Marie-Gabrielle Moreau

Hettangian and Sinemurian magnetostratigraphy from Paris Basin

A long core (1110 m) drilled at Montcornet (northeastern Paris Basin) provides early Jurassic magnetostratigraphic data coupled with biochronological control. About 600 paleomagnetic samples were obtained from a 148-m-thick series of Hettangian and Sinemurian rocks. A composite demagnetization using thermal (up to 300°C) followed by alternating field technique (up to 100 mT) is used to separate the magnetic components. A low unblocking temperature component (<250°C) with an inclination of about 65° is interpreted as a present-day field overprint. The characteristic remanent component with both normal and reversed antipodal directions is then isolated between 5 and 70 mT. Eighty-one polarity intervals are recognized in this study. The higher reversal frequency of the late Hettangian/early Sinemurian time interval contrasts with a lower reversal rate observed in the rest of the early Liassic. A rough mean estimate of about 5 reversals/m.y. can be proposed for the earliest Jurassic. These results represent a significant contribution to the magnetic polarity reversal timescale for a time interval hitherto poorly known and add to the magnetic reversal frequency curve of the last 350 m.y.

Biostratigraphy and magnetostratigraphy of the Cretaceous/Tertiary Sopelana section (Basque country)

Remarkably thick sequences of upper Cretaceous and lower Tertiary sediments outcrop on the northern coast of the Basque country. We have sampled the well exposed section along the Sopelana beach over 150 m, spanning roughly from middle Maastrichtian (71 Ma) to lower Paleocene (66 Ma), for both biostratigraphic and magnetostratigraphic studies. Thermal demagnetization of ∼ 400 specimens revealed both normal and reversed recent overprints unblocked below 200°C, and both normal and reversed characteristic directions at higher temperatures (200–450°C in the grey limestones; 350–550°C in the redder marls and limestones). Thermal and AF demagnetization and monitoring of weak field susceptibility are consistent with some form of (titano-) magnetite as the main carrier of magnetization. 259 demagnetization diagrams yielded two nearly antipodal clusters of directions, which are still polluted by some 20% remaining recent overprint. The overall mean direction in stratigraphic coordinates is D = 356°, I = 51° (α = 3°), consistent with what is expected for Eurasia at KTB time. Magnetic stratigraphy outlines a succession of eight polarity intervals and is rather straightforward, except for two highly complex zones (52 to 40 m, and 28 to 26 m below the KTB) where the magnetic polarity appears to flip at an unreasonable rate. Biostratigraphic check allows unambiguous assignment of several chrons, with the recognition of the Gansserina gansseri, Abathomphalus mayaroensis, “Globigerina” eugubina, Eoglobigerina edita ( = E. pseudobulloides) and E. trinidadensis zones. The magnetostratigraphic section therefore begins in chron 31R and ends in 29N. Further detailed study of the complex zones reveals that specimens there have a distinct magnetic behaviour: susceptibility increases beyond 400°C, indicating instability and mineralogical change, intensities are higher than elsewhere and more scattered, and a higher unblocking temperature/higher coercivity magnetic component is uncovered. This component, which is absent from the rest of the section, could be an early chemical remagnetization. Indeed the complex zones contain larger amounts of chlorite, a sign of more intense diagenesis. We have therefore discarded samples showing this anomalous behaviour, and have retained only the 223 samples where demagnetization was achieved by 450°C. The thicker, lower complex zone reduces to a reversed chron which we propose to correlate with 30R. We have identified an extra short reversed event within chron 30N, for which independent support is found in DSDP Sites 524 and 577a, and possibly in Gubbio. Finally, the high-sedimentation rate section at Sopelana (25 m/Ma in the Maastrichtian, 7 m/Ma in the Danian) displays one of the best resolved on-land magnetostratigraphies around KTB time. The two complex zones of early remagnetization form an “echo” which may have been generated some 100 kyr after deposition of the sediments. The KTB itself occurs approximately halfway to 3/5 up 29R in terms of time.

On the possibility of a widespread remagnetization of pre-Oligocene rocks from Northeast Japan and the Miocene rotational opening of the Japan Sea

Abstract We present new paleomagnetic results from the well dated Miyako Cretaceous sediments (100–110 Ma) from Northeast Japan. These results, combined with those of Tosha [1], yield an in-situ characteristic directionD = 321°,I = 54.5° (α95 = 4.5°),N = 14 sites; reduced to a reference point at 40°N, 142°E). This direction is found to coincide with that of most older plutonic and sedimentary rocks of Devonian to lower Cretaceous age. It is also identical with the westerly pre-folding direction which is preserved in many Oligocene (20–40 Ma) formations from Northeast Japan [1,2]. In contrast, all recent formations (0–17 Ma) have been magnetized in the direction of the present axial dipole field. Only the Oligocene and Miocene results appear to be primary, or at least pre-folding. The Miyako sulfide-bearing sediments and lower Cretaceous (110–125 Ma) magnetite-bearing granites could either still bear a primary magnetization or be completely remagnetized by a low temperature chemical event. Evidence for such events is now found in many places, and as close as South Korea. Available data constrain the Oligo-Miocene history of Northeast Japan and indicate at least20/30° counterclockwise rotation with respect to mainland Asia during the opening of the Sea of Japan. On the other hand, the pre-40 Ma history of Northeast Japan is not well constrained and three models are proposed which are compatible with various interpretations of the data. None of them can presently document pro-Oligocene motion of Northeast Japan with respect to Asia. The most “economical” model implies widespread remagnetization. We conclude that, because of the scarcity of well tested primary magnetization directions, the classical bending of the Japanese Islands rests on weaker grounds than generally realized and that no pre-40 Ma apparent polar wander path of the Japanese Islands can safely be proposed.

Mechanisms of remanent magnetization acquisition in marl and limestone alternations. Case study: Upper Cretaceous (Chron 31–30), Sopelana, Basque Country

Abstract The marl and limestone (M/L) alternations of the Cretaceous/Tertiary section in Sopelana in the Basque Country provide a good example of two chemical magnetizations which have been acquired at different times. A detailed study was conducted on a core which contained an unusual paleomagnetic record in the vicinity of Chron 30 R. The magnetic polarity switches far more rapidly than expected from the reversal time scale. A 10 cm sampling interval provided, on average, six samples in each M/L couplet that was identified from a curve of carbonate content. The lithostratigraphic NRM pattern is related to the lithology. The intervals (three to four M/L couplets) along which the contrast in the carbonate content between the limestone beds and the marly layers is low ( 20%) are characterized, in normal polarity intervals, by the expected NRM in the limestone beds and a reverse or blurred NRM in the marly layers. The dominant magnetic carrier in this complex zone is diagenetic hematite, whereas titanomagnetite was identified below this interval. The characteristics of the detrital minerals indicate an erosion of a mature landscape (dominance of weathered ilmenite), probably related to deformation in the Pyrenees. Diagenetic hematite is characterized by two families. The first (microgranular size, only tenths of a micrometre) is present in all the samples and is interpreted as a very early chemical remanent magnetization (CRM) acquired in the oxic zone that probably extended over a few decimetres below the sediment surface. The second is restricted to the marly layers of intervals with large carbonate content contrasts in the M/L couplets. The grains in this family are larger (very fine, some micrometres in size) and carry a reverse or blurred polarity in a normal polarity zone. It is proposed that this chemical remagnetization was acquired during the circulation of oxygenated fluids. The timing for this process is at least 2 m.y. after deposition, and more likely > 7 m.y. The second diagenetic family grew at least 90 m below the seafloor, and its distribution shows that the marly layers in intervals with large contrasts in the carbonate content between the marls and the limestones were preferential drains along which circulation was favoured. This suggests that, although the porosity was still high (40–60%), the diagenetic evolution in the M/L alternations was already being expressed by different physical properties. Limestone beds and marly layers in intervals with low carbonate content contrasts in the M/L couplets were impermeable to fluid circulation and the early diagenetic CRM signal has been preserved. In contrast, marls in intervals with a large carbonate content contrast in the M/L couplets were sites of later diagenesis and their early CRM was overprinted by another magnetization.

Magnetobiostratigraphy of a Late Miocene section from the Moroccan Atlantic margin

Abstract Paleomagnetic study of a Late Miocene section from Atlantic Morocco correlated with detailed biostratigraphic data suggests an assignment of the observed polarity sequence to the uppermost part of chron 7, to entire chron 6 and to the beginning of chron 5. Comparison with time equivalent sections in the Southern Hemisphere does not suggest noticeable delay in long faunal migration (between the Northern and Southern Hemisphere) during the late Miocene. Thus the postulation of a delay in arrival of species in the Eastern Mediterranean at the base of the Messinnian has to be explained by the presence of selective barriers which could have been located either between the Mediterranean and Atlantic or between the Western and Eastern Mediterranean in the basins delineated during the final evolution of the Tethys. This study also indicates a possible application of biomagnetostratigraphy to the reconstruction of basin interconnections.

Pliensbachian magnetostratigraphy: new data from Paris Basin (France)

Sampling of an industrial drill string from the northeastern Paris Basin (Montcornet, France) provides early Jurassic magnetostratigraphic data coupled with biochronological control. About 375 paleomagnetic samples were obtained from a 145 m thick series of Pliensbachian rocks. A composite demagnetization thermal up to 300°C and an alternating field up to 80 mT were used to separate the magnetic components. A low unblocking temperature component (<250°C) with an inclination of about 64° is interpreted as a present-day field overprint. The characteristic remanent component with both normal and reversed antipodal directions was isolated between 5 and 50 mT. Twenty-nine polarity intervals were recognized. Correlation of these new results from the Paris Basin with data from the Breggia Gorge section (Ticino, southern Alps, Switzerland), which is generally considered as the reference section for Pliensbachian magnetostratigraphy, reveals almost identical patterns of magnetic polarity reversals. However, the correlation implies significant paleontological age discrepancies. Revised age assignments of biostratigraphic data of Breggia as well as an objective evaluation of the uncertainties on zonal boundaries in both Breggia and Moncornet resolve the initial discrepancies between magnetostratigraphic correlations and biostratigraphic ages. Hence, the sequence of magnetic reversals is significantly strengthened and the age calibration is notably improved for the Pliensbachian, a stage for which sections combining adequate magnetic signal and biostratigraphic constraints are still very few.