Magnetic properties of basalts and sediments of ODP Leg 135 sites

Paleomagnetic and rock-magnetic investigations of basalts from Hole 834B in the Lau backarc basin and of sediments from Holes 841A and 841B at the Tonga Ridge are reported. Three groups of blocking temperatures in the basalts suggest the presence of at least three magnetic phases: pure magnetite, a Ti-poor titanomagnetite, and a Ti-rich phase. The drill-string-induced remanence in the basalts is typically between three and six times the original normal remanent magnetization intensity, but it is mostly removed by alternating-field (AF) cleaning in 5 mT. Volume susceptibility values range from 0.04 * 10**-3 to 4 * 10**-3 cgs. The modified Q-ratio J5/sus ranges from 0.5 to 10. The drill-string-induced remanence behaves different in the two sediment cores from Holes 841A and 841B, which may be the result of differences in the sediment or caused by the different drilling equipment used. The AF-cleaned inclinations of the sediment in Holes 841A and 841B suggest a slight flattening with increasing depth (up to 6° under a load of 400 m of sediment) to be present. This flattening is likely to be caused by the differential rotation of detrital particles under compaction during diagenesis.

Sediment paleomagnetism at DSDP Leg 78 Holes

The results of paleomagnetic studies of samples from DSDP Leg 78A are reported. For Site 541, the interval from 60 to 200 m sub-bottom was correlated with the Matuyama through Gilbert polarity epochs. For Site 543, the interval from 150 to 190 m sub-bottom was correlated with marine magnetic Anomalies 5C through 5E. Down-dip directions of tilted beds inferred from declination values for Sites 541 and 542 suggest a pattern of monoclinal folding. Results from basalt samples are comparable to those from other DSDP sites in relatively old basalts.

Paleomagnetic of ODP Holes 122-762B and 122-763A

The magnetostratigraphy of Neogene sediments from Holes 762B and 763A are presented in this paper. Hole 762B contains 17 reversals. All reversals above the base of the Gilbert are correlated with the magnetic polarity time scale (Haq et al., 1988). Hole 763A yields a record of about 20 reversals that can be correlated to the magnetic polarity time scale, documenting all reversals to the base of Chron 4A. Based on the correlation, the sediment accumulation vs. time for Holes 762B and 763A are determined. The age-depth curves obtained show a similar pattern of sedimentation rate since 6.8 Ma. The study also indicates a correlation between the fluctuations in the magnetic parameters (natural remanent magnetization intensity and susceptibility), the lithologic changes, and changes in iron content at both holes. This correlation suggests that the natural remanent magnetization intensity and susceptibility changes observed in Holes 762B and 763A are controlled by changes in depositional processes probably associated with climatic variations.

Iridium and other element concentrations and magnetostratigraphy across the Cretaceous/Tertiarty boundary in ODP Hole 122-761C

Samples from Ocean Drilling Program Hole 761C, collected on both sides of the Cretaceous/Tertiary boundary have been analyzed for their chemical and mineralogical content. The sediment consists of nannofossil ooze with variable amounts of clay. The boundary is marked by a color change associated with a nearly step-like decrease of the carbonate fraction. Paleomagnetic data and the drop of the carbonate content indicate that a strong reduction of the sedimentation rate occurred at the boundary and persisted for million of years. An iridium anomaly of 80 ng/cm**2, together with overabundances of Cr and Fe, are found in close coincidence with the planktonic crisis. These enrichments can be explained by the infall of =0.16 g/cm2 of Cl-like chondritic material. Co and Ni enrichments and a great quantity of Ni-rich magnetites are also observed in the basal Danian. These elements and minerals excepted, the composition of the insoluble fraction appears to be nearly unchanged across the boundary. Chemical and mineralogical observations support a cosmic origin for the Cretaceous/Tertiary event but do not reveal the presence of any significant impact ejecta.

Paleomagnetic of lower Cretaceous sediments from the Argo and Gascoyne Abyssal Plain

Lower Cretaceous sediments were sampled for magnetostratigraphy at three sites. ODP Site 765 and DSDP Site 261, in the Argo Abyssal Plain, consist primarily of brownish-red to gray claystone having hematite and magnetite carriers of characteristic magnetization. ODP Site 766, in the Gascoyne Abyssal Plain, consists mainly of dark greenish-gray volcaniclastic turbidites with magnetite as the carrier of characteristic magnetization. Progressive thermal demagnetization (Sites 765 and 261) or alternating field demagnetization (Site 766) yielded well-defined polarity zones and a set of reliable paleolatitudes. Magnetic polarity chrons were assigned to polarity zones using biostratigraphic correlations. Late Aptian chron M"-1"r, a brief reversed-polarity chron younger than MOr, is a narrow, 40-cm feature delimited in Hole 765C. Early Aptian reversed-polarity chron MOr is also present in Hole 765C. Polarity chrons Mir through M3r were observed in the Barremian of all three sites. Valanginian and Hauterivian polarity chrons can be tentatively assigned to polarity zones only in Hole 766A. The paleolatitude of this region remained at 35° to 37°S from the Berriasian through the Aptian. During this interval, there was approximately 16° of clockwise rotation, with the oriented sample suites of Site 765 displaying a Berriasian declination of 307° to an Aptian declination of 323°. These results are consistent with the interpolated Early Cretaceous apparent polar wander for Australia, but indicate that this region was approximately 5? farther north than predicted.

Characteristic magnetization of upper Cretaceous and lower Tertiary sediments

Lower Campanian to middle Eocene chalks and oozes were recovered at Sites 761 and 762 of Ocean Drilling Program Leg 122 on the Exmouth Plateau, northwest Australia. Paleomagnetic analyses were made on 125 samples from Hole 761B and 367 samples from Hole 762C. Thermal cleaning, alternating field demagnetization, or mixed treatment reveals a stable remanent component of normal or reversed polarity. Correlation of the magnetic polarity sequences established for these holes with the standard magnetic polarity time scale was aided by nannofossil zonation. At Hole 761B, the sequence extends from Subchron C32-N (upper Campanian) through Subchron C17-R (middle Eocene), but given the low sedimentation rate, not all the subchrons of the standard magnetic polarity sequence were recognized. The sequence at Hole 762C extends from Subchron C13-R (middle Eocene) to the boundary between Chrons C33 and C34 (lower Campanian). The sedimentation rate is higher at Hole 762C, and all the magnetic polarity subchrons of the Campanian and Maestrichtian stages were identified. Thus, this hole could be a reference section to refine the Upper Cretaceous time scale.

Paleomagnetic of ODP Hole 130-807C basalts

Characteristic remanent magnetizations derived from detailed thermal and alternating-field demagnetization of basalts recovered at Ocean Drilling Program (ODP) Site 807 on the Ontong Java Plateau reveal constant normal polarity consistent with paleontological ages from overlying sediments, suggesting deposition in early Aptian times at the beginning of the Cretaceous Normal Polarity Superchron (K-N). The paleomagnetic data can be divided into 14 distinct inclination groups, which together define a paleolatitude of 18°S, some 16° shallower than expected from a Pacific apparent polar wander path (APWP) based on nonsedimentary data. The data display a trend in paleomagnetic inclination, showing shallower values with increasing depth. We conclude that this trend is a result of local tectonic tilting during the waning phases of volcanism on the plateau. Hotspot-based plate reconstructions for the Early Cretaceous place the Ontong Java Plateau on the Louisville hotspot, presently located at 51°S, whereas the paleolatitude for Site 807 based on the Pacific APWP is 34°S. Because the nominal mean inclination from Site 807 and values derived from Deep Sea Drilling Project (DSDP) sediments of other sites predict shallower paleolatitudes for the Ontong Java Plateau, values from the Pacific APWP provide lower bounds on true polar wander. Considering mantle plume sources on the southern and northern portions of the plateau (DSDP Site 288 and ODP Site 807, respectively), the Louisville hotspot appears to have moved 9°-17° to the south relative to the spin axis since the Early Cretaceous. This sense of motion is consistent with previous results for the Suiko Seamount (65 Ma) of the Hawaiian-Emperor Chain.

(Appendix) Characteristic directions at DSDP Hole 76-534A

Kimmeridgian-Tithonian red marly limestones and Berriasian white limestones were recovered at Site 534 of DSDP Leg 76 in the western North Atlantic. These yielded a well-defined magnetostratigraphy with the characteristic magnetization carried by hematite in red sediments and magnetite in white sediments. The polarity sequence is correlated to the magnetostratigraphy of Kimmeridgian-Tithonian-Berriasian pelagic carbonates of northern Italy and southern Spain, allowing precise biostratigraphic age correlations. The Berriasian/Tithonian boundary occurs within the upper half of Core 90, the late Tithonian/early Tithonian boundary at the base of Core 96, and the Tithonian/Kimmeridgian boundary at the top of Core 102. Correlations are also made to M-16 through M-22 of the marine magnetic anomaly M-sequence. Poor recovery and irregular magnetic properties of the underlying Kimmeridgian-Oxfordian-Callovian marls and claystones prevented determination of a polarity sequence, but the entire interval has mixed polarity. Valanginian gray marly limestones have very weak magnetizations, and preliminary results are inadequate to determine the polarity pattern.

Magnetic properties of Triassic sediments of the Wombat Plateau

During drilling at Sites 759, 760, and 761 of Leg 122 (Exmouth Plateau, northwest Australia), a thick section of Upper Triassic sediments was recovered. Paleomagnetic analyses were made on 398 samples from Holes 759B, 760A, 760B, and 761C. Progressive thermal demagnetization, alternating field demagnetization, or mixed treatment removed an initial unstable component and isolated a characteristic remanent magnetization which is of normal or reversed polarity. The magnetostratigraphic results allow us to propose a magnetic polarity sequence which extends from the upper Carnian to lower Rhaetian. This sequence reveals many more reversals than previously suggested from paleomagnetic studies. The magnetostratigraphic data also allow us to suggest correlations between Sites 759 and 760.

Paleomagnetic of Jurassic basalts from ODP Hole 129-801C

The paleomagnetic and rock magnetic properties of 51 Jurassic basalts from Ocean Drilling Program (ODP) Hole 801C have been examined. Magnetic properties vary with lithologic composition; alkalic rocks and hydrothermally-altered tholeiites are much weaker in intensity and generally contain higher coercivity magnetic components than the older and less-altered tholeiites at the base of the hole. For the entire column, the Jurassic basalts have an average initial natural remanent magnetization (NRM) intensity of approximately 1.24 A/m and average median destructive fields (MDF) of 8.31 mT. These values and the mean Koenigsberger ratio of 1.7 are very similar to results obtained for Jurassic basalts from the Atlantic (DSDP Leg 76). The similarities suggest that the basalts of both sites and their remanence characteristics are representative of Jurassic oceanic crust. The most profound discovery in these samples was the presence of 5 inclination zones, each showing consistent positive (or negative) polarity opposite the overlying and underlying polarity bands. We interpret these to represent a record of change in polarity of the EarthÆs magnetic field and, because of the large number over such a short interval (60 m) of crust, we assert that the rapid change in polarity during the Jurassic is the probable reason behind the origin of the Jurassic Quiet Zone.