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Downhole measurements

The focus in this section is to provide a description of the downhole data sets collected in Hole M0028A and their significant characteristics and variations. A few examples are given of log trends and boundaries. Some measurements display variation primarily with lithology, whereas others are controlled more by postdepositional factors such as the degree of cementation and diagenesis and the type of interstitial formation water. Integration of the core and logging petrophysical data sets allow calibration of core data with in situ borehole properties and provides an assessment of the precise depth from which core was collected. These correlations are described briefly in "Physical properties." At the end of the section, a synthesis of petrophysical data, downhole logs, and derived quantities are presented for each lithostratigraphic unit.

Downhole measurements in Hole M0028A

In Hole M0028A, 1379.67 m of wireline logging data was acquired, in addition to VSP measurements (see Tables T7 and T8 in the "Methods" chapter). Spectral gamma ray logs were acquired through the steel drill pipe down to 670.27 m wireline log depth below seafloor (WSF).

Two overlapping sections of open hole were logged in Hole M0028A, from 426.3 to 657.7 m WSF (lower section) and from 396 to 429.2 m WSF (middle section). In the lower interval in open hole, acoustic, conductivity, and magnetic susceptibility logs were run. In the middle interval, acoustic and conductivity logs were acquired over a ~30 m interval in open hole. Acoustic logs in this interval were the highest resolution and quality of all boreholes (see description below). VSP measurements were acquired at 404–656 m WSF in open hole and 0–668 m WSF through pipe (see Table T8 in the "Methods" chapter). Figure F48 provides an overview of the downhole measurements for Hole M0028A.

In all figures, downhole geophysical data and images are plotted in meters below present-day seafloor based on the wireline scale (WSF) and, where appropriate, boundaries between lithostratigraphic units (I–VII) are indicated to facilitate comparison with other chapters.

Spectral gamma ray logs

Spectral gamma ray logs allow continuous characterization of the entire borehole and are influenced primarily by lithology. The TGR signal from the downhole probe and the NGR measurement run on whole cores correlate very well and in some cases allow accurate repositioning of core depth below seafloor (see "Stratigraphic correlation"). In general, trends in TGR, K, Th, and U curves are relatively similar in the upper 500 m of Hole M0028A and illustrate alternating coarse-grained intervals with clay-rich intervals (Fig. F48). Total counts in the upper 220 m WSF are generally low. TGR values below 50 cps correspond to dominantly coarse-grained sand intervals (21–53, 90–114, 126–179, and 189–218 m WSF), whereas higher values (>100 cps) correspond to clay-rich levels and stable Th/K and U/K ratios (~0.1). The top of Unit II is also dominated by clays as evidenced by TGR values >150 cps down to 256 m WSF. The TGR signal then decreases drastically in the sands below, with values <30 cps down to 270 m WSF. The lower part of Unit II is dominated by clays except at the base, where TGR increases slightly down to 324 m WSF (with few incursions of low counts) with a decrease in total counts over the lowest 11 m of the unit, when entering sands. In Unit III, TGR values are relatively low (<80 cps), with the smallest counts encountered between 391.5 and 412 m WSF, which ties with intervals of poor core recovery. From 412 to 495 m WSF, the signal is relatively constant, with a step around 475.8 m WSF. A peak at 498 m WSF is related to an increase in Th content, whereas the high-value interval covering Unit IV and the upper part of Unit V (519–545 m WSF) is related to high K content, reflecting the presence of glauconite within dominantly coarse-grained sediment (see "Lithostratigraphy"). TGR values decrease in the lower part of Unit V, with higher K content between 562 and 575 m WSF. Between 592 and 612 m WSF, TGR gradually rises because of a progressive increase in Th content. Unit V is rich in Th with a constant TGR signal around 200 cps, except at the lower limit, where an increased TGR count is associated with increased K content and decreased Th content, reflecting the presence of glauconite in the sediment. TGR then gradually decreases in Unit VII.

Gamma ray logs recorded in Hole M0028A can be interpreted using the same guidelines as in Hole M0027A (see "Downhole measurements" in the "Site M0027" chapter), based on TGR counts and the concentration of the spectral elements K, U, and Th and their ratios (Figs. F48, F49). In the upper 220 m of Hole M0028A, no core was recovered, and the only basis for lithological inference is the gamma ray log collected through pipe and drilling observations or assumed correlations with units recovered elsewhere. The proportion of clay versus sand in a given sedimentary body can be evaluated by the change in TGR when Th/K ratio is stable. The occurrence of glauconite can be assessed by U/K and Th/K ratios (here, <0.05 is assumed to indicate glauconite). Increasing contents of marine organic matter can possibly be assessed by a U/K ratio higher than the Th/K ratio.

Using these guidelines, the interpretation of the gamma ray logs is generally consistent with the sedimentological observations on cores (see "Lithostratigraphy"), with a few exceptions:

  • From 311 to 314 m WSF is the only interval where glauconite was identified in cores but not in gamma ray measurements.

  • From 340 to 392 m WSF (top of Unit III), U/K and Th/K ratios suggest that the glauconitic content in sands slightly decreases progressively downhole, as evidenced by a gradual decrease in K content and increase in Th/K ratio.

  • From 410 to 515 m WSF, the Th/K ratio is very stable. The 476 m WSF boundary does not correspond to a change in sediment mineralogy but instead to a sudden change in the clay/sand ratio together with higher U/K ratio, possibly indicating an increase in marine organic matter that decreases progressively downhole to 492 m WSF. Two gamma ray spikes at 495 and 507 m WSF can also be related to high marine organic matter content.

  • From 515 to 605 m WSF, glauconitic sands exhibit higher U/K than Th/K ratios, possibly suggesting that this unit is rich in organic matter.

  • In contrast, sands from 605 to 612 m WSF and the underlying claystones are depleted in U compared to Th. The K content appears very low in this unit, suggesting either diagenetic K washout or different dominant clay mineralogy.

Based on these guidelines, it is possible to propose a lithological interpretation of the upper part of the hole (0–220 mbsf), where no cores were retrieved (Fig. F50). This section is interpreted to be dominated by sands (often caved below 136 mbsf), with potentially some glauconitic and silty successions and some clays. At 220 m WSF, coring started in an abruptly coarsening-upward clayey unit. Above this, there appears to be a change to sands at 214 m WSF, possibly similar to the uphole coarsening observed in offshore to shoreface–offshore transition successions from 440 to 400 m WSF lower in Hole M0028A or from 440 to 380 m WSF in Hole M0027A.

Another noteworthy feature is the 3 m thick clay layer (133–136 m WSF) with upper and lower boundaries underlined by strong U depletion, suggesting chemical reaction at the sand/clay boundaries, possibly related to redox changes.

Magnetic susceptibility logs

Commonly, magnetic susceptibility is a clear indicator of lithological variations. In all holes, magnetic susceptibility signals from wireline measurements (EM51 probe) and the MSCL on whole cores correlate well, allowing, in some cases, accurate repositioning of core depth below seafloor. As the magnetic susceptibility wireline log in Hole M0028A begins at ~425 m WSF in Subunit IIIC, a more extensive analysis of magnetic susceptibility can be found in "Physical properties."

Acoustic image logs

The ABI40 was run at two different resolutions in Hole M0028A, the lower resolution (72 ppt) in the lower open-hole section and the higher resolution (288 ppt) between 396 and 426 m WSF. Acoustic amplitude and traveltime images provide an estimate of the induration and texture of the borehole walls.

In Hole M0028A, the high-resolution interval in Unit III enables detailed features to be viewed in the borehole wall, in several places supporting sedimentological observations. In this region, the succession grades downhole from coarser grained sandier sediments to finer grained sediments. The acoustic image indicates hardening over this interval by a general increase in amplitudes and faster traveltimes. The clearest change is at ~414.6 m WSF, where amplitude increases and the acoustic caliper changes from variable to a smaller constant diameter when encountering siltier sediments. This change correlates the Subunit IIIB/IIIC boundary. Smaller packets of high- to low-amplitude intervals evident on the images below 415 m WSF are consistent with normal graded storm beds identified in the cores (Fig. F51; see "Lithostratigraphy"). The mottled texture observed on the images from this interval is interpreted as possible burrows, in agreement with observations from core descriptions. Burrows are generally either harder if cemented or otherwise softer than the surrounding formation and would be expected to show up on a high-resolution acoustic log. In Section 313-M0028A-79R-1, a large shell was identified in the archive half of the core but was not present in the working half. On the acoustic image, several low- and high-traveltime "patches" could indicate the location of shells. Below 426 m WSF, the ABI40 was run at a lower resolution (image color changes at this depth are thus artificial). In this interval, it was not possible to get good tool centralization, affecting the image quality and accuracy of the acoustic caliper. However, a generally large and variable diameter borehole is indicated in softer intervals and a more consistent smaller diameter in more indurated zones. A major change in image impedance is clearly observed around 550.2 m WSF, corresponding with the appearance of glauconitic-free sands.

Sonic velocity logs

The sonic tool was not run in Hole M0028A (see "Downhole measurements" in the "Methods" chapter). Velocity measurements from whole cores and discrete samples are described in "Physical properties."

Conductivity logs

Conductivity in the borehole is influenced by a variety of aspects, including lithology, pore water content, and salinity. In Hole M0028A, the conductivity signal shows high variability (Fig. F48). The highest values are observed in Unit III between 395 and 411 m WSF in an interval of poor core recovery that corresponds to poorly sorted unconsolidated medium sands (e.g., Core 313-M0028A-70R). Below 411 m WSF, conductivity decreases abruptly in more competent and finer gained sediments containing clayey silt. This facies transition is also clearly seen on the spectral natural gamma probe (ASGR) curve and ABI40 images (Fig. F48). Conductivity then increases relatively progressively downhole to 475 m WSF, where it shows a low. Below this depth, conductivity is relatively constant until 497.9 m WSF, where it displays a significant low that correlates with a U- and Th-rich interval located just below the m5.4 boundary (see "Stratigraphic correlation"). Below this boundary, conductivity generally increases downhole to ~610 m WSF, with numerous excursions to much lower values. In this interval, conductivity lows often tie to cemented horizons described in the cores (e.g., calcite cemented at 526 m WSF [top 35 cm of Section 313-M0028A-116R-1] and coarse-grained sandstones at 554.45 m WSF [lower part of Sections 129R-1 and 129R-2]). A sharp decrease in conductivity is observed at 610 m WSF, associated with a major change in facies from sands to the silty Th-rich clays of Unit VI. In this unit, conductivity slightly increases to 648.7 m WSF.

As in Hole M0027A, variations in the electrical conductivity curve fit relatively well with chlorinity changes in interstitial waters (Fig. F41; see "Geochemistry"), with the exception of the 430–470 m WSF interval, where electrical conductivity increases downhole and chlorinity slightly decreases. This exception fits with a downhole decrease in density measured in both MSCL and discrete samples and an increase in porosity over this interval. This suggests that the increased conductivity may reflect either an increase in surface conductivity related to changes in the clay fraction and/or a higher volume of pore water stored in the sediment.

Vertical seismic profiling

VSP data were acquired through pipe from 668 m WSF to the seafloor (Fig. F52). Downgoing waves were picked and time-depth relationships calculated. The resultant time-depth curve can be used to calibrate the time to depth of the seismic reflection profile.

Example of multi-log interpretation

Petrophysical and downhole measurements can provide evidence for sequence stratigraphic boundaries. Figure F53 is a compilation of several parameters around the m5.8 boundary (see "Stratigraphic correlation") located at the interface between lithologic Units VI and VII. It marks the abrupt transition between brown sandy siltstone with glauconite-filled burrows below and glauconitic silty medium sand above. This boundary was located at 662.98 mbsf in the core by the sedimentologists (see "Lithostratigraphy"). In Figure F53, core and data measured on the whole core match the wireline signal relatively well, and no core depth shift was required. The m5.8 boundary is marked on the petrophysical data by a clear decrease in P-wave velocity and resistivity and by slight changes in density when passing from siltstone to sands. The magnetic susceptibility signal also increases significantly at this boundary, reflecting the presence of glauconite in the sand. These sands, which extend from 662.98 to 660.7 m WSF, are characterized by low TGR counts (with low Th content). TGR counts decrease toward the top of the sands, with a minimum at 661 m WSF, fitting with a peak in magnetic susceptibility and a slight increase in density. The top of the sands is described in the core as an abrupt but transitional change to dark sandy clays, becoming tan colored moving uphole. This transition is marked by a progressive increase in TGR counts and Th content, a slight increase in resistivity, and a decrease in magnetic susceptibility and density. The tan-colored sandy clays above show low total counts, high Th content, and high magnetic susceptibility not related to glauconite, as testified by the low K content in these deposits.

Downhole log and physical property interpretation

This section combines results of the logging and physical property measurements with the main characteristics of the lithostratigraphic units. A brief summary shows the links between key petrophysical intervals (Table T12) and stratigraphic surfaces (Fig. F36A, F36B, F36C, F36D, F36E). Lithostratigraphy depths are given in meters below seafloor and ignore small differences that may exist between core (mbsf) and log (m WSF) depths. More details can be found in "Physical properties" or elsewhere in this chapter. Numerical data are accessible online; see "Publisher's notes" for links to the database.

Lithostratigraphic Unit VII

Unit VII is only 7 m thick. From 669 mbsf to the top of the unit, gamma ray, magnetic susceptibility, and density increase, the latter peaking at the Unit VII/VI boundary (surface m5.8). These changes reflect increasing glauconite content.

The transition to Unit VI shows a significant peak in TGR, a porosity low, and a density peak that corresponds to a sandy siltstone. The drop in amplitude of all parameters above corresponds to a change in lithology to glauconite sand.

Lithostratigraphic Unit VI

Unit IV TGR data show a flat serrated shape in high values corresponding to clay and siltstone sediments. Magnetic susceptibility displays a bow shape in the middle part of the unit that does not correlate to any change in K content or to glauconite content. However, as the mineralogy here is again fine grained (as in the high–magnetic susceptibility clays of Holes M0027A and M0028A in Unit II), the signal may correlate with fine-grained silt and clay glauconite. This hypothesis is supported by two XRD measurements on samples on both sides of this interval indicative of a high glauconite content. The boundary with Unit V shows a sharp decrease in TGR and an increase in conductivity.

Lithostratigraphic Unit V

The Unit V TGR curve shows a complex pattern with a large bow-shaped trend overlain by three irregular bell-shaped trends in low to high values. This, together with parallel variations in K and low Th/K ratios, corresponds to a change in glauconite content in an overall sandy material. Large variations in conductivity values appear to relate to changing chlorinity (see "Thermal conductivity") bounded by high-density/low-conductivity horizons in numerous cemented intervals. Most often, sediments in this unit contain some glauconite, with a general increase through the unit. This is shown by a slight increase in magnetic susceptibility from the base of the unit to 246 mbsf (surface m5.6), where there is a step increase at a marked increase in glauconite content. There appears to be an interval anomalous to this trend from 546 to 565 mbsf, where magnetic susceptibility (and the K content) is slightly lower but the glauconite content remains high.

At the boundary with Unit IV, TGR values show a distinct kick with an increase, a sharp decrease at the boundary, and then an immediate increase above the boundary. Density shows a peak and conductivity a low at this boundary.

Lithostratigraphic Unit IV

Unit IV is a thin unit that shows considerable variations in gamma ray and conductivity values. Magnetic susceptibility decreases throughout this unit, mirrored by decreasing density, which corresponds to a glauconite decrease.

The boundary with Unit III is characterized by an abrupt but gradational decrease in K content (and increase in Th/K ratio) from below the boundary with a sharp step decrease at the boundary. This is accompanied by a low in MSCL velocity and porosity.

Lithostratigraphic Unit III

The Unit III TGR curve shows high values with a monotonous serrated shape at the base overlain by low values in a series of boxcar-shaped trends becoming more serrated with higher values uphole. This corresponds to the overall superposition of clayey silts with sand interbeds overlain by clean sand with possibly some glauconite. In detail, however, gamma ray values are variable, with a large peak at 498 mbsf just below the m5.4 surface. This kick is mirrored by a peak in density and a low in conductivity. In Subunit IIC, conductivity shows a gradual decrease, whereas chloride content increases, which contrasts with the trends in the remainder of the unit that follow chlorinity (see "Thermal conductivity"). The more variable gamma values in the upper subunit reflect the presence of glauconite. Magnetic susceptibility throughout this unit is low to 336 mbsf in Subunit IIA (correlating with K content), where glauconite appears in the formation.

The boundary with Unit II (surface m5.3) is characterized by an increase in gamma ray, which corresponds to an increase in glauconite content across the boundary. Density decreases toward the boundary to a low just below the boundary, and then, as with gamma ray values, increasing values respond to glauconite.

Lithostratigraphic Unit II

The TGR curve in Unit II shows the superposition, from base to top, of high values with a serrated-shaped curve (335–295 mbsf), alternations of intermediate and low values with funnel and boxcar-shaped trends (295–255 mbsf), and high values with a serrated-shape trend (255–225 m). These trends correspond, respectively, to clayey sediments overlain by sand/silty clay alternations ending with a thick sand package capped by a fining-upward succession from silt/sand to clays. The thick sand with a boxcar character shows high Th values that may relate to the presence of glauconite despite very low magnetic susceptibility. The unit shows three highs in magnetic susceptibility (336–320, 276–284, and 232–244 mbsf). The lower two correspond to high densities and as such indicate the presence of probable fine-grained glauconite together with magnetite, as demonstrated by XRD analyses. The upper one, with low density and a low Th/K ratio, does not correspond to glauconite but rather to ferrimagnetic mineral in the sediment. Four specific levels at 316, 288, 272, and 246 mbsf exhibit high density and low conductivity and may correspond to permeability barriers for the diffusion of fresh and salty waters in the sedimentary column. The upper two and the lower one correlate, respectively, with unconformities recognized both in cores and in the seismic data: m5, m5.45, and m5.2. The boundary with Unit I is characterized by an abrupt decrease in TGR, from high values below to low and constant values above, reflecting an abrupt increase in grain size from silty clays to clean sands. However, the maximum decrease in TGR occurs at 217 mbsf, a few meters above the boundary. This difference in interpretation is probably due to the pick in the first recovered core by the sedimentologists.

Lithostratigraphic Unit I

The TGR curve in Unit I shows alternation of ~10 m thick low values, boxcar trends and meter scale high values in a stack of serrated-, bow-, bell, and funnel-shaped patterns. These characters correspond to alternation of shallow shelf and shoreface clean sands and coastal plain silts and clays. The Th/K ratio shows a number of peaks from 225 to 110 mbsf; these tend to disappear in the upper part of the unit. The lack of cores and MSCL logs in this unit prevent, however, any further interpretation of these changes in K content.

Table T12 summarizes the key petrophysical surfaces and intervals defined in this chapter. Many of these relate to stratigraphic surfaces (last column of Table T12). An exception to this are the m5.3 sequence boundary, not picked by petrophysical characters, and a candidate for an unconformity (600–604 mbsf). Possibly, a minor peak in the Th/K ratio and a slight decrease in density may justify the lithostratigraphic peak of the surface. Other changes of the petrophysical characteristics not recorded in the lithostratigraphy section correspond to cemented horizons or minor facies variations in the units.

Stratigraphic surfaces and correlation with petrophysical intervals

Table T12 lists the key petrophysical surfaces and intervals defined. Many of these relate to stratigraphic surfaces (see penultimate right-hand column on Table T12), although typically there is a small difference in depth that can be attributed to the precise location of the surface (i.e., at the start of an increase in a parameter value or at the peak of the change). Petrophysical surfaces and intervals are defined using the full suite of logs and petrophysical parameters, generally chosen where more than one property displays a significant change. For Site M0028, almost all stratigraphic surfaces are also characterized by a change in petrophysical parameters at or adjacent to the surface. There are apparent exceptions to this, as the m5.2 and m5.3 sequence boundaries along with the possible sequence boundary identified at 600.3–604.42 mbsf (see "Stratigraphic correlation") do not correlate with a key surface identified from petrophysical logs. However, there is a minor impedance contrast at the m5.2 boundary and a minor K/Th peak at the m5.3 surface, along with a density decrease just below, but these were not considered significant in relation to larger variations above and below. Again at this site, several of the petrophysical surfaces/intervals that are identified do not clearly correlate with stratigraphic surfaces or lithological units/subunits. Some of these relate to cemented horizons (see notes in last column of Table T12), whereas some reflect minor sedimentological changes rather than changes on a unit/subunit scale.