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doi:10.2204/iodp.proc.320321.106.2010

Stratigraphic correlation and composite section

STMSL data were collected at 5 cm intervals from Holes U1334B and U1334C and compared to the WRMSL data obtained at 2.5 cm resolution from Hole U1334A. In this way we monitored drilling in Holes U1334B and U1334C in real time to recover and construct a stratigraphically complete composite section. Several intervals between Holes U1334A and U1334B did not overlap sufficiently to cover gaps between cores. Thus, coring of Hole U1334C was designed to recover the missing intervals, as well as to provide additional material for high-resolution studies. The coring effort in Hole U1334C was successful at covering gaps between cores in Holes U1334A and U1334B to ~222 m CCSF-A (Figs. F33, F34) and from 250 to 336 m CCSF-A, almost to the bottom of the section. Stratigraphic correlation between the three holes at Site U1334 was challenging in the light greenish gray interval (Cores 320-U1334A-15H through 22H, 320-U1334B-14H through 22H, and 320-U1334C-14H through 22H), which is characterized by very low magnetic susceptibilities, and in the bottom ~80 m, where coring with the XCB compromised core quality. The correlation between the three holes for the chosen parameters was adequate to good and, in some depth intervals, excellent. The gaps between successive cores in any of the holes are on the order of 1 to 2 m, with a maximum of ~4 m between Cores 320-U1334C-3H and 4H and ~14 m between Cores 320-U1334A-21H and 22H (see discussion below).

The correlation was refined once magnetic susceptibility and GRA density data were available at 2.5 cm resolution from the WRMSL, and NGR and color reflectance data were available from the NGR track and the SHMSL (see "Physical properties"). Visual inspection, comparison with core imagery, and biostratigraphic datums were used to establish and verify hole to hole correlation where track data lacked clearly identifiable features. Magnetic susceptibility and GRA density proved most useful for correlating between holes at Site U1334 (Figs. F33, F34). Features in the magnetic susceptibility and GRA density are well aligned between Holes U1334A–U1334C to ~155 m CCSF-A. From ~155 to ~222 m CCSF-A, GRA density data allow confident alignment of cores despite very low magnetic susceptibility values. In the interval from ~222 to ~250 m CCSF-A (Cores 320-U1334A-21H through 22H, 320-U1334B-20H through 22H, and 320-U1334C-20H through 22H), no features in any of the measurements available could be correlated. Several attempts to match the records did not provide convincing results. We suggest that this interval has to undergo detailed shore-based investigation to attempt the construction of a complete stratigraphic sequence. It cannot be ruled out that the apparent intensive geochemical alteration (see "Geochemistry" for discussion) in this interval has canceled out any signal detectable with the shipboard instrumentation. It is interesting to note that Cores 320-U1334A-22H, 320-U1334B-22H, and 320-U1334C-22H are the last APC cores in each hole and had to be recovered by overdrilling. The following cores (320-U1334A-23X, 320-U1334B-23X, and 320-U1334C-23X) are the first XCB cores and are therefore very likely to be affected by severe coring disturbance. In addition to switching to the XCB, a geochemical transition occurs in Cores 320-U1334A-23X and 320-U1334B-23X and between Cores 320-U1334C-22H and 23X (see "Geochemistry" for discussion). It is characterized by a color change and the reappearance of a good-quality magnetic susceptibility signal. This color transition occurs at substantially different CSF depths in the three holes cored at Site U1334 (onset at 204.7 m CSF in Hole U1334A, 211.8 m CSF in Hole U1334B, and between 208 and 209 m CSF in Hole U1334C). Aligning the color transition leads to a ~14 m core gap in Hole U1334A from 229 to 243 m CCSF-A (Figs. F33E, F34E). The top of Core 320-U1334A-22H exhibits unusually strong coring disturbance in the first two sections, suggesting that drilling conditions might have contributed to the coring gap. Biostratigraphic datum levels imply that the bottom of Core 320-U1334A-22H aligns with the middle of Cores 320-U1334B-22H and 320-U1334C-22H (compare Figs. F33E, F34E), suggesting that the geochemical transition does not occur at the same depth in the Site U1334 holes. A tentative comparison to the Site 1218 GRA record (Shipboard Scientific Party, 2002b) reveals no apparent correlation to Cores 320-U1334B-22H and 320-U1334C-22H and thus suggests disturbance or interruption of the stratigraphic sequence by undetected or unidentifiable causes. Low-amplitude variations of all track data, caused presumably by geochemical alteration, hinders construction of a complete stratigraphic section throughout this interval with the shipboard data available. We decided to append the splice in the interval between ~222 and ~250 m CCSF-A. Below this depth, magnetic susceptibility and GRA data correlate well and have been used to construct a robust composite section (cf. Figs. F33, F34).

Offsets and composite depths are listed in Table T30. Following construction of the composite depth section for Site U1334, a single spliced record was assembled for the aligned cores to Section 320-U1334B-30X-2 at 336.45 m CCSF-A (Fig. F33). The sections of core used for the splice are identified in Table T31 and displayed in Figures F33 and F34. The spliced composite section consists of almost equal proportions from all three holes.

We avoided intervals with significant disturbance or distortion and intervals where whole-round samples for interstitial water chemistry were taken (see "Paleomagnetism;" Table T11). The Site U1334 splice can be used as a sampling guide to recover a single sedimentary sequence from 0 to 336 m CCSF-A with gaps between 222 and 250 m CCSF-A, although it is advisable to overlap a few decimeters from different holes when sampling to accommodate anticipated ongoing development of the depth scale. Stretching and compression of sedimentary features in aligned cores indicates distortion of the cored sequence. Because much of the distortion occurs within individual cores on depth scales of <9 m, it was not possible to align every single feature in the magnetic susceptibility, GRA, NGR, and color reflectance records. However, at crossover points along the splice (Table T31), care was taken to align highly identifiable features from cores in each hole.

A growth factor of 1.16 is calculated by linear regression for all holes at Site U1334, indicating a 16% increase in CCSF-A relative to CSF depth (Fig. F35). We used this value to calculate the CCSF-B depth (see "Corrected core composite depth scale" in the "Methods" chapter) presented in Table T30 to calculate sedimentation rates and aid in the calculation of mass accumulation rates.

Sedimentation rates

All the principal biostratigraphic datums and a set of 61 paleomagnetic reversals (restricted to the APC-cored section of the site) are defined in Holes U1334A–U1334C (Table T32; see "Biostratigraphy" and "Paleomagnetism") and were used in establishing age control (Fig. F14). Only the paleomagnetic reversals were used to calculate the average linear sedimentation rates (LSRs) for the APC section of Site U1334 from the CCSF-B depth scale, as depicted in Figure F14. In XCB cores, all available biostratigraphic datums were used to calculate the average LSRs.

The LSR at Site U1334 in the nannofossil oozes and chalks of lithologic Units II and III between the basement and the lower Oligocene section are ~8 m/m.y., increase in the lower Oligocene to 24 m/m.y., and then decrease throughout the Oligocene and Miocene to 4 m/m.y. (Fig. F14).

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