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doi:10.2204/iodp.proc.308.209.2009

Results

Paleomagnetic analysis of Brazos-Trinity Basin IV (Holes U1319A and U1320A)

Individual sample AF demagnetization curves can be divided into three general direction intervals (Figs. F2 and F3):

1. Low-field steps (0–30 mT) show an overprint due to the presence of a near vertical drilling-induced remanence (DIRM),
2. Intermediate-field steps (30–60 mT) represent characteristic remanent magnetization (ChRM) directions and point toward the origin of the coordinate system.
3. High-field steps (~60–100 mT) can develop a strong horizontal deviation (see also Fu et al., 2008). Typically after the 60 mT AF demagnetization treatment, a strong increase of overall remanence attributed to gyroremanence can be observed in numerous samples. According to Fu et al. (2008), these depth intervals have a mixed magnetomineralogy of (titano)magnetite and greigite or even a dominant greigite composition.

Figure F4A compares NRM intensities of Hole U1319A for the 0 mT (light gray curve) and 30 mT steps (dark gray curve). It becomes clear that the 0 mT step (which is actually equivalent to the original NRM before any demagnetization) includes a relatively large contribution from DIRM, which was removed from the signal after the 30 mT treatment. Figure F4B shows the inclination record of Hole U1319A. Values (Table T1) average ~45°, an expected inclination for this latitude (27°16′N) and time interval. The inclination signal appears to be much noisier below ~60 mbsf, coinciding with the change from advanced piston coring to the extended core barrel drilling technique. This effect can also be seen in the azimuth-corrected ChRM declination signal (Fig. F4C) which averages, ~0°, as expected. Nevertheless, because of the higher noise level in particular below 100 mbsf, the mean declination value D_tot = 345.9° (Table T1) deviates ~14° from the expected GAD value. To test the influence of the drilling disturbance on the error in declination we calculated the mean value for the obviously less disturbed sediment section between 0 and 100 mbsf and the section below separately. This results in a mean declination at Site U1319 of 1.9° for the upper and 346.2° (–13.8°) for the lower drill section. We can therefore conclude that the lower drill cores are heavily disturbed, whereas for the upper drill section the deviation from the theoretical assumed declination is <2°.

The NRM intensity signal of Hole U1320A (Fig. F5A) is patchy and incomplete because of the relatively poor core recovery in certain depth horizons. But a correlation of Hole U1320A to U1319A is still possible via a combination of the stratigraphic core description and the magnetic susceptibility and NRM intensity records as already outlined in the “Methods” chapter. Inclinations (Fig. F5B) and declinations (Fig. F5C) in Hole U1320A generally show nearly the same average values (Table T1) as in Hole U1319A but exhibit locally higher noise levels, which particularly affect the declination record between 120 and 165 mbsf as well as between 185 and 230 mbsf.

Paleomagnetic analysis of the Ursa Basin (Holes U1322B and U1324B)

NRM intensity records of Holes U1322B and U1324B (Figs. F6A, F7A) correlate reasonably well with each other up to the lithologic depth horizon marked by seismic layer S30 (defined in Expedition 308 Scientists, 2005). The values are rather low within the first lithologic layer (between the seafloor and seismic reflector S10). Below this depth, NRM intensity values are much higher for both Ursa sites and also show a higher signal variance comparable to NRM intensities for Brazos-Trinity sites. In contrast, the Ursa sites comprise generally tenfold higher NRM intensities than the Brazos-Trinity sites.

The directional information for Holes U1322B and U1324B is more varied than for the Brazos-Trinity drill sites presumably because of the complicated levee-channel paleosedimentation system and the postdepositional tectonics (cf. “Expedition 308 summary” chapter and Fig. F33 therein) as well as to higher drilling disturbances (Expedition 308 Scientists, 2005). When using the same declination correction technique as described above, the data still yield reasonable results (Table T1) for Hole U1324B and for most sections of Hole U1322B. For the latter site, the interval between ~20 and 120 mbsf seems to be more disturbed than the rest of the sedimentary succession and therefore causes a much larger deviation from 0° than observed at Site U1324. Unfortunately the drilling disturbance for Site U1322 seems to be too important to be able to correct sufficiently for this azimuth disorientation. Nevertheless, the NRM intensity signals between both Ursa sites seem to correlate reasonably well so that comparison between both signals is still possible since Hole U1324B may be used as a reference site for Hole U1322B. The inclination of Hole U1322B (latitude 28°06′N) is on average lower than expected with values ~32°, whereas the inclination of Hole U1324B (latitude 28°05′N) averages ~68°, much steeper than expected for this region.

SEM analyses of the magnetic mineral assemblage (Hole U1319A)

SEM analyses were performed on dispersed samples of magnetic extracts from core catcher Samples 308-U1319A-2H-CC (14.4 mbsf) and 3H-CC (23.8 mbsf) of Hole U1319A. Micrographs (Fig. F8) and element spectra (Table T3) show detrital magnetite (particles 7, 10, 14, 15, and 18 in Fig. F8 and Table T3), titanomagnetite (particles 4, 8, 9, 14, and 17 in Fig. F8 and Table T3), and Tirich hemoilmenite (particles 5 and 6 in Fig. F8 and Table T3) within a 0.1–10 µm grain size range. The presumably terrigenous magnetite grains have irregular fragmental or anhedral shapes and weathered, slightly knobby surfaces (e.g., Freeman, 1986). Detrital titanomagnetites exhibit a fragmented appearance with smooth conchoidal surfaces and sharp curved edges related to shrinkage cracks (Petersen and Vali, 1987). The (semi)quantitative EDS spectra typically show lower Fe:Ti ratios for hemoilmenites than for titanomagnetites; these mineral phases of the magnetite-ulvöspinel solid solution series can be identified by the intensity of the characteristic Fe-Kα and Ti-Kα lines as reported by Dillon and Franke (2009). Some Fe oxides show signs of reductive dissolution or even mixed oxide/sulfide elemental compositions (e.g., particle 16 in Fig. F8 and Table T3). Clusters of fine-grained Fe sulfides were observed next to much coarser FeTi oxides. Their respective element spectra identify these mineral phases as pyrite (FeS₂) and greigite (Fe₃S₄). Greigite occurs in homogeneous clusters composed of particles a few nanometers in size (particles 1–3, 11, and 13 in Fig. F8 and Table T3), whereas clusters of pyrite particles (e.g., particle 12 in Fig. F8 and Table T3) generally consist of larger grains with varying sizes (~1 µm down to a few hundred nanometers). The observed pyrite crystals have typical octahedral shapes and show a brighter contrast in BSE micrographs.

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