DOWNHOLE MEASUREMENTS

Downhole measurements in Hole 1149B were made during Leg 185 after completion of drilling with the RCB. Five logging runs were performed, consisting of one pass with the triple combo tool string, two passes with the geochemical tool string (GLT), and two passes with the Formation MicroScanner (FMS)/sonic tool string. The triple combo tool string provided the most physically complete logging run (342 m of section) from ~10 m above the volcanic basement contact (~410 mbsf; Core 185-1149B-29R) to within the clay-volcanic ash section (~63 mbsf; Core 185-1149A-8H). The subsequent logs were limited by additional fill in the bottom of the hole and/or by a 20-m-thick section of tight hole in the pelagic clays (Cores 185-1149A-16H to 18H) just above the uppermost cherts. It is likely that this tight section of hole resulted from overburden pressure that forced the incompetent clays into the open hole rather than from "swelling clays" caused by an uptake of water within the clay matrix. A more detailed account of the five logging runs in Hole 1149B is provided below.

The triple combo tool string was deployed first and measured porosity, density, resistivity, and natural gamma-ray emissions to within ~10 m above the volcanic basement contact and 45 m above the total hole depth. The tool string included the hostile environment natural gamma sonde (HNGS), hostile environment lithodensity sonde (HLDS), accelerator porosity sonde, phasor dual induction tool (DIT-E), and temperature acceleration pressure sensor. Tool deployment was hampered by a 20-m-thick tight spot in the dark brown clay section just above the uppermost cherts (see "Unit II"); however, repeated working of the tool string eventually allowed penetration through this tight zone to a point 10 m above volcanic basement, where soft fill prevented further lowering of the tool string. The upward logging run through the tight spot in the clay section was uneventful.

The GLT, deployed next, measured some of the major element constituents of the sedimentary section. The GLT included the natural-gamma spectrometry tool (NGT), the compensated neutron log, the aluminum activation clay tool (AACT) and the induced gamma-ray spectrometry tool (GST). The tool string was deployed, and we attempted to reach the same level in the hole as the triple combo; however, the lighter weight of the GST and possible additional tightening of the hole in the clay section just above the uppermost cherts did not allow it to pass below that level. Therefore, the first GLT run was conducted from the top of the constricted clay section (~145 mbsf; Core 185-1149A-16H) to the bottom of pipe within the clay-volcanic ash section (~63 mbsf; Core 185-1149A-8H). This short logging run of ~79 m was uneventful.

Before the second GLT run, the drill string was lowered through the tight spot in the pelagic clays and positioned ~10 m below the top of the uppermost cherts with the elevator in the down position so that a full stand of pipe could be raised during the logging run. The GLT tool string was then run into the open hole and through the chert section without incident, bottoming ~25 m above the top of volcanic basement in the lithologic Unit IV (~385 mbsf; Core 185-1149B-26R). Logging proceeded without incident up ~227 m to the lowermost pelagic clays (~160 mbsf; Core 185-1149A-18H) after the drill string was raised ~30 m to accommodate the uppermost portion of the logging run. A repeat run was conducted in the lowermost 70 m of the logged interval because a few erratic spots were detected in the primary GLT logging run.

The FMS/sonic tool string was used to measure microresistivity, seismic velocity, inclination of the hole from vertical, magnetic field intensities, and natural gamma-ray emissions. The tool string included the NGT, dipole shear sonic imager (DSI), FMS, and general purpose inclinometer tool. Problems with the DSI prevented the measurement of seismic velocities during this lowering; however, the other tools provided continuous data. The strategy for logging the FMS/sonic runs was the same as in the second geochemical logging run—namely, to position the bottom of the drill string just below the top of the uppermost cherts, send the tool string to the bottom of the hole, and log up while raising the drill string 30 m to log the contact between the uppermost cherts and overlying pelagic clays. In this first FMS/sonic run, the bottom of the hole was reached only a few meters higher than in the geochemical run, still equivalent to Core 185-1149B-26R (~385 mbsf). Logging proceeded without incident up ~223 m to the lowermost pelagic clays (~160 mbsf; Core 185-1149A-18H).

For the second FMS/sonic run, the tool string was brought on deck and the long spacing sonic sonde (LSS) was inserted in place of the DSI tool to obtain seismic velocity measurements. The modified tool string then was lowered to the bottom of the hole. This was still within the carbonate marl section (~369 mbsf; Core 185-1149B-24R) but at a level 16 m shallower than on the previous attempt because of additional fill in the hole. Logging proceeded ~207 m without incident to the lowermost pelagic clays (~160 mbsf; Core 185-1149A-18H). Because the LSS record was very noisy in several sections within the chert interval, it was decided to make a repeat sonic run with the FMS tool arms closed on the possibility that this might eliminate some "road noise" in the sonic measurements. Thus, the tool string was again lowered to the bottom of the hole, 4 m higher than before, but still within Core 185-1149B-24R. Logging proceeded ~219 m without incident to the lowermost pelagic clays (~160 mbsf; Core 185-1149A-18H). The LSS record, however, was still quite noisy.

The postprocessing of the data produced nearly continuous and good-quality records of downhole logging measurements. Degraded borehole conditions, ranging from washed out to constricted, in the uppermost portion of the logged interval resulted in logs of decreased quality. Fortunately, core recovery within this region was >95%. In deeper portions of the borehole, where core recovery was poor (<10%), borehole conditions were very good and yielded very high quality logs. The data from the various measurement tools are in good agreement, and the general character of the logging data matches the observations made from the recovered cores.

The size, shape, and borehole deviation with respect to north can be described from data recorded by the calipers within the inclinometer section of the FMS/sonic tool string and within the slim-hole lithodensity logging tool (HLDT) section of the triple combo tool string. These data are important for the calculation of correction factors required for postprocessing of certain data types (e.g., HNGS, seismic velocity, and magnetic field data). In addition, the uniformity and smoothness of a borehole can often be an indicator of the quality of data collected as well as the integrity and rock type comprising the borehole walls.

The borehole walls generally become smoother and more uniform with depth (Fig. F72) and suggest that the quality of the logging data improves with depth within the interval logged. The upper portion of the borehole (80-180 mbsf) is very rugose with borehole dimensions ranging from 6 in (less than the drilled diameter) to 18 in (maximum limit of the calipers). The main source of the varied borehole character is a layer of dark brown pelagic clay and volcanic ash (lithologic Unit II) (see "Unit II") that appears to be "flowing" back into the borehole. The middle portion of the borehole (180-270 mbsf) is moderately rugose and is associated with the carbonate-free interbedded chert and clay layers comprising lithologic Unit III. The lowermost portion of the logged interval (270-395 mbsf) is significantly smoother and more uniform. This section of the borehole appears to be associated with the lowermost chert layer from lithologic Unit III and the chalk, chert, and marl layers in lithologic Unit IV.

On each logging run, natural radioactivity was measured continuously downhole with either the HNGS or NGT. Both tools utilize scintillation detectors to determine the gamma radiation emitted by the radioactive decay of materials within the formation. Spectral processing of the measured gamma radiation identifies characteristic radiation peaks that are used to determine the concentrations of potassium (in weight percent), thorium (in parts per million), and uranium (in parts per million). These values are combined to provide a measure of the total gamma-ray counts and uranium-free or computed gamma-ray counts. Shipboard corrections to the HNGS are made to account for variability in borehole size and borehole-fluid potassium concentrations.

Except for a broad peak in the central portion of the logged interval (155-170 mbsf), the overall character of the total gamma-ray record exhibits a general, but not monotonic, decrease in radioactivity with depth (Fig. F72). The interbedded clay and ash layers of lithologic Unit I and Subunit IIA in the shallow part of the hole (63-155 mbsf) exhibit intermediate, but variable, gamma-ray values. The ash-free pelagic clays in the deeper portion of lithologic Subunit IIB (160-180 mbsf) have relatively high gamma-ray values with distinctive peaks at 163 and 172 mbsf. These peaks are especially noticeable in the thorium log. The remainder of the deep portion of the borehole exhibits relatively low gamma-ray values that steadily decrease to the deepest part of the logged interval.

The potassium and thorium concentrations closely mimic the total gamma-ray pattern, but the uranium log is significantly different (Fig. F72). The uranium concentration appears to maintain intermediate concentrations from the top of the logged interval to the bottom of the carbonate-free cherts and clays of lithologic Unit III. A broad swell in uranium concentration occurs in concert with the other increases in radioactive elements in lithologic Subunit IIB; however, the elevated values appear to be maintained into the upper Si-rich portion of lithologic Unit III. A distinctive peak in uranium also is observed at the top of the carbonate-rich chert and marl layers in lithologic Unit IV (290-300 mbsf). The source of this peak is uncertain, but may be related to organic-rich sediments similar to those sampled slightly deeper in the section (Core 185-1149B-19R).

The downhole electrical resistivity was measured with the DIT-E tool and provides a rough, inverse estimate of the porosity of the formation. The DIT-E provides three measurements of the formation electrical resistivity, labeled "deep," "medium," and "shallow" on the basis of respective depth of penetration of the current into the formation. All of these values are virtually identical throughout the depth of the measured borehole except when the DIT-E entered the drill pipe (Fig. F73).

The resistivity data nicely delineate the main lithology types observed in the sediment column at Site 1149 (Fig. F73). The predominantly pelagic clay and interbedded ash layers in the upper part of the logged interval (63-180 mbsf) are characterized by very uniform resistivity values (<1 m). Slightly elevated and variable resistivity values (1-10 m) are observed within the carbonate-free interbedded chert and clay layers (180-300 mbsf) throughout lithologic Unit III. The largest resistivity values in this interval are observed at the top and bottom of this section and are associated with two Si-rich zones identified in the geochemical logs described below. The chalk, chert, and marl unit (300-395 mbsf) exhibits resistivity values that are relatively uniform but vary with depth over a wavelength of ~40 m.

The HLDT uses the detection of scattered gamma rays from a radioactive cesium source to determine the bulk density of rock units. These measurements are very sensitive to the integrity and smoothness of the borehole walls; so the bulk density values along the more rugose sections of the borehole may be of lower quality than the data obtained along the more uniform sections.

The density data also nicely delineate the main lithology types observed in the sediment column at Site 1149 (Fig. F73). Extremely low density values (~1.5 g/cm³) characterize the predominantly pelagic clay and interbedded ash layers in the upper part of the logged interval (63-180 mbsf), suggesting a relatively high porosity for these units. A sudden increase in density occurs at the interface between lithologic Units II and III (~180 mbsf), and the density continues to increase with depth throughout the unit. The short wavelength variability of the density in this section may be indicative of interbedded chert and clay layers. It is surprising that the transition from lithologic Unit III to Unit IV is gradual in the density log, in contrast to the sonic log where a sharp increase occurs at 270 mbsf. The density continues to increase with depth within the chalk, chert, and marl unit, but the rate of increase is noticeably reduced. The amplitude of the short wavelength variability is also reduced in this section.

The LSS uses two acoustic transmitters and two receivers to record the full waveform of sound waves that travel along the borehole wall. Compressional wave velocity (V_p) is determined through the depth-derived compensation principle where acoustic traveltimes recorded at one depth are combined with a second set of readings at another given depth.

Compressional wave velocities delineate three distinct sections within the logged interval. These sections, however, do not correspond exactly with the identified lithologic units (Fig. F73). The shallowest interval (160-180 mbsf) is characterized by extremely uniform, low seismic velocities and corresponds to the predominantly pelagic clay unit of lithologic Subunit IIB. An increase in magnitude and variability of the compressional wave velocities is associated with the interbedded chert and clay in Unit III, although a significant increase in velocities is observed at the base of this unit (~270 mbsf). This sudden increase in velocity is associated with absolute maximum values of silicon (Fig. F74) and may be related to changes in the physical properties of the chert layers as they evolve from amorphous opal to quartz. These elevated compressional wave velocities are maintained within the chalk, chert, and marl layers of lithologic Unit IV.

Shipboard processing of the logging data obtained with the AACT and the GST provides initial measurements or yields of the relative abundances of Al, Ca, Si, Fe, Cl, H, and S within the logged formation. Postcruise processing also provides measurements of Ti, Gd, and K. Additional shore-based processing is necessary to compute the absolute dry-weight fractions of the major oxides. Although the GST can be used inside the drill pipe and the effect of the pipe can be estimated, only the data obtained outside of the drill pipe for the various logging runs are included for preliminary analysis (Fig. F74). Overall, the relative changes in the yields of the various elements correspond well with the lithologies obtained from the cored intervals.

Elevated Si yields are observed in depth intervals in which chert was recovered (>180 mbsf), and relatively low Si levels are present in the predominantly pelagic clay intervals (60-180 mbsf) (Fig. F74). It is notable that the average Si yield through the chert interval is not uniform. The highest Si yields are observed at the top (180-205 mbsf) and bottom (270-305 mbsf) of lithologic Unit III, and these depth intervals correspond to regions of high resistivity values. The lower Si-rich interval also corresponds to the region in which elevated compressional wave velocities are observed.

The relative yield of Ca is fairly uniform and low for the majority of the logged interval, but the relative concentration begins to increase markedly at the top of the chalk, chert, and marl interval (Fig. F74). The Ca yield increases rapidly downward at first within a transition zone (295-305 mbsf) and then continues to increase more slowly with depth for the remainder of the interval logged (305-370 mbsf).

The Fe yield is puzzling and does not appear to correlate to any of the identified lithologic boundaries. In the upper half of the logged interval (60-210 mbsf), the Fe yield is fairly featureless except for the possible increase within the dark brown pelagic clay unit (Fig. F74). In the lower half (210-370 mbsf), the Fe yield reaches a maximum at ~240 mbsf and then slowly decreases with depth.

The Al yield exhibits significantly high values within the pelagic clay intervals and slightly elevated values in the deeper portion of the borehole (230-310 mbsf) (Fig. F74). The Al yields are especially high within the lower portion of lithologic Unit I (80-120 mbsf). The broad region of elevated Al abundance appears to correspond to the lower half of lithologic Unit III and the upper portion of lithologic Unit IV.

The Cl yield is relatively high in the pelagic clay and ash layers and exhibits a general decrease with depth (Fig. F74). A noticeable increase in Cl yield is associated with the lowermost portion of the dark brown pelagic clay unit (160-180 mbsf). Slightly lower abundances of Cl are observed at 180-210 and 290-310 mbsf and appear to exhibit an inverse relationship with the Si yields. This relationship is much more apparent for the shallower of the two intervals. Since the measured Cl is primarily associated with the formation pore waters, these changes in Cl yield may be used as proxy for the porosity of the formation.

The two passes of the FMS in Hole 1149B provided almost complete coverage of lithologic Units III and IV; recovery was very poor in these units dominated by interbedded pelagic clay, chert, chalk, and marl. Figure F75 shows the entire image recorded during the first pass.

The dynamic normalization performed underlines the very fine alternation of conductive (dark) and resistive (light) layers, with the more resistive features likely being chert layers. The results of the dipmeter analysis of these images is shown (Fig. F75), with "tadpoles" indicating the dip and strike of individual features and "fan plots" indicating the dip azimuth distribution averaged over 20-m intervals. Figure F76 shows smaller intervals where individual dipping features are identified by sinusoids and the associated tadpoles. The different images indicate a change in the general dipping orientation from north-northeast above ~280 mbsf (Fig. F76A, F76B) to south below this depth (Fig. F76C, F76D). The transition between the two structural regimes seems to coincide with the lithologic change between lithologic Units III and IV. The origin of this structural change is not clear, but postcruise biostratigraphic studies might provide evidence for a hiatus. The sedimentation rates calculated from paleomagnetic data in the upper section of the hole and from calcareous nannofossils in the deeper chalks suggest that there could have been a hiatus during the Late Cretaceous and the Miocene. If such a hiatus can be tied with the base of Unit III, it could be associated with some tectonic event that might explain the change in the dipping orientations between the two units.