Magnetostratigraphy of DSDP Leg 94 sediments

The primary objective of DSDP Leg 94 was to obtain continuous paleoclimatic records along a roughly north-south transect in the North Atlantic. The magnetostratigraphy of 21 holes at 6 sites cored with the hydraulic piston corer and extended-core-barrel corer is presented here and establishes an independent chronology for these sediments. Nearly complete records were obtained for the last 2.5 m.y.; in addition, deeper drilling at three sites to satisfy tectonic and paleoceanographic objectives produced older sections suitable for magnetostratigraphic study, allowing first-order correlations of the polarity sequences with calcareous and siliceous micro fossil events. The sections with high sediment accumulation rates yielded very detailed records of polarity history and allowed three short normal-polarity zones within the Matuyama Chronozone to be detected, in addition to the Jaramillo and Olduvai subchronozones. A short reversed-polarity zone also occurs, within the upper intervals of the Gauss Chronozone. These short zones are present in multiple holes, ruling out the possibility that they might be of local origin. Correlation of these short zones with radiometrically dated polarity zones in igneous rocks strongly supports the interpretation of these polarity zones as records of true geomagnetic polarity chrons.

Magnetostratigraphy and biostratigraphy of ODP Leg 105 sites and DSDP Hole 12-112

During Ocean Drilling Program (ODP) Leg 105, three sites (Sites 645 through 647) were drilled in Baffin Bay and the Labrador Sea to examine the tectonic evolution and the climatic and oceanic histories of this region. Biostratigraphic and magnetostratigraphic results vary at each site, while stratigraphic resolution depends on the limited abundance of marker species and the completeness of the paleomagnetic record. Because of the paucity of planktonic microfossils and the poor paleomagnetic record signatures, stratigraphic determinations at Site 645 often rely on defining minimum temporal constraints on specific samples or stratigraphic intervals. The completed stratigraphy indicates that the sedimentary sequence recovered at Site 645 is early Miocene to Holocene in age. The magnetostratigraphy and biostratigraphies are better defined at Sites 646 and 647 in the Labrador Sea. Site 646 generally contains a well-developed magnetostratigraphy and calcareous microfossil biostratigraphy. This biostratigraphy is based on calcareous nannofossils and planktonic foraminifers typical of the North Atlantic Ocean. Siliceous microfossils are also present at Site 646, but they are restricted to upper Pliocene through Holocene sediments. The stratigraphic sequence recovered at Site 646 is late Miocene to Holocene in age. Based primarily on the calcareous nannofossil stratigraphy, the sequence recovered at Site 647 consists of lower Eocene to lower Oligocene, lower Miocene, upper Miocene, and upper Pliocene through Holocene sediments. Three hiatuses are present in this sequence: the older hiatus separates lower Oligocene sediments from lower Miocene sediments, another hiatus separates lower Miocene sediments from upper Miocene sediments, and the youngest one separates upper Miocene from upper Pliocene sediments. A magnetostratigraphy is defined for the interval from the Gauss/Matuyama boundary through the Brunhes (Clement et al., this volume). Both planktonic foraminifers and siliceous microfossils have restricted occurrences. Planktonic foraminifers occur in Pliocene and younger sediments, and siliceous microfossils are present in lower Miocene and lower Oligocene sediments. The near-continuous Eocene through lower Oligocene sequence recovered at Site 647 allows the calcareous nannofossils and diatom stratigraphies at this site to act as a Paleogene stratigraphic framework. This framework can be compared with the stratigraphy previously completed for DSDP Site 112.

Biostratigraphy and magnetostratigraphy of ODP Leg 124 sites

During ODP Leg 124, late middle Eocene to Quaternary sediment sequences were recovered from 13 holes drilled at five sites in the Celebes and Sulu basins. Paleomagnetic measurements and biostratigraphic studies using calcareous nannofossils, planktonic and benthic foraminifers, radiolarians, and diatoms were completed and summarized here. Two Neogene sediment sections recovered in the Sulu Basin yielded excellent core recoveries and magnetic reversal records, allowing direct magnetobiostratigraphic correlations for the Pliocene and Quaternary at Site 768 and for the middle Miocene to Quaternary at Site 769. The interpolated ages of biohorizons are not consistent between sites and only a few of them are in good agreement with previous calibrations. The differences may be the results of redeposition by turbidity currents and selective dissolution of key fossils.

Magnetostratigraphy of ODP Leg 108 sites

This paper presents the magnetostratigraphic results from Leg 108 of the Ocean Drilling Program. Measurements made with the shipboard "pass-through" cryogenic magnetometer on whole cores and archive halves are combined with those made on discrete samples; these measurements constitute the paleomagnetic data base for the Leg 108 cores. Polarity determination on unoriented, low-latitude cores is somewhat subjective; we rely heavily on the available biostratigraphic data and document our line of reasoning where appropriate. The interpretations presented here, therefore, are compatible with the available biostratigraphic information; they are also in substantial agreement with orientation information where available.

Magnetostratigraphy of ODP Leg 178 holes

We present revised magnetostratigraphic interpretations for Ocean Drilling Program Sites 1095, 1096, and 1101, cored in sediment drifts located off the Pacific margin of the Antarctic Peninsula. The revised interpretations incorporate a variety of observations and results obtained since the end of Leg 178, of which the most significant are new paleomagnetic measurements from U-channel samples, composite depth scales that allow stratigraphic correlation between multiple holes cored at a site, and revised biostratigraphic interpretations. The U-channel data, which include more than 102,000 paleomagnetic observations from more than 13,400 intervals along U-channel samples, are included as electronic files. The magnetostratigraphic records at all three sites are consistent with sedimentation being continuous over the intervals cored, although the data resolution does not preclude short hiatuses less than a few hundred thousand years in duration. The magnetostratigraphic records start at the termination of Subchron C4Ar.2n (9.580 Ma) at ~515 meters composite depth (mcd) for Site 1095, at the onset of Subchron C3n.2n (4.620 Ma) at ~489.68 mcd for Site 1096, and at the onset of Subchron C2An.1n (3.040 Ma) at 209.38 meters below seafloor for Site 1101. All three sites provide paleomagnetic records that extend upward through the Brunhes Chron.

Paleomagnetic study of the Messejana Plasencia dyke (Portugal and Spain): A lower Jurassic paleopole for the Iberian plate

The only Iberian lower Jurassic palcomagnetic pole come from the "Central Atlantic Magmatic Province"-related Messejana Plasencia dyke, but the age and origin of its remanence have been a matter of discussion. With the aim of solving this uncertainty, and to go further into a better understanding of its emplacement and other possible tectonic features, a systematic paleomagnetic investigation of 40 sites (625 specimens) distributed all along the 530 kin of the Messejana Plasencia dyke has been carried out. Rock magnetic experiments indicate PSD low Ti-titanomagnetite and magnetite as the minerals carrying the NRM. The samples were mostly thermally demagnetized. Most sites exhibit a characteristic remanent component of normal polarity with the exception of two sites, where samples with reversed polarities have been observed. The paleomagnetic pole derived from a total of 35 valid sites is representative of the whole structure of the dyke, and statistically well defined, with values of PLa = 70.4 degrees N, PLo = 237.6 degrees E, K= 47.9 and A(95) = 3.5 degrees. Paleomagnetic data indicates that: (i) there is no evidence of a Cretaceous remagnetization in the dyke, as it was suggested; (ii) most of the dyke had a brief emplacement time; furthermore, two dyke intrusion events separated in time from it by at least 10,000 y have been detected; (iii) the high grouping of the VGPs directions suggests no important tectonic perturbations of the whole structure of the dyke since its intrusion time; (iv) the pole derived from this study is a good quality lower Jurassic paleopole for the Iberian plate; and (v) the Messejana Plasencia dyke paleopole for the Iberian plate is also in agreement with quality-selected European and North American lower Jurassic paleopoles and the magnetic anomalies data sets that are available for rotate them to Iberia.

Palaeomagnetic study of a subaerial volcanic ridge (Sao Jorge Island, Azores) for the past 1.3 Myr: evidence for the Cobb Mountain Subchron, volcano flank instability and tectonomagmatic implications

We present a palaeomagnetic study on 38 lava flows and 20 dykes encompassing the past 1.3 Myr on S. Jorge Island (Azores ArchipelagoNorth Atlantic Ocean). The sections sampled in the southeastern and central/western parts of the island record reversed and normal polarities, respectively. They indicate a mean palaeomagnetic pole (81.3 degrees N, 160.7 degrees E, K= 33 and A95= 3.4 degrees) with a latitude shallower than that expected from Geocentric Axial Dipole assumption, suggesting an effect of non-dipolar components of the Earth magnetic field. Virtual Geomagnetic Poles of eight flows and two dykes closely follow the contemporaneous records of the Cobb Mountain Subchron (ODP/DSDP programs) and constrain the age transition from reversed to normal polarity at ca. 1.207 +/- 0.017 Ma. Volcano flank instabilities, probably related to dyke emplacement along an NNWSSE direction, led to southwestward tilting of the lava pile towards the sea. Two spatially and temporally distinct dyke systems have been recognized on the island. The eastern is dominated by NNWSSE trending dykes emplaced before the end of the Matuyama Chron, whereas in the central/western parts the eruptive fissures oriented WNWESE controlled the westward growth of the S. Jorge Island during the Brunhes Chron. Both directions are consistent with the present-day regional stress conditions deduced from plate kinematics and tectonomorphology and suggest the emplacement of dykes along pre-existing fractures. The distinct timing and location of each dyke system likely results from a slight shift of the magmatic source.

Palaeomagnetic study of a subaerial volcanic ridge (São Jorge Island, Azores) for the cobb mountain subhron, volcano flank instability and tectonomagmatic implications

We present a palaeomagnetic study on 38 lava flows and 20 dykes encompassing the past 1.3 Myr on S. Jorge Island (Azores ArchipelagoNorth Atlantic Ocean). The sections sampled in the southeastern and central/western parts of the island record reversed and normal polarities, respectively. They indicate a mean palaeomagnetic pole (81.3 degrees N, 160.7 degrees E, K= 33 and A95= 3.4 degrees) with a latitude shallower than that expected from Geocentric Axial Dipole assumption, suggesting an effect of non-dipolar components of the Earth magnetic field. Virtual Geomagnetic Poles of eight flows and two dykes closely follow the contemporaneous records of the Cobb Mountain Subchron (ODP/DSDP programs) and constrain the age transition from reversed to normal polarity at ca. 1.207 +/- 0.017 Ma. Volcano flank instabilities, probably related to dyke emplacement along an NNWSSE direction, led to southwestward tilting of the lava pile towards the sea. Two spatially and temporally distinct dyke systems have been recognized on the island. The eastern is dominated by NNWSSE trending dykes emplaced before the end of the Matuyama Chron, whereas in the central/western parts the eruptive fissures oriented WNWESE controlled the westward growth of the S. Jorge Island during the Brunhes Chron. Both directions are consistent with the present-day regional stress conditions deduced from plate kinematics and tectonomorphology and suggest the emplacement of dykes along pre-existing fractures. The distinct timing and location of each dyke system likely results from a slight shift of the magmatic source.