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doi:10.2204/iodp.proc.320321.103.2010

Stratigraphic correlation and composite section

All cores from Site U1331 were initially depth-shifted on the basis of magnetic susceptibility and GRA bulk density data collected at 5 cm resolution with the Special Task Multisensor Logger (STMSL; "fast track") soon after recovery. Data from the STMSL were used to monitor and direct coring in Holes U1331B and U1331C in real time to provide complete recovery and construction of the composite section. Several intervals between Holes U1331A and U1331B did not overlap sufficiently to cover gaps between cores. Thus, coring of Hole U1331C was designed to cover the missing intervals, as well as to provide additional material for high-resolution studies. The coring effort in Hole U1331C was successful at covering gaps between cores in Holes U1331A and U1331B to ~134 m CSF (149 m CCSF-A). Below 149 m CCSF-A it was only possible to tentatively correlate features in the track data to Core 320-U1331A-17X for a total composite section of ~172 m (Fig. F32). The correlation between the three holes for the chosen parameters was good or, in some depth intervals, excellent. The gaps between successive cores are on the order of 1 to 2 m, with a maximum of 2.5 m.

The correlation was refined once magnetic susceptibiliy and GRA density were available at 2.5 cm resolution from the WRMSL. NGR and color reflectance data were also available from the NGR track and the SHMSL (see "Physical properties"). Magnetic susceptibility proved most useful for correlating between holes at Site U1331. Features in the magnetic susceptibility are well aligned among Holes U1331A–U1331C to 140 m CCSF-A, as these cores were not significantly stretched, squeezed, or disturbed during the coring process (Fig. F32). Offsets and composite depths are listed in Table T38. Good weather conditions during the entire occupation of Site U1331 led to excellent recovery and fairly small coring gaps in all three holes. Chert intervals caused poor core recovery below ~160 m CSF.

Following construction of the composite depth section at Site U1331, a single spliced record was assembled for the aligned cores to 172 m CCSF-A. Sections of core used for the splice are identified in Table T39. The spliced composite section mainly consists of sections from Holes U1331A and U1331B. Four segments from Hole U1331C were needed in the splice to fill core breaks in Holes U1331A and U1331B (Fig. F32). Cores from Site U1331 provide a continuous stratigraphic sequence to ~149 m CCSF-A with four potential gaps at ~39.7, ~45.8, ~115.0, and ~143.7 m CCSF-A. Preliminary paleomagnetic reversal stratigraphy (see "Paleomagnetism") suggests that the hole to hole correlation based on magnetic susceptibility data in the interval between ~32.6 and ~44.0 m CCSF-A might be affected by the occurrence of a sharp erosional contact in the radiolarian ooze that is overlain by multiple graded beds with multicolored coarse sand grains. This interval is easily recognized as a sharp peak in the magnetic susceptibility data at 38.45 m CCSF-A at 320-U1331B-4H-5, 90 cm, and 320-U1331C-4H-6, 34 cm. Below ~150 m CCSF-A the shipboard composite splice is not well constrained and might need postcruise refinement. We avoided intervals with significant disturbance or distortion and intervals where whole-round samples for interstitial water chemistry and microbiology were taken (see "Paleomagnetism;" Table T13).

The Site U1331 splice can be used as a sampling guide to recover a single sedimentary sequence between 0 and 172 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 T39), care was taken to align highly identifiable features from cores in each hole.

A growth factor of 1.10 is calculated by linear regression for all holes at Site U1331, indicating a 10% increase in CCSF-A relative to CSF depth (Fig. F33). We used this value to calculate the CCSF-B (see "Corrected core composite depth scale" in the "Methods" chapter) depths presented in Table T38 to aid in the calculation of mass accumulation rates.

Sedimentation rates

All the principal biostratigraphies, plus a set of ~30 paleomagnetic reversals, are defined in Holes U1331A–U1331C (Table T40; see "Biostratigraphy" and "Paleomagnetism"). Paleomagnetic reversals are used to calculate the average linear sedimentation rates (LSRs) for Site U1331 using the CCSF-B scale through most of the section. Calcareous nannofossils, foraminifers, and radiolarians are present throughout the entire section and were used in addition to the magnetostratigraphy in establishing age control (Fig. F13). The LSR at Site U1331 in the radiolarian oozes of lithologic Units II and III decreases from ~16 m/m.y. in the middle Eocene part of the section to 5.5 m/m.y. in the late Eocene and to 3.2 m/m.y. in the early Oligocene (Fig. F13). Based on a simple linear interpolation from the sediment surface (assumed to be zero age) and Chron C2n (Table T40), the clays of lithologic Unit I (see "Lithostratigraphy") have a LSR of 2.7 m/m.y.

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