DOWNHOLE MEASUREMENTS

Logging Operations

Logging operations began on 3 February 2001 at 1030 hr (Table T14). Strong currents and waves caused significant vibration of the drill pipe. The triple combination tool string with the LDEO multisensor gamma ray tool (MGT) at the top and the LDEO temperature tool at the bottom experienced an electrical malfunction when it was lowered in the pipe. Therefore, the entire tool string was brought back on the rig floor, and the MGT was removed. At 1215 hr, the shortened tool string was lowered to a tight spot at ~520 mbsf, which is 152.2 m above TD (672.2 mbsf). Because the tool experienced overpull, logging started only at 510 mbsf. During the run, vibration loosened several joints and unscrewed the end piece (~4 in long) of the temperature tool.

For the second run, the FMS/Sonic combination was lowered. Unexpectedly, the tool encountered an obstacle in the pipe at ~135 mbrf that might have been the lost piece of the temperature tool. Pumping did not free the pipe, and the tools were pulled back on deck. A core barrel was lowered to punch the obstacle into the hole. This attempt was successful; the FMS/Sonic combination was lowered again into the hole to 838 mbrf (524 mbsf), and logging started. To check the repeatability and to improve data recovery in the enlarged intervals of the hole, the FMS/Sonic pass was repeated.

The third run involved the check shot survey with the WST with a 300-in³ air gun as a source in 7 m water depth next to the stern of the ship and 49 m horizontal distance away from the drill string. Traveltime was successfully measured at 13 stations between 408.4 and 837.5 mbrf (94.4 and 523.5 mbsf) (Table T15). The end of logging operations was 4 February 2001 at 0530 hr.

Data Quality

The combination of a wide hole and a hard formation impacted the data quality (Fig. F61). The hole was more than 17 in for long intervals so that the hostile environment lithodensity logging tool, accelerator porosity sonde detectors, and FMS pads did not always come in contact with the borehole wall. In particular, in the interval between 446 and 472.5 mbsf, neutron porosity values reach a maximum of 100 porosity units (pu) and density values are ~1 g/cm³, indicating that the tools measured only borehole fluid. Nevertheless, at least one of the FMS pads usually had contact during both passes so that features like fractures or sediment textures could be recorded. The second pass of the FMS showed good repeatability of the images, thus improving confidence in the reliability of the FMS data set.

The velocity log was noisy because of acoustic reverberation in the high-velocity formation. After the LDEO Borehole Research Group processed for cycle skipping, the data quality improved significantly. Log velocities are generally lower than those from discrete core measurements (see Fig. F62). The WST produced clear first arrivals for the time-depth conversion (Table T15).

Logging Results

The most useful geological information was collected by the natural gamma ray and resistivity tools (Fig. F61). Additionally, FMS images reveal the bedding character that was difficult to discern in low-recovery intervals. They are particularly helpful for identifying unit boundaries, mainly because these boundaries are related to hardgrounds or firmgrounds where the hole was less enlarged. Based on the logs and the FMS images, four log units, which appear to accurately record the development of multiple platform phases at this site, can be distinguished. Within these log units, subunits are recognized that may be related to internal phases of individual platform growth or facies changes during a growth phase (Fig. F63).

Log Unit 1 (76-128.4 mbsf)

Logging Unit 1 extends from the top of the measured interval to 128.4 mbsf, where uranium and resistivity values dramatically drop (Fig. F63). To this depth, uranium and resistivity values increase gradually with several peaks, increasing variability downhole. The boundary bed (128.4 to 125 mbsf) causes a high peak in total spectral gamma ray (HSGR), with an increase of the uranium content to 10 ppm, which is also accompanied by resistivity and velocity peaks. The FMS image of the 2.5-m-thick boundary bed shows a sharp lower boundary (Fig. F64A).

Logging Unit 1 correlates with the dolomitic floatstone of lithologic Subunit IB and the dolomitic floatstone/rudstone of lithologic Subunit IA (see "Lithostratigraphy and Sedimentology"). Based on the FMS image and the high peaks in HSGR and resistivity, the lower boundary of Subunit IB is sharp and reminiscent of a flooding surface. The variability in the log signatures, particularly in several resistivity peaks within logging Unit 1, is probably the result of hard cemented layers below exposure surfaces.

Logging Unit 2 (128.4-163 mbsf)

Both the upper and lower boundary of logging Unit 2 are marked by an abrupt shift in resistivity and velocity values. At the upper boundary, HSGR also decreases dramatically downhole, whereas only a slight shift downhole to higher values occurs at the lower boundary. Internally, the unit shows uniform low resistivity values and generally low HSGR values, with few peaks (Fig. F63).

The boundary between lithologic Subunits IB and IC is correlated with the log change separating logging Units 1 and 2. The log unit itself coincides with lithologic Subunit IC, which is a reefal facies composed of skeletal rudstone/boundstone alternating with dolomitic floatstone (see "Lithostratigraphy and Sedimentology").

Logging Unit 3 (163-345.5 mbsf)

The upper boundary of logging Unit 3 is placed at 163 mbsf, where both resistivity and velocity rapidly increase downhole and HSGR values start to fluctuate around a higher median (Figs. F61, F63). The unit can be subdivided into three intervals based on the variation in the uranium log. The top interval (163-213 mbsf) is characterized by high-frequency, high-amplitude resistivity fluctuations that decrease in amplitude and frequency within the interval. The velocity log displays a similar pattern of high variability (Fig. F63). The uranium content fluctuates moderately around a near-linear trend, with a basal 6.5-m-thick package of reduced uranium values of about 2 ppm. Some very conductive layers, seen in the FMS log, with low HSGR and velocity streaks between 163 and 175 mbsf might be dissolution features in this portion of the platform.

The boundary to the underlying logging interval at 213 mbsf is marked by a downhole increase in the HSGR (~20 gAPI), increased resistivity, and a drop in velocity (Fig. F63). The lower boundary at 309.5 mbsf is placed at the base of a distinct peak in resistivity (~20 m) and velocity (~5 km/s) and very low neutron porosities (~10 pu). The FMS log images this peak as a bed with a sharp lower boundary and with characteristics similar to the overlying sediments, whereas the sediments below have an overall finer character caused either by reduced porosity or component size (Fig. F65). Above this basal layer, the uranium is generally high (>3 ppm). Peaks in the uranium log repeatedly correspond to increased resistivity values. Distinct peaks in the two logs occur at about 235 mbsf, which might be related to a major event of exposure and subsequent flooding.

The lowermost interval (309.3-345.5 mbsf) is characterized by uranium contents that steadily increase uphole from 4 to 15 ppm. The major uranium peaks in this interval are positively correlated with small resistivity and velocity peaks.

Logging Unit 3 is correlated with lithologic Subunits ID, IIA, and IIB. The top of lithologic Subunit ID coincides with the upper boundary, and the base of lithologic Subunit IIB coincides with the lower boundary of logging Unit 3. However, the three above-described logging intervals of logging Unit 3 do not coincide with the three lithologic subunits. Lithologic Subunit IIB is the base of the entire unit (Fig. F63). Lithologic Subunit ID is only part of the top interval of the logging unit. Nevertheless, the dolomitic floatstone of lithologic Subunit ID corresponds to the high resistivity and velocity values at the top of logging Unit 3. High peaks are interpreted to mark several exposure events toward the end of this platform growth phase. Moldic porosity observed in the cores supports the interpretation of exposure. In addition, this top part shows pervasive dolomitization that partly explains the high velocity and resistivity values in this log interval. The low resistivity and HSGR at the base of this top logging interval might be the record of a recovered grainstone with fenestral porosity indicative of a beach(?) deposit (see "Lithostratigraphy and Sedimentology").

The logging interval of logging Unit 3 with increased uranium content correlates to the middle part of the skeletal floatstone with grainstone matrix of lithologic Subunit IIA that is interpreted to represent open lagoonal platform facies (see "Lithostratigraphy and Sedimentology"). A likely reason for the increased accumulation of uranium in this lagoonal facies might be the more anoxic environment and an increased organic carbon content.

The trend of increasing HSGR values of the lowermost logging interval of logging Unit 3 (309.3-345.5 mbsf) indicates changing facies, potentially a regressive trend. Sedimentologically, no trend is discernable, possibly because of low recovery. Cores from this interval record a thin-bedded unit with mottled mudstones, grainstones, and exposure horizons probably deposited in very shallow water (see "Lithostratigraphy and Sedimentology").

Logging Unit 4 (345.5-524 mbsf)

The upper boundary to logging Unit 4 is placed at 345.5 mbsf, where both HSGR and velocity decrease markedly. Logging Unit 4 shows, on average, the lowest gamma ray values for the logged interval. Based on variations in the resistivity, velocity, and density logs, a subdivision can be made at 472.5 mbsf. From 345.5 to 472.5 mbsf, the hole is enlarged to a diameter of more than 17 in, which is the maximum opening of the caliper (Fig. F61). It is narrower only at ~460 mbsf, where a peak in resistivity indicates a densely cemented bed or hardground that was not recovered in core. Despite low resistivity values above this horizon, velocity log values are high. Above 446 mbsf, the HSGR increases slightly and the resistivity log shows only few excursions to higher values.

From 472.5 mbsf to the bottom of the logged interval at 524 mbsf, the downhole increasing high density (2.6 g/cm³), resistivity (100 m) and velocity values (5 km/s) indicate a hard formation. This change is corroborated by the narrower borehole diameter starting at this depth, resulting in clear and complete FMS images (Fig. F65). In the images, a highly fractured interval is recognized, in which fractures have an average dip orientation of 160°/60° (Fig. F66).

Logging Unit 4 corresponds to lithologic Subunits IIIA to IIIC. Overall the logs do not show much variation, and a discrimination of lithologic Subunit IIIA (dolomitic rudstone) from Subunit IIIB (sucrosic dolomite) is not possible in the logs. The top of lithologic Subunit IIIC consists of well-lithified dolostones and is characterized by high velocity and resistivity values (Fig. F63).

Discussion

The main objective at Site 1196 was to determine the initiation and facies development of the SMP. Lithology and ages of the recovered rocks clearly indicate that the SMP grew in several phases (see "Lithostratigraphy and Sedimentology", and "Biostratigraphy and Paleoenvironments"). Consequently, cores at Site 1196 penetrated an edifice of multiple amalgamated platforms, which were established on a glauconite-rich substrate (see "Lithostratigraphy and Sedimentology"). The log facies record the evolution in these multiple platforms. Logging Unit 4 provides the rather uniform log signature of a pervasively dolomitized platform. The high-resistivity portion below 472.5 mbsf might be part of the oldest platform phase, which is early Miocene in age. Logging Unit 3, with its three log intervals, is interpreted to be the middle Miocene growth phase as indicated by biostratigraphic data (see "Age Model"). The facies of this middle Miocene SMP starts with a reef that is overlain by high-energy platform margin sediments (lithologic Subunit IIB and the bottom of Subunit IIA; see "Lithostratigraphy and Sedimentology"). Increasing HSGR and velocity values from 345.5 to 309.5 mbsf are the log expressions of this regressive trend. Subsequent flooding of the platform establishes a thick lagoonal to open shelf facies (lithologic Subunit IIA), which is overlain by shallower (beach) facies and packages with exposure surfaces (lithologic Subunit IIB). The increased resistivity and velocity values are the physical expression of this sedimentologic facies succession (Fig. F63). The interpreted dissolution features in the FMS images at 170.2-173 mbsf and 174.5-174.7 mbsf indicate karstification at the top of this middle Miocene platform.

Because the middle/late Miocene boundary is biostratigraphically not constrained, the base of the late Miocene SMP, the youngest platform growth phase, is at the base of either lithologic Subunit IC or IB. If the boundary is at the base of lithologic Subunit IC, the late Miocene platform would start at 163 mbsf at the base of logging Unit 2, with low resistivity, velocity, and HSGR in a reefal facies. The karst features and the related variable thickness of lithologic Subunit ID below 168 mbsf, as recognized in Holes 1196A and 1196B, give evidence of long-term exposure on top of the dolomitized interval of lithologic Subunit ID (see "Lithostratigraphy and Sedimentology"). A slight trend of increasing HSGR values in the first 20 m is reminiscent of the regressive trend at the bottom of the middle Miocene platform between 345.5 and 309.5 mbsf. Peaks in HSGR, resistivity, and velocity mark a flooding surface at 128.4 mbsf, above the reef, and the establishment of a floatstone unit that is similar in the log expression of the lagoonal facies of the middle Miocene platform. This flooding surface at the base of logging Unit 1 could alternatively be the base of the late Miocene platform. Such an interpretation is supported by seismic stratigraphy data (see "Seismic Stratigraphy") and the occurrence of a major exposure horizon at the equivalent interval at Site 1199.

Time-Depth Conversion

A check shot survey was performed using the single-channel WST and a 300-in³ air gun. The tool was lowered to a maximum depth of 523.5 mbsf in Hole 1196A, and the traveltime was measured at 12 stations (Table T15). The resulting traveltime-depth curve is displayed in Figure F62. This time-depth relationship is similar to estimates obtained by integrating the sonic log but is vastly different when the velocities of the discrete samples measured with the P-wave sensor (PWS) are used. The values from discrete samples are generally much higher; in many cases, the difference is over 2 km/s (Fig. F62). Although such differences can be caused by shipboard sampling bias (hard vs. soft), pressure differences (burial vs. surface), or dispersion (ultrasonic vs. seismic frequency), the observed large difference might be mainly explained by the sampling bias produced as a result of the selective recovery of the hard portions of the formation. In addition, FMS images document fractures at certain intervals of the platform, which also slow down the formation velocity. Nevertheless, the offset of the downhole and lab velocity values is unusually large when compared to other data sets, in which PWS data often are very close to the velocities determined in the WST experiment (Eberli, Swart, Malone et al., 1997). At Site 1194, for example, the PWS velocities were generally lower than the log data (see "Downhole Measurements" in the "Site 1194" chapter). The significance of the large difference at this site becomes evident when the time-depth conversion of both data sets is compared: 300 ms (two-way traveltime [TWT]) would translate into 345 mbsf using the integrated sonic log, 365 mbsf using the check shot interval velocities, and 655 mbsf using the integrated traveltimes of the PWS data. Because the reliability of the shipboard velocity data is generally difficult to estimate, a WST experiment is critical for a precise time-depth conversion.

Logging Operations

Logging operations started at 1700 hr on 21 February 2001 with rigging up the wireline. At 1830 hr the triple combo tool string was deployed in Hole 1199A. Two narrow spots were encountered at 118 mbsf and 204 mbsf, but were passed after few attempts. The tool string was lowered to a depth of 418 mbsf, which was 1.5 m above the TD of 419.5 mbsf of Hole 1199A. A first logging run started from this depth uphole. For the second run, the FMS/Sonic tools were lowered into the hole (Table T16) but could not pass a tight spot at 129.2 mbsf, and the tools were pulled back onto the rig floor. The drill pipe was advanced to ~210 mbsf to widen the narrow hole. The tools were lowered again in the hole but could not pass beneath 129.2 mbsf. Hence, logging started above this horizon. The tool measured a hole deviation of 7.5°, which is probably the cause of repeated failure to pass the critical spot, most likely a cave at 129.2 mbsf. In a last attempt to reach the bottom of the hole, two sections with bowsprings were added to the lower part of the tool string. This attempt also failed, and logging operations terminated at 1000 hr.

Log Quality

Caliper readings of the first run showed a nearly constant hole diameter of 11 in for most of the measured sections (Fig. F67). In three intervals, the caliper opened to its maximum range, indicating a hole larger than 17 in. Two of these intervals between 118-129.2 mbsf and 155-162 mbsf, respectively, are most likely karst caves, as bit drops were reported during coring. The third interval (185-210 mbsf) is an enlarged borehole. The otherwise good hole conditions resulted in very good log quality for the first run. The second short logging run covers part of the top of the first cave, resulting in poor-quality FMS and velocity data. However, the transition from the karstified reef into lithologic Subunit IB, which could be the base of the late Miocene growth phase of the SMP, is well imaged.

Log Results

The logging data in Hole 1199A provide important information on the lithology and architecture of the middle to late Miocene SMP edifice, especially in the lower part of the hole where core recovery was low. Site 1199 is only 5 km east-northeast of Site 1196, on the SMP, but significant differences between this site and Site 1196 can be identified, despite similarities in the log responses for individual units. A similar subdivision into three logging units is recognized, although thickness variations in logging Units 2 and 3 provide evidence for significant lateral facies heterogeneities and a complicated architecture of the entire platform edifice. As at Site 1196, the dominant contribution to the HSGR signal in Hole 1199A is caused by high uranium concentrations.

Logging Unit 1 (75-115 mbsf)

Logging Unit 1 consists of an interval with high resistivity and HSGR values (Fig. F68). A peak of high resistivity marks the boundary to the next logging unit. The FMS image shows a sharp boundary and overall little open pore space (Fig. F69). The first peak of resistivity and HSGR at 115 mbsf correlates to the base of the overlying dolomitic floatstone of lithologic Subunit IB. The highly dolomitized rocks show the lowest porosity and density measured on Leg 194.

Logging Unit 2 (115-130.5 mbsf)

Readings of 100 pu porosity and density around 1 g/cm³ in logging Unit 2 indicate the presence of a water-filled karst cave of ~10 m thickness. This zone corresponds to the reefal interval of lithologic Subunit IC that is capped by an exposure horizon (see "Lithostratigraphy and Sedimentology"). Bit drops were reported while cutting in Core 194-1199A-14R, supporting the interpretation that major karst caves occur in this interval. The low recovery of Core 194-1199A-15R and the log responses imply a greater thickness for lithologic Subunit IC.

Logging Unit 3 (130.5-275 mbsf)

Logging Unit 3 corresponds to lithologic Subunits ID and IIA, likely representing the middle Miocene platform growth phase. Based on changes in uranium concentration, density, porosity, and resistivity, four intervals can be distinguished (Fig. F68). The top interval of logging Unit 3 (130.5-154 mbsf) is characterized by high resistivity (~20 m) and density (~2.6 g/cm³) and low neutron porosity (15 pu). Below 154 mbsf, resistivity and density decrease drastically, which again is interpreted as a karst cave. This section extends down to 172 mbsf, with slowly decreasing neutron porosity from 100 pu down to 50 pu. The HSGR does not correlate with the other logs. It increases dramatically at 161 mbsf and stays generally high, with large variations (20-140 gAPI) down to 208.2 mbsf, where it shows a sharp drop to low values around 20 gAPI. In the lowermost interval, the HSGR shows variations but with lower amplitudes (15-40 gAPI).

The top interval of logging Unit 3 corresponds to floatstone (lithologic Subunit ID), which is pervasively dolomitized and densely cemented. The karst cave below, indicated by 100% porosity, separates the top 24.5 m from a 200-m-thick section of skeletal floatstone and grainstone (lithologic Subunit IIA), which builds a major part of the platform succession.

Logging Unit 4 (275-418 mbsf)

Logging Unit 4 displays low variability and a near linear trend of slightly decreasing resistivity and density and increasing porosity and HSGR (Fig. F68). Based on the HSGR, two intervals can be subdivided. Between 275 and 363 mbsf, the HSGR values are on average low (~15 gAPI) with small variations. The boundary to the lower interval is placed at 363 mbsf at the top of a downhole decreasing trend in the HSGR log. Furthermore, this boundary is also characterized by a small peak in density, porosity, and resistivity, indicating the development of a hardground. Logging Unit 4 corresponds to lithologic Subunit IIB that is composed of skeletal grainstone and floatstone. Because of low core recovery, a detailed core-log correlation is somewhat tenuous.

Correlation of Site 1196 and 1199

Lithologic correlation of Sites 1196 and 1199, both drilled into the SMP, is difficult because of low recovery. Only logging data provide a continuous record of the multiple platform growth phases. In both holes, four logging units were distinguished that are related to the Miocene platform growth phases. Preliminary shipboard analysis shows a good correlation in logging Units 1 and 2 to a depth of 163 and 115 mbsf, respectively (Figs. F70, F71). Below these depths, the correlation is more interpretative. Two scenarios of correlating log responses between Sites 1196 and 1199 are presented below. The favorite one is discussed in detail, whereas the second possibility is only briefly addressed. The most useful logs for the correlation are the resistivity and uranium logs.

In both Holes 1196A and 1199A, logging Units 1 and 2 show a similar log response pattern (Fig. F70). Logging Unit 2 shows low uranium values and is abruptly overlain by a succession in logging Unit 1, displaying at its base very high uranium values that decrease upward. The correlation of logging Unit 3 is more complicated. At both sites, the top of logging Unit 3 is a high resistivity interval, corresponding to a dolomitic floatstone (lithologic Subunit ID). The range of porosity, density, and resistivity data is similar in this unit at both sites, but the uranium signature differs. For example, extremely high and strongly fluctuating uranium values (up to 15 ppm) in the top 50 m of Hole 1199A are not present in Hole 1196A. Consequently, the resistivity is used for correlation. In both holes, resistivity in the top of logging Unit 3 shows high-frequency, high-amplitude variations that decrease and, at 172 mbsf, change into a low-frequency resistivity signal that fluctuates downhole around a near-linear median. This change is taken as a correlation horizon (purple line in Fig. F70). Toward the bottom of logging Unit 3, resistivity shows two peaks. The lower of these peaks is the log unit boundary and is used as another correlation horizon. The strata in between the peaks has a high uranium content but varies in thickness between the two sites by nearly 30 m. If this correlation between the two holes is correct, then logging Unit 3, which records the strata from the middle Miocene growth phase, is 39.5 m thinner at Site 1199 than it is at Site 1196. Karstification with its base at 172 mbsf at Site 1199 seems not to follow this difference, as it is found at the same depth at Site 1196 (Fig. F70).

In logging Unit 4, resistivity shows a slightly decreasing downhole trend with few peaks in both holes, whereas uranium differs. At Site 1199, uranium has increasing values and variability from 386.2 to 363 mbsf, above which an abrupt decrease to lower values and variability occurs. This log character is similar, although with lower values, to the corresponding interval in Hole 1196A between 410 and 346 mbsf. Thus the top of the uranium increase at 363 mbsf, in connection with a resistivity peak, is used as a correlation horizon. This correlation makes lithologic Unit III (dolomitized rudstone) in Hole 1196A equivalent to lithologic Subunit IIB (skeletal grainstone and floatstone) in Hole 1199A. Recovery of lithologic Subunit IIIA (Hole 1196A) is, however, minimal, which might produce a biased recovery of dolomitized rocks (see "Lithostratigraphy and Sedimentology"). Assuming that the correlation is correct, a thickness variation of ~50 m of equivalent facies can be postulated in the two holes (Fig. F70).

The second possibility of correlating log signatures based on the uranium would result in variable thickness of correlated packages and no general dip from one site to the other (Fig. F71). The most consistent correlative surface for both interpretations is the boundary between logging Units 1 and 2. Below this surface, four major correlation surfaces (a, d, e, and f) can be traced. Two additional surfaces (b and c) are distinguished but considered to be of minor prominence. Surface a marks the lower boundary of an interval with low uranium values. The sharp downhole increase in uranium with a following interval of higher uranium concentrations down to surface d at Site 1199 is related to a similar interval with elevated uranium concentrations at Site 1196 between surfaces a and d (Fig. F71). Surfaces e and f mark the upper and lower boundaries of an interval with a log signature that is very characteristic of increasing uranium concentrations. This pattern can be recognized in both holes and triggered the correlation based on uranium.

The resulting correlation scheme would be more consistent with the lithologic correlation, as it relates most of lithologic Subunit IIB in Hole 1199A to lithologic Subunits IIA and IIB in Hole 1196A. However, the scheme would correlate different log units in the two holes, which were determined independently. In addition, the role of uranium as a primary facies indicator is not unambiguous, as it can be remobilized with migrating fluids. Furthermore, this correlation would create up to 70 m of topography on the shallow-water platform over a distance of only ~3 km, which is unlikely, considering the overall shallow-water facies retrieved in the cores.