PHYSICAL PROPERTIES

Physical properties at Site 1209 were measured on both whole-round sections and discrete samples from split core sections. Continuous whole-round measurements were made of magnetic susceptibility, GRA bulk density, and compressional P-wave velocity using the MST. Whole-round sections of some cores were also measured for natural gamma radiation (using the MST) and discrete measurements of thermal conductivity. Discrete compressional P-wave velocity was measured at a frequency of at least one measurement per split-core section in Holes 1209A, 1209B, and 1209C. Index properties were measured on discrete samples from split core sections at a frequency of one measurement per section throughout Hole 1209A and in Cores 198-1209B-27H and 31H.

MST Measurements

All core sections from Holes 1209A, 1209B, and 1209C were routinely measured on the MST for magnetic susceptibility and GRA density at 2.5-cm (Hole 1209A) or 3-cm (Holes 1209B and 1209C) intervals (Figs. F32, F33). MST P-wave velocity was routinely measured at 10-cm intervals in all APC cores (Fig. F34), but was not measured on XCB cores because of the poor contact between the sediment and core liner. Natural gamma radiation was measured on the MST at 30-cm intervals in some Cenozoic cores and at 20-cm intervals in some cores of late Eocene, late Paleocene, and Late Cretaceous age (Fig. F35). All collected MST data are archived in the Ocean Drilling Program Janus database.

Magnetic susceptibility values (Fig. F32) are generally higher in the uppermost ~115 m of the Site 1209 sedimentary column. The low magnetic susceptibility values in the sediments below ~115 mbsf correlate with the collection of weak magnetic inclination data from these sediments (see "Paleomagnetism"). Peaks in magnetic susceptibility in lithologic Subunits IA and IB may correlate with distinct ash layers (see "Lithostratigraphy"). In the Pleistocene-Pliocene section an excellent correlation is observed between magnetic susceptibility data and color reflectance measurements, primarily the total reflectance value (L*) and the 550-nm wavelength (see "Lithostratigraphy"). Both magnetic susceptibility and color reflectance data in this interval reveal a pronounced cyclicity, which may be useful in identifying astronomically controlled depositional processes. Magnetic susceptibility values are higher in lithologic Subunit IC, relative to Subunit IB. At ~130 mbsf there is a small downhole increase in magnetic susceptibility, which corresponds closely to the Eocene/Oligocene boundary. Subunits IIA and IIB are generally characterized by fairly constant magnetic susceptibility values. Small magnitude peaks in magnetic susceptibility within Subunits IIA and IIB are generally short-lived events such as that at ~198 mbsf that correlates with the Paleocene/Eocene boundary (see "Lithostratigraphy" and "Biostratigraphy"). The Cretaceous/Tertiary boundary is marked in Holes 1209A and 1209C by a large, sharp peak in magnetic susceptibility, associated with an increase in clay content and perhaps corresponding to a microspherule horizon (see "Lithostratigraphy" and "Organic Geochemistry"). In lithologic Unit III magnetic susceptibility values are generally close to background and do not exhibit any consistent downhole variation, except some excursions that are related to chert horizons.

MST GRA bulk density data exhibit a general downhole increase in magnitude from the seafloor to ~111 mbsf (Fig. F33), resulting from sediment compaction and dewatering processes with increased overburden pressure. Cyclical variation in GRA bulk density values, similar to that evident in magnetic susceptibility and color data (see "Lithostratigraphy"), is found within Pleistocene-Miocene lithologic Unit I. At the boundary between lithologic Subunit IC and Subunit IIA (~111 mbsf; Paleogene/Neogene boundary) there is a distinct downhole increase in GRA bulk density. GRA bulk density values then gradually increase, reach a peak, and then decrease between ~111 and 150 mbsf. From 150 mbsf, GRA bulk density values increase rapidly to a maximum value at ~185 mbsf. Between 185 and 200 mbsf, GRA bulk density values decrease somewhat between 200 and 235 mbsf. At the boundary between Subunit IIB and Unit III (~235 mbsf; Cretaceous/Tertiary boundary) a pronounced high peak in GRA density values is recorded.

GRA bulk density values are consistently higher than the discrete wet bulk density measurements (Fig. F33; Table T12) throughout Hole 1209A. These overestimated GRA bulk density values can be explained by the relatively high carbonate content, porosity, and moisture content of sediments; the calibration procedure for the MST GRA sensor is optimized for mixed-lithology sediments. Consequently, the GRA method overestimates the density in carbonate-rich sediments of all lithologic units. This phenomenon is most pronounced in lithologic Units II and III because these sediments have the highest carbonate contents (see "Lithostratigraphy" and "Organic Geochemistry"). During the acquisition of GRA bulk density data from Site 1209, a number of steps to extremely high values (~>2.4 g/cm3) were noted and these data are clearly offset from the general downhole trends within the data set. These anomalous data were subsequently traced to a malfunction of the GRA detector, and this problem was rectified during the collection of Site 1209 MST data.

MST P-wave velocities were recorded at 10-cm intervals in Holes 1209A, 1209B, and 1209C to a depth of ~300 mbsf (Fig. F34). Despite some obviously "out of range" values, a general trend to higher velocities with increased depth in the sediment column can be discerned from values lying between 1500 and 1600 m/s. MST P-wave values generally increase in magnitude with depth through lithologic Unit I and most of Subunit IIA. Between ~150 and ~165 mbsf, P-wave values increase relatively abruptly from ~1525 to ~1550 m/s. In the lower part of lithologic Subunits IIA and IIB, between ~165 and ~235 mbsf, MST P-wave values maintain an almost constant velocity of ~1550 m/s. At ~235 mbsf there is a short-lived increase in P-wave velocity that is associated with an interval just above the K/T boundary and the boundary between lithologic Units II and III. In the lower half of lithologic Unit III, P-wave velocities are highly variable relative to other parts of the sedimentary column, possibly due to localized lithification. The downhole trend recorded by the reliable MST P-wave logger (PWL) values also compare well with the discrete measurements of P-wave velocity (Table T13; Fig. F36). However, MST PWL values are consistently lower than discrete values; this may be due to an assumption in the calibration of the MST PWL that the core liner is full of sediment and that there is no water between the liner and the sediment.

Natural gamma radiation data were collected at 30-cm intervals for some cores of Cenozoic age and at 20-cm intervals in cores of late Eocene, late Paleocene, and Late Cretaceous age from Holes 1209A and 1209B (Fig. F35). Natural gamma radiation data were not collected in the upper 100 m of Hole 1209A. Below 100 mbsf, natural gamma radiation values are generally constant. Peak natural gamma radiation values occur at the Oligocene/Miocene boundary, in the upper Eocene sediments, and at the Cretaceous/Paleocene boundary, reflecting a small increase in clay content in the sedimentary record (see "Lithostratigraphy").

P-Wave Velocity

Discrete measurements of compressional P-wave velocity were made on Site 1209 split-core sections using the modified Hamilton frame (PWS3) velocimeter. These data are listed in Table T13 and illustrated in Figure F36. Data were collected at a routine sampling frequency of one measurement per section. Velocities vary between ~1500 m/s in the soft surface sediments and ~1600 m/s in the more consolidated sediments at Site 1209. Discrete P-wave measurements show an increase in velocity with depth between 0 and ~130 mbsf, which is similar to the trend in the reliable data obtained with the MST PWL. The lack of evidence for early diagenetic cementation near the seafloor, as shown by high-percentage porosity in the interval 0-110 mbsf (Fig. F37; Table T12), suggests that increasing P-wave velocity with depth in the upper 110 m of the sedimentary column is primarily the consequence of compaction and pore fluid expulsion. A downhole increase in discrete P-wave velocities between 0 and 110 mbsf broadly correlates with an increase in the magnitude of discrete bulk density values through this stratigraphic interval (Fig. F38). P-wave values then exhibit a slight increase in magnitude between ~110 and ~300 mbsf, from ~1540 to ~1570 m/s. An exception to this general trend occurs (in Holes 1209A and 1209C) at ~235 mbsf, where P-wave velocities of up to 1650 m/s are recorded over a thin stratigraphic interval. This peak in P-wave velocity is at the boundary between lithologic Subunit IIB and Unit III, which marks the K/T boundary. Recorded P-wave velocities in the upper part of lithologic Unit III are slower than those measured at the base of this lithologic unit. The variation in P-wave velocity in lithologic Unit III may be due to localized lithification of the Cretaceous sediments increasing with depth in the sedimentary column (see "Lithostratigraphy").

Thermal Conductivity

Thermal conductivity data from Site 1209, obtained using the TK04, are listed in Table T14 and shown in Figure F39. Measurements were made on Sections 1 and 3 of each core in Hole 1209A and in Cores 1209B-8H, 10H, and 12H. Average thermal conductivity for the 79 data points is 1.01 W/(m·K), with a standard deviation of 0.18. Thermal conductivity values also exhibit a general increase in magnitude with increasing depth below seafloor from ~0.9 W/(m·K) near the seafloor to ~1.20 W/(m·K) at ~250 mbsf. The downhole increase does not strongly correlate with a decrease in porosity (R2 = 0.37) (Fig. F40), which could be due to the high degree of scatter in the thermal conductivity data set (Fig. F39).

In Situ Temperature Measurements and Heat Flow

The Adara tool was deployed four times in Hole 1209B, but reliable in situ temperature data were only collected for Cores 198-1209B-8H, 10H, and 12H (Table T15). The bottom-water temperature (~2.2°C) was calculated using the temperature data collected when the Adara tool was equilibrated with ambient temperatures at the mudline. The temperature record from Core 198-1209B-8H (a typical deployment record) shows a well-developed thermal decay subsequent to the penetration of the tool into the sediment (Fig. F41). Penetration of the Adara tool into the formation results in an instantaneous rise in recorded temperature, the result of frictional heating, which is followed by an exponential decrease in temperature as the tool equilibrates with the ambient in situ temperature. The calculated Hole 1209B bottom-water and in situ sediment temperatures are illustrated in Figure F42. The geothermal gradient in Hole 1209B was determined from the four data points shown on the temperature profile in Figure F42. These data values can be fitted with a linear least-squares regression (R2 = 0.98), with the solution resulting in a 26.65°C/km geothermal gradient. Average heat flow at Site 1209B, determined using an average thermal conductivity value of 1.11 W/(m·K), is 29.58 mW/m2.

Index Properties

Index properties were determined for discrete samples from Hole 1209A and Cores 198-1209B-27H and 31H. These data are listed in Table T12 and shown in Figures F37 and F43. Index properties primarily reflect progressive sediment compaction and fluid expulsion with depth in the sediment column, but also indicate changes in sediment composition as defined by lithologic units and subunits (see "Lithostratigraphy"). Bulk and dry density increase in magnitude between the seafloor and ~110 mbsf, within lithologic Unit I. Bulk and dry density continue to increase to maximum values within the lower part of lithologic Subunit IIA (~180 mbsf). Between 180 mbsf and the bottom of Hole 1209A (~300 mbsf), both bulk and dry density exhibit a trend to decreasing magnitude. By comparison, grain density exhibits a small general downhole decrease in magnitude. Water content, porosity, and void ratio all show a general decrease in magnitude downhole between the seafloor and ~190 mbsf (lithologic Unit I [Pleistocene-Miocene] and the upper part of Subunit IIA [Oligocene-middle Eocene]). Water content, porosity, and void ratio increase slightly in magnitude between ~200 and ~300 mbsf within lithologic Subunit IIB (Paleocene) and Unit III (Cretaceous). These trends are of interest as the sediments in this interval are less indurated than those recovered above, and this observation is somewhat counterintuitive to the expected general trend of increasing lithification with greater burial depth.

Summary

Physical properties data at Site 1209 show variation with depth below seafloor to ~200 mbsf that suggests control by progressive compaction and fluid expulsion. The physical properties data further suggest that below ~200 mbsf the expected trend of increasing compaction with depth is absent (i.e., downhole, between ~200-300 mbsf, there is generally decreasing GRA density, constant P-wave velocity and increasing porosity, void ratio, and water content). A long temporal break in sedimentation at Site 1209, followed by rapid deposition, could explain these phenomena (such as is evident at Site 1207), but at Site 1209 sedimentation rates were almost constant from the Maastrichtian to the middle Eocene (see "Sedimentation and Accumulation Rates"). Thus, the unusual physical properties data from below ~200 mbsf cannot be explained solely by the sediment burial history at this location. The physical properties data from below ~200 mbsf suggest that there has been less diagenesis and compaction in the sediments below 200 mbsf, relative to those in the interval ~111-200 mbsf. This observation may be a reflection of varying microfossil composition within the Site 1209 sediments and the so-called "diagenetic potential" of different sediments (Schlanger and Douglas, 1974), which in the case of the Site 1209 sediments below ~200 mbsf is obviously low.

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