43. paleomagnetism of cenozoic sediments in ......762b contains 17 reversals. all reversals above...

von Rad, U., Haq, B. U., et al., 1992Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 122

43. PALEOMAGNETISM OF CENOZOIC SEDIMENTS IN HOLES 762B AND 763A, CENTRALEXMOUTH PLATEAU, NORTHWEST AUSTRALIA1

Cheng Tang2

ABSTRACT

The magnetostratigraphy of Neogene sediments from Holes 762B and 763A are presented in this paper. Hole762B contains 17 reversals. All reversals above the base of the Gilbert are correlated with the magnetic polarity timescale (Haq et al., 1988). Hole 763A yields a record of about 20 reversals that can be correlated to the magneticpolarity time scale, documenting all reversals to the base of Chron 4A. Based on the correlation, the sedimentaccumulation vs. time for Holes 762B and 763A are determined. The age-depth curves obtained show a similarpattern of sedimentation rate since 6.8 Ma.

The study also indicates a correlation between the fluctuations in the magnetic parameters (natural remanentmagnetization intensity and susceptibility), the lithologic changes, and changes in iron content at both holes. Thiscorrelation suggests that the natural remanent magnetization intensity and susceptibility changes observed in Holes762B and 763A are controlled by changes in depositional processes probably associated with climatic variations.

INTRODUCTION

Geologic Setting

The Exmouth Plateau is a rifted and deeply subsided piece ofcontinental crust covered by more than 8 km of Phanerozoicsediments. Two sites (762 and 763) were drilled on the westernpart of the central Exmouth Plateau (Fig. 1) during Leg 122. Themajor objectives of drilling at these sites included documentingthe Cretaceous to Quaternary depositional sequences and study-ing the Cretaceous and Tertiary paleoenvironments of thissediment-starved passive continental margin. Over 1.63 km ofsediments were drilled from Sites 762 and 763, including morethan 300 m of Neogene sediments. It has been proved since theearly days of deep-sea coring that magnetostratigraphy canprovide a basis for core correlation and for establishing a precisetime calibration (Hays et al., 1969, Opdyke et al., 1974). Thischapter summarizes the magnetostratigraphy of the Neogenesediment sequence and the susceptibility record from the Cen-ozoic sediment at both sites.

Hole 762B is located on the western part of the centralExmouth Plateau (19°53'S, 112°15'E) at a water depth of 1360m. This hole was continuously cored by the advanced pistoncorer (APC) to a depth of 175.4 meters below seafloor (mbsf)with an excellent recovery rate of 99%. The lithology of the145 m of Neogene sediments is relatively simple, character-ized by pelagic deposition of foraminiferal nannofossil oozewith a small amount of terrigenous material. Sedimentationrates are generally low and sedimentation is interrupted byseveral hiatuses (see "Biostratigraphy" section, "Site 762"chapter, Haq, von Rad, O'Connell, et al., 1990). Hole 763A islocated 84 km south of Hole 762B (20°19'S, 112°52'E) at awater depth of 1367.5 m. Twenty APC cores were routinelytaken from this hole to a depth of 185.4 mbsf with an excellentrecovery rate. Hole 763A comprises Quaternary throughupper Eocene ooze and semi-indurated chalk, both with clay.The lithology of the 160 m of Neogene sediments is dominated

1 von Rad, U., Haq, B. U., et al., 1992. Proc. ODP, Sci. Results, 122:College Station, TX (Ocean Drilling Program).

2 Earth Science Board, University of California, Santa Cruz, Santa Cruz,CA 95064, U.S.A.

by nannofossil oozes with abundant foraminifers and verysmall amounts of terrigenous quartz and clay minerals.

MethodsAboard the drillship JOIDES Resolution we measured the

natural remanent magnetization (NRM) of archive sectionsfrom each hole, excluding badly disturbed intervals, with a2G-Enterprises 760R cryogenic magnetometer at 5- or 10-cmintervals. The sections were then demagnetized in a 9-mTalternating field (AF) and their remanence was measuredagain. Unfortunately, the AF demagnetization at 9 mT wasnot sufficient to remove the common viscous remanent mag-netization (VRM) overprint for a large number of core sec-tions. For this reason, two discrete samples from each sectionwere taken by carefully pushing plastic 7-cm3 boxes into thesplit, visually undisturbed sections. About 335 discrete sam-ples were taken from both holes for shore-based study.

All discrete samples were subjected to stepwise AF demag-netization at higher demagnetization levels above 35 mT in thepaleomagnetic laboratory at the University of California,Santa Cruz. If the remanence appeared unstable, additionalmeasurements were made at higher demagnetization levels(up to 80 mT in some cases) in an effort to isolate the stableremanence. The reported incünation and declination in Tables1 and 2 are based on directions of demagnetization levelsabove 15 mT. Both shipboard measurements of archive halvesand results of discrete samples were used to determine themagnetostratigraphy in Holes 762B and 763A.

Because the shipboard core-orienting device was not oper-ating during the entire cruise, the absolute declination is notknown. Consequently, the reversals were picked largely onthe basis of the inclination data and confirmed by relativedeclination changes. The expected inclinations for the twosites (assuming an axial geocentric dipole) were -36° (normal)and 36° (reversed). This inclination is sufficiently steep todetermine the polarity of cleaned samples.

Measurement of volume susceptibility was made at 5- to10-cm intervals on whole 1.5-m sections from Holes 762B and763A. A Bartington Instruments' MS-1 magnetic susceptibilitymeter with a 80-mm loop sensor was employed. This instrumentis capable of measuring susceptibility with a sensitivity of 1 ×10~7 cgs. Susceptibility is controlled by changes in magneticmineral assemblages and concentration. Although susceptibility

717

C. TANG

15°S

17°

19P

21C

Argo Abyssal Plain

0DepuchIff Picard I

{S Ronsard I0 Sable I

Ange

ExmouthPlateau

West Tryal Rocksp

Tryal Rock I

CuvierAbyssal

™ % Plai

109°E 113° 115° 117° 119°

Figure 1. Bathymetric map (in meters) of the central Exmouth Plateau with location of Sites 762 and 763, as well as other Leg 122 (solid circles)and Leg 123 (open circles) drill sites. Commercial petroleum exploration wells are denoted by standard industry symbols.

is independent of magnetic mineral grain size, it is indicative ofbulk magnetic mineral content when magnetic mineralogy is heldconstant. Whole-core susceptibility is proportional to the vol-ume concentration of the most susceptible magnetic materialthat the sediment contains. Thus, the susceptibility profiles oftenreflect downhole variation in lithology—that is, changes in theratio of magnetic to nonmagnetic constituents in the sediments.Therefore, whole-core susceptibility measurements have beenused in previous studies to provide a rapid, nondestructivemethod of regional-scale lithostratigraphic correction betweensites (e.g., Thompson et al., 1980; King, 1986; Robinson, 1986;Bloemendal et al., 1988; Doh et al., 1988; Hall et al., 1989;Bloemendal et al., 1989). The objective of the whole-core sus-ceptibility measurements made at Sites 762 and 763, therefore,was to correlate Hole 762B to Hole 763A with a high degree oflithostratigraphic resolution.

RESULTS

Demagnetization BehaviorFigure 2 shows representative Zijderveld diagrams for

samples of Neogene sediments from Holes 762B and 763A.The demagnetization curves commonly fall into one of fourgroups (types A-D). One group (32% of the samples), shownin Figure 2A, has a linear demagnetization path and a mediandestructive field between 30 and 40 mT, which suggests astable, single component of magnetization. In contrast, somesamples (12%) exhibit demagnetization paths similar to that inFigure 2D. This diagram indicates an unstable magnetization.

The polarity of this specimen may be reversed, though astraight line segment cannot be obtained from the plot. Mostof samples (37%) have Zijderveld diagrams similar to that inFigure 2B. These samples are characterized by the removal ofa soft secondary component by 5 mT or 9 mT, and subsequentAF demagnetization results in little or no change in thedirection of magnetization but a steady decrease in intensitywith increasing AF strength. Finally, some samples (19%)behave like the one shown in Figure 2C, displaying a largenormal component that is removed at 20 mT. This componentis most likely a current field overprint that is viscous in originbecause it is removed at low AF fields.

MagnetostratigraphyTables 1 and 2 list the paleomagnetic data of the discrete

samples from Holes 762B and 763A. Figures 3 and 4 show theinclination and relative declination of all cleaned samplesplotted against depth for Holes 762B and 763A, respectively.

Hole 762B

No discrete samples were taken above 25 mbsf. Thereversal boundaries within this interval were identified basedon shipboard measurements of archive sections, suggestingtwo reversals at 11.2 mbsf and 12.1 mbsf, respectively (see"Paleomagnetics" section, "Site 762" chapter, Haq, vonRad, 0'Connell, et al., 1990). Below 25 mbsf, a combinationof 100 discrete samples and the shipboard measurementsyields an excellent record. This shows the polarity sequencefrom the base of the Gilbert at 100.1 mbsf to the upper Olduvai

718

PALEOMAGNETISM OF CENOZOIC SEDIMENTS

Table 1. Paleomagnetic results of discrete samples from Hole 762B.

Core, section,interval (cm)a

122-762B-4H-1, 1304H-2, 1304H-3, 1304H-4, 1304H-5, 1304H-6, 1305H-1, 1345H-2, 1345H-3, 1345H-4, 1345H-5, 1345H-6, 1026H-1, 1306H-2, 1306H-3, 1306H-4, 1306H-5, 1306H-6, 1306H-7, 487H-1, 1307H-2, 1307H-3, 1307H-4, 1307H-5, 1307H-6, 1307H-7, 428H-1, 1308H-2, 1308H-3, 1308H-4, 1308H-5, 1308H-6, 1069H-1, 1299H-2, 1299H-3, 1289H-4, 1259H-5, 1109H-6, 12810H-1, 12910H-2, 12910H-3, 12810H-4, 12910H-5, 13110H-6, 13110H-7, 7111H-1, 12911H-2, 12911H-3, 13311H-4, 13411H-5, 13511H-6, 13412H-1, 12912H-2, 12812H-3, 13012H-4,10812H-5, 13012H-6, 13013H-1, 12813H-2, 6613H-3, 7013H-4, 12613H-5, 7513H-6, 6014H-1, 12814H-2, 12814H-3, 12814H-4, 12814H-5, 12514H-6, 12815H-1, 11115H-2, 13415H-3, 11315H-4, 12815H-6, 13516H-1, 13516H-2, 13516H-3, 13716H-4, 13516H-5, 13516H-6, 128

Depth(mbsf)

24.7026.2027.7029.2030.7032.2034.2435.7437.2438.7440.2441.5243.7045.2046.7048.2049.7051.2052.7053.2054.7056.2057.7059.2060.7061.2362.7064.2065.7067.3068.7069.6072.1973.5975.1876.6578.0079.6881.6983.1984.6886.1987.7189.2190.191.1992.6994.2395.7497.2598.74100.69101.97103.49104.77106.70108.20110.18111.06112.52114.66115.65117.00119.68121.18122.68124.18125.68127.68129.01130.74132.13133.68137.38138.78140.28141.70143.37144.87146.30

Declination

O

157.10102.60359.20192.40208.30147.40145.30166.00327.00187.30220.30218.90144.40230.40197.40217.00185.30252.1045.80198.70211.30207.30176.00159.30172.7070.50235.00178.40176.70200.70167.90162.20269.80184.00265.40305.60295.60270.90270.50280.20234.70290.20213.50218.301160.50263.40225.20323.30135.30194.40200.80177.7026.20281.90254.30231.90258.60222.70201.00357.30303.80302.7037.20182.50187.00200.40197.10195.70133.40167.50228.3096.90193.90183.90180.20210.70184.60193.70181.30191.80

InclinationO

49.30-23.00-23.6034.0040.5030.3018.2053.5042.80-0.20-18.30-62.70-59.60-20.3044.40

-28.80-49.70-22.1039.40-6.00-22.80-46.80-19.30-58.80-73.90-45.00-56.6027.4025.5030.2016.7056.80

-76.20-21.20-71.2019.1023.60

-35.8042.1042.30

-62.0019.3058.3015.208.70

-59.30-35.90-63.4028.3069.5030.70

-54.5031.70

-28.4050.70

-32.5037.40

-42.80-60.20-53.40-83.30-57.30-55.20-34.00-35.60-34.30-29.20-27.80-18.7048.4020.6031.6042.90

-61.2024.10

-28.10-50.60-23.40-76.70-40.00

NRM(emu)

1.13 E-087.13 E-082.65 E-081.79 E-082.01 E-088.05 E-084.93 E-076.18 E-077.80 E-071.56 E-061.97 E-067.11 E-064.23 E-063.37 E-066.39 E-068.91 E-075.25 E-069.52 E-074.76 E-064.47 E-066.01 E-063.97 E-061.67 E-065.37 E-061.01 E-054.32 E-069.91 E-063.57 E-063.87 E-061.73 E-063.58 E-061.03 E-067.99 E-063.48 E-061.63 E-061.95 E-062.76 E-062.79 E-064.70 E-062.35 E-069.41 E-071.85 E-061.30 E-063.84 E-071.63 E-063.68 E-061.52 E-061.99 E-061.35 E-063.09 E-063.93 E-064.51 E-064.09 E-064.42 E-063.11 E-068.82 E-071.77 E-066.81 E-064.63 E-061.38 E-065.75 E-071.08 E-064.10 E-071.98 E-061.25 E-064.70 E-067.10 E-073.18 E-064.91 E-072.93 E-072.84 E-072.80 E-071.18 E-063.61 E-064.70 E-061.38 E-061.42 E-062.26 E-061.41 E-061.60 E-06

Peak A F field(mT)

8080807575757575756060606070707070707070707085909090908080707070707070707575757580808090906075756575757070656575657070757565656565656570858585607060657575707575

Demagnetizationtypeb

DACCcBBBBBAAABBCABAAABBAABABBBBCAABBAAAABABDBABBABBBAAABBBABBACAAABADDCBABCCBADA

a Interval is measured from the top of the section.See Figure 2 for explanation.

719

Table 2. Paleomagnetic results of discrete samples from Hole 763A.

Core, section, Depth Declination Inclination NRM Peak AF field Demagnetizationinterval (cm)a (mbsf) (°) (°) (emu) (mT) typeb

122-763A-1H-1,74 0.74 30.30 -35.80 1.24 E-07 70 C1H-1, 134 1.34 2.80 -49.70 1.76 E-06 75 A1H-2,74 2.24 331.60 -49.80 1.94 E-06 75 A1H-2, 133 2.83 316.20 -60.20 3.17 E-06 75 A1H-3,72 3.72 350.70 -13.00 7.48 E-07 75 A1H-3, 134 4.34 342.60 -59.90 1.75 E-06 75 A2H-l,70 5.65 242.20 -48.70 2.65 E-06 75 B2H-1, 128 6.18 274.50 -48.00 3.43 E-07 75 A2H-2, 66 7.06 287.60 -42.50 9.96 E-08 75 B2H-3,66 8.56 87.90 48.60 5.26 E-08 60 C2H-3, 129 9.19 16.40 -36.00 3.21 E-08 85 D2H-4, 64 10.04 321.00 -40.00 5.19 E-08 75 C2H-4, 128 10.68 288.30 -36.70 5.77 E-07 65 A2H-5,46 11.36 250.50 36.00 2.45 E-08 65 A2H-5, 130 12.20 296.30 -43.10 5.77 E-08 65 B2H-6, 70 13.10 289.80 -44.70 1.92 E-07 65 B2H-6, 127 13.68 293.20 -56.00 6.28 E-07 65 A2H-7, 22 14.12 335.50 -36.30 1.88 E-07 65 B2H-7,72 14.62 272.20 -65.60 2.31 E-07 75 B3H-1, 129 15.69 156.70 -12.50 3.65 E-08 75 B3H-2,46 16.36 174.50 -14.70 5.67 E-08 75 B3H-2, 130 17.20 259.20 29.50 1.55 E-08 75 D3H-3,45 17.85 198.80 40.70 1.05 E-08 70 C3H-3, 125 18.66 312.70 59.50 1.88 E-08 70 C3H-4,45 19.35 337.10 -13.40 2.87 E-08 70 C3H-4, 125 20.15 36.60 -64.40 2.66 E-08 70 C3H-5,44 20.84 67.50 24.10 1.76 E-08 75 C3H-5, 126 21.66 228.10 64.60 9.93 E-09 85 D3H-6,48 22.38 267.40 -42.10 3.52 E-08 70 B3H-6, 125 23.15 260.50 45.10 1.42 E-08 90 D4H-1.36 24.26 275.30 41.00 4.59 E-08 65 C4H-1, 123 25.13 215.00 39.60 1.29 E-08 70 B4H-2, 34 25.71 97.40 6.20 4.76 E-08 65 D4H-2, 129 26.69 207.20 48.50 2.63 E-08 75 B4H-3, 34 27.21 215.70 37.80 2.80 E-08 75 D4H-3, 130 28.20 90.30 37.00 3.23 E-08 75 D4H-4, 35 28.75 240.60 28.40 2.37 E-08 70 A4H-4, 130 29.43 270.20 63.80 9.06 E-09 65 D4H-5, 36 30.26 338.00 66.40 2.12 E-08 65 D4H-5, 130 31.20 183.90 -26.40 1.31 E-08 70 D4H-6, 36 31.76 182.60 -62.80 1.76 E-08 70 D4H-6, 130 32.70 196.10 -37.30 3.92 E-08 75 C5H-1.47 33.87 308.70 38.20 1.99 E-08 75 D5H-1, 129 34.69 50.50 38.80 1.06 E-08 55 D5H-2,43 35.33 125.60 34.60 1.64 E-08 65 C5H-2, 133 36.23 216.60 61.30 9.06 E-09 55 D5H-3,43 36.83 343.70 22.10 3.66 E-08 65 B5H-3, 132 37.72 242.60 64.30 9.37 E-09 75 C5H-4,47 38.37 133.20 5.90 2.47 E-08 55 C5H-4, 132 39.20 217.60 23.20 8.39 E-09 75 D5H-5,45 39.85 154.10 24.40 1.94 E-08 75 D5H-5, 132 40.72 157.70 59.50 2.12 E-08 75 D5H-6, 45 41.35 122.30 64.80 1.33 E-08 75 D5H-6, 133 42.23 268.90 35.40 1.75 E-08 75 C5H-7,45 42.85 0.80 33.90 1.92 E-08 75 C6H-1, 35 43.25 29.60 54.20 2.25 E-08 75 D6H-1, 125 44.15 12.00 35.90 3.26 E-08 75 C6H-2,35 44.75 221.20 48.00 2.30 E-07 75 A6H-2, 125 45.65 218.70 -11.40 8.45 E-07 80 A6H-3,35 46.25 191.10 -14.00 9.43 E-07 85 A6H-3, 125 47.15 231.50 57.20 1.68 E-08 70 B6H-4, 35 47.75 127.50 59.60 8.27 E-08 65 B6H-4, 125 48.65 264.50 48.90 4.27 E-07 75 C6H-5, 35 49.25 187.00 32.90 8.12 E-07 65 C6H-5, 125 50.15 206.70 44.90 1.94 E-06 75 B6H-6,35 50.75 36.50 54.30 1.70 E-06 80 B6H-6, 125 51.65 224.10 52.40 2.17 E-06 75 A6H-7, 15 52.05 204.10 42.70 1.00 E-06 75 C6H-7, 53 52.43 2.40 28.50 7.93 E-07 75 A7H-1, 125 53.65 250.20 -55.30 2.14 E-06 75 C7H-2, 35 54.25 97.20 -52.10 2.06 E-06 75 C7H-2, 126 55.16 117.10 -48.00 2.31 E-06 75 C7H-3,35 55.75 302.00 -45.40 1.77 E-06 75 B7H-3, 126 56.66 125.30 -72.40 2.01 E-06 75 A7H-4, 35 57.25 273.60 -80.60 8.29 E-07 80 C7H-4, 116 58.06 64.50 -62.60 1.74 E-06 80 B

C. TANG


Table 2 (continued).


7H-5, 357H-5, 1267H-6, 357H-6, 1268H-1, 358H-1, 1258H-2, 358H-2, 1278H-3, 358H-3, 1268H-4, 358H-4, 1268H-5, 358H-5, 1268H-6, 358H-6, 1268H-7, 358H-7, 759H-1, 709H-1, 1259H-2, 359H-2, 1259H-3, 359H-3, 1259H-4, 359H-4, 1259H-5, 359H-5, 1259H-6, 359H-6, 1259H-7, 20

10H-1, 5010H-1, 12610H-2, 3610H-2, 12510H-3, 3510H-3, 12410H-4, 3510H-4, 11510H-5, 3510H-5, 12510H-6, 3510H-6, 12510H-7, 5211H-2, 9011H-3, 3411H-3, 12011H-4, 3411H-4, 12011H-5, 3411H-5, 12011H-6, 3411H-6, 12011H-7, 3512H-1, 3412H-1, 12412H-2, 3412H-2, 12412H-3, 3412H-3, 12412H-4, 3412H-4, 12512H-5, 3412H-5, 12412H-6, 3412H-6, 12412H-7, 3412H-7, 6713H-1, 7713H-1, 12513H-2, 7713H-2, 12713H-3, 7813H-3, 12713H-4, 7613H-4, 10513H-5, 78

Depth(mbsf)

58.7559.6660.2561.1662.2563.1563.7564.6765.2566.1666.7567.6668.2569.1669.7570.6671.2571.6572.1072.6573.2574.1574.7575.6576.2577.1577.7578.6579.2580.1580.6081.4082.1682.7683.6584.2585.1485.7586.5587.2588.1588.7589.6590.4293.0093.7494.4095.2595.9096.7497.4098.2598.9099.75

100.24101.14101.74102.64103.24104.14104.74105.65106.24107.14107.74108.64109.24109.57110.17110.65111.67112.17113.18113.67114.66114.95116.18

DeclinationO

88.90240.9020.50

121.3091.50

134.8095.3077.50

159.90120.70105.80190.10109.20103.2063.6084.8073.20

132.8081.00

312.8043.0096.9089.50

168.10122.10152.6068.40

241.3031.20

154.50333.10137.50150.10181.70149.70184.6077.00

145.40163.20134.70187.70114.50175.3099.90

136.60161.90164.10141.90146.10118.60161.20119.60165.6064.30

162.80195.60179.10188.40125.80171.90151.60221.10102.50166.4074.20

182.2011.90

121.00189.40322.90255.80211.90275.80294.30220.90166.90163.30

Inclination

o-28.20-27.90-28.80-72.40-76.70-15.30-26.70

51.5022.00

-31.40-46.70

17.5018.50

-39.70-11.20-13.20-58.30-49.30-64.70-74.40-26.30-59.50-32.40-45.60-41.20-30.60-21.10-23.80-10.90-63.40

56.8011.5033.2030.9024.8017.6020.7028.7020.2039.003.80

36.202.80

-1.10-63.40

20.30-68.40

12.606.50

24.405.40

18.60-2.50

-14.60-51.40

32.6015.5010.00

-11.90-48.90-46.20

27.1046.90-6.70

-34.2031.1070.2025.20

-29.50-56.00-69.00-49.90

53.7023.1043.90

-17.90-23.10

NRM(emu)

4.32 E-063.42 E-061.61 E-067.42 E-064.58 E-06,2.54 E-061.12 E-066.43 E-071.85 E-063.43 E-062.44 E-061.16 E-063.56 E-061.70 E-062.73 E-062.03 E-065.57 E-063.63 E-069.46 E-061.88 E-062.05 E-061.16 E-061.99 E-062.57 E-061.41 E-061.74 E-061.34 E-061.48 E-061.41 E-062.10 E-061.33 E-061.86 E-061.76 E-066.95 E-071.40 E-061.43 E-061.02 E-062.40 E-062.01 E-063.36 E-062.43 E-062.93 E-064.78 E-063.14 E-063.19 E-066.95 E-072.65 E-063.57 E-063.63 E-069.39 E-073.60 E-061.13 E-063.81 E-062.72 E-061.90 E-061.22 E-069.97 E-071.78 E-062.33 E-072.27 E-063.13 E-061.13 E-062.53 E-062.46 E-061.80 E-064.00 E-061.75 E-063.07 E-062.66 E-063.45 E-062.22 E-062.78 E-068.76 E-073.12 E-065.61 E-072.07 E-061.65 E-06

Peak AF field(mT)

8080808080707070707070707070707070656060657575606065656565606060606065656575707070707070706565656565656060607570757580808070707070706565656565606060707070


BCCBBBCDBAACBABCAAAAABBAAABBBAAABBBBBAAABAABCABBCBBBCBABBCCBABABAABAABBACADBB

721

C. TANG

Table 2 (continued).


13H-5, 12713H-6, 6813H-6, 12713H-7, 4614H-1, 8014H-1, 12814H-2, 7814H-2, 12914H-3, 7914H-3, 12914H-4, 4814H-4, 12714H-5, 4814H-5, 12914H-6, 4914H-6, 12914H-7, 4915H-1,4515H-1, 13215H-2, 4515H-2, 13015H-3, 4515H-3, 13015H-4, 4315H-4, 13315H-5, 4715H-5, 13315H-6, 4615H-6, 13515H-7, 4716H-1.4416H-1, 12716H-2, 4416H-2, 12816H-3, 4516H-3, 12816H-4, 4516H-4, 11416H-5, 4616H-5, 14416H-6, 5316H-6, 13316H-7, 2317H-1, 2417H-1, 13017H-2, 2517H-2, 11217H-3, 5217H-3, 10417H-4, 5417H-4, 13217H-5, 5417H-5, 12117H-6, 2717H-6, 12717H-7, 2118H-1, 5118H-1, 13418H-2, 5018H-2, 131

Depth(mbsf)

116.67117.58118.17118.86119.70120.18121.18121.69122.69123.19123.88124.67125.38126.19126.89127.69128.39128.85129.72130.35131.20131.85132.70133.33134.23134.87135.73136.36137.25137.87138.34139.17139.84140.68141.35142.18142.85143.54144.36145.34145.93146.73147.13147.64148.70149.15150.02150.92151.42152.44153.22153.94154.61155.17156.17156.61157.41158.24158.90159.71

Declination(°)

196.70124.70105.3058.40

289.70270.00242.40315.50154.20276.50110.20318.00113.3067.5084.90

171.9066.60

349.40315.00308.50322.40347.80259.40168.30247.90276.80164.3087.50

147.4043.80

311.40187.80105.50167.70132.90166.90161.20156.80131.60160.60103.50156.0069.50

198.70193.30201.90217.00152.40168.2038.0081.80

148.20172.6078.00

167.00123.70130.20336.10176.60139.90

Inclination(°)

-24.80-23.90-37.80-13.30-52.00

21.0043.4030.4066.4071.0043.3060.7051.5072.50

-31.9067.8053.60

-43.90-19.20

1.8014.5024.5029.60

-54.1048.7058.70

-43.90-42.60-13.20-43.60-62.40

28.7069.2034.0054.0023.5032.7024.8028.7052.4027.0015.5076.50

-34.10-38.90

0.801.40

25.2012.30

-18.0061.70

-10.5038.5051.7020.4060.80

-35.0045.0015.60

-35.80

NRM(emu)

8.07 E-072.06 E-061.63 E-062.21 E-069.59 E-071.18 E-076.06 E-078.21 E-076.39 E-078.35 E-077.02 E-071.84 E-061.16 E-061.06 E-061.07 E-061.23 E-061.69 E-066.23 E-071.79 E-078.59 E-074.32 E-075.34 E-071.95 E-073.09 E-072.09 E-061.23 E-061.10 E-063.47 E-062.14 E-061.64 E-062.07 E-061.43 E-061.07 E-061.57 E-066.33 E-071.53 E-062.42 E-062.08 E-069.04 E-077.78 E-075.59 E-079.69 E-078.67 E-078.32 E-071.10 E-067.79 E-073.17 E-071.80 E-065.91 E-061.10 E-061.53 E-064.10 E-061.39 E-063.37 E-062.15 E-063.20 E-066.47 E-077.06 E-071.73 E-061.68 E-06

Peak AF field(mT)

707070707070707070707070707070707070707070707070707070707070707070707070707070707070707070707070606060606060606060608080


ABBBCBCBCCCAABCCAADBBABCABBAABBBBBCBABCAABABBADBCACBCACACCAA

a Interval is measured from the top of the section.b See Figure 2 for explanation.

at 25.2 mbsf (Fig. 5 and Table 3). Below 100.1 mbsf, thereversal stratigraphy becomes uninterpretable due to thepresence of three hiatuses at 112, 122, and 132 mbsf (see"Biostratigraphy" section, "Site 762" chapter, Haq, vonRad, O'Connell, et al., 1990).

Hole 763A

Reliable paleomagnetic results (Table 2 and Fig. 4) wereobtained from Hole 763A. All major polarity reversals from 18Ma to the present (Table 3) were observed. The reversalstratigraphy is based primarily on the inclination data fromdiscrete samples because of disagreement between paleomag-netic results obtained from shipboard measurements and

discrete samples. The discrepancy is probably due to theinability of removing secondary overprints with 9 mT alter-nating field. Above 141 mbsf, polarity assignments are lessambiguous and can be correlated to the geomagnetic polaritytime scale, from Chron 1 to Chron 4A (Fig. 5).

Downhole Fluctuations of NRM Intensity andSusceptibility

Hole 762BFigure 3 also shows the whole-core susceptibility, the

NRM intensity, and intensity after 9-mT demagnetization ofarchive sections plotted against depth. Although the demag-

722

N ™ W

T

U P WUP TN.UP / N R M N ) U P

A ^ \ B 45 mT C " / D T\ 9 ml "" A /

V 2 0 m T "• \ A TΛ \ o -r - - Λ5 mT

\ "" V35mT-- -– \ i<T 1 5 m T - -

\ \ W.WH 1 "lrT~~t> I m|—I E,E w,W I—I—I—b^-W-l 1 E E35 m T ^ -• \ -• _50mTQ \J Af

A - NRMT 60mi\ -• \ X NRM,/ A50 m l \ \ \ V f \ ip mT /

70mT\-- v •• r\> so ^ ^ T Λ ^ ^C Q \ an mT l / *^?0 mT \ PApO m •- -S'T I I I I I O ^ W ^ S,S| I I A I I L N̂,N _ i25mt \umT

35 mlj/'-- \ / -– \ \^ ^ \ / \ VNv30mT--

•NRM \15 mT

E.DOWN NRM E.DOWN S.DOWNS.DOWN

0 Vertical Component Horizontal Component

Figure 2. Zijderveld orthogonal projection diagrams of discrete samples from Holes 762B and 763A subjected to alternating field demagnetization. A. Sample 122-762B-7H-5, 130-132 cm.B. Sample 122-763A-11H-3, 120-122 cm. C. Sample 122-763A-2H-3, 66-68 cm. D. Sample 122-763A-4H-4, 130-132 cm.

C. TANG

Declination inclination Intensity (NRM) Intensity (9mT)(degree) (degree) ( m A / m ) ( m A /m )

Susceptibility(10E-6cgs)

. 1 ? ° . . 3P° ' 9 0 "45 0 45 90 0 20 40 60 0 10 20 30 40 -8 0 8 16 24u

20-

4 0 -

6 0 "

E 80-.c

a

100-

120"

140-

160"

1 öU

_oo°c

o

o

<o <

<

6i

o

oo

o

o

b

<P

°o°

o °o

ob

o

o

o

oh

o°°o

8

8

o>

I

1

• -

-

• -

- .

-

- .

- .

1 • • I

B

D α

cG

ααtP

aG

a

α

Q

αGα

•

αD°

• • 1 i

α

α

•aB

αα

α

n

α

α

- 2 0

- 4 0

- 6 0

- 8 0

- 100 H

- 120

- 140

- 160 -

180 -4

•• •1 ' ' ' ' • | 0

-20 -

-40 "

- 60 -

-80 -

100 •

- 120 -

- 140 "

- 160 -

180

- 20

- 40

- 60

- 80

- 100

• 120

- 140

- 160

Figure 3. Inclination and relative declination of discrete samples and whole-core NRM intensity, intensity after 9-mTdemagnetization, and susceptibility as a function of depth in Hole 762B. Solid circles and squares represent the samples ofdemagnetization type D (Fig. 2). Wavy lines represent the hiatuses seen from biostratigraphic data.

netization in 9-mT AF reduces the NRM intensity by approx-imately 37.5%, the main features of the NRM intensity remainunchanged. The spikes of the NRM intensity shown in Figure3 are considered to be rust contamination from the corecatcher during coring because their positions can be corre-lated precisely to the bottom of each core. Despite thepresence of these spikes, a relatively high value of the NRMintensity can still be seen in the interval 33-115 mbsf. Whole-

core susceptibility shown in Figure 3 also displays similardownhole fluctuations.

Comparing the magnetic results with lithology (see"Lithostratigraphy" section, "Site 762" chapter, Haq, vonRad, O'Connell, et al., 1990) and geochemistry (De Carlo,this volume) from this hole, the NRM intensity and thesusceptibility profiles appear to correlate to lithologic unitsand fluctuations in iron content. For instance, Subunit IA

724


Declination Inclination Intensity (NRM) Intensity (9mT) Susceptibility(degree) (degree) (mA/m) (mA/m) (10E-6cgs)

0 180 360-90-45 0 45 90 0 20 40 60 0 10 20 30 40 0 8 16 24 32

CΛ

CL

Q

100"

120-

140-

160-

180

-20 -

-40 -

- 6 0 i

-80 -

- 100 -

- 120

- 140

- 160

180

- 2 0

-40 "

-60 ~

-80 -

" 100 "

" 120 -

- 140 :

- 160 "

180

- 2 0

- 4 0

- 60

- 8 0

- 100

- 120

- 140

- 160

180

Figure 4. Inclination and relative declination of discrete samples and whole-core NRM intensity, intensity after 9-mTdemagnetization, and susceptibility as a function of depth in Hole 763A. Solid circles and squares represent the samples ofdemagnetization type D (Fig. 2). Wavy lines represent the hiatuses seen from biostratigraphic data.

(0-61.4 mbsf) is represented mainly by foraminifer-nanno-fossil ooze with a small amount of terrigenous material (e.g.,clay minerals). The NRM intensity and the susceptibilityprofiles show an extremely low value in the interval 5-37mbsf and relatively high value with fluctuations in theinterval 37-61.4 mbsf. The high-value interval correlates toincreased clay content (iron content around 0.12 parts permillion, or ppm) and the low-value interval to dominantforaminifer and low iron content (~0.03 ppm). Within Sub-

unit IB (61.4-118.4 mbsf) the NRM intensity and suscepti-bility values stay relatively high with a maximum at the topand bottom of the subunit. Similarly, the high values ofsusceptibility correlate well to the occurrence of terrigenousmaterial (e.g., mica, clay, and accessory minerals) from thelithology and higher iron content (—0.38 ppm) from thegeochemistry. Furthermore, a positive correlation betweeniron and aluminum content is found in the geochemistrystudy (De Carlo, this volume), where aluminum is an index

725

C. TANG

Ages(Ma)

0 —i

1 -

2 -

3 .

4 -

5 -

6 -

7 -

8 .

9 -

10 -

11 .

12 -

13 -

14 -

15 .

16 -

17 -

18 -

19 -

20 -

Time Scale ofHaq (1988)

Jaramillo

C5C

Figure 5. Correlations of the magnetostratigraphy of Holes 762B and 763Awith the magnetic time scale of Haq et al. (1988) and the age of the Reunionevent from Berggren et al. (1985). Wavy lines represent the hiatuses seen frombiostratigraphic data.

of clay content (Kennett, 1982). Therefore, changes indepositional processes seem to control the downhole varia-tion of magnetic record.

Below 118.4 mbsf, the color of the lithology changes fromlight grey to white and pale yellow until 127.9 mbsf. Extremelylow values of susceptibility in this interval correspond to themaximum of carbonate content (—90% CaCO3). The correla-tion between the downhole fluctuations of magnetic record

and the lithology (especially clay content) suggests the differ-ent sediment transport mechanisms that allow terrigenousmaterial to be deposited.

Hole 763'A

Figure 4 also shows the whole-core susceptibility, NRMintensity, and intensity after 9-mT demagnetization of archivesections plotted against depth. For the same reason, spikes of

726


Table 3. Magnetic polarity reversal boundariesidentified in APC cores from Holes 762B and 763A.

Age(Ma)

0.73

0.910.981.651.882.012.502.923.083.403.904.104.414.575.406.707.808.9511.7016.10

Depth (mbsf)

Hole 762B

11.20

12.1013.1025.0027.50

37.6047.2051.4062.1071.6079.5084.5090.00

100.10110.40

126.20134.30

Hole 763A

15.10

18.3021.1031.1035.7045.8052.3064.2067.8081.5090.3090.1094.5099.50

107.90115.80127.60139.60

147.10

Boundary or event

Brunhes/Matuyamaupper Jaramillolower Jaramilloupper Olduvailower OlduvaiReunion?Matuyama/GaussKaenaMammothGauss/GilbertCochitiNunivakSidufjallThveraBase of GilbertBase of C3A?Base of C4?Base of C4A?Base of C5?Base of C5B?

NRM intensity can be considered to be rust contaminationfrom the core catcher. The variation in NRM intensity paral-lels the variation seen in the susceptibility measurement andcan be correlated well to terrigenous material.

In general, susceptibility values are low (mostly between 1and 15 × 10~6 cgs), with the exception of a rust-contaminatedtop section (0-4 mbsf) that yielded susceptibility values inexcess of 3.5 × 10~5 cgs. The interval between 15 and 40 mbsfcorresponds to a section of white or light gray sediment(foraminifer nannofossil oozes with clay) of early Quaternaryage. The color of this sediment suggests a low concentrationof magnetic material (iron content around 0.08 ppm), which isconsistent with the extremely low values of NRM intensity(around 0.1 mA/m) and susceptibility (around 0.5-3 × 10~6

cgs). Below this interval to a depth of 110 mbsf, susceptibilityand NRM intensity values are generally higher than elsewherein the profile (i.e., between 5 and 15 × 10~6 cgs, 1-8 mA/m,respectively), which corresponds to color changes from lightgray to light greenish brown and higher iron content (—0.55ppm). This correspondence is particularly clear in the intervalfrom 150 to 155 mbsf, which is characterized by olive yellowto pale yellow sediment (clayey foraminifer-nannofossil oozeswith chalk) and a high content of clay and quartz (see"Lithostratigraphy" section, "Site 763" chapter, Haq, vonRad, O'Connell, et al., 1990). The clear lithologic control ofmagnetic susceptibility variations in Holes 762B and 763Aprovides a potential basis for a correlation between Hole 762Band 763A.

Correlation Between Holes 762B and 763AThe high-resolution magnetic susceptibility profiles ob-

tained from Holes 762B and 763A (Figs. 3 and 4) yield a goodcorrelation between these holes. It is extremely difficult tomake visual identification of fine-scale lithostratigraphic cor-relation when the susceptibility profiles are plotted on thescales used in Figure 6. Therefore, I have replotted selectedportions from each profile on the expanded scales in Figure 7in order to better illustrate the extent of correlation.

Correlation between the susceptibility profiles of each holeis based on the relative position, intensity, and configurationof first-order peaks (20-40 × 10~6 cgs) and second-order peaks(5-10 × 10~6 cgs). Figures 6 and 7 show possible correlations

between the two holes. The reliability of specific correlationsis variable. Susceptibility values of less than 1 × 10~6 cgs areclose to the limit of detection of the whole-core sensor coil.Therefore, erratic fluctuations in background noise interferewith the susceptibility signal in several intervals between 5and 33 mbsf in Hole 762B and the interval 8-45 mbsf in Hole763A. A negative susceptibility peak (about-6 × lO"6 cgs) wasobserved at 31 mbsf from Hole 762B, though there is noapparent change in lithology except that a significant amount(1%) of zeolite (a nonmagnetic mineral) was identified in thesmear slides in the interval between 23 and 33 mbsf where theclay content is very low. Biogenic carbonate and siliceouscomponents are diamagnetic. They have a negative suscepti-bility value. Study of sediment magnetism (King et al., 1982)show that ferrimagnetic minerals (e.g., magnetite and titano-magnetite) dominate the magnetic properties of naturallyoccurring sediments. However, diamagnetic minerals couldhave a significant effect on the susceptibility measurementswhen ferrimagnetic mineral concentration is low. Silica isusually regarded as an indicator of biogenic material in apelagic region (Dymond et al., 1977). This negative featurewas not seen in susceptibility profile from Hole 763A, suggest-ing changes in biogenic processes.

DISCUSSIONIn general, the paleomagnetic data from the Neogene

sediments in Holes 762B and 763A appear reliable, especiallyfrom discrete samples with AF demagnetization types A, B,and C. On the basis of nannofossil data (see "Biostratigra-phy" section, "Site 762" and "Site 763" chapters, Haq, vonRad, O'Connell, et al., 1990), all reversals are correlated withthe latest magnetic polarity time scale (Haq et al., 1988). Thecorrelations based on magnetostratigraphy between the twoholes are shown in Figure 5. The correlations based on thesusceptibility measurements are also shown in Figures 6 and7. To check the consistency of the correlations between thetwo different magnetic methods, a Shaw diagram of depth inHole 763A vs. depth in Hole 762B was drawn (Fig. 8) bycomparing the correlations shown in Figure 5 with those madein Figures 6 and 7. Figure 8 shows a very good consistencybetween the two methods.

The sequence of about 37 reversals in Holes 762B and 763 A(Table 3) yields the age-depth curve shown in Figure 9A.Differentiation of these curves gives sedimentation rate as afunction of time (Fig. 9B). The age-depth curves for Holes762B and 763A show a similar trend except that the sedimen-tation rate in Hole 763A is slightly higher than that in Hole762B. In both holes, there has been decreased sedimentationin the last 3.5 m.y., implying a similar depositional environ-ment. The sedimentation rate at both holes has been increas-ing since 6.8 Ma. The highest sedimentation rate for Hole762B (~20 m/m.y.) occurred at -4.6 Ma and for Hole 763A(-24 m/m.y.) at 3.3 Ma. The periods of maximum sedimenta-tion rate correlate with high values of susceptibility (10-15 ×10-6 cgs) in the intervals of 40-50 mbsf in Hole 762B and55-65 mbsf in Hole 763A, which also indicates depositionalcontrol on the intensity of susceptibility.

I believe the changes in both NRM intensity and suscepti-bility from the Holes 762B and 763A reflect changes inconcentration of magnetic materials supplied to the sedi-ments. In Figure 6, the susceptibility, iron content, andaluminum content (from De Carlo, this volume) are plottedagainst depth for both holes. Figure 10 shows the relationshipbetween susceptibility and iron content for Holes 762B and763A. The reasonably good correlations shown in Figure 10support the hypothesis that magnetic concentration observedat both holes reflects fluctuations in terrigenous influx. Based

727

C. TANG

Fe Al(mg/g) (mg/g)

Hole 762B Hole 763ASusceptibility

(1OE•6C9S) ‰Al

(mg/g)

4 8 12 6 10 14 -8 0 8 16 24 0 8 16 24 32 2 6 10 4 8 12 16

2 0 -

4 0 -

60 1to

E

Q-Φ

Q 80-

100-

120-

140-

160-

180•

- 2 0 -

- 4 0 -

- 6 0 -

- 8 0 -

-100-

-120-

-140-

J 6 0 -

.180-

-20 -

-40 -

- 60 -

-80 -

- 100-

- 120-

- 140-

160-

1 8 O J 180.

- 20 -

- 40 "

- 60 "

- 8 0 1

- 100

- 120"

- 140"

- 160

180

Figure 6. Whole-core susceptibility profile, iron content, and aluminum content vs. depth for Holes 762B and 763A.

on the paleoceanographic record of the Maurice Ewing Bank(Ciesielski et al., 1982), one can speculate that climaticchanges accompanied by a reduction in the vigor of oceaniccirculation may cause an abrupt decrease in the supply ofmagnetic material from continental sources.

Evidence supporting the existence of an association be-tween magnetic mineral concentration and climatic variationis strongest in the interval between 115 and 135 mbsf in Hole762B (Fig. 6). The susceptibility in this interval is extremelylow and three major hiatuses are present. Studies of oxygen

and carbon isotopes are needed to support this speculation.Moreover, at 137-145 mbsf in Hole 762B and at 150-165 mbsfin Hole 763A, the susceptibility is fairly high (~5 to 15 × lO~6

cgs). The interval of high magnetic susceptibility coincideswith the well known early Miocene (—21-24 m.y.) lowstandperiod of eustatic level (Haq et al., 1988). Although inconclu-sive, this relationship supports the suggestion that the NRMintensity and magnetic susceptibility profiles observed inHoles 762B and 763A reflect changes in depositional pro-cesses associated with climatic change.

728


Hole 763A

E

C LCDQ

- 65

- 7 0

62"

6 6 -

7 0 -

- 75

- 8 0

Figure 7. Suggested correlations between the whole-core magnetic susceptibility profiles of the middleportions of Hole 762B and 763A.

CONCLUSIONS

This study contains a record of magnetostratigraphy of theNeogene sediments from Holes 762B and 763A. Approximately40 reversals are found and can be correlated to the magneticpolarity time scale, yielding an age-depth curve for each hole.

Hole 762B and 763A both appear to have had a similarsedimentation rate prior to 6.8 Ma. In Hole 762B, the sedi-mentation rate at 6.8 Ma was 14 m/m.y. It then increaseduniformly to a maximum value of about 20 m/m.y. at 4.6 Ma.Following a fairly uniform decay, it decreased to a minimum

value of 13 m/m.y. at 0.7 Ma. Similarly, the sedimentation ratein Hole 763A reached a maximum value of 24 m/m.y. at 3.3Ma and decreased to a minimum value of 18 m/m.y. with fairlylarge fluctuations.

The fluctuations of the susceptibility and NRM intensityobserved at both holes correlate well with changes in lithologyand sedimentation rate of sediments since 6.8 Ma. Thiscorrelation suggests changes in the supply of the terrigenousinflux to the sites, probably controlled by depositionalchanges. In at least one instance, these changes seem tocorrelate with a climatic change.

729

C. TANG

ACKNOWLEDGMENTS

I sincerely thank E. De Carlo for providing his unpublishedgeochemistry data, P. Haeussler and R. S. Coe for helpfuldiscussions, S. O'Connell and the reviewers for their helpfulreviews and many valuable suggestions, and the Joint Ocean-ographic Institutions, Inc., for partial financial support.

REFERENCES

Berggren, W. A., Kent, D. V., and Van Couvering, J. A., 1985. TheNeogene: Part 2. Neogene geochronology and chronostratigraphy.In Snelliπg, N. J. (Ed.), The Chronology of the Geological Record.Geol. Soc. London Mem., 10:211-260.

Bloemendal, J., King, J. W., Tauxe, L., and Valet, J.-P., 1989.Rock-magnetic stratigraphy of Leg 108 Sites 658, 659, 661, and665, eastern tropical Atlantic. In Ruddiman, W., Sarnthein, M., etal., Proc. ODP, Sci. Results, 108: College Station, TX (OceanDrilling Program), 415-428.

Bloemendal, J., Lamb, B., and King, J., 1988. Paleoenvironmentalimplications of rock-magnetic properties of late Quaternary sedi-ment cores from the eastern equatorial Atlantic. Paleoceanogra-phy, 3:61-87.

Ciesielski, P. F., Ledbetter, M. T., and Ellwood, B. B., 1982. Thedevelopment of Antarctic glaciation and the Neogene paleoenvi-ronment of the Maurice Ewing Bank. Mar. Geol., 46:1-51.

Doh, S.-J., King, J., and Lienen, M., 1988. A rock-magnetic study ofGPC-3 from the central North Pacific and its paleoceanographicimplications. Paleoceanography, 3:89-112.

Dymond, J., Corliss, J. B., and Health, G. R., 1977. History ofmetalliferous sediments at DSDP site 319 in the south easternPacific. Geochim Cosmochim. Acta, A\:1A1.

Hall, F. R., Bloemendal, J., King, J. W., Arthur, M. A., and Aksu,A. E., 1989. Middle to Late Quaternary sediment fluxes in theLabrador Sea, ODP Leg 105, Site 646: a synthesis of rock-magnetic, oxygen-isotopic, carbonate, and planktonic foraminif-

eral data. In Srivastava, S. P., Arthur, M., Clement, B., et al.,Proc. ODP, Sci. Results, 105: College Station, TX (Ocean DrillingProgram), 653-688.

Haq, B. U., Hardenbol, J., and Vail, P. R., 1988. Mesozoic andCenozoic chronostratigraphy and cycles of sea-level change. InWilgus, C , et al., (Eds.), Sea-Level Change—An IntegratedApproach. Soc. Econ. Paleontol. Mineral. Spec. Publ., 42:71-108.

Haq, B. U., von Rad, U., O'Connell, S., et al., 1990. Proc. ODP, Init.Repts., 122: College Station, TX (Ocean Drilling Program).

Hays, J. D., Saito, T., Opdyke, N. D., and Burckle, L. H., 1969.Pliocene-Pleistocene sediments of the equatorial Pacific: theirpaleomagnetic, biostratigraphic, and climatic record. Geol. Soc.Am. Bull, 80:1481-1513.

Kennett, J. P., 1982. Marine Geology: Englewood Cliffs, NJ (PrenticeHall).

King, J. W., 1986. Paleomagnetic and rock-magnetic stratigraphy ofPigmy Basin, Deep Sea Drilling Project Site 619, Leg 96. InBouma, A. H., Coleman, J. M, Meyer, A. W., et al., Init. Repts.DSDP, 96: Washington (U.S. Govt. Printing Office), 677-684.

King, J. W., Leskee, W., Marvin, J., and Banerjee, S. K., 1982.Identification of magnetite and hematite in natural samples: acomparison of different rock magnetic methods. Eos, 63:917.

Opdyke, N. D., Burckle, L., and Todd, A., 1974. The extension of themagnetic time scale in sediments of the Central Pacific Ocean.Earth Planet. Sci. Lett., 22:300-306.

Robison, S. G., 1986. The Late Pleistocene paleomagnetic record ofNorth Atlantic deep-sea sediments revealed by mineral-magneticmeasurements. Phys. Earth Planet. Inter., 42:22-47.

Thompson, R., Bloemendal, J., Dealing, J. A., Oldfield, F., Rum-mery, T. A., Stober, J. C , and Turner, G. M., 1980. Environmen-tal applications of magnetic measurements. Science, 207:481-486.

Date of initial receipt: 4 May 1990Date of acceptance: 10 December 1990Ms 122B-153

730


140

120-

100-

Reversal Boundary

Susceptibility

80 100 120 140

Depth (mbsf) in Hole 762B

Figure 8. Shaw correlation diagram showing the relationship between the lithostratigraphicrecords of Holes 762B and 763A, based on magnetostratigraphy and susceptibility of eachhole.

731

C. TANG

Age (m.y.)0 1 2 3 4 5 6 7 8

Depth

(m

o-

2 0 -

4 0 -

60-

80"

100"

120-

763A'

\

\

V 762B

B3 0 -

2 5 "

E20 i

1 5 -

o"cöH 10Φ

ET3Φ 5

CO ö

763A

'62B

0 1 2 3 4 5 6 7 8

Age (m.y.)

Figure 9. A. Age-depth curve. B. Sedimentation rate as a function ofage for Holes 762B and 763A.

732


A 16

2 4 6 8

Susceptibility (10E-6 cgs)

B 12

10-

8 "

4 -

Hole 763A

π Interval 0-39.4 mbsf

• Interval 48.5-143.5 mbsf

8 10

Susceptibility (10E-6 cgs)

Figure 10. Whole-core susceptibility vs. iron content (De Carlo, thisvolume) for (A) Hole 762B and (B) Hole 763A, showing positive correla-tions between susceptibility and concentration of terrigenous material inthe interval 48.5-143.5 mbsf in Hole 763A.

733

43. paleomagnetism of cenozoic sediments in ......762b contains 17 reversals. all reversals above...

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