°2
) are listed in Table
5. Relative clay mineral abundance are listed in Table
6. In general, fluctuations in mineral abundance are erratic with depth and age; contrasts among lithostratigraphic units and sites are subtle.
Point counting was used to analyze 134 thin sections. Statistical comparisons among sites are shown in Table 2. Table 3 contains complete point-count data, and Table 4 lists all detrital modes. In general, fluctuations of detrital modes are within the limits of point-counting precision. The modal values of total quartz, total feldspar, and lithic fragments (Q-F-L) are approximately equal in most samples (Fig. 4, Fig. 6). The largest variations generally occur within the polycrystalline constituents (Qp-Lv-Lsm), but some of this scatter may be a statistical artifact of small subset populations.
One of the more interesting discoveries from the Hydrothermal Transition Transect is that sandy silt layers from the lower part of Subunit IA and from Subunit IB contain over 70% (typically over 90%) of what we interpret to be intraformational mud chips. One sample from Site 1028, immediately above the 0.28-Ma datum, also contains abundant intrabasinal mudstone clasts. The mudstone particles are typically brownish yellow in color and contain silt-sized grains of monocrystalline quartz, plagioclase, calcite, microfossils, opaque grains, mica, and mafic minerals in a clay matrix. These clasts of cohesive sediment probably were remobilized off adjacent basement highs.
Temporal variations in sand composition within and among the Leg 168 sites are not pronounced. Except for the lithic-rich intraformational deposits, sand composition within the Hydrothermal Transition Transect is quite homogeneous (Fig. 4). Sand samples from Sites 1028 and 1029 are also fairly homogeneous in Q-F-L modes; variations in polycrystalline constituents are larger and more erratic (Fig. 5). Systematic temporal changes in sand composition occur only within the Rough Basement Transect. Polycrystalline quartz and volcanic-rock fragments at Site 1026 (above basement high) increase steadily above the 0.28-Ma datum, whereas sedimentary-rock plus metamorphic-rock fragments decrease upsection from 75% to 27% (Fig. 6). These trends, however, are not apparent at Site 1027 (above adjacent basement low). Above the 0.09-Ma datum, Sites 1026 and 1027 show consistent increases in sedimentary-rock plus metamorphic-rock fragments (Fig. 6).
Clay mineral abundance was determined from 130 samples of mud (both hemipelagic and turbidite) and 47 samples of matrix clay from sand and silt turbidites. Values of peak intensity (counts), integrated peak area (total counts), and peak width at half height (°2) are listed in Table
5. Relative clay mineral abundance are listed in Table
6. In general, fluctuations in mineral abundance are erratic with depth and age; contrasts among lithostratigraphic units and sites are subtle.
Smectite content is consistently low in all of the samples analyzed (Table 2). At Site 1023, for example, values for hemipelagic mud samples range between 2% and 20% (Fig. 7). Values range from 3% to 20% within Subunit IA, from 2% to 16% within Subunit IB, and from 2% to 5% within Unit II. Similar results were obtained for samples from Sites 1024 and Site 1025 (Table 2). At Site 1028, Subunit IA contains 5%-35% smectite (Fig. 7), whereas values in Unit II increase from 5% to 20%. Smectite abundance at Site 1029 also increases in Unit II and upsection within Subunit IA. Relative abundance at Site 1026 (Rough Basement Transect) are 4%-16% in Subunit IA, 2%-13% in Subunit IB, and 0%-25% within Unit II (Fig. 7). Mud samples from Site 1027 contain less than 12% smectite throughout the section.
The relative abundance of illite in mud deposits at the Hydrothermal Transition sites ranges from 30% to 50%, and chlorite + kaolinite ranges from 40% to 75%. Similar scatter occurs across the Rough Basement and Buried Basement Transects (Fig. 7). At Site 1026, for example, illite abundance is 32%-57%, changing very little among stratigraphic units, whereas chlorite + kaolinite varies between 38% and 63%. At Site 1027, mud samples contain 19%-49% illite (increasing upsection) and 39%-78% chlorite + kaolinite (decreasing upsection).
The relatively thin accumulations of hemipelagic mud at Sites 1030 and 1031 are situated above a basement high; these strata are intriguing because their grain fabrics have not compacted and their pore-fluid compositions are consistent with fluid upflow (Shipboard Scientific Party, 1997a). Our results show that the clay minerals in these muds are not unusual (Fig. 7). When compared to the other sites, samples from Sites 1030 and 1031 contain slightly less smectite (mean values of 5% and 3%), slightly more illite (mean values of 45% and 48%), and similar amounts of chlorite + kaolinite (mean values of 50% and 49%). These subtle compositional shifts are probably not enough to account for the documented differences in compaction behavior (Giambalvo et al., in press).
We also analyzed the <2-µm-size fractions from coarse turbidites to determine whether or not their relative mineral abundance change systematically with respect to those of silty clay interbeds (Table 5, Table 6). Differences in clay mineralogy are <10% in more than half of the sample pairs analyzed, but shifts >20% also occur (Fig. 8). As an extreme example, the highest content of smectite from our study (38%) occurs within a turbidite sand at Site 1027, whereas the overlying mud contains only 12% smectite. None of the mineral groups, however, exhibits a consistent sense of change from one lithology to the other, so we cannot attribute differences in clay composition solely to the effects of selective partitioning by grain size or depositional process.
