Three holes (1098A, 1098B, and 1098C) were drilled at Site 1098, and the data from all three were highly comparable. Hole 1098C was drilled to a depth of 46.7 meters below seafloor (mbsf) and was chosen for this study, as it had additional discrete samples taken for physical properties measurements. Two holes were drilled at Site 1099, Hole 1099A (0-62.3 mbsf) and Hole 1099B (60.0-107.5 mbsf). These holes were geographically close enough to enable them to be joined at the chosen depth of 60 mbsf.
Sediments recovered at Sites 1098 and 1099 consist of alternating massive muddy diatom oozes, laminated mud-bearing diatom oozes, and diatom-bearing silty clays and clayey silts with a generally low sand content. In Hole 1098C, ice-rafted pebbles increase in number downhole and at the base of the hole a further increase in coarse clastic material and pebbles is observed. Figures F3 and F4 give the lithostratigraphy of Sites 1098 and 1099, respectively; the lithologic key is given in Figure F5.
Multisensor track (MST) measurements were collected at both Sites 1098 and 1099 (Barker, Camerlenghi, Acton, et al., 1999). These included magnetic susceptibility (MGS) taken every 2 cm (averaged over 2 s), gamma ray attenuation (GRA) density, again taken every 2 cm (averaged over 2 s), and natural gamma radiation counts taken every 15 cm (averaged over 15 s). In addition, physical properties were measured on discrete samples (5-10 cm3), taken on approximately two sections per 10 m of core. On these samples, a saturated mass, dry mass, and dry volume were measured, from which porosity and bulk density were calculated (Blum, 1997).
The derived porosity and bulk density values made on discrete samples taken from the core have been crossplotted (Fig. F6A) and give a significant relationship (r = 0.998 for both sites); 1 known erroneous point was removed out of 48. The values of porosity and bulk density are generated from the same set of three measurements made on the discrete samples and are related by definition; therefore, the relationship between the variables is only quasi-independent.
Evaluation of the bulk density was achieved by crossplotting values against the GRA density (Fig. F6B). Two extra data points were removed from Figure F6B, which gave a significant relationship for both Hole 1098C and Site 1099 (r = 0.967 and 0.889, respectively). Site 1099 tends to give consistently lower GRA density values than the bulk density. Given the strong correlation between bulk density and GRA density, a high-frequency bulk density data set was generated, and from the significant relationship between bulk density and porosity (Fig. F6A), a high-frequency porosity data set was calculated from the newly generated bulk density.
Petrophysical units have been defined to confirm and constrain the lithologic sequence and also to determine whether there is additional detail present that is not captured within the lithologic logs (Figs. F3, F4).
The groups for Hole 1098C (Fig. F7A) are split as follows: Group 1 has MGS values approximately <20; Group 2 has values with a MGS between 20 and 100 and GRA values approximately >1.35 g/cm3. Group 3 MGS values fall between 10 and 600 with varying GRA values. Group 4 is characterized by MGS values >600. The groups for Site 1099 (Fig. F7B) are split in a similar, but not identical, manner to those in Hole 1098C. Group 1 has MGS values between 1 and 13; Group 2, between 13 and 60; Group 3, between 60 and 600; and Group 4, all points with MGS values >600.
The extent to which the clustering of data shown in Figure F7 may be attributed to lithologic variation is demonstrated in Figures F3 and F4. These figures show the lithologic log, MGS, GRA density, and porosity curves for both sites. The MGS and GRA density data are split into the four petrophysical groups defined in the previous section. The lithologic and petrophysical key is given in Figure F5.
The physical properties shown in Figures F3 and F4 do not show smooth graduation downhole. Zones can be picked out visually using the dominant petrophysical groups and character changes within the data. Use of these features enables a series of petrophysical units to be defined.
The porosity curve for Figure F3 decreases steadily downhole (0-38 mbsf) from ~85% to 70%, when it then shows an obvious positive step; the GRA density illustrates the opposite. The MGS data is heterogeneous downhole and has been used in many instances to define the petrophysical units. The turbidites at ~25 and 30 mbsf show graded densities, and this is reflected in the MGS, which generally increases with increasing grain size.
Figure F3 shows how the lithologic log for Hole 1098C, in which one lithostratigraphic unit is defined (divided into Subunits IA and IB), can be subdivided into six petrophysical units using the petrophysical Groups 1-4 defined in Figure F7A. The six petrophysical units are labeled A-F (Fig. F3), and their defining features are listed in Table T1.
The petrophysical Units A and B were recorded as the same lithologies (Barker, Camerlenghi, Acton, et al., 1999), but the response of the MGS data illustrates that the sediment in Unit B is different and is possibly derived from a different source material than Unit A. Preliminary investigations of diatom species assemblages recovered from Sites 1098 and 1099 contribute toward explaining environmental variation in the basins.
A 0- to 9-mbsf piston core recovered at the Site 1098 location (Leventer et al., 1996) revealed that laminated sediments (low magnetic susceptibility) and massive, bioturbated sediments (high magnetic susceptibility) were characterized by different diatom assemblages. Comparison between similar lithologies in petrophysical Units A and B show that the variation cannot simply be due to a change in the percentage of different lithologies. There is real intralithologic variation between the two petrophysical units.
The lithologic and petrophysical logs for Holes 1099A and 1099B are shown in Figure F4. The porosity curve for Figure F4 is very heterogeneous with a decrease of 10% from 0 to 110 mbsf, but values vary by 35% in between and the data show a high degree of scatter. The MGS data vary between three orders of magnitude, but the changes are sharp and scatter is limited. The one lithologic unit can be subdivided into nine units using the petrophysical Groups 1-4 defined in Figure F7. The nine units are labeled A-I (Fig. F4), and their defining features are listed in Table T2.