31. PALEOMAGNETIC RESULTS FROM DEEP SEA DRILLING …31. PALEOMAGNETIC RESULTS FROM DEEP SEA DRILLING PROJECT LEG 78A1 Douglas S. Wilson, Geophysics Department, Stanford University2

31. PALEOMAGNETIC RESULTS FROM DEEP SEA DRILLING PROJECT LEG 78A1

Douglas S. Wilson, Geophysics Department, Stanford University2

ABSTRACT

The results of paleomagnetic studies of samples from DSDP Leg 78A are reported. For Site 541, the interval from 60to 200 m sub-bottom was correlated with the Matuyama through Gilbert polarity epochs. For Site 543, the interval from150 to 190 m sub-bottom was correlated with marine magnetic Anomalies 5C through 5E. Down-dip directions of tiltedbeds inferred from declination values for Sites 541 and 542 suggest a pattern of monoclinal folding. Results from basaltsamples are comparable to those from other DSDP sites in relatively old basalts.

INTRODUCTION

The primary objective of Deep Sea Drilling ProjectLeg 78A was to study the accretionary processes of theBarbados Ridge, at the convergent margin between theAmerican and Caribbean plates. Site locations are shownin Figure 1. Sites 541 and 542 are near the toe of the ac-cretionary prism and about a kilometer apart; coring atboth recovered only sediment and failed to penetrate theinterplate thrust fault. Site 543 was chosen as a referenceeast (seaward) of the deformation front, on presumablynormal, undeformed ocean crust; drilling there bottomedin basalt overlain by and interbedded with Upper Creta-ceous (Campanian) sediments. Because of time con-straints, rotary coring was used throughout the cruise.

Paleomagnetic sampling on Leg 78A was carried outwith three objectives in mind: to help determine age bypolarity stratigraphy; to constrain in situ structure withestimates of bed strikes; and to characterize the mag-netic properties of the basaltic basement, thereby sup-plementing data from other DSDP legs on the generalnature of the source layer for marine magnetic anoma-lies. Recovery at all the sites was fairly high, but practi-cal considerations limited the number of sediment sam-ples suitable for paleomagnetic work. To avoid problemswith drilling disturbance or rotation resulting from un-detected tectonic tilting, samples were taken only at visi-ble beds. The drilling disturbance was sufficiently severein the upper 50 m or so of each hole that no sampleswere taken in the upper parts of any of the holes. Atbest, the sample interval was 1-2 m, with occasionalgaps of several meters or more owing to lack of recoveryor the disturbed nature of the core. In Hole 541, be-cause of the exceptionally high sediment accumulationrate, this rather sparse sample interval was adequate forpolarity stratigraphy over much of the hole.

METHODSIn the less consolidated sediments of the core, samples were taken

by pressing 2.4 cm × 2.4 cm × 2 cm plastic cubes into the core. Thecubes, scribed with arrows pointing upcore, were pried from the rest

1 Biju-Duval, B., Moore, J. C , et al., Init. Repts. DSDP, Vol. 78A: Washington (U.S.Govt. Printing Office).

2 Address: Geophysics Department, Stanford University, Stanford, CA 94305.

of the core. In the more consolidated regions, an upcore arrow wasscribed directly on the core, and a sample of measurable size was sawnor drilled. The basalt samples were standard 2.5 cm × 2.5 cm cores.The samples were measured on the shipboard Digico fluxgate spinnermagnetometer; alternating-field (AF) demagnetization studies were car-ried out using the Schonstedt AF demagnetizer. For some of the speci-mens, follow-up studies were done at Stanford University on the Su-perconducting Technology two-axis cryogenic magnetometer withbuilt-in AF demagnetizer. For the shipboard measurements, the moststraightforward procedure was to imagine a hypothetical arrow on thetop (upcore side) of each specimen pointing to the actual upcore arrowscribed on the side of the sample. The hypothetical arrow correspondsto the fiducial line referred to in the Digico manual, and allows thesamples to be measured without using the time-consuming field orbedding corrections. The Digico convention defines the coordinatesystem for relative declinations: the direction normal to and awayfrom the cut surface of the working half of the core is 0°, increasingclockwise looking downcore. In this coordinate system, the workinghalf of the core is the "south" half. This same coordinate system wasused to record strikes and dips of bedding for structural analysis. Ateach measurable bed, the core was sliced to give apparent dips in twodifferent planes, defining the bedding plane. The strike of the bed wastaken to be the line in the bedding plane normal to the axis of the core.By convention, the strikes recorded are 90° counterclockwise from thedown-dip direction. The magnetization directions are corrected forbedding by rotating the direction about the strike line by the amountof the dip.

The measurement results are tabulated in Table 1, which lists, foreach sample, natural remanent magnetization (NRM) (in A/m), mag-netization at optimum demagnetization level (A/m), optimum demag-netization level (mT), polarity interpretation (normal or reversed), andreliability estimate. For samples taken at dipping beds, a second linegives strike, dip, and corrected inclinations and declinations. For thebasalt samples, susceptibility (cgs) is listed instead of polarity and reli-ability. The reliability categories are modeled after U.S. academic coursegrades: A represents a stable magnetization inferred from at least twodemagnetizations; B indicates that stable magnetization was inferredfrom the agreement between a single demagnetization and the NRM;C indicates that the magnetization direction was still changing system-atically at the highest demagnetization used; D indicates a marginallystable sample; and F indicates an unstable one. The H designation in-dicates that the magnetization was too close to horizontal to deter-mine the polarity, the latitude for the sites is 15°N, which should bevalid for the entire Neogene. This latitude predicts a dipole inclinationof 28°, which is far enough from horizontal for polarity interpreta-tion, but shallow enough that some horizontal magnetizations shouldbe expected either from normal secular variation or from an increasein scatter caused by rotary coring. On this basis, a sample with a stableinclination greater than + 10° was judged to be normal and less than- 10° was considered reversed. Inclinations between +10° and - 10°were not used for polarity determinations. The expected inclinationfor older times is not clear, and depends on whether the sites have beenon the North American Plate or South American Plate. (The activeboundary between these two plates has never been clearly recognized.)

583

D. S. WILSON

Venezuela

64° 62° 60° 58°

Figure 1. Locations of Sites 541, 542, and 543 on and near the Barbados Ridge.

56°

Since the only reliable inclinations in the older samples are from thebasalts, where secular variation has not been adequately averaged andtectonic tilting is probably significant, this problem has not been con-sidered further.

POLARITY STRATIGRAPHY

Site 541The sedimentary section at site 541 (Hole 541) yield-

ed by far the most success in terms of polarity stratigra-phy, with good determinations for the interval from 59to 200 m sub-bottom (Cores 8 to 22; see Fig. 2). (Alldepths hereafter are sub-bottom.) The first sample (B-l,141 cm; 59.4 m) is normal, corresponding to either theoldest Brunhes (0.7 Ma) or the Jaramillo event (0.91-0.97 Ma; ages are from Ness et al., 1980). Other recog-nizable markers are the Olduvai event (1.66-1.87 Ma) at88-90 m (Sections 11-2 and 11-3), the Matuyama/Gausstransition (2.47 Ma) at 120.5 m (Section 14-4), and theGauss/Gilbert transition (3.40 Ma) at about 141 m (Sec-tion 16-6). The identification of the Olduvai event corre-sponds extremely well to the Pliocene/Pleistocene bound-ary at 90-91 m (Section 11-3), identified from both nan-

nofossils and foraminifers (Site Report for Site 541, thisvolume). The first normal event in the Gilbert epoch(3.86 Ma) occurs at 156 m (Section 18-3), and if there isa fault at the base of Core 19 (172 m), as indicated by aninversion in the nannofossil stratigraphy (see Site Report),the polarity information would indicate about 20 m ofrepeated section, with the first normal event in the Gil-bert occurring again at 178 m (Section 20-5). This corre-lation is consistent with the two thick ash beds in Core18 (18-1, 140 cm [154.4 m] and 18-2, 135 cm [155.8 m])being equivalent to the two thick beds in Core 20 (20-1,119 cm [173.2 m] and 20-3, 15 cm [175.2 m]). The po-larity transition at 195 m (Section 22-3) probably corre-sponds to the last normal event in the Gilbert (4.79 Ma).It is very unfortunate that operational constraints pre-vented the use of the hydraulic piston corer, since the ex-ceptionally high sediment accumulation rate, the reason-able magnetic stability, and the presence of datable ashesprovide one of the best known opportunities to study po-larity subchrons and excursions in the Brunhes and Matu-yama polarity epochs.

From 289 to 340 m (Cores 32 to 37) a high percentageof the samples were horizontally magnetized, and the

584

PALEOMAGNETIC RESULTS

Table 1A. Sediment paleomagnetic data summary.

Sample(core-section,

cm level)

Hole 541

8-1, 1418-2, 368-5, 148-7, 138-7,649-7,5110-1,9110-3, 12310-4, 6710-6, 12510-7, 4711-1, 14611-2,2011-2, 10211-3, 1711-3, 12711-4,511-4, 7311-4, U012-1, 2212-7, 2212-7, 7013-1, 7713-3, 10313-4, 11313-5, 6513-5, 13513-6, 8913-7, 6014-1,4014-1, 11614-2, 6114-2, 13214-3, 15014-3, 12114-4, 1014-4,7014-4, 14114-5, 5414-6,5614-6, 12014-7,6015-1, 10515-2, 5515-3, 5215-4, 5515-5, 1615-5, 9015-6, 1616-2, 5816-2, 13816-3, 5816-4,3116-4, 10016-5, 4716-6, 8817-1, 4217-2, 12817-3, 2817-3, 10217-4, 7418-1, 39

s 0 d 2018-1, 14118-3, 56

s 180 d 2018-4, 116

s 0 d 2018-5, 133

s 0 d 2018-6, 7519-1, 77

s 130 d 4019-1, 146

s 0 d 2519-*, 110

s 0 d 1020-1, 3520-1, 12320-3, 8120-4, 14120-5, 1620-6,9220-7,621-1,921-1, 9821-2, 88

Sub-bottomdepth(m)

59.4159.8664.1467.1367.6477.0177.9181.2382.1785.7586.4787.9688.2089.0289.6790.7791.0591.7392.1096.22

105.22105.70106.27109.53111.13112.15112.85113.89115.10115.40116.16117.11117.82118.50119.21119.60120.20120.91121.54123.06123.70124.50125.55126.55128.02129.55130.66131.40132.16136.08136.88137.58138.89139.50140.47142.38143.92146.28146.78147.52148.74153.39

154.41156.56

158.56

160.33

161.25163.27

163.%

171.10

172.35173.23175.81177.91178.16180.42181.06181.59182.48183.88

Incli-nation

(degrees)

4768

- 1 2- 1 2

11- 2 6

- 3- 3 4

2- 4 3

- 828

- 517

- 3- 1 2- 8- 6

- 5 4?13

- 2 4- 2 8- 1 1

14

- 1 5- 6

- 3 836

- 3 2- 3

- 3 529

- 4- 2 6

93

30332

681614363520323

5834534

- 1 5- 5329

- 68

173

13563823

3604210203043

1013

- 5 2- 2- 8

- 1 42344

- 1 229

727

NRM

Relativedecli-nation

(degrees)

2323123315573

18241

1473502230

1881333296

30012622423327216416112810414531735778

248210177101149336

5120178180316104113101359250115248

5513395

18431226924117354

23133816950

1581308583

255299289

9494

125113108332314116

193191922222

327263438999

166268

Intensity

×io-*(A/m)

3.01.77.95.84.84.02.4

10.02.3

14.80.69.73.46.3

14.43.72.16.06.94.37.90.8

12.02.14.86.58.23.03.50.80.474.84.51.32.78.44.84.47.9

12.77.7

12.04.54.95.92.32.94.25.95.0

10.91.41.00.935.42.29.11.93.80.302.24.8

14.41.5

5.9

5.1

4.72.1

0.2

35.7

0.252.10.573.5

28.72.91.42.86.5

15.0

Incli-nation

(degrees)

33- 2 0

- 3- 1 4- 2 2- 7

- 3 2- 1 3- 6 3- 6

- 2 0- 1 1

33- 531

- 2- 2 6- 8

- 1 6- 5 9- 1 4- 1 7- 2 4- 3 1

- 612

- 3 0- 1 5- 3 4

6- 2 1

2- 3 6

6- 6

- 4 610

- 126

3- 3- 8651413194014

- 1 6- 1 4- 3 4

- 4129

- 3 2- 1 2- 2 3- 2 0- 2 4- 2 3

2- 1 7- 3 4

504070514845

- 1 04

326

271318

- 1 0- 2 8

0- 1 3

2060

- 1 244

- 1 43

Optimum Demag


(degrees)

222244330

5963

11231

148323

3211619113435

107297100224229259165158134137147312359103233190157107147330

3912618218332810699

106355292117234

7313810932

304324215322321196272482155793636

2813383204967

18316212035834830330821321040

326329329300

59422

137300

Intensity×IO-S(A/m)

1.03.64.94.74.33.23.68.51.39.40.85.01.74.07.72.31.66.06.13.04.31.7

13.02.82.93.95.63.13.51.50.273.93.81.61.24.33.43.75.47.62.98.51.23.42.81.12.32.51.10.553.10.350.620.630.980.661.90.783.00.200.711.4

4.30.62

0.95

1.7

0.351.4

0.14

11.6

0.800.480.801.72.91.71.11.23.55.7

Demagne-tization

level(mT)

2016241610201010102020161316161313131313131313131616131316161616161630232313161625161613161016161640303030302525162513203016

3045

50

23

2616

25

25

40503020403010202025

Polarity

NR7RRR?RRR7RRN?N?RRRRRRRR???RRR?

R7

R?7RN7N7?7NNNNNNR?RR7N?N?RRRRRR

RR

N

N

NR

N

N?

NRR7RNNRNR9

Relia-bility3

AA

A/HAA

C/HABC

A/HCAA

A/HA

A/HCAAAACAC

A/HA/HA/H

CAA

A/HA

B/HA

A/HA/H

AA

A/HA

A/HA/HA/H

AABAAAC -AC

A/HC +CACC +AC +C +

AA

C

C +

AA

A

C

AAC +

A/HACAAA -C

A/H

585

D. S. WILSON

Table 1A. (Continued).


cm level)

Hole 541 (Cent.)

21-3, 2521-3, 10222-1,64

s 150 d 2522-1, 12822-2, 6222-3, 5822-4,722-4, 12822-5, 6022-6,4222-6, 13023-1, 78

s 310 d IS23-5, 35

s 150 d 4023-6,9432-3, 29

s 210 d 4533-5, 1133-6, 15

s 0 d 2033-6, 75

s 50 d 4533-7, 45

s 150 d 2534-1, 32

s 60 d 3034-5, 67

s 90 d 2034-6,46

s 180 d 3035-5, 66

s 40 d 4537-2, 109

s 200 d 2037-3, 81

s 330 d 2537-5, 78

s 0 d 1541-4, 5741-4, 10742-1, 10342-2, 9842-3, 10642-4,6442-5, 343-5, 69

s 120 d 12543-5, 77

s ? d 12543-6, 10944-4, 14844-5, 7844-6,8244-7,5545-1, 14145-2, 2745-3, 6145-7, 1748-4, 1048-4, 10848-6,6548-6, 10248.CC (15)49-2, 11549-2, 12949-3, 4249-4, 3249-4, 13849-5, 52

Hole 542

H2-2, 108H2-7, 98H3-1, 145

s 300 d 25H3-3, 971-1, 141-2,991-3, 521-4,351-4, 103

s 330 d 121-6,692-2, 119

Sub-bottomdepth(m)

184.75185.52191.64

192.28193.12194.58195.57196.78197.60198.92199.80201.28

207.28

208.94289.29

301.61303.15

303.75

304.95

305.32

311.67

312.96

321.16

336.09

337.31

340.28

376.57377.07382.03383.48385.06386.14387.03397.19

397.27

399.09405.98406.78408.32409.55410.91411.27413.11418.67442.60443.58446.15446.52447.65450.15450.29450.92452.32453.38454.02

202.14204.49205.52206.85207.53

210.19214.19

Incli-nation

(degrees)

41117

233124

- 1 67

- 4 40

- 2- 1 5

48015

204135413908

1257793625361232

- 74

22185

18182336363636233341

- 524

2732283240392738372

12- 2 1

1115

23331666

- 1 94

- 1 3- 1 3- 1 1

23- 1 8- 3 6- 1 3

23

11-36

NRMRelativedecli-nation

(degrees)

92104292

153230

92260

8411683188785

316312

55343

2672041891771681371423092482432333103021641598684

12111718317812214627621410565

358328252108

2226740

17413635829762

255260

4134923213566

80167302333

18612411712313333710913528149

76337

Intensity×10-3(A/m)

3.610.54.1

4.88.35.2

50.53.70.800.88

15.94.6

7.1

39.56.2

22.69.8

4.6

0.3

20.3

75.9

45.7

7.7

0.64

1.0

11.1

9.214.543.547.818.827.229.366.0

59.3

63.618.9

118.847.186.219.298.768.718.39.7

12.325.518.67.5

17.6

1.218.120.58.6

0.9112.548.3

4.112.718.846.139.00.77

25.112.4

Incli-nation

(degrees)

- 2 5162017143213

- 2 8- 3 4

4- 2 0- 3 2

205

- 1 6- 8

314

- 3 047

- 242

- 45329

3- 621

267420

- 3 9- 7

- 2 1- 1 2- 3 7

182110163537

- 9233431

120

25- 3 6

24383232313836

- 6500

- 1 5- 5 6

6- 61325

6

- 70

- 1 1- 1 4- 817

- 1 0- 3 6- 1 5- 9- 6

422

Optimum Demag


(degrees)

522

32932014528934125435135821

3254646

336345

5314315149154153161157217224

7776

19018922324810495

3333366769

1931886091

283220

2068

322327259113

22833140

142132

2304

5725928444

33623025885

22793

149286

23712010811414334497

1312633163148959

Intensity×K>-3(A/m)

2.11.40.70

0.491.91.9

23.51.60.260.487.41.8

0.81

19.20.95

8.20.60

0.95

0.13

19.1

6.0

6.4

1.5

0.10

0.9

4.9

2.65.2

26.123.3

3.213.38.5

27.0

41.0

28.63.7

86.57.6

57.413.464.646.413.50.37.2

10.011.70.61.83.40.59.69.0

0.405.9

30.7

0.962.1

18.932.918.80.18

5.44.2

Demagne-tization

level(mT)

203535

403025353030252516

60

2023

2320

13

35

26

30

30

26

25

30

30

1616161630161610

10

16451025101010101016101610161316131616

51616

101016101650

1616

Polarity

RN

NNNNRR?

RR

?

R??

R?

?

7

N

7

?

N

R

R?

R

NN?NNNR?NN

7

?NRNNNNNNN7N?R???

N?N?

??

RR?NRRR

??N

Relia-bility*

AC +

AA -AAAA

A/HAA

A/H

A-/HA/H

A -A/H

C/H

A/H

A -

A/H

C/H

C

C

D

A

ACCAACBB

A/H

ABCBCBBBBBCBCBF

A/H

CCC

CA/H

AABABA

C/HA/H

C

586




cm level)

Hole 542 (Cont.)

2-3, 502-5, 1003-2, 95

s 0 d 103-3, 67

s 30 d 103-5, 120

s 0 d 203-6,38

s 180 d 104-1,42

s 150 d 304-5,55

s 180 d 18

Hole 542A

2-4, 132s 300 d 25

2-6,76s 300 d 35

3-2, 25s 340 d 25

3-6,72s 0 d 18

4-1,324-1, 824-4, 108

s 40 d 15

Hole 543

6-6, 1427-4,88-5, 1239-3, 2210-5, 3311-3, 13312-1, 812-1,4813-2, 1913-3, 13113-5, 12514-1,714-2, 13915-2, 517-1, 13817-2, 9917-3, 3217-4, 1617-4, 14118-1, 3518-2, 11318-3, 12618-4, 5318-6, 9319-1, 7819-3, 2619-4, 8019-5, 9219-7, 320-1, 8920-2, 5220-3, 2720-3, 11320-4, 1820-4, 12223-2, 13923-3, 2324-2, 14424-4, 1824-4,9825-1, 4825-2, 4925-2, 11525-3, 2926-2,4626-2, 11426-3, 3526-3,8526-4,4326-4, 10326-5, 7326-6,4426-6, 12127-2, 12127-3, 39

Sub-bottomdepth(m)

215.00218.50223.95

224.67

228.20

228.88

230.92

237.05

245.82

248.26

251.25

257.72

259.32259.82264.58

57.4262.5874.7380.2292.83

100.33105.58105.98116.69119.31122.25124.57127.39135.55154.38155.49156.32157.66158.91162.85165.13166.76167.53170.93172.78175.26177.30178.92181.03182.39183.52184.77185.63186.18187.22212.89213.23222.44224.18224.98229.48230.99231.65232.29240.46241.14241.85242.35243.43244.03245.23246.44247.21250.71251.39

Incli-nation

(degrees)

- 5 414

- 4 0

- 2 0- 2 8- 4 3- 5 5- 3 5- 2 5- 2 2

18

20

1612

- 6 2- 6 4

25276

24- 2 6

20- 4 9- 3 6

- 3 8- 1 1- 1 1

152424

- 2 3- 5 3

1140264

12- 7 0

354

10276034264764

- 8388

6142

- 5- 1 4- 4 4

3631

- 1371533

- 6 6- 3 1- 7 1- 6 8- 1 8

9255649

- 1 53746206228733138

NRM


(degrees)

9444

176

9391

1361551101089996

134130

306312

80156331341269269162295347340

401151731103612

35125027810510545

291203

8053

35230633131834622467

11632334919689

273218426535

35832311825032131224535228

33933035626331919018118

1041377

297344

Intensity×IO~*(A/m)

0.7411.012.0

2.2

0.95

1.7

2.6

8.8

18.4

12.0

15.7

10.2

26.320.32.5

3.89.14.72.3

11.64.54.64.67.10.52.4

11.41.63.58.21.68.78.92.8

10.60.6

10.23.51.73.83.55.72.91.41.90.9

10.51.42.48.11.01.60.420.550.240.321.63.11.62.94.22.73.33.14.03.62.12.13.83.2

Incli-nation

(degrees)

- 4 610

- 2 8- 2 9- 2 1- 3 0- 3 8- 2 6

129

- 1 2- 2 9

1933

1916

- 5 4- 3 4

30327

24- 2 6

36- 3 8- 4 2

- 4 5- 2 0- 1 2- 9157

- 2 9- 2 8

121514

- 593

34- 2 0

02239214040

- 1 8- 2 5- 1 3

104436

- 2 2- 3 9- 7 0

19- 2 8- 3 2

6- 526

- 7 1- 6 7- 1 3- 5 9- 3 0- 3 8- 1 9

1- 3 4

7- 3 4- 1 9

- 4- 2 4- 3

— 34- 9

- 7 3

Optimum Demag


(degrees)

9459

185190101100329319348347292302124130

302310138167330342267266156287197209

3499

1661514212

310271277

271016

3333239523

336312331319

12208184113216334219115244206

1969

358258337153243340

424

3252942461953351032859813

349312198353126215

Intensity× I O - J(A/m)

0.401.27.7

1.8

0.09

0.05

0.27

2.3

12.6

14.0

11.8

3.8

26.62.62.7

2.45.66.01.45.23.32.05.15.15.83.46.35.23.81.82.51.71.51.12.30.53.20.94.80.71.01.00.340.523.40.571.91.71.91.10.790.720.400.300.080.360.250.870.782.31.40.430.570.740.161.40.571.12.40.43

Demagne-tization

level(mT)

51616

10

15

40

25

13

16

20

16

13

161620

16161616162513161620203025301616201620161316252020161320131613161616252016131320131616162016251613251620131616

Polarity

R?N

R

R

R

?

R

N

N

R

N

NRN

R

RRRR?N7RRNNN??

7?NR?

NNNNNRRR?NNNRRRNRR7?NRRR?RRRR7R7RR?

R7RRR

Relia-bility*

CA

A

A

C

C

C

A

A +

A +

A +

A +AC

A +

AAAAACCAACCCCCAA

A/HACACACACAAAACCACA

C/HC/H

AA -CCC

cAC

C/HA

A/HCAC -A

A/HCAA

587

D. S. WILSON



cm level)

Hole 543 (Cont.)

27-4, 11327-5, 428-2, 2728-3, 2728-3, 13128-4, 3828-4, 9428-5, 5429-1, 11129-2, 11129-3, 2429-3, 11129-4, 11129-5, 11129-6, 2429-6, 11129-7, 2030-1,9930-2, 9430-3, 9330-4, 10530-5, 10730-6, 5631-1,3231-2, 1432-1, 9132-2, 10332-3, 9232-4, 12232-5, 8933-1, 9433-2, 7034-1, 10234-2, 23

Hole 543A

3-1,33-1,405-1, 1045-2, 815-3, 946-1, 187-2, 1037-3, 77-3, 147-3, 858-1, 769-1,4610-1, 11

Sub-bottomdepth(m)

253.63254.04259.27260.77261.81262.38262.94264.04268.11269.61270.24271.11272.61274.11274.74275.61276.20277.49278.94280.43282.05283.57284.56286.32287.64296.41298.03299.42301.22302.39305.94307.20315.52316.23

341.53341.90361.54362.81364.44370.18382.03382.57382.64383.35389.76398.96408.11

Incli-nation

(degrees)

69663043192619321866442730803536

1182919

- 1 06085272542

- 1 15

- 6- 5 9

148

- 2 5- 6 1

2132

- 2 65

11252048

- 517

- 1 54

52

NRM


(degrees)

7960

19140

30719

33323633620434324

34056

31689134

334263114

310264

53248285280115351181266133266

21869

337326

16303342290358

633

35175

Intensity

×io-S(A/m)

4.64.2

10.67.56.67.97.87.27.84.65.25.13.62.31.66.84.4

10.05.45.63.94.24.86.75.91.21.30.791.00.622.52.61.51.1

4.63.64.35.3

11.23.9

11.70.39

12.318.611.023.1

5.3

Incli-nation

(degrees)

- 2 1- 1 5- 1 1

22- 2 6- 4 0- 3 3

- 1- 1 4- 1 0- 3 5- 1 6- 1 7- 1 0

7- 7

5- 4

- 8 3- 5- 5

- 1 4- 1 3- 1 1- 1 5

11- 1 8- 1 3- 2 0- 7 1

1- 1 0- 2 5- 5 9

69

- 5 8- 917

- 43531

- 2 339

- 3 1- 4 8

24

Optimum Demag


(degrees)

30029219084

313350182217327

1018320324225330817210236

32573

231346292245

5863

28224512152

187267128271

20615216632639

311262157335

17354

33156

Intensity×10-5(A/m)

1.42.11.80.540.300.20.161.61.81.70.550.540.413.30.171.31.21.40.321.01.30.952.31.10.820.461.70.741.40.821.41.51.41.2

2.01.11.10.281.30.470.460.091.21.52.50.633.3

Demagne-tization

level(mT)

16163520161616252020161320163020202016202025161620161616161325251616

20252525254020352525202025

Polarity

RRRNRR?R?7RRRRRR?R???

R?77RRRRN?RRRR?

RRR

7N?RR?N?NNRNR7N

Relia-bility*

AACACDD

A/HCAC

cc+ccc

A/HA/H

DA/HA/H

AAACCACAA -

A/HCA +A +

A + / HA + / H

AC + / H

A +C/H

A -CAA -AFA

Note: For samples from dipping beds, a second line of data gives strike (s), dip (d), and corrected inclinations and declinations.a Reliability categories: A = stable, multiple demagnetizations; B = stable, single demagnetization; C = direction changing systemati-

cally at highest demagnetization level; D = marginally stable; F = unstable; H = too close to horizontal for polarity determination.(See text.)

few confident polarity determinations were too far apartfor polarity stratigraphy. The samples from 382 to 419 m(Cores 42 to 44) at first looked promising. Fourteen of15 samples have inclinations between 21° and 38° (with-out tectonic correction), and all of these samples havevery high intensities and show exceptional directional sta-bility. However, applying the tectonic correction to thetwo samples from the overturned section in Section 43-5results in nearly horizontal magnetizations, strongly sug-gesting a post-deformational magnetization. This interpre-tation is supported by the failure to observe similarproperties of intensity or stability in any samples thatmight be the same age at Site 543. The most likely mech-anism for resetting the magnetization is chemical orthermal changes associated with the warm water ob-served at the bottom of the hole.

Site 542The number of sediment specimens available from

Site 542 was inadequate for polarity stratigraphy, even

though the samples were quite stable and horizontal mag-netizations were rare. In Hole 542, the predominantly re-versed interval from 200 to 230 m (Cores 1 to 4) proba-bly corresponds to the Gilbert epoch, but this assign-ment is based more on the nannofossil age than on thepolarity intervals.

Site 543

At Site 543, drilling deformation prevented adequatesampling of the sediments in the upper 150 m (Hole 543,Cores 1 to 16). Sampling was adequate in Cores 17 to 20(150-190 m), and a correlation can be drawn withAnomalies 5C through 5E (16 to 19 Ma) of the reversaltime scale (Fig. 3). This is in agreement with the radio-larian age assignments of D. alata to S. wolffii (see SiteReport for Site 543, this volume) based on the time scaleof Theyer and Hammond (1974). Cores 21 and 22 hadno recovery, and Cores 23 through 34 (the bottom ofHole 543) show a dramatic change in directional proper-ties. The vast majority of the samples have reversed sta-

588


Table IB. Basalt paleomagnetic data, Hole 543A.


cm level)

10-1, 12710-2, 3211-1, 4011-1, 14011-2, 8512-1, 3212-2, 612-2, 6412-3, 4012-4, 13513-1, 2613-2, 9313-3, 7413-5, 4013-6, 11413-7, 4214-1, 2815-1, 3515-2, 1415-3, 7015-4, 11715-5, 6516-1, 9416-2, 3816-3, 12716-4,9116-5, 7816-6, 8816-7, 86

Sub-bottomdepth(m)

409.27409.82417.90418.90419.85420.32421.56422.14423.40425.85427.26429.43430.74433.40435.64436.42436.28438.35439.64441.70443.67444.65445.94446.88449.27450.41451.78453.38454.86

Incli-nation

(degrees)

-11.1-29.5-36.9-34.9-28.8-31.9-20.5-19.3-21.9-21.6-21.4-22.1-32.7-27.9—23.7-26.2-23.6-25.4-28.8-22.5-27.7-23.2-28.0-20.7-21.7-21.5-17.9-21.1-32.9

NRM


(degrees)

163.1268.7359.1272.5198.2210.6139.1202.9107.1132.088.6

211.9234.2

4.0276.5172.090.3

335.3250.3196.4224.2177.7232.496.2

180.5145.8191.1

14.1272.4

Intensity(A/m)

2.715.745.168.565.168.257.566.764.065.336.705.415.039.676.126.106.94

14.55.56

10.68.357.958.989.479.019.23

10.510.95.70

Incli-nation

(degrees)

-11.8-29.4-35.9-32.5-30.8-31.8-20.9-19.8-23.7-20.9-21.0

-31.4-27.8-24.0-24.4-19.2-26.4-27.1-21.2-28.3-23.2-27.7-19.3-20.8-19.6-19.4-22.8-31.5

Optimum Demag


(degrees)

161.4270.2

0.1274.1198.6211.6135.6204.2107.8130.988.4

238.37.5

276.4168.386.2

335.8255.6197.4229.9177.2235.1

94.4180.5144.3191.514.1

273.6

Intensity(A/m)

1.823.953.715.052.284.744.453.092.823.665.74

1.376.422.140.891.29

10.91.252.952.382.723.994.112.392.522.535.183.16

Demagne-tization

level(mT)

2020

450°Ca

2020

450°C2020202020

20340°C202020

34O°C202020202020202020

34O°C20

Susceptibility(10- 4 cgs)

3.17.66.17.29.16.8

10.08.3

6.13.2

12.36.8

10.716.812.86.0

13.711.210.49.28.57.8

10.69.7

11.25.16.6

Note: See Table 1A footnotes for reliability catetories and other information.a Temperature of thermal demagnetization.

50

75

100

•e 125

I 1503

175

200

225

V•

r

-90 -60 -30 0 30 60

Inclination (degrees)90

Figure 2. Inclinations and polarity interpretations for Site 541. Thedotted lines indicate the 10° cutoff for polarity determinations.Black indicates normal polarity; white, reversed; and hachured, in-determinate.

ble directions, though most have normal NRM's. Sincethe age range of these samples covers most of the Eo-cene and Oligocene, some type of post-depositional re-

magnetization is clearly indicated. A likely cause of theremagnetization is the diagenetic changes associated withthe formation of manganese staining and of authigenicrhodochrosite. There is very little information as to theage of this remagnetization.

The Paleocene(?) and Upper Cretaceous sediments ofHole 543A were only sparsely sampled, owing to drill-ing deformation. Polarities are mixed; about a third ofthe samples have nearly horizontal magnetization. Theage of magnetization of these iron-rich sediments is opento question, however, since reddish staining fronts wereobserved to crosscut bedding features.

STRUCTURAL INTERPRETATION

The interpretation of in situ structure provided inter-esting results. The objective is to use the magnetic decli-nation of samples taken at dipping beds to estimate theoriginal orientation and thereby determine approximatelythe true dip directions of the beds. Of the samples col-lected at dipping beds for Site 541, twelve have very sta-ble directions, and eight of these have unambiguous po-larity. For Site 542, nine samples at apparently dippingbeds have very stable magnetizations. However, Hole 542was abandoned because it was measured to deviate 7.6°from vertical, so three samples with only moderate dipsfrom this hole have been excluded. For the samples withclearly determined polarity, true north was assumed to bethe declination direction for normal samples, and declina-tion minus 180° for reversed rocks. The apparent in situstrikes and down-dip directions in Table 2 and Figure 4are relative to this north. For any one sample, the uncer-tainty in this rather crude technique is estimated to beabout 40°. The main sources of error are the lack of

589

D. S. WILSON

50

75

100

125

150

J•6 175

|I 200

225

250

275

300

325

••4

3?

Table 2. Structural interpretation.

5C

Remagnetizedreversed

-90 -60 -30 0 30 60 90Inclination (degrees)

Figure 3. Inclinations and polarity interpretations for Site 543. Sym-bols as in Figure 2.

correspondence between the declination direction andtrue north, and the uncertainty in measuring the orien-tation of a slightly deformed bed on half of a 10-cm-di-ameter core. From Site 541, the eight samples with clearpolarity scatter about dip directions to the southwest,and the samples with ambiguous polarity are not incon-sistent with this. For Site 542, located about 1.5 km tothe east, the six samples dip predominantly to the east.The scatter within each site is remarkably small, consid-ering the uncertainties. These dipping beds, often withdips of 30° to 45°, alternate with horizontal beds overdepth ranges of typically 5 to 10 m. The in situ patternsuggested by horizontal beds alternating with dippingbeds with the same down-dip direction is that the re-gions of horizontal bedding are separated by bands ofmonoclinal folds, with the axial plane of the folds prob-ably dipping in the opposite direction from the beddingdip. These fold bands may well be controlled by sub-parallel thrust faults.


cm level)

Hole 541

18-1, 3918-5, 13319-1, 7719-6, 11022-1, 6432-3, 2937-3, 8137-5, 78

Bedding(relative to core)

Strike(degrees)

3000

1300

150210330

0

Polarity ambiguous

23-1, 7823-5, 7833-6, 7534-1, 32

Hole 542

H3-1, 145

Hole 542A

2-4, 1322-6, 763-2, 253-6,724-4, 108

3101505060

300

300300340

040

Site 541N

Dip(degrees)

2020401025452515

15404530

25

2535251815

Magnetization(corrected

for bedding)Incli-nation

(degrees)

- 1 745321817

- 3 0- 3 7

21

54

- 4- 6

- 1 4

16- 3 4

3224

- 4 2

Decli-nation

(degrees)

3616234821032031569

188

4631715776

114

310167342266209

Polarity

RNNNNRRN

R

NRNNR

Site 542N

Apparentin situ

down-dipdirection(degrees)

174288232240280165171262

174 or 354103 or 283103 or 28374 or 254

%

804388

184101

Figure 4. Estimates of in situ down-dip direction, based on the differ-ence between magnetic declination and bedding strike relative tothe core. Each radial line represents the down-dip direction for asingle bed.

BASALT MAGNETIZATION

Coring of Hole 543A recovered 35.9 m of basalt at anaverage recovery rate of 81%. The lithology was moder-ately altered plagioclase-olivine phyric pillow basalt. Theseflows are dated as Campanian (approx 80 Ma old) onthe basis of overlying and interbedded sediment, andcompositionally are typical of normal ocean crust (e.g.,Bougault et al., this volume). The freedom from limita-tion of the sample interval and the high degree of mag-netic stability provided a pleasant contrast to the sedi-mentary section. The sample interval averaged about1.5 m, and the remanence direction rarely moved morethan 3° to 4° during either AF or thermal demagnetiza-tion. There is no indication of drilling-induced, viscous,or other type of unstable magnetization. The samplesfrom Hole 543A can be divided into three groups ac-

590


cording to their inclinations: group 1 consists of a singlesample from Section 10-1 (409 m), with an inclinationof -11°; group 2 includes Sections 10-2 to 12-1 (410-420 m), with inclinations of -29° to -36°; and group3 includes Sections 12-2 to 16-7 (421-455 m), with incli-nations of -19° to -31°. These groupings are verysimilar to those based on petrography (Natland et al.,this volume). The interpretation of these distinct miner-alogic and magnetic groups is that they represent brieferuptive events separated by fairly long time intervals.

The mean NRM intensity of the basalt samples is 7.45A/m, with a standard deviation of 2.50. Using the resultsfrom Sites 417 and 418 (Levi et al., 1979) as a standardfor Cretaceous basalts, these intensities fall well withinthe normal scatter. The mean susceptibility is 8.8 × 10~4

cgs units, with standard deviation 3.1 × 10~4. This isagain within the normal range, perhaps below average.The Königsberger ratio Q of remanent to induced mag-netization scatters between 10 and 50; most values arenear 20. The median destructive fields determined fromAF demagnetizations range from iθ to 25 mT, again,these values are not unusual.

SUMMARY

Magnetostratigraphy for Leg 78A met with limitedsuccess. For Site 541, the interval from 60 to 200 m was

correlated with the Matuyama through Gilbert polarityepochs. For Site 543, the interval from 150 to 190 m wascorrelated with marine magnetic Anomalies 5C through5E. Down-dip directions of tilted beds inferred from decli-nation values for Sites 541 and 542 suggest a pattern ofmonoclinal folding. Results from basalts are compara-ble to those for other DSDP sites.

ACKNOWLEDGMENTS

Reviews by Shaul Levi and Fritz Theyer improved the manuscript.

REFERENCES

Levi, S., Bleil, U, Smith, B. M., and Rigotti, P. A., 1979. Compila-tion of paleomagnetic and rock magnetic results of basalt samplesfrom DSDP Legs 51, 52, and 53. In Donnelly, T., Francheteau, J.,Bryan, W., Robinson, P., Flower, M., Salisbury, M., et al., Init.Repts. DSDP, 51, 52, 53, Pt. 2: Washington (U.S. Govt. PrintingOffice), 1337-1350.

Ness, G., Levi, S., and Couch, R., 1980. Marine magnetic anomalytimescales for the Cenozoic and Late Cretaceous: a precis, critique,and synthesis. Rev. Geophys. Space Phys., 18:753-770.

Theyer, F., and Hammond, S. R., 1974. Paleomagnetic polarity se-quence and radiolarian zones, Brunhes to polarity epoch 20. EarthPlanet. Sci. Lett., 22:307-319.

Date of Initial Receipt: July 25, 1983Date of Acceptance: August 19, 1983

591

Documents

31. PALEOMAGNETIC RESULTS FROM DEEP SEA DRILLING …31. PALEOMAGNETIC RESULTS FROM DEEP SEA DRILLING PROJECT LEG 78A1 Douglas S. Wilson, Geophysics Department, Stanford University2