PHYSICAL PROPERTIES

At Site 1228 we collected a full range of physical property data from all available cores in Hole 1228A, which extended from the seafloor to a depth of 200.9 mbsf. All cores except for two (Core 201-1228A-13M and the deepest one, Core 23P) were taken by APC. Four additional holes were drilled at Site 1228 to address high-resolution objectives and spot-coring needs. Each core was run through the multisensor track (MST), with Hole 1228E at high resolution and Holes 1228B, 1228C, and 1228D at standard resolution, but no discrete moisture and density (MAD) samples were collected from these subsequent holes.

The physical property data from these cores are described below and compared with those from Site 680 (Shipboard Scientific Party, 1988b). The stratigraphic record was extended marginally from Leg 112, Hole 680B (195.5 mbsf) to 200.9 mbsf in Hole 1228A, with much improved recovery below 92.0 mbsf because of APC coring. Dolomitic hardground and gravels resulted in caving and out-of-sequence responses in the downhole trend of physical logs, but in most cases these are located at or near the ends of cores. Nonetheless, we were able to acquire a more complete interpretable record at a higher spatial resolution than Leg 112 scientists.

Whole-round cores were degassed for up to 2 hr on the catwalk when necessary for safety, were equilibrated to laboratory temperature (2-4 hr), and then each available section was run on the MST. The standard measurements were magnetic susceptibility (spacing = 5 cm, data acquisition scheme [DAQ] = 2 x 1 s), gamma ray attenuation (GRA) density (spacing = 10 cm, count time = 5 s), P-wave velocity (spacing = 10 cm, DAQ = 10), and NGR (spacing = 30 cm, count time = 15 s). Thermal conductivity measurements were made on the third section of each whole-round core in Hole 1228A, where possible. Some sections were removed from the catwalk for microbiology and interstitial water sampling. Physical properties were measured on these sections only if intact parts remained following the sampling. This limited the continuity and, hence, spatial resolution of the physical property record below ~90 mbsf.

MAD, P-wave velocity from the digital velocimeter, and resistance data (translated to formation factor, as detailed in "Formation Factor" in "Physical Properties" in the "Explanatory Notes" chapter) were collected regularly only from Hole 1228A. MAD samples were taken at one per section and at higher resolution in sections with many voids or lithologic transitions. MAD samples were co-located with the methane headspace extractions, where possible, to facilitate the volumetric analysis of methane concentrations. Spot sampling for MAD was also carried out in Hole 1228D, in order to confirm measurements from Hole 1228A. Even though core recovery decreased significantly below ~90 mbsf, the supplementary data are sufficient to allow characterization of the physical parameters of each lithostratigraphic unit and to be confident of the correspondence of our data to those from Site 680.

Instrumentation, measurement principles, and data transformations are further discussed in "Physical Properties" in the "Explanatory Notes" chapter.

The infrared scanner was not employed at this site, due to expected rapid recovery.

Low-field volume magnetic susceptibility was measured on the MST using the Bartington loop sensor as described in "Magnetic Susceptibility" in "MST Measurements" in "Physical Properties" in the "Explanatory Notes" chapter. The magnetic susceptibility record is shown in Figure F9. We have divided the cored interval into five susceptibility "zones" based on the downhole record. These zones compare readily with the lithostratigraphic subdivisions (see "Description of Lithostratigraphic Units" in "Lithostratigraphy"):

1. From 0 to 43 mbsf (Subunits IA and IB), the signal mostly varies between 2.5 x 10^-5 and 10 x 10^-5 SI units. Within this interval there are several peaks of up to 25 x 10^-5 SI units (including at 2.5, 11, 21, and 35 mbsf). This record corresponds closely to the Site 680 data set (Fig. F9B) (Merrill et al., 1990). The peak at 2.5 mbsf emanates from a glauconite-bearing quartz-rich lithic silt. There is a significant increase in susceptibility at 24 mbsf, corresponding to the Subunit IA/IB boundary, which is best seen in the high-resolution data set (Fig. F9C). The remainder of the peaks in this interval are located near core or section ends and appear to be artifacts that do not correspond to obvious lithologic features.
2. Across the interval from 43 to 57 mbsf (lithostratigraphic Subunit IC), magnetic susceptibility decreases from 10 x 10^-5 to 0 x 10^-5 SI units, with peaks at 43 and 47 mbsf. As with zone 1 above, the higher susceptibility values are adjacent to the end of a core, making the reliability of the signal questionable.
3. At 57 mbsf there is an abrupt increase in magnetic susceptibility from ~0 x 10^-5 to 15-20 x 10^-5 SI units. The magnetic susceptibility high is located within a 5-m interval that corresponds to a lithologic change from overlying diatom oozes of Unit I to the clay- and silt-dominated lithologies of Unit II (see "Description of Lithostratigraphic Units" in "Lithostratigraphy"). From 57 to ~72 mbsf, susceptibility decreases from 20 x 10^-5 to 5 x 10^-5 SI units.
4. There is a peak of 30 x 10^-5 SI units at 72 mbsf. This peak is located at the top of Core 201-1228A-9H and is possibly a result of accumulated gravel in the bottom of the hole prior to coring the next interval, although Section 201-1228A-9H-1 contained the stratigraphically highest recorded gravel in the sequence. Below 72 mbsf, magnetic susceptibility increases from 5 x 10^-5 to 20 x 10^-5 SI units at 120 mbsf. Although the record is incomplete, the most significant increase in the susceptibility record is located between 90 and 110 mbsf.
5. From 120 mbsf to the base of Hole 1228A, magnetic susceptibility is consistently higher, varying between 15 x 10^-5 and 35 x 10^-5 SI units. Core recovery was much lower below 90 mbsf, and several narrow peaks are present adjacent to missing sections and core tops. We interpret the highest readings across this interval as drilling artifacts.

At Site 1228 we collected 21 discrete samples for paleomagnetic measurements. The sampling frequency was two samples from each core in Cores 201-1228A-2H through 5H (6.8-42.9 mbsf) and one sample from each core below this interval to the bottom of the hole (Cores 201-1228A-6H through 22H; 42.9-194.9 mbsf). Alternating-field (AF) demagnetization of the natural remanent magnetization (NRM) was conducted up to 40 mT in 10- or 5-mT steps. Anhysteretic remanent magnetization (ARM) was measured to 40 mT in 10-mT steps with a 29-µT direct current-biasing field. AF demagnetization of the ARM was conducted to 40 mT in 10-mT steps.

Lithostratigraphic Subunits IA through IB and the lower part of Subunit IIC (see "Description of Lithostratigraphic Units" in "Lithostratigraphy") exhibit higher NRM intensity (Fig. F10). Dark brown sand-sized foraminifer-bearing siliciclastic-rich diatom ooze (Samples 201-1228A-2H-2, 65-67 cm, and 4H-5, 32-34 cm) in lithostratigraphic Subunits IA and IB show stable magnetization (Fig. F11). Gray volcanogenic material containing silt (Sample 201-1228A-3H-7, 5-7 cm) shows stable magnetization after removing the downward drilling-induced overprint by 10-mT AF demagnetization (Fig. F12). Higher NRM intensity and susceptibility in lithostratigraphic Subunit IIC correlated with the coarse lithology, which is gray diatom and quartz-rich feldspar silt and sand. This interval has a low magnetic intensity after 20-mT AF demagnetization (Fig. F10).

Density data were measured on the MST by the GRA densitometer (spacing = 10 cm, count time = 5 s) and calculated from split-core mass/volume measurements. Porosity was calculated from the split-core samples. The overall trend in the GRA density is increasing, from 1.2 g/cm³ at the top of Hole 1228A to ~1.7 g/cm³ at the base (Fig. F13). Moving averages of 5 m of Hole 1228A and Site 680 GRA densities illustrate consistent general trends, with both data sets highlighting the abrupt lithologic boundary at 57 mbsf (Fig. F14). The discrete sample MAD bulk densities track the MST density record very well, except for a few outliers in Unit I (Fig. F15A). The porosity decreases from ~80% at 0 mbsf to ~50% by 120 mbsf (Fig. F15C). At 90 mbsf, coincident with the Subunit IIB/IIC boundary, amplitude variations in the porosity data decrease. Below 120 mbsf, average porosity is relatively constant to the bottom of the hole. There is a strong correlation among the density, porosity, and magnetic susceptibility records, and the same five zones seen in magnetic susceptibility are apparent in both of the other records, delineated by noticeable peaks in density and lows in porosity:

1. The top 43 m, incorporating Subunits IA and IB, has a stationary downhole trend with density consistently between 1.2 and 1.5 g/cm³. The bottom of this zone is identified by a clear density peak of 2.0 g/cm³ at 42 mbsf, which corresponds to a porosity low of 35%.
2. From 43 to 57 mbsf (Subunit IC), the average MST bulk density decreases slightly from 1.3 to 1.2 g/cm³, with a similar decrease observed in the discrete sample data. We attribute this to a systematic increase in biogenic components of the sediment (grain densities fall to ~2.06 g/cm³) (Fig. F15B). Bulk density abruptly increases to ~2.0 g/cm³ at 57 mbsf, coincident with the Unit I/II boundary and a sharp porosity decrease.
3. From 57 to 72 mbsf (Subunit IIA), bulk density decreases from 1.8 to 1.4 g/cm³, with one excursion at 72 mbsf that corresponds to the stratigraphically highest gravel layer.
4. The zone from 72 to 120 mbsf has at least four cycles characterized by an upward increase in density and corresponding upward decrease in porosity, although the porosity record is more symmetrical than the density, especially in the lowermost of these cycles. Each cycle is 10-15 m thick. Core recovery greatly decreased below 90 mbsf, and gravel layers were consistently found at the top of cores, suggesting core disturbance. The wireline log density (Fig. F13) and porosity (Fig. F15C) records, however, show the same cycles, and partial records from the discrete samples support recognition of these trends.
5. From 120 mbsf to the base of the hole, the average density is nearly constant at 1.8 g/cm³.

Grain density data from Holes 680A and 680B extend from 0 to ~90 mbsf (Shipboard Scientific Party, 1988b). Both sets of their density measurements are highly variable, and we have not included these data in our plots. The values are ~12% higher than Hole 1228A measurements shown in Figure F15B. In Hole 680B they range from 2.2 to almost 3.0 g/cm³. The poor resolution of their data shows no clear downhole pattern. A positive excursion is visible in the Hole 680A data at 60 mbsf, coincident with the one we identified in Hole 1228A, but apparently grain density lows were not detected between 45 and 50 mbsf or at ~72 mbsf.

P-wave data from the MST P-wave logger (PWL) were recorded (spacing = 10 cm, DAQ = 10) for all available APC cores from Holes 1228A, 1228C, and 1228D and at 2-cm spacing for Hole 1228B. The PWS3 velocimeter was used to measure P-wave velocities on split cores from Hole 1228A, with measurements taken at a minimum of one per section, depending on lithologic boundaries or evidence of diagenetic or other petrophysical changes. Most measurements were taken in the x-axis direction using the PWS3 contact probe. In intervals containing suspension-like material, most of which were silt or sand, insertion P-wave sensors 1 and 2 (PWS1 and PWS2), which measure along the core axis (z-axis) and across the core axis (y-axis), respectively, were used. These were used in Cores 201-1228A-16H, 19H, and 20H.

Between 0 and 40 mbsf, PWL measurements range from 1445 to 1680 m/s, whereas PWS3 velocities are within the range 1520-1625 m/s (Fig. F16). The PWS measurements were generally 30-40 m/s faster (similar to measurement differences at Site 1227). Between 40 and 90 mbsf, PWL measurements range from 1420 to 1680 m/s, whereas PWS3 velocities are bounded within 1520-1680 m/s (Fig. F16). This depth interval is characterized by more variable measurements. Below Core 201-1228A-11H (~90 mbsf), the PWL and PWS3 measurements are discontinuous but generally increase downcore from ~1550 to 1650 m/s, to a depth of ~160 mbsf.

There are several peaks in P-wave velocities that correspond to peaks found in other physical property measurements at 42, 57, and 72 mbsf. However, the resolution of small peaks (such as those at 2.5, 11, and 25 mbsf) is too poor to make any conclusive interpretation based on P-wave data alone. Below 90 mbsf, velocity peaks may be less dependable because the core liners were commonly not completely filled.

NGR was measured on the MST for all Site 1228 holes (spacing = 30 cm, count time = 15 s), except for Hole 1228B, which was run at a higher spatial resolution (spacing = 15 cm, count time = 15 s). In addition, natural gamma radiation was recorded with the NGR sonde during the triple combo wireline logging run. Both data sets are shown in Figure F17. There is reasonable correspondence between the trends in the wireline and MST NGR traces above 70 mbsf, where the wireline sonde was within the drill pipe and the record is attenuated. The most prominent peaks (~17 American Petroleum Institute gamma ray units [gAPI] at 3 mbsf and 24 gAPI at 57 mbsf) match those on the MST gamma log, but many of several other lesser peaks on the wireline record do not align easily with the MST data. This lack of alignment suggests minor drilling disruption or noise in the MST data. Below 70 mbsf the MST record is much less complete, but where intervals of five or more continuous meters were recorded, the trends match well with the corresponding depth-equivalent parts of the wireline record (Fig. F17B). From 70 to ~92 mbsf there are at least five spikes in the wireline data that may correlate with quartzo-feldspathic tuffs. The wireline record below 96 mbsf shows a succession of at least five cycles, each 10-15 m thick. Because the cores are incomplete below 90 mbsf, the record of a single cycle has not been entirely preserved.

Thermal conductivity measurements were made on Hole 1228A sediments at a rate of one per core (usually the third section, at 75 cm, if this was available). Values range between 0.71 and 1.29 W/(m·K) (average = 0.96 W/[m·K]) (Fig. F18). Thermal conductivities in excess of 1.20 W/(m·K) are present from 130 to 170 mbsf. This interval corresponds to the quartz-rich feldspar silts and sands of Subunit IIC, the interval of low downhole porosity. Average normalized thermal conductivity and bulk density show a high correlation (Fig. F18B), indicating that the thermal conductivity is a direct function of water content of the sediments. We note that the change in lithology between Unit I and II coincides with an abrupt change in the dominant grain thermal characteristics. Measurements where unfilled core liner or gravel slurry are present are flagged in Figure F18A.

Formation factor (longitudinal and transverse) was determined for Hole 1228A as described in "Formation Factor" in "Physical Properties" in the "Explanatory Notes" chapter with a minimum of one sample per section or corresponding to distinct lithologic changes.

Longitudinal (parallel to core axis) formation factors range from 1.7 to 2.3 in the mainly biogenic sediments and 2.7 to 4.9 in the interbedded siliciclastic sediments of Unit I (Fig. F19). The layered character of Subunits IIA and IIB produce steplike changes in apparent conductivity, with formation factor measurements in quartz-bearing clays up to 5.8. Below 110 mbsf, the dominant lithology of feldspar silt and sand produces formation factors of 2.9-3.3. Low formation factors (~2.5) in this interval arise from the diatom ooze components of Subunit IIC. Electrical conductivity anisotropy typically ranges from 0% to 12% (average = 5%). Overall, the formation factor measurements track the changing lithostratigraphy, clearly delineating the sharp sedimentary changes in the upper 90 mbsf.

Site 1228 has a complex physical property record. The signals result from a variety of lithologies—including silt and clay layers, ash layers, and gravel—and reflect drilling disturbance and include common hiatuses resulting from whole-round sampling. Discrete samples and wireline logs were essential for the interpretation of MST data. The characteristics of five distinct zones are summarized below:

1. 0-43 mbsf. The seafloor and a peak in magnetic susceptibility, density, and thermal conductivity at 43 mbsf bound this zone. It is characterized by fairly low magnetic susceptibility, density, and average P-wave velocities. It has common peaks that are present in magnetic susceptibility, density, porosity, and NGR records. The peaks correlate with silt layers in this interval. The zone corresponds to Subunits IA and IB.
2. 43-57 mbsf. This zone is bounded by peaks in magnetic susceptibility and density. Decreasing magnetic susceptibility and bulk density, low grain density, and high porosity are characteristic. It corresponds to Subunit IC. The boundary at 57 mbsf clearly marks the lithologic break between Units I and II.
3. 57-72 mbsf. This zone is bounded by peaks in magnetic susceptibility and density. High but variable bulk density and magnetic susceptibility, high grain density, and low porosity are characteristic. The peak at 72 mbsf is gravel located at the top of the core, and although this recovery position makes its apparent stratigraphic location suspect, it does represent the highest indication of gravel in the sequence. It is slightly below the Subunit IIA/IIB boundary.
4. 72-120 mbsf. This zone is bounded at the top by gravel at 72 mbsf and at the base by the termination of a series of physical property cycles. The cycles, each of which is at least 10 m thick, are characterized by gradual upward trends from low to high density and resistivity. Porosity is correspondingly low at the top of each cycle and high at the base. The partially complete lithostratigraphic record shows that the interval comprises interstratified clastic silts and silty sands, ashes, and biogenic layers. Although we have yet to correlate the lithologic and physical property records at a detailed scale, it seems clear that the cycles represent a variation of the terrigenous clastic input.
5. 120-200 mbsf. This zone is characterized by less clearly defined but still discernable trends in the partially complete physical property record. Overall magnetic susceptibility, bulk and grain density, and P-wave velocity are relatively higher than in the overlying sequence. Cycles present in the overlying zone are still present from 120 to 200 mbsf but are poorly defined by the density data. Instead, they stand out clearly on the NGR and are present in the resistivity logs. The cyclic pattern is also supported by the partial MAD and MST data.

Overall, the physical property data sets correlate well between the different measurements. A record of the compactional history is present with porosity decreasing downhole, whereas density and P-wave velocity increase. Superimposed on this general pattern at 10-m frequency is a record of the environmental fluctuation between marine and terrigenous sediment input within an overall transition from marginal to open-marine conditions. The wireline log and less complete MST and discrete sample physical data provide a template within which a sequence stratigraphic interpretation can be constructed.