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Magnetization of Jurassic red deep-sea sediments in the Atlantic (DSDP site 105)
Earth and Planetary Science Letters, 35 (1977) 205-214
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© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands [1]
MAGNETIZATION OF JURASSIC RED DEEP-SEA SEDIMENTS IN THE ATLANTIC (DSDP SITE 105) 1
MAUREEN B. STEINER 2 Institute for Geosciences, The University of Texas at Dallas, Richardson, Texas 75080 (USA) Received December 16, 1976 Revised version received March 7, 1977
Measurement of the remanent magnetization of samples of Jurassic oceanic red sediments recovered in the western Atlantic on Leg 11, site 105 of the Deep Sea Drilling Project yields quite different results, depending on the demagnetization processes used. Both the Jurassic section and the Berrlasian-Valanginian part of the Lower Cretaceous were measured, but with less satisfactory results for the Lower Cretaceous. The natural remanent magnetization of the Jurassic section is almost entirely normal, with 44.6 ° inclinations (standard deviation = 13.9 °) and is not changed by 1000 Oe alternating field (AF) demagnetization. Thermal demagnetization to temperatures of 630°C brings the inclination and polarity sequence in line with that expected for Oxfordian through Tithonian time at this site. The average inclination after thermal demagnetization is 22.10 , standard deviation = 12.1 ° , and the polarity pattern is one of frequently alternating polarity, much more similar to published reversal patterns for this time than the all normal results of AF demagnetization. The polarity pattern is not identical to the published ones as a result of insufficiently detailed sampling. Thermomagnetic and X-ray analyses were ambiguous, but suggest the presence of titanomagnetite, hematite, and possibly titanomaghemite and pyrrhotite. The primary remanence is carded by hematite.
1. Introduction
Site 105 of Leg 11 of the Deep Sea Drilling Project (JOIDES) was drilled east of Cape Hatteras in the central Atlantic (Fig. 1), near the north end of the Hatteras Abyssal Plain (34.9°N, 69.2°W). The hole was drilled in sea floor between Late Jurassic anomalies M24 and M25 [1 ]. The Oxfordian age of the bottom of this hole has served to place the lower limit on the age of the oldest observed oceanic anomalies [1,2]. Overlying basaltic basement are 67 m (cores 39 to 33) of fairly well lithified reddish brown to very dark red-brown (10 R 3/4 to 10 R 6/6 on the Geological Society of America Rock-Color Chart) clayey chalk. The sediment color is well reproduced in the frontispiece of the Initial Reports of the Deep Sea Drilling
Project, Volume IX [3]. Above the red sediments the color gradually changes through shades of pink (10 R 7 / 4 to 10 R 8/2 in section 1 of core 33 and core 32) to white in core 31 and above. The age of the red
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Fig. 1. Location of site 105 (modified from fig. 5 of Larson and Hilde [ 1 ]).
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sediments ranges from Oxfordian through Tithonian [3] but with the Kimmeridgian being extremely thin or absent [4]. The Jurassic-Cretaceous boundary (Tithonian-Berdasian) coincides with the change in color. Nannofossils indicate an unconformity at this boundary [4]. The Oxfordian through Tithonian age of these red sediments suggests that they should contain a magnetic polarity reversal sequence of approximately the pattern seen in MI 6 through M25 of the M sequence of lineated magnetic anomalies [1] and in the continental Oxfordian-Kimmeridgian Morrison Formation [8]. The strata of site 105 were sampled to determine whether they record the same magnetic record as that observed from the equivalent-aged sea-floor anomalies. A second objective was to investigate the magnetic stability of these somewhat uncommon red oceanic sediments.
2. Sampling JOIDES cores are recovered in 9.5-m core barrel lengths and cut into six sections. The position of a sample is thus designated by a core barrel number and section number. Permission was obtained for paleomagnetic samples at an interval of approximately two samples per recovered section of each core barrel. This gives a spacing of about 6 0 - 1 0 0 cm between samples in continuous sections of the core. However, only 49 m of the 67 m cored were recovered, so larger gaps exist between some of the samples. Paleomagnetic samples were also taken from 82 m of the overlying white chalks (57 white samples, 68 red samples). The sedimentary layers are indurated, and the JOIDES cores had to be cored with a diamond bit mounted in a drill press. Samples are 2.5 cm diameter, 2.5 cm long cores oriented horizontally with respect to the axis of the JOIDES core. Samples were measured with a cryogenic magnetometer (Superconducting Technology). Demagnetization was done largely on a 400-Hz alternating field system with a 3-axis tumbler; but some was also done on a Schonsted single-axis AF demagnetizer, demagnetizing each axis and alternating the polarity of the axes with each step. Thermal demagnetization utilized a non-inductively wound furnace with separate cooling chamber which was field free to within 10 7.
3. Magnetization The inclination of the natural remanent magnetization (NRM) directions of the red sediments (Fig. 2A, lower part) were fairly well grouped around a mean inclination of 44.6 ° (standard deviation = 13.9°). All but two of the red samples (total of 68) show normal polarity. The inclination is distinct from that of the present observed field at the site (65 ° ) and also the axial dipole field (54.5°). The NRM inclinations of the white sediments were quite scattered. Color seems to bear a relationship to magnetic properties. Beneath the white strata, as the red sediments range down section from light pink to red to dark red, NRM intensity increases with increasing red color. White samples average 0.10 × 10 -5 G, while red samples have average intensities of 0.52 X 10 - s G. The few pinkish samples (representing about 11 m) have NRM and demagnetized intensities intermediate between the red and white. Dark red samples frequently have higher intensities, ranging up to 0.35 X 10 -4 G. There also is a general correlation of greater resistance to AF demagnetization with increasing depth and with increasing red color. In the lowest portion of the section an increase in brown in the red color correlates with polarity becoming much less distinct and difficult to define (see thermal demagnetization discussion). In alternating field (AF) demagnetization, low fields quickly removed the magnetization of the white samples. These samples frequently followed non-linear, rather erratic, demagnetization paths. Of 39 white samples treated, 22 could not be demagnetized in fields higher than 100 Oe (peak field) without directions becoming spurious. Nine were completely erratic and determination of a true direction was difficult. The rest were more coherent and cleaned in fields of 150-250 Oe. The results are shown in Fig. 2B, the points plotted being the stable end point for each sample or the point beyond which directions were not coherent. After demagnetization the directions of magnetization of the white samples are still quite scattered. Thus, they are not considered further in this study. On the other hand, alternating fields up to 1000 Oe caused little change in the magnetization directions of the red samples (Fig. 213). The predominant effect of AF demagnetization on the red samples was
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Fig. 3. Directional response of samples to AF demagnetization (triangles) and subsequent thermal demagnetization (crosses and dots), in equal area projection. a slight shallowing of inclinations, although intensities decreased markedly. AF demagnetization response is shown in Figs. 3, 4, and 5. All red samples were AF demagnetized to 350 Oe. All have a pronounced low-stability component which is largely erased by 200 Oe peak field. After 350 Oe field demagnetization, the intensities have decreased to an average of 33% (range of 20-65%) of the NRM value. By 500 Oe, the curves are fairly flat and the remanence is 2 0 - 5 0 % of the original. 1000 Oe do not
destroy the remanence; usually 20-30% remains. The basal 5 m in the hole (core 38, section 5 to the bottom) have a much higher AF stability, having decreased to only about 60% by 350 Oe. Some samples were remeasured after storage in the earth's field subsequent to 350 Oe demagnetization. They showed an ability to acquire a small remanence (10-20% of NRM value) during storage. Despite the large low-stability component removed by AF demagnetization, such treatment did not af-
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210 demagnetization to as much as 1000 Oe, it seems that this magnetization is not the Jurassic magnetization that sediments of this age would have received at the time of their deposition. The inclination for this site relative to the Cretaceous pole [9] would be 45 ° . The NRM and AF mean inclination of these samples suggest a Cretaceous magnetization. Because of this inclination discrepancy, the nearly completely normal polarity sequence, and the red color of the sediments, 24 pilot samples were stepwise thermally demagnetized at temperatures ranging from 150 ° to 645°C. Steps varied from 50 ° to 100°C. (Thermal demagnetization was done on the same samples that had already received AF treatment to 350 Oe as a result of the limited amount of JOIDES material available.) Before heating, a 2- to 5-mm slice from each sample was set aside for magnetic properties studies. Behavior upon thermal demagnetization is shown.
fect the directions significantly. The result is a distribution of inclinations similar to the NRM distribution as Fig. 4A and B show. The samples still show normal polarity, with two exceptions (see Fig. 2 and 6B) and the inclinations are tightly grouped with a mean of 41.2 ° (standard deviation = 14.7°). Steiner [5] and Steiner and Helsley [6] have discussed the position of the paleomagnetic pole with respect to North America during the Jurassic. The continental Morrison Formation is thought to be approximately the same age (Oxfordian) as a large portion of the red sediments at the b o t t o m of site 105 [7]. The poles obtained from the Morrison Formation [8] were used in calculating the expected Jurassic inclination for this site during the early Late Jurassic. The inclination should be between 22 ° and 37 °, depending on the exact age of the samples in relation to the Morrison data (see Steiner and Helsley [6]). Since the inclination of the site 105 samples is 41 ° after
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211 in Figs. 3 and 5. As a result of heating, a large number of samples (48%) inverted their polarity. Directional changes begin to take place around 150 ° to 200°C, are nearly complete by 370°C, and the final direction has usually been obtained by 570°C. However, about 30% continue to move when heated to 630°C, and most all samples are stable up to 645°C. About 20% seem to be completely demagnetized if heated above 580°C. Since large directional changes often took place as a result of heating to only 200 ° C, two samples were heated in 10 ° increments from 26 ° to 190°C. One sample contained NRM (reversed) that had been demagnetized to 350 Oe while the other had been AF demagnetized to less 0.05% of the NRM intensity and given an ARM (0.25 Oe/1000 Oe). Both samples showed a very pronounced remanence loss when heated from 100 ° to 130°C. No further intensity loss occurred between 130 ° and 190°C, and the curves are very flat. No accompanying direction change occurred in the NRM sample; nor was any seen up to 190 ° C. During thermal demagnetization, intensity of the normal samples steadily decreased while that of the reversed samples rose, generally an average of 1.4 times the intensity prior to heating (Fig. 5). Even the pale pink samples which had already reversed their polarity as a result of AF treatment, increased in intensity upon being heated. As with AF demagnetization, a color dependence of thermal demagnetization behavior was exhibited. The pale pink display very flat thermal demagnetization curves with little loss of intensity by 630°C while the underlying red samples show irregular curves and frequently exhibit a more distributed set of blocking temperatures. The lowest portion of the core, the basal 8.5 m, shows peculiar shallow inclinations, unlike the samples higher in the core. NRM inclinations were generally shallower than those above and were not changed by AF demagnetization. Upon thermal demagnetization, they move to very shallow positive and negative inclinations around 10 °. These shallow inclinations are associated with the dark red-brown color and rather high AF stability, but their age is not significantly different from the overlying core barrels [3]. The inclination pattern of the red (and pink) samples after thermal demagnetization is shown in Fig.
6B. The pattern is a coherent series of normal and reversed intervals. As is obvious from Fig. 6C, the reversal sequence now has similarity to the corresponding pattern of sea-floor magnetic anomalies, whereas after AF demagnetization (Fig. 6A) there was no similarity at all between these data and the supposedly time-equivalent sea-floor magnetic anomaly record and the Morrison Formation magnetic record. The average inclination is now 22.1 o (standard deviation = 12.1°), as is predicted for this site by the Morrison data.
4. Magnetic properties As a result of the rather unexpected response of these samples to thermal demagnetization, an attempt was made to determine their magnetic mineralogy. A few additional samples situated very close to original samples in the core were obtained from JOIDES. In addition to the detailed AF and thermal demagnetization, thermomagnetic curves and X-ray diffraction studies were performed. Four samples were magnetically separated, closely following the method of l_¢vhe et al. [10, pp. 1 5 5 159], using a vertical water-filled tube attached to a Franz Isodynamic Separator. Problems were encountered as a result of the very large content o f red clay. At the highest fields on the separator, most of this red clay was attracted to the magnet. The gravitational force then exceeded the magnetic attractive force and caused everything to slide out of the separation tube. A 5- to 6-kOe field (1.0-1.3 A) was found to attract much less clay and to give appreciable quantities of a black mineral; however, separates still contained considerable quantities of red clay. Powder camera X-ray diffraction on the magnetic separates was not successful in identifying the magnetic fraction. Primarily quartz and feldspar lines were obtained. Some of the lines of maghemite in one sample, and magnetite in another, were observed but the data are too poor to be certain. Either the samples were not crushed to a fine enough grain size, or had not been magnetically separated well enough. Hysteresis loops were measured for six samples, using a vibrating sample magnetometer (VSM) of PAR design. Two discrete specimens were run on two of the samples. Two samples that already had been
212 thermally demagnetized to 630°C were also run. Unseparated crushed samples were run because it was felt that the red minerals adhering to clay and other grains might be responsible for a large part of the magnetization. The number of samples that had sufficient intensity to give a signal on the VSM was quite limited. Of the magnetic separates, only one yielded enough material to give a signal. Intensity as a function of temperature (J/T) was then measured. In order to involve as little paramagnetic component as possible, a field of 2.5-5.0 KOe, usually 3 KOe, was used. This value was obtained from the flattening point of the hysteresis curves. As a result of the low intensities of these samples, the J/T curves (Fig. 7) are.quite noisy. The separate gave no less noisy a signal than the unseparated samples. During heating, flowing air was forced into the sample chamber in an attempt to keep the red samples
from reducing. This effort was unsuccessful, and many samples altered irreversibly at high temperatures (to magnetite, as indicated by orders of magnitude increase in intensity upon cooling and appearance of black coloration). Therefore, very few reliable cooling curves were obtained. A number of temperatures at which intensity decreased on heating were repeatedly observed for different samples; between 120 and 150 ° C, 275-310 ° C, and 340-370°C. The 120°C abrupt drop to a very low moment is also seen in one sample which previously had been thermally demagnetized to 630°C. Similarly, in the two cooling curves obtained (one from 700°C and the other from 520°C), the 3 4 0 370°C drop is still obvious. These intensity losses will be discussed in the next section. It is significant however, that both normal and reversed samples exhibited the intensity losses mentioned.
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At first the possibility was considered that a selfreversal mechanism might be occurring during thermal demagnetization of these samples. Self-reversals can occur in some highly oxidized (but probably highly metastable) titanomagnetites [12], in ilmenohematites of grain size larger than 4/am [13], and in pyrrhotite [ 14,15 ]. However, after heating, site 105 samples exhibited a polarity .pattern similar to that of the anomaly sequence, arguing'against a self reversal mechanism. Further, the closer agreement of the inclination values after thermal versus AF treatment argues that these reversed samples recorded reversed fields. The notable consistency of the inclination from sample to sample prior to thermal demagnetization (Figs. 2 and 3) is typical of the situation of a secondary component having been added to a primary magnetization. One of the significant magnetic characteristics of these red sediments is their AF stability. Although a large proportion of the remanent intensity has AF stabilities less than 200 Oe, the direction carrying remanence in almost all samples has AF stabilities in excess of 1000 Oe. The low AF stabilities of NRM intensity suggest the presence of large grains (multidomain) of magnetite or titanomagnetite. The very high AF directional stabilities and red color suggest
213 hematite as the main carrier of the remanence. The remanence remaining after thermal demagnetization is stable to 645°C, also suggesting that hematite carties this remanence. The pronounced drop at about 120-150 ° C, in the J/T curves also suggests the presence of titanomagnetire, or perhaps goethite. Titanomagnetite is favored since no directional change occurs at this temperature and since large portions of the remanence are removed by 200 Oe AF demagnetization. Titanomagnetite is also indicated because the sample previously heated to 630°C showed a marked intensity loss at 120°C in J/T measurement. Goethite would not have survived heating to 630°C. The observed hardness to alternating fields and the large directional shift at low temperatures are characteristics observed by both Biquand and" Pr6vot [11 ] and Avchyan and Faustov [16]. In their studies of fine-grained red sedimentary rocks, AF demagnetization failed to clean a hard viscous remanence, but low temperature heating removed it. Biquand and Pr6vot attributed it to very fine-grained hematite, but conceivably, according to Neel theory (see Dunlop and West [17]), any other magnetic mineral of fine enough particle size could exhibit this high stability to alternating fields and low stability to increasing temperatures. Another significant characteristic of these samples is the large directional shift, and frequently a shift to opposite polarity, at low temperatures. The directional movement persists past the 370°C heating, i.e., to temperatures above the stability field of goethite. Furthermore, no directional change at 120-130°C was observed in the one reversed sample heated in detail at those temperatures. This suggests that the secondary component is perhaps carried by an oxidized titanomagnetite. The J/T curves support the presence of oxidized titanomagnetite. Most exhibit the intensity loss at 340-370°C, followed by a rise in intensity, suggesting the creation of a new magnetic phase with a greater spontaneous magnetization. A similar thermomagnetic curve was obtained by Readman and O'ReiUy [19] on heating a highly oxidized titanomagnetite (x = 0.4, z = 0.95). Kent and Lowrie [20] observed an apparent Curie point at 360 ° in brown-red deep sea sediments, also attributing it to inversion of an oxidized magnetite. Pyrrhotite is suggested in these samples (perhaps
later formed in locally reduced patches) by the frequently observed intensity drop in the J/T curves between 275 ° and 310°C. This is sometimes followed by a small rise in intensity and another dro p subsequent to 300°C, similar to that observed by Schwarz
[181. The polarity sequence delineated by these samples does not match in detail published reversal patterns for this time, the older portion of the M anomaly sequence [1 ] and Morrison Formation [22]. This is the result of the fact that sampling in this core was not dense enough to allow precise placing of reversal boundaries and the large breaks in the record due to non-recovery during drilling. Compounding the problem is the fact that about 10% of the samples could not be relied upon after thermal demagnetization for true polarity determination, as a result of the hard secondary magnetization they contained. With the sparseness of the sampling, these 10% are significant. The nannofossil ages [3,4] combined with the magnetic polarity pattern obtained from this sedimentary section do not match Larson and Hilde's combination of age and polarity pattern [1 ]. This is to be expected as the ages determined for the anomaly sequence pattern are based on the assumptions of constant spreading rate and equal length (in millions of years) of the stages of the Jurassic, neither of which are certain. Those ages, then, are determined by linear extrapolation between two known end points, using these assumptions (see Larson and Hilde [1 ]). Since the ages and magnetic pattern for site 105 are both obtained from the same source, the sediments, further sampling in this core might refine the ages to be placed on the magnetic anomalies. The red samples have a mean inclination after thermal demagnetization (22.1 °) in agreement with the similar-aged Morrison Formation. The mean inclination of the pink and white Cretaceous samples is 26.7 °, standard deviation = 20.0*. It is interesting that these samples of very lowest Cretaceous age (Berriasian to Valanginian) have inclinations similar to the Upper Jurassic samples. Although the white samples cannot be considered very reliable (because of the scatter in the inclinations and peculiar demagnetization behavior) they may indicate that the paleomagnetic pole position relative to North America did not change until sometime in the Lower Cretaceous.
214
6. Conclusion Thermal demagnetization o f red deep-sea sediments reveals a sequence of alternating polarity that matches considerably better the magnetization expected for that time than does the remanence after A F demagnetization. The magnetization prior to thermal cleaning is not that o f the Late Jurassic at this locality. Magnetic mineralogy studies are complex, but suggest the presence o f titanomagnetite, oxidized titanomagnetite, pyrrhotite, and hematite. The high stabilities o f the remanence with respect to A F and the thermal demagnetization stability suggest that the primary remanence o f these red sediments is carried by hematite. The core from site 105 was not sampled in sufficient detail to define the polarity sequence well enough to compare in detail with published polarity patterns for this time. Nevertheless, the remanence most stable to b o t h A F and thermal treatment best matches the expected inclination and polarity pattern for this time. In this particular study it is clear that without thermal demagnetization it would not have been possible to remove the secondary remanence and an erroneous and puzzling result would have been obtained.
3 4
5 6 7 8 9 10 11 12 13 14
Acknowledgements This study was supported b y NSF grant E A R 7422750 (formerly DES 74-22750) to the University o f Texas at Dallas and by NSF grant DES 75-14800 to the University o f Minnesota. Drs. Charles Helsley and Barbara Keating made the initial collection o f sampies. I am grateful to Drs. S.K. Banerjee, Shaul Levi, and Peter Shive for many helpful discussions and for critically reading the manuscript. The studies o f magnetic properties were performed in the University o f Minnesota laboratory. The University of Wyoming provided facilities for preparation of the final draft o f this report.
15
References
21
1 R.L, Larson and T.W.C. Hilde, A revised time scale of magnetic reversals for the Early Cretaceous and Late Jurassic, J. Geophys. Res. 80 (1975) 2586-2594. 2 R.L. Larson and W.C. Pitman III, World-wide correlation
16 17 18 19 20
22
of Mesozoic magnetic anomalies, and its implications, Geol. Soc. Am. Bull. 83 (1972) 3645-3662. C.D. Hollister and J.I. Ewing, eds., Initial Reports of The Deep Sea Drilling Project, IX (U.S. Government Printing Office, Washington, D.C.) 1205 pp. H.R. Thierstein, Calcareous nannoplankton biostratigraphy at the Jurassic-Cretaceous boundary, Colioque sur la limite Jurassique-Cr6tace, Mere. Bur. Rech. G6ol. Min. 86 (1975). M.B. Steiner, Mesozoic apparent polar wander and Atlantic plate tectonics, Nature 254 (1975) 107-109. M.B. Steiner and C,E. Helsley, Jurassic polar movement relative to North America, J. Geophys. Res. 77 (1972) 4981-4993. R.W. Imlay, personal communication (1975). M.B. Steiner and C.E. Helsley, Reversal pattern and apparent polar wander for the Late Jurassic, Geol. Soc. Am. Bull. 86 (1975) 1537-1543. M.W. McElhinny, Paleomagnetism and Plate Tectonics (Cambridge University Press, Cambridge, 1973) 202. R. L~vlie, W. Lowrie and M. Jacobs, Magnetic properties and mineralogy of four deep-sea cores, Earth Planet. Sci. Lett. 15 (1972) 157-168. D. Biquand and M. Pr6vot, A.F. demagnetization of viscous remanent magnetization in rocks, Z. Geophys. 37 (1971) 471-485. W. O'ReiUy and S.K. Banerjee, Cation distribution in titanomagnetites (1 - x)Fe304 - xFe2TiO4, Phys. Lett. 17 (1965) 237-238. M.F. Westcott-Lewis and L.G. Parry, Thermoremanence in synthetic rhombohedral iron-titanium oxides, Aust. J. Phys. 24 (1971) 735-742. C.W.F. Everitt, Self-reversal of magnetization in a shale containing pyrrhotite, Philos. Mag. (Set. 8) 7 (1962) 821-843. V.L.S. Bhimasankaram, Partial magnetic self-reversal of pyrrhotite, Nature 202 (1964) 478-480. G.M. Avchyan and S.S. Faustov, On the stability of viscous magnetization in variable magnetic field, Izv. Earth Pys. 5 (1966) 96-104. D.J. Dunlop and G.F. West, An experimental evaluation of single domain theories, Rev. Geophys. 7 (1969) 709-754. E.J. Schwarz, Magnetic phases in natural pyrrhotite Feo.89S and Feo.91S, J. Geomagn. Geoelectr. 20 (1968) 67-74. P.W. Readman and W. O'Reilly, The synthesis and inversion of non-stoichiometric titanomagnetites, Phys. Earth Planet. Inter. 4 (1970) 121-128. D.V. Kent and W. Lowrie, Origin of magnetic instability in sediment cores from the central North Pacific, J. Geophys. Res. 79 (1974) 2987-3000. P.W. Readman and W. O'Reilly, Magnetic properties of oxidized (cation-deficient) titanomagnetites (Fe, Ti, n) 304, J. Geomagn. Geoelectr. 24 (1972) 69-90. M.B. Steiner and C.E. Helsley, Late Jurassic magnetic polarity sequence, Earth Planet. Sci. Lett. 27 (1975) 108-112.
205
© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands [1]
MAGNETIZATION OF JURASSIC RED DEEP-SEA SEDIMENTS IN THE ATLANTIC (DSDP SITE 105) 1
MAUREEN B. STEINER 2 Institute for Geosciences, The University of Texas at Dallas, Richardson, Texas 75080 (USA) Received December 16, 1976 Revised version received March 7, 1977
Measurement of the remanent magnetization of samples of Jurassic oceanic red sediments recovered in the western Atlantic on Leg 11, site 105 of the Deep Sea Drilling Project yields quite different results, depending on the demagnetization processes used. Both the Jurassic section and the Berrlasian-Valanginian part of the Lower Cretaceous were measured, but with less satisfactory results for the Lower Cretaceous. The natural remanent magnetization of the Jurassic section is almost entirely normal, with 44.6 ° inclinations (standard deviation = 13.9 °) and is not changed by 1000 Oe alternating field (AF) demagnetization. Thermal demagnetization to temperatures of 630°C brings the inclination and polarity sequence in line with that expected for Oxfordian through Tithonian time at this site. The average inclination after thermal demagnetization is 22.10 , standard deviation = 12.1 ° , and the polarity pattern is one of frequently alternating polarity, much more similar to published reversal patterns for this time than the all normal results of AF demagnetization. The polarity pattern is not identical to the published ones as a result of insufficiently detailed sampling. Thermomagnetic and X-ray analyses were ambiguous, but suggest the presence of titanomagnetite, hematite, and possibly titanomaghemite and pyrrhotite. The primary remanence is carded by hematite.
1. Introduction
Site 105 of Leg 11 of the Deep Sea Drilling Project (JOIDES) was drilled east of Cape Hatteras in the central Atlantic (Fig. 1), near the north end of the Hatteras Abyssal Plain (34.9°N, 69.2°W). The hole was drilled in sea floor between Late Jurassic anomalies M24 and M25 [1 ]. The Oxfordian age of the bottom of this hole has served to place the lower limit on the age of the oldest observed oceanic anomalies [1,2]. Overlying basaltic basement are 67 m (cores 39 to 33) of fairly well lithified reddish brown to very dark red-brown (10 R 3/4 to 10 R 6/6 on the Geological Society of America Rock-Color Chart) clayey chalk. The sediment color is well reproduced in the frontispiece of the Initial Reports of the Deep Sea Drilling
Project, Volume IX [3]. Above the red sediments the color gradually changes through shades of pink (10 R 7 / 4 to 10 R 8/2 in section 1 of core 33 and core 32) to white in core 31 and above. The age of the red
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Fig. 1. Location of site 105 (modified from fig. 5 of Larson and Hilde [ 1 ]).
206
sediments ranges from Oxfordian through Tithonian [3] but with the Kimmeridgian being extremely thin or absent [4]. The Jurassic-Cretaceous boundary (Tithonian-Berdasian) coincides with the change in color. Nannofossils indicate an unconformity at this boundary [4]. The Oxfordian through Tithonian age of these red sediments suggests that they should contain a magnetic polarity reversal sequence of approximately the pattern seen in MI 6 through M25 of the M sequence of lineated magnetic anomalies [1] and in the continental Oxfordian-Kimmeridgian Morrison Formation [8]. The strata of site 105 were sampled to determine whether they record the same magnetic record as that observed from the equivalent-aged sea-floor anomalies. A second objective was to investigate the magnetic stability of these somewhat uncommon red oceanic sediments.
2. Sampling JOIDES cores are recovered in 9.5-m core barrel lengths and cut into six sections. The position of a sample is thus designated by a core barrel number and section number. Permission was obtained for paleomagnetic samples at an interval of approximately two samples per recovered section of each core barrel. This gives a spacing of about 6 0 - 1 0 0 cm between samples in continuous sections of the core. However, only 49 m of the 67 m cored were recovered, so larger gaps exist between some of the samples. Paleomagnetic samples were also taken from 82 m of the overlying white chalks (57 white samples, 68 red samples). The sedimentary layers are indurated, and the JOIDES cores had to be cored with a diamond bit mounted in a drill press. Samples are 2.5 cm diameter, 2.5 cm long cores oriented horizontally with respect to the axis of the JOIDES core. Samples were measured with a cryogenic magnetometer (Superconducting Technology). Demagnetization was done largely on a 400-Hz alternating field system with a 3-axis tumbler; but some was also done on a Schonsted single-axis AF demagnetizer, demagnetizing each axis and alternating the polarity of the axes with each step. Thermal demagnetization utilized a non-inductively wound furnace with separate cooling chamber which was field free to within 10 7.
3. Magnetization The inclination of the natural remanent magnetization (NRM) directions of the red sediments (Fig. 2A, lower part) were fairly well grouped around a mean inclination of 44.6 ° (standard deviation = 13.9°). All but two of the red samples (total of 68) show normal polarity. The inclination is distinct from that of the present observed field at the site (65 ° ) and also the axial dipole field (54.5°). The NRM inclinations of the white sediments were quite scattered. Color seems to bear a relationship to magnetic properties. Beneath the white strata, as the red sediments range down section from light pink to red to dark red, NRM intensity increases with increasing red color. White samples average 0.10 × 10 -5 G, while red samples have average intensities of 0.52 X 10 - s G. The few pinkish samples (representing about 11 m) have NRM and demagnetized intensities intermediate between the red and white. Dark red samples frequently have higher intensities, ranging up to 0.35 X 10 -4 G. There also is a general correlation of greater resistance to AF demagnetization with increasing depth and with increasing red color. In the lowest portion of the section an increase in brown in the red color correlates with polarity becoming much less distinct and difficult to define (see thermal demagnetization discussion). In alternating field (AF) demagnetization, low fields quickly removed the magnetization of the white samples. These samples frequently followed non-linear, rather erratic, demagnetization paths. Of 39 white samples treated, 22 could not be demagnetized in fields higher than 100 Oe (peak field) without directions becoming spurious. Nine were completely erratic and determination of a true direction was difficult. The rest were more coherent and cleaned in fields of 150-250 Oe. The results are shown in Fig. 2B, the points plotted being the stable end point for each sample or the point beyond which directions were not coherent. After demagnetization the directions of magnetization of the white samples are still quite scattered. Thus, they are not considered further in this study. On the other hand, alternating fields up to 1000 Oe caused little change in the magnetization directions of the red samples (Fig. 213). The predominant effect of AF demagnetization on the red samples was
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Fig. 3. Directional response of samples to AF demagnetization (triangles) and subsequent thermal demagnetization (crosses and dots), in equal area projection. a slight shallowing of inclinations, although intensities decreased markedly. AF demagnetization response is shown in Figs. 3, 4, and 5. All red samples were AF demagnetized to 350 Oe. All have a pronounced low-stability component which is largely erased by 200 Oe peak field. After 350 Oe field demagnetization, the intensities have decreased to an average of 33% (range of 20-65%) of the NRM value. By 500 Oe, the curves are fairly flat and the remanence is 2 0 - 5 0 % of the original. 1000 Oe do not
destroy the remanence; usually 20-30% remains. The basal 5 m in the hole (core 38, section 5 to the bottom) have a much higher AF stability, having decreased to only about 60% by 350 Oe. Some samples were remeasured after storage in the earth's field subsequent to 350 Oe demagnetization. They showed an ability to acquire a small remanence (10-20% of NRM value) during storage. Despite the large low-stability component removed by AF demagnetization, such treatment did not af-
208
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209
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210 demagnetization to as much as 1000 Oe, it seems that this magnetization is not the Jurassic magnetization that sediments of this age would have received at the time of their deposition. The inclination for this site relative to the Cretaceous pole [9] would be 45 ° . The NRM and AF mean inclination of these samples suggest a Cretaceous magnetization. Because of this inclination discrepancy, the nearly completely normal polarity sequence, and the red color of the sediments, 24 pilot samples were stepwise thermally demagnetized at temperatures ranging from 150 ° to 645°C. Steps varied from 50 ° to 100°C. (Thermal demagnetization was done on the same samples that had already received AF treatment to 350 Oe as a result of the limited amount of JOIDES material available.) Before heating, a 2- to 5-mm slice from each sample was set aside for magnetic properties studies. Behavior upon thermal demagnetization is shown.
fect the directions significantly. The result is a distribution of inclinations similar to the NRM distribution as Fig. 4A and B show. The samples still show normal polarity, with two exceptions (see Fig. 2 and 6B) and the inclinations are tightly grouped with a mean of 41.2 ° (standard deviation = 14.7°). Steiner [5] and Steiner and Helsley [6] have discussed the position of the paleomagnetic pole with respect to North America during the Jurassic. The continental Morrison Formation is thought to be approximately the same age (Oxfordian) as a large portion of the red sediments at the b o t t o m of site 105 [7]. The poles obtained from the Morrison Formation [8] were used in calculating the expected Jurassic inclination for this site during the early Late Jurassic. The inclination should be between 22 ° and 37 °, depending on the exact age of the samples in relation to the Morrison data (see Steiner and Helsley [6]). Since the inclination of the site 105 samples is 41 ° after
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211 in Figs. 3 and 5. As a result of heating, a large number of samples (48%) inverted their polarity. Directional changes begin to take place around 150 ° to 200°C, are nearly complete by 370°C, and the final direction has usually been obtained by 570°C. However, about 30% continue to move when heated to 630°C, and most all samples are stable up to 645°C. About 20% seem to be completely demagnetized if heated above 580°C. Since large directional changes often took place as a result of heating to only 200 ° C, two samples were heated in 10 ° increments from 26 ° to 190°C. One sample contained NRM (reversed) that had been demagnetized to 350 Oe while the other had been AF demagnetized to less 0.05% of the NRM intensity and given an ARM (0.25 Oe/1000 Oe). Both samples showed a very pronounced remanence loss when heated from 100 ° to 130°C. No further intensity loss occurred between 130 ° and 190°C, and the curves are very flat. No accompanying direction change occurred in the NRM sample; nor was any seen up to 190 ° C. During thermal demagnetization, intensity of the normal samples steadily decreased while that of the reversed samples rose, generally an average of 1.4 times the intensity prior to heating (Fig. 5). Even the pale pink samples which had already reversed their polarity as a result of AF treatment, increased in intensity upon being heated. As with AF demagnetization, a color dependence of thermal demagnetization behavior was exhibited. The pale pink display very flat thermal demagnetization curves with little loss of intensity by 630°C while the underlying red samples show irregular curves and frequently exhibit a more distributed set of blocking temperatures. The lowest portion of the core, the basal 8.5 m, shows peculiar shallow inclinations, unlike the samples higher in the core. NRM inclinations were generally shallower than those above and were not changed by AF demagnetization. Upon thermal demagnetization, they move to very shallow positive and negative inclinations around 10 °. These shallow inclinations are associated with the dark red-brown color and rather high AF stability, but their age is not significantly different from the overlying core barrels [3]. The inclination pattern of the red (and pink) samples after thermal demagnetization is shown in Fig.
6B. The pattern is a coherent series of normal and reversed intervals. As is obvious from Fig. 6C, the reversal sequence now has similarity to the corresponding pattern of sea-floor magnetic anomalies, whereas after AF demagnetization (Fig. 6A) there was no similarity at all between these data and the supposedly time-equivalent sea-floor magnetic anomaly record and the Morrison Formation magnetic record. The average inclination is now 22.1 o (standard deviation = 12.1°), as is predicted for this site by the Morrison data.
4. Magnetic properties As a result of the rather unexpected response of these samples to thermal demagnetization, an attempt was made to determine their magnetic mineralogy. A few additional samples situated very close to original samples in the core were obtained from JOIDES. In addition to the detailed AF and thermal demagnetization, thermomagnetic curves and X-ray diffraction studies were performed. Four samples were magnetically separated, closely following the method of l_¢vhe et al. [10, pp. 1 5 5 159], using a vertical water-filled tube attached to a Franz Isodynamic Separator. Problems were encountered as a result of the very large content o f red clay. At the highest fields on the separator, most of this red clay was attracted to the magnet. The gravitational force then exceeded the magnetic attractive force and caused everything to slide out of the separation tube. A 5- to 6-kOe field (1.0-1.3 A) was found to attract much less clay and to give appreciable quantities of a black mineral; however, separates still contained considerable quantities of red clay. Powder camera X-ray diffraction on the magnetic separates was not successful in identifying the magnetic fraction. Primarily quartz and feldspar lines were obtained. Some of the lines of maghemite in one sample, and magnetite in another, were observed but the data are too poor to be certain. Either the samples were not crushed to a fine enough grain size, or had not been magnetically separated well enough. Hysteresis loops were measured for six samples, using a vibrating sample magnetometer (VSM) of PAR design. Two discrete specimens were run on two of the samples. Two samples that already had been
212 thermally demagnetized to 630°C were also run. Unseparated crushed samples were run because it was felt that the red minerals adhering to clay and other grains might be responsible for a large part of the magnetization. The number of samples that had sufficient intensity to give a signal on the VSM was quite limited. Of the magnetic separates, only one yielded enough material to give a signal. Intensity as a function of temperature (J/T) was then measured. In order to involve as little paramagnetic component as possible, a field of 2.5-5.0 KOe, usually 3 KOe, was used. This value was obtained from the flattening point of the hysteresis curves. As a result of the low intensities of these samples, the J/T curves (Fig. 7) are.quite noisy. The separate gave no less noisy a signal than the unseparated samples. During heating, flowing air was forced into the sample chamber in an attempt to keep the red samples
from reducing. This effort was unsuccessful, and many samples altered irreversibly at high temperatures (to magnetite, as indicated by orders of magnitude increase in intensity upon cooling and appearance of black coloration). Therefore, very few reliable cooling curves were obtained. A number of temperatures at which intensity decreased on heating were repeatedly observed for different samples; between 120 and 150 ° C, 275-310 ° C, and 340-370°C. The 120°C abrupt drop to a very low moment is also seen in one sample which previously had been thermally demagnetized to 630°C. Similarly, in the two cooling curves obtained (one from 700°C and the other from 520°C), the 3 4 0 370°C drop is still obvious. These intensity losses will be discussed in the next section. It is significant however, that both normal and reversed samples exhibited the intensity losses mentioned.
5. Discussion 04
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At first the possibility was considered that a selfreversal mechanism might be occurring during thermal demagnetization of these samples. Self-reversals can occur in some highly oxidized (but probably highly metastable) titanomagnetites [12], in ilmenohematites of grain size larger than 4/am [13], and in pyrrhotite [ 14,15 ]. However, after heating, site 105 samples exhibited a polarity .pattern similar to that of the anomaly sequence, arguing'against a self reversal mechanism. Further, the closer agreement of the inclination values after thermal versus AF treatment argues that these reversed samples recorded reversed fields. The notable consistency of the inclination from sample to sample prior to thermal demagnetization (Figs. 2 and 3) is typical of the situation of a secondary component having been added to a primary magnetization. One of the significant magnetic characteristics of these red sediments is their AF stability. Although a large proportion of the remanent intensity has AF stabilities less than 200 Oe, the direction carrying remanence in almost all samples has AF stabilities in excess of 1000 Oe. The low AF stabilities of NRM intensity suggest the presence of large grains (multidomain) of magnetite or titanomagnetite. The very high AF directional stabilities and red color suggest
213 hematite as the main carrier of the remanence. The remanence remaining after thermal demagnetization is stable to 645°C, also suggesting that hematite carties this remanence. The pronounced drop at about 120-150 ° C, in the J/T curves also suggests the presence of titanomagnetire, or perhaps goethite. Titanomagnetite is favored since no directional change occurs at this temperature and since large portions of the remanence are removed by 200 Oe AF demagnetization. Titanomagnetite is also indicated because the sample previously heated to 630°C showed a marked intensity loss at 120°C in J/T measurement. Goethite would not have survived heating to 630°C. The observed hardness to alternating fields and the large directional shift at low temperatures are characteristics observed by both Biquand and" Pr6vot [11 ] and Avchyan and Faustov [16]. In their studies of fine-grained red sedimentary rocks, AF demagnetization failed to clean a hard viscous remanence, but low temperature heating removed it. Biquand and Pr6vot attributed it to very fine-grained hematite, but conceivably, according to Neel theory (see Dunlop and West [17]), any other magnetic mineral of fine enough particle size could exhibit this high stability to alternating fields and low stability to increasing temperatures. Another significant characteristic of these samples is the large directional shift, and frequently a shift to opposite polarity, at low temperatures. The directional movement persists past the 370°C heating, i.e., to temperatures above the stability field of goethite. Furthermore, no directional change at 120-130°C was observed in the one reversed sample heated in detail at those temperatures. This suggests that the secondary component is perhaps carried by an oxidized titanomagnetite. The J/T curves support the presence of oxidized titanomagnetite. Most exhibit the intensity loss at 340-370°C, followed by a rise in intensity, suggesting the creation of a new magnetic phase with a greater spontaneous magnetization. A similar thermomagnetic curve was obtained by Readman and O'ReiUy [19] on heating a highly oxidized titanomagnetite (x = 0.4, z = 0.95). Kent and Lowrie [20] observed an apparent Curie point at 360 ° in brown-red deep sea sediments, also attributing it to inversion of an oxidized magnetite. Pyrrhotite is suggested in these samples (perhaps
later formed in locally reduced patches) by the frequently observed intensity drop in the J/T curves between 275 ° and 310°C. This is sometimes followed by a small rise in intensity and another dro p subsequent to 300°C, similar to that observed by Schwarz
[181. The polarity sequence delineated by these samples does not match in detail published reversal patterns for this time, the older portion of the M anomaly sequence [1 ] and Morrison Formation [22]. This is the result of the fact that sampling in this core was not dense enough to allow precise placing of reversal boundaries and the large breaks in the record due to non-recovery during drilling. Compounding the problem is the fact that about 10% of the samples could not be relied upon after thermal demagnetization for true polarity determination, as a result of the hard secondary magnetization they contained. With the sparseness of the sampling, these 10% are significant. The nannofossil ages [3,4] combined with the magnetic polarity pattern obtained from this sedimentary section do not match Larson and Hilde's combination of age and polarity pattern [1 ]. This is to be expected as the ages determined for the anomaly sequence pattern are based on the assumptions of constant spreading rate and equal length (in millions of years) of the stages of the Jurassic, neither of which are certain. Those ages, then, are determined by linear extrapolation between two known end points, using these assumptions (see Larson and Hilde [1 ]). Since the ages and magnetic pattern for site 105 are both obtained from the same source, the sediments, further sampling in this core might refine the ages to be placed on the magnetic anomalies. The red samples have a mean inclination after thermal demagnetization (22.1 °) in agreement with the similar-aged Morrison Formation. The mean inclination of the pink and white Cretaceous samples is 26.7 °, standard deviation = 20.0*. It is interesting that these samples of very lowest Cretaceous age (Berriasian to Valanginian) have inclinations similar to the Upper Jurassic samples. Although the white samples cannot be considered very reliable (because of the scatter in the inclinations and peculiar demagnetization behavior) they may indicate that the paleomagnetic pole position relative to North America did not change until sometime in the Lower Cretaceous.
214
6. Conclusion Thermal demagnetization o f red deep-sea sediments reveals a sequence of alternating polarity that matches considerably better the magnetization expected for that time than does the remanence after A F demagnetization. The magnetization prior to thermal cleaning is not that o f the Late Jurassic at this locality. Magnetic mineralogy studies are complex, but suggest the presence o f titanomagnetite, oxidized titanomagnetite, pyrrhotite, and hematite. The high stabilities o f the remanence with respect to A F and the thermal demagnetization stability suggest that the primary remanence o f these red sediments is carried by hematite. The core from site 105 was not sampled in sufficient detail to define the polarity sequence well enough to compare in detail with published polarity patterns for this time. Nevertheless, the remanence most stable to b o t h A F and thermal treatment best matches the expected inclination and polarity pattern for this time. In this particular study it is clear that without thermal demagnetization it would not have been possible to remove the secondary remanence and an erroneous and puzzling result would have been obtained.
3 4
5 6 7 8 9 10 11 12 13 14
Acknowledgements This study was supported b y NSF grant E A R 7422750 (formerly DES 74-22750) to the University o f Texas at Dallas and by NSF grant DES 75-14800 to the University o f Minnesota. Drs. Charles Helsley and Barbara Keating made the initial collection o f sampies. I am grateful to Drs. S.K. Banerjee, Shaul Levi, and Peter Shive for many helpful discussions and for critically reading the manuscript. The studies o f magnetic properties were performed in the University o f Minnesota laboratory. The University of Wyoming provided facilities for preparation of the final draft o f this report.
15
References
21
1 R.L, Larson and T.W.C. Hilde, A revised time scale of magnetic reversals for the Early Cretaceous and Late Jurassic, J. Geophys. Res. 80 (1975) 2586-2594. 2 R.L. Larson and W.C. Pitman III, World-wide correlation
16 17 18 19 20
22
of Mesozoic magnetic anomalies, and its implications, Geol. Soc. Am. Bull. 83 (1972) 3645-3662. C.D. Hollister and J.I. Ewing, eds., Initial Reports of The Deep Sea Drilling Project, IX (U.S. Government Printing Office, Washington, D.C.) 1205 pp. H.R. Thierstein, Calcareous nannoplankton biostratigraphy at the Jurassic-Cretaceous boundary, Colioque sur la limite Jurassique-Cr6tace, Mere. Bur. Rech. G6ol. Min. 86 (1975). M.B. Steiner, Mesozoic apparent polar wander and Atlantic plate tectonics, Nature 254 (1975) 107-109. M.B. Steiner and C,E. Helsley, Jurassic polar movement relative to North America, J. Geophys. Res. 77 (1972) 4981-4993. R.W. Imlay, personal communication (1975). M.B. Steiner and C.E. Helsley, Reversal pattern and apparent polar wander for the Late Jurassic, Geol. Soc. Am. Bull. 86 (1975) 1537-1543. M.W. McElhinny, Paleomagnetism and Plate Tectonics (Cambridge University Press, Cambridge, 1973) 202. R. L~vlie, W. Lowrie and M. Jacobs, Magnetic properties and mineralogy of four deep-sea cores, Earth Planet. Sci. Lett. 15 (1972) 157-168. D. Biquand and M. Pr6vot, A.F. demagnetization of viscous remanent magnetization in rocks, Z. Geophys. 37 (1971) 471-485. W. O'ReiUy and S.K. Banerjee, Cation distribution in titanomagnetites (1 - x)Fe304 - xFe2TiO4, Phys. Lett. 17 (1965) 237-238. M.F. Westcott-Lewis and L.G. Parry, Thermoremanence in synthetic rhombohedral iron-titanium oxides, Aust. J. Phys. 24 (1971) 735-742. C.W.F. Everitt, Self-reversal of magnetization in a shale containing pyrrhotite, Philos. Mag. (Set. 8) 7 (1962) 821-843. V.L.S. Bhimasankaram, Partial magnetic self-reversal of pyrrhotite, Nature 202 (1964) 478-480. G.M. Avchyan and S.S. Faustov, On the stability of viscous magnetization in variable magnetic field, Izv. Earth Pys. 5 (1966) 96-104. D.J. Dunlop and G.F. West, An experimental evaluation of single domain theories, Rev. Geophys. 7 (1969) 709-754. E.J. Schwarz, Magnetic phases in natural pyrrhotite Feo.89S and Feo.91S, J. Geomagn. Geoelectr. 20 (1968) 67-74. P.W. Readman and W. O'Reilly, The synthesis and inversion of non-stoichiometric titanomagnetites, Phys. Earth Planet. Inter. 4 (1970) 121-128. D.V. Kent and W. Lowrie, Origin of magnetic instability in sediment cores from the central North Pacific, J. Geophys. Res. 79 (1974) 2987-3000. P.W. Readman and W. O'Reilly, Magnetic properties of oxidized (cation-deficient) titanomagnetites (Fe, Ti, n) 304, J. Geomagn. Geoelectr. 24 (1972) 69-90. M.B. Steiner and C.E. Helsley, Late Jurassic magnetic polarity sequence, Earth Planet. Sci. Lett. 27 (1975) 108-112.