RESULTS

Downhole Measurements

Downhole measurements reveal a distinct anomaly centered at ~975.5 mbsf, which consists of local maxima in total gamma-ray counts, SiO₂, Al₂O₃, and FeO, with local minima in sonic velocity, density, resistivity, and CaCO₃ values (Fig. 4). These co-varying extreme logging values extend for less than a meter. There is also a distinct change in mean log values below and above the anomaly (e.g., natural gamma ray, resistivity, velocity, Al₂O₃). This abrupt change in logging values extends uniformly for 10-15 m above and below the anomaly.

The FMS image reveals a 97-cm-thick interval (975.09-976.06 mbsf; Fig. 4, Fig. 5) of relatively low resistivities (dark gray-black). The anomaly in standard downhole logs is centered on this interval (~975.5 mbsf; Fig. 4). The darkest area (lowest resistivity) in the FMS image from 975.69 to 976.06 mbsf (37 cm thick) lies at the base of this generally low resistivity zone.

The calibrated FMS resistivity curve reveals a 1.5-2.0 m transition zone between the minimum resistivity values (<2 ohm-m) at the log anomaly, which grade up to greater than 3.0 ohm-m at ~974 mbsf without returning to pre-anomaly values (Fig. 5). The calibrated FMS resistivity curve also shows four distinct minima within the 37-cm-thick, low resistivity zone.

Core Logs

Two characteristic low GRAPE density peaks were identified at 975.85 and 976.02 mbsf within a narrow zone of nearly constant density from 975.6 to 976.25 mbsf (Fig. 5). The density values above and below this zone show much more variability, especially the overlying interval.

The calcium intensities (in cps) reflect the relative variation of this element. The Ca intensities are relatively constant from 976.06 to 977 mbsf. An abrupt decrease is at 976.05 mbsf, and a transitional zone back to higher values starts at 975.8 mbsf (Fig. 5). Both the GRAPE density data and the Ca intensities are more variable above 975.6 mbsf than below 976.2 mbsf. The iron intensities (Fe, cps) are negatively correlated to the calcium values. Between 976.06 and 977 mbsf there are more variations in the Fe intensities than within the interval characterized by low-Ca intensities. Three Fe intensity peaks are at 975.85, 975.94, and 976.05 mbsf; the sharp peak at 975.94 mbsf is the most prominent one. The magnetic susceptibility values generally correlate well with the Fe intensities (Fig. 5). Sharp peaks are easy to identify at 975.94 and 976.05 mbsf.

Core Log Integration and Interpretation

The correlation of core data to downhole measurements is essential for holes with incomplete recovery. Fortunately, at Site 999 more than three quarters of the drilled hole is represented by cores (76.1% overall and 87.7% over the LPTM interval). Log-to-core correlations for the LPTM interval at Site 999 are most easily obtained by first matching the general character of the largest anomaly in the data from downhole measurements to core-log measurements and interpreting the logging response in terms of varying lithology and porosity.

The co-varying, extreme downhole logging values that extend for less than a meter at ~975.5 mbsf (Fig. 4) are indicative of a much less indurated zone with relatively high clay content. A significant anomaly is also apparent in the core log data. The minimum in Ca counts correspond to maxima in Fe counts and magnetic susceptibility. This response is also indicative of a carbonate-poor, clay-rich interval (original core depth = 975.23-975.65 mbsf). It is clear that there is a dramatic anomaly in measurements common to both downhole and core logs (e.g., CaCO₃ vs. Ca, FeO vs. Fe; Fig. 4) as well as co-varying measurements (e.g., high Fe counts and gamma-ray counts both responding to increased clay content). The nearly identical match in downhole log depth and core depth makes this an unequivocal correlation. In general, all these logging responses are consistent with the recovery of a claystone layer (975.15-975.7 mbsf core depth) within bioturbated, clayey, calcareous, mixed sediment (Sigurdsson, Leckie, Acton, et al., 1997).

Both the core logs and downhole logs show that an abrupt lithologic, physical, and chemical transition occurred in association with the recovered claystone. The identification of this claystone as the LPTM claystone is confirmed by shipboard biostratigraphy and, more critically, by the distinct negative ¹³C excursion that characterizes the LPTM at other drill sites worldwide (Bralower et al., 1997). The LPTM claystone (or LPTM clay) is defined as the interval containing the anomalous ¹³C minima. The ¹³C anomaly has an abrupt onset as it does at other sites, and this marks the base of the LPTM clay. The depth of the LPTM clay base is most accurately determined by the FMS in cases where recovery is not 100% and is well defined at 976.06 mbsf by the abrupt onset of low resistivities (Fig. 4, Fig. 5). Thus, core log depths are shifted by 40 cm to align the onset of core log anomalies (e.g., Ca, Fe) and the ¹³C anomaly to the base of the FMS-defined LPTM claystone interval.

The generally good correlation between GRAPE density, carbonate content (Ca, cps), and high FMS resistivity reflects more indurated, less porous nature of the higher carbonate sediment. In contrast, the correlation of iron intensities, magnetic susceptibility, and lower FMS resistivity characterizes the relatively soft, highly porous, clayey sediments with relatively high amounts of pore water.

The LPTM anomaly in standard downhole logs is centered on the 97-cm-thick zone (975.09-976.06 mbsf, Fig. 5) of relatively low FMS resistivity, and encompasses the 55-cm-thick claystone recovered in core interval 165-999B-51R-5, 75-150 cm (Sigurdsson, Leckie, Acton, et al., 1997). In addition, XRF Ca and Fe counts remain both below and above pre-LPTM levels throughout this interval, respectively. The darkest area (lowest resistivity) in the FMS image from 975.69 to 976.06 mbsf (37 cm thick) lies at the base of this generally low-resistivity zone and correlates to the negative ¹³C excursion, Ca minimum, Fe and magnetic susceptibility maximum, and an interval of less variable GRAPE density values (Fig. 5).

The nearly constant minimum Ca intensities and maximum Fe intensities, as well as the extreme ¹³C values extend over an ~25 cm interval. Thus, we define the thickness of the LPTM clay as 25-37 cm at the base of a broader region (97 cm) referred to as the LPTM interval where carbonate content remains relatively low. The LPTM consists of a greenish gray, partly fine-laminated claystone (see core image, Fig. 5). Comparison with shipboard carbonate data obtained from analyses of discrete samples (Sigurdsson, Leckie, Acton, et al., 1997 and Bralower et al., 1997) with measured Ca intensity variations of ~200 to >8000 cps reflect carbonate contents from <0.5% (in the LPTM clay) to up to 68% CaCO₃ (in the post-LPTM interval). The abrupt onset of the FMS resistivity anomaly followed by a transition zone is similar to the pattern displayed by Ca, Fe, and magnetic susceptibility values (Fig. 4, Fig. 5).

Macroscopically we identified three mm-thick ash layers within the LPTM clay (layers "Ash 1" to "Ash 3" in fig. 5 of Bralower et al., 1997). The calibrated FMS resistivity curve shows four distinct minima within the LPTM claystone. Three of these minima are interpreted to be caused by the three observed mm-thick volcanic ash layers with characteristic low resistivity values. The local maxima in gamma-ray counts marking the LPTM anomaly is dominated by large concentrations of thorium (rather than potassium or uranium), which supports a volcanic contribution to the claystone. These low-resistivity "ash peaks" of the FMS curve have counterparts in the GRAPE density, magnetic susceptibility, and iron intensity curves. The two minima in GRAPE density appear correlated to "Ash 3" and "Ash 1." In the Fe intensity and magnetic susceptibility curves we identified all three ash layers: "Ash 3" is more pronounced in the Fe and "Ash 2" and "Ash 1" in the magnetic susceptibility curve. The fourth resistivity minima in the FMS resistivity curve at 975.75 mbsf may indicate a fourth ash layer, which is not lithologically apparent in the cores.

Downhole Measurements

Downhole measurements (note: geochemical data were not collected at Site 1001) reveal a relatively broad and less distinct anomaly compared to that observed at Site 999. The anomaly is best represented by a maximum in natural gamma-ray counts and minimum sonic velocity and FMS resistivity (Fig. 7) and is centered at ~240-241 mbsf.

The absolute minimum in FMS resistivity represented by the darkest area in the FMS image extends for only 9.2 cm, from 240.775 to 240.864 mbsf (Fig. 6). The broad anomaly in standard downhole logs is centered on this interval (~240.8 mbsf, Fig. 7).

The calibrated FMS resistivity curve displays distinctly different mean values below and above the anomaly (Fig. 7) and these trends extend uniformly for 10-15 m. In addition, the calibrated FMS resistivity curve reveals a 3-m-long transition prior to the anomaly rather than post-LPTM as observed at Site 999 (Fig. 5).

Core Logs

The magnetic susceptibility and the calcium intensity curves for both LPTM Cores 165-1001A-27R and 165-1001B-6R are negatively correlated (Fig. 8). In general, the magnetic susceptibility values are relatively low, but some zones have magnetic susceptibility values up to 200 SI. The calcium intensities show a range from ~100 to 10,000 cps. The highest Ca intensity values found between 239.45 and 240.3 mbsf of Section 165-1001A-27R-3 could not be identified in Core 165-1001B-6R. The core data from 238.55 to 239.45 mbsf in Hole 1001A and 237.8 to 238.4 mbsf in Hole 1001B show generally low Ca intensities and higher susceptibility values.

Core Correlation and Core Log Integration

Core recovery at Site 1001 was considerably poorer than at Site 999 (54.7% in Hole 1001A, 66.8% in Hole 1001B). The cores covering the LPTM interval in Holes 1001A and 1001B are only 5.34 m (55% recovery) and 4.23 m long (44% recovery), respectively. The correlation of core data to downhole measurements is essential for reconstructing a single section from two adjacent cores in holes with such poor recovery. Reconstruction of a more complete section from Holes 1001A and 1001B was accomplished using FMS, core log data, and matching of distinguishing features, such as ash layers. Log-to-core correlations for the LPTM interval in particular are initiated by identifying log responses that distinguished the LPTM so clearly at Site 999.

The logging response in the upper Paleocene-lower Eocene portion of Hole 1001A can be interpreted in terms of the varying proportions of the primary constituents in the recovered samples (i.e., clay, chalk, chert, and varying amounts of volcanic ash) and to their relative porosities. The poor recovery is probably related to the generally higher frequency of lithologic alternations and the presence of chert interbedded with much less indurated material (i.e., clay rather than claystone, chalk rather than limestone). The magnetic susceptibility is generally low because of the relatively high carbonate content (up to 82.9%, Bralower et al., 1997), but the more clayey intervals, the LPTM clay, and altered volcanic ash horizons show magnetic susceptibility maxima. The wide variety of different lithologies ranging from pure limestones to pure claystones with transitional mixed sediment types is mirrored by the large range of Ca and Fe intensities and is apparent on the FMS image.

We identified characteristic altered volcanic ash or clay layers in the cores of the two holes both macroscopically and in detail by their typical magnetic susceptibility peak pattern; these horizons are labeled "D" to "P" in Figure 8 (same scheme as in Bralower et al., 1997). Some of these layers are up to several centimeters thick and can be used as index layers (e.g., the double peak "E," the typically greenish ash layer "F," and the "triple" "N1" to "N3" could be identified in both holes). But in detail and for the complete cores it was quite difficult to correlate core piece by core piece between the two holes. This was only possible using all the different data sets available. In Figure 9 the magnetic susceptibility curves are shown for Cores 165-1001A-27R and 165-1001B-6R. For correlation purposes we shifted the index layers "D," "E," "F," "G," and "N1" to "N3" to a common depth. The resulting gaps in core range from several cm to up to 90 cm (Fig. 9). Afterward, we examined the cores directly for potential gaps and drilling disturbances to make sure core sections could actually be missing at these positions.

The next step was to assign the resulting core and data pieces to the continuous FMS image and resistivity curve. The index clay/altered ash layers "E" to "K" and especially characteristic series of peaks (e.g., "E" to "G") were used to identify corresponding index horizons in each hole using different physical and chemical parameters. Magnetic susceptibility and iron intensity show characteristic maxima, and Ca intensity, GRAPE density, and FMS resistivity show distinct minima (Fig. 6). Carbonate-rich, high-resistivity layers were also used for correlating the core and downhole logs. This correlation indicates that the highest resistivity layers (very light gray to white) were not recovered. These horizons probably represent chert and/or silicified limestone layers.

In general there is a good correlation between Ca intensities and GRAPE densities. For example, in the upper part of the 4 m sections shown in Figure 6 (238-238.2 mbsf), where these two curves are almost parallel and just minor variations are found. This is different downhole (e.g., 240.55-240.8 mbsf), where single pronounced GRAPE density peaks have no equivalent within the Ca intensity curve. This pattern appears to be related to the presence of chert, characterized by high-resistivity values in these intervals. As soon as light gray to white colors in the FMS image (and therefore the chert layers) disappear, the Ca intensity and GRAPE density curves correlate quite well (e.g., 240.86-241.1 mbsf).

There is an overall good correlation between the magnetic susceptibility and iron intensity curves. The double peak "E" in the magnetic susceptibility curve (at ~238.3 mbsf) is irregular in the Fe intensity curve: the upper peak is more pronounced compared to the lower one. The LPTM clay shows a characteristic minimum in the FMS resistivity curve. The Fe intensity shows a differentiation of layer "I," which is not visible in the magnetic susceptibility curve.

The anomaly in sonic velocity and gamma-ray counts is centered on the absolute minimum in FMS resistivity, which extends for only 9.2 cm (240.775-240.867 mbsf; Fig. 6, Fig. 7). The co-varying extreme logging values indicate a zone with relatively high clay content and low induration and are interpreted as a clay and/or claystone layer within chalk and mixed sedimentary rock with clay. The 60 cm (Hole 1001B) to 90 cm (Hole 1001A) thick intervals with generally low Ca intensities and higher magnetic susceptibility values are, as at Site 999, expected to contain the LPTM event. Shipboard biostratigraphy and the fortuitous recovery of a claystone unit containing a negative carbon isotope excursion (Bralower et al., 1997) locate the LPTM at ~239.5 mbsf (core depth) at the base of index layer "I" (Fig. 8, dotted region; Fig. 6). After the core composite was created as described above, the section of the core containing the ¹³C anomaly corresponded to the base of the LPTM clay at 240.86 mbsf as defined by the FMS image without any further depth shifting of the core. In other words, depth shifts and composite core construction were accomplished independently of matching the ¹³C anomaly to the FMS defined claystone, further supporting the interpretation of the FMS resistivity minimum as the LPTM clay. The extreme minimum ¹³C and uniformly low Ca intensities overlap the 9.2 cm FMS resistivity minimum and extend for a total of 30 cm, which we interpret as the LPTM clay thickness at Site 1001.