Logging Summary

IODP Expedition 320-321:

Pacific Equatorial Age Transect (PEAT)

Introduction

Figure 1. Map showing the location of sites drilled during Expeditions 320-321.

The Pacific Equatorial Age Transect, or PEAT, program (IODP Expeditions 320/321) was designed to recover a continuous sedimentary record of the Cenozoic by drilling a transect at the paleoposition of the equator at successive crustal ages. The sites were located to maximize the recovery of well-preserved carbonate-rich sediments (Figure 1). The first expedition of the PEAT program (Exp. 320) obtained records from six sites (U1331-U1336) spanning the Paleogene, while the two sites drilled in Exp. 321 (U1337 and U1338) were focused on the Neogene time interval. The key objectives of the PEAT program were to reconstruct the Cenozoic history of changes in the calcium carbonate compensation depth and paleoproductivity in the equatorial Pacific; to calibrate the Cenozoic geological timescale using orbitally forced variations in sediment composition; to determine ocean paleotemperatures and nutrient profiles; to better constrain Pacific plate tectonic motion and better locate the Cenozoic equatorial region in plate reconstructions; and to correlate sedimentary sections with existing seismic stratigraphy to develop an improved model of sedimentation in the equatorial Pacific.

The primary logging objectives of Expeditions 320-321 were to collect high-resolution downhole physical property data and integrate them with core measurements.

Wireline logging was planned for the first hole at all sites. The purpose of this was to provide near real-time information about the sediment column to guide drilling operations in subsequent holes and maximize recovery.

Downhole measurements of natural gamma-ray radioactivity, bulk density, electrical resistivity, and elastic wave velocity were taken to complement the physical property measurements made on cores and to assist in establishing a detailed lithostratigraphy at each site. In particular, downhole measurements were helpful in reconstructing the lithostratigraphy in intervals that contained chert and porcellanite and had incomplete core recovery. The logging program also included vertical seismic profile (VSP) operations at both sites. The VSP “checkshot” data (measurements of the arrival time of seismic pulses generated at the sea surface) gave a relationship between depth in the borehole and travel time in seismic sections. This travel time-depth relationship will be a critical input to the seismic stratigraphy study that is one of the PEAT objectives.

Logging Operations Summary

Figure 2. Tool strings used during IODP Expeditions 320 and 321. The "modified Triple Combo" configuration shown is that used in Exp. 321; in Exp.320, the DIT tool was replaced by the Magnetic Susceptibility Sonde (MSS).

The planned logging program included two logging runs at each site: a modified Triple Combo tool string (natural gamma-ray, density, and resistivity or magnetic susceptibility) and the Formation Microscanner (FMS)-Sonic tool string. The Versatile Seismic Imager (VSI) was deployed on two sites during Expedition 321 (Figure 2).

The FMS tool obtains high-resolution (~1 cm) resistivity images with arrays of electrodes on four pads that are pressed against the borehole wall. Two overlapping passes of the Triple Combo and of the sonic-FMS tool string were attempted in each hole; the measurements were generally repeatable and of high quality.

The Triple Combo takes a measurement of the borehole diameter and was always run first to assess the hole quality for subsequent operations. The order of the sonic-FMS and VSI runs was determined by the need to shoot the VSP airgun source during daylight hours. Daytime shooting was required to maintain a marine mammal watch.

Two sites (U1331, U1332) were logged as part of Expedition 320 and two (U1337, U1338) were logged during Expedition 321.

The deployment of two tool strings was planned for Hole 1331A: the modified Triple Combo with density, natural gamma and magnetic susceptibility tools and the Formation Micro Scanner (FMS)–Sonic string. However, when the new Active Wireline Heave Compensation System was turned on, once the modified Triple Combo reached the base of pipe, it failed to work. Given the relatively calm sea state we proceeded to run without active heave compensation. The hole was logged down with the modified Triple Combo to the base reaching total depth at 190.6 m WSF. During the uplog it became apparent that the wireline winch unit was experiencing problems and we had some difficulty in retrieving the tool string. Due to these problems the deployment of the FMS-Sonic tool string was abandoned.

Two deployments were planned for Hole U1332A: the modified Triple Combo and the FMS-Sonic string. The modified Triple Combo tool string was lowered and logged down to ~150 m WSF, almost to the bottom of the hole. Two upward logging passes were made up to the base of the pipe. The tools provided continuous and good quality log data, but they are affected by ship heave (typically 2m peak to peak), because the wireline heave compensator (WHC) was not working.

At the end of the second upward pass at Hole U1332A we encountered difficulties when attempting to pull the tool string back into the pipe. During this procedure, communications with the tool string were lost, and shortly after that the wireline lost ~800 lbs of weight, corresponding to the weight of the tool string. When the wireline was retrieved it was confirmed that the tool string was severed from the wireline. Fishing attempts to retrieve the tool string were unsuccessful and Hole U1332A was cemented and abandoned.

At Site U1334, while the tool string (DSI/DIT) was being lowered through the pipe about 1700m below the ship, the transmission on the wireline winch failed, and the tool string had to be retrieved using the coring winch. No further logging operations were possible during the expedition.

Logging Results

We present here a summary of the logging data and some highlights of the results. Because of hole instability in the shallow sediments, the base of the drill pipe was lowered to 70-80 m below the sea floor during logging, and wireline logs were generally not recorded in this interval. The exception was the natural gamma-ray log: although the measurement of natural radioactivity was attenuated by the drill pipe, the gamma-ray log was recorded to detect the depth of the seafloor and obtain a wireline depth scale below sea floor (WSF). In the Expedition 320 and 321 sites, the sea floor was clearly marked by a peak in natural radioactivity due to a relatively high uranium content. After data acquisition, the measurement depths were adjusted to match across different logging runs, obtaining a wireline depth scale matched below seafloor (WMSF).

Hole U1331A

Figure 3. Summary of downhole log measurements in Hole U1331A.

The successful deployment of the modified Triple Combo tool string at U1331A provided complete coverage of the ~190 meter open hole interval with good physical property and lithologic information for density, natural gamma ray, photoelectric effect, conductivity and magnetic susceptibility (Figure 3). The downhole logs provided important information about the interval between 157 m and 175 m WMSF which consisted of interbedded cherts and porcellanites with very little core recovery.

Figure 4. Summary of downhole log measurements in Hole U1332A.

The modified Triple Combo tool string once again provided complete coverage of the open hole section at Hole U1332A. Good physical property and lithologic information was obtained for the ~150 meter section including density, natural gamma ray, photoelectric effect, conductivity and magnetic susceptibility (Figure 4). The interval between 136-146 m WMSF was characterized by a series of peaks in density. The corresponding lithostratigraphy, although only partially recovered in this interval, is a mixture of radiolarian oozes and porcellanites.

Hole U1337A

Figure 5 . Summary of downhole log measurements in Hole U1337A.

Site U1337 was targeted to recover the early middle Miocene interval on ~24 Ma crust, and the column drilled at Site U1337 represents a nearly complete and continuous Neogene sedimentary section. A summary of the downhole log measurements taken in Hole U1337A is in Figure 5. Three logging units were identified: Unit I (77-212 m WMSF) and Unit II (212-339 m WMSF) have average densities of ~1.3 and ~1.6 g/cm³, respectively, that do not show any trend with depth, whereas in Unit III (339-442 m WMSF) density increases with depth reaching 1.85 g/cm³ at the base of the hole. Resistivity and P-wave velocity follow a pattern similar to that of density, suggesting that the major control on these physical properties are variations in sediment composition that give rise to corresponding variations of porosity. Changes in porosity affect similarly these logged properties, with high-porosity sediments having low bulk density, low resistivity, and low P-wave velocity.

Figure 6 . VSP data in Hole U1337A.

Natural gamma ray measurements were low throughout the logged interval (~5 gAPI), except for two pronounced peaks caused by uranium, one at the seafloor and the other at 240 m WMSF. The gamma ray peak at 240 m WMSF corresponds to a ~40 cm thick chert layer that has only been recovered as rubble in the cores but can be clearly identified in the downhole logs and borehole images as an interval of high density and resistivity.

A VSP experiment was also performed in Hole U1337A, and the arrival times of the seismic pulse from the sea surface were measured at 16 stations in the 214-439 m WSF depth interval (Figure 6). Together with the travel time to the seafloor, VSP measurements are the basis for a travel time-depth conversion that allows seismic reflectors to be correlated to stratigraphic events. Finally, downhole temperature measurements and thermal conductivities of core samples were combined to estimate a geothermal gradient of 32.4°C/km and a heat flow of 28.4 mW/m² at Site U1337.

Hole U1338B

Figure 7. Summary of downhole log measurements in Hole U1338B.

Site U1338 was planned to collect a 3-18 Ma sediment interval, and the drilled sediment column is a nearly complete and continuous early Miocene to Holocene section. The log measurements taken in Hole U1338B are summarized in Figure 7. Downhole log data were used to define three logging units: Unit I (139-244 m WMSF) and Unit II (244-380 m WMSF) have average densities of ~1.45 and ~1.6 g/cm³, respectively, that do not show any trend with depth, whereas in Unit III (from 380 m WMSF) density increases with depth, reaching 1.7 g/cm³ at the base of the hole. Resistivity and P-wave velocity follow a pattern similar to that of density throughout the logged interval, suggesting that the major control on these physical properties are variations in sediment porosity. Both resistivity and density measurements show a small-scale peak at 280 m WMSF. This peak is clearly visible in the borehole resistivity images as a high-resistivity layer 16 cm thick, and it corresponds to a chert layer that has only been recovered as rubble in the cores. Natural gamma ray measurements are low throughout (~4 gAPI) but do show a pronounced high at the seafloor caused by a local increase in uranium concentration.

In the Hole U1338B VSP experiment, the arrival time of a seismic pulse from the sea surface was measured at 14 stations in the 189.5-414.5 m WSF depth interval. As at Site U1337, these arrival times are the basis for correlation of stratigraphic events to seismic reflectors. Downhole temperature measurements and thermal conductivities of core samples were combined to estimate a geothermal gradient of 34.4°C/km and a heat flow of 33.6 mW/m² at Site U1338.

Scientific Highlights

Lithostratigraphic Correlation

Figure 8. High-resolution bulk density downhole logs from Holes U1337A (black) and U1338B (red) with depth scales shifted and stretched to match the different sedimentation rates at the two sites.

Previous drilling showed that sediment properties in coeval equatorial Pacific sequences are highly correlated between sites separated by large distances. Figure 8 compares the downhole density logs measured in Sites U1337 and U1338, which are ~600 km apart. The depth scales in the two holes have been shifted and stretched to account for the different overall sedimentation rates at the two sites, and the depths of several nannofossil events from the shipboard stratigraphy are shown for comparison. The continuous density log curves are ideal to establish a lithostratigraphic correlation between the two sites.he successful deployment of the modified Triple Combo tool string at U1331A provided complete coverage of the ~190 meter open hole interval with good physical property and lithologic information for density, natural gamma ray, photoelectric effect, conductivity and magnetic susceptibility (Figure 3). The downhole logs provided important information about the interval between 157 m and 175 m WMSF which consisted of interbedded cherts and porcellanites with very little core recovery.

The downhole log depth scale, however, has been established independently from the core depth scale, and the depths of stratigraphic events observed in the cores do not exactly correspond to depths in the log. To address this issue, post-cruise work will concentrate on developing an accurate core-log depth correlation by comparing composite density curves measured on cores with the density logs. Once this correlation is established, the downhole log data can be plotted in the composite depth scale and can be used to aid the detailed correlation of stratigraphic events from the core data.

In addition, the interval 260-360 m WMSF in the Hole U1337A density and resistivity logs shows prominent sediment cycles. This interval should correspond to ~13-19 Ma, a time interval of broad interest that encompasses the end of the mid-Miocene climatic optimum and the subsequent cooling that led to the formation of a stable East Antarctic ice sheet. The geological time scale is not well constrained in this interval, and during post-cruise research downhole log data will be correlated to an astronomical tuning curve to derive an age model. By establishing an accurate core-depth correlation (see above), this age model will be able to date key stratigraphic events (e.g., ages of geomagnetic polarity reversals).

Cherts and Porcellanite Layers

Intervals of chert/porcellanite occurred in both Sites U1331 and U1332. Only small amounts of these lithologies were recovered at both sites. In Hole U1331A downhole logs show ~18 meters of interbedded chert/porcellanite and radiolarian oozes (Figure 2). The chert layers show higher density, gamma ray and photo-electric factor values than the surrounding sediment. Of particular interest is the increased magnetic susceptibility and low conductivity in the chert/porcellanite layers. The depth of the chert-rich interval was identified in the logs (157-175 m WSF), which allowed the coring strategy in Holes U1331B and U1331C to be adjusted to maximize recovery of the target sediment interval beneath the chert.

Figure 9. Downhole logs in the interval containing a thin chert layer, Holes U1337A and U1338B.

Figure 9 compares the Formation MicroScanner (FMS) images measured at Sites U1337 and U1338 in the respective intervals that contain a thin chert layer. The images show high resistivities in light shades and low resistivities in dark shades. The chert layer shows up as a thin high resistivity layer (40 cm at Site U1337 and 16 cm at Site U1338) sandwiched between two low resistivity beds that are diatom-rich oozes. The intermediate resistivity intervals above and below the diatom-rich beds are carbonate oozes. The preliminary shipboard stratigraphy suggests that these thin chert layers may be coeval (~12 Ma, upper middle Miocene). Core recovery was poor around the chert intervals, and chert samples were only retrieved as rubble. Nonetheless, a comparison of core images with the FMS logs showed that there was only a small amount of disturbed and missing section (<1 m) in the sediments cored around the chert.odified Triple Combo tool string once again provided complete coverage of the open hole section at Hole U1332A. Good physical property and lithologic information was obtained for the ~150 meter section including density, natural gamma ray, photoelectric effect, conductivity and magnetic susceptibility (Figure 4). The interval between 136-146 m WMSF was characterized by a series of peaks in density. The corresponding lithostratigraphy, although only partially recovered in this interval, is a mixture of radiolarian oozes and porcellanites.

Seismic Stratigraphy Correlation and Synthetic Seismograms

Figure 10. Correlation between the seismic reflection record at Site U1337 (line 4 of the AMAT-03 site survey in the PEAT-7 area) and depth in Hole U1337A.

Figure 10 shows an initial correlation between depth in Site U1337 and seismic reflectors in a site survey profile. This correlation is based on the first arrival travel times measured in the VSP at Site U1337 (Figure 6). In reflection surveys, seismic waves are reflected at interfaces where there is a contrast in density and P-wave velocity. The downhole logs in Figure 10 confirm that heterogeneities in density and velocity give rise to seismic reflectors. During post-cruise research, the initial travel time-depth correlation of Figure 10 will be refined and extended to other sites. The goal is to calibrate the seismic stratigraphy and better constrain reconstructions of the sedimentation history in the Pacific equatorial region.

Figure 11. Synthetic seismogram for Hole U1331A

Synthetic seismograms were made from bulk density and sonic velocity data from core material and downhole logs using the IESX software by Schlumberger GeoQuest. The synthetic seismogram was stretched and squeezed slightly to line up the synthetic reflections with those in the seismic section. Typically, only a small number of adjustments were needed to give a good match. The synthetics help to match features in the lithology to reflections in the seismic section. An example from Site U1331 is given in Figure 11.

Summary

A downhole logging program was successfully carried out on IODP Expedition 321. Downhole logs measured profiles of natural gamma-ray radioactivity, density, electrical resistivity, and velocity together with high-resolution electrical resistivity images of the borehole wall. VSP experiments were also run at both Exp. 321 sites. The downhole measurements have been useful in reconstructing the drilled sequence in intervals of difficult core recovery, and will help to address some of the major objectives of the PEAT program. Post-cruise activities will concentrate on developing an accurate core-log depth correlation.

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

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

Alberto Malinverno: Principal Scientist, Borehole Research Group, Lamont-Doherty Earth Observatory of Columbia University, PO Box 1000, 61 Route 9W, Palisades, NY 10964, USA

Louise Anderson: Logging Staff Scientist, Department of Geology, University of Leicester, Leicester LE1 7RH, United Kingdom