11. A PRELIMINARY PALEOMAGNETIC STRATIGRAPHY FOR LOWER EOCENE SEDIMENTSAT SITE 366 (SIERRA LEONE RISE) AND MIOCENE AND OLIGOCENE SEDIMENTS AT SITE 368

(CAPE VERDE RISE), NORTHWEST AFRICAN CONTINENTAL MARGIN

Ernest A. Hailwood, Oceanography Department, University of Southhampton, U.K.

INTRODUCTION

In recent years magnetic polarity stratigraphy hasemerged as an accurate tool for absolute correlationbetween rapidly and continuously depositedsedimentary sequences (e.g., Opdyke, 1972).Furthermore, under favorable circumstances,comparison of such polarity sequences with theestablished paleomagnetic reversal time scale can allowthe assignment of absolute ages to the sedimentarysequence. Attempts have been made to apply thismethod of dating to DSDP cores with varying degreesof success (e.g., Ryan, 1973; Allis et al., 1974). Inprinciple the technique is limited by the followingfactors:

1) Assuming an axial geocentric dipole geomagneticfield model, a magnetic polarity reversal should,theoretically, be represented by a change in declinationof 180°, and a change in sign of inclination.Unfortunately, DSDP cores are not azimuthallyoriented, and in any case the rotary technique used forcoring is likely to lead to relative rotation of differentparts of the sediment core, so that even relativeazimuthal orientation is not necessarily preservedwithin a single core barrel. Consequently,measurements of magnetic declination are of verylimited use in attempting to define the paleomagneticstratigraphy.

2) Since magnetic inclination is a function oflatitude, it is important that the site is situated at asufficiently high latitude to allow accuratedeterminations of inclination to be made.

3) The precision of paleomagnetic determinationswill generally depend on the intensity of remnantmagnetization, which in turn is related to thesedimentary lithology. In general, clastic sediments aremore strongly magnetized than biogenic pelagicdeposits.

4) The validity of the magnetic stratigraphy dependson a clear demonstration that the magnetization isstable and of primary origin.

5) The existence of hiatuses in the sedimentaryrecord will generally lead to ambiguities in correlationof the observed magnetostratigraphy with theestablished polarity time scale.

6) It is important that samples should be taken atsufficiently close intervals to allow the detection of allmajor magnetic epochs and events. A suitable samplinginterval would be in the range of 104 to 105 yr.

An attempt has been made to determine apreliminary paleomagnetic stratigraphy for a portion of

the continuously cored Eocene sediments at Site 366(Sierra Leone Rise) and Miocene/Oligocene sedimentsat Site 368 (Cape Verde Rise). Particular problemsinherent in this study are:

1) Site 366 is located at a relatively low magneticlatitude (5°41'N) and is unlikely to have undergonesignificant latitudinal movement during the Cenozoic.The corresponding low paleomagnetic inclination(approximately 11°) places severe limits on thereliability of polarity changes determined frommagnetic inclination alone;

2) At both sites the sedimentary sequences arecomprised predominantly, of pelagic oozes, with veryweak intensities of magnetization.

MEASUREMENTSAll paleomagnetic determinations were performed

on a Digico Spinner magnetometer (Molyneux, 1971),and, in most cases, triplicate measurements were madefor each sample, in order to assess the reliability of themeasured paleomagnetic directions. The precision isspecified in terms of the Circular Standard Deviation(C.S.D. = 81/VÀ;) of the triplicate measurements,where k is the Fisher precision parameter (Fisher,1953).

Alternating field (AF) demagnetization was carriedout by means of a "Highmoor" electronicallycontrolled demagnetizer.

SITE 368

General RemarksThe sediments cored at this site are predominantly

terrigenous, and the biostratigraphic record is veryincomplete. This provided a stimulus for undertakingpaleomagnetic measurements, in the hope of being ableto place additional age constraints on certain parts ofthe sedimentary record. A preliminary set of 44 sampleshas been measured, principally to evaluate thesuitability of these sediments for more detailed study.Where possible, these samples were taken in groups ofthree from the least-disturbed portions of continuouslycored Miocene and Oligocene sediments. Threesamples were taken from the Pleistocene and Pliocenesections, and eight samples from the middle and upperEocene sections.

ResultsNRM measurements reveal consistently high

intensities of magnetization (>1.5 × 10~6 Gauss) downto a depth of about 200 meters, below which there is an

987

E.A. HAILWOOD

abrupt drop in value by at least one order of magnitude(Table 1), correlating with the lithological change fromnanno oozes and marls to nanno-bearing silty clays.Pilot AF demagnetization at 50 oe increments to 300 oehas been performed on each of the samples from above200 meters depth, but attempts to demagnetize theweaker samples from below this depth were largelyunsuccessful, due to problems in measuring theextremely low intensities of magnetization.

In general, treatment at 50 oe leads to changes inremanence direction of ca 10°, and further treatment athigher fields produces only small additional changes(Figure 1[A]). This behavior is consistent with thesuccessful removal of a weak viscous component. In afew cases larger directional changes were observed,sometimes resulting in a switch of polarity ofremanence after AF cleaning (e.g., Samples 368-5-6, 32-34 cm and 368-7-6, 135-137 cm). The stability of thesingle sample treated from below 200 meters depth

(Figure 1[B]) is apparently lower than that of samplesabove this level, but this is considered to reflect thelower precision in measurement of its very weakremanence.

Remanence directions after AF demagnetization atthe optimum field (chosen by visual inspection ofFigure 1) are listed in Table 1. Below the 200 meterdepth level treatment was restricted to 50 oe for allexcept three samples. Of these, 368-13-4, 92-94 cm wasdemagnetized to 200 oe, and 368-17-4, 33-35 cm and368-19-3, 58-60 cm were too weak to demagnetize at all.The repeatability of measurements after AF cleaning isgood (C.S.D. of triplicate measurements < 7°,commonly < 2°) above the 200-meter level, butvariable, and sometimes poor below this depth (Table1).

The remanence directions are plotted as a function ofdepth in Figure 2. The declination values for differentsamples from the same core barrel are linked, since the

TABLE 1Results of Magnetic Measurements for Site 368

Sample(Interval in cm)

2-3, 8-104-5, 84-865-6, 32-3464,47-496-5,104-1066-6, 44466-6, 90-927-6, 135-1379-2, 101-1039-3, 91-9394,51-539-5, 67-699-5, 70-7210-1,62-6410-2, 32-3410-2, 137-13911-2, 19-2113-2,51-5313-2, 86-8813-4, 92-9415-2, 87-8915-3, 19-21154, 37-39174, 33-35174, 130-13217-5, 100-10217-6,48-50184, 101-10318-5, 110-11218-6, 33-3519-1, 138-14019-2, 62-6419-3, 54-5619-3, 58-6020-1, 135-13621-1, 96-9821-2, 21-23224, 130-13222-5, 100-10222-6,48-5022-6, 72-7425-1, 14-1627-2, 75-77

Depth inHole (m)

12.09'91.85130.83165.98168.05168.95169.41179.36192.02193.42194.52196.18196.21199.63200.83201.88210.20229.52229.87232.93248.88249.70251.38270.34271.31272.51273.49280.52282.11282.84295.89286.63288.05288.09295.35313.97314.72328.31329.51330.49330.73370.15387.26

Dec.

165.2220.9202.5

5.7326.9359.848.1

283.025.6

103.52.5

114.0328.421.5

348.4347.9196.2147.413.5

187.873.45.03.5

316.523.117.5

3.912.024.276.146.337.3_1.1

171.3345.8297.421.264.865.432.2

298.6226.3

Inc.

+61.6+12.2+21.6

-7.311.24.3

-25.1+24.6

-1.6-38.3

-1.3+36.8-26.3-0.8-2.9

-15.2+77.1

+2.0-14.9+12.4

13.6-7.8-0.7

+42.3-20.2-15.0-17.0-24.3

-9.732.3

-41.340.4

_+20.441.3

^0 .8-23.1-14.8-74.2-32.320.5

+54.4+40.1

NRM

C.S.D.

1.02.33.72.53.00.41.33.42.63.00.91.51.21.00.41.04.74.10.91.20.82.61.0

34.525.222.312.711.98.48.8

10.017.9_8.74.5

23.066.150.281.013.248.477.242.7

INT +S.D.(µg)

6.21 ±0.221.48 ±0.031.81 ±0.024.19 ±0.442.37 ±0.08

14.40 ±0.384.69 ±0.106.75 ±0.073.78 ±0.134.66 ±0.017.74 ±0.155.45 ±0.156.52 ±0.127.87 ±0.21

17.69 ±0.5826.96 ±0.700.03 ±0.010.44 ±0.051.93 ±0.080.92 ±0.02

68.62+0.202.01 ±0.113.48 ±0.400.07 ±0.060.17 ±0.050.27 ±0.040.15 ±0.020.10+0.040.07 +0.000.28+0.140.07 ±0.010.15 +0.06

_.0.02 +0.010.34 +0.030.10+0.020.10+0.040.12+0.070.06 ±0.050.06 ±0.040.08 +0.060.08+0.020.05 +0.03

Demag.Field

50(P)50(P)

100(P)50505050(P)

300(P)250(P)

5050

150(P)5050505050(P)50(P)50

100(P)505050u

0 b

505050505050505050K

0 b

5050

50

5050

Dec.

160.6213.1177.819.6

254.71.8

47.7292.2

24.6139.3

0.797.7

315.744.1

335.7314.2163.460.6

1.6152.277.4

0.110.7

316.58.3

38.2358.243.450.799.1

3.218.3

338.71.1

163.8333.3

74.4

359.5197.8

After Demag.

Inc.

+53.0+6.5

-10.9+0.6

+24.6+7.7

-30.0-36.4-16.8-29.5+0.9

+27.6-32.9

2.50.0

-39.0+42.1-19.5-17.3-24.2+15.9-15.5

4 . 3+42.3-36.3-37.645.8-60.3-27.5+35.6-52.8-33.2-62.9+20.4

35.8-21.9

-18.7

+63.5+15.7

C.S.D.

0.52.11.6a

3.93.00.60.96.92.6a

1.00.71.5a

0.61.11.12.3

25.62.8

11.48.9a

1.25.82.9

34.54.7

13.710.122.9

7.94.4

13.47.2

24.58.71.9

17.2

31.1

19.239.5

INT±S D.(µg)

5.49 ±0.301.38 ±0.031.991.34 ±0.181.43 ±0.089.23 ±0.083.96 ±0.040.47 ±0.042.096.18 ±0.113.88 ±0.183.304.21 ±0.394.06 ±0.11

10.25 ±0.126.74 ±0.100.03 ±0.020.17 ±0.050.39 ±0.150.14

69.18 ±0.680.33 ±0.010.92 ±0.080.07 ±0.060.09 ±0.010.08 ±0.010.14 ±0.020.05 ±0.020.08 ±0.020.27 ±0.020.08 ±0.020.17 ±0.010.16 ±0.080.02 ±0.010.49 +0.090.07 ±0.02

0.08 +0.02

0.05 ±0.030.02 ±0.01

NOTE: (P) indicates sample was incrementally demagnetized to 300 oe, and results are listed in Table 2.aC.S.D. based on NRM or 50 oe measurement."Too weak for measurement after demagnetization, so NRM direction used.

988

PALEOMAGNETIC STRATIGRAPHY FOR SEDIMENTS, SITES 366 AND 368

Figure 1. Representative pilot AF demagnetization stability behavior for Site 368 (A) above and (B) below 200 meters sub-bottom. Plotted on polar sterographic projection, with arbitrary declination for each sample.

relative azimuthal orientation within each barrel hadbeen preserved. However, the possibility remains thatapparent changes in declination may reflect relativerotation (i.e., "disking") of the sediment core duringdrilling, as discussed above.

The Miocene and Oligocene magnetic polaritysequence inferred from these measurements is plottedin the left-hand column of Figure 2(C), together with theposition of nannofossil and foram zones. The magneticpolarity is assigned on the basis of the sign of themagnetic inclination (positive = Normal, negative =Reversed). In some instances changes in sign ofinclination are corroborated by changes in declinationof about 100° (e.g., at 169 m).

The magnetic reversal time scale of Ryan et al. (1974)is plotted in the right-hand column of Figure 2(C), and itis clear that the polarity determinations from Site 368are too widely spaced to allow an unambiguouscorrelation with the "standard" time scale, on the basisof magnetic measurements alone. However, using theposition of microfossil zones as an additionalconstraint, a highly provisional and speculativecorrelation is attempted by means of the broken lines.In view of the provisional nature of this correlation, nofurther discussion is presented here, but the generalmagnetic stability characteristics of these sedimentssuggest that a more detailed paleomagnetic study,based on a considerably closer sampling interval wouldprobably yield a meaningful magnetostratigraphy(Table 2).

SITE 366

General RemarksA very complete sequence of Cenozoic pelagic oozes

was recovered from this site and the general absence ofhiatuses and the co-occurrence of different microfossilgroups contribute to making this a very suitablereference section for Cenozoic biostratigraphiczonation in tropical and subtropical latitudes.

Detailed paleomagnetic measurements have beencarried out on part of the Eocene section, in whichdrilling disturbance is virtually absent, in an attempt toassign an absolute chronology to this biostratigraphicsequence.

The accumulation rate for these sediments isapproximately 14 m/m.y., so that a sampling intervalof 1 to 2 meters should correspond with a time intervalof approximately 105 yr. A total of 178 samples wastaken from the depth range 575 to 800 meters, giving anaverage sampling interval of 1.25 meters. This shouldbe sufficient to allow the detection of most importantmagnetic events. Results from 44 samples in the depthrange 575 to 645 meters are presented here, and work isin progress on the remaining samples, to be publishedelsewhere.

ResultsPaleomagnetic measurements at Site 366 are

summarized in Table 3. The intensities of NRM arevery low, with 70% of the determinations lying in the

989

(A) NRM (B) AFTER DEMAGSUBBOTTOM

DEPTH DECLINATION INCLINATION DECLINATION INCLINATION(m)-180 -90 0 90 180 90 -45 0 45 90 -180 -90 0 90 180-90 -45 0 45 90

160

170

1 8 0

190

(C) SITE 369 < r - z<o

2 0 0

210

2 2 0

2 3 0

2 4 0

2 5 0

2 6 0

2 7 0

2 8 0

290

< uj

DC ZOO

N 17

NI6

HI

U

6.0

7.0

8.0

-9.0

•10.0

11.0

12.0

13.0

14.0

15.0

16.0

17.0

18.0

19.0

20.0

Figure 2. Paleomagnetic measurements for Site 368. (A) NRM directions; (B) after demagnetization at 50 oe, (below 200 meters) or optimum field determined frompilot sample behavior (above 200 meters). Error bars represent the Circular Standard Deviation of triplicate measurements (see Tables 1 and 2); (C) summary ofpolarity determinations for Site 368 and highly tentative correlation with the Ryan et al. (1974) time scale, constrained by microfossil zones. Black shadingrepresents Normal magnetization, and open circles, Reversed.

PALEOMAGNETIC STRATIGRAPHY FOR SEDIMENTS, SITES 366 AND 368

TABLE 2Pilot AF Demagnetization Results, Site 368

Demag.Field

02550

100150200250300

Demag.Field

02550

100150200250300

De.

165.2161.5160.6160.9160.1160.7159.3157.1

De.

283.0

264.4268.7268.4259.3288.9292.2

368-2-3,/O = (6.21

In.

+61.6+54.6+53.0+53.0+52.1+52.4+51.4+50.5

8-10 cm±0.22)µg

C.S.D.

1.0

0.5

368-7-6, 135-137 cmJo - (6.75

In.

+24.6

+ 16.4-A.9-5.2-8.1

-29.7-36.4

±0.07)µg

C.S.D.

3.4

1.1

2.3

6.9

J/Jo

1.000.990.880.760.660.630.550.48

JU0

1.00

0.600.600.200.150.100.07

De.

220.9212.9213.1212.8217.0217.9216.3212.9

De.

25.624.725.127.128.227.024.623.0

3684-5.Jo = (1.4

In.

+ 12.2+6.9+6.5+7.4+4.0+6.9+3.4+5.8

368-9-2,

, 84-86 cm8 ±0.03)µg

C.S.D.

2.3

2.1

101-103 cm/O = (3.78±0.13)µg

In.

-1.6-8.4

-11.5-13.2-12.1-13.4-16.8-16.8

C.S.D.

2.6

JU0

1.000.950.930.890.820.570.410.34

J/Jo

1.001.151.231.170.960.750.550.43

De.

202.5

180.4177.8177.8180.8183.5184.5

De.

114.099.896.995.397.796.596.295.3

368-5-6,J0 = (l•8

In.

+21.6

-7.7-10.9-10.9-11.3-11.0-11.7

368-9-5,JO = (5A

In.

+36.8+34.8+32.7+30.9+27.6+27.7+26.8+30.7

32-34 cm1 ±0.02)µg

C.S.D.

3.7

1.6

67-69 cm5 =0.15)µg

C.S.D.

1.5

J/Jo

1.00

1.241.100.870.630.510.41

J/Jo

1.000.900.830.680.600.510.420.32

De.

48.147.947.746.848.144.243.742.3

De.

187.8170.7174.4152.2167.5137.7

368-6-6, 90-92 cmJo = (4.69 ±0.10)µg

In. C.S.D.

-25.1 1.3-29.2-30.0 0.9-31.8-30.6-32.5-31.0-31.4

368-134,92-94 cmJo - (0.92 ±0.02)µg

In. C.S.D.

+ 12.4 1.2+ 1.8-5.3

-24.2-48.7 8.9-26.0

JUO

1.000.950.840.680.540.350.240.17

JU0

1.000.340.260.150.100.13

TABLE 3Results of Magnetic Measurements for Site 366

Sample(Interval in cm)

30-1, 135-13731-1,82-8431-2, 30-3231-2, 133-13531-3,91-9331-4,48-5031-4, 146-14832-1, 120-12232-2, 72-7432-3, 26-2832-3, 128-13032-4, 80-8232-5, 32-3432-5, 140-14233-1,108-11033-2, 75-7733-3, 30-3233-3, 127-12933-4, 86-8833-5, 38-4033-5, 142-14434-1, 106-10834-2, 72-7434-3, 26-2834-3, 132-13427-1, 2427-1, 104-10627-2, 73-7527-3, 30-3227-3, 130-13227-4, 84-8627-5, 35-3727-5, 143-14528-1, 122-12428-2, 77-7928-3, 30-3228-3, 133-13528-4, 86-8828-5, 42-4428-5, 148-15029-1, 85-8729-2, 33-3529-2, 134-13629-3, 91-9329-4, 42-4429-4, 145-147

Depth inHole (m)

604.86613.83614.81615.84616.92617.99618.976"23.71624.73625.77626.79627.81628.3629.91633.09634.26635.31636.27637.37638.39639.43642.57643.73644.77645.83575.03576.05577.24578.31579.31580.35581.36582.44585.73586.78587.81588.84589.87590.93591.99594.86595.84596.85597.92599.11599.96

De.

38.7146.049.442.960.2

115.946.6

316.88.4

186.8347.2169.085.4

338.9111.536.5

316.944.4

345.26.8

98.8359.8

329.6

10.9352.6

18.1105.0112.5354.8

13.2240.2112.3326.398.1

107.2321.5

18.976.646.5

168.6100.159.435.133.4

In.

-28.1-14.2

1.3-18.8-2.4

-39.1-10.7-17.5-6.3

-17.89.2

-33.614.56.3

-12.1-4.122.9-4.3

5.4-8.0-1.526.1

34.7

-6.517.535.3

-23.1-40.5-31.7

16.4-13.4

17.3-22.428.4

-61.2-72.456.213.8

7.0-44.3-35.123.919.3

-47.0

NRM

C.S.D.

42.119.2

13.99.6

23.062.115.0

13.8

5.1

38.6

21.3

18.9

8.722.8

7.342.742.134.838.616.017.2

2.07.17.2

62.115.818.5

9.877.217.642.7

2.84.8

INT ±S.D.(µg)

0.056 ±0.0080.027 ±0.0010.0300.051 ±0.0080.037 ±0.0070.0240.0370.0120.026 ±0.0050.023 ±0.0090.050 ±0.0060.0130.0320.044 ±0.0050.0360.0160.069 ±0.0100.0190.3090.024 ±0.0180.1080.047 ±0.012

0.038 ±0.004

0.274 ±0.0590.019 ±0.0150.046 ±0.0310.016 ±0.0060.031 ±0.0140.028 ±0.0030.026 ±0.0170.098 ±0.0240.218 ±0.0680.133 ±0.0160.266 ±0.0440.120 ±0.0340.015 ±0.0070.060 ±0.0190.041 ±0.0260.135 ±0.0450.023 ±0.0070.082 ±0.0220.255 ±0.3240.482 ±0.0590.154 ±0.044

De.

73.4130.032.648.991.7

148.121.5

281.83.8

191.2313.6251.7111.0280.392.1

388.6305.593.3

323.0315.5100.2346.3

334.6

3.1348.729.3

207.3121.7

7.3114.0223.7126.4330.1100.876.0

131.837.2

126.736.7

356.7108.426.843.333.7

After Demag.

In.

3.6-19.7

1.012.331.1

9.4-8.2

1.421.950.223.714.618.020.5-1.716.122.0-4.417.235.612.052.7

38.9

15.525.944.173.8

0.754.829.4

-12.621.6

-52.730.0

-45.764.277.2

-13.13.8

-21.4-50.6

-1.120.9

-41.7

C.S.D.

27.516.79.2

16.233.138.2

9.551.215.823.731.535.914.910.726.146.0

8.622.0

4.811.7

6.739.5

13.0

8.529.8

9.725.529.281.041.5

5.93.6

12.21.86.8

50.23.1

39.116.690.0

5.824.5

7.35.2

INT rS.D.(µg)

0.046 ±0.0190.023 ±0.0070.034 ±0.0040.019 ±0.0070.020 ±0.0000.031 ±0.0120.037 ±0.0190.015 ±0.0060.023 ±0.0180.018 ±0.0060.015 ±0.0080.015 ±0.0090.024 ±0.0030.032 ±0.0030.024 ±0.0010.009 ±0.0040.111 ±0.0180.015 ±0.0030.066 ±0.0070.016 ±0.0050.065 ±0.0080.034 ±0.015

0.051 ±0.013

0.171 ±0.0430.032 ±0.0090.039 ±0.0080.011 ±0.0040.023 ±0.0170.023 ±0.0060.031 ±0.0090.068 ±0.0160.169 ±0.0150.079 ±0.0180.142 ±0.0110.073 ±0.0280.014 ±0.0070.048 ±0.0190.021 ±0.0020.069 ±0.0080.016 ±0.0100.044 ±0.0050.047 ±0.0100.522 ±0.1470.128 ±0.016

991

NRMSUBBOTTOM

DEPTH DECLINATION INCLINATION(m) -180 -90 0 90 180-90 -45 0 45

575

5 8 0

5 8 5

590

595

6 0 0

605

6 1 0

615

6 2 0

6 2 5

6 3 0

635

6 4 0

6 4 5

>b

X'"~J J>

t < & -

90 -180DECLINATION

-90 0 90

50 oe DEMAGINCLINATION

180-90 -45 0 45

P9-P8

P10

-44--

- 4 6

Figure 3. Paleomagnetic measurements for Site 366. Symbols as in Figure 2, Normal polarity black, Reversed polarity, white.

PALEOMAGNETIC STRATIGRAPHY FOR SEDIMENTS, SITES 366 AND 368

range 1 to 6 × lO~8 Gauss. In general, demagnetizationat 50 oe reduced the intensity to a value close to thelimit of measurement with the available magnetometer,and so demagnetization at higher fields was notattempted.

The directions of magnetization are plotted in Figure3. The inclinations after magnetic cleaning at 50 oe arehighly variable and generally low, with an overall meanvalue of 26 ±20°. Interpretation of the results ishampered by a shortage of samples in the 600 to 612meter depth range, but there is an apparent tendencyfor a concentration of Normal polarities near to the topand bottom of the plotted section, with a zone ofmainly Reversed samples between. The inferredpolarity sequence is plotted in Figure 3, and using theforam zones as an additional constraint, the upper andlower Normal zones are tentatively correlated withanomalies 21 and 22, respectively, on the Tarling andMitchell (1976) polarity time scale. None of theinclination values used to define the restricted Normalzones centered on 605 and 616 meters are significantlydifferent from zero, and so the presence of these zonesmust be regarded as questionable. Consequently, thetop of anomaly 22 is taken to correspond with the topof the thick Normal sequence beginning at 621 meters.

CONCLUSIONThe proposed correlation between the Site 366

polarity sequence and the Tarling and Mitchell (1976)time scale is shown in Figure 4 and implies a change inaccumulation rate at some depth between 621 and 645meters, from 23 m/m.y. to a minimum value of 52m/m.y. below this depth.

Since the correlation and determination ofaccumulation rates are based on only four points, theymust be regarded as highly provisional, and detailedmeasurements on the rest of the Eocene section,currently in progress, are likely to produce a morereliable absolute chronology for the biostratigraphicsequence.

ACKNOWLEDGMENTSI would like to thank Mrs. Pat Copland for undertaking a

substantial proportion of the remanence measurements, andDr. Norman Hamilton for reviewing the manuscript.

REFERENCES

Allis, R.G., Barrett, P.J., and Christoffel, D.A., 1974. Apaleomagnetic stratigraphy for Oligocene and earlyMiocene marine glacial sediments at Site 270, Ross Sea,Antarctica. In Hayes, D., Frakes, L.H., et al , Initial

DEPTH

575η

45-0 44-5 4 4 0

TARLING AND MITCHELL 119761 t

Figure 4. Proposed correlation between the inferred polaritysequence at Site 366 and the Tarling and Mitchell (1976)time scale.

Reports of the Deep Sea Drilling Project, Volume 28:Washington (U.S. Government Printing Office), p.

Fisher, R.A., 1953. Dispersion on a sphere: Proc. Roy. Soc.Lond., Ser. A, v. 217, p. 295-305.

Molyneux, L., 1971. A complete results magnetometer formeasuring the remanent magnetization of rocks. Geophys.J. Roy. Astron. Soc, v. 24, p. 429-433.

Opdyke, N.D., 1972. Paleo magnetism of deep sea cores: Rev.Geophys. Space Phys., v. 10, p. 213-249.

Ryan, W.B.F., 1973. Palaeomagnetic stratigraphy. In Ryan,W.B.F., Hsu, K.J., et al., Initial Reports of the Deep SeaDrilling Project, Volume 13: Washington (U.S.Government Printing Office), p.

Ryan, W.F.B., Cita, M.B., Dreyfus, R.M., Burckle, L.H.,and Saito, T., 1974. A paleomagnetic assignment ofNeogene stage boundaries and the development ofisochronous datum planes between the Mediterranean, thePacific and Indian Oceans in order to investigate theresponse of the world ocean to the Mediterranean "salinitycrisis": Riv. Ital. Paleontol., v. 80, p. 631-688.

Tarling, D.H. and Mitchell, J.G., 1976. Revised Cenozoicpolarity time scale: Geology, v. 4, p. 133-136.

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