After completion of drilling operations at Hole 1128D, the hole was prepared for logging (see "Operations"). An initial logging attempt was made with the base of pipe at 90 mbsf. This proved to be too shallow and resulted in a niche being carved into the sidewall as the pipe moved in the complex ~3-m heave. After this initial attempt, the lower limit of the BHA was placed at 110 mbsf for both logging runs (Fig. F33). Two logging tool strings were run in the following order: (1) triple combo, phasor dual induction-spherically focused resistivity tool (DITE-SFL), hostile environment lithodensity sonde, accelerator porosity sonde (APS), hostile environment natural gamma-ray sonde; and (2) FMS/sonic (see "Logging Tools and Tool Strings" in the "Explanatory Notes" chapter). Neither logging run reached the total depth drilled (452.5 mbsf), with the first run touching down at 424 mbsf and the second at 419 mbsf (Fig. F33). Because of time constraints, the geologic high-resolution magnetic tool and WST were not run.
The diameter of the borehole was within the limits of both FMS (38 cm) and triple combo (42 cm) calipers for the entire interval logged and, with only a few exceptions, data quality was good. The caliper record showed the presence of a tight spot in the upper part of the section (252 mbsf), and some small-scale washouts between 242 and 295 mbsf, resulting from thinly bedded alternations of indurated and unindurated sediments (Figs. F34, F35). Extremely high porosities in some unconsolidated layers resulted in poor statistical validity of the APS data, despite the high-resolution (275 m/hr) logging speed. The DITE-SFL malfunctioned, resulting in several unrepeatable small spikes in the deep-resistivity log below 337 mbsf. Despite a functioning wireline heave compensator, ship movement affected the FMS data. Shipboard processing of the FMS was able to remove some of the influences of heave, although postcruise processing will be needed to entirely remove these effects. The analog modes on the sonic tool performed poorly because of the slow formation velocities, which were confirmed by discrete physical properties measurements (see "Physical Properties"). The sonic digital tool (DTCO output) provided good quality data, except within a few meters of the pipe.
Downhole data trends in the logging data at Site 1128 are closely correlated to lithologic and sediment physical properties data. As sediment recovery below 250 m was variable, averaging only ~34%, the downhole logs provided valuable stratigraphic information and enabled more accurate placement of lithostratigraphic boundaries. Downhole logging data, when correlated to the incomplete core record during postcruise analysis, will provide a more complete understanding of depositional processes occurring from the lower middle Eocene to Holocene on the upper continental rise adjacent to the cool-water carbonate margin in the Great Australian Bight.
The compatibility of wireline log and physical properties data measured on both whole-core and discrete samples attests to the integrity of the logging data sets (see "Physical Properties"). Overall, downhole measurement trends show the effects of compaction overprinted with the influence of diagenesis and sediment redeposition. Compaction effects cause a gradual increase in sonic velocity and bulk density and a decrease in porosity with depth (Figs. F34, F35). Variations in the importance of diagenesis and sediment redeposition enable the downhole logs to be divided into four units on the basis of general trends in the data.
Logging Unit 1 (0-242 mbsf) was divided into two subunits. Subunit 1A (0-154 mbsf) is characterized by relatively uniform values in all parameters measured, with the exception of gamma radiation, which shows a general increase throughout the unit (Figs. F34, F35). Uniform values within logging Unit 1A are punctuated by excursions of decreased gamma radiation and photoelectric effect (PEF) and increased porosity, bulk density, resistivity, and sonic velocity. These excursions represent thin units (5-8 m) of redeposited nannofossil ooze, with the most prominent of these layers occurring at 133 and 153 mbsf (Figs. F34, F35). The upper part of Subunit 1A (0-110 mbsf) includes the section logged by the spectral gamma-ray (SGR) within the pipe (Fig. F36). The SGR variations within this upper interval correlate well with those from the MST NGR record (see "Physical Properties"). Despite this correlation, the low amplitude of the SGR signal in pipe precluded any unequivocal division of units in the top 100 mbsf of the section. The base of logging Subunit 1A correlates with the first appearance of debris flows and redeposited layers within the upper part of the sedimentary section (see "Lithostratigraphy").
Logging Subunit 1B (154-242 mbsf) is characterized by a downhole decrease in gamma-ray values and nearly constant values for all other logs (Figs. F34, F35). This decrease in gamma radiation is mainly because of a loss of thorium (Fig. F35), indicating downhole dilution of clay minerals. This conclusion is supported by the close correlation of decreased gamma-ray values with increased calcium carbonate content between 160 and 255 mbsf (see "Organic Geochemistry"). The generally homogenous logging values in logging Subunit 1B are related to monotonous, dominantly unlithified, homogenous clays and chalks, which become increasingly lithified below 203 mbsf (see "Lithostratigraphy"). This increased lithification was not observed in the porosity and density logs but correlates moderately well with a slight increase of sonic velocity near 203 mbsf (Fig. F34). The base of logging Unit 1 correlates well with the base of lithostratigraphic Subunit IIB (see "Lithostratigraphy").
The top of logging Unit 2 (242-295 mbsf) occurs at a sharp change in character in all data sets, with an increase in density, sonic velocity, and resistivity and a decrease in gamma radiation and porosity (Figs. F34, F35). All data sets within logging Unit 2 have high variability corresponding to an equivalent interval of high variability in physical properties data from recovered sediments (see "Physical Properties"). This variability in lithology and sedimentation was imaged in FMS data as well-bedded alternations of conductive and resistive layers. Logging Unit 2 can be divided into two subunits on the basis of the nature of variability seen in the data. Logging Subunit 2A correlates with lithostratigraphic Subunit IIC, which is composed of claystone with chert and variable lithification. Within logging Subunit 2A, peaks in the different logs are not well correlated, as the thin-bedded turbidites and cherts each result in different log responses (Fig. F34). However, data peaks in logging Subunit 2B correlate well between parameters and reflect thickly deposited nannofossil ooze turbidites. These sharply bounded, high-carbonate turbidites result in decreased gamma radiation and porosity and increased density, resistivity, PEF, and sonic velocity (Fig. F34). Nannofossil ooze turbidites alternate with high gamma-ray, low density, reduced velocity, conductive intervals corresponding to terrigenous-rich sandstones (see "Lithostratigraphy"). The presence of terrigenous-rich intervals is reflected by an increase in thorium and potassium within SGR values of logging Subunit 2B (Fig. F35). Logging Subunit 2B correlates well with lithostratigraphic Unit III, although low recovery prevented the accurate placement of the lower boundary of this lithostratigraphic unit. The downhole logs indicate that this boundary was 11 m lower than described from core data (see "Lithostratigraphy"). The base of logging Unit 2 is marked by a return to less variable values in all parameters (Figs. F34, F35).
Logging Unit 3 (295-362 mbsf) corresponds to lithostratigraphic Subunit IVA, which is composed of relatively monotonous silty clays and claystones. Increased thorium and potassium concentrations within logging Unit 3 reflect an increased concentration of terrigenous sediment between 295 and 262 mbsf (Fig. F35). This conclusion is supported by X-ray diffraction data from this interval, which shows an increase of noncarbonate sediment in samples measured (see "Inorganic Geochemistry"). Bulk density and resistivity are relatively constant throughout logging Unit 3, with the exception of a density peak at 342 mbsf correlating to a peak in PEF (Fig. F34). This layer was described in the recovered sediments as an interval of coarse terrigenous-rich sandstones (see "Lithostratigraphy"). Within logging Unit 3, gamma-ray values increase gradually between 295 and 338 mbsf, and then decrease in a stepwise fashion toward the base of the unit (Figs. F34, F35). Sonic velocity is relatively constant downhole until a slight increase near 350 mbsf, coincident with an increase in sediment induration (see "Lithostratigraphy").
Logging Unit 4 (363-414 mbsf) has consistent values for all parameters, with the exception of gamma radiation and porosity that exhibit some cyclic variability (Figs. F34, F35). An interval of increased carbonate content is seen between 363 and 378 mbsf, as indicated by decreased gamma-ray values and a closer correspondence of porosity and density traces that have been plotted so that the two curves will overlie each other in a clean limestone (Fig. F34). The close correspondence of porosity and density curves for the remainder of the interval indicate calcium carbonate contents in logging Unit 4 are greater than in Unit 3 (Fig. F34). Terrigenous components are also present in the sediments of logging Unit 4, as shown by the moderately high thorium concentrations (Fig. F35). These conclusions are supported by the limited recovery of an indurated clayey siltstone in this interval (see "Lithostratigraphy").
Acoustic impedance was calculated (Fig. F37) using downhole velocity and density data and passed through a 5-m smoothing function to produce a resolution comparable to that of the site-survey seismic data (see "Seismic Stratigraphy"). Two prominent peaks in impedance occur near 130 and 152 mbsf, which correlate well with reflectors occurring above Horizon 1c_h3. The Horizon 1c_h3 reflector (173 mbsf) is not obvious in the impedance data and, thus, may result from interference effects (Fig. F37). A prominent shift in impedance near 236 mbsf does not appear as a significant reflector in the seismic data. The most significant impedance peak occurs near 282 mbsf (Fig. F37) and corresponds to the lower boundary of lithostratigraphic Unit II. The remainder of the impedance record shows peaks that correlate with prominent reflectors below Horizon 1c_3a.