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Logging Summary
IODP Expedition 340:

Lesser Antilles Volcanism and Landslides

Expedition 340 Scientific Party
Introduction

    Figure 1. Location of the sites drilled during IODP Expedition 340.

    IODP Expedition 340 drilled nine sites along the Lesser Antilles arc (see Figure 1) to explore the constructive and destructive processes related to island arc volcanism. The Lesser Antilles arc offers a diverse range of magmatic, eruptive, and destructive styles across a relatively small geographic area. The dataset collected during Expedition 340 includes nearshore submarine cores that penetrate 100s of meters into volcanic island landslides, providing a long-term record of volcanic eruption cycles and magmatic evolution. Previously such records have been limited to the results of subaerial geologic studies, remote sensing and shallow piston cores. The combination of land-based knowledge with information from related seafloor deposits will provide a more complete picture of volcanic activity in this geologic setting. Data acquired during this expedition may also provide a better understanding of the mechanisms involved in both transport and deposition of volcanic debris avalanche deposits, and be used to better assess the potential for future volcanic hazards associated with future avalanches. A more complete overview of the expedition operations and preliminary scientific results are available in the Expedition 340 Preliminary Report.

Logging Tools and Operations

    Figure 2. Logging tool strings planned for IODP Expedition 340.

    Four of the nine drilled sites were logged with the standard suite of wireline logging tools, in order to complement the coring program by measuring in situ physical properties of the previously unsampled debris avalanche deposits and to characterize the formation through intervals of incomplete core recovery. Two logged sites (U1394 and U1395) are located near Montserrat in the northern part of the arc, and two logged sites (U1397 and U1399) are located in the vicinity of Martinique in the southern part of the arc (see Figure 1). At Sites U1394 and U1399, the logged intervals cover a region with nearly full core recovery and provide a complementary data set. Conversely, at Sites U1395 and U1397, the logged intervals correspond to regions of very poor core recovery and thus provide characterization of lithologies that were not recovered in cores.

    The logging tool strings deployed during Expedition 340 are shown in Figure 2: the triple combo; the FMS-Sonic tool string; and the VSI tool string for vertical seismic profiles (VSPs). Although initially planned for deployment at only a few sites, magnetic susceptibility logs from the Magnetic Susceptibility Sonde (MSS) were found to be valuable for the entire region so the MSS was included in every run of the triple combo tool string. In addition to running the triple combo and FMS-sonic tool strings, VSPs were planned for three sites to refine the velocity model used for interpretation of regional seismic surveys as well as to facilitate integration of the logs and coring data with seismic data. Ultimately, challenging drilling and borehole conditions and forced abandonment of multiple holes limited the number of sites that were logged and the tools and tool strings that were run during the expedition.
    Figure 3. Logged intervals and tool strings run in each hole during IODP Expedition 340.

    Due to unstable borehole conditions, Holes U1394B, U1395B, and U1399C were displaced with heavy logging mud (~10.5 ppg) prior to logging. Hole U1397B was displaced with seawater rather than heavy mud; the unstable hole conditions made it necessary to circulate while pulling pipe up to logging depth, which would have flushed out any special displacement fluid. All logging operations took place under favorable sea conditions, with low heave (< 1.0 m, peak-to-peak).

    Logging operations are summarized in the following table and Figure 3.

     table

 

Data and Results

    Overview

    We present here a summary of the logging data and some highlights from each site. The drill pipe was raised to ~80 meters below seafloor prior to logging because of hole instability in shallow sediments, so logs are recorded only below this depth. Logging data are initially referenced to depth below the rig floor; after logging is completed, all data are shifted to a seafloor reference and depth-matched to remove offsets between different logging runs. The resulting depth scale, used for all data presented here, is meters below seafloor (mbsf).

    Montserrat - Site U1394

    Figure 4. Summary of logging data recorded in Hole U1394B.

    Site U1394 was drilled as a site proximal to the island of Montserrat, at 1114 m water depth (coring), to study multiple units of debris avalanche Deposit 2 and associated erosional processes. The primary objective of drilling this deposit was to provide information to help determine whether mass-wasting processes occurred as a single event or as a series of closely-spaced, separate events. Two holes were drilled and average core recovery in the most complete hole (U1394B) was 78%. Average recovery was >100% in the logged interval in Hole U1394B, allowing full integration with the log data. The caliper log in Hole U1394B shows that hole size rarely exceeded the bit size, indicating generally high quality data (see Figure 4). There is a short interval (99-106 mbsf) where the FMS pads were closed due to excessive sticking over which no FMS images were recorded. Additionally, due to concerns about borehole stability, the HLDS was run without a source so no density data were recorded.

    Comparison of logs with gamma ray and magnetic susceptibility measurements made on the recovered core shows that logs and core measurements are in good agreement, allowing for reliable core-log integration. There are not enough large-scale differences in this ~100-m interval of open hole to warrant subdivision into distinct units, but some trends are detected in the logs. Coinciding peaks in resistivity, magnetic susceptibility, and often Vp, may be associated with similarly-scaled turbidite units identified in the core. The logs likely reflect changing grain size and/or variations in dominant lithology within these units. FMS images also show some strong resistivity contrasts that correspond to changing grain size/lithology in the cores.

    A VSP experiment was attempted in Hole U1394B, but the tool string could not be passed below the drill bit and the run was abandoned after significant efforts.

Montserrat - Site U1395
Figure 5. Summary of logging data recorded in Hole U1395B.

The primary objective of drilling Site U1395 was to characterize the distal turbidite deposits associated with Montserrat landslides. Turbidites in this location should provide records of recent pyroclastic flow events, and dome and flank collapses, as well as help constrain the long-term background sedimentation rate in the northern part of the Lesser Antilles arc. The average core recovery in the two holes drilled was 65%, with significant gaps in recovery below ~120 mbsf. The caliper log in Hole U1395B shows that borehole diameter ranges from 17 inches just below the pipe to 11 inches at the base of the hole, indicating that the tools maintained good contact with borehole wall over most of the logged interval, ensuring generally good quality data. The FMS calipers were closed for several meters between 100 mbsf and 110 mbsf due to high cable tension during pipe reentry, but the images recorded below 110 mbsf are of high quality. Vp logs show high coherence, indicating good quality, but Vs logs will require some postcruise processing (particularly between 85 and 125 mbsf) to improve their quality. The MSS malfunctioned during this logging run, so no magnetic susceptibility data were recorded in this hole.

Combined analysis of the logs allows for the identification of three logging units in Hole U1395B, based on characteristic features and trends (See Figure 5). Logging Unit 1 (85-112 mbsf) is characterized by consistent, low-variability profiles in density, resistivity, and Vp. Density appears to have a step increase at 92 mbsf, but this is likely a response to the closing of the HLDS caliper at the end of the logging run. This unit coincides with Lithologic Unit G, which is dominated by hemipelagic sediment.

The top of Logging Unit 2 (112-163 mbsf) is defined by abrupt changes in gamma ray, density, and resistivity. Gamma ray, density, and Vp all display high-amplitude variability throughout this unit, relative to Unit 1. Logging Unit 2 is further subdivided into two subunits: Subunit 2A (112-137 mbsf) and Subunit 2B (137-163 mbsf). Several intervals of decreased gamma ray and increased density, resistivity, and Vp in Subunit 2A likely correspond to pumiceous turbidites, which are separated by intervals of hemipelagic sediment. Logging Subunit 2B is characterized by a series of gamma ray highs that, in some cases coincide with high density, resistivity, and Vp, but in others, coincide with decreased density and resistivity. It is difficult to interpret these features in terms of lithology due to the poor core recovery through this interval, but they may reflect different types of turbidites (e.g. pumiceous vs. low-pumice).

Logging Unit 3 (163-204 mbsf) is characterized by a return to lower frequency variations in most of the downhole measurements, similar to what is observed in Logging Unit 1. This may indicate a return to greater abundance of hemipelagic sediment. However, distinct variability in gamma ray and velocity, and some sharp boundaries between layers in FMS images, suggest that there may be lithological changes throughout this unit. A highly resistive layer centered on ~188 mbsf coincides with the recovery of a partial core containing lithified material at roughly the same depth, supporting the idea of lithological heterogeneity at depth in the hole.

    Martinique - Site U1397
    Figure 6. Summary of logging data recorded in Hole U1397B.

    Site U1397 is located on a local topographic high, bounded by canyons, to the northwest of Martinique. The objectives for drilling this site were to provide a marine record of the eruptive history of Dominica and Martinique's major volcanoes by studying tephra deposits, and to characterize a distinctive interval in the seismic survey data identified by chaotic reflections. The average core recovery in the two holes drilled was 54%, with the most significant gaps occurring below ~120 mbsf. The caliper log in Hole U1397B indicated that the borehole was significantly larger than bit size, and both HLDS and FMS calipers were opened to their full extent through most of the open hole, indicating that data quality is likely compromised. Due to uncertain hole conditions, the HLDS was run without a source so no density data were recorded. Despite the enlarged borehole, comparison between gamma ray logs and natural gamma ray (NGR) measurements made on recovered core shows good agreement, indicating that core-log integration is possible. There are also some intervals of high coherence in the velocity logs, particularly Vp, indicating that compressional waves were adequately recorded over a significant portion of the logged interval.

    Based on characteristic features and trends in the logs, four logging units were identified (see Figure 6). Logging Unit 1 (85-90 mbsf) is characterized by relatively consistent resistivity and gamma ray values, and a decreasing trend in magnetic susceptibility with depth. In contrast, Logging Unit 2 (90-127 mbsf) is distinguished by four intervals on the scale of 5-10 m that each exhibit increasing resistivity and Vp downhole with sharp boundaries between intervals. These boundaries correspond to distinct changes in the magnetic susceptibility log, with a general trend of local increases in magnetic susceptibility and elevated resistivity and Vp at the base of each interval. These features likely correspond with a series of volcaniclastic turbidites recovered in cores.

    Logging Unit 3 (127-185 mbsf) is identified by a return to lower variability in resistivity and Vp logs. This unit is further subdivided based on subtle differences in magnetic susceptibility log character into Subunit 3A (127-155 mbsf) and Subunit 3B (155-185 mbsf). There is higher-amplitude variability in magnetic susceptibility in Subunit 3A than in 3B. However, there are higher-amplitude changes in resistivity and Vp in Subunit 3B. Based on limited core recovery, the Logging Unit 2/3 boundary may reflect a lithological change from volcaniclastic turbidites above to mud-rich sandstones and consolidated mudstones below. Logging Unit 4 (185-223 mbsf) is characterized by increased resistivity, magnetic susceptibility, and Vp, relative to the overlying unit. There is a ~5 m thick interval with high resistivity and Vp at the top of Logging Unit 4, but below this interval, resistivity curves become increasingly separated, indicating that logs in this unit may be affected by degraded hole conditions at the base of the hole.

    Martinique - Site U1399
    Figure 7. Summary of logging data recorded with the Triple Combo in Hole U1399C.
    Figure 8. Summary of logging data recorded with the FMS-sonic tool string in Hole U1399C.

    The primary objective of drilling Site U1399, located west of Martinique, was to characterize the processes of debris avalanche emplacement and the associated erosion. The site is located on one of the oldest chaotic deposits imaged in seismic survey data, and coring was planned to penetrate volcanic and biogenic sediment as well as the chaotic debris avalanche deposits. The average core recovery in the two cored holes at this site was 94%, providing a nearly complete record of the targeted deposits to compare with the logs recorded in logging-dedicated Hole U1399C (drilled when coring in Hole U1399B became too unstable to continue operations). The HLDS caliper log shows that Hole U1399C was relatively in gauge, with the exception of three zones where borehole size was significantly larger than bit size (see Figure 7). This indicates that the tools maintained good contact with the borehole wall for most of the logged interval, ensuring good quality data from the tools in the triple combo. FMS caliper data from subsequent passes suggest that hole conditions deteriorated significantly between the VSP (run #2) and the FMS-sonic runs (run #3), with multiple narrowed zones, allowing the FMS-sonic tool string to reach a total depth ~26 m shallower than the earliest triple combo run (see Figure 8). Despite deteriorating hole conditions, the repeatability of measurements is good between multiple passes for both tool strings, and the gamma ray measurements from both tool strings show good agreement. No radioactive sources were used in this hole as a consequence of unstable conditions.

    Combined analysis of the different logs allows for the identification of four logging units, primarily on the basis of responses from the triple combo data (gamma ray, resistivity, and magnetic susceptibility). Logging Unit 1 (80-106 mbsf) is characterized mainly by an increase with depth in gamma ray. Magnetic susceptibility and resistivity are similar to one another in character, and show small net decreases with depth. The transition from Logging Unit 1 to Logging Unit 2 (106-150 mbsf) is marked by a sharp decrease in gamma ray, with sharp increases in resistivity and magnetic susceptibility. All log responses are affected by the three washed out zones in Logging Unit 2. Between 140 and 150 mbsf (at the base of this unit) there is an interval of increased gamma ray values, repeated in all logging runs. This unit may correspond to an interval of highly deformed and contorted sediment recognized in cores from Hole U1399A.

    Logging Unit 3 (150-181 mbsf) is characterized by higher amplitude variability in gamma ray, resistivity, and magnetic susceptibility. Similar to the Logging Unit 1/2 boundary, the transition from Logging Unit 2 to Logging Unit 3 is marked by a step decrease in gamma ray and increases in resistivity and magnetic susceptibility. Generally lower gamma ray values coincide with high resistivity and high magnetic susceptibility features in this unit, which may reflect the presence of a thick sequence of pumice-rich turbidites interbedded with mud.

    Higher amplitude and more regular variability in resistivity and magnetic susceptibility characterize Logging Unit 4 (181-237 mbsf). This unit exhibits the most variable magnetic susceptibility profile in Hole U1399C, including some significant peaks that often correspond to similarly elevated values of resistivity. One such feature (~205-208 mbsf) coincides with the depth in Hole U1399A at which the coring system was changed from APC to XCB, indicating a significant change in lithology and/or induration at this depth.

    Figure 9. VSP data recorded in Hole U1399C.

    A VSP experiment was conducted in Hole U1399C in order to help constrain the seismic stratigraphy of mass transport deposits. Eight stations between 99 mbsf and the bottom of the hole (237 mbsf) yielded reliable check shot travel times (see Figure 9). A comparison of data from both the VSP and the sonic tool with predictions of velocity from the seismic data at this site (1800-2200 m/s) indicates that measured velocity is slower than predicted.

    Angela Slagle: Logging Staff Scientist, Borehole Research Group, Lamont-Doherty Earth Observatory of Columbia University, PO Box 1000, 61 Route 9W, Palisades, NY 10964 USA.

    Sally Morgan: Logging Staff Scientist, Borehole Research Group, Department of Geology, University of Leicester, University Road, Leicester, LE1 7RH.