50-m/s velocity increase in Unit II to the distinct lithology change at ~138 mbsf, below which the sequence is dominated by coarse-grained hornblende-bearing feldspathic sand.
At Site 1229, we collected a full range of physical property data from Hole 1229A, which extended from the seafloor to a depth of 194.4 mbsf. All cores in Hole 1229A except Core 201-1229A-20M were taken by APC. Samples were taken from four additional holes drilled at Site 1229 to address high-resolution objectives and spot-coring needs. All cores from each hole were run through the multisensor track (MST), with Hole 1229E at higher resolution and Holes 1229B, 1229C, and 1229D at standard resolution. No discrete moisture and density (MAD) samples or split-core measurements were collected from these subsequent holes.
The physical property data from these cores are described below and compared with those from Site 681 (Shipboard Scientific Party, 1988). The local stratigraphic record was extended from 187.0 mbsf in Hole 681A to 194.4 mbsf in Hole 1229A, though core recovery at Hole 1229A was very poor below 130 mbsf where sediments consisted of semiconsolidated clayey silts and unconsolidated feldspar- and magnetite-bearing quartz sands. Above 130 mbsf, Holes 1229A and 1229D provide >90% recovery for MST and split-core physical property profiles. We have incorporated wireline logs from the triple combo tool string, which provide a continuous record between ~70 and 170 mbsf, with the shipboard physical property logs to provide an almost complete physical description of the sediments at Site 1229.
Each section of WRC that was analyzed for physical properties was first degassed for up to 2 hr on the catwalk, if necessary, for safety (because of high hydrogen sulfide levels), was equilibrated to laboratory temperature (2-4 hr), and then was run on the MST. The standard-resolution 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 1229A, 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.
MAD, P-wave velocity from the digital velocimeter, and resistance data (translated to formation factors as detailed in "Formation Factor" in "Physical Properties" in the "Explanatory Notes" chapter) were collected regularly only from Hole 1229A. MAD samples were taken at a frequency of 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.
Instrumentation, measurement principles, and data transformations are discussed further in "Physical Properties" in the "Explanatory Notes" chapter.
In general, the wireline logs and physical data that record burial history (bulk density, resistivity, and P-wave velocity) show expected downhole trends controlled by interstitial dewatering with increasing overburden. The measurements most responsive to lithologic variations (magnetic susceptibility, grain density, and NGR) have preserved a record of cyclic sedimentation, and these higher-frequency variations are clearly superimposed on the burial signatures.
We recognize three broad zones in the characteristics of the physical property data: 0-40, 40-138, and 138-194.4 mbsf. The first boundary (40 mbsf) is a distinct lithologic change, whereas the second boundary (138 mbsf) has been chosen at the base of a transitional sequence extending from 125 to 138 mbsf. Within the 40- to 138-mbsf zone, four sedimentary sequences of biogenic siliciclastic deposition are identified, between 40 and 62 mbsf, 62 and 88 mbsf, 88 and 125 mbsf, and 125 and 138 mbsf. In the following sections, we describe the main characteristics of each physical property in terms of these intervals, relating them (where possible) to the lithostratigraphic divisions given in "Description of Lithostratigraphic Units" in "Lithostratigraphy."
The infrared scanner was not employed at this site.
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. Data were collected on WRC sections from deep Holes 1229A and 1229D at standard resolution (spacing = 5 cm, DAQ = 2 x 1 s). The interval of low recovery in Hole 1229A from 40 to 60 mbsf was subsequently covered by sediments from Hole 1229D (Fig. F10A). Hole 1229D terminated at 112.8 mbsf, and data are missing from Hole 1229A over the intervals 113-118, 128-138, and 140-155 mbsf. Both data sets match well, especially above 70 mbsf, but both are increasingly noisy from 80 to 112 mbsf. Despite this interference, we were still able to discern the underlying trends.
The magnetic susceptibility record can be divided into the three zones described above. The uppermost, from 0 to 40 mbsf in Hole 1229A, corresponds to Subunit IA (see "Description of Lithostratigraphic Units" in "Lithostratigraphy"). It is characterized by a low response of 2 x 10-5 to 5 x 10-5 SI units but with three narrow peaks (at 2.5, 15, and 20 mbsf) that reach between 10 x 10-5 and 45 x 10-5 SI units. The 15- and 20-mbsf peaks appear in the records of both Holes 1229A and 1229D and coincide with silt interlayers. At 40 mbsf, there is a sharp increase in average magnetic susceptibility to ~23 x 10-5 SI units; from 40 to 42 mbsf, the signal fluctuates widely. We consider the horizon delineated by this increase to be a significant physical property boundary.
We define a second physical property zone between 40 and 138 mbsf. This zone incorporates Subunits IB and IC. Across this interval all physical property records are characterized by a cyclic pattern that is repeated every 10 to 30 m. The pattern is not as clear in the susceptibility record as it is in the other property profiles. Data from Hole 1229D indicate that the high-susceptibility interval at the top of this zone is 8-9 m thick. Susceptibility generally decreases from ~23 x 10-5 SI units at 45 mbsf to ~0 x 10-5 SI units at 85 mbsf. There is increasing variation in the signal, averaging ~15 x 10-5 SI units to 108 mbsf. From 108 to 120 mbsf, susceptibility remains ~0-10 x 10-5 SI units. From 120 to 138 mbsf, the average susceptibility increases but exhibits large fluctuations, probably as a result of closely spaced interbeds of diatomaceous ooze and clastic silt or sand.
The lowermost zone extends from 138 mbsf to the base of the hole. Over this interval, magnetic susceptibility averages 35 x 10-5 to 45 x 10-5 SI units. Missing data from 140 to 155 mbsf and from most of the section below 170 mbsf make it impossible to determine a trend across this interval.
Magnetic susceptibility data from Site 681 were collected postcruise by Merrill et al. (1990). They are reproduced here for comparison in Figure F10B. Their record is complete only across the uppermost 50 mbsf, where they show good agreement with Site 1229 data. Below 50 mbsf, Site 681 records are too incomplete to enable detailed comparisons.
At Site 1229, we collected 16 discrete samples for paleomagnetic measurements. The sampling frequency was two samples from each core in Cores 201-1229A-2H through 8H (2.9-61.0 mbsf) and one sample from each core below this interval to the bottom of the hole (Cores 201-1229A-9H through 22H; 61.0-194.4 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.
Samples from lithostratigraphic Subunits IA and IC through Unit II (see "Description of Lithostratigraphic Units" in "Lithostratigraphy") indicate higher magnetic intensity after 20-mT AF demagnetization (Fig. F11). The intensity peak in lithostratigraphic Subunit IA at 23 mbsf correlates with the intensity peak at the lower Brunhes/Matuyama boundary (18 mbsf) identified from Hole 681A (Shipboard Scientific Party, 1988). Although stepwise AF demagnetization shows a downward drilling-induced overprint, we were able to isolate the original magnetic direction in lithostratigraphic Subunit IA in Cores 201-1229A-3H through 4H and in Subunit IC in Core 201-1229A-14H. Subunit IA is characterized by dark brown diatom- and clay-rich silt with pale yellow nannofossil-rich laminae and layers of gray silt. Demagnetization of a sample from the pale yellow laminae (Sample 201-1229A-4H-6, 70-72 cm) reveals an original magnetic direction (Fig. F12). Demagnetization of the dark brown diatom- and clay-rich silt (Sample 201-1229A-3H-3, 65-67cm) in lithostratigraphic Subunit IA and laminated clay- and silt-rich diatom ooze (Sample 201-1229A-14H-3, 110-112 cm) in the middle part of lithostratigraphic Subunit IC reveals shallow inclinations (Figs. F13, F14) near that expected for a geocentric axial dipole at this latitude (21°) after removal of the drilling-induced overprint.
Gray siliciclastic layers in lithostratigraphic Subunit IB and Unit II consist of feldspar- and quartz-rich sand with variable proportions of clay. This interval is coarser grained than the rest of lithostratigraphic Unit I. The sediments in this interval have a relatively high NRM intensity (Fig. F11) and magnetic susceptibility; however, we cannot resolve the original magnetic component because of the strong drilling-induced overprint.
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 density and porosity data sets show expected changes with lithostatic loading. Both MST GRA and discrete-sample bulk density increase from ~1.2-1.3 g/cm3 at the top of the hole to ~1.8 g/cm3 at the base (Fig. F15A). Porosity decreases from 80% to 40% over the same interval (Fig. F15C). The three physical property zones are clearly identified, with good resolution of the sedimentary sequences between 40 and138 mbsf (Subunits IB and IC). The most complete records in the lowermost two-thirds of Hole 1229A are the wireline records of bulk density, porosity, and resistivity (Figs. F15, F16), but the same patterns are evident in the less continuous shipboard GRA and MAD bulk density, grain density, and porosity data.
In combination, these data show that the interval from 40 to 138 mbsf is composed of four cycles of variation in the physical properties. The cycles shown in Figures F16 and F17 range from 13 m (125-138 mbsf) up to 37 m (88-125 mbsf) in thickness. Each cycle consists of two parts (see Fig. F17). In the upper part, bulk densities range from 1.6 to 1.8 g/cm3, with porosities averaging ~60%. The lower part is characterized by lower bulk densities (between 1.2 and 1.4 g/cm3) with porosities ranging up to 80%. Grain densities show a similar pattern, with the upper part of a cycle consistently >2.6 g/cm3 and the lower parts between 2.3 and 2.4 g/cm3. Each cycle has a sharp base and gradual change in physical properties upward to the base of the next cycle. Wireline resistivity values exhibit a similar pattern (Fig. F16).
Examination of the lithostratigraphy (see "Lithostratigraphy") shows that the overall interval from 40 to 138 mbsf consists of two main interbedded lithologies: (1) diatomaceous ooze, which forms the principal component of the lower part of the cycle described above, and (2) quartz- and feldspar-rich clay and silt, defined by the higher bulk and grain densities and the lower porosity, making up the upper part of each cycle.
The interval from 138 to 194.4 mbsf is the third and lowermost zone consistently delineated by changes in physical properties, and it corresponds to Unit II (Fig. F15). This interval is characterized by high grain and bulk densities (>2.6 and 1.7-1.9 g/cm3, respectively) and low porosity (<50%). These density and porosity variations are consistent with the dominant sand and silt lithologies.
MAD data for Site 681 were collected at very low resolution but generally fall within the range of our data. GRA data from Site 681 (Shipboard Scientific Party, 1988) show mean values and variance similar to those from Site 1229 over the common intervals. The Site 681 data only extend to a depth of 138 mbsf and were too sparse to allow for a full description of the downhole variation.
P-wave data from the MST P-wave logger (PWL) were recorded at a 10-cm spacing for all available APC cores from Holes 1229A, 1229B, 1229C, and 1229D and at 2-cm spacing for Hole 1229E. The PWS3 velocimeter was used to measure P-wave velocities on split cores from Hole 1229A, with measurements taken at least once per section. Closer-spaced measurements were made at lithologic boundaries and in sedimentary intervals marked by evidence of diagenetic or other petrophysical changes.
P-wave data show a consistent increase in velocity across lithostratigraphic Unit I (physical property zone 1), from 1510 m/s at the seafloor to ~1580 m/s at 138 mbsf (Fig. F18). Several diagenetic anomalies are superimposed on the background velocity increase at 13-15, 42-44, 82-84, and ~94 mbsf (see also "Lithostratigraphy"). The base of Subunit IA at 40 mbsf is particularly evident, with sharply increasing velocities ranging between 1500 and 1750 m/s over the short interval to 43 mbsf. This horizon is marked by interlayered dolomitic and phosphate cements and nodules.
The bases of the two best-defined cycles within the lower part of Unit I (physical property zone 2) at ~83 and 125 mbsf are marked by thin intervals of higher P-wave velocities. The tops of these cycles are indicated by slightly higher velocities relative to the immediately adjacent strata, but the cyclic patterns are not as clearly discerned as in the other data (e.g., bulk density and natural gamma ray measurements).
There is a large gap in our MST and discrete sample data over the interval 128-155 mbsf, but the wireline logs all support a boundary at ~138 mbsf. Below this level, P-wave velocity is on average much faster than would be predicted from the downhole trend extrapolated from Unit I. Most of the PWS3 velocimeter measurements yielded velocities in the range of 1720-1780 m/s. The lower end of this range was the highest value recorded over the same interval on the MST PWL, where most of the values ranged from 1620 to 1720 m/s. We attribute the 50-m/s velocity increase in Unit II to the distinct lithology change at ~138 mbsf, below which the sequence is dominated by coarse-grained hornblende-bearing feldspathic sand.
NGR was measured on the MST for all Site 1229 holes (spacing = 30 cm, count time = 15 s) except for Hole 1229E, which was run at a higher spatial resolution (spacing = 15 cm, count time = 30 s) for the first five cores and then at 20-cm spacing and 30-s count times (from Section 201-1229E-5H-7). In addition, NGR was recorded with the NGR sonde during the wireline logging run. Both data sets are shown in Figure F19. The wireline response is suppressed above ~68 mbsf because of attenuation by the drill pipe. Within Subunit IA, the MST gamma ray response is relatively low at ~20 counts per second (cps), but two spikes at 13 and 20 mbsf appear on both Holes 1229A and 1229D NGR records and in the wireline data. These correspond to quartz- and feldspar-rich silty interbeds. The base of Subunit IA is clearly delineated on all NGR records. It is a narrow interval characterized by large-amplitude fluctuations between ~40 and 42 mbsf. MST and wireline profiles indicate a break at ~56 mbsf, where the trend changes from an upward-decreasing background response (0-56 mbsf) to an upward-increasing background gamma response (56-125 mbsf) seen in both the GRA and wireline data (Fig. F19B).
Between 40 and 138 mbsf, the MST NGR data are overall somewhat noisy and do not track the wireline log closely, although common peaks and troughs are apparent. The wireline log, on the other hand, clearly shows the second and third cycles defined by the other physical property measurements (Fig. F19B). The lowermost cycle (from 125 to 138 mbsf) is different in NGR character from those above; we suggest this interval is a transition between the underlying sand- and silt-dominated clastic unit (Unit II) and the sequences within Subunit IB and the upper part of Subunit IC.
The gamma ray wireline log suggests that the downward-increasing trend seen at the base of Unit I continues in Unit II to a depth of at least 153 mbsf, at which point the wireline logging terminates. This is coincidently the depth at which we were again able to collect MST and MAD data. The MST NGR record indicates that below 155 mbsf the emissions are again low, which is consistent with the quartz-rich sand and silt lithologies recovered.
Thermal conductivity measurements were made on Hole 1229A 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 = 0.86 W/[m·K]) (Fig. F20A). Average normalized thermal conductivity and bulk density show a high correlation (Fig. F20B), indicating that the thermal conductivity is a direct function of water content of the sediments. The combination of high clastic content (i.e., increased grain-scale thermal conductivity) and low porosity in Unit II results in the thermal conductivity anomaly at ~160 mbsf.
Formation factor (longitudinal and transverse) was determined for Hole 1229A as described in "Formation Factor" in "Physical Properties" in the "Explanatory Notes" chapter, with a minimum sample interval of one per section, increasing if distinct lithologic changes were observed.
The trend of decreasing formation factor with depth for a given lithology represents increasing interstitial water salinity, which causes the sediments to appear more conductive relative to the seawater standard used in the resistivity-formation factor transform. This effect will be corrected in postcruise analysis once the interstitial water chemistry has been completely determined.
Longitudinal (parallel to core axis) formation factors range from 1.1 to 2.5 in the mainly biogenic sediments and 2.5 to 4.9 in the interbedded siliciclastic sediments of Unit I (Fig. F21). The steplike changes in apparent conductivity are consistent with changes seen in other physical properties. Below 138 mbsf (Unit II), the dominant lithology of feldspathic silt and sand produces formation factors of 2.9-3.3. Low formation factors (<2.7) reflect recovery artifacts. Electrical conductivity anisotropy typically ranges from 0% to 12% (average = 6%). Overall, the formation factor measurements track the changing lithostratigraphy, clearly delineating the sedimentary sequences in Unit I.
At Site 1229 there are three physical property zones over the 194.4-m interval drilled. The lowermost of these is dominated by terrigenous quartz- and feldspar-rich sand and silt. Overlying this basal epiclastic unit is an interval of mixed terrigenous and hemipelagic sediments arranged into cycles ranging from 13 to 37 m thick. The uppermost unit is dominated by a hemipelagic facies. The three zones are visible in all physical property data sets. The characteristics of each are summarized below (with the depths taken from Hole 1229A):
Overall, the physical property data sets correlate well between the different measurements. The sedimentary environmental record is overprinted by a simple burial pattern showing increasing density and P-wave velocity with depth and a progressive decrease in porosity. Superimposed on this general pattern, on 10-m scales, 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 continuous MST and discrete sample physical property data provide a template within which a composite lithostratigraphic sequence stratigraphy can be constructed.
