model t o t h a t of H a r r i s o n and R a m i r e z , Denhamet aZ. ( 1 9 7 6 ) showed t h a t t h e B l a k e e v e n t may n o t h a v e b e e n f e l t o v e r m o r e t h a n 9 % o f t h e e a r t h ' s s u r f a c e . On t h e o t h e r h a n d , S m i t h ( 1 9 6 7 ) and Verosub and Cox (1971) h a v e shown t h a t p a l e o m a g n e t i c e x c u r s i o n s may r e f l e c t g l o b a l g e o m a g n e t i c phenomena r e s u l t i n g from t h e change i n t h e r e l a t i v e s i z e s o f t h e d i p o l e and n o n - d i p o l e f i e l d s .

A l l t h i s i s not t o say t h a t localized excursions a r e not useful. T h e Q u a t e r n a r y s t r a t i g r a p h e r must f i r s t come t o r e a l i z e t h a t t h e y can b e l o c a l i n n a t u r e o r may h a v e a w o r l d w i d e o c c u r r e n c e , and t h e n b e a b l e t o d e t e r m i n e t h e i r a g e s o t h a t t h e y can b e u s e d a s s t r a t i g r a p h i c m a r k e r s . I f a n e x c u r s i o n c a n b e shown t o b e o f g l o b a l e x t e n t and d i p o l a r i n

o r i g i n , a v e r y u s e f u l marker h o r i z o n h a s i n d e e d been found.

F i n a l l y , i f e a c h of t h e r e p o r t e d e x c u r s i o n s l i s t e d i n Table 1 r e p r e s e n t s a d i s t i n c t e x c u r s i o n , t h e n t h e g e o m a g n e t i c f i e l d i s much l e s s s t a b l e t h a n had p r e v i o u s l y b e e n t h o u g h t . Such a h i g h d e g r e e of i n s t a b i l i t y when e x t e n d e d b a c k o v e r g e o l o g i c a l time would p r o d u c e a

117

m a g n e t i c f i e l d f a r more c o m p l e x t h a n h a s b e e n o b s e r v e d i n t h e r e c o r d ( V e r o s u b , 1975). C 0 NC iED I NG RE MARKS

D a t i n g by p a l e o m a g n e t i c c h a r a c t e r i z a t i o n a n d g e o m a g n e t i c p o l a r i t y h i s t o r y i s a r e l a t i v e l y riew t e c h n i q u e . The l a r g e - s c a l e f e a t u r e s o f t h e e a r t h ‘ s m a g n e t i c f i e l d c h a r a c t e r have been well worked o u t f o r t h e p a s t 5 m i l l i o n y e a r s o r s o . The d e t a i l e d s m a l l - s c a l e f e a t u r e s f o r t h i s p e r i o d a r e s t i l l b e i n g d i s c o v e r e d and d e f i n e d t h r o u g h a n a l y s i s of t e r r e s t r i a l s e d i m e n t s . B e c a u s e o f t h e much g r e a t e r s e d i m e n t a t i o n r a t e on l a n d , t h e s e a r e n o r e l i k e l y t o show s h o r t - l i v e d e v e n t s a n d r e c o r d t h e e x c u r s i o n s w h i c h u l t i m a t e l y w i l l become o s e f o i c o r r e l a t i v e t o o l s . The o n l y p r a c t i c a l way of d e m o n s t r a t i n g t h e v a l i d i t y o f i n t e r p r e t e d m a g n e t o - s t r a t i g r a p h y i s t o show t h a t r e s u l t s a r e r e p r o d u c i b l e i r ? w i d e l y s e p a r a t e d s e c t i o n s w i t h d i f f e r e n t l i t h o l o g y and s e d i m e n t a t i o n r a t e s . I n Canada where P l e i s t o c e n e d e p o s i t s a r e l a r g e l y g l a c i a l i n o r i g i n , a n d w e r e t h u s e p i s o d i c , o n e m u s t b e aware t h a t t h e s e d e p o s i t s may o n l y h a v e recorded t h e earth’s magnetic f i e l d i n s h o r t time i n t e r v a l s . P o s s i b l e s u b s e q u e n t a l t e r a t i o n of t h e s e s e d i m e n t s by t h e p r o c e s s e s o u t l i n e d e a r l i e r , must be b o r n e i n mind. Great Lake s e d i m e n t s , g l a c i a l l a k e s e d i m e n t s a n d d e p o s i t s s u c h as t h o s e d e s c r i b e d b y S t a l k e r i n w e s t e r n Canada ( F o s t e r a n d S t a l k e r , 1 9 7 6 ) p r o v i d e e x c e l l e n t o p p o r t u n i t y f o r m a g n e t o - s t r a t i g r a p h i c c o r r e l a t i o n and d a t i n g .

ACKNOWLEDGEMENTS The a u t h o r ’ s w o r k i n Q u a t e r n a r y p a l e o r n a g n e t i s m h a s b e e n s u p p o r t e d by g r a n t s f r o m t h e N a t i o n a l R e s e a r c h C o u n c i l o f C a n a d a a n d t h e D e p a r t ment o f E n e r g y , M i n e s a n d R e s o u r c e s . I a m g r a t e f u l t o Drs. J.H. F o s t e r , K . L . V e r o s u b , D . P a c k e r , a n d A . M . S t a l k e r for t h e i r c o n s i d e r a b l e h e l p i n b r o a d e n i n g my u n d e r s t a n d i n g o f p a l e o r n a g n e t i s m a s a u s e f u l t o o l i n Quaternary research. I a m i n d e b t e d t o D r s . A . L a t h a m a n d H . C . Palmer for c r i t i c a l l y r e v i e w i n g t h e m a n u s c r i p t a n d m a k i n g many h e l p f u l c o m r n e n t s . REFERENCES C I T E D A b r a h a m s e n , N . a n d K n u d s e n , K . L . , 1 9 7 9 , I n d i c a t i o n o f a g e o m a g n e t i c lowi n c l i n a t i o n excursion i n supposed middle Weichselian I n t e r s t a d i a l m a r i n e c l a y a t R u b j e r g , Denmark: P h y s i c s of t h e E a r t h and P l a n e t a r y I n t e r i o r s , v . 1 8 , p . 238-246. Abrahamsen, N . a n d Readman, P . W . , 1 9 8 0 , G e o m a g n e t i c v a r i a t i o n s r e c o r d e d i n O l d e r ( 2 2 3 , 0 0 0 BP) a n d Y o u n g e r Y o l d i a c l a y ( - 1 4 , 0 0 0 BP) a t N o r r e Lyngby, Denmark: G e o p h y s . J. R . A s t r . S O C . , v . 6 2 , p . 345366. A l l d r e d g e , L . R . and H u r w i t z , L . H . , 1964, R a d i a l d i p o l e s as t h e s o u r c e s of t h e E a r t h ’ s main m a g n e t i c f i e l d : J . G e o p h y s . Res.., v . 6 9 , n o . 1 2 , p . 2631-2640. As,

and Z i i d e r v e l d , J . D . A . , i n paleomagnetic research: 78, p. 1-56.

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1958, A c l a s s of s e l f - s u s t a i n i n g d i s s i p a t i v e s p h e r i c a l Ann. P h y s . , v . 4 , p . 3 7 2 - 4 4 7 .

B a n e r j e e , S.K. and Lund, S . P . , 1979, Gothenburg and E r i e a u e x c u r s i o n s q u e s t i o n s r e g a r d i n g t h e i r v a l u e as m a g n e t o s t r a t i g r a p h i c markers: EOS T r a n s . Am. G e o p h y s . U . , v . 6 0 , n o . 1 8 , p . 2 3 8 . B a n e r j e e , S . K . e t az., 1 9 7 9 , G e o m a g n e t i c r e c o r d i n M i n n e s o t a l a k e sediments-absence of t h e Gothenburg and E r i e a u e x c u r s i o n s : Geology V. 7 , p. 588-591. B a r b e t t i , N.F. a n d M c E l h i n n y , M . C . , 1 9 7 6 , T h e L a k e Nungo g e o m a g n e t i c London, v . 281, p . 515-542. excursion: P h i l . T r a n s . R . Soc,,

118 B a r e n d r e g t , R.W. e t a l . , 1 9 7 6 , D l f f e r e n t i a t i o n o f t i l l s i n t h e PakowkiP i n h o r n area of s o u t h e r n A l b e r t a on t h e b a s i s o f t h e i r magnetic susceptibility: Geol. S u r v . Can. P a p e r 76-1C, p . 189-190.

, 1977, Paleomagnetic remanence c h a r a c t e r i s t i c s of t i l l s found i n Pakowki-Pinhorn area of s o u t h e r n A l b e r t a : Can. P a p e r 77-1B, p . 271-272.

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B a r e n d r e g t , R.W. and S t a l k e r , A . M . , 1978, C h a r a c t e r i s t i c magnetization o f some m i d d l e P l e i s t o c e n e s e d i m e n t s f r o m t h e M e d i c i n e H a t area o f southern Alberta: G e o l . S u r v . Can. P a p e r 78-1A, p . 487-488. Bonhommet, N . and B a b k i n e , J . , 1 9 6 7 , S u r l a p r e s e n c e d ' a i m a n t a t i o n s inversges dans l a chaines des puys: C . R . Acad. S c L - , v . 2 6 4 , p . 9294. Bonhommet, N . and Z a h r i n g e r , J . , 1 9 6 9 , P a l e o m a g n e t i s m and p o t a s s i u m a r g o n d a t e d e t e r m i n a t i o n s of t h e Laschamp geomagnetic p o l a r i t y event: E a r t h P l a n e t . S c i . L e t t . , v . 6 , p . 43-46. Bullard, E.C., 1 9 5 5 , The s t a b i l i t y of a h o m o p o l a r dynamo: C a m b r i d g e P h i l . Sot.-, v . 5 1 , n o . 7 4 4 . Nature of Bullard,E.C. field:

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C a r r i g a n , C.R. and Gubbins, D . , 1 9 7 9 , The s o u r c e o f t h e E a r t h ' s m a g n e t i c field: S c i e n t i f i c American, v. 240, no. 2 , p . 118-130. C h i l d r e s s , S . , 1969, A class of s o l u t i o n s of t h e magnetohydrodynamic dynamo p r o b l e m , i n R u n c o r n , S . K . , e d . , The A p p l i c a t i o n o f Modern P h y s i c s t o t h e E a r t h and P l a n e t a r y I n t e r i o r s , N . Y . , Wiley, 629 p . Clark, H.C. and Kennett, J.P., 1973, Paleomagnetic excursion recorded i n l a t e s t P l e i s t o c e n e deep-sea s e d i m e n t s , Gulf of Mexico: m h and P l a n e t , S c i . L e t t . , v . 1 9 , p . 267-274. Cox, A . , 1968, L e n g t h s of geomagnetic p o l a r i t y i n t e r v a l s : R e s . , v . 73, p . 3247-3260.

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Creer, K.M., 1974, Geomagnetic v a r i a t i o n s f o r t h e i n t e r v a l 7,000-25,000 y r . BP a s r e c o r d e d i n a c o r e o f s e d i m e n t f r o m S t a t i o n 1 4 7 4 o f t h e B l a c k S e a c r u i s e o f A t l a n t i s 11: E a r t h P l a n e t . S c i . L e t t . , v . 2 3 , p . 34-42.

, 1977, Geomagnetic s e c u l a r - v a r i a t i o n s during t h e l a s t 25,000 y e a r s : a n i n t e r p r e t a t i o n of d a t a o b t a i n e d from r a p i d l y deposited sediments: Geophys. J . R . A s t r o n . SOC., v . 4 8 , p . 91-109. C r e e r . K . M . e t al.. 1 9 7 2 , G e o m a-g n e t i c s e c u l a r v a r i a t i o n r e c o r d e d i n t h e s t a b l e m a g n e t i c remanence of Recent s e d i m e n t s : Earth Planet. Sci. L e t t . , v . 1 4 , p. 105-127.

, 1976, Late Quaternary geomagnetic s t r a t i g r a p h y recorded i n Lake Erie sediments: E a r t h P l a n e t . S c i . L e t t . , v . 3 1 , p . 37-47. C u r t i s , G.H. and Hay, R.L., 1972, F u r t h e r g e o l o g i c s t u d i e s and K-Ar Bishop, W.W. d a t i n g of O l d u v a i Gorge and Ngorogoro Crater, i n and Miller, J . A . , e d s . , C a l i b r a t i o n of Hominoid E v o l u t i o n : Edinburgh, S c o t t i s h Academic P r e s s , p . 289-302.

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Denham, C . R . , 1 9 7 4 , C o u n t e r c l o c k w i s e m o t i o n o f 2 4 , 0 0 0 y e a r s a g o a t Mono L a k e , C a l i f o r n i a : V. 26, p . 487-498.

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Denham, C . R . a n d C o x , A . , 1 9 7 1 , E v i d e n c e t h a t t h e L a s c h a m p p o l a r i t y event did not occur 13,300-30,400 years ago: Earth Planet. Sci. L e t t . , v. 13, p . 181-190. D e n h a m , C . R . e t al., 1 9 7 6 , B l a k e p o l a r i t y e p i s o d e i n t w o c o r e s f r o m t h e Greater A n t i l l e s o u t e r r i d g e : Earth Planet. Sci. L e t t . , v. 29, p . 422-434.

, 1977, Paleomagnetism and r a d i o c h e m i c a l a g e estimates f o r Earth and P l a n e t . S c i . L e t t . , v . Late Brunhes p o l a r i t y episodes: 35, p . 384-397. Foster, J.H. and S t a l k e r , A.M., 1976, Paleomagnetic s t r a t i g r a p h y of t h e Wellsch v a l l e y site, Saskatchewan: G e o l . S u r v . Can. P a p e r 76-1C, p . 191-193. F r e e d , W.K. and H e a l y , N . , 1 9 7 4 , E x c u r s i o n s o f t h e P l e i s t o c e n e geom a g n e t i c f i e l d r e c o r d e d i n Gulf of Mexico s e d i m e n t s : E a r t h and Planet. Sci. Lett., v . 24, p . 99-104. Gaibar-Puertas, C., Tortosa, Spain:

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